_content/doc: temporarily add Go language specification documents
The canonical location for these documents is in the main Go repository
in the doc directory. Due to go.dev/issue/51686, the website is using
Go content from release-branch.go1.17 rather than release-branch.go1.18.
Temporarily add a copy of the spec docs to x/website so it's possible to
view them at go.dev (in addition to tip.golang.org) while that issue is
being resolved.
For golang/go#51686.
Change-Id: Ia6640d6c32af631f562db1294d191f461373b9ee
Reviewed-on: https://go-review.googlesource.com/c/website/+/393115
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Trust: Dmitri Shuralyov <dmitshur@google.com>
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+<!--{
+ "Title": "The Go Programming Language Specification",
+ "Subtitle": "Version of Oct 15, 2021"
+}-->
+
+<h2 id="Introduction">Introduction</h2>
+
+<p>
+This is a reference manual for the Go programming language. For
+more information and other documents, see <a href="/">golang.org</a>.
+</p>
+
+<p>
+Go is a general-purpose language designed with systems programming
+in mind. It is strongly typed and garbage-collected and has explicit
+support for concurrent programming. Programs are constructed from
+<i>packages</i>, whose properties allow efficient management of
+dependencies.
+</p>
+
+<p>
+The grammar is compact and simple to parse, allowing for easy analysis
+by automatic tools such as integrated development environments.
+</p>
+
+<h2 id="Notation">Notation</h2>
+<p>
+The syntax is specified using Extended Backus-Naur Form (EBNF):
+</p>
+
+<pre class="grammar">
+Production = production_name "=" [ Expression ] "." .
+Expression = Alternative { "|" Alternative } .
+Alternative = Term { Term } .
+Term = production_name | token [ "…" token ] | Group | Option | Repetition .
+Group = "(" Expression ")" .
+Option = "[" Expression "]" .
+Repetition = "{" Expression "}" .
+</pre>
+
+<p>
+Productions are expressions constructed from terms and the following
+operators, in increasing precedence:
+</p>
+<pre class="grammar">
+| alternation
+() grouping
+[] option (0 or 1 times)
+{} repetition (0 to n times)
+</pre>
+
+<p>
+Lower-case production names are used to identify lexical tokens.
+Non-terminals are in CamelCase. Lexical tokens are enclosed in
+double quotes <code>""</code> or back quotes <code>``</code>.
+</p>
+
+<p>
+The form <code>a … b</code> represents the set of characters from
+<code>a</code> through <code>b</code> as alternatives. The horizontal
+ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various
+enumerations or code snippets that are not further specified. The character <code>…</code>
+(as opposed to the three characters <code>...</code>) is not a token of the Go
+language.
+</p>
+
+<h2 id="Source_code_representation">Source code representation</h2>
+
+<p>
+Source code is Unicode text encoded in
+<a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
+canonicalized, so a single accented code point is distinct from the
+same character constructed from combining an accent and a letter;
+those are treated as two code points. For simplicity, this document
+will use the unqualified term <i>character</i> to refer to a Unicode code point
+in the source text.
+</p>
+<p>
+Each code point is distinct; for instance, upper and lower case letters
+are different characters.
+</p>
+<p>
+Implementation restriction: For compatibility with other tools, a
+compiler may disallow the NUL character (U+0000) in the source text.
+</p>
+<p>
+Implementation restriction: For compatibility with other tools, a
+compiler may ignore a UTF-8-encoded byte order mark
+(U+FEFF) if it is the first Unicode code point in the source text.
+A byte order mark may be disallowed anywhere else in the source.
+</p>
+
+<h3 id="Characters">Characters</h3>
+
+<p>
+The following terms are used to denote specific Unicode character classes:
+</p>
+<pre class="ebnf">
+newline = /* the Unicode code point U+000A */ .
+unicode_char = /* an arbitrary Unicode code point except newline */ .
+unicode_letter = /* a Unicode code point classified as "Letter" */ .
+unicode_digit = /* a Unicode code point classified as "Number, decimal digit" */ .
+</pre>
+
+<p>
+In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
+Section 4.5 "General Category" defines a set of character categories.
+Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
+as Unicode letters, and those in the Number category Nd as Unicode digits.
+</p>
+
+<h3 id="Letters_and_digits">Letters and digits</h3>
+
+<p>
+The underscore character <code>_</code> (U+005F) is considered a letter.
+</p>
+<pre class="ebnf">
+letter = unicode_letter | "_" .
+decimal_digit = "0" … "9" .
+binary_digit = "0" | "1" .
+octal_digit = "0" … "7" .
+hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
+</pre>
+
+<h2 id="Lexical_elements">Lexical elements</h2>
+
+<h3 id="Comments">Comments</h3>
+
+<p>
+Comments serve as program documentation. There are two forms:
+</p>
+
+<ol>
+<li>
+<i>Line comments</i> start with the character sequence <code>//</code>
+and stop at the end of the line.
+</li>
+<li>
+<i>General comments</i> start with the character sequence <code>/*</code>
+and stop with the first subsequent character sequence <code>*/</code>.
+</li>
+</ol>
+
+<p>
+A comment cannot start inside a <a href="#Rune_literals">rune</a> or
+<a href="#String_literals">string literal</a>, or inside a comment.
+A general comment containing no newlines acts like a space.
+Any other comment acts like a newline.
+</p>
+
+<h3 id="Tokens">Tokens</h3>
+
+<p>
+Tokens form the vocabulary of the Go language.
+There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators
+and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from
+spaces (U+0020), horizontal tabs (U+0009),
+carriage returns (U+000D), and newlines (U+000A),
+is ignored except as it separates tokens
+that would otherwise combine into a single token. Also, a newline or end of file
+may trigger the insertion of a <a href="#Semicolons">semicolon</a>.
+While breaking the input into tokens,
+the next token is the longest sequence of characters that form a
+valid token.
+</p>
+
+<h3 id="Semicolons">Semicolons</h3>
+
+<p>
+The formal grammar uses semicolons <code>";"</code> as terminators in
+a number of productions. Go programs may omit most of these semicolons
+using the following two rules:
+</p>
+
+<ol>
+<li>
+When the input is broken into tokens, a semicolon is automatically inserted
+into the token stream immediately after a line's final token if that token is
+<ul>
+ <li>an
+ <a href="#Identifiers">identifier</a>
+ </li>
+
+ <li>an
+ <a href="#Integer_literals">integer</a>,
+ <a href="#Floating-point_literals">floating-point</a>,
+ <a href="#Imaginary_literals">imaginary</a>,
+ <a href="#Rune_literals">rune</a>, or
+ <a href="#String_literals">string</a> literal
+ </li>
+
+ <li>one of the <a href="#Keywords">keywords</a>
+ <code>break</code>,
+ <code>continue</code>,
+ <code>fallthrough</code>, or
+ <code>return</code>
+ </li>
+
+ <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a>
+ <code>++</code>,
+ <code>--</code>,
+ <code>)</code>,
+ <code>]</code>, or
+ <code>}</code>
+ </li>
+</ul>
+</li>
+
+<li>
+To allow complex statements to occupy a single line, a semicolon
+may be omitted before a closing <code>")"</code> or <code>"}"</code>.
+</li>
+</ol>
+
+<p>
+To reflect idiomatic use, code examples in this document elide semicolons
+using these rules.
+</p>
+
+
+<h3 id="Identifiers">Identifiers</h3>
+
+<p>
+Identifiers name program entities such as variables and types.
+An identifier is a sequence of one or more letters and digits.
+The first character in an identifier must be a letter.
+</p>
+<pre class="ebnf">
+identifier = letter { letter | unicode_digit } .
+</pre>
+<pre>
+a
+_x9
+ThisVariableIsExported
+αβ
+</pre>
+
+<p>
+Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>.
+</p>
+
+
+<h3 id="Keywords">Keywords</h3>
+
+<p>
+The following keywords are reserved and may not be used as identifiers.
+</p>
+<pre class="grammar">
+break default func interface select
+case defer go map struct
+chan else goto package switch
+const fallthrough if range type
+continue for import return var
+</pre>
+
+<h3 id="Operators_and_punctuation">Operators and punctuation</h3>
+
+<p>
+The following character sequences represent <a href="#Operators">operators</a>
+(including <a href="#Assignments">assignment operators</a>) and punctuation:
+</p>
+<pre class="grammar">
++ & += &= && == != ( )
+- | -= |= || < <= [ ]
+* ^ *= ^= <- > >= { }
+/ << /= <<= ++ = := , ;
+% >> %= >>= -- ! ... . :
+ &^ &^=
+</pre>
+
+<h3 id="Integer_literals">Integer literals</h3>
+
+<p>
+An integer literal is a sequence of digits representing an
+<a href="#Constants">integer constant</a>.
+An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code>
+for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal,
+and <code>0x</code> or <code>0X</code> for hexadecimal.
+A single <code>0</code> is considered a decimal zero.
+In hexadecimal literals, letters <code>a</code> through <code>f</code>
+and <code>A</code> through <code>F</code> represent values 10 through 15.
+</p>
+
+<p>
+For readability, an underscore character <code>_</code> may appear after
+a base prefix or between successive digits; such underscores do not change
+the literal's value.
+</p>
+<pre class="ebnf">
+int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
+decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
+binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
+octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
+hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
+
+decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
+binary_digits = binary_digit { [ "_" ] binary_digit } .
+octal_digits = octal_digit { [ "_" ] octal_digit } .
+hex_digits = hex_digit { [ "_" ] hex_digit } .
+</pre>
+
+<pre>
+42
+4_2
+0600
+0_600
+0o600
+0O600 // second character is capital letter 'O'
+0xBadFace
+0xBad_Face
+0x_67_7a_2f_cc_40_c6
+170141183460469231731687303715884105727
+170_141183_460469_231731_687303_715884_105727
+
+_42 // an identifier, not an integer literal
+42_ // invalid: _ must separate successive digits
+4__2 // invalid: only one _ at a time
+0_xBadFace // invalid: _ must separate successive digits
+</pre>
+
+
+<h3 id="Floating-point_literals">Floating-point literals</h3>
+
+<p>
+A floating-point literal is a decimal or hexadecimal representation of a
+<a href="#Constants">floating-point constant</a>.
+</p>
+
+<p>
+A decimal floating-point literal consists of an integer part (decimal digits),
+a decimal point, a fractional part (decimal digits), and an exponent part
+(<code>e</code> or <code>E</code> followed by an optional sign and decimal digits).
+One of the integer part or the fractional part may be elided; one of the decimal point
+or the exponent part may be elided.
+An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.
+</p>
+
+<p>
+A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code>
+prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
+and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits).
+One of the integer part or the fractional part may be elided; the radix point may be elided as well,
+but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
+An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.
+</p>
+
+<p>
+For readability, an underscore character <code>_</code> may appear after
+a base prefix or between successive digits; such underscores do not change
+the literal value.
+</p>
+
+<pre class="ebnf">
+float_lit = decimal_float_lit | hex_float_lit .
+
+decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
+ decimal_digits decimal_exponent |
+ "." decimal_digits [ decimal_exponent ] .
+decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
+
+hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
+hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
+ [ "_" ] hex_digits |
+ "." hex_digits .
+hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
+</pre>
+
+<pre>
+0.
+72.40
+072.40 // == 72.40
+2.71828
+1.e+0
+6.67428e-11
+1E6
+.25
+.12345E+5
+1_5. // == 15.0
+0.15e+0_2 // == 15.0
+
+0x1p-2 // == 0.25
+0x2.p10 // == 2048.0
+0x1.Fp+0 // == 1.9375
+0X.8p-0 // == 0.5
+0X_1FFFP-16 // == 0.1249847412109375
+0x15e-2 // == 0x15e - 2 (integer subtraction)
+
+0x.p1 // invalid: mantissa has no digits
+1p-2 // invalid: p exponent requires hexadecimal mantissa
+0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent
+1_.5 // invalid: _ must separate successive digits
+1._5 // invalid: _ must separate successive digits
+1.5_e1 // invalid: _ must separate successive digits
+1.5e_1 // invalid: _ must separate successive digits
+1.5e1_ // invalid: _ must separate successive digits
+</pre>
+
+
+<h3 id="Imaginary_literals">Imaginary literals</h3>
+
+<p>
+An imaginary literal represents the imaginary part of a
+<a href="#Constants">complex constant</a>.
+It consists of an <a href="#Integer_literals">integer</a> or
+<a href="#Floating-point_literals">floating-point</a> literal
+followed by the lower-case letter <code>i</code>.
+The value of an imaginary literal is the value of the respective
+integer or floating-point literal multiplied by the imaginary unit <i>i</i>.
+</p>
+
+<pre class="ebnf">
+imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
+</pre>
+
+<p>
+For backward compatibility, an imaginary literal's integer part consisting
+entirely of decimal digits (and possibly underscores) is considered a decimal
+integer, even if it starts with a leading <code>0</code>.
+</p>
+
+<pre>
+0i
+0123i // == 123i for backward-compatibility
+0o123i // == 0o123 * 1i == 83i
+0xabci // == 0xabc * 1i == 2748i
+0.i
+2.71828i
+1.e+0i
+6.67428e-11i
+1E6i
+.25i
+.12345E+5i
+0x1p-2i // == 0x1p-2 * 1i == 0.25i
+</pre>
+
+
+<h3 id="Rune_literals">Rune literals</h3>
+
+<p>
+A rune literal represents a <a href="#Constants">rune constant</a>,
+an integer value identifying a Unicode code point.
+A rune literal is expressed as one or more characters enclosed in single quotes,
+as in <code>'x'</code> or <code>'\n'</code>.
+Within the quotes, any character may appear except newline and unescaped single
+quote. A single quoted character represents the Unicode value
+of the character itself,
+while multi-character sequences beginning with a backslash encode
+values in various formats.
+</p>
+
+<p>
+The simplest form represents the single character within the quotes;
+since Go source text is Unicode characters encoded in UTF-8, multiple
+UTF-8-encoded bytes may represent a single integer value. For
+instance, the literal <code>'a'</code> holds a single byte representing
+a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
+<code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
+a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
+</p>
+
+<p>
+Several backslash escapes allow arbitrary values to be encoded as
+ASCII text. There are four ways to represent the integer value
+as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
+digits; <code>\u</code> followed by exactly four hexadecimal digits;
+<code>\U</code> followed by exactly eight hexadecimal digits, and a
+plain backslash <code>\</code> followed by exactly three octal digits.
+In each case the value of the literal is the value represented by
+the digits in the corresponding base.
+</p>
+
+<p>
+Although these representations all result in an integer, they have
+different valid ranges. Octal escapes must represent a value between
+0 and 255 inclusive. Hexadecimal escapes satisfy this condition
+by construction. The escapes <code>\u</code> and <code>\U</code>
+represent Unicode code points so within them some values are illegal,
+in particular those above <code>0x10FFFF</code> and surrogate halves.
+</p>
+
+<p>
+After a backslash, certain single-character escapes represent special values:
+</p>
+
+<pre class="grammar">
+\a U+0007 alert or bell
+\b U+0008 backspace
+\f U+000C form feed
+\n U+000A line feed or newline
+\r U+000D carriage return
+\t U+0009 horizontal tab
+\v U+000B vertical tab
+\\ U+005C backslash
+\' U+0027 single quote (valid escape only within rune literals)
+\" U+0022 double quote (valid escape only within string literals)
+</pre>
+
+<p>
+All other sequences starting with a backslash are illegal inside rune literals.
+</p>
+<pre class="ebnf">
+rune_lit = "'" ( unicode_value | byte_value ) "'" .
+unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
+byte_value = octal_byte_value | hex_byte_value .
+octal_byte_value = `\` octal_digit octal_digit octal_digit .
+hex_byte_value = `\` "x" hex_digit hex_digit .
+little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
+big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
+ hex_digit hex_digit hex_digit hex_digit .
+escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
+</pre>
+
+<pre>
+'a'
+'ä'
+'本'
+'\t'
+'\000'
+'\007'
+'\377'
+'\x07'
+'\xff'
+'\u12e4'
+'\U00101234'
+'\'' // rune literal containing single quote character
+'aa' // illegal: too many characters
+'\xa' // illegal: too few hexadecimal digits
+'\0' // illegal: too few octal digits
+'\uDFFF' // illegal: surrogate half
+'\U00110000' // illegal: invalid Unicode code point
+</pre>
+
+
+<h3 id="String_literals">String literals</h3>
+
+<p>
+A string literal represents a <a href="#Constants">string constant</a>
+obtained from concatenating a sequence of characters. There are two forms:
+raw string literals and interpreted string literals.
+</p>
+
+<p>
+Raw string literals are character sequences between back quotes, as in
+<code>`foo`</code>. Within the quotes, any character may appear except
+back quote. The value of a raw string literal is the
+string composed of the uninterpreted (implicitly UTF-8-encoded) characters
+between the quotes;
+in particular, backslashes have no special meaning and the string may
+contain newlines.
+Carriage return characters ('\r') inside raw string literals
+are discarded from the raw string value.
+</p>
+
+<p>
+Interpreted string literals are character sequences between double
+quotes, as in <code>"bar"</code>.
+Within the quotes, any character may appear except newline and unescaped double quote.
+The text between the quotes forms the
+value of the literal, with backslash escapes interpreted as they
+are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and
+<code>\"</code> is legal), with the same restrictions.
+The three-digit octal (<code>\</code><i>nnn</i>)
+and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual
+<i>bytes</i> of the resulting string; all other escapes represent
+the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.
+Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent
+a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>,
+<code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent
+the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character
+U+00FF.
+</p>
+
+<pre class="ebnf">
+string_lit = raw_string_lit | interpreted_string_lit .
+raw_string_lit = "`" { unicode_char | newline } "`" .
+interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
+</pre>
+
+<pre>
+`abc` // same as "abc"
+`\n
+\n` // same as "\\n\n\\n"
+"\n"
+"\"" // same as `"`
+"Hello, world!\n"
+"日本語"
+"\u65e5本\U00008a9e"
+"\xff\u00FF"
+"\uD800" // illegal: surrogate half
+"\U00110000" // illegal: invalid Unicode code point
+</pre>
+
+<p>
+These examples all represent the same string:
+</p>
+
+<pre>
+"日本語" // UTF-8 input text
+`日本語` // UTF-8 input text as a raw literal
+"\u65e5\u672c\u8a9e" // the explicit Unicode code points
+"\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points
+"\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes
+</pre>
+
+<p>
+If the source code represents a character as two code points, such as
+a combining form involving an accent and a letter, the result will be
+an error if placed in a rune literal (it is not a single code
+point), and will appear as two code points if placed in a string
+literal.
+</p>
+
+
+<h2 id="Constants">Constants</h2>
+
+<p>There are <i>boolean constants</i>,
+<i>rune constants</i>,
+<i>integer constants</i>,
+<i>floating-point constants</i>, <i>complex constants</i>,
+and <i>string constants</i>. Rune, integer, floating-point,
+and complex constants are
+collectively called <i>numeric constants</i>.
+</p>
+
+<p>
+A constant value is represented by a
+<a href="#Rune_literals">rune</a>,
+<a href="#Integer_literals">integer</a>,
+<a href="#Floating-point_literals">floating-point</a>,
+<a href="#Imaginary_literals">imaginary</a>,
+or
+<a href="#String_literals">string</a> literal,
+an identifier denoting a constant,
+a <a href="#Constant_expressions">constant expression</a>,
+a <a href="#Conversions">conversion</a> with a result that is a constant, or
+the result value of some built-in functions such as
+<code>unsafe.Sizeof</code> applied to any value,
+<code>cap</code> or <code>len</code> applied to
+<a href="#Length_and_capacity">some expressions</a>,
+<code>real</code> and <code>imag</code> applied to a complex constant
+and <code>complex</code> applied to numeric constants.
+The boolean truth values are represented by the predeclared constants
+<code>true</code> and <code>false</code>. The predeclared identifier
+<a href="#Iota">iota</a> denotes an integer constant.
+</p>
+
+<p>
+In general, complex constants are a form of
+<a href="#Constant_expressions">constant expression</a>
+and are discussed in that section.
+</p>
+
+<p>
+Numeric constants represent exact values of arbitrary precision and do not overflow.
+Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
+and not-a-number values.
+</p>
+
+<p>
+Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
+Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
+and certain <a href="#Constant_expressions">constant expressions</a>
+containing only untyped constant operands are untyped.
+</p>
+
+<p>
+A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
+or <a href="#Conversions">conversion</a>, or implicitly when used in a
+<a href="#Variable_declarations">variable declaration</a> or an
+<a href="#Assignments">assignment</a> or as an
+operand in an <a href="#Expressions">expression</a>.
+It is an error if the constant value
+cannot be <a href="#Representability">represented</a> as a value of the respective type.
+</p>
+
+<p>
+An untyped constant has a <i>default type</i> which is the type to which the
+constant is implicitly converted in contexts where a typed value is required,
+for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
+such as <code>i := 0</code> where there is no explicit type.
+The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
+<code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
+respectively, depending on whether it is a boolean, rune, integer, floating-point,
+complex, or string constant.
+</p>
+
+<p>
+Implementation restriction: Although numeric constants have arbitrary
+precision in the language, a compiler may implement them using an
+internal representation with limited precision. That said, every
+implementation must:
+</p>
+
+<ul>
+ <li>Represent integer constants with at least 256 bits.</li>
+
+ <li>Represent floating-point constants, including the parts of
+ a complex constant, with a mantissa of at least 256 bits
+ and a signed binary exponent of at least 16 bits.</li>
+
+ <li>Give an error if unable to represent an integer constant
+ precisely.</li>
+
+ <li>Give an error if unable to represent a floating-point or
+ complex constant due to overflow.</li>
+
+ <li>Round to the nearest representable constant if unable to
+ represent a floating-point or complex constant due to limits
+ on precision.</li>
+</ul>
+
+<p>
+These requirements apply both to literal constants and to the result
+of evaluating <a href="#Constant_expressions">constant
+expressions</a>.
+</p>
+
+
+<h2 id="Variables">Variables</h2>
+
+<p>
+A variable is a storage location for holding a <i>value</i>.
+The set of permissible values is determined by the
+variable's <i><a href="#Types">type</a></i>.
+</p>
+
+<p>
+A <a href="#Variable_declarations">variable declaration</a>
+or, for function parameters and results, the signature
+of a <a href="#Function_declarations">function declaration</a>
+or <a href="#Function_literals">function literal</a> reserves
+storage for a named variable.
+
+Calling the built-in function <a href="#Allocation"><code>new</code></a>
+or taking the address of a <a href="#Composite_literals">composite literal</a>
+allocates storage for a variable at run time.
+Such an anonymous variable is referred to via a (possibly implicit)
+<a href="#Address_operators">pointer indirection</a>.
+</p>
+
+<p>
+<i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
+and <a href="#Struct_types">struct</a> types have elements and fields that may
+be <a href="#Address_operators">addressed</a> individually. Each such element
+acts like a variable.
+</p>
+
+<p>
+The <i>static type</i> (or just <i>type</i>) of a variable is the
+type given in its declaration, the type provided in the
+<code>new</code> call or composite literal, or the type of
+an element of a structured variable.
+Variables of interface type also have a distinct <i>dynamic type</i>,
+which is the concrete type of the value assigned to the variable at run time
+(unless the value is the predeclared identifier <code>nil</code>,
+which has no type).
+The dynamic type may vary during execution but values stored in interface
+variables are always <a href="#Assignability">assignable</a>
+to the static type of the variable.
+</p>
+
+<pre>
+var x interface{} // x is nil and has static type interface{}
+var v *T // v has value nil, static type *T
+x = 42 // x has value 42 and dynamic type int
+x = v // x has value (*T)(nil) and dynamic type *T
+</pre>
+
+<p>
+A variable's value is retrieved by referring to the variable in an
+<a href="#Expressions">expression</a>; it is the most recent value
+<a href="#Assignments">assigned</a> to the variable.
+If a variable has not yet been assigned a value, its value is the
+<a href="#The_zero_value">zero value</a> for its type.
+</p>
+
+
+<h2 id="Types">Types</h2>
+
+<p>
+A type determines a set of values together with operations and methods specific
+to those values. A type may be denoted by a <i>type name</i>, if it has one,
+or specified using a <i>type literal</i>, which composes a type from existing types.
+</p>
+
+<pre class="ebnf">
+Type = TypeName | TypeLit | "(" Type ")" .
+TypeName = identifier | QualifiedIdent .
+TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
+ SliceType | MapType | ChannelType .
+</pre>
+
+<p>
+The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
+Others are introduced with <a href="#Type_declarations">type declarations</a>.
+<i>Composite types</i>—array, struct, pointer, function,
+interface, slice, map, and channel types—may be constructed using
+type literals.
+</p>
+
+<p>
+Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
+is one of the predeclared boolean, numeric, or string types, or a type literal,
+the corresponding underlying
+type is <code>T</code> itself. Otherwise, <code>T</code>'s underlying type
+is the underlying type of the type to which <code>T</code> refers in its
+<a href="#Type_declarations">type declaration</a>.
+</p>
+
+<pre>
+type (
+ A1 = string
+ A2 = A1
+)
+
+type (
+ B1 string
+ B2 B1
+ B3 []B1
+ B4 B3
+)
+</pre>
+
+<p>
+The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
+and <code>B2</code> is <code>string</code>.
+The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
+</p>
+
+<h3 id="Method_sets">Method sets</h3>
+<p>
+A type has a (possibly empty) <i>method set</i> associated with it.
+The method set of an <a href="#Interface_types">interface type</a> is its interface.
+The method set of any other type <code>T</code> consists of all
+<a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
+The method set of the corresponding <a href="#Pointer_types">pointer type</a> <code>*T</code>
+is the set of all methods declared with receiver <code>*T</code> or <code>T</code>
+(that is, it also contains the method set of <code>T</code>).
+Further rules apply to structs containing embedded fields, as described
+in the section on <a href="#Struct_types">struct types</a>.
+Any other type has an empty method set.
+In a method set, each method must have a
+<a href="#Uniqueness_of_identifiers">unique</a>
+non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
+</p>
+
+<p>
+The method set of a type determines the interfaces that the
+type <a href="#Interface_types">implements</a>
+and the methods that can be <a href="#Calls">called</a>
+using a receiver of that type.
+</p>
+
+<h3 id="Boolean_types">Boolean types</h3>
+
+<p>
+A <i>boolean type</i> represents the set of Boolean truth values
+denoted by the predeclared constants <code>true</code>
+and <code>false</code>. The predeclared boolean type is <code>bool</code>;
+it is a <a href="#Type_definitions">defined type</a>.
+</p>
+
+<h3 id="Numeric_types">Numeric types</h3>
+
+<p>
+A <i>numeric type</i> represents sets of integer or floating-point values.
+The predeclared architecture-independent numeric types are:
+</p>
+
+<pre class="grammar">
+uint8 the set of all unsigned 8-bit integers (0 to 255)
+uint16 the set of all unsigned 16-bit integers (0 to 65535)
+uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
+uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
+
+int8 the set of all signed 8-bit integers (-128 to 127)
+int16 the set of all signed 16-bit integers (-32768 to 32767)
+int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
+int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
+
+float32 the set of all IEEE-754 32-bit floating-point numbers
+float64 the set of all IEEE-754 64-bit floating-point numbers
+
+complex64 the set of all complex numbers with float32 real and imaginary parts
+complex128 the set of all complex numbers with float64 real and imaginary parts
+
+byte alias for uint8
+rune alias for int32
+</pre>
+
+<p>
+The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
+<a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
+</p>
+
+<p>
+There is also a set of predeclared numeric types with implementation-specific sizes:
+</p>
+
+<pre class="grammar">
+uint either 32 or 64 bits
+int same size as uint
+uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
+</pre>
+
+<p>
+To avoid portability issues all numeric types are <a href="#Type_definitions">defined
+types</a> and thus distinct except
+<code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
+<code>rune</code>, which is an alias for <code>int32</code>.
+Explicit conversions
+are required when different numeric types are mixed in an expression
+or assignment. For instance, <code>int32</code> and <code>int</code>
+are not the same type even though they may have the same size on a
+particular architecture.
+
+
+<h3 id="String_types">String types</h3>
+
+<p>
+A <i>string type</i> represents the set of string values.
+A string value is a (possibly empty) sequence of bytes.
+The number of bytes is called the length of the string and is never negative.
+Strings are immutable: once created,
+it is impossible to change the contents of a string.
+The predeclared string type is <code>string</code>;
+it is a <a href="#Type_definitions">defined type</a>.
+</p>
+
+<p>
+The length of a string <code>s</code> can be discovered using
+the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
+The length is a compile-time constant if the string is a constant.
+A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
+0 through <code>len(s)-1</code>.
+It is illegal to take the address of such an element; if
+<code>s[i]</code> is the <code>i</code>'th byte of a
+string, <code>&s[i]</code> is invalid.
+</p>
+
+
+<h3 id="Array_types">Array types</h3>
+
+<p>
+An array is a numbered sequence of elements of a single
+type, called the element type.
+The number of elements is called the length of the array and is never negative.
+</p>
+
+<pre class="ebnf">
+ArrayType = "[" ArrayLength "]" ElementType .
+ArrayLength = Expression .
+ElementType = Type .
+</pre>
+
+<p>
+The length is part of the array's type; it must evaluate to a
+non-negative <a href="#Constants">constant</a>
+<a href="#Representability">representable</a> by a value
+of type <code>int</code>.
+The length of array <code>a</code> can be discovered
+using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
+The elements can be addressed by integer <a href="#Index_expressions">indices</a>
+0 through <code>len(a)-1</code>.
+Array types are always one-dimensional but may be composed to form
+multi-dimensional types.
+</p>
+
+<pre>
+[32]byte
+[2*N] struct { x, y int32 }
+[1000]*float64
+[3][5]int
+[2][2][2]float64 // same as [2]([2]([2]float64))
+</pre>
+
+<h3 id="Slice_types">Slice types</h3>
+
+<p>
+A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
+provides access to a numbered sequence of elements from that array.
+A slice type denotes the set of all slices of arrays of its element type.
+The number of elements is called the length of the slice and is never negative.
+The value of an uninitialized slice is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+SliceType = "[" "]" ElementType .
+</pre>
+
+<p>
+The length of a slice <code>s</code> can be discovered by the built-in function
+<a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
+execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
+0 through <code>len(s)-1</code>. The slice index of a
+given element may be less than the index of the same element in the
+underlying array.
+</p>
+<p>
+A slice, once initialized, is always associated with an underlying
+array that holds its elements. A slice therefore shares storage
+with its array and with other slices of the same array; by contrast,
+distinct arrays always represent distinct storage.
+</p>
+<p>
+The array underlying a slice may extend past the end of the slice.
+The <i>capacity</i> is a measure of that extent: it is the sum of
+the length of the slice and the length of the array beyond the slice;
+a slice of length up to that capacity can be created by
+<a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
+The capacity of a slice <code>a</code> can be discovered using the
+built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
+</p>
+
+<p>
+A new, initialized slice value for a given element type <code>T</code> is
+made using the built-in function
+<a href="#Making_slices_maps_and_channels"><code>make</code></a>,
+which takes a slice type
+and parameters specifying the length and optionally the capacity.
+A slice created with <code>make</code> always allocates a new, hidden array
+to which the returned slice value refers. That is, executing
+</p>
+
+<pre>
+make([]T, length, capacity)
+</pre>
+
+<p>
+produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
+it, so these two expressions are equivalent:
+</p>
+
+<pre>
+make([]int, 50, 100)
+new([100]int)[0:50]
+</pre>
+
+<p>
+Like arrays, slices are always one-dimensional but may be composed to construct
+higher-dimensional objects.
+With arrays of arrays, the inner arrays are, by construction, always the same length;
+however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
+Moreover, the inner slices must be initialized individually.
+</p>
+
+<h3 id="Struct_types">Struct types</h3>
+
+<p>
+A struct is a sequence of named elements, called fields, each of which has a
+name and a type. Field names may be specified explicitly (IdentifierList) or
+implicitly (EmbeddedField).
+Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
+be <a href="#Uniqueness_of_identifiers">unique</a>.
+</p>
+
+<pre class="ebnf">
+StructType = "struct" "{" { FieldDecl ";" } "}" .
+FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
+EmbeddedField = [ "*" ] TypeName .
+Tag = string_lit .
+</pre>
+
+<pre>
+// An empty struct.
+struct {}
+
+// A struct with 6 fields.
+struct {
+ x, y int
+ u float32
+ _ float32 // padding
+ A *[]int
+ F func()
+}
+</pre>
+
+<p>
+A field declared with a type but no explicit field name is called an <i>embedded field</i>.
+An embedded field must be specified as
+a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
+and <code>T</code> itself may not be
+a pointer type. The unqualified type name acts as the field name.
+</p>
+
+<pre>
+// A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
+struct {
+ T1 // field name is T1
+ *T2 // field name is T2
+ P.T3 // field name is T3
+ *P.T4 // field name is T4
+ x, y int // field names are x and y
+}
+</pre>
+
+<p>
+The following declaration is illegal because field names must be unique
+in a struct type:
+</p>
+
+<pre>
+struct {
+ T // conflicts with embedded field *T and *P.T
+ *T // conflicts with embedded field T and *P.T
+ *P.T // conflicts with embedded field T and *T
+}
+</pre>
+
+<p>
+A field or <a href="#Method_declarations">method</a> <code>f</code> of an
+embedded field in a struct <code>x</code> is called <i>promoted</i> if
+<code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
+that field or method <code>f</code>.
+</p>
+
+<p>
+Promoted fields act like ordinary fields
+of a struct except that they cannot be used as field names in
+<a href="#Composite_literals">composite literals</a> of the struct.
+</p>
+
+<p>
+Given a struct type <code>S</code> and a <a href="#Type_definitions">defined type</a>
+<code>T</code>, promoted methods are included in the method set of the struct as follows:
+</p>
+<ul>
+ <li>
+ If <code>S</code> contains an embedded field <code>T</code>,
+ the <a href="#Method_sets">method sets</a> of <code>S</code>
+ and <code>*S</code> both include promoted methods with receiver
+ <code>T</code>. The method set of <code>*S</code> also
+ includes promoted methods with receiver <code>*T</code>.
+ </li>
+
+ <li>
+ If <code>S</code> contains an embedded field <code>*T</code>,
+ the method sets of <code>S</code> and <code>*S</code> both
+ include promoted methods with receiver <code>T</code> or
+ <code>*T</code>.
+ </li>
+</ul>
+
+<p>
+A field declaration may be followed by an optional string literal <i>tag</i>,
+which becomes an attribute for all the fields in the corresponding
+field declaration. An empty tag string is equivalent to an absent tag.
+The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
+and take part in <a href="#Type_identity">type identity</a> for structs
+but are otherwise ignored.
+</p>
+
+<pre>
+struct {
+ x, y float64 "" // an empty tag string is like an absent tag
+ name string "any string is permitted as a tag"
+ _ [4]byte "ceci n'est pas un champ de structure"
+}
+
+// A struct corresponding to a TimeStamp protocol buffer.
+// The tag strings define the protocol buffer field numbers;
+// they follow the convention outlined by the reflect package.
+struct {
+ microsec uint64 `protobuf:"1"`
+ serverIP6 uint64 `protobuf:"2"`
+}
+</pre>
+
+<h3 id="Pointer_types">Pointer types</h3>
+
+<p>
+A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
+type, called the <i>base type</i> of the pointer.
+The value of an uninitialized pointer is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+PointerType = "*" BaseType .
+BaseType = Type .
+</pre>
+
+<pre>
+*Point
+*[4]int
+</pre>
+
+<h3 id="Function_types">Function types</h3>
+
+<p>
+A function type denotes the set of all functions with the same parameter
+and result types. The value of an uninitialized variable of function type
+is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+FunctionType = "func" Signature .
+Signature = Parameters [ Result ] .
+Result = Parameters | Type .
+Parameters = "(" [ ParameterList [ "," ] ] ")" .
+ParameterList = ParameterDecl { "," ParameterDecl } .
+ParameterDecl = [ IdentifierList ] [ "..." ] Type .
+</pre>
+
+<p>
+Within a list of parameters or results, the names (IdentifierList)
+must either all be present or all be absent. If present, each name
+stands for one item (parameter or result) of the specified type and
+all non-<a href="#Blank_identifier">blank</a> names in the signature
+must be <a href="#Uniqueness_of_identifiers">unique</a>.
+If absent, each type stands for one item of that type.
+Parameter and result
+lists are always parenthesized except that if there is exactly
+one unnamed result it may be written as an unparenthesized type.
+</p>
+
+<p>
+The final incoming parameter in a function signature may have
+a type prefixed with <code>...</code>.
+A function with such a parameter is called <i>variadic</i> and
+may be invoked with zero or more arguments for that parameter.
+</p>
+
+<pre>
+func()
+func(x int) int
+func(a, _ int, z float32) bool
+func(a, b int, z float32) (bool)
+func(prefix string, values ...int)
+func(a, b int, z float64, opt ...interface{}) (success bool)
+func(int, int, float64) (float64, *[]int)
+func(n int) func(p *T)
+</pre>
+
+
+<h3 id="Interface_types">Interface types</h3>
+
+<p>
+An interface type specifies a <a href="#Method_sets">method set</a> called its <i>interface</i>.
+A variable of interface type can store a value of any type with a method set
+that is any superset of the interface. Such a type is said to
+<i>implement the interface</i>.
+The value of an uninitialized variable of interface type is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+InterfaceType = "interface" "{" { ( MethodSpec | InterfaceTypeName ) ";" } "}" .
+MethodSpec = MethodName Signature .
+MethodName = identifier .
+InterfaceTypeName = TypeName .
+</pre>
+
+<p>
+An interface type may specify methods <i>explicitly</i> through method specifications,
+or it may <i>embed</i> methods of other interfaces through interface type names.
+</p>
+
+<pre>
+// A simple File interface.
+interface {
+ Read([]byte) (int, error)
+ Write([]byte) (int, error)
+ Close() error
+}
+</pre>
+
+<p>
+The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
+and not <a href="#Blank_identifier">blank</a>.
+</p>
+
+<pre>
+interface {
+ String() string
+ String() string // illegal: String not unique
+ _(x int) // illegal: method must have non-blank name
+}
+</pre>
+
+<p>
+More than one type may implement an interface.
+For instance, if two types <code>S1</code> and <code>S2</code>
+have the method set
+</p>
+
+<pre>
+func (p T) Read(p []byte) (n int, err error)
+func (p T) Write(p []byte) (n int, err error)
+func (p T) Close() error
+</pre>
+
+<p>
+(where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
+then the <code>File</code> interface is implemented by both <code>S1</code> and
+<code>S2</code>, regardless of what other methods
+<code>S1</code> and <code>S2</code> may have or share.
+</p>
+
+<p>
+A type implements any interface comprising any subset of its methods
+and may therefore implement several distinct interfaces. For
+instance, all types implement the <i>empty interface</i>:
+</p>
+
+<pre>
+interface{}
+</pre>
+
+<p>
+Similarly, consider this interface specification,
+which appears within a <a href="#Type_declarations">type declaration</a>
+to define an interface called <code>Locker</code>:
+</p>
+
+<pre>
+type Locker interface {
+ Lock()
+ Unlock()
+}
+</pre>
+
+<p>
+If <code>S1</code> and <code>S2</code> also implement
+</p>
+
+<pre>
+func (p T) Lock() { … }
+func (p T) Unlock() { … }
+</pre>
+
+<p>
+they implement the <code>Locker</code> interface as well
+as the <code>File</code> interface.
+</p>
+
+<p>
+An interface <code>T</code> may use a (possibly qualified) interface type
+name <code>E</code> in place of a method specification. This is called
+<i>embedding</i> interface <code>E</code> in <code>T</code>.
+The <a href="#Method_sets">method set</a> of <code>T</code> is the <i>union</i>
+of the method sets of <code>T</code>’s explicitly declared methods and of
+<code>T</code>’s embedded interfaces.
+</p>
+
+<pre>
+type Reader interface {
+ Read(p []byte) (n int, err error)
+ Close() error
+}
+
+type Writer interface {
+ Write(p []byte) (n int, err error)
+ Close() error
+}
+
+// ReadWriter's methods are Read, Write, and Close.
+type ReadWriter interface {
+ Reader // includes methods of Reader in ReadWriter's method set
+ Writer // includes methods of Writer in ReadWriter's method set
+}
+</pre>
+
+<p>
+A <i>union</i> of method sets contains the (exported and non-exported)
+methods of each method set exactly once, and methods with the
+<a href="#Uniqueness_of_identifiers">same</a> names must
+have <a href="#Type_identity">identical</a> signatures.
+</p>
+
+<pre>
+type ReadCloser interface {
+ Reader // includes methods of Reader in ReadCloser's method set
+ Close() // illegal: signatures of Reader.Close and Close are different
+}
+</pre>
+
+<p>
+An interface type <code>T</code> may not embed itself
+or any interface type that embeds <code>T</code>, recursively.
+</p>
+
+<pre>
+// illegal: Bad cannot embed itself
+type Bad interface {
+ Bad
+}
+
+// illegal: Bad1 cannot embed itself using Bad2
+type Bad1 interface {
+ Bad2
+}
+type Bad2 interface {
+ Bad1
+}
+</pre>
+
+<h3 id="Map_types">Map types</h3>
+
+<p>
+A map is an unordered group of elements of one type, called the
+element type, indexed by a set of unique <i>keys</i> of another type,
+called the key type.
+The value of an uninitialized map is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+MapType = "map" "[" KeyType "]" ElementType .
+KeyType = Type .
+</pre>
+
+<p>
+The <a href="#Comparison_operators">comparison operators</a>
+<code>==</code> and <code>!=</code> must be fully defined
+for operands of the key type; thus the key type must not be a function, map, or
+slice.
+If the key type is an interface type, these
+comparison operators must be defined for the dynamic key values;
+failure will cause a <a href="#Run_time_panics">run-time panic</a>.
+
+</p>
+
+<pre>
+map[string]int
+map[*T]struct{ x, y float64 }
+map[string]interface{}
+</pre>
+
+<p>
+The number of map elements is called its length.
+For a map <code>m</code>, it can be discovered using the
+built-in function <a href="#Length_and_capacity"><code>len</code></a>
+and may change during execution. Elements may be added during execution
+using <a href="#Assignments">assignments</a> and retrieved with
+<a href="#Index_expressions">index expressions</a>; they may be removed with the
+<a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
+</p>
+<p>
+A new, empty map value is made using the built-in
+function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
+which takes the map type and an optional capacity hint as arguments:
+</p>
+
+<pre>
+make(map[string]int)
+make(map[string]int, 100)
+</pre>
+
+<p>
+The initial capacity does not bound its size:
+maps grow to accommodate the number of items
+stored in them, with the exception of <code>nil</code> maps.
+A <code>nil</code> map is equivalent to an empty map except that no elements
+may be added.
+
+<h3 id="Channel_types">Channel types</h3>
+
+<p>
+A channel provides a mechanism for
+<a href="#Go_statements">concurrently executing functions</a>
+to communicate by
+<a href="#Send_statements">sending</a> and
+<a href="#Receive_operator">receiving</a>
+values of a specified element type.
+The value of an uninitialized channel is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
+</pre>
+
+<p>
+The optional <code><-</code> operator specifies the channel <i>direction</i>,
+<i>send</i> or <i>receive</i>. If no direction is given, the channel is
+<i>bidirectional</i>.
+A channel may be constrained only to send or only to receive by
+<a href="#Assignments">assignment</a> or
+explicit <a href="#Conversions">conversion</a>.
+</p>
+
+<pre>
+chan T // can be used to send and receive values of type T
+chan<- float64 // can only be used to send float64s
+<-chan int // can only be used to receive ints
+</pre>
+
+<p>
+The <code><-</code> operator associates with the leftmost <code>chan</code>
+possible:
+</p>
+
+<pre>
+chan<- chan int // same as chan<- (chan int)
+chan<- <-chan int // same as chan<- (<-chan int)
+<-chan <-chan int // same as <-chan (<-chan int)
+chan (<-chan int)
+</pre>
+
+<p>
+A new, initialized channel
+value can be made using the built-in function
+<a href="#Making_slices_maps_and_channels"><code>make</code></a>,
+which takes the channel type and an optional <i>capacity</i> as arguments:
+</p>
+
+<pre>
+make(chan int, 100)
+</pre>
+
+<p>
+The capacity, in number of elements, sets the size of the buffer in the channel.
+If the capacity is zero or absent, the channel is unbuffered and communication
+succeeds only when both a sender and receiver are ready. Otherwise, the channel
+is buffered and communication succeeds without blocking if the buffer
+is not full (sends) or not empty (receives).
+A <code>nil</code> channel is never ready for communication.
+</p>
+
+<p>
+A channel may be closed with the built-in function
+<a href="#Close"><code>close</code></a>.
+The multi-valued assignment form of the
+<a href="#Receive_operator">receive operator</a>
+reports whether a received value was sent before
+the channel was closed.
+</p>
+
+<p>
+A single channel may be used in
+<a href="#Send_statements">send statements</a>,
+<a href="#Receive_operator">receive operations</a>,
+and calls to the built-in functions
+<a href="#Length_and_capacity"><code>cap</code></a> and
+<a href="#Length_and_capacity"><code>len</code></a>
+by any number of goroutines without further synchronization.
+Channels act as first-in-first-out queues.
+For example, if one goroutine sends values on a channel
+and a second goroutine receives them, the values are
+received in the order sent.
+</p>
+
+<h2 id="Properties_of_types_and_values">Properties of types and values</h2>
+
+<h3 id="Type_identity">Type identity</h3>
+
+<p>
+Two types are either <i>identical</i> or <i>different</i>.
+</p>
+
+<p>
+A <a href="#Type_definitions">defined type</a> is always different from any other type.
+Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
+structurally equivalent; that is, they have the same literal structure and corresponding
+components have identical types. In detail:
+</p>
+
+<ul>
+ <li>Two array types are identical if they have identical element types and
+ the same array length.</li>
+
+ <li>Two slice types are identical if they have identical element types.</li>
+
+ <li>Two struct types are identical if they have the same sequence of fields,
+ and if corresponding fields have the same names, and identical types,
+ and identical tags.
+ <a href="#Exported_identifiers">Non-exported</a> field names from different
+ packages are always different.</li>
+
+ <li>Two pointer types are identical if they have identical base types.</li>
+
+ <li>Two function types are identical if they have the same number of parameters
+ and result values, corresponding parameter and result types are
+ identical, and either both functions are variadic or neither is.
+ Parameter and result names are not required to match.</li>
+
+ <li>Two interface types are identical if they have the same set of methods
+ with the same names and identical function types.
+ <a href="#Exported_identifiers">Non-exported</a> method names from different
+ packages are always different. The order of the methods is irrelevant.</li>
+
+ <li>Two map types are identical if they have identical key and element types.</li>
+
+ <li>Two channel types are identical if they have identical element types and
+ the same direction.</li>
+</ul>
+
+<p>
+Given the declarations
+</p>
+
+<pre>
+type (
+ A0 = []string
+ A1 = A0
+ A2 = struct{ a, b int }
+ A3 = int
+ A4 = func(A3, float64) *A0
+ A5 = func(x int, _ float64) *[]string
+)
+
+type (
+ B0 A0
+ B1 []string
+ B2 struct{ a, b int }
+ B3 struct{ a, c int }
+ B4 func(int, float64) *B0
+ B5 func(x int, y float64) *A1
+)
+
+type C0 = B0
+</pre>
+
+<p>
+these types are identical:
+</p>
+
+<pre>
+A0, A1, and []string
+A2 and struct{ a, b int }
+A3 and int
+A4, func(int, float64) *[]string, and A5
+
+B0 and C0
+[]int and []int
+struct{ a, b *T5 } and struct{ a, b *T5 }
+func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
+</pre>
+
+<p>
+<code>B0</code> and <code>B1</code> are different because they are new types
+created by distinct <a href="#Type_definitions">type definitions</a>;
+<code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
+are different because <code>B0</code> is different from <code>[]string</code>.
+</p>
+
+
+<h3 id="Assignability">Assignability</h3>
+
+<p>
+A value <code>x</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
+("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
+</p>
+
+<ul>
+<li>
+<code>x</code>'s type is identical to <code>T</code>.
+</li>
+<li>
+<code>x</code>'s type <code>V</code> and <code>T</code> have identical
+<a href="#Types">underlying types</a> and at least one of <code>V</code>
+or <code>T</code> is not a <a href="#Type_definitions">defined</a> type.
+</li>
+<li>
+<code>T</code> is an interface type and
+<code>x</code> <a href="#Interface_types">implements</a> <code>T</code>.
+</li>
+<li>
+<code>x</code> is a bidirectional channel value, <code>T</code> is a channel type,
+<code>x</code>'s type <code>V</code> and <code>T</code> have identical element types,
+and at least one of <code>V</code> or <code>T</code> is not a defined type.
+</li>
+<li>
+<code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
+is a pointer, function, slice, map, channel, or interface type.
+</li>
+<li>
+<code>x</code> is an untyped <a href="#Constants">constant</a>
+<a href="#Representability">representable</a>
+by a value of type <code>T</code>.
+</li>
+</ul>
+
+
+<h3 id="Representability">Representability</h3>
+
+<p>
+A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
+by a value of type <code>T</code> if one of the following conditions applies:
+</p>
+
+<ul>
+<li>
+<code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
+</li>
+
+<li>
+<code>T</code> is a floating-point type and <code>x</code> can be rounded to <code>T</code>'s
+precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
+negative zero further simplified to an unsigned zero. Note that constant values never result
+in an IEEE negative zero, NaN, or infinity.
+</li>
+
+<li>
+<code>T</code> is a complex type, and <code>x</code>'s
+<a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
+are representable by values of <code>T</code>'s component type (<code>float32</code> or
+<code>float64</code>).
+</li>
+</ul>
+
+<pre>
+x T x is representable by a value of T because
+
+'a' byte 97 is in the set of byte values
+97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
+"foo" string "foo" is in the set of string values
+1024 int16 1024 is in the set of 16-bit integers
+42.0 byte 42 is in the set of unsigned 8-bit integers
+1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
+2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
+-1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
+0i int 0 is an integer value
+(42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
+</pre>
+
+<pre>
+x T x is not representable by a value of T because
+
+0 bool 0 is not in the set of boolean values
+'a' string 'a' is a rune, it is not in the set of string values
+1024 byte 1024 is not in the set of unsigned 8-bit integers
+-1 uint16 -1 is not in the set of unsigned 16-bit integers
+1.1 int 1.1 is not an integer value
+42i float32 (0 + 42i) is not in the set of float32 values
+1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
+</pre>
+
+
+<h2 id="Blocks">Blocks</h2>
+
+<p>
+A <i>block</i> is a possibly empty sequence of declarations and statements
+within matching brace brackets.
+</p>
+
+<pre class="ebnf">
+Block = "{" StatementList "}" .
+StatementList = { Statement ";" } .
+</pre>
+
+<p>
+In addition to explicit blocks in the source code, there are implicit blocks:
+</p>
+
+<ol>
+ <li>The <i>universe block</i> encompasses all Go source text.</li>
+
+ <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
+ Go source text for that package.</li>
+
+ <li>Each file has a <i>file block</i> containing all Go source text
+ in that file.</li>
+
+ <li>Each <a href="#If_statements">"if"</a>,
+ <a href="#For_statements">"for"</a>, and
+ <a href="#Switch_statements">"switch"</a>
+ statement is considered to be in its own implicit block.</li>
+
+ <li>Each clause in a <a href="#Switch_statements">"switch"</a>
+ or <a href="#Select_statements">"select"</a> statement
+ acts as an implicit block.</li>
+</ol>
+
+<p>
+Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
+</p>
+
+
+<h2 id="Declarations_and_scope">Declarations and scope</h2>
+
+<p>
+A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
+<a href="#Constant_declarations">constant</a>,
+<a href="#Type_declarations">type</a>,
+<a href="#Variable_declarations">variable</a>,
+<a href="#Function_declarations">function</a>,
+<a href="#Labeled_statements">label</a>, or
+<a href="#Import_declarations">package</a>.
+Every identifier in a program must be declared.
+No identifier may be declared twice in the same block, and
+no identifier may be declared in both the file and package block.
+</p>
+
+<p>
+The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
+in a declaration, but it does not introduce a binding and thus is not declared.
+In the package block, the identifier <code>init</code> may only be used for
+<a href="#Package_initialization"><code>init</code> function</a> declarations,
+and like the blank identifier it does not introduce a new binding.
+</p>
+
+<pre class="ebnf">
+Declaration = ConstDecl | TypeDecl | VarDecl .
+TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
+</pre>
+
+<p>
+The <i>scope</i> of a declared identifier is the extent of source text in which
+the identifier denotes the specified constant, type, variable, function, label, or package.
+</p>
+
+<p>
+Go is lexically scoped using <a href="#Blocks">blocks</a>:
+</p>
+
+<ol>
+ <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
+
+ <li>The scope of an identifier denoting a constant, type, variable,
+ or function (but not method) declared at top level (outside any
+ function) is the package block.</li>
+
+ <li>The scope of the package name of an imported package is the file block
+ of the file containing the import declaration.</li>
+
+ <li>The scope of an identifier denoting a method receiver, function parameter,
+ or result variable is the function body.</li>
+
+ <li>The scope of a constant or variable identifier declared
+ inside a function begins at the end of the ConstSpec or VarSpec
+ (ShortVarDecl for short variable declarations)
+ and ends at the end of the innermost containing block.</li>
+
+ <li>The scope of a type identifier declared inside a function
+ begins at the identifier in the TypeSpec
+ and ends at the end of the innermost containing block.</li>
+</ol>
+
+<p>
+An identifier declared in a block may be redeclared in an inner block.
+While the identifier of the inner declaration is in scope, it denotes
+the entity declared by the inner declaration.
+</p>
+
+<p>
+The <a href="#Package_clause">package clause</a> is not a declaration; the package name
+does not appear in any scope. Its purpose is to identify the files belonging
+to the same <a href="#Packages">package</a> and to specify the default package name for import
+declarations.
+</p>
+
+
+<h3 id="Label_scopes">Label scopes</h3>
+
+<p>
+Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
+used in the <a href="#Break_statements">"break"</a>,
+<a href="#Continue_statements">"continue"</a>, and
+<a href="#Goto_statements">"goto"</a> statements.
+It is illegal to define a label that is never used.
+In contrast to other identifiers, labels are not block scoped and do
+not conflict with identifiers that are not labels. The scope of a label
+is the body of the function in which it is declared and excludes
+the body of any nested function.
+</p>
+
+
+<h3 id="Blank_identifier">Blank identifier</h3>
+
+<p>
+The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
+It serves as an anonymous placeholder instead of a regular (non-blank)
+identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
+as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>.
+</p>
+
+
+<h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
+
+<p>
+The following identifiers are implicitly declared in the
+<a href="#Blocks">universe block</a>:
+</p>
+<pre class="grammar">
+Types:
+ bool byte complex64 complex128 error float32 float64
+ int int8 int16 int32 int64 rune string
+ uint uint8 uint16 uint32 uint64 uintptr
+
+Constants:
+ true false iota
+
+Zero value:
+ nil
+
+Functions:
+ append cap close complex copy delete imag len
+ make new panic print println real recover
+</pre>
+
+
+<h3 id="Exported_identifiers">Exported identifiers</h3>
+
+<p>
+An identifier may be <i>exported</i> to permit access to it from another package.
+An identifier is exported if both:
+</p>
+<ol>
+ <li>the first character of the identifier's name is a Unicode upper case
+ letter (Unicode class "Lu"); and</li>
+ <li>the identifier is declared in the <a href="#Blocks">package block</a>
+ or it is a <a href="#Struct_types">field name</a> or
+ <a href="#MethodName">method name</a>.</li>
+</ol>
+<p>
+All other identifiers are not exported.
+</p>
+
+
+<h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
+
+<p>
+Given a set of identifiers, an identifier is called <i>unique</i> if it is
+<i>different</i> from every other in the set.
+Two identifiers are different if they are spelled differently, or if they
+appear in different <a href="#Packages">packages</a> and are not
+<a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
+</p>
+
+<h3 id="Constant_declarations">Constant declarations</h3>
+
+<p>
+A constant declaration binds a list of identifiers (the names of
+the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
+The number of identifiers must be equal
+to the number of expressions, and the <i>n</i>th identifier on
+the left is bound to the value of the <i>n</i>th expression on the
+right.
+</p>
+
+<pre class="ebnf">
+ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
+ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
+
+IdentifierList = identifier { "," identifier } .
+ExpressionList = Expression { "," Expression } .
+</pre>
+
+<p>
+If the type is present, all constants take the type specified, and
+the expressions must be <a href="#Assignability">assignable</a> to that type.
+If the type is omitted, the constants take the
+individual types of the corresponding expressions.
+If the expression values are untyped <a href="#Constants">constants</a>,
+the declared constants remain untyped and the constant identifiers
+denote the constant values. For instance, if the expression is a
+floating-point literal, the constant identifier denotes a floating-point
+constant, even if the literal's fractional part is zero.
+</p>
+
+<pre>
+const Pi float64 = 3.14159265358979323846
+const zero = 0.0 // untyped floating-point constant
+const (
+ size int64 = 1024
+ eof = -1 // untyped integer constant
+)
+const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
+const u, v float32 = 0, 3 // u = 0.0, v = 3.0
+</pre>
+
+<p>
+Within a parenthesized <code>const</code> declaration list the
+expression list may be omitted from any but the first ConstSpec.
+Such an empty list is equivalent to the textual substitution of the
+first preceding non-empty expression list and its type if any.
+Omitting the list of expressions is therefore equivalent to
+repeating the previous list. The number of identifiers must be equal
+to the number of expressions in the previous list.
+Together with the <a href="#Iota"><code>iota</code> constant generator</a>
+this mechanism permits light-weight declaration of sequential values:
+</p>
+
+<pre>
+const (
+ Sunday = iota
+ Monday
+ Tuesday
+ Wednesday
+ Thursday
+ Friday
+ Partyday
+ numberOfDays // this constant is not exported
+)
+</pre>
+
+
+<h3 id="Iota">Iota</h3>
+
+<p>
+Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
+<code>iota</code> represents successive untyped integer <a href="#Constants">
+constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
+in that constant declaration, starting at zero.
+It can be used to construct a set of related constants:
+</p>
+
+<pre>
+const (
+ c0 = iota // c0 == 0
+ c1 = iota // c1 == 1
+ c2 = iota // c2 == 2
+)
+
+const (
+ a = 1 << iota // a == 1 (iota == 0)
+ b = 1 << iota // b == 2 (iota == 1)
+ c = 3 // c == 3 (iota == 2, unused)
+ d = 1 << iota // d == 8 (iota == 3)
+)
+
+const (
+ u = iota * 42 // u == 0 (untyped integer constant)
+ v float64 = iota * 42 // v == 42.0 (float64 constant)
+ w = iota * 42 // w == 84 (untyped integer constant)
+)
+
+const x = iota // x == 0
+const y = iota // y == 0
+</pre>
+
+<p>
+By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
+</p>
+
+<pre>
+const (
+ bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
+ bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
+ _, _ // (iota == 2, unused)
+ bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
+)
+</pre>
+
+<p>
+This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
+of the last non-empty expression list.
+</p>
+
+
+<h3 id="Type_declarations">Type declarations</h3>
+
+<p>
+A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
+Type declarations come in two forms: alias declarations and type definitions.
+</p>
+
+<pre class="ebnf">
+TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
+TypeSpec = AliasDecl | TypeDef .
+</pre>
+
+<h4 id="Alias_declarations">Alias declarations</h4>
+
+<p>
+An alias declaration binds an identifier to the given type.
+</p>
+
+<pre class="ebnf">
+AliasDecl = identifier "=" Type .
+</pre>
+
+<p>
+Within the <a href="#Declarations_and_scope">scope</a> of
+the identifier, it serves as an <i>alias</i> for the type.
+</p>
+
+<pre>
+type (
+ nodeList = []*Node // nodeList and []*Node are identical types
+ Polar = polar // Polar and polar denote identical types
+)
+</pre>
+
+
+<h4 id="Type_definitions">Type definitions</h4>
+
+<p>
+A type definition creates a new, distinct type with the same
+<a href="#Types">underlying type</a> and operations as the given type,
+and binds an identifier to it.
+</p>
+
+<pre class="ebnf">
+TypeDef = identifier Type .
+</pre>
+
+<p>
+The new type is called a <i>defined type</i>.
+It is <a href="#Type_identity">different</a> from any other type,
+including the type it is created from.
+</p>
+
+<pre>
+type (
+ Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
+ polar Point // polar and Point denote different types
+)
+
+type TreeNode struct {
+ left, right *TreeNode
+ value *Comparable
+}
+
+type Block interface {
+ BlockSize() int
+ Encrypt(src, dst []byte)
+ Decrypt(src, dst []byte)
+}
+</pre>
+
+<p>
+A defined type may have <a href="#Method_declarations">methods</a> associated with it.
+It does not inherit any methods bound to the given type,
+but the <a href="#Method_sets">method set</a>
+of an interface type or of elements of a composite type remains unchanged:
+</p>
+
+<pre>
+// A Mutex is a data type with two methods, Lock and Unlock.
+type Mutex struct { /* Mutex fields */ }
+func (m *Mutex) Lock() { /* Lock implementation */ }
+func (m *Mutex) Unlock() { /* Unlock implementation */ }
+
+// NewMutex has the same composition as Mutex but its method set is empty.
+type NewMutex Mutex
+
+// The method set of PtrMutex's underlying type *Mutex remains unchanged,
+// but the method set of PtrMutex is empty.
+type PtrMutex *Mutex
+
+// The method set of *PrintableMutex contains the methods
+// Lock and Unlock bound to its embedded field Mutex.
+type PrintableMutex struct {
+ Mutex
+}
+
+// MyBlock is an interface type that has the same method set as Block.
+type MyBlock Block
+</pre>
+
+<p>
+Type definitions may be used to define different boolean, numeric,
+or string types and associate methods with them:
+</p>
+
+<pre>
+type TimeZone int
+
+const (
+ EST TimeZone = -(5 + iota)
+ CST
+ MST
+ PST
+)
+
+func (tz TimeZone) String() string {
+ return fmt.Sprintf("GMT%+dh", tz)
+}
+</pre>
+
+
+<h3 id="Variable_declarations">Variable declarations</h3>
+
+<p>
+A variable declaration creates one or more <a href="#Variables">variables</a>,
+binds corresponding identifiers to them, and gives each a type and an initial value.
+</p>
+
+<pre class="ebnf">
+VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
+VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
+</pre>
+
+<pre>
+var i int
+var U, V, W float64
+var k = 0
+var x, y float32 = -1, -2
+var (
+ i int
+ u, v, s = 2.0, 3.0, "bar"
+)
+var re, im = complexSqrt(-1)
+var _, found = entries[name] // map lookup; only interested in "found"
+</pre>
+
+<p>
+If a list of expressions is given, the variables are initialized
+with the expressions following the rules for <a href="#Assignments">assignments</a>.
+Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
+</p>
+
+<p>
+If a type is present, each variable is given that type.
+Otherwise, each variable is given the type of the corresponding
+initialization value in the assignment.
+If that value is an untyped constant, it is first implicitly
+<a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
+if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
+The predeclared value <code>nil</code> cannot be used to initialize a variable
+with no explicit type.
+</p>
+
+<pre>
+var d = math.Sin(0.5) // d is float64
+var i = 42 // i is int
+var t, ok = x.(T) // t is T, ok is bool
+var n = nil // illegal
+</pre>
+
+<p>
+Implementation restriction: A compiler may make it illegal to declare a variable
+inside a <a href="#Function_declarations">function body</a> if the variable is
+never used.
+</p>
+
+<h3 id="Short_variable_declarations">Short variable declarations</h3>
+
+<p>
+A <i>short variable declaration</i> uses the syntax:
+</p>
+
+<pre class="ebnf">
+ShortVarDecl = IdentifierList ":=" ExpressionList .
+</pre>
+
+<p>
+It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
+with initializer expressions but no types:
+</p>
+
+<pre class="grammar">
+"var" IdentifierList = ExpressionList .
+</pre>
+
+<pre>
+i, j := 0, 10
+f := func() int { return 7 }
+ch := make(chan int)
+r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
+_, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
+</pre>
+
+<p>
+Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
+variables provided they were originally declared earlier in the same block
+(or the parameter lists if the block is the function body) with the same type,
+and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
+As a consequence, redeclaration can only appear in a multi-variable short declaration.
+Redeclaration does not introduce a new variable; it just assigns a new value to the original.
+</p>
+
+<pre>
+field1, offset := nextField(str, 0)
+field2, offset := nextField(str, offset) // redeclares offset
+a, a := 1, 2 // illegal: double declaration of a or no new variable if a was declared elsewhere
+</pre>
+
+<p>
+Short variable declarations may appear only inside functions.
+In some contexts such as the initializers for
+<a href="#If_statements">"if"</a>,
+<a href="#For_statements">"for"</a>, or
+<a href="#Switch_statements">"switch"</a> statements,
+they can be used to declare local temporary variables.
+</p>
+
+<h3 id="Function_declarations">Function declarations</h3>
+
+<p>
+A function declaration binds an identifier, the <i>function name</i>,
+to a function.
+</p>
+
+<pre class="ebnf">
+FunctionDecl = "func" FunctionName Signature [ FunctionBody ] .
+FunctionName = identifier .
+FunctionBody = Block .
+</pre>
+
+<p>
+If the function's <a href="#Function_types">signature</a> declares
+result parameters, the function body's statement list must end in
+a <a href="#Terminating_statements">terminating statement</a>.
+</p>
+
+<pre>
+func IndexRune(s string, r rune) int {
+ for i, c := range s {
+ if c == r {
+ return i
+ }
+ }
+ // invalid: missing return statement
+}
+</pre>
+
+<p>
+A function declaration may omit the body. Such a declaration provides the
+signature for a function implemented outside Go, such as an assembly routine.
+</p>
+
+<pre>
+func min(x int, y int) int {
+ if x < y {
+ return x
+ }
+ return y
+}
+
+func flushICache(begin, end uintptr) // implemented externally
+</pre>
+
+<h3 id="Method_declarations">Method declarations</h3>
+
+<p>
+A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
+A method declaration binds an identifier, the <i>method name</i>, to a method,
+and associates the method with the receiver's <i>base type</i>.
+</p>
+
+<pre class="ebnf">
+MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
+Receiver = Parameters .
+</pre>
+
+<p>
+The receiver is specified via an extra parameter section preceding the method
+name. That parameter section must declare a single non-variadic parameter, the receiver.
+Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
+pointer to a defined type <code>T</code>. <code>T</code> is called the receiver
+<i>base type</i>. A receiver base type cannot be a pointer or interface type and
+it must be defined in the same package as the method.
+The method is said to be <i>bound</i> to its receiver base type and the method name
+is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
+or <code>*T</code>.
+</p>
+
+<p>
+A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
+<a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
+If the receiver's value is not referenced inside the body of the method,
+its identifier may be omitted in the declaration. The same applies in
+general to parameters of functions and methods.
+</p>
+
+<p>
+For a base type, the non-blank names of methods bound to it must be unique.
+If the base type is a <a href="#Struct_types">struct type</a>,
+the non-blank method and field names must be distinct.
+</p>
+
+<p>
+Given defined type <code>Point</code>, the declarations
+</p>
+
+<pre>
+func (p *Point) Length() float64 {
+ return math.Sqrt(p.x * p.x + p.y * p.y)
+}
+
+func (p *Point) Scale(factor float64) {
+ p.x *= factor
+ p.y *= factor
+}
+</pre>
+
+<p>
+bind the methods <code>Length</code> and <code>Scale</code>,
+with receiver type <code>*Point</code>,
+to the base type <code>Point</code>.
+</p>
+
+<p>
+The type of a method is the type of a function with the receiver as first
+argument. For instance, the method <code>Scale</code> has type
+</p>
+
+<pre>
+func(p *Point, factor float64)
+</pre>
+
+<p>
+However, a function declared this way is not a method.
+</p>
+
+
+<h2 id="Expressions">Expressions</h2>
+
+<p>
+An expression specifies the computation of a value by applying
+operators and functions to operands.
+</p>
+
+<h3 id="Operands">Operands</h3>
+
+<p>
+Operands denote the elementary values in an expression. An operand may be a
+literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
+non-<a href="#Blank_identifier">blank</a> identifier denoting a
+<a href="#Constant_declarations">constant</a>,
+<a href="#Variable_declarations">variable</a>, or
+<a href="#Function_declarations">function</a>,
+or a parenthesized expression.
+</p>
+
+<p>
+The <a href="#Blank_identifier">blank identifier</a> may appear as an
+operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
+</p>
+
+<pre class="ebnf">
+Operand = Literal | OperandName | "(" Expression ")" .
+Literal = BasicLit | CompositeLit | FunctionLit .
+BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
+OperandName = identifier | QualifiedIdent .
+</pre>
+
+<h3 id="Qualified_identifiers">Qualified identifiers</h3>
+
+<p>
+A qualified identifier is an identifier qualified with a package name prefix.
+Both the package name and the identifier must not be
+<a href="#Blank_identifier">blank</a>.
+</p>
+
+<pre class="ebnf">
+QualifiedIdent = PackageName "." identifier .
+</pre>
+
+<p>
+A qualified identifier accesses an identifier in a different package, which
+must be <a href="#Import_declarations">imported</a>.
+The identifier must be <a href="#Exported_identifiers">exported</a> and
+declared in the <a href="#Blocks">package block</a> of that package.
+</p>
+
+<pre>
+math.Sin // denotes the Sin function in package math
+</pre>
+
+<h3 id="Composite_literals">Composite literals</h3>
+
+<p>
+Composite literals construct values for structs, arrays, slices, and maps
+and create a new value each time they are evaluated.
+They consist of the type of the literal followed by a brace-bound list of elements.
+Each element may optionally be preceded by a corresponding key.
+</p>
+
+<pre class="ebnf">
+CompositeLit = LiteralType LiteralValue .
+LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
+ SliceType | MapType | TypeName .
+LiteralValue = "{" [ ElementList [ "," ] ] "}" .
+ElementList = KeyedElement { "," KeyedElement } .
+KeyedElement = [ Key ":" ] Element .
+Key = FieldName | Expression | LiteralValue .
+FieldName = identifier .
+Element = Expression | LiteralValue .
+</pre>
+
+<p>
+The LiteralType's underlying type must be a struct, array, slice, or map type
+(the grammar enforces this constraint except when the type is given
+as a TypeName).
+The types of the elements and keys must be <a href="#Assignability">assignable</a>
+to the respective field, element, and key types of the literal type;
+there is no additional conversion.
+The key is interpreted as a field name for struct literals,
+an index for array and slice literals, and a key for map literals.
+For map literals, all elements must have a key. It is an error
+to specify multiple elements with the same field name or
+constant key value. For non-constant map keys, see the section on
+<a href="#Order_of_evaluation">evaluation order</a>.
+</p>
+
+<p>
+For struct literals the following rules apply:
+</p>
+<ul>
+ <li>A key must be a field name declared in the struct type.
+ </li>
+ <li>An element list that does not contain any keys must
+ list an element for each struct field in the
+ order in which the fields are declared.
+ </li>
+ <li>If any element has a key, every element must have a key.
+ </li>
+ <li>An element list that contains keys does not need to
+ have an element for each struct field. Omitted fields
+ get the zero value for that field.
+ </li>
+ <li>A literal may omit the element list; such a literal evaluates
+ to the zero value for its type.
+ </li>
+ <li>It is an error to specify an element for a non-exported
+ field of a struct belonging to a different package.
+ </li>
+</ul>
+
+<p>
+Given the declarations
+</p>
+<pre>
+type Point3D struct { x, y, z float64 }
+type Line struct { p, q Point3D }
+</pre>
+
+<p>
+one may write
+</p>
+
+<pre>
+origin := Point3D{} // zero value for Point3D
+line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
+</pre>
+
+<p>
+For array and slice literals the following rules apply:
+</p>
+<ul>
+ <li>Each element has an associated integer index marking
+ its position in the array.
+ </li>
+ <li>An element with a key uses the key as its index. The
+ key must be a non-negative constant
+ <a href="#Representability">representable</a> by
+ a value of type <code>int</code>; and if it is typed
+ it must be of integer type.
+ </li>
+ <li>An element without a key uses the previous element's index plus one.
+ If the first element has no key, its index is zero.
+ </li>
+</ul>
+
+<p>
+<a href="#Address_operators">Taking the address</a> of a composite literal
+generates a pointer to a unique <a href="#Variables">variable</a> initialized
+with the literal's value.
+</p>
+
+<pre>
+var pointer *Point3D = &Point3D{y: 1000}
+</pre>
+
+<p>
+Note that the <a href="#The_zero_value">zero value</a> for a slice or map
+type is not the same as an initialized but empty value of the same type.
+Consequently, taking the address of an empty slice or map composite literal
+does not have the same effect as allocating a new slice or map value with
+<a href="#Allocation">new</a>.
+</p>
+
+<pre>
+p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
+p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
+</pre>
+
+<p>
+The length of an array literal is the length specified in the literal type.
+If fewer elements than the length are provided in the literal, the missing
+elements are set to the zero value for the array element type.
+It is an error to provide elements with index values outside the index range
+of the array. The notation <code>...</code> specifies an array length equal
+to the maximum element index plus one.
+</p>
+
+<pre>
+buffer := [10]string{} // len(buffer) == 10
+intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
+days := [...]string{"Sat", "Sun"} // len(days) == 2
+</pre>
+
+<p>
+A slice literal describes the entire underlying array literal.
+Thus the length and capacity of a slice literal are the maximum
+element index plus one. A slice literal has the form
+</p>
+
+<pre>
+[]T{x1, x2, … xn}
+</pre>
+
+<p>
+and is shorthand for a slice operation applied to an array:
+</p>
+
+<pre>
+tmp := [n]T{x1, x2, … xn}
+tmp[0 : n]
+</pre>
+
+<p>
+Within a composite literal of array, slice, or map type <code>T</code>,
+elements or map keys that are themselves composite literals may elide the respective
+literal type if it is identical to the element or key type of <code>T</code>.
+Similarly, elements or keys that are addresses of composite literals may elide
+the <code>&T</code> when the element or key type is <code>*T</code>.
+</p>
+
+<pre>
+[...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
+[][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
+[][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
+map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
+map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
+
+type PPoint *Point
+[2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
+[2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
+</pre>
+
+<p>
+A parsing ambiguity arises when a composite literal using the
+TypeName form of the LiteralType appears as an operand between the
+<a href="#Keywords">keyword</a> and the opening brace of the block
+of an "if", "for", or "switch" statement, and the composite literal
+is not enclosed in parentheses, square brackets, or curly braces.
+In this rare case, the opening brace of the literal is erroneously parsed
+as the one introducing the block of statements. To resolve the ambiguity,
+the composite literal must appear within parentheses.
+</p>
+
+<pre>
+if x == (T{a,b,c}[i]) { … }
+if (x == T{a,b,c}[i]) { … }
+</pre>
+
+<p>
+Examples of valid array, slice, and map literals:
+</p>
+
+<pre>
+// list of prime numbers
+primes := []int{2, 3, 5, 7, 9, 2147483647}
+
+// vowels[ch] is true if ch is a vowel
+vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
+
+// the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
+filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
+
+// frequencies in Hz for equal-tempered scale (A4 = 440Hz)
+noteFrequency := map[string]float32{
+ "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
+ "G0": 24.50, "A0": 27.50, "B0": 30.87,
+}
+</pre>
+
+
+<h3 id="Function_literals">Function literals</h3>
+
+<p>
+A function literal represents an anonymous <a href="#Function_declarations">function</a>.
+</p>
+
+<pre class="ebnf">
+FunctionLit = "func" Signature FunctionBody .
+</pre>
+
+<pre>
+func(a, b int, z float64) bool { return a*b < int(z) }
+</pre>
+
+<p>
+A function literal can be assigned to a variable or invoked directly.
+</p>
+
+<pre>
+f := func(x, y int) int { return x + y }
+func(ch chan int) { ch <- ACK }(replyChan)
+</pre>
+
+<p>
+Function literals are <i>closures</i>: they may refer to variables
+defined in a surrounding function. Those variables are then shared between
+the surrounding function and the function literal, and they survive as long
+as they are accessible.
+</p>
+
+
+<h3 id="Primary_expressions">Primary expressions</h3>
+
+<p>
+Primary expressions are the operands for unary and binary expressions.
+</p>
+
+<pre class="ebnf">
+PrimaryExpr =
+ Operand |
+ Conversion |
+ MethodExpr |
+ PrimaryExpr Selector |
+ PrimaryExpr Index |
+ PrimaryExpr Slice |
+ PrimaryExpr TypeAssertion |
+ PrimaryExpr Arguments .
+
+Selector = "." identifier .
+Index = "[" Expression "]" .
+Slice = "[" [ Expression ] ":" [ Expression ] "]" |
+ "[" [ Expression ] ":" Expression ":" Expression "]" .
+TypeAssertion = "." "(" Type ")" .
+Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
+</pre>
+
+
+<pre>
+x
+2
+(s + ".txt")
+f(3.1415, true)
+Point{1, 2}
+m["foo"]
+s[i : j + 1]
+obj.color
+f.p[i].x()
+</pre>
+
+
+<h3 id="Selectors">Selectors</h3>
+
+<p>
+For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
+that is not a <a href="#Package_clause">package name</a>, the
+<i>selector expression</i>
+</p>
+
+<pre>
+x.f
+</pre>
+
+<p>
+denotes the field or method <code>f</code> of the value <code>x</code>
+(or sometimes <code>*x</code>; see below).
+The identifier <code>f</code> is called the (field or method) <i>selector</i>;
+it must not be the <a href="#Blank_identifier">blank identifier</a>.
+The type of the selector expression is the type of <code>f</code>.
+If <code>x</code> is a package name, see the section on
+<a href="#Qualified_identifiers">qualified identifiers</a>.
+</p>
+
+<p>
+A selector <code>f</code> may denote a field or method <code>f</code> of
+a type <code>T</code>, or it may refer
+to a field or method <code>f</code> of a nested
+<a href="#Struct_types">embedded field</a> of <code>T</code>.
+The number of embedded fields traversed
+to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
+The depth of a field or method <code>f</code>
+declared in <code>T</code> is zero.
+The depth of a field or method <code>f</code> declared in
+an embedded field <code>A</code> in <code>T</code> is the
+depth of <code>f</code> in <code>A</code> plus one.
+</p>
+
+<p>
+The following rules apply to selectors:
+</p>
+
+<ol>
+<li>
+For a value <code>x</code> of type <code>T</code> or <code>*T</code>
+where <code>T</code> is not a pointer or interface type,
+<code>x.f</code> denotes the field or method at the shallowest depth
+in <code>T</code> where there
+is such an <code>f</code>.
+If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
+with shallowest depth, the selector expression is illegal.
+</li>
+
+<li>
+For a value <code>x</code> of type <code>I</code> where <code>I</code>
+is an interface type, <code>x.f</code> denotes the actual method with name
+<code>f</code> of the dynamic value of <code>x</code>.
+If there is no method with name <code>f</code> in the
+<a href="#Method_sets">method set</a> of <code>I</code>, the selector
+expression is illegal.
+</li>
+
+<li>
+As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
+pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
+(but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
+</li>
+
+<li>
+In all other cases, <code>x.f</code> is illegal.
+</li>
+
+<li>
+If <code>x</code> is of pointer type and has the value
+<code>nil</code> and <code>x.f</code> denotes a struct field,
+assigning to or evaluating <code>x.f</code>
+causes a <a href="#Run_time_panics">run-time panic</a>.
+</li>
+
+<li>
+If <code>x</code> is of interface type and has the value
+<code>nil</code>, <a href="#Calls">calling</a> or
+<a href="#Method_values">evaluating</a> the method <code>x.f</code>
+causes a <a href="#Run_time_panics">run-time panic</a>.
+</li>
+</ol>
+
+<p>
+For example, given the declarations:
+</p>
+
+<pre>
+type T0 struct {
+ x int
+}
+
+func (*T0) M0()
+
+type T1 struct {
+ y int
+}
+
+func (T1) M1()
+
+type T2 struct {
+ z int
+ T1
+ *T0
+}
+
+func (*T2) M2()
+
+type Q *T2
+
+var t T2 // with t.T0 != nil
+var p *T2 // with p != nil and (*p).T0 != nil
+var q Q = p
+</pre>
+
+<p>
+one may write:
+</p>
+
+<pre>
+t.z // t.z
+t.y // t.T1.y
+t.x // (*t.T0).x
+
+p.z // (*p).z
+p.y // (*p).T1.y
+p.x // (*(*p).T0).x
+
+q.x // (*(*q).T0).x (*q).x is a valid field selector
+
+p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
+p.M1() // ((*p).T1).M1() M1 expects T1 receiver
+p.M2() // p.M2() M2 expects *T2 receiver
+t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
+</pre>
+
+<p>
+but the following is invalid:
+</p>
+
+<pre>
+q.M0() // (*q).M0 is valid but not a field selector
+</pre>
+
+
+<h3 id="Method_expressions">Method expressions</h3>
+
+<p>
+If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
+<code>T.M</code> is a function that is callable as a regular function
+with the same arguments as <code>M</code> prefixed by an additional
+argument that is the receiver of the method.
+</p>
+
+<pre class="ebnf">
+MethodExpr = ReceiverType "." MethodName .
+ReceiverType = Type .
+</pre>
+
+<p>
+Consider a struct type <code>T</code> with two methods,
+<code>Mv</code>, whose receiver is of type <code>T</code>, and
+<code>Mp</code>, whose receiver is of type <code>*T</code>.
+</p>
+
+<pre>
+type T struct {
+ a int
+}
+func (tv T) Mv(a int) int { return 0 } // value receiver
+func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
+
+var t T
+</pre>
+
+<p>
+The expression
+</p>
+
+<pre>
+T.Mv
+</pre>
+
+<p>
+yields a function equivalent to <code>Mv</code> but
+with an explicit receiver as its first argument; it has signature
+</p>
+
+<pre>
+func(tv T, a int) int
+</pre>
+
+<p>
+That function may be called normally with an explicit receiver, so
+these five invocations are equivalent:
+</p>
+
+<pre>
+t.Mv(7)
+T.Mv(t, 7)
+(T).Mv(t, 7)
+f1 := T.Mv; f1(t, 7)
+f2 := (T).Mv; f2(t, 7)
+</pre>
+
+<p>
+Similarly, the expression
+</p>
+
+<pre>
+(*T).Mp
+</pre>
+
+<p>
+yields a function value representing <code>Mp</code> with signature
+</p>
+
+<pre>
+func(tp *T, f float32) float32
+</pre>
+
+<p>
+For a method with a value receiver, one can derive a function
+with an explicit pointer receiver, so
+</p>
+
+<pre>
+(*T).Mv
+</pre>
+
+<p>
+yields a function value representing <code>Mv</code> with signature
+</p>
+
+<pre>
+func(tv *T, a int) int
+</pre>
+
+<p>
+Such a function indirects through the receiver to create a value
+to pass as the receiver to the underlying method;
+the method does not overwrite the value whose address is passed in
+the function call.
+</p>
+
+<p>
+The final case, a value-receiver function for a pointer-receiver method,
+is illegal because pointer-receiver methods are not in the method set
+of the value type.
+</p>
+
+<p>
+Function values derived from methods are called with function call syntax;
+the receiver is provided as the first argument to the call.
+That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
+as <code>f(t, 7)</code> not <code>t.f(7)</code>.
+To construct a function that binds the receiver, use a
+<a href="#Function_literals">function literal</a> or
+<a href="#Method_values">method value</a>.
+</p>
+
+<p>
+It is legal to derive a function value from a method of an interface type.
+The resulting function takes an explicit receiver of that interface type.
+</p>
+
+<h3 id="Method_values">Method values</h3>
+
+<p>
+If the expression <code>x</code> has static type <code>T</code> and
+<code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
+<code>x.M</code> is called a <i>method value</i>.
+The method value <code>x.M</code> is a function value that is callable
+with the same arguments as a method call of <code>x.M</code>.
+The expression <code>x</code> is evaluated and saved during the evaluation of the
+method value; the saved copy is then used as the receiver in any calls,
+which may be executed later.
+</p>
+
+<pre>
+type S struct { *T }
+type T int
+func (t T) M() { print(t) }
+
+t := new(T)
+s := S{T: t}
+f := t.M // receiver *t is evaluated and stored in f
+g := s.M // receiver *(s.T) is evaluated and stored in g
+*t = 42 // does not affect stored receivers in f and g
+</pre>
+
+<p>
+The type <code>T</code> may be an interface or non-interface type.
+</p>
+
+<p>
+As in the discussion of <a href="#Method_expressions">method expressions</a> above,
+consider a struct type <code>T</code> with two methods,
+<code>Mv</code>, whose receiver is of type <code>T</code>, and
+<code>Mp</code>, whose receiver is of type <code>*T</code>.
+</p>
+
+<pre>
+type T struct {
+ a int
+}
+func (tv T) Mv(a int) int { return 0 } // value receiver
+func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
+
+var t T
+var pt *T
+func makeT() T
+</pre>
+
+<p>
+The expression
+</p>
+
+<pre>
+t.Mv
+</pre>
+
+<p>
+yields a function value of type
+</p>
+
+<pre>
+func(int) int
+</pre>
+
+<p>
+These two invocations are equivalent:
+</p>
+
+<pre>
+t.Mv(7)
+f := t.Mv; f(7)
+</pre>
+
+<p>
+Similarly, the expression
+</p>
+
+<pre>
+pt.Mp
+</pre>
+
+<p>
+yields a function value of type
+</p>
+
+<pre>
+func(float32) float32
+</pre>
+
+<p>
+As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
+using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
+</p>
+
+<p>
+As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
+using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
+</p>
+
+<pre>
+f := t.Mv; f(7) // like t.Mv(7)
+f := pt.Mp; f(7) // like pt.Mp(7)
+f := pt.Mv; f(7) // like (*pt).Mv(7)
+f := t.Mp; f(7) // like (&t).Mp(7)
+f := makeT().Mp // invalid: result of makeT() is not addressable
+</pre>
+
+<p>
+Although the examples above use non-interface types, it is also legal to create a method value
+from a value of interface type.
+</p>
+
+<pre>
+var i interface { M(int) } = myVal
+f := i.M; f(7) // like i.M(7)
+</pre>
+
+
+<h3 id="Index_expressions">Index expressions</h3>
+
+<p>
+A primary expression of the form
+</p>
+
+<pre>
+a[x]
+</pre>
+
+<p>
+denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
+The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
+The following rules apply:
+</p>
+
+<p>
+If <code>a</code> is not a map:
+</p>
+<ul>
+ <li>the index <code>x</code> must be of integer type or an untyped constant</li>
+ <li>a constant index must be non-negative and
+ <a href="#Representability">representable</a> by a value of type <code>int</code></li>
+ <li>a constant index that is untyped is given type <code>int</code></li>
+ <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
+ otherwise it is <i>out of range</i></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
+</p>
+<ul>
+ <li>a <a href="#Constants">constant</a> index must be in range</li>
+ <li>if <code>x</code> is out of range at run time,
+ a <a href="#Run_time_panics">run-time panic</a> occurs</li>
+ <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
+ <code>a[x]</code> is the element type of <code>A</code></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
+</p>
+<ul>
+ <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
+</p>
+<ul>
+ <li>if <code>x</code> is out of range at run time,
+ a <a href="#Run_time_panics">run-time panic</a> occurs</li>
+ <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
+ <code>a[x]</code> is the element type of <code>S</code></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#String_types">string type</a>:
+</p>
+<ul>
+ <li>a <a href="#Constants">constant</a> index must be in range
+ if the string <code>a</code> is also constant</li>
+ <li>if <code>x</code> is out of range at run time,
+ a <a href="#Run_time_panics">run-time panic</a> occurs</li>
+ <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
+ <code>a[x]</code> is <code>byte</code></li>
+ <li><code>a[x]</code> may not be assigned to</li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
+</p>
+<ul>
+ <li><code>x</code>'s type must be
+ <a href="#Assignability">assignable</a>
+ to the key type of <code>M</code></li>
+ <li>if the map contains an entry with key <code>x</code>,
+ <code>a[x]</code> is the map element with key <code>x</code>
+ and the type of <code>a[x]</code> is the element type of <code>M</code></li>
+ <li>if the map is <code>nil</code> or does not contain such an entry,
+ <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
+ for the element type of <code>M</code></li>
+</ul>
+
+<p>
+Otherwise <code>a[x]</code> is illegal.
+</p>
+
+<p>
+An index expression on a map <code>a</code> of type <code>map[K]V</code>
+used in an <a href="#Assignments">assignment</a> or initialization of the special form
+</p>
+
+<pre>
+v, ok = a[x]
+v, ok := a[x]
+var v, ok = a[x]
+</pre>
+
+<p>
+yields an additional untyped boolean value. The value of <code>ok</code> is
+<code>true</code> if the key <code>x</code> is present in the map, and
+<code>false</code> otherwise.
+</p>
+
+<p>
+Assigning to an element of a <code>nil</code> map causes a
+<a href="#Run_time_panics">run-time panic</a>.
+</p>
+
+
+<h3 id="Slice_expressions">Slice expressions</h3>
+
+<p>
+Slice expressions construct a substring or slice from a string, array, pointer
+to array, or slice. There are two variants: a simple form that specifies a low
+and high bound, and a full form that also specifies a bound on the capacity.
+</p>
+
+<h4>Simple slice expressions</h4>
+
+<p>
+For a string, array, pointer to array, or slice <code>a</code>, the primary expression
+</p>
+
+<pre>
+a[low : high]
+</pre>
+
+<p>
+constructs a substring or slice. The <i>indices</i> <code>low</code> and
+<code>high</code> select which elements of operand <code>a</code> appear
+in the result. The result has indices starting at 0 and length equal to
+<code>high</code> - <code>low</code>.
+After slicing the array <code>a</code>
+</p>
+
+<pre>
+a := [5]int{1, 2, 3, 4, 5}
+s := a[1:4]
+</pre>
+
+<p>
+the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
+</p>
+
+<pre>
+s[0] == 2
+s[1] == 3
+s[2] == 4
+</pre>
+
+<p>
+For convenience, any of the indices may be omitted. A missing <code>low</code>
+index defaults to zero; a missing <code>high</code> index defaults to the length of the
+sliced operand:
+</p>
+
+<pre>
+a[2:] // same as a[2 : len(a)]
+a[:3] // same as a[0 : 3]
+a[:] // same as a[0 : len(a)]
+</pre>
+
+<p>
+If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
+<code>(*a)[low : high]</code>.
+</p>
+
+<p>
+For arrays or strings, the indices are <i>in range</i> if
+<code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
+otherwise they are <i>out of range</i>.
+For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
+A <a href="#Constants">constant</a> index must be non-negative and
+<a href="#Representability">representable</a> by a value of type
+<code>int</code>; for arrays or constant strings, constant indices must also be in range.
+If both indices are constant, they must satisfy <code>low <= high</code>.
+If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<p>
+Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
+the result of the slice operation is a non-constant value of the same type as the operand.
+For untyped string operands the result is a non-constant value of type <code>string</code>.
+If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
+and the result of the slice operation is a slice with the same element type as the array.
+</p>
+
+<p>
+If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
+is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
+array with the operand.
+</p>
+
+<pre>
+var a [10]int
+s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
+s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
+s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
+</pre>
+
+
+<h4>Full slice expressions</h4>
+
+<p>
+For an array, pointer to array, or slice <code>a</code> (but not a string), the primary expression
+</p>
+
+<pre>
+a[low : high : max]
+</pre>
+
+<p>
+constructs a slice of the same type, and with the same length and elements as the simple slice
+expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
+by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
+After slicing the array <code>a</code>
+</p>
+
+<pre>
+a := [5]int{1, 2, 3, 4, 5}
+t := a[1:3:5]
+</pre>
+
+<p>
+the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
+</p>
+
+<pre>
+t[0] == 2
+t[1] == 3
+</pre>
+
+<p>
+As for simple slice expressions, if <code>a</code> is a pointer to an array,
+<code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
+If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
+</p>
+
+<p>
+The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
+otherwise they are <i>out of range</i>.
+A <a href="#Constants">constant</a> index must be non-negative and
+<a href="#Representability">representable</a> by a value of type
+<code>int</code>; for arrays, constant indices must also be in range.
+If multiple indices are constant, the constants that are present must be in range relative to each
+other.
+If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<h3 id="Type_assertions">Type assertions</h3>
+
+<p>
+For an expression <code>x</code> of <a href="#Interface_types">interface type</a>
+and a type <code>T</code>, the primary expression
+</p>
+
+<pre>
+x.(T)
+</pre>
+
+<p>
+asserts that <code>x</code> is not <code>nil</code>
+and that the value stored in <code>x</code> is of type <code>T</code>.
+The notation <code>x.(T)</code> is called a <i>type assertion</i>.
+</p>
+<p>
+More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
+that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
+to the type <code>T</code>.
+In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
+otherwise the type assertion is invalid since it is not possible for <code>x</code>
+to store a value of type <code>T</code>.
+If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
+of <code>x</code> implements the interface <code>T</code>.
+</p>
+<p>
+If the type assertion holds, the value of the expression is the value
+stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+In other words, even though the dynamic type of <code>x</code>
+is known only at run time, the type of <code>x.(T)</code> is
+known to be <code>T</code> in a correct program.
+</p>
+
+<pre>
+var x interface{} = 7 // x has dynamic type int and value 7
+i := x.(int) // i has type int and value 7
+
+type I interface { m() }
+
+func f(y I) {
+ s := y.(string) // illegal: string does not implement I (missing method m)
+ r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
+ …
+}
+</pre>
+
+<p>
+A type assertion used in an <a href="#Assignments">assignment</a> or initialization of the special form
+</p>
+
+<pre>
+v, ok = x.(T)
+v, ok := x.(T)
+var v, ok = x.(T)
+var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
+</pre>
+
+<p>
+yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
+if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
+the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
+No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
+</p>
+
+
+<h3 id="Calls">Calls</h3>
+
+<p>
+Given an expression <code>f</code> of function type
+<code>F</code>,
+</p>
+
+<pre>
+f(a1, a2, … an)
+</pre>
+
+<p>
+calls <code>f</code> with arguments <code>a1, a2, … an</code>.
+Except for one special case, arguments must be single-valued expressions
+<a href="#Assignability">assignable</a> to the parameter types of
+<code>F</code> and are evaluated before the function is called.
+The type of the expression is the result type
+of <code>F</code>.
+A method invocation is similar but the method itself
+is specified as a selector upon a value of the receiver type for
+the method.
+</p>
+
+<pre>
+math.Atan2(x, y) // function call
+var pt *Point
+pt.Scale(3.5) // method call with receiver pt
+</pre>
+
+<p>
+In a function call, the function value and arguments are evaluated in
+<a href="#Order_of_evaluation">the usual order</a>.
+After they are evaluated, the parameters of the call are passed by value to the function
+and the called function begins execution.
+The return parameters of the function are passed by value
+back to the caller when the function returns.
+</p>
+
+<p>
+Calling a <code>nil</code> function value
+causes a <a href="#Run_time_panics">run-time panic</a>.
+</p>
+
+<p>
+As a special case, if the return values of a function or method
+<code>g</code> are equal in number and individually
+assignable to the parameters of another function or method
+<code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
+will invoke <code>f</code> after binding the return values of
+<code>g</code> to the parameters of <code>f</code> in order. The call
+of <code>f</code> must contain no parameters other than the call of <code>g</code>,
+and <code>g</code> must have at least one return value.
+If <code>f</code> has a final <code>...</code> parameter, it is
+assigned the return values of <code>g</code> that remain after
+assignment of regular parameters.
+</p>
+
+<pre>
+func Split(s string, pos int) (string, string) {
+ return s[0:pos], s[pos:]
+}
+
+func Join(s, t string) string {
+ return s + t
+}
+
+if Join(Split(value, len(value)/2)) != value {
+ log.Panic("test fails")
+}
+</pre>
+
+<p>
+A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
+of (the type of) <code>x</code> contains <code>m</code> and the
+argument list can be assigned to the parameter list of <code>m</code>.
+If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
+set contains <code>m</code>, <code>x.m()</code> is shorthand
+for <code>(&x).m()</code>:
+</p>
+
+<pre>
+var p Point
+p.Scale(3.5)
+</pre>
+
+<p>
+There is no distinct method type and there are no method literals.
+</p>
+
+<h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
+
+<p>
+If <code>f</code> is <a href="#Function_types">variadic</a> with a final
+parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
+the type of <code>p</code> is equivalent to type <code>[]T</code>.
+If <code>f</code> is invoked with no actual arguments for <code>p</code>,
+the value passed to <code>p</code> is <code>nil</code>.
+Otherwise, the value passed is a new slice
+of type <code>[]T</code> with a new underlying array whose successive elements
+are the actual arguments, which all must be <a href="#Assignability">assignable</a>
+to <code>T</code>. The length and capacity of the slice is therefore
+the number of arguments bound to <code>p</code> and may differ for each
+call site.
+</p>
+
+<p>
+Given the function and calls
+</p>
+<pre>
+func Greeting(prefix string, who ...string)
+Greeting("nobody")
+Greeting("hello:", "Joe", "Anna", "Eileen")
+</pre>
+
+<p>
+within <code>Greeting</code>, <code>who</code> will have the value
+<code>nil</code> in the first call, and
+<code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
+</p>
+
+<p>
+If the final argument is assignable to a slice type <code>[]T</code> and
+is followed by <code>...</code>, it is passed unchanged as the value
+for a <code>...T</code> parameter. In this case no new slice is created.
+</p>
+
+<p>
+Given the slice <code>s</code> and call
+</p>
+
+<pre>
+s := []string{"James", "Jasmine"}
+Greeting("goodbye:", s...)
+</pre>
+
+<p>
+within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
+with the same underlying array.
+</p>
+
+
+<h3 id="Operators">Operators</h3>
+
+<p>
+Operators combine operands into expressions.
+</p>
+
+<pre class="ebnf">
+Expression = UnaryExpr | Expression binary_op Expression .
+UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
+
+binary_op = "||" | "&&" | rel_op | add_op | mul_op .
+rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
+add_op = "+" | "-" | "|" | "^" .
+mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
+
+unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
+</pre>
+
+<p>
+Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
+For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
+unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
+For operations involving constants only, see the section on
+<a href="#Constant_expressions">constant expressions</a>.
+</p>
+
+<p>
+Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
+and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
+to the type of the other operand.
+</p>
+
+<p>
+The right operand in a shift expression must have integer type
+or be an untyped constant <a href="#Representability">representable</a> by a
+value of type <code>uint</code>.
+If the left operand of a non-constant shift expression is an untyped constant,
+it is first implicitly converted to the type it would assume if the shift expression were
+replaced by its left operand alone.
+</p>
+
+<pre>
+var a [1024]byte
+var s uint = 33
+
+// The results of the following examples are given for 64-bit ints.
+var i = 1<<s // 1 has type int
+var j int32 = 1<<s // 1 has type int32; j == 0
+var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
+var m int = 1.0<<s // 1.0 has type int; m == 1<<33
+var n = 1.0<<s == j // 1.0 has type int32; n == true
+var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
+var p = 1<<s == 1<<33 // 1 has type int; p == true
+var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
+var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
+var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
+var v float32 = 1<<s // illegal: 1 has type float32, cannot shift
+var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
+var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
+var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
+
+// The results of the following examples are given for 32-bit ints,
+// which means the shifts will overflow.
+var mm int = 1.0<<s // 1.0 has type int; mm == 0
+var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
+var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
+var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
+var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
+</pre>
+
+<h4 id="Operator_precedence">Operator precedence</h4>
+<p>
+Unary operators have the highest precedence.
+As the <code>++</code> and <code>--</code> operators form
+statements, not expressions, they fall
+outside the operator hierarchy.
+As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
+<p>
+There are five precedence levels for binary operators.
+Multiplication operators bind strongest, followed by addition
+operators, comparison operators, <code>&&</code> (logical AND),
+and finally <code>||</code> (logical OR):
+</p>
+
+<pre class="grammar">
+Precedence Operator
+ 5 * / % << >> & &^
+ 4 + - | ^
+ 3 == != < <= > >=
+ 2 &&
+ 1 ||
+</pre>
+
+<p>
+Binary operators of the same precedence associate from left to right.
+For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
+</p>
+
+<pre>
++x
+23 + 3*x[i]
+x <= f()
+^a >> b
+f() || g()
+x == y+1 && <-chanInt > 0
+</pre>
+
+
+<h3 id="Arithmetic_operators">Arithmetic operators</h3>
+<p>
+Arithmetic operators apply to numeric values and yield a result of the same
+type as the first operand. The four standard arithmetic operators (<code>+</code>,
+<code>-</code>, <code>*</code>, <code>/</code>) apply to integer,
+floating-point, and complex types; <code>+</code> also applies to strings.
+The bitwise logical and shift operators apply to integers only.
+</p>
+
+<pre class="grammar">
++ sum integers, floats, complex values, strings
+- difference integers, floats, complex values
+* product integers, floats, complex values
+/ quotient integers, floats, complex values
+% remainder integers
+
+& bitwise AND integers
+| bitwise OR integers
+^ bitwise XOR integers
+&^ bit clear (AND NOT) integers
+
+<< left shift integer << integer >= 0
+>> right shift integer >> integer >= 0
+</pre>
+
+
+<h4 id="Integer_operators">Integer operators</h4>
+
+<p>
+For two integer values <code>x</code> and <code>y</code>, the integer quotient
+<code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
+relationships:
+</p>
+
+<pre>
+x = q*y + r and |r| < |y|
+</pre>
+
+<p>
+with <code>x / y</code> truncated towards zero
+(<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
+</p>
+
+<pre>
+ x y x / y x % y
+ 5 3 1 2
+-5 3 -1 -2
+ 5 -3 -1 2
+-5 -3 1 -2
+</pre>
+
+<p>
+The one exception to this rule is that if the dividend <code>x</code> is
+the most negative value for the int type of <code>x</code>, the quotient
+<code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
+due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
+</p>
+
+<pre>
+ x, q
+int8 -128
+int16 -32768
+int32 -2147483648
+int64 -9223372036854775808
+</pre>
+
+<p>
+If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
+If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
+If the dividend is non-negative and the divisor is a constant power of 2,
+the division may be replaced by a right shift, and computing the remainder may
+be replaced by a bitwise AND operation:
+</p>
+
+<pre>
+ x x / 4 x % 4 x >> 2 x & 3
+ 11 2 3 2 3
+-11 -2 -3 -3 1
+</pre>
+
+<p>
+The shift operators shift the left operand by the shift count specified by the
+right operand, which must be non-negative. If the shift count is negative at run time,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+The shift operators implement arithmetic shifts if the left operand is a signed
+integer and logical shifts if it is an unsigned integer.
+There is no upper limit on the shift count. Shifts behave
+as if the left operand is shifted <code>n</code> times by 1 for a shift
+count of <code>n</code>.
+As a result, <code>x << 1</code> is the same as <code>x*2</code>
+and <code>x >> 1</code> is the same as
+<code>x/2</code> but truncated towards negative infinity.
+</p>
+
+<p>
+For integer operands, the unary operators
+<code>+</code>, <code>-</code>, and <code>^</code> are defined as
+follows:
+</p>
+
+<pre class="grammar">
++x is 0 + x
+-x negation is 0 - x
+^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
+ and m = -1 for signed x
+</pre>
+
+
+<h4 id="Integer_overflow">Integer overflow</h4>
+
+<p>
+For unsigned integer values, the operations <code>+</code>,
+<code>-</code>, <code>*</code>, and <code><<</code> are
+computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
+the <a href="#Numeric_types">unsigned integer</a>'s type.
+Loosely speaking, these unsigned integer operations
+discard high bits upon overflow, and programs may rely on "wrap around".
+</p>
+<p>
+For signed integers, the operations <code>+</code>,
+<code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
+overflow and the resulting value exists and is deterministically defined
+by the signed integer representation, the operation, and its operands.
+Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
+A compiler may not optimize code under the assumption that overflow does
+not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
+</p>
+
+
+<h4 id="Floating_point_operators">Floating-point operators</h4>
+
+<p>
+For floating-point and complex numbers,
+<code>+x</code> is the same as <code>x</code>,
+while <code>-x</code> is the negation of <code>x</code>.
+The result of a floating-point or complex division by zero is not specified beyond the
+IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
+occurs is implementation-specific.
+</p>
+
+<p>
+An implementation may combine multiple floating-point operations into a single
+fused operation, possibly across statements, and produce a result that differs
+from the value obtained by executing and rounding the instructions individually.
+An explicit floating-point type <a href="#Conversions">conversion</a> rounds to
+the precision of the target type, preventing fusion that would discard that rounding.
+</p>
+
+<p>
+For instance, some architectures provide a "fused multiply and add" (FMA) instruction
+that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
+These examples show when a Go implementation can use that instruction:
+</p>
+
+<pre>
+// FMA allowed for computing r, because x*y is not explicitly rounded:
+r = x*y + z
+r = z; r += x*y
+t = x*y; r = t + z
+*p = x*y; r = *p + z
+r = x*y + float64(z)
+
+// FMA disallowed for computing r, because it would omit rounding of x*y:
+r = float64(x*y) + z
+r = z; r += float64(x*y)
+t = float64(x*y); r = t + z
+</pre>
+
+<h4 id="String_concatenation">String concatenation</h4>
+
+<p>
+Strings can be concatenated using the <code>+</code> operator
+or the <code>+=</code> assignment operator:
+</p>
+
+<pre>
+s := "hi" + string(c)
+s += " and good bye"
+</pre>
+
+<p>
+String addition creates a new string by concatenating the operands.
+</p>
+
+
+<h3 id="Comparison_operators">Comparison operators</h3>
+
+<p>
+Comparison operators compare two operands and yield an untyped boolean value.
+</p>
+
+<pre class="grammar">
+== equal
+!= not equal
+< less
+<= less or equal
+> greater
+>= greater or equal
+</pre>
+
+<p>
+In any comparison, the first operand
+must be <a href="#Assignability">assignable</a>
+to the type of the second operand, or vice versa.
+</p>
+<p>
+The equality operators <code>==</code> and <code>!=</code> apply
+to operands that are <i>comparable</i>.
+The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
+apply to operands that are <i>ordered</i>.
+These terms and the result of the comparisons are defined as follows:
+</p>
+
+<ul>
+ <li>
+ Boolean values are comparable.
+ Two boolean values are equal if they are either both
+ <code>true</code> or both <code>false</code>.
+ </li>
+
+ <li>
+ Integer values are comparable and ordered, in the usual way.
+ </li>
+
+ <li>
+ Floating-point values are comparable and ordered,
+ as defined by the IEEE-754 standard.
+ </li>
+
+ <li>
+ Complex values are comparable.
+ Two complex values <code>u</code> and <code>v</code> are
+ equal if both <code>real(u) == real(v)</code> and
+ <code>imag(u) == imag(v)</code>.
+ </li>
+
+ <li>
+ String values are comparable and ordered, lexically byte-wise.
+ </li>
+
+ <li>
+ Pointer values are comparable.
+ Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
+ Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
+ </li>
+
+ <li>
+ Channel values are comparable.
+ Two channel values are equal if they were created by the same call to
+ <a href="#Making_slices_maps_and_channels"><code>make</code></a>
+ or if both have value <code>nil</code>.
+ </li>
+
+ <li>
+ Interface values are comparable.
+ Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
+ and equal dynamic values or if both have value <code>nil</code>.
+ </li>
+
+ <li>
+ A value <code>x</code> of non-interface type <code>X</code> and
+ a value <code>t</code> of interface type <code>T</code> are comparable when values
+ of type <code>X</code> are comparable and
+ <code>X</code> implements <code>T</code>.
+ They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
+ and <code>t</code>'s dynamic value is equal to <code>x</code>.
+ </li>
+
+ <li>
+ Struct values are comparable if all their fields are comparable.
+ Two struct values are equal if their corresponding
+ non-<a href="#Blank_identifier">blank</a> fields are equal.
+ </li>
+
+ <li>
+ Array values are comparable if values of the array element type are comparable.
+ Two array values are equal if their corresponding elements are equal.
+ </li>
+</ul>
+
+<p>
+A comparison of two interface values with identical dynamic types
+causes a <a href="#Run_time_panics">run-time panic</a> if values
+of that type are not comparable. This behavior applies not only to direct interface
+value comparisons but also when comparing arrays of interface values
+or structs with interface-valued fields.
+</p>
+
+<p>
+Slice, map, and function values are not comparable.
+However, as a special case, a slice, map, or function value may
+be compared to the predeclared identifier <code>nil</code>.
+Comparison of pointer, channel, and interface values to <code>nil</code>
+is also allowed and follows from the general rules above.
+</p>
+
+<pre>
+const c = 3 < 4 // c is the untyped boolean constant true
+
+type MyBool bool
+var x, y int
+var (
+ // The result of a comparison is an untyped boolean.
+ // The usual assignment rules apply.
+ b3 = x == y // b3 has type bool
+ b4 bool = x == y // b4 has type bool
+ b5 MyBool = x == y // b5 has type MyBool
+)
+</pre>
+
+<h3 id="Logical_operators">Logical operators</h3>
+
+<p>
+Logical operators apply to <a href="#Boolean_types">boolean</a> values
+and yield a result of the same type as the operands.
+The right operand is evaluated conditionally.
+</p>
+
+<pre class="grammar">
+&& conditional AND p && q is "if p then q else false"
+|| conditional OR p || q is "if p then true else q"
+! NOT !p is "not p"
+</pre>
+
+
+<h3 id="Address_operators">Address operators</h3>
+
+<p>
+For an operand <code>x</code> of type <code>T</code>, the address operation
+<code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
+The operand must be <i>addressable</i>,
+that is, either a variable, pointer indirection, or slice indexing
+operation; or a field selector of an addressable struct operand;
+or an array indexing operation of an addressable array.
+As an exception to the addressability requirement, <code>x</code> may also be a
+(possibly parenthesized)
+<a href="#Composite_literals">composite literal</a>.
+If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
+then the evaluation of <code>&x</code> does too.
+</p>
+
+<p>
+For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
+indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
+to by <code>x</code>.
+If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
+will cause a <a href="#Run_time_panics">run-time panic</a>.
+</p>
+
+<pre>
+&x
+&a[f(2)]
+&Point{2, 3}
+*p
+*pf(x)
+
+var x *int = nil
+*x // causes a run-time panic
+&*x // causes a run-time panic
+</pre>
+
+
+<h3 id="Receive_operator">Receive operator</h3>
+
+<p>
+For an operand <code>ch</code> of <a href="#Channel_types">channel type</a>,
+the value of the receive operation <code><-ch</code> is the value received
+from the channel <code>ch</code>. The channel direction must permit receive operations,
+and the type of the receive operation is the element type of the channel.
+The expression blocks until a value is available.
+Receiving from a <code>nil</code> channel blocks forever.
+A receive operation on a <a href="#Close">closed</a> channel can always proceed
+immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
+after any previously sent values have been received.
+</p>
+
+<pre>
+v1 := <-ch
+v2 = <-ch
+f(<-ch)
+<-strobe // wait until clock pulse and discard received value
+</pre>
+
+<p>
+A receive expression used in an <a href="#Assignments">assignment</a> or initialization of the special form
+</p>
+
+<pre>
+x, ok = <-ch
+x, ok := <-ch
+var x, ok = <-ch
+var x, ok T = <-ch
+</pre>
+
+<p>
+yields an additional untyped boolean result reporting whether the
+communication succeeded. The value of <code>ok</code> is <code>true</code>
+if the value received was delivered by a successful send operation to the
+channel, or <code>false</code> if it is a zero value generated because the
+channel is closed and empty.
+</p>
+
+
+<h3 id="Conversions">Conversions</h3>
+
+<p>
+A conversion changes the <a href="#Types">type</a> of an expression
+to the type specified by the conversion.
+A conversion may appear literally in the source, or it may be <i>implied</i>
+by the context in which an expression appears.
+</p>
+
+<p>
+An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
+where <code>T</code> is a type and <code>x</code> is an expression
+that can be converted to type <code>T</code>.
+</p>
+
+<pre class="ebnf">
+Conversion = Type "(" Expression [ "," ] ")" .
+</pre>
+
+<p>
+If the type starts with the operator <code>*</code> or <code><-</code>,
+or if the type starts with the keyword <code>func</code>
+and has no result list, it must be parenthesized when
+necessary to avoid ambiguity:
+</p>
+
+<pre>
+*Point(p) // same as *(Point(p))
+(*Point)(p) // p is converted to *Point
+<-chan int(c) // same as <-(chan int(c))
+(<-chan int)(c) // c is converted to <-chan int
+func()(x) // function signature func() x
+(func())(x) // x is converted to func()
+(func() int)(x) // x is converted to func() int
+func() int(x) // x is converted to func() int (unambiguous)
+</pre>
+
+<p>
+A <a href="#Constants">constant</a> value <code>x</code> can be converted to
+type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
+by a value of <code>T</code>.
+As a special case, an integer constant <code>x</code> can be explicitly converted to a
+<a href="#String_types">string type</a> using the
+<a href="#Conversions_to_and_from_a_string_type">same rule</a>
+as for non-constant <code>x</code>.
+</p>
+
+<p>
+Converting a constant yields a typed constant as result.
+</p>
+
+<pre>
+uint(iota) // iota value of type uint
+float32(2.718281828) // 2.718281828 of type float32
+complex128(1) // 1.0 + 0.0i of type complex128
+float32(0.49999999) // 0.5 of type float32
+float64(-1e-1000) // 0.0 of type float64
+string('x') // "x" of type string
+string(0x266c) // "♬" of type string
+MyString("foo" + "bar") // "foobar" of type MyString
+string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
+(*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
+int(1.2) // illegal: 1.2 cannot be represented as an int
+string(65.0) // illegal: 65.0 is not an integer constant
+</pre>
+
+<p>
+A non-constant value <code>x</code> can be converted to type <code>T</code>
+in any of these cases:
+</p>
+
+<ul>
+ <li>
+ <code>x</code> is <a href="#Assignability">assignable</a>
+ to <code>T</code>.
+ </li>
+ <li>
+ ignoring struct tags (see below),
+ <code>x</code>'s type and <code>T</code> have <a href="#Type_identity">identical</a>
+ <a href="#Types">underlying types</a>.
+ </li>
+ <li>
+ ignoring struct tags (see below),
+ <code>x</code>'s type and <code>T</code> are pointer types
+ that are not <a href="#Type_definitions">defined types</a>,
+ and their pointer base types have identical underlying types.
+ </li>
+ <li>
+ <code>x</code>'s type and <code>T</code> are both integer or floating
+ point types.
+ </li>
+ <li>
+ <code>x</code>'s type and <code>T</code> are both complex types.
+ </li>
+ <li>
+ <code>x</code> is an integer or a slice of bytes or runes
+ and <code>T</code> is a string type.
+ </li>
+ <li>
+ <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
+ </li>
+ <li>
+ <code>x</code> is a slice, <code>T</code> is a pointer to an array,
+ and the slice and array types have <a href="#Type_identity">identical</a> element types.
+ </li>
+</ul>
+
+<p>
+<a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
+for identity for the purpose of conversion:
+</p>
+
+<pre>
+type Person struct {
+ Name string
+ Address *struct {
+ Street string
+ City string
+ }
+}
+
+var data *struct {
+ Name string `json:"name"`
+ Address *struct {
+ Street string `json:"street"`
+ City string `json:"city"`
+ } `json:"address"`
+}
+
+var person = (*Person)(data) // ignoring tags, the underlying types are identical
+</pre>
+
+<p>
+Specific rules apply to (non-constant) conversions between numeric types or
+to and from a string type.
+These conversions may change the representation of <code>x</code>
+and incur a run-time cost.
+All other conversions only change the type but not the representation
+of <code>x</code>.
+</p>
+
+<p>
+There is no linguistic mechanism to convert between pointers and integers.
+The package <a href="#Package_unsafe"><code>unsafe</code></a>
+implements this functionality under
+restricted circumstances.
+</p>
+
+<h4>Conversions between numeric types</h4>
+
+<p>
+For the conversion of non-constant numeric values, the following rules apply:
+</p>
+
+<ol>
+<li>
+When converting between integer types, if the value is a signed integer, it is
+sign extended to implicit infinite precision; otherwise it is zero extended.
+It is then truncated to fit in the result type's size.
+For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
+The conversion always yields a valid value; there is no indication of overflow.
+</li>
+<li>
+When converting a floating-point number to an integer, the fraction is discarded
+(truncation towards zero).
+</li>
+<li>
+When converting an integer or floating-point number to a floating-point type,
+or a complex number to another complex type, the result value is rounded
+to the precision specified by the destination type.
+For instance, the value of a variable <code>x</code> of type <code>float32</code>
+may be stored using additional precision beyond that of an IEEE-754 32-bit number,
+but float32(x) represents the result of rounding <code>x</code>'s value to
+32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
+of precision, but <code>float32(x + 0.1)</code> does not.
+</li>
+</ol>
+
+<p>
+In all non-constant conversions involving floating-point or complex values,
+if the result type cannot represent the value the conversion
+succeeds but the result value is implementation-dependent.
+</p>
+
+<h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
+
+<ol>
+<li>
+Converting a signed or unsigned integer value to a string type yields a
+string containing the UTF-8 representation of the integer. Values outside
+the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
+
+<pre>
+string('a') // "a"
+string(-1) // "\ufffd" == "\xef\xbf\xbd"
+string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
+type MyString string
+MyString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
+</pre>
+</li>
+
+<li>
+Converting a slice of bytes to a string type yields
+a string whose successive bytes are the elements of the slice.
+
+<pre>
+string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
+string([]byte{}) // ""
+string([]byte(nil)) // ""
+
+type MyBytes []byte
+string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
+</pre>
+</li>
+
+<li>
+Converting a slice of runes to a string type yields
+a string that is the concatenation of the individual rune values
+converted to strings.
+
+<pre>
+string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
+string([]rune{}) // ""
+string([]rune(nil)) // ""
+
+type MyRunes []rune
+string(MyRunes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
+</pre>
+</li>
+
+<li>
+Converting a value of a string type to a slice of bytes type
+yields a slice whose successive elements are the bytes of the string.
+
+<pre>
+[]byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+[]byte("") // []byte{}
+
+MyBytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+</pre>
+</li>
+
+<li>
+Converting a value of a string type to a slice of runes type
+yields a slice containing the individual Unicode code points of the string.
+
+<pre>
+[]rune(MyString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
+[]rune("") // []rune{}
+
+MyRunes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
+</pre>
+</li>
+</ol>
+
+<h4 id="Conversions_from_slice_to_array_pointer">Conversions from slice to array pointer</h4>
+
+<p>
+Converting a slice to an array pointer yields a pointer to the underlying array of the slice.
+If the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<pre>
+s := make([]byte, 2, 4)
+s0 := (*[0]byte)(s) // s0 != nil
+s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
+s2 := (*[2]byte)(s) // &s2[0] == &s[0]
+s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
+
+var t []string
+t0 := (*[0]string)(t) // t0 == nil
+t1 := (*[1]string)(t) // panics: len([1]string) > len(t)
+
+u := make([]byte, 0)
+u0 := (*[0]byte)(u) // u0 != nil
+</pre>
+
+<h3 id="Constant_expressions">Constant expressions</h3>
+
+<p>
+Constant expressions may contain only <a href="#Constants">constant</a>
+operands and are evaluated at compile time.
+</p>
+
+<p>
+Untyped boolean, numeric, and string constants may be used as operands
+wherever it is legal to use an operand of boolean, numeric, or string type,
+respectively.
+</p>
+
+<p>
+A constant <a href="#Comparison_operators">comparison</a> always yields
+an untyped boolean constant. If the left operand of a constant
+<a href="#Operators">shift expression</a> is an untyped constant, the
+result is an integer constant; otherwise it is a constant of the same
+type as the left operand, which must be of
+<a href="#Numeric_types">integer type</a>.
+</p>
+
+<p>
+Any other operation on untyped constants results in an untyped constant of the
+same kind; that is, a boolean, integer, floating-point, complex, or string
+constant.
+If the untyped operands of a binary operation (other than a shift) are of
+different kinds, the result is of the operand's kind that appears later in this
+list: integer, rune, floating-point, complex.
+For example, an untyped integer constant divided by an
+untyped complex constant yields an untyped complex constant.
+</p>
+
+<pre>
+const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
+const b = 15 / 4 // b == 3 (untyped integer constant)
+const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
+const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
+const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
+const d = 1 << 3.0 // d == 8 (untyped integer constant)
+const e = 1.0 << 3 // e == 8 (untyped integer constant)
+const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
+const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
+const h = "foo" > "bar" // h == true (untyped boolean constant)
+const j = true // j == true (untyped boolean constant)
+const k = 'w' + 1 // k == 'x' (untyped rune constant)
+const l = "hi" // l == "hi" (untyped string constant)
+const m = string(k) // m == "x" (type string)
+const Σ = 1 - 0.707i // (untyped complex constant)
+const Δ = Σ + 2.0e-4 // (untyped complex constant)
+const Φ = iota*1i - 1/1i // (untyped complex constant)
+</pre>
+
+<p>
+Applying the built-in function <code>complex</code> to untyped
+integer, rune, or floating-point constants yields
+an untyped complex constant.
+</p>
+
+<pre>
+const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
+const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
+</pre>
+
+<p>
+Constant expressions are always evaluated exactly; intermediate values and the
+constants themselves may require precision significantly larger than supported
+by any predeclared type in the language. The following are legal declarations:
+</p>
+
+<pre>
+const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
+const Four int8 = Huge >> 98 // Four == 4 (type int8)
+</pre>
+
+<p>
+The divisor of a constant division or remainder operation must not be zero:
+</p>
+
+<pre>
+3.14 / 0.0 // illegal: division by zero
+</pre>
+
+<p>
+The values of <i>typed</i> constants must always be accurately
+<a href="#Representability">representable</a> by values
+of the constant type. The following constant expressions are illegal:
+</p>
+
+<pre>
+uint(-1) // -1 cannot be represented as a uint
+int(3.14) // 3.14 cannot be represented as an int
+int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
+Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
+Four * 100 // product 400 cannot be represented as an int8 (type of Four)
+</pre>
+
+<p>
+The mask used by the unary bitwise complement operator <code>^</code> matches
+the rule for non-constants: the mask is all 1s for unsigned constants
+and -1 for signed and untyped constants.
+</p>
+
+<pre>
+^1 // untyped integer constant, equal to -2
+uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
+^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
+int8(^1) // same as int8(-2)
+^int8(1) // same as -1 ^ int8(1) = -2
+</pre>
+
+<p>
+Implementation restriction: A compiler may use rounding while
+computing untyped floating-point or complex constant expressions; see
+the implementation restriction in the section
+on <a href="#Constants">constants</a>. This rounding may cause a
+floating-point constant expression to be invalid in an integer
+context, even if it would be integral when calculated using infinite
+precision, and vice versa.
+</p>
+
+
+<h3 id="Order_of_evaluation">Order of evaluation</h3>
+
+<p>
+At package level, <a href="#Package_initialization">initialization dependencies</a>
+determine the evaluation order of individual initialization expressions in
+<a href="#Variable_declarations">variable declarations</a>.
+Otherwise, when evaluating the <a href="#Operands">operands</a> of an
+expression, assignment, or
+<a href="#Return_statements">return statement</a>,
+all function calls, method calls, and
+communication operations are evaluated in lexical left-to-right
+order.
+</p>
+
+<p>
+For example, in the (function-local) assignment
+</p>
+<pre>
+y[f()], ok = g(h(), i()+x[j()], <-c), k()
+</pre>
+<p>
+the function calls and communication happen in the order
+<code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
+<code><-c</code>, <code>g()</code>, and <code>k()</code>.
+However, the order of those events compared to the evaluation
+and indexing of <code>x</code> and the evaluation
+of <code>y</code> is not specified.
+</p>
+
+<pre>
+a := 1
+f := func() int { a++; return a }
+x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
+m := map[int]int{a: 1, a: 2} // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified
+n := map[int]int{a: f()} // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified
+</pre>
+
+<p>
+At package level, initialization dependencies override the left-to-right rule
+for individual initialization expressions, but not for operands within each
+expression:
+</p>
+
+<pre>
+var a, b, c = f() + v(), g(), sqr(u()) + v()
+
+func f() int { return c }
+func g() int { return a }
+func sqr(x int) int { return x*x }
+
+// functions u and v are independent of all other variables and functions
+</pre>
+
+<p>
+The function calls happen in the order
+<code>u()</code>, <code>sqr()</code>, <code>v()</code>,
+<code>f()</code>, <code>v()</code>, and <code>g()</code>.
+</p>
+
+<p>
+Floating-point operations within a single expression are evaluated according to
+the associativity of the operators. Explicit parentheses affect the evaluation
+by overriding the default associativity.
+In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
+is performed before adding <code>x</code>.
+</p>
+
+<h2 id="Statements">Statements</h2>
+
+<p>
+Statements control execution.
+</p>
+
+<pre class="ebnf">
+Statement =
+ Declaration | LabeledStmt | SimpleStmt |
+ GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
+ FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
+ DeferStmt .
+
+SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
+</pre>
+
+<h3 id="Terminating_statements">Terminating statements</h3>
+
+<p>
+A <i>terminating statement</i> interrupts the regular flow of control in
+a <a href="#Blocks">block</a>. The following statements are terminating:
+</p>
+
+<ol>
+<li>
+ A <a href="#Return_statements">"return"</a> or
+ <a href="#Goto_statements">"goto"</a> statement.
+ <!-- ul below only for regular layout -->
+ <ul> </ul>
+</li>
+
+<li>
+ A call to the built-in function
+ <a href="#Handling_panics"><code>panic</code></a>.
+ <!-- ul below only for regular layout -->
+ <ul> </ul>
+</li>
+
+<li>
+ A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
+ <!-- ul below only for regular layout -->
+ <ul> </ul>
+</li>
+
+<li>
+ An <a href="#If_statements">"if" statement</a> in which:
+ <ul>
+ <li>the "else" branch is present, and</li>
+ <li>both branches are terminating statements.</li>
+ </ul>
+</li>
+
+<li>
+ A <a href="#For_statements">"for" statement</a> in which:
+ <ul>
+ <li>there are no "break" statements referring to the "for" statement, and</li>
+ <li>the loop condition is absent, and</li>
+ <li>the "for" statement does not use a range clause.</li>
+ </ul>
+</li>
+
+<li>
+ A <a href="#Switch_statements">"switch" statement</a> in which:
+ <ul>
+ <li>there are no "break" statements referring to the "switch" statement,</li>
+ <li>there is a default case, and</li>
+ <li>the statement lists in each case, including the default, end in a terminating
+ statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
+ statement</a>.</li>
+ </ul>
+</li>
+
+<li>
+ A <a href="#Select_statements">"select" statement</a> in which:
+ <ul>
+ <li>there are no "break" statements referring to the "select" statement, and</li>
+ <li>the statement lists in each case, including the default if present,
+ end in a terminating statement.</li>
+ </ul>
+</li>
+
+<li>
+ A <a href="#Labeled_statements">labeled statement</a> labeling
+ a terminating statement.
+</li>
+</ol>
+
+<p>
+All other statements are not terminating.
+</p>
+
+<p>
+A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
+is not empty and its final non-empty statement is terminating.
+</p>
+
+
+<h3 id="Empty_statements">Empty statements</h3>
+
+<p>
+The empty statement does nothing.
+</p>
+
+<pre class="ebnf">
+EmptyStmt = .
+</pre>
+
+
+<h3 id="Labeled_statements">Labeled statements</h3>
+
+<p>
+A labeled statement may be the target of a <code>goto</code>,
+<code>break</code> or <code>continue</code> statement.
+</p>
+
+<pre class="ebnf">
+LabeledStmt = Label ":" Statement .
+Label = identifier .
+</pre>
+
+<pre>
+Error: log.Panic("error encountered")
+</pre>
+
+
+<h3 id="Expression_statements">Expression statements</h3>
+
+<p>
+With the exception of specific built-in functions,
+function and method <a href="#Calls">calls</a> and
+<a href="#Receive_operator">receive operations</a>
+can appear in statement context. Such statements may be parenthesized.
+</p>
+
+<pre class="ebnf">
+ExpressionStmt = Expression .
+</pre>
+
+<p>
+The following built-in functions are not permitted in statement context:
+</p>
+
+<pre>
+append cap complex imag len make new real
+unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice
+</pre>
+
+<pre>
+h(x+y)
+f.Close()
+<-ch
+(<-ch)
+len("foo") // illegal if len is the built-in function
+</pre>
+
+
+<h3 id="Send_statements">Send statements</h3>
+
+<p>
+A send statement sends a value on a channel.
+The channel expression must be of <a href="#Channel_types">channel type</a>,
+the channel direction must permit send operations,
+and the type of the value to be sent must be <a href="#Assignability">assignable</a>
+to the channel's element type.
+</p>
+
+<pre class="ebnf">
+SendStmt = Channel "<-" Expression .
+Channel = Expression .
+</pre>
+
+<p>
+Both the channel and the value expression are evaluated before communication
+begins. Communication blocks until the send can proceed.
+A send on an unbuffered channel can proceed if a receiver is ready.
+A send on a buffered channel can proceed if there is room in the buffer.
+A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
+A send on a <code>nil</code> channel blocks forever.
+</p>
+
+<pre>
+ch <- 3 // send value 3 to channel ch
+</pre>
+
+
+<h3 id="IncDec_statements">IncDec statements</h3>
+
+<p>
+The "++" and "--" statements increment or decrement their operands
+by the untyped <a href="#Constants">constant</a> <code>1</code>.
+As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
+or a map index expression.
+</p>
+
+<pre class="ebnf">
+IncDecStmt = Expression ( "++" | "--" ) .
+</pre>
+
+<p>
+The following <a href="#Assignments">assignment statements</a> are semantically
+equivalent:
+</p>
+
+<pre class="grammar">
+IncDec statement Assignment
+x++ x += 1
+x-- x -= 1
+</pre>
+
+
+<h3 id="Assignments">Assignments</h3>
+
+<pre class="ebnf">
+Assignment = ExpressionList assign_op ExpressionList .
+
+assign_op = [ add_op | mul_op ] "=" .
+</pre>
+
+<p>
+Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
+a map index expression, or (for <code>=</code> assignments only) the
+<a href="#Blank_identifier">blank identifier</a>.
+Operands may be parenthesized.
+</p>
+
+<pre>
+x = 1
+*p = f()
+a[i] = 23
+(k) = <-ch // same as: k = <-ch
+</pre>
+
+<p>
+An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
+<code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
+is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
+<code>(y)</code> but evaluates <code>x</code>
+only once. The <i>op</i><code>=</code> construct is a single token.
+In assignment operations, both the left- and right-hand expression lists
+must contain exactly one single-valued expression, and the left-hand
+expression must not be the blank identifier.
+</p>
+
+<pre>
+a[i] <<= 2
+i &^= 1<<n
+</pre>
+
+<p>
+A tuple assignment assigns the individual elements of a multi-valued
+operation to a list of variables. There are two forms. In the
+first, the right hand operand is a single multi-valued expression
+such as a function call, a <a href="#Channel_types">channel</a> or
+<a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
+The number of operands on the left
+hand side must match the number of values. For instance, if
+<code>f</code> is a function returning two values,
+</p>
+
+<pre>
+x, y = f()
+</pre>
+
+<p>
+assigns the first value to <code>x</code> and the second to <code>y</code>.
+In the second form, the number of operands on the left must equal the number
+of expressions on the right, each of which must be single-valued, and the
+<i>n</i>th expression on the right is assigned to the <i>n</i>th
+operand on the left:
+</p>
+
+<pre>
+one, two, three = '一', '二', '三'
+</pre>
+
+<p>
+The <a href="#Blank_identifier">blank identifier</a> provides a way to
+ignore right-hand side values in an assignment:
+</p>
+
+<pre>
+_ = x // evaluate x but ignore it
+x, _ = f() // evaluate f() but ignore second result value
+</pre>
+
+<p>
+The assignment proceeds in two phases.
+First, the operands of <a href="#Index_expressions">index expressions</a>
+and <a href="#Address_operators">pointer indirections</a>
+(including implicit pointer indirections in <a href="#Selectors">selectors</a>)
+on the left and the expressions on the right are all
+<a href="#Order_of_evaluation">evaluated in the usual order</a>.
+Second, the assignments are carried out in left-to-right order.
+</p>
+
+<pre>
+a, b = b, a // exchange a and b
+
+x := []int{1, 2, 3}
+i := 0
+i, x[i] = 1, 2 // set i = 1, x[0] = 2
+
+i = 0
+x[i], i = 2, 1 // set x[0] = 2, i = 1
+
+x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
+
+x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
+
+type Point struct { x, y int }
+var p *Point
+x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
+
+i = 2
+x = []int{3, 5, 7}
+for i, x[i] = range x { // set i, x[2] = 0, x[0]
+ break
+}
+// after this loop, i == 0 and x == []int{3, 5, 3}
+</pre>
+
+<p>
+In assignments, each value must be <a href="#Assignability">assignable</a>
+to the type of the operand to which it is assigned, with the following special cases:
+</p>
+
+<ol>
+<li>
+ Any typed value may be assigned to the blank identifier.
+</li>
+
+<li>
+ If an untyped constant
+ is assigned to a variable of interface type or the blank identifier,
+ the constant is first implicitly <a href="#Conversions">converted</a> to its
+ <a href="#Constants">default type</a>.
+</li>
+
+<li>
+ If an untyped boolean value is assigned to a variable of interface type or
+ the blank identifier, it is first implicitly converted to type <code>bool</code>.
+</li>
+</ol>
+
+<h3 id="If_statements">If statements</h3>
+
+<p>
+"If" statements specify the conditional execution of two branches
+according to the value of a boolean expression. If the expression
+evaluates to true, the "if" branch is executed, otherwise, if
+present, the "else" branch is executed.
+</p>
+
+<pre class="ebnf">
+IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
+</pre>
+
+<pre>
+if x > max {
+ x = max
+}
+</pre>
+
+<p>
+The expression may be preceded by a simple statement, which
+executes before the expression is evaluated.
+</p>
+
+<pre>
+if x := f(); x < y {
+ return x
+} else if x > z {
+ return z
+} else {
+ return y
+}
+</pre>
+
+
+<h3 id="Switch_statements">Switch statements</h3>
+
+<p>
+"Switch" statements provide multi-way execution.
+An expression or type is compared to the "cases"
+inside the "switch" to determine which branch
+to execute.
+</p>
+
+<pre class="ebnf">
+SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
+</pre>
+
+<p>
+There are two forms: expression switches and type switches.
+In an expression switch, the cases contain expressions that are compared
+against the value of the switch expression.
+In a type switch, the cases contain types that are compared against the
+type of a specially annotated switch expression.
+The switch expression is evaluated exactly once in a switch statement.
+</p>
+
+<h4 id="Expression_switches">Expression switches</h4>
+
+<p>
+In an expression switch,
+the switch expression is evaluated and
+the case expressions, which need not be constants,
+are evaluated left-to-right and top-to-bottom; the first one that equals the
+switch expression
+triggers execution of the statements of the associated case;
+the other cases are skipped.
+If no case matches and there is a "default" case,
+its statements are executed.
+There can be at most one default case and it may appear anywhere in the
+"switch" statement.
+A missing switch expression is equivalent to the boolean value
+<code>true</code>.
+</p>
+
+<pre class="ebnf">
+ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
+ExprCaseClause = ExprSwitchCase ":" StatementList .
+ExprSwitchCase = "case" ExpressionList | "default" .
+</pre>
+
+<p>
+If the switch expression evaluates to an untyped constant, it is first implicitly
+<a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
+The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
+The switch expression type must be <a href="#Comparison_operators">comparable</a>.
+</p>
+
+<p>
+If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
+to the type of the switch expression.
+For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
+of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
+</p>
+
+<p>
+In other words, the switch expression is treated as if it were used to declare and
+initialize a temporary variable <code>t</code> without explicit type; it is that
+value of <code>t</code> against which each case expression <code>x</code> is tested
+for equality.
+</p>
+
+<p>
+In a case or default clause, the last non-empty statement
+may be a (possibly <a href="#Labeled_statements">labeled</a>)
+<a href="#Fallthrough_statements">"fallthrough" statement</a> to
+indicate that control should flow from the end of this clause to
+the first statement of the next clause.
+Otherwise control flows to the end of the "switch" statement.
+A "fallthrough" statement may appear as the last statement of all
+but the last clause of an expression switch.
+</p>
+
+<p>
+The switch expression may be preceded by a simple statement, which
+executes before the expression is evaluated.
+</p>
+
+<pre>
+switch tag {
+default: s3()
+case 0, 1, 2, 3: s1()
+case 4, 5, 6, 7: s2()
+}
+
+switch x := f(); { // missing switch expression means "true"
+case x < 0: return -x
+default: return x
+}
+
+switch {
+case x < y: f1()
+case x < z: f2()
+case x == 4: f3()
+}
+</pre>
+
+<p>
+Implementation restriction: A compiler may disallow multiple case
+expressions evaluating to the same constant.
+For instance, the current compilers disallow duplicate integer,
+floating point, or string constants in case expressions.
+</p>
+
+<h4 id="Type_switches">Type switches</h4>
+
+<p>
+A type switch compares types rather than values. It is otherwise similar
+to an expression switch. It is marked by a special switch expression that
+has the form of a <a href="#Type_assertions">type assertion</a>
+using the keyword <code>type</code> rather than an actual type:
+</p>
+
+<pre>
+switch x.(type) {
+// cases
+}
+</pre>
+
+<p>
+Cases then match actual types <code>T</code> against the dynamic type of the
+expression <code>x</code>. As with type assertions, <code>x</code> must be of
+<a href="#Interface_types">interface type</a>, and each non-interface type
+<code>T</code> listed in a case must implement the type of <code>x</code>.
+The types listed in the cases of a type switch must all be
+<a href="#Type_identity">different</a>.
+</p>
+
+<pre class="ebnf">
+TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
+TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
+TypeCaseClause = TypeSwitchCase ":" StatementList .
+TypeSwitchCase = "case" TypeList | "default" .
+TypeList = Type { "," Type } .
+</pre>
+
+<p>
+The TypeSwitchGuard may include a
+<a href="#Short_variable_declarations">short variable declaration</a>.
+When that form is used, the variable is declared at the end of the
+TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
+In clauses with a case listing exactly one type, the variable
+has that type; otherwise, the variable has the type of the expression
+in the TypeSwitchGuard.
+</p>
+
+<p>
+Instead of a type, a case may use the predeclared identifier
+<a href="#Predeclared_identifiers"><code>nil</code></a>;
+that case is selected when the expression in the TypeSwitchGuard
+is a <code>nil</code> interface value.
+There may be at most one <code>nil</code> case.
+</p>
+
+<p>
+Given an expression <code>x</code> of type <code>interface{}</code>,
+the following type switch:
+</p>
+
+<pre>
+switch i := x.(type) {
+case nil:
+ printString("x is nil") // type of i is type of x (interface{})
+case int:
+ printInt(i) // type of i is int
+case float64:
+ printFloat64(i) // type of i is float64
+case func(int) float64:
+ printFunction(i) // type of i is func(int) float64
+case bool, string:
+ printString("type is bool or string") // type of i is type of x (interface{})
+default:
+ printString("don't know the type") // type of i is type of x (interface{})
+}
+</pre>
+
+<p>
+could be rewritten:
+</p>
+
+<pre>
+v := x // x is evaluated exactly once
+if v == nil {
+ i := v // type of i is type of x (interface{})
+ printString("x is nil")
+} else if i, isInt := v.(int); isInt {
+ printInt(i) // type of i is int
+} else if i, isFloat64 := v.(float64); isFloat64 {
+ printFloat64(i) // type of i is float64
+} else if i, isFunc := v.(func(int) float64); isFunc {
+ printFunction(i) // type of i is func(int) float64
+} else {
+ _, isBool := v.(bool)
+ _, isString := v.(string)
+ if isBool || isString {
+ i := v // type of i is type of x (interface{})
+ printString("type is bool or string")
+ } else {
+ i := v // type of i is type of x (interface{})
+ printString("don't know the type")
+ }
+}
+</pre>
+
+<p>
+The type switch guard may be preceded by a simple statement, which
+executes before the guard is evaluated.
+</p>
+
+<p>
+The "fallthrough" statement is not permitted in a type switch.
+</p>
+
+<h3 id="For_statements">For statements</h3>
+
+<p>
+A "for" statement specifies repeated execution of a block. There are three forms:
+The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
+</p>
+
+<pre class="ebnf">
+ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
+Condition = Expression .
+</pre>
+
+<h4 id="For_condition">For statements with single condition</h4>
+
+<p>
+In its simplest form, a "for" statement specifies the repeated execution of
+a block as long as a boolean condition evaluates to true.
+The condition is evaluated before each iteration.
+If the condition is absent, it is equivalent to the boolean value
+<code>true</code>.
+</p>
+
+<pre>
+for a < b {
+ a *= 2
+}
+</pre>
+
+<h4 id="For_clause">For statements with <code>for</code> clause</h4>
+
+<p>
+A "for" statement with a ForClause is also controlled by its condition, but
+additionally it may specify an <i>init</i>
+and a <i>post</i> statement, such as an assignment,
+an increment or decrement statement. The init statement may be a
+<a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
+Variables declared by the init statement are re-used in each iteration.
+</p>
+
+<pre class="ebnf">
+ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
+InitStmt = SimpleStmt .
+PostStmt = SimpleStmt .
+</pre>
+
+<pre>
+for i := 0; i < 10; i++ {
+ f(i)
+}
+</pre>
+
+<p>
+If non-empty, the init statement is executed once before evaluating the
+condition for the first iteration;
+the post statement is executed after each execution of the block (and
+only if the block was executed).
+Any element of the ForClause may be empty but the
+<a href="#Semicolons">semicolons</a> are
+required unless there is only a condition.
+If the condition is absent, it is equivalent to the boolean value
+<code>true</code>.
+</p>
+
+<pre>
+for cond { S() } is the same as for ; cond ; { S() }
+for { S() } is the same as for true { S() }
+</pre>
+
+<h4 id="For_range">For statements with <code>range</code> clause</h4>
+
+<p>
+A "for" statement with a "range" clause
+iterates through all entries of an array, slice, string or map,
+or values received on a channel. For each entry it assigns <i>iteration values</i>
+to corresponding <i>iteration variables</i> if present and then executes the block.
+</p>
+
+<pre class="ebnf">
+RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
+</pre>
+
+<p>
+The expression on the right in the "range" clause is called the <i>range expression</i>,
+which may be an array, pointer to an array, slice, string, map, or channel permitting
+<a href="#Receive_operator">receive operations</a>.
+As with an assignment, if present the operands on the left must be
+<a href="#Address_operators">addressable</a> or map index expressions; they
+denote the iteration variables. If the range expression is a channel, at most
+one iteration variable is permitted, otherwise there may be up to two.
+If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
+the range clause is equivalent to the same clause without that identifier.
+</p>
+
+<p>
+The range expression <code>x</code> is evaluated once before beginning the loop,
+with one exception: if at most one iteration variable is present and
+<code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
+the range expression is not evaluated.
+</p>
+
+<p>
+Function calls on the left are evaluated once per iteration.
+For each iteration, iteration values are produced as follows
+if the respective iteration variables are present:
+</p>
+
+<pre class="grammar">
+Range expression 1st value 2nd value
+
+array or slice a [n]E, *[n]E, or []E index i int a[i] E
+string s string type index i int see below rune
+map m map[K]V key k K m[k] V
+channel c chan E, <-chan E element e E
+</pre>
+
+<ol>
+<li>
+For an array, pointer to array, or slice value <code>a</code>, the index iteration
+values are produced in increasing order, starting at element index 0.
+If at most one iteration variable is present, the range loop produces
+iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
+or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
+</li>
+
+<li>
+For a string value, the "range" clause iterates over the Unicode code points
+in the string starting at byte index 0. On successive iterations, the index value will be the
+index of the first byte of successive UTF-8-encoded code points in the string,
+and the second value, of type <code>rune</code>, will be the value of
+the corresponding code point. If the iteration encounters an invalid
+UTF-8 sequence, the second value will be <code>0xFFFD</code>,
+the Unicode replacement character, and the next iteration will advance
+a single byte in the string.
+</li>
+
+<li>
+The iteration order over maps is not specified
+and is not guaranteed to be the same from one iteration to the next.
+If a map entry that has not yet been reached is removed during iteration,
+the corresponding iteration value will not be produced. If a map entry is
+created during iteration, that entry may be produced during the iteration or
+may be skipped. The choice may vary for each entry created and from one
+iteration to the next.
+If the map is <code>nil</code>, the number of iterations is 0.
+</li>
+
+<li>
+For channels, the iteration values produced are the successive values sent on
+the channel until the channel is <a href="#Close">closed</a>. If the channel
+is <code>nil</code>, the range expression blocks forever.
+</li>
+</ol>
+
+<p>
+The iteration values are assigned to the respective
+iteration variables as in an <a href="#Assignments">assignment statement</a>.
+</p>
+
+<p>
+The iteration variables may be declared by the "range" clause using a form of
+<a href="#Short_variable_declarations">short variable declaration</a>
+(<code>:=</code>).
+In this case their types are set to the types of the respective iteration values
+and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
+statement; they are re-used in each iteration.
+If the iteration variables are declared outside the "for" statement,
+after execution their values will be those of the last iteration.
+</p>
+
+<pre>
+var testdata *struct {
+ a *[7]int
+}
+for i, _ := range testdata.a {
+ // testdata.a is never evaluated; len(testdata.a) is constant
+ // i ranges from 0 to 6
+ f(i)
+}
+
+var a [10]string
+for i, s := range a {
+ // type of i is int
+ // type of s is string
+ // s == a[i]
+ g(i, s)
+}
+
+var key string
+var val interface{} // element type of m is assignable to val
+m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
+for key, val = range m {
+ h(key, val)
+}
+// key == last map key encountered in iteration
+// val == map[key]
+
+var ch chan Work = producer()
+for w := range ch {
+ doWork(w)
+}
+
+// empty a channel
+for range ch {}
+</pre>
+
+
+<h3 id="Go_statements">Go statements</h3>
+
+<p>
+A "go" statement starts the execution of a function call
+as an independent concurrent thread of control, or <i>goroutine</i>,
+within the same address space.
+</p>
+
+<pre class="ebnf">
+GoStmt = "go" Expression .
+</pre>
+
+<p>
+The expression must be a function or method call; it cannot be parenthesized.
+Calls of built-in functions are restricted as for
+<a href="#Expression_statements">expression statements</a>.
+</p>
+
+<p>
+The function value and parameters are
+<a href="#Calls">evaluated as usual</a>
+in the calling goroutine, but
+unlike with a regular call, program execution does not wait
+for the invoked function to complete.
+Instead, the function begins executing independently
+in a new goroutine.
+When the function terminates, its goroutine also terminates.
+If the function has any return values, they are discarded when the
+function completes.
+</p>
+
+<pre>
+go Server()
+go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
+</pre>
+
+
+<h3 id="Select_statements">Select statements</h3>
+
+<p>
+A "select" statement chooses which of a set of possible
+<a href="#Send_statements">send</a> or
+<a href="#Receive_operator">receive</a>
+operations will proceed.
+It looks similar to a
+<a href="#Switch_statements">"switch"</a> statement but with the
+cases all referring to communication operations.
+</p>
+
+<pre class="ebnf">
+SelectStmt = "select" "{" { CommClause } "}" .
+CommClause = CommCase ":" StatementList .
+CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
+RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
+RecvExpr = Expression .
+</pre>
+
+<p>
+A case with a RecvStmt may assign the result of a RecvExpr to one or
+two variables, which may be declared using a
+<a href="#Short_variable_declarations">short variable declaration</a>.
+The RecvExpr must be a (possibly parenthesized) receive operation.
+There can be at most one default case and it may appear anywhere
+in the list of cases.
+</p>
+
+<p>
+Execution of a "select" statement proceeds in several steps:
+</p>
+
+<ol>
+<li>
+For all the cases in the statement, the channel operands of receive operations
+and the channel and right-hand-side expressions of send statements are
+evaluated exactly once, in source order, upon entering the "select" statement.
+The result is a set of channels to receive from or send to,
+and the corresponding values to send.
+Any side effects in that evaluation will occur irrespective of which (if any)
+communication operation is selected to proceed.
+Expressions on the left-hand side of a RecvStmt with a short variable declaration
+or assignment are not yet evaluated.
+</li>
+
+<li>
+If one or more of the communications can proceed,
+a single one that can proceed is chosen via a uniform pseudo-random selection.
+Otherwise, if there is a default case, that case is chosen.
+If there is no default case, the "select" statement blocks until
+at least one of the communications can proceed.
+</li>
+
+<li>
+Unless the selected case is the default case, the respective communication
+operation is executed.
+</li>
+
+<li>
+If the selected case is a RecvStmt with a short variable declaration or
+an assignment, the left-hand side expressions are evaluated and the
+received value (or values) are assigned.
+</li>
+
+<li>
+The statement list of the selected case is executed.
+</li>
+</ol>
+
+<p>
+Since communication on <code>nil</code> channels can never proceed,
+a select with only <code>nil</code> channels and no default case blocks forever.
+</p>
+
+<pre>
+var a []int
+var c, c1, c2, c3, c4 chan int
+var i1, i2 int
+select {
+case i1 = <-c1:
+ print("received ", i1, " from c1\n")
+case c2 <- i2:
+ print("sent ", i2, " to c2\n")
+case i3, ok := (<-c3): // same as: i3, ok := <-c3
+ if ok {
+ print("received ", i3, " from c3\n")
+ } else {
+ print("c3 is closed\n")
+ }
+case a[f()] = <-c4:
+ // same as:
+ // case t := <-c4
+ // a[f()] = t
+default:
+ print("no communication\n")
+}
+
+for { // send random sequence of bits to c
+ select {
+ case c <- 0: // note: no statement, no fallthrough, no folding of cases
+ case c <- 1:
+ }
+}
+
+select {} // block forever
+</pre>
+
+
+<h3 id="Return_statements">Return statements</h3>
+
+<p>
+A "return" statement in a function <code>F</code> terminates the execution
+of <code>F</code>, and optionally provides one or more result values.
+Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
+are executed before <code>F</code> returns to its caller.
+</p>
+
+<pre class="ebnf">
+ReturnStmt = "return" [ ExpressionList ] .
+</pre>
+
+<p>
+In a function without a result type, a "return" statement must not
+specify any result values.
+</p>
+<pre>
+func noResult() {
+ return
+}
+</pre>
+
+<p>
+There are three ways to return values from a function with a result
+type:
+</p>
+
+<ol>
+ <li>The return value or values may be explicitly listed
+ in the "return" statement. Each expression must be single-valued
+ and <a href="#Assignability">assignable</a>
+ to the corresponding element of the function's result type.
+<pre>
+func simpleF() int {
+ return 2
+}
+
+func complexF1() (re float64, im float64) {
+ return -7.0, -4.0
+}
+</pre>
+ </li>
+ <li>The expression list in the "return" statement may be a single
+ call to a multi-valued function. The effect is as if each value
+ returned from that function were assigned to a temporary
+ variable with the type of the respective value, followed by a
+ "return" statement listing these variables, at which point the
+ rules of the previous case apply.
+<pre>
+func complexF2() (re float64, im float64) {
+ return complexF1()
+}
+</pre>
+ </li>
+ <li>The expression list may be empty if the function's result
+ type specifies names for its <a href="#Function_types">result parameters</a>.
+ The result parameters act as ordinary local variables
+ and the function may assign values to them as necessary.
+ The "return" statement returns the values of these variables.
+<pre>
+func complexF3() (re float64, im float64) {
+ re = 7.0
+ im = 4.0
+ return
+}
+
+func (devnull) Write(p []byte) (n int, _ error) {
+ n = len(p)
+ return
+}
+</pre>
+ </li>
+</ol>
+
+<p>
+Regardless of how they are declared, all the result values are initialized to
+the <a href="#The_zero_value">zero values</a> for their type upon entry to the
+function. A "return" statement that specifies results sets the result parameters before
+any deferred functions are executed.
+</p>
+
+<p>
+Implementation restriction: A compiler may disallow an empty expression list
+in a "return" statement if a different entity (constant, type, or variable)
+with the same name as a result parameter is in
+<a href="#Declarations_and_scope">scope</a> at the place of the return.
+</p>
+
+<pre>
+func f(n int) (res int, err error) {
+ if _, err := f(n-1); err != nil {
+ return // invalid return statement: err is shadowed
+ }
+ return
+}
+</pre>
+
+<h3 id="Break_statements">Break statements</h3>
+
+<p>
+A "break" statement terminates execution of the innermost
+<a href="#For_statements">"for"</a>,
+<a href="#Switch_statements">"switch"</a>, or
+<a href="#Select_statements">"select"</a> statement
+within the same function.
+</p>
+
+<pre class="ebnf">
+BreakStmt = "break" [ Label ] .
+</pre>
+
+<p>
+If there is a label, it must be that of an enclosing
+"for", "switch", or "select" statement,
+and that is the one whose execution terminates.
+</p>
+
+<pre>
+OuterLoop:
+ for i = 0; i < n; i++ {
+ for j = 0; j < m; j++ {
+ switch a[i][j] {
+ case nil:
+ state = Error
+ break OuterLoop
+ case item:
+ state = Found
+ break OuterLoop
+ }
+ }
+ }
+</pre>
+
+<h3 id="Continue_statements">Continue statements</h3>
+
+<p>
+A "continue" statement begins the next iteration of the
+innermost <a href="#For_statements">"for" loop</a> at its post statement.
+The "for" loop must be within the same function.
+</p>
+
+<pre class="ebnf">
+ContinueStmt = "continue" [ Label ] .
+</pre>
+
+<p>
+If there is a label, it must be that of an enclosing
+"for" statement, and that is the one whose execution
+advances.
+</p>
+
+<pre>
+RowLoop:
+ for y, row := range rows {
+ for x, data := range row {
+ if data == endOfRow {
+ continue RowLoop
+ }
+ row[x] = data + bias(x, y)
+ }
+ }
+</pre>
+
+<h3 id="Goto_statements">Goto statements</h3>
+
+<p>
+A "goto" statement transfers control to the statement with the corresponding label
+within the same function.
+</p>
+
+<pre class="ebnf">
+GotoStmt = "goto" Label .
+</pre>
+
+<pre>
+goto Error
+</pre>
+
+<p>
+Executing the "goto" statement must not cause any variables to come into
+<a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
+For instance, this example:
+</p>
+
+<pre>
+ goto L // BAD
+ v := 3
+L:
+</pre>
+
+<p>
+is erroneous because the jump to label <code>L</code> skips
+the creation of <code>v</code>.
+</p>
+
+<p>
+A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
+For instance, this example:
+</p>
+
+<pre>
+if n%2 == 1 {
+ goto L1
+}
+for n > 0 {
+ f()
+ n--
+L1:
+ f()
+ n--
+}
+</pre>
+
+<p>
+is erroneous because the label <code>L1</code> is inside
+the "for" statement's block but the <code>goto</code> is not.
+</p>
+
+<h3 id="Fallthrough_statements">Fallthrough statements</h3>
+
+<p>
+A "fallthrough" statement transfers control to the first statement of the
+next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
+It may be used only as the final non-empty statement in such a clause.
+</p>
+
+<pre class="ebnf">
+FallthroughStmt = "fallthrough" .
+</pre>
+
+
+<h3 id="Defer_statements">Defer statements</h3>
+
+<p>
+A "defer" statement invokes a function whose execution is deferred
+to the moment the surrounding function returns, either because the
+surrounding function executed a <a href="#Return_statements">return statement</a>,
+reached the end of its <a href="#Function_declarations">function body</a>,
+or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
+</p>
+
+<pre class="ebnf">
+DeferStmt = "defer" Expression .
+</pre>
+
+<p>
+The expression must be a function or method call; it cannot be parenthesized.
+Calls of built-in functions are restricted as for
+<a href="#Expression_statements">expression statements</a>.
+</p>
+
+<p>
+Each time a "defer" statement
+executes, the function value and parameters to the call are
+<a href="#Calls">evaluated as usual</a>
+and saved anew but the actual function is not invoked.
+Instead, deferred functions are invoked immediately before
+the surrounding function returns, in the reverse order
+they were deferred. That is, if the surrounding function
+returns through an explicit <a href="#Return_statements">return statement</a>,
+deferred functions are executed <i>after</i> any result parameters are set
+by that return statement but <i>before</i> the function returns to its caller.
+If a deferred function value evaluates
+to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
+when the function is invoked, not when the "defer" statement is executed.
+</p>
+
+<p>
+For instance, if the deferred function is
+a <a href="#Function_literals">function literal</a> and the surrounding
+function has <a href="#Function_types">named result parameters</a> that
+are in scope within the literal, the deferred function may access and modify
+the result parameters before they are returned.
+If the deferred function has any return values, they are discarded when
+the function completes.
+(See also the section on <a href="#Handling_panics">handling panics</a>.)
+</p>
+
+<pre>
+lock(l)
+defer unlock(l) // unlocking happens before surrounding function returns
+
+// prints 3 2 1 0 before surrounding function returns
+for i := 0; i <= 3; i++ {
+ defer fmt.Print(i)
+}
+
+// f returns 42
+func f() (result int) {
+ defer func() {
+ // result is accessed after it was set to 6 by the return statement
+ result *= 7
+ }()
+ return 6
+}
+</pre>
+
+<h2 id="Built-in_functions">Built-in functions</h2>
+
+<p>
+Built-in functions are
+<a href="#Predeclared_identifiers">predeclared</a>.
+They are called like any other function but some of them
+accept a type instead of an expression as the first argument.
+</p>
+
+<p>
+The built-in functions do not have standard Go types,
+so they can only appear in <a href="#Calls">call expressions</a>;
+they cannot be used as function values.
+</p>
+
+<h3 id="Close">Close</h3>
+
+<p>
+For a channel <code>c</code>, the built-in function <code>close(c)</code>
+records that no more values will be sent on the channel.
+It is an error if <code>c</code> is a receive-only channel.
+Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
+Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
+After calling <code>close</code>, and after any previously
+sent values have been received, receive operations will return
+the zero value for the channel's type without blocking.
+The multi-valued <a href="#Receive_operator">receive operation</a>
+returns a received value along with an indication of whether the channel is closed.
+</p>
+
+
+<h3 id="Length_and_capacity">Length and capacity</h3>
+
+<p>
+The built-in functions <code>len</code> and <code>cap</code> take arguments
+of various types and return a result of type <code>int</code>.
+The implementation guarantees that the result always fits into an <code>int</code>.
+</p>
+
+<pre class="grammar">
+Call Argument type Result
+
+len(s) string type string length in bytes
+ [n]T, *[n]T array length (== n)
+ []T slice length
+ map[K]T map length (number of defined keys)
+ chan T number of elements queued in channel buffer
+
+cap(s) [n]T, *[n]T array length (== n)
+ []T slice capacity
+ chan T channel buffer capacity
+</pre>
+
+<p>
+The capacity of a slice is the number of elements for which there is
+space allocated in the underlying array.
+At any time the following relationship holds:
+</p>
+
+<pre>
+0 <= len(s) <= cap(s)
+</pre>
+
+<p>
+The length of a <code>nil</code> slice, map or channel is 0.
+The capacity of a <code>nil</code> slice or channel is 0.
+</p>
+
+<p>
+The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
+<code>s</code> is a string constant. The expressions <code>len(s)</code> and
+<code>cap(s)</code> are constants if the type of <code>s</code> is an array
+or pointer to an array and the expression <code>s</code> does not contain
+<a href="#Receive_operator">channel receives</a> or (non-constant)
+<a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
+Otherwise, invocations of <code>len</code> and <code>cap</code> are not
+constant and <code>s</code> is evaluated.
+</p>
+
+<pre>
+const (
+ c1 = imag(2i) // imag(2i) = 2.0 is a constant
+ c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
+ c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
+ c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
+ c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
+)
+var z complex128
+</pre>
+
+<h3 id="Allocation">Allocation</h3>
+
+<p>
+The built-in function <code>new</code> takes a type <code>T</code>,
+allocates storage for a <a href="#Variables">variable</a> of that type
+at run time, and returns a value of type <code>*T</code>
+<a href="#Pointer_types">pointing</a> to it.
+The variable is initialized as described in the section on
+<a href="#The_zero_value">initial values</a>.
+</p>
+
+<pre class="grammar">
+new(T)
+</pre>
+
+<p>
+For instance
+</p>
+
+<pre>
+type S struct { a int; b float64 }
+new(S)
+</pre>
+
+<p>
+allocates storage for a variable of type <code>S</code>,
+initializes it (<code>a=0</code>, <code>b=0.0</code>),
+and returns a value of type <code>*S</code> containing the address
+of the location.
+</p>
+
+<h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
+
+<p>
+The built-in function <code>make</code> takes a type <code>T</code>,
+which must be a slice, map or channel type,
+optionally followed by a type-specific list of expressions.
+It returns a value of type <code>T</code> (not <code>*T</code>).
+The memory is initialized as described in the section on
+<a href="#The_zero_value">initial values</a>.
+</p>
+
+<pre class="grammar">
+Call Type T Result
+
+make(T, n) slice slice of type T with length n and capacity n
+make(T, n, m) slice slice of type T with length n and capacity m
+
+make(T) map map of type T
+make(T, n) map map of type T with initial space for approximately n elements
+
+make(T) channel unbuffered channel of type T
+make(T, n) channel buffered channel of type T, buffer size n
+</pre>
+
+
+<p>
+Each of the size arguments <code>n</code> and <code>m</code> must be of integer type
+or an untyped <a href="#Constants">constant</a>.
+A constant size argument must be non-negative and <a href="#Representability">representable</a>
+by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
+If both <code>n</code> and <code>m</code> are provided and are constant, then
+<code>n</code> must be no larger than <code>m</code>.
+If <code>n</code> is negative or larger than <code>m</code> at run time,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<pre>
+s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
+s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
+s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
+s := make([]int, 10, 0) // illegal: len(s) > cap(s)
+c := make(chan int, 10) // channel with a buffer size of 10
+m := make(map[string]int, 100) // map with initial space for approximately 100 elements
+</pre>
+
+<p>
+Calling <code>make</code> with a map type and size hint <code>n</code> will
+create a map with initial space to hold <code>n</code> map elements.
+The precise behavior is implementation-dependent.
+</p>
+
+
+<h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
+
+<p>
+The built-in functions <code>append</code> and <code>copy</code> assist in
+common slice operations.
+For both functions, the result is independent of whether the memory referenced
+by the arguments overlaps.
+</p>
+
+<p>
+The <a href="#Function_types">variadic</a> function <code>append</code>
+appends zero or more values <code>x</code>
+to <code>s</code> of type <code>S</code>, which must be a slice type, and
+returns the resulting slice, also of type <code>S</code>.
+The values <code>x</code> are passed to a parameter of type <code>...T</code>
+where <code>T</code> is the <a href="#Slice_types">element type</a> of
+<code>S</code> and the respective
+<a href="#Passing_arguments_to_..._parameters">parameter passing rules</a> apply.
+As a special case, <code>append</code> also accepts a first argument
+assignable to type <code>[]byte</code> with a second argument of
+string type followed by <code>...</code>. This form appends the
+bytes of the string.
+</p>
+
+<pre class="grammar">
+append(s S, x ...T) S // T is the element type of S
+</pre>
+
+<p>
+If the capacity of <code>s</code> is not large enough to fit the additional
+values, <code>append</code> allocates a new, sufficiently large underlying
+array that fits both the existing slice elements and the additional values.
+Otherwise, <code>append</code> re-uses the underlying array.
+</p>
+
+<pre>
+s0 := []int{0, 0}
+s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
+s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
+s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
+s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
+
+var t []interface{}
+t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"}
+
+var b []byte
+b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
+</pre>
+
+<p>
+The function <code>copy</code> copies slice elements from
+a source <code>src</code> to a destination <code>dst</code> and returns the
+number of elements copied.
+Both arguments must have <a href="#Type_identity">identical</a> element type <code>T</code> and must be
+<a href="#Assignability">assignable</a> to a slice of type <code>[]T</code>.
+The number of elements copied is the minimum of
+<code>len(src)</code> and <code>len(dst)</code>.
+As a special case, <code>copy</code> also accepts a destination argument assignable
+to type <code>[]byte</code> with a source argument of a string type.
+This form copies the bytes from the string into the byte slice.
+</p>
+
+<pre class="grammar">
+copy(dst, src []T) int
+copy(dst []byte, src string) int
+</pre>
+
+<p>
+Examples:
+</p>
+
+<pre>
+var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
+var s = make([]int, 6)
+var b = make([]byte, 5)
+n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
+n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
+n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
+</pre>
+
+
+<h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
+
+<p>
+The built-in function <code>delete</code> removes the element with key
+<code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
+type of <code>k</code> must be <a href="#Assignability">assignable</a>
+to the key type of <code>m</code>.
+</p>
+
+<pre class="grammar">
+delete(m, k) // remove element m[k] from map m
+</pre>
+
+<p>
+If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
+does not exist, <code>delete</code> is a no-op.
+</p>
+
+
+<h3 id="Complex_numbers">Manipulating complex numbers</h3>
+
+<p>
+Three functions assemble and disassemble complex numbers.
+The built-in function <code>complex</code> constructs a complex
+value from a floating-point real and imaginary part, while
+<code>real</code> and <code>imag</code>
+extract the real and imaginary parts of a complex value.
+</p>
+
+<pre class="grammar">
+complex(realPart, imaginaryPart floatT) complexT
+real(complexT) floatT
+imag(complexT) floatT
+</pre>
+
+<p>
+The type of the arguments and return value correspond.
+For <code>complex</code>, the two arguments must be of the same
+floating-point type and the return type is the complex type
+with the corresponding floating-point constituents:
+<code>complex64</code> for <code>float32</code> arguments, and
+<code>complex128</code> for <code>float64</code> arguments.
+If one of the arguments evaluates to an untyped constant, it is first implicitly
+<a href="#Conversions">converted</a> to the type of the other argument.
+If both arguments evaluate to untyped constants, they must be non-complex
+numbers or their imaginary parts must be zero, and the return value of
+the function is an untyped complex constant.
+</p>
+
+<p>
+For <code>real</code> and <code>imag</code>, the argument must be
+of complex type, and the return type is the corresponding floating-point
+type: <code>float32</code> for a <code>complex64</code> argument, and
+<code>float64</code> for a <code>complex128</code> argument.
+If the argument evaluates to an untyped constant, it must be a number,
+and the return value of the function is an untyped floating-point constant.
+</p>
+
+<p>
+The <code>real</code> and <code>imag</code> functions together form the inverse of
+<code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
+<code>z == Z(complex(real(z), imag(z)))</code>.
+</p>
+
+<p>
+If the operands of these functions are all constants, the return
+value is a constant.
+</p>
+
+<pre>
+var a = complex(2, -2) // complex128
+const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
+x := float32(math.Cos(math.Pi/2)) // float32
+var c64 = complex(5, -x) // complex64
+var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
+_ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
+var rl = real(c64) // float32
+var im = imag(a) // float64
+const c = imag(b) // untyped constant -1.4
+_ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
+</pre>
+
+<h3 id="Handling_panics">Handling panics</h3>
+
+<p> Two built-in functions, <code>panic</code> and <code>recover</code>,
+assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
+and program-defined error conditions.
+</p>
+
+<pre class="grammar">
+func panic(interface{})
+func recover() interface{}
+</pre>
+
+<p>
+While executing a function <code>F</code>,
+an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
+terminates the execution of <code>F</code>.
+Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
+are then executed as usual.
+Next, any deferred functions run by <code>F's</code> caller are run,
+and so on up to any deferred by the top-level function in the executing goroutine.
+At that point, the program is terminated and the error
+condition is reported, including the value of the argument to <code>panic</code>.
+This termination sequence is called <i>panicking</i>.
+</p>
+
+<pre>
+panic(42)
+panic("unreachable")
+panic(Error("cannot parse"))
+</pre>
+
+<p>
+The <code>recover</code> function allows a program to manage behavior
+of a panicking goroutine.
+Suppose a function <code>G</code> defers a function <code>D</code> that calls
+<code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
+is executing.
+When the running of deferred functions reaches <code>D</code>,
+the return value of <code>D</code>'s call to <code>recover</code> will be the value passed to the call of <code>panic</code>.
+If <code>D</code> returns normally, without starting a new
+<code>panic</code>, the panicking sequence stops. In that case,
+the state of functions called between <code>G</code> and the call to <code>panic</code>
+is discarded, and normal execution resumes.
+Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
+execution terminates by returning to its caller.
+</p>
+
+<p>
+The return value of <code>recover</code> is <code>nil</code> if any of the following conditions holds:
+</p>
+<ul>
+<li>
+<code>panic</code>'s argument was <code>nil</code>;
+</li>
+<li>
+the goroutine is not panicking;
+</li>
+<li>
+<code>recover</code> was not called directly by a deferred function.
+</li>
+</ul>
+
+<p>
+The <code>protect</code> function in the example below invokes
+the function argument <code>g</code> and protects callers from
+run-time panics raised by <code>g</code>.
+</p>
+
+<pre>
+func protect(g func()) {
+ defer func() {
+ log.Println("done") // Println executes normally even if there is a panic
+ if x := recover(); x != nil {
+ log.Printf("run time panic: %v", x)
+ }
+ }()
+ log.Println("start")
+ g()
+}
+</pre>
+
+
+<h3 id="Bootstrapping">Bootstrapping</h3>
+
+<p>
+Current implementations provide several built-in functions useful during
+bootstrapping. These functions are documented for completeness but are not
+guaranteed to stay in the language. They do not return a result.
+</p>
+
+<pre class="grammar">
+Function Behavior
+
+print prints all arguments; formatting of arguments is implementation-specific
+println like print but prints spaces between arguments and a newline at the end
+</pre>
+
+<p>
+Implementation restriction: <code>print</code> and <code>println</code> need not
+accept arbitrary argument types, but printing of boolean, numeric, and string
+<a href="#Types">types</a> must be supported.
+</p>
+
+<h2 id="Packages">Packages</h2>
+
+<p>
+Go programs are constructed by linking together <i>packages</i>.
+A package in turn is constructed from one or more source files
+that together declare constants, types, variables and functions
+belonging to the package and which are accessible in all files
+of the same package. Those elements may be
+<a href="#Exported_identifiers">exported</a> and used in another package.
+</p>
+
+<h3 id="Source_file_organization">Source file organization</h3>
+
+<p>
+Each source file consists of a package clause defining the package
+to which it belongs, followed by a possibly empty set of import
+declarations that declare packages whose contents it wishes to use,
+followed by a possibly empty set of declarations of functions,
+types, variables, and constants.
+</p>
+
+<pre class="ebnf">
+SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
+</pre>
+
+<h3 id="Package_clause">Package clause</h3>
+
+<p>
+A package clause begins each source file and defines the package
+to which the file belongs.
+</p>
+
+<pre class="ebnf">
+PackageClause = "package" PackageName .
+PackageName = identifier .
+</pre>
+
+<p>
+The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
+</p>
+
+<pre>
+package math
+</pre>
+
+<p>
+A set of files sharing the same PackageName form the implementation of a package.
+An implementation may require that all source files for a package inhabit the same directory.
+</p>
+
+<h3 id="Import_declarations">Import declarations</h3>
+
+<p>
+An import declaration states that the source file containing the declaration
+depends on functionality of the <i>imported</i> package
+(<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
+and enables access to <a href="#Exported_identifiers">exported</a> identifiers
+of that package.
+The import names an identifier (PackageName) to be used for access and an ImportPath
+that specifies the package to be imported.
+</p>
+
+<pre class="ebnf">
+ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
+ImportSpec = [ "." | PackageName ] ImportPath .
+ImportPath = string_lit .
+</pre>
+
+<p>
+The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
+to access exported identifiers of the package within the importing source file.
+It is declared in the <a href="#Blocks">file block</a>.
+If the PackageName is omitted, it defaults to the identifier specified in the
+<a href="#Package_clause">package clause</a> of the imported package.
+If an explicit period (<code>.</code>) appears instead of a name, all the
+package's exported identifiers declared in that package's
+<a href="#Blocks">package block</a> will be declared in the importing source
+file's file block and must be accessed without a qualifier.
+</p>
+
+<p>
+The interpretation of the ImportPath is implementation-dependent but
+it is typically a substring of the full file name of the compiled
+package and may be relative to a repository of installed packages.
+</p>
+
+<p>
+Implementation restriction: A compiler may restrict ImportPaths to
+non-empty strings using only characters belonging to
+<a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
+L, M, N, P, and S general categories (the Graphic characters without
+spaces) and may also exclude the characters
+<code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
+and the Unicode replacement character U+FFFD.
+</p>
+
+<p>
+Assume we have compiled a package containing the package clause
+<code>package math</code>, which exports function <code>Sin</code>, and
+installed the compiled package in the file identified by
+<code>"lib/math"</code>.
+This table illustrates how <code>Sin</code> is accessed in files
+that import the package after the
+various types of import declaration.
+</p>
+
+<pre class="grammar">
+Import declaration Local name of Sin
+
+import "lib/math" math.Sin
+import m "lib/math" m.Sin
+import . "lib/math" Sin
+</pre>
+
+<p>
+An import declaration declares a dependency relation between
+the importing and imported package.
+It is illegal for a package to import itself, directly or indirectly,
+or to directly import a package without
+referring to any of its exported identifiers. To import a package solely for
+its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
+identifier as explicit package name:
+</p>
+
+<pre>
+import _ "lib/math"
+</pre>
+
+
+<h3 id="An_example_package">An example package</h3>
+
+<p>
+Here is a complete Go package that implements a concurrent prime sieve.
+</p>
+
+<pre>
+package main
+
+import "fmt"
+
+// Send the sequence 2, 3, 4, … to channel 'ch'.
+func generate(ch chan<- int) {
+ for i := 2; ; i++ {
+ ch <- i // Send 'i' to channel 'ch'.
+ }
+}
+
+// Copy the values from channel 'src' to channel 'dst',
+// removing those divisible by 'prime'.
+func filter(src <-chan int, dst chan<- int, prime int) {
+ for i := range src { // Loop over values received from 'src'.
+ if i%prime != 0 {
+ dst <- i // Send 'i' to channel 'dst'.
+ }
+ }
+}
+
+// The prime sieve: Daisy-chain filter processes together.
+func sieve() {
+ ch := make(chan int) // Create a new channel.
+ go generate(ch) // Start generate() as a subprocess.
+ for {
+ prime := <-ch
+ fmt.Print(prime, "\n")
+ ch1 := make(chan int)
+ go filter(ch, ch1, prime)
+ ch = ch1
+ }
+}
+
+func main() {
+ sieve()
+}
+</pre>
+
+<h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
+
+<h3 id="The_zero_value">The zero value</h3>
+<p>
+When storage is allocated for a <a href="#Variables">variable</a>,
+either through a declaration or a call of <code>new</code>, or when
+a new value is created, either through a composite literal or a call
+of <code>make</code>,
+and no explicit initialization is provided, the variable or value is
+given a default value. Each element of such a variable or value is
+set to the <i>zero value</i> for its type: <code>false</code> for booleans,
+<code>0</code> for numeric types, <code>""</code>
+for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
+This initialization is done recursively, so for instance each element of an
+array of structs will have its fields zeroed if no value is specified.
+</p>
+<p>
+These two simple declarations are equivalent:
+</p>
+
+<pre>
+var i int
+var i int = 0
+</pre>
+
+<p>
+After
+</p>
+
+<pre>
+type T struct { i int; f float64; next *T }
+t := new(T)
+</pre>
+
+<p>
+the following holds:
+</p>
+
+<pre>
+t.i == 0
+t.f == 0.0
+t.next == nil
+</pre>
+
+<p>
+The same would also be true after
+</p>
+
+<pre>
+var t T
+</pre>
+
+<h3 id="Package_initialization">Package initialization</h3>
+
+<p>
+Within a package, package-level variable initialization proceeds stepwise,
+with each step selecting the variable earliest in <i>declaration order</i>
+which has no dependencies on uninitialized variables.
+</p>
+
+<p>
+More precisely, a package-level variable is considered <i>ready for
+initialization</i> if it is not yet initialized and either has
+no <a href="#Variable_declarations">initialization expression</a> or
+its initialization expression has no <i>dependencies</i> on uninitialized variables.
+Initialization proceeds by repeatedly initializing the next package-level
+variable that is earliest in declaration order and ready for initialization,
+until there are no variables ready for initialization.
+</p>
+
+<p>
+If any variables are still uninitialized when this
+process ends, those variables are part of one or more initialization cycles,
+and the program is not valid.
+</p>
+
+<p>
+Multiple variables on the left-hand side of a variable declaration initialized
+by single (multi-valued) expression on the right-hand side are initialized
+together: If any of the variables on the left-hand side is initialized, all
+those variables are initialized in the same step.
+</p>
+
+<pre>
+var x = a
+var a, b = f() // a and b are initialized together, before x is initialized
+</pre>
+
+<p>
+For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
+variables are treated like any other variables in declarations.
+</p>
+
+<p>
+The declaration order of variables declared in multiple files is determined
+by the order in which the files are presented to the compiler: Variables
+declared in the first file are declared before any of the variables declared
+in the second file, and so on.
+</p>
+
+<p>
+Dependency analysis does not rely on the actual values of the
+variables, only on lexical <i>references</i> to them in the source,
+analyzed transitively. For instance, if a variable <code>x</code>'s
+initialization expression refers to a function whose body refers to
+variable <code>y</code> then <code>x</code> depends on <code>y</code>.
+Specifically:
+</p>
+
+<ul>
+<li>
+A reference to a variable or function is an identifier denoting that
+variable or function.
+</li>
+
+<li>
+A reference to a method <code>m</code> is a
+<a href="#Method_values">method value</a> or
+<a href="#Method_expressions">method expression</a> of the form
+<code>t.m</code>, where the (static) type of <code>t</code> is
+not an interface type, and the method <code>m</code> is in the
+<a href="#Method_sets">method set</a> of <code>t</code>.
+It is immaterial whether the resulting function value
+<code>t.m</code> is invoked.
+</li>
+
+<li>
+A variable, function, or method <code>x</code> depends on a variable
+<code>y</code> if <code>x</code>'s initialization expression or body
+(for functions and methods) contains a reference to <code>y</code>
+or to a function or method that depends on <code>y</code>.
+</li>
+</ul>
+
+<p>
+For example, given the declarations
+</p>
+
+<pre>
+var (
+ a = c + b // == 9
+ b = f() // == 4
+ c = f() // == 5
+ d = 3 // == 5 after initialization has finished
+)
+
+func f() int {
+ d++
+ return d
+}
+</pre>
+
+<p>
+the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
+Note that the order of subexpressions in initialization expressions is irrelevant:
+<code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
+order in this example.
+</p>
+
+<p>
+Dependency analysis is performed per package; only references referring
+to variables, functions, and (non-interface) methods declared in the current
+package are considered. If other, hidden, data dependencies exists between
+variables, the initialization order between those variables is unspecified.
+</p>
+
+<p>
+For instance, given the declarations
+</p>
+
+<pre>
+var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
+var _ = sideEffect() // unrelated to x, a, or b
+var a = b
+var b = 42
+
+type I interface { ab() []int }
+type T struct{}
+func (T) ab() []int { return []int{a, b} }
+</pre>
+
+<p>
+the variable <code>a</code> will be initialized after <code>b</code> but
+whether <code>x</code> is initialized before <code>b</code>, between
+<code>b</code> and <code>a</code>, or after <code>a</code>, and
+thus also the moment at which <code>sideEffect()</code> is called (before
+or after <code>x</code> is initialized) is not specified.
+</p>
+
+<p>
+Variables may also be initialized using functions named <code>init</code>
+declared in the package block, with no arguments and no result parameters.
+</p>
+
+<pre>
+func init() { … }
+</pre>
+
+<p>
+Multiple such functions may be defined per package, even within a single
+source file. In the package block, the <code>init</code> identifier can
+be used only to declare <code>init</code> functions, yet the identifier
+itself is not <a href="#Declarations_and_scope">declared</a>. Thus
+<code>init</code> functions cannot be referred to from anywhere
+in a program.
+</p>
+
+<p>
+A package with no imports is initialized by assigning initial values
+to all its package-level variables followed by calling all <code>init</code>
+functions in the order they appear in the source, possibly in multiple files,
+as presented to the compiler.
+If a package has imports, the imported packages are initialized
+before initializing the package itself. If multiple packages import
+a package, the imported package will be initialized only once.
+The importing of packages, by construction, guarantees that there
+can be no cyclic initialization dependencies.
+</p>
+
+<p>
+Package initialization—variable initialization and the invocation of
+<code>init</code> functions—happens in a single goroutine,
+sequentially, one package at a time.
+An <code>init</code> function may launch other goroutines, which can run
+concurrently with the initialization code. However, initialization
+always sequences
+the <code>init</code> functions: it will not invoke the next one
+until the previous one has returned.
+</p>
+
+<p>
+To ensure reproducible initialization behavior, build systems are encouraged
+to present multiple files belonging to the same package in lexical file name
+order to a compiler.
+</p>
+
+
+<h3 id="Program_execution">Program execution</h3>
+<p>
+A complete program is created by linking a single, unimported package
+called the <i>main package</i> with all the packages it imports, transitively.
+The main package must
+have package name <code>main</code> and
+declare a function <code>main</code> that takes no
+arguments and returns no value.
+</p>
+
+<pre>
+func main() { … }
+</pre>
+
+<p>
+Program execution begins by initializing the main package and then
+invoking the function <code>main</code>.
+When that function invocation returns, the program exits.
+It does not wait for other (non-<code>main</code>) goroutines to complete.
+</p>
+
+<h2 id="Errors">Errors</h2>
+
+<p>
+The predeclared type <code>error</code> is defined as
+</p>
+
+<pre>
+type error interface {
+ Error() string
+}
+</pre>
+
+<p>
+It is the conventional interface for representing an error condition,
+with the nil value representing no error.
+For instance, a function to read data from a file might be defined:
+</p>
+
+<pre>
+func Read(f *File, b []byte) (n int, err error)
+</pre>
+
+<h2 id="Run_time_panics">Run-time panics</h2>
+
+<p>
+Execution errors such as attempting to index an array out
+of bounds trigger a <i>run-time panic</i> equivalent to a call of
+the built-in function <a href="#Handling_panics"><code>panic</code></a>
+with a value of the implementation-defined interface type <code>runtime.Error</code>.
+That type satisfies the predeclared interface type
+<a href="#Errors"><code>error</code></a>.
+The exact error values that
+represent distinct run-time error conditions are unspecified.
+</p>
+
+<pre>
+package runtime
+
+type Error interface {
+ error
+ // and perhaps other methods
+}
+</pre>
+
+<h2 id="System_considerations">System considerations</h2>
+
+<h3 id="Package_unsafe">Package <code>unsafe</code></h3>
+
+<p>
+The built-in package <code>unsafe</code>, known to the compiler
+and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
+provides facilities for low-level programming including operations
+that violate the type system. A package using <code>unsafe</code>
+must be vetted manually for type safety and may not be portable.
+The package provides the following interface:
+</p>
+
+<pre class="grammar">
+package unsafe
+
+type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
+type Pointer *ArbitraryType
+
+func Alignof(variable ArbitraryType) uintptr
+func Offsetof(selector ArbitraryType) uintptr
+func Sizeof(variable ArbitraryType) uintptr
+
+type IntegerType int // shorthand for an integer type; it is not a real type
+func Add(ptr Pointer, len IntegerType) Pointer
+func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
+</pre>
+
+<p>
+A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
+value may not be <a href="#Address_operators">dereferenced</a>.
+Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be converted to
+a type of underlying type <code>Pointer</code> and vice versa.
+The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
+</p>
+
+<pre>
+var f float64
+bits = *(*uint64)(unsafe.Pointer(&f))
+
+type ptr unsafe.Pointer
+bits = *(*uint64)(ptr(&f))
+
+var p ptr = nil
+</pre>
+
+<p>
+The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
+of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
+as if <code>v</code> was declared via <code>var v = x</code>.
+</p>
+<p>
+The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
+<code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
+or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
+If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
+without pointer indirections through fields of the struct.
+For a struct <code>s</code> with field <code>f</code>:
+</p>
+
+<pre>
+uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
+</pre>
+
+<p>
+Computer architectures may require memory addresses to be <i>aligned</i>;
+that is, for addresses of a variable to be a multiple of a factor,
+the variable's type's <i>alignment</i>. The function <code>Alignof</code>
+takes an expression denoting a variable of any type and returns the
+alignment of the (type of the) variable in bytes. For a variable
+<code>x</code>:
+</p>
+
+<pre>
+uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
+</pre>
+
+<p>
+Calls to <code>Alignof</code>, <code>Offsetof</code>, and
+<code>Sizeof</code> are compile-time constant expressions of type <code>uintptr</code>.
+</p>
+
+<p>
+The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
+and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
+The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
+A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
+if it is an untyped constant it is given type <code>int</code>.
+The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
+</p>
+
+<p>
+The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
+and whose length and capacity are <code>len</code>.
+<code>Slice(ptr, len)</code> is equivalent to
+</p>
+
+<pre>
+(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
+</pre>
+
+<p>
+except that, as a special case, if <code>ptr</code>
+is <code>nil</code> and <code>len</code> is zero,
+<code>Slice</code> returns <code>nil</code>.
+</p>
+
+<p>
+The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
+A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
+if it is an untyped constant it is given type <code>int</code>.
+At run time, if <code>len</code> is negative,
+or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
+
+<p>
+For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
+</p>
+
+<pre class="grammar">
+type size in bytes
+
+byte, uint8, int8 1
+uint16, int16 2
+uint32, int32, float32 4
+uint64, int64, float64, complex64 8
+complex128 16
+</pre>
+
+<p>
+The following minimal alignment properties are guaranteed:
+</p>
+<ol>
+<li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
+</li>
+
+<li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
+ all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
+</li>
+
+<li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
+ the alignment of a variable of the array's element type.
+</li>
+</ol>
+
+<p>
+A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.
+</p>
diff --git a/_content/ref/spec.html b/_content/ref/spec.html
new file mode 100644
index 0000000..197acc5
--- /dev/null
+++ b/_content/ref/spec.html
@@ -0,0 +1,8060 @@
+<!--{
+ "Title": "The Go Programming Language Specification",
+ "Subtitle": "Version of March 10, 2022"
+}-->
+
+<h2 id="Introduction">Introduction</h2>
+
+<p>
+This is the reference manual for the Go programming language.
+The pre-Go1.18 version, without generics, can be found
+<a href="/doc/go1.17_spec.html">here</a>.
+For more information and other documents, see <a href="/">golang.org</a>.
+</p>
+
+<p>
+Go is a general-purpose language designed with systems programming
+in mind. It is strongly typed and garbage-collected and has explicit
+support for concurrent programming. Programs are constructed from
+<i>packages</i>, whose properties allow efficient management of
+dependencies.
+</p>
+
+<p>
+The grammar is compact and simple to parse, allowing for easy analysis
+by automatic tools such as integrated development environments.
+</p>
+
+<h2 id="Notation">Notation</h2>
+<p>
+The syntax is specified using Extended Backus-Naur Form (EBNF):
+</p>
+
+<pre class="grammar">
+Production = production_name "=" [ Expression ] "." .
+Expression = Alternative { "|" Alternative } .
+Alternative = Term { Term } .
+Term = production_name | token [ "…" token ] | Group | Option | Repetition .
+Group = "(" Expression ")" .
+Option = "[" Expression "]" .
+Repetition = "{" Expression "}" .
+</pre>
+
+<p>
+Productions are expressions constructed from terms and the following
+operators, in increasing precedence:
+</p>
+<pre class="grammar">
+| alternation
+() grouping
+[] option (0 or 1 times)
+{} repetition (0 to n times)
+</pre>
+
+<p>
+Lower-case production names are used to identify lexical tokens.
+Non-terminals are in CamelCase. Lexical tokens are enclosed in
+double quotes <code>""</code> or back quotes <code>``</code>.
+</p>
+
+<p>
+The form <code>a … b</code> represents the set of characters from
+<code>a</code> through <code>b</code> as alternatives. The horizontal
+ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various
+enumerations or code snippets that are not further specified. The character <code>…</code>
+(as opposed to the three characters <code>...</code>) is not a token of the Go
+language.
+</p>
+
+<h2 id="Source_code_representation">Source code representation</h2>
+
+<p>
+Source code is Unicode text encoded in
+<a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
+canonicalized, so a single accented code point is distinct from the
+same character constructed from combining an accent and a letter;
+those are treated as two code points. For simplicity, this document
+will use the unqualified term <i>character</i> to refer to a Unicode code point
+in the source text.
+</p>
+<p>
+Each code point is distinct; for instance, upper and lower case letters
+are different characters.
+</p>
+<p>
+Implementation restriction: For compatibility with other tools, a
+compiler may disallow the NUL character (U+0000) in the source text.
+</p>
+<p>
+Implementation restriction: For compatibility with other tools, a
+compiler may ignore a UTF-8-encoded byte order mark
+(U+FEFF) if it is the first Unicode code point in the source text.
+A byte order mark may be disallowed anywhere else in the source.
+</p>
+
+<h3 id="Characters">Characters</h3>
+
+<p>
+The following terms are used to denote specific Unicode character classes:
+</p>
+<pre class="ebnf">
+newline = /* the Unicode code point U+000A */ .
+unicode_char = /* an arbitrary Unicode code point except newline */ .
+unicode_letter = /* a Unicode code point classified as "Letter" */ .
+unicode_digit = /* a Unicode code point classified as "Number, decimal digit" */ .
+</pre>
+
+<p>
+In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
+Section 4.5 "General Category" defines a set of character categories.
+Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
+as Unicode letters, and those in the Number category Nd as Unicode digits.
+</p>
+
+<h3 id="Letters_and_digits">Letters and digits</h3>
+
+<p>
+The underscore character <code>_</code> (U+005F) is considered a letter.
+</p>
+<pre class="ebnf">
+letter = unicode_letter | "_" .
+decimal_digit = "0" … "9" .
+binary_digit = "0" | "1" .
+octal_digit = "0" … "7" .
+hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
+</pre>
+
+<h2 id="Lexical_elements">Lexical elements</h2>
+
+<h3 id="Comments">Comments</h3>
+
+<p>
+Comments serve as program documentation. There are two forms:
+</p>
+
+<ol>
+<li>
+<i>Line comments</i> start with the character sequence <code>//</code>
+and stop at the end of the line.
+</li>
+<li>
+<i>General comments</i> start with the character sequence <code>/*</code>
+and stop with the first subsequent character sequence <code>*/</code>.
+</li>
+</ol>
+
+<p>
+A comment cannot start inside a <a href="#Rune_literals">rune</a> or
+<a href="#String_literals">string literal</a>, or inside a comment.
+A general comment containing no newlines acts like a space.
+Any other comment acts like a newline.
+</p>
+
+<h3 id="Tokens">Tokens</h3>
+
+<p>
+Tokens form the vocabulary of the Go language.
+There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators
+and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from
+spaces (U+0020), horizontal tabs (U+0009),
+carriage returns (U+000D), and newlines (U+000A),
+is ignored except as it separates tokens
+that would otherwise combine into a single token. Also, a newline or end of file
+may trigger the insertion of a <a href="#Semicolons">semicolon</a>.
+While breaking the input into tokens,
+the next token is the longest sequence of characters that form a
+valid token.
+</p>
+
+<h3 id="Semicolons">Semicolons</h3>
+
+<p>
+The formal grammar uses semicolons <code>";"</code> as terminators in
+a number of productions. Go programs may omit most of these semicolons
+using the following two rules:
+</p>
+
+<ol>
+<li>
+When the input is broken into tokens, a semicolon is automatically inserted
+into the token stream immediately after a line's final token if that token is
+<ul>
+ <li>an
+ <a href="#Identifiers">identifier</a>
+ </li>
+
+ <li>an
+ <a href="#Integer_literals">integer</a>,
+ <a href="#Floating-point_literals">floating-point</a>,
+ <a href="#Imaginary_literals">imaginary</a>,
+ <a href="#Rune_literals">rune</a>, or
+ <a href="#String_literals">string</a> literal
+ </li>
+
+ <li>one of the <a href="#Keywords">keywords</a>
+ <code>break</code>,
+ <code>continue</code>,
+ <code>fallthrough</code>, or
+ <code>return</code>
+ </li>
+
+ <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a>
+ <code>++</code>,
+ <code>--</code>,
+ <code>)</code>,
+ <code>]</code>, or
+ <code>}</code>
+ </li>
+</ul>
+</li>
+
+<li>
+To allow complex statements to occupy a single line, a semicolon
+may be omitted before a closing <code>")"</code> or <code>"}"</code>.
+</li>
+</ol>
+
+<p>
+To reflect idiomatic use, code examples in this document elide semicolons
+using these rules.
+</p>
+
+
+<h3 id="Identifiers">Identifiers</h3>
+
+<p>
+Identifiers name program entities such as variables and types.
+An identifier is a sequence of one or more letters and digits.
+The first character in an identifier must be a letter.
+</p>
+<pre class="ebnf">
+identifier = letter { letter | unicode_digit } .
+</pre>
+<pre>
+a
+_x9
+ThisVariableIsExported
+αβ
+</pre>
+
+<p>
+Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>.
+</p>
+
+
+<h3 id="Keywords">Keywords</h3>
+
+<p>
+The following keywords are reserved and may not be used as identifiers.
+</p>
+<pre class="grammar">
+break default func interface select
+case defer go map struct
+chan else goto package switch
+const fallthrough if range type
+continue for import return var
+</pre>
+
+<h3 id="Operators_and_punctuation">Operators and punctuation</h3>
+
+<p>
+The following character sequences represent <a href="#Operators">operators</a>
+(including <a href="#Assignments">assignment operators</a>) and punctuation:
+</p>
+<pre class="grammar">
++ & += &= && == != ( )
+- | -= |= || < <= [ ]
+* ^ *= ^= <- > >= { }
+/ << /= <<= ++ = := , ;
+% >> %= >>= -- ! ... . :
+ &^ &^= ~
+</pre>
+
+<h3 id="Integer_literals">Integer literals</h3>
+
+<p>
+An integer literal is a sequence of digits representing an
+<a href="#Constants">integer constant</a>.
+An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code>
+for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal,
+and <code>0x</code> or <code>0X</code> for hexadecimal.
+A single <code>0</code> is considered a decimal zero.
+In hexadecimal literals, letters <code>a</code> through <code>f</code>
+and <code>A</code> through <code>F</code> represent values 10 through 15.
+</p>
+
+<p>
+For readability, an underscore character <code>_</code> may appear after
+a base prefix or between successive digits; such underscores do not change
+the literal's value.
+</p>
+<pre class="ebnf">
+int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
+decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
+binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
+octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
+hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
+
+decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
+binary_digits = binary_digit { [ "_" ] binary_digit } .
+octal_digits = octal_digit { [ "_" ] octal_digit } .
+hex_digits = hex_digit { [ "_" ] hex_digit } .
+</pre>
+
+<pre>
+42
+4_2
+0600
+0_600
+0o600
+0O600 // second character is capital letter 'O'
+0xBadFace
+0xBad_Face
+0x_67_7a_2f_cc_40_c6
+170141183460469231731687303715884105727
+170_141183_460469_231731_687303_715884_105727
+
+_42 // an identifier, not an integer literal
+42_ // invalid: _ must separate successive digits
+4__2 // invalid: only one _ at a time
+0_xBadFace // invalid: _ must separate successive digits
+</pre>
+
+
+<h3 id="Floating-point_literals">Floating-point literals</h3>
+
+<p>
+A floating-point literal is a decimal or hexadecimal representation of a
+<a href="#Constants">floating-point constant</a>.
+</p>
+
+<p>
+A decimal floating-point literal consists of an integer part (decimal digits),
+a decimal point, a fractional part (decimal digits), and an exponent part
+(<code>e</code> or <code>E</code> followed by an optional sign and decimal digits).
+One of the integer part or the fractional part may be elided; one of the decimal point
+or the exponent part may be elided.
+An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.
+</p>
+
+<p>
+A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code>
+prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
+and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits).
+One of the integer part or the fractional part may be elided; the radix point may be elided as well,
+but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
+An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.
+</p>
+
+<p>
+For readability, an underscore character <code>_</code> may appear after
+a base prefix or between successive digits; such underscores do not change
+the literal value.
+</p>
+
+<pre class="ebnf">
+float_lit = decimal_float_lit | hex_float_lit .
+
+decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
+ decimal_digits decimal_exponent |
+ "." decimal_digits [ decimal_exponent ] .
+decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
+
+hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
+hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
+ [ "_" ] hex_digits |
+ "." hex_digits .
+hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
+</pre>
+
+<pre>
+0.
+72.40
+072.40 // == 72.40
+2.71828
+1.e+0
+6.67428e-11
+1E6
+.25
+.12345E+5
+1_5. // == 15.0
+0.15e+0_2 // == 15.0
+
+0x1p-2 // == 0.25
+0x2.p10 // == 2048.0
+0x1.Fp+0 // == 1.9375
+0X.8p-0 // == 0.5
+0X_1FFFP-16 // == 0.1249847412109375
+0x15e-2 // == 0x15e - 2 (integer subtraction)
+
+0x.p1 // invalid: mantissa has no digits
+1p-2 // invalid: p exponent requires hexadecimal mantissa
+0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent
+1_.5 // invalid: _ must separate successive digits
+1._5 // invalid: _ must separate successive digits
+1.5_e1 // invalid: _ must separate successive digits
+1.5e_1 // invalid: _ must separate successive digits
+1.5e1_ // invalid: _ must separate successive digits
+</pre>
+
+
+<h3 id="Imaginary_literals">Imaginary literals</h3>
+
+<p>
+An imaginary literal represents the imaginary part of a
+<a href="#Constants">complex constant</a>.
+It consists of an <a href="#Integer_literals">integer</a> or
+<a href="#Floating-point_literals">floating-point</a> literal
+followed by the lower-case letter <code>i</code>.
+The value of an imaginary literal is the value of the respective
+integer or floating-point literal multiplied by the imaginary unit <i>i</i>.
+</p>
+
+<pre class="ebnf">
+imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
+</pre>
+
+<p>
+For backward compatibility, an imaginary literal's integer part consisting
+entirely of decimal digits (and possibly underscores) is considered a decimal
+integer, even if it starts with a leading <code>0</code>.
+</p>
+
+<pre>
+0i
+0123i // == 123i for backward-compatibility
+0o123i // == 0o123 * 1i == 83i
+0xabci // == 0xabc * 1i == 2748i
+0.i
+2.71828i
+1.e+0i
+6.67428e-11i
+1E6i
+.25i
+.12345E+5i
+0x1p-2i // == 0x1p-2 * 1i == 0.25i
+</pre>
+
+
+<h3 id="Rune_literals">Rune literals</h3>
+
+<p>
+A rune literal represents a <a href="#Constants">rune constant</a>,
+an integer value identifying a Unicode code point.
+A rune literal is expressed as one or more characters enclosed in single quotes,
+as in <code>'x'</code> or <code>'\n'</code>.
+Within the quotes, any character may appear except newline and unescaped single
+quote. A single quoted character represents the Unicode value
+of the character itself,
+while multi-character sequences beginning with a backslash encode
+values in various formats.
+</p>
+
+<p>
+The simplest form represents the single character within the quotes;
+since Go source text is Unicode characters encoded in UTF-8, multiple
+UTF-8-encoded bytes may represent a single integer value. For
+instance, the literal <code>'a'</code> holds a single byte representing
+a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
+<code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
+a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
+</p>
+
+<p>
+Several backslash escapes allow arbitrary values to be encoded as
+ASCII text. There are four ways to represent the integer value
+as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
+digits; <code>\u</code> followed by exactly four hexadecimal digits;
+<code>\U</code> followed by exactly eight hexadecimal digits, and a
+plain backslash <code>\</code> followed by exactly three octal digits.
+In each case the value of the literal is the value represented by
+the digits in the corresponding base.
+</p>
+
+<p>
+Although these representations all result in an integer, they have
+different valid ranges. Octal escapes must represent a value between
+0 and 255 inclusive. Hexadecimal escapes satisfy this condition
+by construction. The escapes <code>\u</code> and <code>\U</code>
+represent Unicode code points so within them some values are illegal,
+in particular those above <code>0x10FFFF</code> and surrogate halves.
+</p>
+
+<p>
+After a backslash, certain single-character escapes represent special values:
+</p>
+
+<pre class="grammar">
+\a U+0007 alert or bell
+\b U+0008 backspace
+\f U+000C form feed
+\n U+000A line feed or newline
+\r U+000D carriage return
+\t U+0009 horizontal tab
+\v U+000B vertical tab
+\\ U+005C backslash
+\' U+0027 single quote (valid escape only within rune literals)
+\" U+0022 double quote (valid escape only within string literals)
+</pre>
+
+<p>
+All other sequences starting with a backslash are illegal inside rune literals.
+</p>
+<pre class="ebnf">
+rune_lit = "'" ( unicode_value | byte_value ) "'" .
+unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
+byte_value = octal_byte_value | hex_byte_value .
+octal_byte_value = `\` octal_digit octal_digit octal_digit .
+hex_byte_value = `\` "x" hex_digit hex_digit .
+little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
+big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
+ hex_digit hex_digit hex_digit hex_digit .
+escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
+</pre>
+
+<pre>
+'a'
+'ä'
+'本'
+'\t'
+'\000'
+'\007'
+'\377'
+'\x07'
+'\xff'
+'\u12e4'
+'\U00101234'
+'\'' // rune literal containing single quote character
+'aa' // illegal: too many characters
+'\xa' // illegal: too few hexadecimal digits
+'\0' // illegal: too few octal digits
+'\uDFFF' // illegal: surrogate half
+'\U00110000' // illegal: invalid Unicode code point
+</pre>
+
+
+<h3 id="String_literals">String literals</h3>
+
+<p>
+A string literal represents a <a href="#Constants">string constant</a>
+obtained from concatenating a sequence of characters. There are two forms:
+raw string literals and interpreted string literals.
+</p>
+
+<p>
+Raw string literals are character sequences between back quotes, as in
+<code>`foo`</code>. Within the quotes, any character may appear except
+back quote. The value of a raw string literal is the
+string composed of the uninterpreted (implicitly UTF-8-encoded) characters
+between the quotes;
+in particular, backslashes have no special meaning and the string may
+contain newlines.
+Carriage return characters ('\r') inside raw string literals
+are discarded from the raw string value.
+</p>
+
+<p>
+Interpreted string literals are character sequences between double
+quotes, as in <code>"bar"</code>.
+Within the quotes, any character may appear except newline and unescaped double quote.
+The text between the quotes forms the
+value of the literal, with backslash escapes interpreted as they
+are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and
+<code>\"</code> is legal), with the same restrictions.
+The three-digit octal (<code>\</code><i>nnn</i>)
+and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual
+<i>bytes</i> of the resulting string; all other escapes represent
+the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.
+Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent
+a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>,
+<code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent
+the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character
+U+00FF.
+</p>
+
+<pre class="ebnf">
+string_lit = raw_string_lit | interpreted_string_lit .
+raw_string_lit = "`" { unicode_char | newline } "`" .
+interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
+</pre>
+
+<pre>
+`abc` // same as "abc"
+`\n
+\n` // same as "\\n\n\\n"
+"\n"
+"\"" // same as `"`
+"Hello, world!\n"
+"日本語"
+"\u65e5本\U00008a9e"
+"\xff\u00FF"
+"\uD800" // illegal: surrogate half
+"\U00110000" // illegal: invalid Unicode code point
+</pre>
+
+<p>
+These examples all represent the same string:
+</p>
+
+<pre>
+"日本語" // UTF-8 input text
+`日本語` // UTF-8 input text as a raw literal
+"\u65e5\u672c\u8a9e" // the explicit Unicode code points
+"\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points
+"\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes
+</pre>
+
+<p>
+If the source code represents a character as two code points, such as
+a combining form involving an accent and a letter, the result will be
+an error if placed in a rune literal (it is not a single code
+point), and will appear as two code points if placed in a string
+literal.
+</p>
+
+
+<h2 id="Constants">Constants</h2>
+
+<p>There are <i>boolean constants</i>,
+<i>rune constants</i>,
+<i>integer constants</i>,
+<i>floating-point constants</i>, <i>complex constants</i>,
+and <i>string constants</i>. Rune, integer, floating-point,
+and complex constants are
+collectively called <i>numeric constants</i>.
+</p>
+
+<p>
+A constant value is represented by a
+<a href="#Rune_literals">rune</a>,
+<a href="#Integer_literals">integer</a>,
+<a href="#Floating-point_literals">floating-point</a>,
+<a href="#Imaginary_literals">imaginary</a>,
+or
+<a href="#String_literals">string</a> literal,
+an identifier denoting a constant,
+a <a href="#Constant_expressions">constant expression</a>,
+a <a href="#Conversions">conversion</a> with a result that is a constant, or
+the result value of some built-in functions such as
+<code>unsafe.Sizeof</code> applied to <a href="#Package_unsafe">certain values</a>,
+<code>cap</code> or <code>len</code> applied to
+<a href="#Length_and_capacity">some expressions</a>,
+<code>real</code> and <code>imag</code> applied to a complex constant
+and <code>complex</code> applied to numeric constants.
+The boolean truth values are represented by the predeclared constants
+<code>true</code> and <code>false</code>. The predeclared identifier
+<a href="#Iota">iota</a> denotes an integer constant.
+</p>
+
+<p>
+In general, complex constants are a form of
+<a href="#Constant_expressions">constant expression</a>
+and are discussed in that section.
+</p>
+
+<p>
+Numeric constants represent exact values of arbitrary precision and do not overflow.
+Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
+and not-a-number values.
+</p>
+
+<p>
+Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
+Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
+and certain <a href="#Constant_expressions">constant expressions</a>
+containing only untyped constant operands are untyped.
+</p>
+
+<p>
+A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
+or <a href="#Conversions">conversion</a>, or implicitly when used in a
+<a href="#Variable_declarations">variable declaration</a> or an
+<a href="#Assignments">assignment</a> or as an
+operand in an <a href="#Expressions">expression</a>.
+It is an error if the constant value
+cannot be <a href="#Representability">represented</a> as a value of the respective type.
+If the type is a type parameter, the constant is converted into a non-constant
+value of the type parameter.
+</p>
+
+<p>
+An untyped constant has a <i>default type</i> which is the type to which the
+constant is implicitly converted in contexts where a typed value is required,
+for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
+such as <code>i := 0</code> where there is no explicit type.
+The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
+<code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
+respectively, depending on whether it is a boolean, rune, integer, floating-point,
+complex, or string constant.
+</p>
+
+<p>
+Implementation restriction: Although numeric constants have arbitrary
+precision in the language, a compiler may implement them using an
+internal representation with limited precision. That said, every
+implementation must:
+</p>
+
+<ul>
+ <li>Represent integer constants with at least 256 bits.</li>
+
+ <li>Represent floating-point constants, including the parts of
+ a complex constant, with a mantissa of at least 256 bits
+ and a signed binary exponent of at least 16 bits.</li>
+
+ <li>Give an error if unable to represent an integer constant
+ precisely.</li>
+
+ <li>Give an error if unable to represent a floating-point or
+ complex constant due to overflow.</li>
+
+ <li>Round to the nearest representable constant if unable to
+ represent a floating-point or complex constant due to limits
+ on precision.</li>
+</ul>
+
+<p>
+These requirements apply both to literal constants and to the result
+of evaluating <a href="#Constant_expressions">constant
+expressions</a>.
+</p>
+
+
+<h2 id="Variables">Variables</h2>
+
+<p>
+A variable is a storage location for holding a <i>value</i>.
+The set of permissible values is determined by the
+variable's <i><a href="#Types">type</a></i>.
+</p>
+
+<p>
+A <a href="#Variable_declarations">variable declaration</a>
+or, for function parameters and results, the signature
+of a <a href="#Function_declarations">function declaration</a>
+or <a href="#Function_literals">function literal</a> reserves
+storage for a named variable.
+
+Calling the built-in function <a href="#Allocation"><code>new</code></a>
+or taking the address of a <a href="#Composite_literals">composite literal</a>
+allocates storage for a variable at run time.
+Such an anonymous variable is referred to via a (possibly implicit)
+<a href="#Address_operators">pointer indirection</a>.
+</p>
+
+<p>
+<i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
+and <a href="#Struct_types">struct</a> types have elements and fields that may
+be <a href="#Address_operators">addressed</a> individually. Each such element
+acts like a variable.
+</p>
+
+<p>
+The <i>static type</i> (or just <i>type</i>) of a variable is the
+type given in its declaration, the type provided in the
+<code>new</code> call or composite literal, or the type of
+an element of a structured variable.
+Variables of interface type also have a distinct <i>dynamic type</i>,
+which is the (non-interface) type of the value assigned to the variable at run time
+(unless the value is the predeclared identifier <code>nil</code>,
+which has no type).
+The dynamic type may vary during execution but values stored in interface
+variables are always <a href="#Assignability">assignable</a>
+to the static type of the variable.
+</p>
+
+<pre>
+var x interface{} // x is nil and has static type interface{}
+var v *T // v has value nil, static type *T
+x = 42 // x has value 42 and dynamic type int
+x = v // x has value (*T)(nil) and dynamic type *T
+</pre>
+
+<p>
+A variable's value is retrieved by referring to the variable in an
+<a href="#Expressions">expression</a>; it is the most recent value
+<a href="#Assignments">assigned</a> to the variable.
+If a variable has not yet been assigned a value, its value is the
+<a href="#The_zero_value">zero value</a> for its type.
+</p>
+
+
+<h2 id="Types">Types</h2>
+
+<p>
+A type determines a set of values together with operations and methods specific
+to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be
+followed by <a href="#Instantiations">type arguments</a> if the type is generic.
+A type may also be specified using a <i>type literal</i>, which composes a type
+from existing types.
+</p>
+
+<pre class="ebnf">
+Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
+TypeName = identifier | QualifiedIdent .
+TypeArgs = "[" TypeList [ "," ] "]" .
+TypeList = Type { "," Type } .
+TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
+ SliceType | MapType | ChannelType .
+</pre>
+
+<p>
+The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
+Others are introduced with <a href="#Type_declarations">type declarations</a>
+or <a href="#Type_parameter_declarations">type parameter lists</a>.
+<i>Composite types</i>—array, struct, pointer, function,
+interface, slice, map, and channel types—may be constructed using
+type literals.
+</p>
+
+<p>
+Predeclared types, defined types, and type parameters are called <i>named types</i>.
+An alias denotes a named type if the type given in the alias declaration is a named type.
+</p>
+
+<h3 id="Boolean_types">Boolean types</h3>
+
+<p>
+A <i>boolean type</i> represents the set of Boolean truth values
+denoted by the predeclared constants <code>true</code>
+and <code>false</code>. The predeclared boolean type is <code>bool</code>;
+it is a <a href="#Type_definitions">defined type</a>.
+</p>
+
+<h3 id="Numeric_types">Numeric types</h3>
+
+<p>
+An <i>integer</i>, <i>floating-point</i>, or <i>complex</i> type
+represents the set of integer, floating-point, or complex values, respectively.
+They are collectively called <i>numeric types</i>.
+The predeclared architecture-independent numeric types are:
+</p>
+
+<pre class="grammar">
+uint8 the set of all unsigned 8-bit integers (0 to 255)
+uint16 the set of all unsigned 16-bit integers (0 to 65535)
+uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
+uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
+
+int8 the set of all signed 8-bit integers (-128 to 127)
+int16 the set of all signed 16-bit integers (-32768 to 32767)
+int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
+int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
+
+float32 the set of all IEEE-754 32-bit floating-point numbers
+float64 the set of all IEEE-754 64-bit floating-point numbers
+
+complex64 the set of all complex numbers with float32 real and imaginary parts
+complex128 the set of all complex numbers with float64 real and imaginary parts
+
+byte alias for uint8
+rune alias for int32
+</pre>
+
+<p>
+The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
+<a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
+</p>
+
+<p>
+There is also a set of predeclared integer types with implementation-specific sizes:
+</p>
+
+<pre class="grammar">
+uint either 32 or 64 bits
+int same size as uint
+uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
+</pre>
+
+<p>
+To avoid portability issues all numeric types are <a href="#Type_definitions">defined
+types</a> and thus distinct except
+<code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
+<code>rune</code>, which is an alias for <code>int32</code>.
+Explicit conversions
+are required when different numeric types are mixed in an expression
+or assignment. For instance, <code>int32</code> and <code>int</code>
+are not the same type even though they may have the same size on a
+particular architecture.
+
+
+<h3 id="String_types">String types</h3>
+
+<p>
+A <i>string type</i> represents the set of string values.
+A string value is a (possibly empty) sequence of bytes.
+The number of bytes is called the length of the string and is never negative.
+Strings are immutable: once created,
+it is impossible to change the contents of a string.
+The predeclared string type is <code>string</code>;
+it is a <a href="#Type_definitions">defined type</a>.
+</p>
+
+<p>
+The length of a string <code>s</code> can be discovered using
+the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
+The length is a compile-time constant if the string is a constant.
+A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
+0 through <code>len(s)-1</code>.
+It is illegal to take the address of such an element; if
+<code>s[i]</code> is the <code>i</code>'th byte of a
+string, <code>&s[i]</code> is invalid.
+</p>
+
+
+<h3 id="Array_types">Array types</h3>
+
+<p>
+An array is a numbered sequence of elements of a single
+type, called the element type.
+The number of elements is called the length of the array and is never negative.
+</p>
+
+<pre class="ebnf">
+ArrayType = "[" ArrayLength "]" ElementType .
+ArrayLength = Expression .
+ElementType = Type .
+</pre>
+
+<p>
+The length is part of the array's type; it must evaluate to a
+non-negative <a href="#Constants">constant</a>
+<a href="#Representability">representable</a> by a value
+of type <code>int</code>.
+The length of array <code>a</code> can be discovered
+using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
+The elements can be addressed by integer <a href="#Index_expressions">indices</a>
+0 through <code>len(a)-1</code>.
+Array types are always one-dimensional but may be composed to form
+multi-dimensional types.
+</p>
+
+<pre>
+[32]byte
+[2*N] struct { x, y int32 }
+[1000]*float64
+[3][5]int
+[2][2][2]float64 // same as [2]([2]([2]float64))
+</pre>
+
+<h3 id="Slice_types">Slice types</h3>
+
+<p>
+A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
+provides access to a numbered sequence of elements from that array.
+A slice type denotes the set of all slices of arrays of its element type.
+The number of elements is called the length of the slice and is never negative.
+The value of an uninitialized slice is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+SliceType = "[" "]" ElementType .
+</pre>
+
+<p>
+The length of a slice <code>s</code> can be discovered by the built-in function
+<a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
+execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
+0 through <code>len(s)-1</code>. The slice index of a
+given element may be less than the index of the same element in the
+underlying array.
+</p>
+<p>
+A slice, once initialized, is always associated with an underlying
+array that holds its elements. A slice therefore shares storage
+with its array and with other slices of the same array; by contrast,
+distinct arrays always represent distinct storage.
+</p>
+<p>
+The array underlying a slice may extend past the end of the slice.
+The <i>capacity</i> is a measure of that extent: it is the sum of
+the length of the slice and the length of the array beyond the slice;
+a slice of length up to that capacity can be created by
+<a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
+The capacity of a slice <code>a</code> can be discovered using the
+built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
+</p>
+
+<p>
+A new, initialized slice value for a given element type <code>T</code> may be
+made using the built-in function
+<a href="#Making_slices_maps_and_channels"><code>make</code></a>,
+which takes a slice type
+and parameters specifying the length and optionally the capacity.
+A slice created with <code>make</code> always allocates a new, hidden array
+to which the returned slice value refers. That is, executing
+</p>
+
+<pre>
+make([]T, length, capacity)
+</pre>
+
+<p>
+produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
+it, so these two expressions are equivalent:
+</p>
+
+<pre>
+make([]int, 50, 100)
+new([100]int)[0:50]
+</pre>
+
+<p>
+Like arrays, slices are always one-dimensional but may be composed to construct
+higher-dimensional objects.
+With arrays of arrays, the inner arrays are, by construction, always the same length;
+however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
+Moreover, the inner slices must be initialized individually.
+</p>
+
+<h3 id="Struct_types">Struct types</h3>
+
+<p>
+A struct is a sequence of named elements, called fields, each of which has a
+name and a type. Field names may be specified explicitly (IdentifierList) or
+implicitly (EmbeddedField).
+Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
+be <a href="#Uniqueness_of_identifiers">unique</a>.
+</p>
+
+<pre class="ebnf">
+StructType = "struct" "{" { FieldDecl ";" } "}" .
+FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
+EmbeddedField = [ "*" ] TypeName .
+Tag = string_lit .
+</pre>
+
+<pre>
+// An empty struct.
+struct {}
+
+// A struct with 6 fields.
+struct {
+ x, y int
+ u float32
+ _ float32 // padding
+ A *[]int
+ F func()
+}
+</pre>
+
+<p>
+A field declared with a type but no explicit field name is called an <i>embedded field</i>.
+An embedded field must be specified as
+a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
+and <code>T</code> itself may not be
+a pointer type. The unqualified type name acts as the field name.
+</p>
+
+<pre>
+// A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
+struct {
+ T1 // field name is T1
+ *T2 // field name is T2
+ P.T3 // field name is T3
+ *P.T4 // field name is T4
+ x, y int // field names are x and y
+}
+</pre>
+
+<p>
+The following declaration is illegal because field names must be unique
+in a struct type:
+</p>
+
+<pre>
+struct {
+ T // conflicts with embedded field *T and *P.T
+ *T // conflicts with embedded field T and *P.T
+ *P.T // conflicts with embedded field T and *T
+}
+</pre>
+
+<p>
+A field or <a href="#Method_declarations">method</a> <code>f</code> of an
+embedded field in a struct <code>x</code> is called <i>promoted</i> if
+<code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
+that field or method <code>f</code>.
+</p>
+
+<p>
+Promoted fields act like ordinary fields
+of a struct except that they cannot be used as field names in
+<a href="#Composite_literals">composite literals</a> of the struct.
+</p>
+
+<p>
+Given a struct type <code>S</code> and a <a href="#Type_definitions">defined type</a>
+<code>T</code>, promoted methods are included in the method set of the struct as follows:
+</p>
+<ul>
+ <li>
+ If <code>S</code> contains an embedded field <code>T</code>,
+ the <a href="#Method_sets">method sets</a> of <code>S</code>
+ and <code>*S</code> both include promoted methods with receiver
+ <code>T</code>. The method set of <code>*S</code> also
+ includes promoted methods with receiver <code>*T</code>.
+ </li>
+
+ <li>
+ If <code>S</code> contains an embedded field <code>*T</code>,
+ the method sets of <code>S</code> and <code>*S</code> both
+ include promoted methods with receiver <code>T</code> or
+ <code>*T</code>.
+ </li>
+</ul>
+
+<p>
+A field declaration may be followed by an optional string literal <i>tag</i>,
+which becomes an attribute for all the fields in the corresponding
+field declaration. An empty tag string is equivalent to an absent tag.
+The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
+and take part in <a href="#Type_identity">type identity</a> for structs
+but are otherwise ignored.
+</p>
+
+<pre>
+struct {
+ x, y float64 "" // an empty tag string is like an absent tag
+ name string "any string is permitted as a tag"
+ _ [4]byte "ceci n'est pas un champ de structure"
+}
+
+// A struct corresponding to a TimeStamp protocol buffer.
+// The tag strings define the protocol buffer field numbers;
+// they follow the convention outlined by the reflect package.
+struct {
+ microsec uint64 `protobuf:"1"`
+ serverIP6 uint64 `protobuf:"2"`
+}
+</pre>
+
+<h3 id="Pointer_types">Pointer types</h3>
+
+<p>
+A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
+type, called the <i>base type</i> of the pointer.
+The value of an uninitialized pointer is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+PointerType = "*" BaseType .
+BaseType = Type .
+</pre>
+
+<pre>
+*Point
+*[4]int
+</pre>
+
+<h3 id="Function_types">Function types</h3>
+
+<p>
+A function type denotes the set of all functions with the same parameter
+and result types. The value of an uninitialized variable of function type
+is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+FunctionType = "func" Signature .
+Signature = Parameters [ Result ] .
+Result = Parameters | Type .
+Parameters = "(" [ ParameterList [ "," ] ] ")" .
+ParameterList = ParameterDecl { "," ParameterDecl } .
+ParameterDecl = [ IdentifierList ] [ "..." ] Type .
+</pre>
+
+<p>
+Within a list of parameters or results, the names (IdentifierList)
+must either all be present or all be absent. If present, each name
+stands for one item (parameter or result) of the specified type and
+all non-<a href="#Blank_identifier">blank</a> names in the signature
+must be <a href="#Uniqueness_of_identifiers">unique</a>.
+If absent, each type stands for one item of that type.
+Parameter and result
+lists are always parenthesized except that if there is exactly
+one unnamed result it may be written as an unparenthesized type.
+</p>
+
+<p>
+The final incoming parameter in a function signature may have
+a type prefixed with <code>...</code>.
+A function with such a parameter is called <i>variadic</i> and
+may be invoked with zero or more arguments for that parameter.
+</p>
+
+<pre>
+func()
+func(x int) int
+func(a, _ int, z float32) bool
+func(a, b int, z float32) (bool)
+func(prefix string, values ...int)
+func(a, b int, z float64, opt ...interface{}) (success bool)
+func(int, int, float64) (float64, *[]int)
+func(n int) func(p *T)
+</pre>
+
+<h3 id="Interface_types">Interface types</h3>
+
+<p>
+An interface type defines a <i>type set</i>.
+A variable of interface type can store a value of any type that is in the type
+set of the interface. Such a type is said to
+<a href="#Implementing_an_interface">implement the interface</a>.
+The value of an uninitialized variable of interface type is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+InterfaceType = "interface" "{" { InterfaceElem ";" } "}" .
+InterfaceElem = MethodElem | TypeElem .
+MethodElem = MethodName Signature .
+MethodName = identifier .
+TypeElem = TypeTerm { "|" TypeTerm } .
+TypeTerm = Type | UnderlyingType .
+UnderlyingType = "~" Type .
+</pre>
+
+<p>
+An interface type is specified by a list of <i>interface elements</i>.
+An interface element is either a <i>method</i> or a <i>type element</i>,
+where a type element is a union of one or more <i>type terms</i>.
+A type term is either a single type or a single underlying type.
+</p>
+
+<h4 id="Basic_interfaces">Basic interfaces</h4>
+
+<p>
+In its most basic form an interface specifies a (possibly empty) list of methods.
+The type set defined by such an interface is the set of types which implement all of
+those methods, and the corresponding <a href="#Method_sets">method set</a> consists
+exactly of the methods specified by the interface.
+Interfaces whose type sets can be defined entirely by a list of methods are called
+<i>basic interfaces.</i>
+</p>
+
+<pre>
+// A simple File interface.
+interface {
+ Read([]byte) (int, error)
+ Write([]byte) (int, error)
+ Close() error
+}
+</pre>
+
+<p>
+The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
+and not <a href="#Blank_identifier">blank</a>.
+</p>
+
+<pre>
+interface {
+ String() string
+ String() string // illegal: String not unique
+ _(x int) // illegal: method must have non-blank name
+}
+</pre>
+
+<p>
+More than one type may implement an interface.
+For instance, if two types <code>S1</code> and <code>S2</code>
+have the method set
+</p>
+
+<pre>
+func (p T) Read(p []byte) (n int, err error)
+func (p T) Write(p []byte) (n int, err error)
+func (p T) Close() error
+</pre>
+
+<p>
+(where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
+then the <code>File</code> interface is implemented by both <code>S1</code> and
+<code>S2</code>, regardless of what other methods
+<code>S1</code> and <code>S2</code> may have or share.
+</p>
+
+<p>
+Every type that is a member of the type set of an interface implements that interface.
+Any given type may implement several distinct interfaces.
+For instance, all types implement the <i>empty interface</i> which stands for the set of all types:
+</p>
+
+<pre>
+interface{}
+</pre>
+
+<p>
+For convenience, the predeclared type <code>any</code> is an alias for the empty interface.
+</p>
+
+<p>
+Similarly, consider this interface specification,
+which appears within a <a href="#Type_declarations">type declaration</a>
+to define an interface called <code>Locker</code>:
+</p>
+
+<pre>
+type Locker interface {
+ Lock()
+ Unlock()
+}
+</pre>
+
+<p>
+If <code>S1</code> and <code>S2</code> also implement
+</p>
+
+<pre>
+func (p T) Lock() { … }
+func (p T) Unlock() { … }
+</pre>
+
+<p>
+they implement the <code>Locker</code> interface as well
+as the <code>File</code> interface.
+</p>
+
+<h4 id="Embedded_interfaces">Embedded interfaces</h4>
+
+<p>
+In a slightly more general form
+an interface <code>T</code> may use a (possibly qualified) interface type
+name <code>E</code> as an interface element. This is called
+<i>embedding</i> interface <code>E</code> in <code>T</code>.
+The type set of <code>T</code> is the <i>intersection</i> of the type sets
+defined by <code>T</code>'s explicitly declared methods and the type sets
+of <code>T</code>’s embedded interfaces.
+In other words, the type set of <code>T</code> is the set of all types that implement all the
+explicitly declared methods of <code>T</code> and also all the methods of
+<code>E</code>.
+</p>
+
+<pre>
+type Reader interface {
+ Read(p []byte) (n int, err error)
+ Close() error
+}
+
+type Writer interface {
+ Write(p []byte) (n int, err error)
+ Close() error
+}
+
+// ReadWriter's methods are Read, Write, and Close.
+type ReadWriter interface {
+ Reader // includes methods of Reader in ReadWriter's method set
+ Writer // includes methods of Writer in ReadWriter's method set
+}
+</pre>
+
+<p>
+When embedding interfaces, methods with the
+<a href="#Uniqueness_of_identifiers">same</a> names must
+have <a href="#Type_identity">identical</a> signatures.
+</p>
+
+<pre>
+type ReadCloser interface {
+ Reader // includes methods of Reader in ReadCloser's method set
+ Close() // illegal: signatures of Reader.Close and Close are different
+}
+</pre>
+
+<h4 id="General_interfaces">General interfaces</h4>
+
+<p>
+In their most general form, an interface element may also be an arbitrary type term
+<code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
+or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
+Together with method specifications, these elements enable the precise
+definition of an interface's type set as follows:
+</p>
+
+<ul>
+ <li>The type set of the empty interface is the set of all non-interface types.
+ </li>
+
+ <li>The type set of a non-empty interface is the intersection of the type sets
+ of its interface elements.
+ </li>
+
+ <li>The type set of a method specification is the set of types
+ whose method sets include that method.
+ </li>
+
+ <li>The type set of a non-interface type term is the set consisting
+ of just that type.
+ </li>
+
+ <li>The type set of a term of the form <code>~T</code>
+ is the set of types whose underlying type is <code>T</code>.
+ </li>
+
+ <li>The type set of a <i>union</i> of terms
+ <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
+ is the union of the type sets of the terms.
+ </li>
+</ul>
+
+<p>
+By construction, an interface's type set never contains an interface type.
+</p>
+
+<pre>
+// An interface representing only the type int.
+interface {
+ int
+}
+
+// An interface representing all types with underlying type int.
+interface {
+ ~int
+}
+
+// An interface representing all types with underlying type int that implement the String method.
+interface {
+ ~int
+ String() string
+}
+
+// An interface representing an empty type set: there is no type that is both an int and a string.
+interface {
+ int
+ string
+}
+</pre>
+
+<p>
+In a term of the form <code>~T</code>, the underlying type of <code>T</code>
+must be itself, and <code>T</code> cannot be an interface.
+</p>
+
+<pre>
+type MyInt int
+
+interface {
+ ~[]byte // the underlying type of []byte is itself
+ ~MyInt // illegal: the underlying type of MyInt is not MyInt
+ ~error // illegal: error is an interface
+}
+</pre>
+
+<p>
+Union elements denote unions of type sets:
+</p>
+
+<pre>
+// The Float interface represents all floating-point types
+// (including any named types whose underlying types are
+// either float32 or float64).
+type Float interface {
+ ~float32 | ~float64
+}
+</pre>
+
+<p>
+In a union, a term cannot be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all
+non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
+Given a type parameter <code>P</code>:
+</p>
+
+<pre>
+interface {
+ P // illegal: P is a type parameter
+ int | P // illegal: P is a type parameter
+ ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
+ float32 | Float // overlapping type sets but Float is an interface
+}
+</pre>
+
+<p>
+Implementation restriction:
+A union (with more than one term) cannot contain the
+<a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code>
+or interfaces that specify methods, or embed <code>comparable</code> or interfaces
+that specify methods.
+</p>
+
+<p>
+Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type
+constraints, or as elements of other interfaces used as constraints.
+They cannot be the types of values or variables, or components of other,
+non-interface types.
+</p>
+
+<pre>
+var x Float // illegal: Float is not a basic interface
+
+var x interface{} = Float(nil) // illegal
+
+type Floatish struct {
+ f Float // illegal
+}
+</pre>
+
+<p>
+An interface type <code>T</code> may not embed any type element
+that is, contains, or embeds <code>T</code>, recursively.
+</p>
+
+<pre>
+// illegal: Bad cannot embed itself
+type Bad interface {
+ Bad
+}
+
+// illegal: Bad1 cannot embed itself using Bad2
+type Bad1 interface {
+ Bad2
+}
+type Bad2 interface {
+ Bad1
+}
+
+// illegal: Bad3 cannot embed a union containing Bad3
+type Bad3 interface {
+ ~int | ~string | Bad3
+}
+</pre>
+
+<h4 id="Implementing_an_interface">Implementing an interface</h4>
+
+<p>
+A type <code>T</code> implements an interface <code>I</code> if
+</p>
+
+<ul>
+<li>
+ <code>T</code> is not an interface and is an element of the type set of <code>I</code>; or
+</li>
+<li>
+ <code>T</code> is an interface and the type set of <code>T</code> is a subset of the
+ type set of <code>I</code>.
+</li>
+</ul>
+
+<p>
+A value of type <code>T</code> implements an interface if <code>T</code>
+implements the interface.
+</p>
+
+<h3 id="Map_types">Map types</h3>
+
+<p>
+A map is an unordered group of elements of one type, called the
+element type, indexed by a set of unique <i>keys</i> of another type,
+called the key type.
+The value of an uninitialized map is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+MapType = "map" "[" KeyType "]" ElementType .
+KeyType = Type .
+</pre>
+
+<p>
+The <a href="#Comparison_operators">comparison operators</a>
+<code>==</code> and <code>!=</code> must be fully defined
+for operands of the key type; thus the key type must not be a function, map, or
+slice.
+If the key type is an interface type, these
+comparison operators must be defined for the dynamic key values;
+failure will cause a <a href="#Run_time_panics">run-time panic</a>.
+</p>
+
+<pre>
+map[string]int
+map[*T]struct{ x, y float64 }
+map[string]interface{}
+</pre>
+
+<p>
+The number of map elements is called its length.
+For a map <code>m</code>, it can be discovered using the
+built-in function <a href="#Length_and_capacity"><code>len</code></a>
+and may change during execution. Elements may be added during execution
+using <a href="#Assignments">assignments</a> and retrieved with
+<a href="#Index_expressions">index expressions</a>; they may be removed with the
+<a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
+</p>
+<p>
+A new, empty map value is made using the built-in
+function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
+which takes the map type and an optional capacity hint as arguments:
+</p>
+
+<pre>
+make(map[string]int)
+make(map[string]int, 100)
+</pre>
+
+<p>
+The initial capacity does not bound its size:
+maps grow to accommodate the number of items
+stored in them, with the exception of <code>nil</code> maps.
+A <code>nil</code> map is equivalent to an empty map except that no elements
+may be added.
+
+<h3 id="Channel_types">Channel types</h3>
+
+<p>
+A channel provides a mechanism for
+<a href="#Go_statements">concurrently executing functions</a>
+to communicate by
+<a href="#Send_statements">sending</a> and
+<a href="#Receive_operator">receiving</a>
+values of a specified element type.
+The value of an uninitialized channel is <code>nil</code>.
+</p>
+
+<pre class="ebnf">
+ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
+</pre>
+
+<p>
+The optional <code><-</code> operator specifies the channel <i>direction</i>,
+<i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
+otherwise it is <i>bidirectional</i>.
+A channel may be constrained only to send or only to receive by
+<a href="#Assignments">assignment</a> or
+explicit <a href="#Conversions">conversion</a>.
+</p>
+
+<pre>
+chan T // can be used to send and receive values of type T
+chan<- float64 // can only be used to send float64s
+<-chan int // can only be used to receive ints
+</pre>
+
+<p>
+The <code><-</code> operator associates with the leftmost <code>chan</code>
+possible:
+</p>
+
+<pre>
+chan<- chan int // same as chan<- (chan int)
+chan<- <-chan int // same as chan<- (<-chan int)
+<-chan <-chan int // same as <-chan (<-chan int)
+chan (<-chan int)
+</pre>
+
+<p>
+A new, initialized channel
+value can be made using the built-in function
+<a href="#Making_slices_maps_and_channels"><code>make</code></a>,
+which takes the channel type and an optional <i>capacity</i> as arguments:
+</p>
+
+<pre>
+make(chan int, 100)
+</pre>
+
+<p>
+The capacity, in number of elements, sets the size of the buffer in the channel.
+If the capacity is zero or absent, the channel is unbuffered and communication
+succeeds only when both a sender and receiver are ready. Otherwise, the channel
+is buffered and communication succeeds without blocking if the buffer
+is not full (sends) or not empty (receives).
+A <code>nil</code> channel is never ready for communication.
+</p>
+
+<p>
+A channel may be closed with the built-in function
+<a href="#Close"><code>close</code></a>.
+The multi-valued assignment form of the
+<a href="#Receive_operator">receive operator</a>
+reports whether a received value was sent before
+the channel was closed.
+</p>
+
+<p>
+A single channel may be used in
+<a href="#Send_statements">send statements</a>,
+<a href="#Receive_operator">receive operations</a>,
+and calls to the built-in functions
+<a href="#Length_and_capacity"><code>cap</code></a> and
+<a href="#Length_and_capacity"><code>len</code></a>
+by any number of goroutines without further synchronization.
+Channels act as first-in-first-out queues.
+For example, if one goroutine sends values on a channel
+and a second goroutine receives them, the values are
+received in the order sent.
+</p>
+
+<h2 id="Properties_of_types_and_values">Properties of types and values</h2>
+
+<h3 id="Underlying_types">Underlying types</h3>
+
+<p>
+Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
+is one of the predeclared boolean, numeric, or string types, or a type literal,
+the corresponding underlying type is <code>T</code> itself.
+Otherwise, <code>T</code>'s underlying type is the underlying type of the
+type to which <code>T</code> refers in its declaration.
+For a type parameter that is the underlying type of its
+<a href="#Type_constraints">type constraint</a>, which is always an interface.
+</p>
+
+<pre>
+type (
+ A1 = string
+ A2 = A1
+)
+
+type (
+ B1 string
+ B2 B1
+ B3 []B1
+ B4 B3
+)
+
+func f[P any](x P) { … }
+</pre>
+
+<p>
+The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
+and <code>B2</code> is <code>string</code>.
+The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
+The underlying type of <code>P</code> is <code>interface{}</code>.
+</p>
+
+<h3 id="Core_types">Core types</h3>
+
+<p>
+Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the
+<a href="#Underlying_types">underlying type</a> of <code>T</code>.
+</p>
+
+<p>
+An interface <code>T</code> has a core type if one of the following
+conditions is satisfied:
+</p>
+
+<ol>
+<li>
+There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a>
+of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or
+</li>
+<li>
+the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a>
+with identical element type <code>E</code>, and all directional channels have the same
+direction.
+</li>
+</ol>
+
+<p>
+No other interfaces have a core type.
+</p>
+
+<p>
+The core type of an interface is, depending on the condition that is satisfied, either:
+</p>
+
+<ol>
+<li>
+the type <code>U</code>; or
+</li>
+<li>
+the type <code>chan E</code> if <code>T</code> contains only bidirectional
+channels, or the type <code>chan<- E</code> or <code><-chan E</code>
+depending on the direction of the directional channels present.
+</li>
+</ol>
+
+<p>
+By definition, a core type is never a <a href="#Type_definitions">defined type</a>,
+<a href="#Type_parameter_declarations">type parameter</a>, or
+<a href="#Interface_types">interface type</a>.
+</p>
+
+<p>
+Examples of interfaces with core types:
+</p>
+
+<pre>
+type Celsius float32
+type Kelvin float32
+
+interface{ int } // int
+interface{ Celsius|Kelvin } // float32
+interface{ ~chan int } // chan int
+interface{ ~chan int|~chan<- int } // chan<- int
+interface{ ~[]*data; String() string } // []*data
+</pre>
+
+<p>
+Examples of interfaces without core types:
+</p>
+
+<pre>
+interface{} // no single underlying type
+interface{ Celsius|float64 } // no single underlying type
+interface{ chan int | chan<- string } // channels have different element types
+interface{ <-chan int | chan<- int } // directional channels have different directions
+</pre>
+
+<h3 id="Type_identity">Type identity</h3>
+
+<p>
+Two types are either <i>identical</i> or <i>different</i>.
+</p>
+
+<p>
+A <a href="#Types">named type</a> is always different from any other type.
+Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
+structurally equivalent; that is, they have the same literal structure and corresponding
+components have identical types. In detail:
+</p>
+
+<ul>
+ <li>Two array types are identical if they have identical element types and
+ the same array length.</li>
+
+ <li>Two slice types are identical if they have identical element types.</li>
+
+ <li>Two struct types are identical if they have the same sequence of fields,
+ and if corresponding fields have the same names, and identical types,
+ and identical tags.
+ <a href="#Exported_identifiers">Non-exported</a> field names from different
+ packages are always different.</li>
+
+ <li>Two pointer types are identical if they have identical base types.</li>
+
+ <li>Two function types are identical if they have the same number of parameters
+ and result values, corresponding parameter and result types are
+ identical, and either both functions are variadic or neither is.
+ Parameter and result names are not required to match.</li>
+
+ <li>Two interface types are identical if they define the same type set.
+ </li>
+
+ <li>Two map types are identical if they have identical key and element types.</li>
+
+ <li>Two channel types are identical if they have identical element types and
+ the same direction.</li>
+
+ <li>Two <a href="#Instantiations">instantiated</a> types are identical if
+ their defined types and all type arguments are identical.
+ </li>
+</ul>
+
+<p>
+Given the declarations
+</p>
+
+<pre>
+type (
+ A0 = []string
+ A1 = A0
+ A2 = struct{ a, b int }
+ A3 = int
+ A4 = func(A3, float64) *A0
+ A5 = func(x int, _ float64) *[]string
+
+ B0 A0
+ B1 []string
+ B2 struct{ a, b int }
+ B3 struct{ a, c int }
+ B4 func(int, float64) *B0
+ B5 func(x int, y float64) *A1
+
+ C0 = B0
+ D0[P1, P2 any] struct{ x P1; y P2 }
+ E0 = D0[int, string]
+)
+</pre>
+
+<p>
+these types are identical:
+</p>
+
+<pre>
+A0, A1, and []string
+A2 and struct{ a, b int }
+A3 and int
+A4, func(int, float64) *[]string, and A5
+
+B0 and C0
+D0[int, string] and E0
+[]int and []int
+struct{ a, b *T5 } and struct{ a, b *T5 }
+func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
+</pre>
+
+<p>
+<code>B0</code> and <code>B1</code> are different because they are new types
+created by distinct <a href="#Type_definitions">type definitions</a>;
+<code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
+are different because <code>B0</code> is different from <code>[]string</code>;
+and <code>P1</code> and <code>P2</code> are different because they are different
+type parameters.
+<code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are
+different because the former is an <a href="#Instantiations">instantiated</a>
+defined type while the latter is a type literal
+(but they are still <a href="#Assignability">assignable</a>).
+</p>
+
+<h3 id="Assignability">Assignability</h3>
+
+<p>
+A value <code>x</code> of type <code>V</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
+("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
+</p>
+
+<ul>
+<li>
+<code>V</code> and <code>T</code> are identical.
+</li>
+<li>
+<code>V</code> and <code>T</code> have identical
+<a href="#Underlying_types">underlying types</a> and at least one of <code>V</code>
+or <code>T</code> is not a <a href="#Types">named type</a>.
+</li>
+<li>
+<code>V</code> and <code>T</code> are channel types with
+identical element types, <code>V</code> is a bidirectional channel,
+and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
+</li>
+<li>
+<code>T</code> is an interface type, but not a type parameter, and
+<code>x</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
+</li>
+<li>
+<code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
+is a pointer, function, slice, map, channel, or interface type,
+but not a type parameter.
+</li>
+<li>
+<code>x</code> is an untyped <a href="#Constants">constant</a>
+<a href="#Representability">representable</a>
+by a value of type <code>T</code>.
+</li>
+</ul>
+
+<p>
+Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code>
+is assignable to a variable of type <code>T</code> if one of the following conditions applies:
+</p>
+
+<ul>
+<li>
+<code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
+a type parameter, and <code>x</code> is assignable to each type in
+<code>T</code>'s type set.
+</li>
+<li>
+<code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
+a type parameter, and <code>x</code> is assignable to each type in
+<code>T</code>'s type set.
+</li>
+<li>
+<code>V</code> is a type parameter and <code>T</code> is not a named type,
+and values of each type in <code>V</code>'s type set are assignable
+to <code>T</code>.
+</li>
+</ul>
+
+<h3 id="Representability">Representability</h3>
+
+<p>
+A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
+by a value of type <code>T</code>,
+where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>,
+if one of the following conditions applies:
+</p>
+
+<ul>
+<li>
+<code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
+</li>
+
+<li>
+<code>T</code> is a <a href="#Numeric_types">floating-point type</a> and <code>x</code> can be rounded to <code>T</code>'s
+precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
+negative zero further simplified to an unsigned zero. Note that constant values never result
+in an IEEE negative zero, NaN, or infinity.
+</li>
+
+<li>
+<code>T</code> is a complex type, and <code>x</code>'s
+<a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
+are representable by values of <code>T</code>'s component type (<code>float32</code> or
+<code>float64</code>).
+</li>
+</ul>
+
+<p>
+If <code>T</code> is a type parameter,
+<code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
+by a value of each type in <code>T</code>'s type set.
+</p>
+
+<pre>
+x T x is representable by a value of T because
+
+'a' byte 97 is in the set of byte values
+97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
+"foo" string "foo" is in the set of string values
+1024 int16 1024 is in the set of 16-bit integers
+42.0 byte 42 is in the set of unsigned 8-bit integers
+1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
+2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
+-1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
+0i int 0 is an integer value
+(42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
+</pre>
+
+<pre>
+x T x is not representable by a value of T because
+
+0 bool 0 is not in the set of boolean values
+'a' string 'a' is a rune, it is not in the set of string values
+1024 byte 1024 is not in the set of unsigned 8-bit integers
+-1 uint16 -1 is not in the set of unsigned 16-bit integers
+1.1 int 1.1 is not an integer value
+42i float32 (0 + 42i) is not in the set of float32 values
+1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
+</pre>
+
+<h3 id="Method_sets">Method sets</h3>
+
+<p>
+The <i>method set</i> of a type determines the methods that can be
+<a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type.
+Every type has a (possibly empty) method set associated with it:
+</p>
+
+<ul>
+<li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all
+<a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
+</li>
+
+<li>
+The method set of a pointer to a defined type <code>T</code>
+(where <code>T</code> is neither a pointer nor an interface)
+is the set of all methods declared with receiver <code>*T</code> or <code>T</code>.
+</li>
+
+<li>The method set of an <a href="#Interface_types">interface type</a> is the intersection
+of the method sets of each type in the interface's <a href="#Interface_types">type set</a>
+(the resulting method set is usually just the set of declared methods in the interface).
+</li>
+</ul>
+
+<p>
+Further rules apply to structs (and pointer to structs) containing embedded fields,
+as described in the section on <a href="#Struct_types">struct types</a>.
+Any other type has an empty method set.
+</p>
+
+<p>
+In a method set, each method must have a
+<a href="#Uniqueness_of_identifiers">unique</a>
+non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
+</p>
+
+<h2 id="Blocks">Blocks</h2>
+
+<p>
+A <i>block</i> is a possibly empty sequence of declarations and statements
+within matching brace brackets.
+</p>
+
+<pre class="ebnf">
+Block = "{" StatementList "}" .
+StatementList = { Statement ";" } .
+</pre>
+
+<p>
+In addition to explicit blocks in the source code, there are implicit blocks:
+</p>
+
+<ol>
+ <li>The <i>universe block</i> encompasses all Go source text.</li>
+
+ <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
+ Go source text for that package.</li>
+
+ <li>Each file has a <i>file block</i> containing all Go source text
+ in that file.</li>
+
+ <li>Each <a href="#If_statements">"if"</a>,
+ <a href="#For_statements">"for"</a>, and
+ <a href="#Switch_statements">"switch"</a>
+ statement is considered to be in its own implicit block.</li>
+
+ <li>Each clause in a <a href="#Switch_statements">"switch"</a>
+ or <a href="#Select_statements">"select"</a> statement
+ acts as an implicit block.</li>
+</ol>
+
+<p>
+Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
+</p>
+
+
+<h2 id="Declarations_and_scope">Declarations and scope</h2>
+
+<p>
+A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
+<a href="#Constant_declarations">constant</a>,
+<a href="#Type_declarations">type</a>,
+<a href="#Type_parameter_declarations">type parameter</a>,
+<a href="#Variable_declarations">variable</a>,
+<a href="#Function_declarations">function</a>,
+<a href="#Labeled_statements">label</a>, or
+<a href="#Import_declarations">package</a>.
+Every identifier in a program must be declared.
+No identifier may be declared twice in the same block, and
+no identifier may be declared in both the file and package block.
+</p>
+
+<p>
+The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
+in a declaration, but it does not introduce a binding and thus is not declared.
+In the package block, the identifier <code>init</code> may only be used for
+<a href="#Package_initialization"><code>init</code> function</a> declarations,
+and like the blank identifier it does not introduce a new binding.
+</p>
+
+<pre class="ebnf">
+Declaration = ConstDecl | TypeDecl | VarDecl .
+TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
+</pre>
+
+<p>
+The <i>scope</i> of a declared identifier is the extent of source text in which
+the identifier denotes the specified constant, type, variable, function, label, or package.
+</p>
+
+<p>
+Go is lexically scoped using <a href="#Blocks">blocks</a>:
+</p>
+
+<ol>
+ <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
+
+ <li>The scope of an identifier denoting a constant, type, variable,
+ or function (but not method) declared at top level (outside any
+ function) is the package block.</li>
+
+ <li>The scope of the package name of an imported package is the file block
+ of the file containing the import declaration.</li>
+
+ <li>The scope of an identifier denoting a method receiver, function parameter,
+ or result variable is the function body.</li>
+
+ <li>The scope of an identifier denoting a type parameter of a function
+ or declared by a method receiver is the function body and all parameter lists of the
+ function.
+ </li>
+
+ <li>The scope of an identifier denoting a type parameter of a type
+ begins after the name of the type and ends at the end
+ of the TypeSpec.</li>
+
+ <li>The scope of a constant or variable identifier declared
+ inside a function begins at the end of the ConstSpec or VarSpec
+ (ShortVarDecl for short variable declarations)
+ and ends at the end of the innermost containing block.</li>
+
+ <li>The scope of a type identifier declared inside a function
+ begins at the identifier in the TypeSpec
+ and ends at the end of the innermost containing block.</li>
+</ol>
+
+<p>
+An identifier declared in a block may be redeclared in an inner block.
+While the identifier of the inner declaration is in scope, it denotes
+the entity declared by the inner declaration.
+</p>
+
+<p>
+The <a href="#Package_clause">package clause</a> is not a declaration; the package name
+does not appear in any scope. Its purpose is to identify the files belonging
+to the same <a href="#Packages">package</a> and to specify the default package name for import
+declarations.
+</p>
+
+
+<h3 id="Label_scopes">Label scopes</h3>
+
+<p>
+Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
+used in the <a href="#Break_statements">"break"</a>,
+<a href="#Continue_statements">"continue"</a>, and
+<a href="#Goto_statements">"goto"</a> statements.
+It is illegal to define a label that is never used.
+In contrast to other identifiers, labels are not block scoped and do
+not conflict with identifiers that are not labels. The scope of a label
+is the body of the function in which it is declared and excludes
+the body of any nested function.
+</p>
+
+
+<h3 id="Blank_identifier">Blank identifier</h3>
+
+<p>
+The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
+It serves as an anonymous placeholder instead of a regular (non-blank)
+identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
+as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>.
+</p>
+
+
+<h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
+
+<p>
+The following identifiers are implicitly declared in the
+<a href="#Blocks">universe block</a>:
+</p>
+<pre class="grammar">
+Types:
+ any bool byte comparable
+ complex64 complex128 error float32 float64
+ int int8 int16 int32 int64 rune string
+ uint uint8 uint16 uint32 uint64 uintptr
+
+Constants:
+ true false iota
+
+Zero value:
+ nil
+
+Functions:
+ append cap close complex copy delete imag len
+ make new panic print println real recover
+</pre>
+
+<h3 id="Exported_identifiers">Exported identifiers</h3>
+
+<p>
+An identifier may be <i>exported</i> to permit access to it from another package.
+An identifier is exported if both:
+</p>
+<ol>
+ <li>the first character of the identifier's name is a Unicode upper case
+ letter (Unicode class "Lu"); and</li>
+ <li>the identifier is declared in the <a href="#Blocks">package block</a>
+ or it is a <a href="#Struct_types">field name</a> or
+ <a href="#MethodName">method name</a>.</li>
+</ol>
+<p>
+All other identifiers are not exported.
+</p>
+
+<h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
+
+<p>
+Given a set of identifiers, an identifier is called <i>unique</i> if it is
+<i>different</i> from every other in the set.
+Two identifiers are different if they are spelled differently, or if they
+appear in different <a href="#Packages">packages</a> and are not
+<a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
+</p>
+
+<h3 id="Constant_declarations">Constant declarations</h3>
+
+<p>
+A constant declaration binds a list of identifiers (the names of
+the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
+The number of identifiers must be equal
+to the number of expressions, and the <i>n</i>th identifier on
+the left is bound to the value of the <i>n</i>th expression on the
+right.
+</p>
+
+<pre class="ebnf">
+ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
+ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
+
+IdentifierList = identifier { "," identifier } .
+ExpressionList = Expression { "," Expression } .
+</pre>
+
+<p>
+If the type is present, all constants take the type specified, and
+the expressions must be <a href="#Assignability">assignable</a> to that type,
+which must not be a type parameter.
+If the type is omitted, the constants take the
+individual types of the corresponding expressions.
+If the expression values are untyped <a href="#Constants">constants</a>,
+the declared constants remain untyped and the constant identifiers
+denote the constant values. For instance, if the expression is a
+floating-point literal, the constant identifier denotes a floating-point
+constant, even if the literal's fractional part is zero.
+</p>
+
+<pre>
+const Pi float64 = 3.14159265358979323846
+const zero = 0.0 // untyped floating-point constant
+const (
+ size int64 = 1024
+ eof = -1 // untyped integer constant
+)
+const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
+const u, v float32 = 0, 3 // u = 0.0, v = 3.0
+</pre>
+
+<p>
+Within a parenthesized <code>const</code> declaration list the
+expression list may be omitted from any but the first ConstSpec.
+Such an empty list is equivalent to the textual substitution of the
+first preceding non-empty expression list and its type if any.
+Omitting the list of expressions is therefore equivalent to
+repeating the previous list. The number of identifiers must be equal
+to the number of expressions in the previous list.
+Together with the <a href="#Iota"><code>iota</code> constant generator</a>
+this mechanism permits light-weight declaration of sequential values:
+</p>
+
+<pre>
+const (
+ Sunday = iota
+ Monday
+ Tuesday
+ Wednesday
+ Thursday
+ Friday
+ Partyday
+ numberOfDays // this constant is not exported
+)
+</pre>
+
+
+<h3 id="Iota">Iota</h3>
+
+<p>
+Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
+<code>iota</code> represents successive untyped integer <a href="#Constants">
+constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
+in that constant declaration, starting at zero.
+It can be used to construct a set of related constants:
+</p>
+
+<pre>
+const (
+ c0 = iota // c0 == 0
+ c1 = iota // c1 == 1
+ c2 = iota // c2 == 2
+)
+
+const (
+ a = 1 << iota // a == 1 (iota == 0)
+ b = 1 << iota // b == 2 (iota == 1)
+ c = 3 // c == 3 (iota == 2, unused)
+ d = 1 << iota // d == 8 (iota == 3)
+)
+
+const (
+ u = iota * 42 // u == 0 (untyped integer constant)
+ v float64 = iota * 42 // v == 42.0 (float64 constant)
+ w = iota * 42 // w == 84 (untyped integer constant)
+)
+
+const x = iota // x == 0
+const y = iota // y == 0
+</pre>
+
+<p>
+By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
+</p>
+
+<pre>
+const (
+ bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
+ bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
+ _, _ // (iota == 2, unused)
+ bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
+)
+</pre>
+
+<p>
+This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
+of the last non-empty expression list.
+</p>
+
+
+<h3 id="Type_declarations">Type declarations</h3>
+
+<p>
+A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
+Type declarations come in two forms: alias declarations and type definitions.
+</p>
+
+<pre class="ebnf">
+TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
+TypeSpec = AliasDecl | TypeDef .
+</pre>
+
+<h4 id="Alias_declarations">Alias declarations</h4>
+
+<p>
+An alias declaration binds an identifier to the given type.
+</p>
+
+<pre class="ebnf">
+AliasDecl = identifier "=" Type .
+</pre>
+
+<p>
+Within the <a href="#Declarations_and_scope">scope</a> of
+the identifier, it serves as an <i>alias</i> for the type.
+</p>
+
+<pre>
+type (
+ nodeList = []*Node // nodeList and []*Node are identical types
+ Polar = polar // Polar and polar denote identical types
+)
+</pre>
+
+
+<h4 id="Type_definitions">Type definitions</h4>
+
+<p>
+A type definition creates a new, distinct type with the same
+<a href="#Types">underlying type</a> and operations as the given type
+and binds an identifier, the <i>type name</i>, to it.
+</p>
+
+<pre class="ebnf">
+TypeDef = identifier [ TypeParameters ] Type .
+</pre>
+
+<p>
+The new type is called a <i>defined type</i>.
+It is <a href="#Type_identity">different</a> from any other type,
+including the type it is created from.
+</p>
+
+<pre>
+type (
+ Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
+ polar Point // polar and Point denote different types
+)
+
+type TreeNode struct {
+ left, right *TreeNode
+ value any
+}
+
+type Block interface {
+ BlockSize() int
+ Encrypt(src, dst []byte)
+ Decrypt(src, dst []byte)
+}
+</pre>
+
+<p>
+A defined type may have <a href="#Method_declarations">methods</a> associated with it.
+It does not inherit any methods bound to the given type,
+but the <a href="#Method_sets">method set</a>
+of an interface type or of elements of a composite type remains unchanged:
+</p>
+
+<pre>
+// A Mutex is a data type with two methods, Lock and Unlock.
+type Mutex struct { /* Mutex fields */ }
+func (m *Mutex) Lock() { /* Lock implementation */ }
+func (m *Mutex) Unlock() { /* Unlock implementation */ }
+
+// NewMutex has the same composition as Mutex but its method set is empty.
+type NewMutex Mutex
+
+// The method set of PtrMutex's underlying type *Mutex remains unchanged,
+// but the method set of PtrMutex is empty.
+type PtrMutex *Mutex
+
+// The method set of *PrintableMutex contains the methods
+// Lock and Unlock bound to its embedded field Mutex.
+type PrintableMutex struct {
+ Mutex
+}
+
+// MyBlock is an interface type that has the same method set as Block.
+type MyBlock Block
+</pre>
+
+<p>
+Type definitions may be used to define different boolean, numeric,
+or string types and associate methods with them:
+</p>
+
+<pre>
+type TimeZone int
+
+const (
+ EST TimeZone = -(5 + iota)
+ CST
+ MST
+ PST
+)
+
+func (tz TimeZone) String() string {
+ return fmt.Sprintf("GMT%+dh", tz)
+}
+</pre>
+
+<p>
+If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>,
+the type name denotes a <i>generic type</i>.
+Generic types must be <a href="#Instantiations">instantiated</a> when they
+are used.
+</p>
+
+<pre>
+type List[T any] struct {
+ next *List[T]
+ value T
+}
+</pre>
+
+<p>
+In a type definition the given type cannot be a type parameter.
+</p>
+
+<pre>
+type T[P any] P // illegal: P is a type parameter
+
+func f[T any]() {
+ type L T // illegal: T is a type parameter declared by the enclosing function
+}
+</pre>
+
+<p>
+A generic type may also have <a href="#Method_declarations">methods</a> associated with it.
+In this case, the method receivers must declare the same number of type parameters as
+present in the generic type definition.
+</p>
+
+<pre>
+// The method Len returns the number of elements in the linked list l.
+func (l *List[T]) Len() int { … }
+</pre>
+
+<h3 id="Type_parameter_declarations">Type parameter declarations</h3>
+
+<p>
+A type parameter list declares the <i>type parameters</i> of a generic function or type declaration.
+The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a>
+except that the type parameter names must all be present and the list is enclosed
+in square brackets rather than parentheses.
+</p>
+
+<pre class="ebnf">
+TypeParameters = "[" TypeParamList [ "," ] "]" .
+TypeParamList = TypeParamDecl { "," TypeParamDecl } .
+TypeParamDecl = IdentifierList TypeConstraint .
+</pre>
+
+<p>
+All non-blank names in the list must be unique.
+Each name declares a type parameter, which is a new and different <a href="#Types">named type</a>
+that acts as a place holder for an (as of yet) unknown type in the declaration.
+The type parameter is replaced with a <i>type argument</i> upon
+<a href="#Instantiations">instantiation</a> of the generic function or type.
+</p>
+
+<pre>
+[P any]
+[S interface{ ~[]byte|string }]
+[S ~[]E, E any]
+[P Constraint[int]]
+[_ any]
+</pre>
+
+<p>
+Just as each ordinary function parameter has a parameter type, each type parameter
+has a corresponding (meta-)type which is called its
+<a href="#Type_constraints"><i>type constraint</i></a>.
+</p>
+
+<p>
+A parsing ambiguity arises when the type parameter list for a generic type
+declares a single type parameter <code>P</code> with a constraint <code>C</code>
+such that the text <code>P C</code> forms a valid expression:
+</p>
+
+<pre>
+type T[P *C] …
+type T[P (C)] …
+type T[P *C|Q] …
+…
+</pre>
+
+<p>
+In these rare cases, the type parameter list is indistinguishable from an
+expression and the type declaration is parsed as an array type declaration.
+To resolve the ambiguity, embed the constraint in an
+<a href="#Interface_types">interface</a> or use a trailing comma:
+</p>
+
+<pre>
+type T[P interface{*C}] …
+type T[P *C,] …
+</pre>
+
+<p>
+Type parameters may also be declared by the receiver specification
+of a <a href="#Method_declarations">method declaration</a> associated
+with a generic type.
+</p>
+
+<!--
+This section needs to explain if and what kind of cycles are permitted
+using type parameters in a type parameter list.
+-->
+
+<h4 id="Type_constraints">Type constraints</h4>
+
+<p>
+A type constraint is an <a href="#Interface_types">interface</a> that defines the
+set of permissible type arguments for the respective type parameter and controls the
+operations supported by values of that type parameter.
+</p>
+
+<pre class="ebnf">
+TypeConstraint = TypeElem .
+</pre>
+
+<p>
+If the constraint is an interface literal of the form <code>interface{E}</code> where
+<code>E</code> is an embedded type element (not a method), in a type parameter list
+the enclosing <code>interface{ … }</code> may be omitted for convenience:
+</p>
+
+<pre>
+[T []P] // = [T interface{[]P}]
+[T ~int] // = [T interface{~int}]
+[T int|string] // = [T interface{int|string}]
+type Constraint ~int // illegal: ~int is not inside a type parameter list
+</pre>
+
+<!--
+We should be able to simplify the rules for comparable or delegate some of them
+elsewhere since we have a section that clearly defines how interfaces implement
+other interfaces based on their type sets. But this should get us going for now.
+-->
+
+<p>
+The <a href="#Predeclared_identifiers">predeclared</a>
+<a href="#Interface_types">interface type</a> <code>comparable</code>
+denotes the set of all non-interface types that are
+<a href="#Comparison_operators">comparable</a>. Specifically,
+a type <code>T</code> implements <code>comparable</code> if:
+</p>
+
+<ul>
+<li>
+ <code>T</code> is not an interface type and <code>T</code> supports the operations
+ <code>==</code> and <code>!=</code>; or
+</li>
+<li>
+ <code>T</code> is an interface type and each type in <code>T</code>'s
+ <a href="#Interface_types">type set</a> implements <code>comparable</code>.
+</li>
+</ul>
+
+<p>
+Even though interfaces that are not type parameters can be
+<a href="#Comparison_operators">compared</a>
+(possibly causing a run-time panic) they do not implement
+<code>comparable</code>.
+</p>
+
+<pre>
+int // implements comparable
+[]byte // does not implement comparable (slices cannot be compared)
+interface{} // does not implement comparable (see above)
+interface{ ~int | ~string } // type parameter only: implements comparable
+interface{ comparable } // type parameter only: implements comparable
+interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (not all types in the type set are comparable)
+</pre>
+
+<p>
+The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
+<code>comparable</code> may only be used as type constraints. They cannot be the types of
+values or variables, or components of other, non-interface types.
+</p>
+
+<h3 id="Variable_declarations">Variable declarations</h3>
+
+<p>
+A variable declaration creates one or more <a href="#Variables">variables</a>,
+binds corresponding identifiers to them, and gives each a type and an initial value.
+</p>
+
+<pre class="ebnf">
+VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
+VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
+</pre>
+
+<pre>
+var i int
+var U, V, W float64
+var k = 0
+var x, y float32 = -1, -2
+var (
+ i int
+ u, v, s = 2.0, 3.0, "bar"
+)
+var re, im = complexSqrt(-1)
+var _, found = entries[name] // map lookup; only interested in "found"
+</pre>
+
+<p>
+If a list of expressions is given, the variables are initialized
+with the expressions following the rules for <a href="#Assignments">assignments</a>.
+Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
+</p>
+
+<p>
+If a type is present, each variable is given that type.
+Otherwise, each variable is given the type of the corresponding
+initialization value in the assignment.
+If that value is an untyped constant, it is first implicitly
+<a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
+if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
+The predeclared value <code>nil</code> cannot be used to initialize a variable
+with no explicit type.
+</p>
+
+<pre>
+var d = math.Sin(0.5) // d is float64
+var i = 42 // i is int
+var t, ok = x.(T) // t is T, ok is bool
+var n = nil // illegal
+</pre>
+
+<p>
+Implementation restriction: A compiler may make it illegal to declare a variable
+inside a <a href="#Function_declarations">function body</a> if the variable is
+never used.
+</p>
+
+<h3 id="Short_variable_declarations">Short variable declarations</h3>
+
+<p>
+A <i>short variable declaration</i> uses the syntax:
+</p>
+
+<pre class="ebnf">
+ShortVarDecl = IdentifierList ":=" ExpressionList .
+</pre>
+
+<p>
+It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
+with initializer expressions but no types:
+</p>
+
+<pre class="grammar">
+"var" IdentifierList = ExpressionList .
+</pre>
+
+<pre>
+i, j := 0, 10
+f := func() int { return 7 }
+ch := make(chan int)
+r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
+_, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
+</pre>
+
+<p>
+Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
+variables provided they were originally declared earlier in the same block
+(or the parameter lists if the block is the function body) with the same type,
+and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
+As a consequence, redeclaration can only appear in a multi-variable short declaration.
+Redeclaration does not introduce a new variable; it just assigns a new value to the original.
+</p>
+
+<pre>
+field1, offset := nextField(str, 0)
+field2, offset := nextField(str, offset) // redeclares offset
+a, a := 1, 2 // illegal: double declaration of a or no new variable if a was declared elsewhere
+</pre>
+
+<p>
+Short variable declarations may appear only inside functions.
+In some contexts such as the initializers for
+<a href="#If_statements">"if"</a>,
+<a href="#For_statements">"for"</a>, or
+<a href="#Switch_statements">"switch"</a> statements,
+they can be used to declare local temporary variables.
+</p>
+
+<h3 id="Function_declarations">Function declarations</h3>
+
+<!--
+ Given the importance of functions, this section has always
+ been woefully underdeveloped. Would be nice to expand this
+ a bit.
+-->
+
+<p>
+A function declaration binds an identifier, the <i>function name</i>,
+to a function.
+</p>
+
+<pre class="ebnf">
+FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
+FunctionName = identifier .
+FunctionBody = Block .
+</pre>
+
+<p>
+If the function's <a href="#Function_types">signature</a> declares
+result parameters, the function body's statement list must end in
+a <a href="#Terminating_statements">terminating statement</a>.
+</p>
+
+<pre>
+func IndexRune(s string, r rune) int {
+ for i, c := range s {
+ if c == r {
+ return i
+ }
+ }
+ // invalid: missing return statement
+}
+</pre>
+
+<p>
+If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>,
+the function name denotes a <i>generic function</i>.
+A generic function must be <a href="#Instantiations">instantiated</a> before it can be
+called or used as a value.
+</p>
+
+<pre>
+func min[T ~int|~float64](x, y T) T {
+ if x < y {
+ return x
+ }
+ return y
+}
+</pre>
+
+<p>
+A function declaration without type parameters may omit the body.
+Such a declaration provides the signature for a function implemented outside Go,
+such as an assembly routine.
+</p>
+
+<pre>
+func flushICache(begin, end uintptr) // implemented externally
+</pre>
+
+<h3 id="Method_declarations">Method declarations</h3>
+
+<p>
+A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
+A method declaration binds an identifier, the <i>method name</i>, to a method,
+and associates the method with the receiver's <i>base type</i>.
+</p>
+
+<pre class="ebnf">
+MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
+Receiver = Parameters .
+</pre>
+
+<p>
+The receiver is specified via an extra parameter section preceding the method
+name. That parameter section must declare a single non-variadic parameter, the receiver.
+Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
+pointer to a defined type <code>T</code>, possibly followed by a list of type parameter
+names <code>[P1, P2, …]</code> enclosed in square brackets.
+<code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be
+a pointer or interface type and it must be defined in the same package as the method.
+The method is said to be <i>bound</i> to its receiver base type and the method name
+is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
+or <code>*T</code>.
+</p>
+
+<p>
+A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
+<a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
+If the receiver's value is not referenced inside the body of the method,
+its identifier may be omitted in the declaration. The same applies in
+general to parameters of functions and methods.
+</p>
+
+<p>
+For a base type, the non-blank names of methods bound to it must be unique.
+If the base type is a <a href="#Struct_types">struct type</a>,
+the non-blank method and field names must be distinct.
+</p>
+
+<p>
+Given defined type <code>Point</code> the declarations
+</p>
+
+<pre>
+func (p *Point) Length() float64 {
+ return math.Sqrt(p.x * p.x + p.y * p.y)
+}
+
+func (p *Point) Scale(factor float64) {
+ p.x *= factor
+ p.y *= factor
+}
+</pre>
+
+<p>
+bind the methods <code>Length</code> and <code>Scale</code>,
+with receiver type <code>*Point</code>,
+to the base type <code>Point</code>.
+</p>
+
+<p>
+If the receiver base type is a <a href="#Type_declarations">generic type</a>, the
+receiver specification must declare corresponding type parameters for the method
+to use. This makes the receiver type parameters available to the method.
+Syntactically, this type parameter declaration looks like an
+<a href="#Instantiations">instantiation</a> of the receiver base type: the type
+arguments must be identifiers denoting the type parameters being declared, one
+for each type parameter of the receiver base type.
+The type parameter names do not need to match their corresponding parameter names in the
+receiver base type definition, and all non-blank parameter names must be unique in the
+receiver parameter section and the method signature.
+The receiver type parameter constraints are implied by the receiver base type definition:
+corresponding type parameters have corresponding constraints.
+</p>
+
+<pre>
+type Pair[A, B any] struct {
+ a A
+ b B
+}
+
+func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B
+func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair
+</pre>
+
+<h2 id="Expressions">Expressions</h2>
+
+<p>
+An expression specifies the computation of a value by applying
+operators and functions to operands.
+</p>
+
+<h3 id="Operands">Operands</h3>
+
+<p>
+Operands denote the elementary values in an expression. An operand may be a
+literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
+non-<a href="#Blank_identifier">blank</a> identifier denoting a
+<a href="#Constant_declarations">constant</a>,
+<a href="#Variable_declarations">variable</a>, or
+<a href="#Function_declarations">function</a>,
+or a parenthesized expression.
+</p>
+
+<pre class="ebnf">
+Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
+Literal = BasicLit | CompositeLit | FunctionLit .
+BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
+OperandName = identifier | QualifiedIdent .
+</pre>
+
+<p>
+An operand name denoting a <a href="#Function_declarations">generic function</a>
+may be followed by a list of <a href="#Instantiations">type arguments</a>; the
+resulting operand is an <a href="#Instantiations">instantiated</a> function.
+</p>
+
+<p>
+The <a href="#Blank_identifier">blank identifier</a> may appear as an
+operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
+</p>
+
+<p>
+Implementation restriction: A compiler need not report an error if an operand's
+type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty
+<a href="#Interface_types">type set</a>. Functions with such type parameters
+cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead
+to an error at the instantiation site.
+</p>
+
+<h3 id="Qualified_identifiers">Qualified identifiers</h3>
+
+<p>
+A <i>qualified identifier</i> is an identifier qualified with a package name prefix.
+Both the package name and the identifier must not be
+<a href="#Blank_identifier">blank</a>.
+</p>
+
+<pre class="ebnf">
+QualifiedIdent = PackageName "." identifier .
+</pre>
+
+<p>
+A qualified identifier accesses an identifier in a different package, which
+must be <a href="#Import_declarations">imported</a>.
+The identifier must be <a href="#Exported_identifiers">exported</a> and
+declared in the <a href="#Blocks">package block</a> of that package.
+</p>
+
+<pre>
+math.Sin // denotes the Sin function in package math
+</pre>
+
+<h3 id="Composite_literals">Composite literals</h3>
+
+<p>
+Composite literals construct new composite values each time they are evaluated.
+They consist of the type of the literal followed by a brace-bound list of elements.
+Each element may optionally be preceded by a corresponding key.
+</p>
+
+<pre class="ebnf">
+CompositeLit = LiteralType LiteralValue .
+LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
+ SliceType | MapType | TypeName .
+LiteralValue = "{" [ ElementList [ "," ] ] "}" .
+ElementList = KeyedElement { "," KeyedElement } .
+KeyedElement = [ Key ":" ] Element .
+Key = FieldName | Expression | LiteralValue .
+FieldName = identifier .
+Element = Expression | LiteralValue .
+</pre>
+
+<p>
+The LiteralType's <a href="#Core_types">core type</a> <code>T</code>
+must be a struct, array, slice, or map type
+(the grammar enforces this constraint except when the type is given
+as a TypeName).
+The types of the elements and keys must be <a href="#Assignability">assignable</a>
+to the respective field, element, and key types of type <code>T</code>;
+there is no additional conversion.
+The key is interpreted as a field name for struct literals,
+an index for array and slice literals, and a key for map literals.
+For map literals, all elements must have a key. It is an error
+to specify multiple elements with the same field name or
+constant key value. For non-constant map keys, see the section on
+<a href="#Order_of_evaluation">evaluation order</a>.
+</p>
+
+<p>
+For struct literals the following rules apply:
+</p>
+<ul>
+ <li>A key must be a field name declared in the struct type.
+ </li>
+ <li>An element list that does not contain any keys must
+ list an element for each struct field in the
+ order in which the fields are declared.
+ </li>
+ <li>If any element has a key, every element must have a key.
+ </li>
+ <li>An element list that contains keys does not need to
+ have an element for each struct field. Omitted fields
+ get the zero value for that field.
+ </li>
+ <li>A literal may omit the element list; such a literal evaluates
+ to the zero value for its type.
+ </li>
+ <li>It is an error to specify an element for a non-exported
+ field of a struct belonging to a different package.
+ </li>
+</ul>
+
+<p>
+Given the declarations
+</p>
+<pre>
+type Point3D struct { x, y, z float64 }
+type Line struct { p, q Point3D }
+</pre>
+
+<p>
+one may write
+</p>
+
+<pre>
+origin := Point3D{} // zero value for Point3D
+line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
+</pre>
+
+<p>
+For array and slice literals the following rules apply:
+</p>
+<ul>
+ <li>Each element has an associated integer index marking
+ its position in the array.
+ </li>
+ <li>An element with a key uses the key as its index. The
+ key must be a non-negative constant
+ <a href="#Representability">representable</a> by
+ a value of type <code>int</code>; and if it is typed
+ it must be of <a href="#Numeric_types">integer type</a>.
+ </li>
+ <li>An element without a key uses the previous element's index plus one.
+ If the first element has no key, its index is zero.
+ </li>
+</ul>
+
+<p>
+<a href="#Address_operators">Taking the address</a> of a composite literal
+generates a pointer to a unique <a href="#Variables">variable</a> initialized
+with the literal's value.
+</p>
+
+<pre>
+var pointer *Point3D = &Point3D{y: 1000}
+</pre>
+
+<p>
+Note that the <a href="#The_zero_value">zero value</a> for a slice or map
+type is not the same as an initialized but empty value of the same type.
+Consequently, taking the address of an empty slice or map composite literal
+does not have the same effect as allocating a new slice or map value with
+<a href="#Allocation">new</a>.
+</p>
+
+<pre>
+p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
+p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
+</pre>
+
+<p>
+The length of an array literal is the length specified in the literal type.
+If fewer elements than the length are provided in the literal, the missing
+elements are set to the zero value for the array element type.
+It is an error to provide elements with index values outside the index range
+of the array. The notation <code>...</code> specifies an array length equal
+to the maximum element index plus one.
+</p>
+
+<pre>
+buffer := [10]string{} // len(buffer) == 10
+intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
+days := [...]string{"Sat", "Sun"} // len(days) == 2
+</pre>
+
+<p>
+A slice literal describes the entire underlying array literal.
+Thus the length and capacity of a slice literal are the maximum
+element index plus one. A slice literal has the form
+</p>
+
+<pre>
+[]T{x1, x2, … xn}
+</pre>
+
+<p>
+and is shorthand for a slice operation applied to an array:
+</p>
+
+<pre>
+tmp := [n]T{x1, x2, … xn}
+tmp[0 : n]
+</pre>
+
+<p>
+Within a composite literal of array, slice, or map type <code>T</code>,
+elements or map keys that are themselves composite literals may elide the respective
+literal type if it is identical to the element or key type of <code>T</code>.
+Similarly, elements or keys that are addresses of composite literals may elide
+the <code>&T</code> when the element or key type is <code>*T</code>.
+</p>
+
+<pre>
+[...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
+[][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
+[][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
+map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
+map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
+
+type PPoint *Point
+[2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
+[2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
+</pre>
+
+<p>
+A parsing ambiguity arises when a composite literal using the
+TypeName form of the LiteralType appears as an operand between the
+<a href="#Keywords">keyword</a> and the opening brace of the block
+of an "if", "for", or "switch" statement, and the composite literal
+is not enclosed in parentheses, square brackets, or curly braces.
+In this rare case, the opening brace of the literal is erroneously parsed
+as the one introducing the block of statements. To resolve the ambiguity,
+the composite literal must appear within parentheses.
+</p>
+
+<pre>
+if x == (T{a,b,c}[i]) { … }
+if (x == T{a,b,c}[i]) { … }
+</pre>
+
+<p>
+Examples of valid array, slice, and map literals:
+</p>
+
+<pre>
+// list of prime numbers
+primes := []int{2, 3, 5, 7, 9, 2147483647}
+
+// vowels[ch] is true if ch is a vowel
+vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
+
+// the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
+filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
+
+// frequencies in Hz for equal-tempered scale (A4 = 440Hz)
+noteFrequency := map[string]float32{
+ "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
+ "G0": 24.50, "A0": 27.50, "B0": 30.87,
+}
+</pre>
+
+
+<h3 id="Function_literals">Function literals</h3>
+
+<p>
+A function literal represents an anonymous <a href="#Function_declarations">function</a>.
+Function literals cannot declare type parameters.
+</p>
+
+<pre class="ebnf">
+FunctionLit = "func" Signature FunctionBody .
+</pre>
+
+<pre>
+func(a, b int, z float64) bool { return a*b < int(z) }
+</pre>
+
+<p>
+A function literal can be assigned to a variable or invoked directly.
+</p>
+
+<pre>
+f := func(x, y int) int { return x + y }
+func(ch chan int) { ch <- ACK }(replyChan)
+</pre>
+
+<p>
+Function literals are <i>closures</i>: they may refer to variables
+defined in a surrounding function. Those variables are then shared between
+the surrounding function and the function literal, and they survive as long
+as they are accessible.
+</p>
+
+
+<h3 id="Primary_expressions">Primary expressions</h3>
+
+<p>
+Primary expressions are the operands for unary and binary expressions.
+</p>
+
+<pre class="ebnf">
+PrimaryExpr =
+ Operand |
+ Conversion |
+ MethodExpr |
+ PrimaryExpr Selector |
+ PrimaryExpr Index |
+ PrimaryExpr Slice |
+ PrimaryExpr TypeAssertion |
+ PrimaryExpr Arguments .
+
+Selector = "." identifier .
+Index = "[" Expression "]" .
+Slice = "[" [ Expression ] ":" [ Expression ] "]" |
+ "[" [ Expression ] ":" Expression ":" Expression "]" .
+TypeAssertion = "." "(" Type ")" .
+Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
+</pre>
+
+
+<pre>
+x
+2
+(s + ".txt")
+f(3.1415, true)
+Point{1, 2}
+m["foo"]
+s[i : j + 1]
+obj.color
+f.p[i].x()
+</pre>
+
+
+<h3 id="Selectors">Selectors</h3>
+
+<p>
+For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
+that is not a <a href="#Package_clause">package name</a>, the
+<i>selector expression</i>
+</p>
+
+<pre>
+x.f
+</pre>
+
+<p>
+denotes the field or method <code>f</code> of the value <code>x</code>
+(or sometimes <code>*x</code>; see below).
+The identifier <code>f</code> is called the (field or method) <i>selector</i>;
+it must not be the <a href="#Blank_identifier">blank identifier</a>.
+The type of the selector expression is the type of <code>f</code>.
+If <code>x</code> is a package name, see the section on
+<a href="#Qualified_identifiers">qualified identifiers</a>.
+</p>
+
+<p>
+A selector <code>f</code> may denote a field or method <code>f</code> of
+a type <code>T</code>, or it may refer
+to a field or method <code>f</code> of a nested
+<a href="#Struct_types">embedded field</a> of <code>T</code>.
+The number of embedded fields traversed
+to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
+The depth of a field or method <code>f</code>
+declared in <code>T</code> is zero.
+The depth of a field or method <code>f</code> declared in
+an embedded field <code>A</code> in <code>T</code> is the
+depth of <code>f</code> in <code>A</code> plus one.
+</p>
+
+<p>
+The following rules apply to selectors:
+</p>
+
+<ol>
+<li>
+For a value <code>x</code> of type <code>T</code> or <code>*T</code>
+where <code>T</code> is not a pointer or interface type,
+<code>x.f</code> denotes the field or method at the shallowest depth
+in <code>T</code> where there is such an <code>f</code>.
+If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
+with shallowest depth, the selector expression is illegal.
+</li>
+
+<li>
+For a value <code>x</code> of type <code>I</code> where <code>I</code>
+is an interface type, <code>x.f</code> denotes the actual method with name
+<code>f</code> of the dynamic value of <code>x</code>.
+If there is no method with name <code>f</code> in the
+<a href="#Method_sets">method set</a> of <code>I</code>, the selector
+expression is illegal.
+</li>
+
+<li>
+As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
+pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
+(but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
+</li>
+
+<li>
+In all other cases, <code>x.f</code> is illegal.
+</li>
+
+<li>
+If <code>x</code> is of pointer type and has the value
+<code>nil</code> and <code>x.f</code> denotes a struct field,
+assigning to or evaluating <code>x.f</code>
+causes a <a href="#Run_time_panics">run-time panic</a>.
+</li>
+
+<li>
+If <code>x</code> is of interface type and has the value
+<code>nil</code>, <a href="#Calls">calling</a> or
+<a href="#Method_values">evaluating</a> the method <code>x.f</code>
+causes a <a href="#Run_time_panics">run-time panic</a>.
+</li>
+</ol>
+
+<p>
+For example, given the declarations:
+</p>
+
+<pre>
+type T0 struct {
+ x int
+}
+
+func (*T0) M0()
+
+type T1 struct {
+ y int
+}
+
+func (T1) M1()
+
+type T2 struct {
+ z int
+ T1
+ *T0
+}
+
+func (*T2) M2()
+
+type Q *T2
+
+var t T2 // with t.T0 != nil
+var p *T2 // with p != nil and (*p).T0 != nil
+var q Q = p
+</pre>
+
+<p>
+one may write:
+</p>
+
+<pre>
+t.z // t.z
+t.y // t.T1.y
+t.x // (*t.T0).x
+
+p.z // (*p).z
+p.y // (*p).T1.y
+p.x // (*(*p).T0).x
+
+q.x // (*(*q).T0).x (*q).x is a valid field selector
+
+p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
+p.M1() // ((*p).T1).M1() M1 expects T1 receiver
+p.M2() // p.M2() M2 expects *T2 receiver
+t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
+</pre>
+
+<p>
+but the following is invalid:
+</p>
+
+<pre>
+q.M0() // (*q).M0 is valid but not a field selector
+</pre>
+
+
+<h3 id="Method_expressions">Method expressions</h3>
+
+<p>
+If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
+<code>T.M</code> is a function that is callable as a regular function
+with the same arguments as <code>M</code> prefixed by an additional
+argument that is the receiver of the method.
+</p>
+
+<pre class="ebnf">
+MethodExpr = ReceiverType "." MethodName .
+ReceiverType = Type .
+</pre>
+
+<p>
+Consider a struct type <code>T</code> with two methods,
+<code>Mv</code>, whose receiver is of type <code>T</code>, and
+<code>Mp</code>, whose receiver is of type <code>*T</code>.
+</p>
+
+<pre>
+type T struct {
+ a int
+}
+func (tv T) Mv(a int) int { return 0 } // value receiver
+func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
+
+var t T
+</pre>
+
+<p>
+The expression
+</p>
+
+<pre>
+T.Mv
+</pre>
+
+<p>
+yields a function equivalent to <code>Mv</code> but
+with an explicit receiver as its first argument; it has signature
+</p>
+
+<pre>
+func(tv T, a int) int
+</pre>
+
+<p>
+That function may be called normally with an explicit receiver, so
+these five invocations are equivalent:
+</p>
+
+<pre>
+t.Mv(7)
+T.Mv(t, 7)
+(T).Mv(t, 7)
+f1 := T.Mv; f1(t, 7)
+f2 := (T).Mv; f2(t, 7)
+</pre>
+
+<p>
+Similarly, the expression
+</p>
+
+<pre>
+(*T).Mp
+</pre>
+
+<p>
+yields a function value representing <code>Mp</code> with signature
+</p>
+
+<pre>
+func(tp *T, f float32) float32
+</pre>
+
+<p>
+For a method with a value receiver, one can derive a function
+with an explicit pointer receiver, so
+</p>
+
+<pre>
+(*T).Mv
+</pre>
+
+<p>
+yields a function value representing <code>Mv</code> with signature
+</p>
+
+<pre>
+func(tv *T, a int) int
+</pre>
+
+<p>
+Such a function indirects through the receiver to create a value
+to pass as the receiver to the underlying method;
+the method does not overwrite the value whose address is passed in
+the function call.
+</p>
+
+<p>
+The final case, a value-receiver function for a pointer-receiver method,
+is illegal because pointer-receiver methods are not in the method set
+of the value type.
+</p>
+
+<p>
+Function values derived from methods are called with function call syntax;
+the receiver is provided as the first argument to the call.
+That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
+as <code>f(t, 7)</code> not <code>t.f(7)</code>.
+To construct a function that binds the receiver, use a
+<a href="#Function_literals">function literal</a> or
+<a href="#Method_values">method value</a>.
+</p>
+
+<p>
+It is legal to derive a function value from a method of an interface type.
+The resulting function takes an explicit receiver of that interface type.
+</p>
+
+<h3 id="Method_values">Method values</h3>
+
+<p>
+If the expression <code>x</code> has static type <code>T</code> and
+<code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
+<code>x.M</code> is called a <i>method value</i>.
+The method value <code>x.M</code> is a function value that is callable
+with the same arguments as a method call of <code>x.M</code>.
+The expression <code>x</code> is evaluated and saved during the evaluation of the
+method value; the saved copy is then used as the receiver in any calls,
+which may be executed later.
+</p>
+
+<pre>
+type S struct { *T }
+type T int
+func (t T) M() { print(t) }
+
+t := new(T)
+s := S{T: t}
+f := t.M // receiver *t is evaluated and stored in f
+g := s.M // receiver *(s.T) is evaluated and stored in g
+*t = 42 // does not affect stored receivers in f and g
+</pre>
+
+<p>
+The type <code>T</code> may be an interface or non-interface type.
+</p>
+
+<p>
+As in the discussion of <a href="#Method_expressions">method expressions</a> above,
+consider a struct type <code>T</code> with two methods,
+<code>Mv</code>, whose receiver is of type <code>T</code>, and
+<code>Mp</code>, whose receiver is of type <code>*T</code>.
+</p>
+
+<pre>
+type T struct {
+ a int
+}
+func (tv T) Mv(a int) int { return 0 } // value receiver
+func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
+
+var t T
+var pt *T
+func makeT() T
+</pre>
+
+<p>
+The expression
+</p>
+
+<pre>
+t.Mv
+</pre>
+
+<p>
+yields a function value of type
+</p>
+
+<pre>
+func(int) int
+</pre>
+
+<p>
+These two invocations are equivalent:
+</p>
+
+<pre>
+t.Mv(7)
+f := t.Mv; f(7)
+</pre>
+
+<p>
+Similarly, the expression
+</p>
+
+<pre>
+pt.Mp
+</pre>
+
+<p>
+yields a function value of type
+</p>
+
+<pre>
+func(float32) float32
+</pre>
+
+<p>
+As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
+using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
+</p>
+
+<p>
+As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
+using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
+</p>
+
+<pre>
+f := t.Mv; f(7) // like t.Mv(7)
+f := pt.Mp; f(7) // like pt.Mp(7)
+f := pt.Mv; f(7) // like (*pt).Mv(7)
+f := t.Mp; f(7) // like (&t).Mp(7)
+f := makeT().Mp // invalid: result of makeT() is not addressable
+</pre>
+
+<p>
+Although the examples above use non-interface types, it is also legal to create a method value
+from a value of interface type.
+</p>
+
+<pre>
+var i interface { M(int) } = myVal
+f := i.M; f(7) // like i.M(7)
+</pre>
+
+
+<h3 id="Index_expressions">Index expressions</h3>
+
+<p>
+A primary expression of the form
+</p>
+
+<pre>
+a[x]
+</pre>
+
+<p>
+denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
+The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
+The following rules apply:
+</p>
+
+<p>
+If <code>a</code> is neither a map nor a type parameter:
+</p>
+<ul>
+ <li>the index <code>x</code> must be an untyped constant or its
+ <a href="#Core_types">core type</a> must be an <a href="#Numeric_types">integer</a></li>
+ <li>a constant index must be non-negative and
+ <a href="#Representability">representable</a> by a value of type <code>int</code></li>
+ <li>a constant index that is untyped is given type <code>int</code></li>
+ <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
+ otherwise it is <i>out of range</i></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
+</p>
+<ul>
+ <li>a <a href="#Constants">constant</a> index must be in range</li>
+ <li>if <code>x</code> is out of range at run time,
+ a <a href="#Run_time_panics">run-time panic</a> occurs</li>
+ <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
+ <code>a[x]</code> is the element type of <code>A</code></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
+</p>
+<ul>
+ <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
+</p>
+<ul>
+ <li>if <code>x</code> is out of range at run time,
+ a <a href="#Run_time_panics">run-time panic</a> occurs</li>
+ <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
+ <code>a[x]</code> is the element type of <code>S</code></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#String_types">string type</a>:
+</p>
+<ul>
+ <li>a <a href="#Constants">constant</a> index must be in range
+ if the string <code>a</code> is also constant</li>
+ <li>if <code>x</code> is out of range at run time,
+ a <a href="#Run_time_panics">run-time panic</a> occurs</li>
+ <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
+ <code>a[x]</code> is <code>byte</code></li>
+ <li><code>a[x]</code> may not be assigned to</li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
+</p>
+<ul>
+ <li><code>x</code>'s type must be
+ <a href="#Assignability">assignable</a>
+ to the key type of <code>M</code></li>
+ <li>if the map contains an entry with key <code>x</code>,
+ <code>a[x]</code> is the map element with key <code>x</code>
+ and the type of <code>a[x]</code> is the element type of <code>M</code></li>
+ <li>if the map is <code>nil</code> or does not contain such an entry,
+ <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
+ for the element type of <code>M</code></li>
+</ul>
+
+<p>
+For <code>a</code> of <a href="#Type_parameter_declarations">type parameter type</a> <code>P</code>:
+</p>
+<ul>
+ <li>The index expression <code>a[x]</code> must be valid for values
+ of all types in <code>P</code>'s type set.</li>
+ <li>The element types of all types in <code>P</code>'s type set must be identical.
+ In this context, the element type of a string type is <code>byte</code>.</li>
+ <li>If there is a map type in the type set of <code>P</code>,
+ all types in that type set must be map types, and the respective key types
+ must be all identical.</li>
+ <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
+ or the map element with key <code>x</code> of the type argument
+ that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
+ the type of the (identical) element types.</li>
+ <li><code>a[x]</code> may not be assigned to if <code>P</code>'s type set
+ includes string types.
+</ul>
+
+<p>
+Otherwise <code>a[x]</code> is illegal.
+</p>
+
+<p>
+An index expression on a map <code>a</code> of type <code>map[K]V</code>
+used in an <a href="#Assignments">assignment</a> or initialization of the special form
+</p>
+
+<pre>
+v, ok = a[x]
+v, ok := a[x]
+var v, ok = a[x]
+</pre>
+
+<p>
+yields an additional untyped boolean value. The value of <code>ok</code> is
+<code>true</code> if the key <code>x</code> is present in the map, and
+<code>false</code> otherwise.
+</p>
+
+<p>
+Assigning to an element of a <code>nil</code> map causes a
+<a href="#Run_time_panics">run-time panic</a>.
+</p>
+
+
+<h3 id="Slice_expressions">Slice expressions</h3>
+
+<p>
+Slice expressions construct a substring or slice from a string, array, pointer
+to array, or slice. There are two variants: a simple form that specifies a low
+and high bound, and a full form that also specifies a bound on the capacity.
+</p>
+
+<h4>Simple slice expressions</h4>
+
+<p>
+The primary expression
+</p>
+
+<pre>
+a[low : high]
+</pre>
+
+<p>
+constructs a substring or slice. The <a href="#Core_types">core type</a> of
+<code>a</code> must be a string, array, pointer to array, or slice.
+The <i>indices</i> <code>low</code> and
+<code>high</code> select which elements of operand <code>a</code> appear
+in the result. The result has indices starting at 0 and length equal to
+<code>high</code> - <code>low</code>.
+After slicing the array <code>a</code>
+</p>
+
+<pre>
+a := [5]int{1, 2, 3, 4, 5}
+s := a[1:4]
+</pre>
+
+<p>
+the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
+</p>
+
+<pre>
+s[0] == 2
+s[1] == 3
+s[2] == 4
+</pre>
+
+<p>
+For convenience, any of the indices may be omitted. A missing <code>low</code>
+index defaults to zero; a missing <code>high</code> index defaults to the length of the
+sliced operand:
+</p>
+
+<pre>
+a[2:] // same as a[2 : len(a)]
+a[:3] // same as a[0 : 3]
+a[:] // same as a[0 : len(a)]
+</pre>
+
+<p>
+If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
+<code>(*a)[low : high]</code>.
+</p>
+
+<p>
+For arrays or strings, the indices are <i>in range</i> if
+<code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
+otherwise they are <i>out of range</i>.
+For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
+A <a href="#Constants">constant</a> index must be non-negative and
+<a href="#Representability">representable</a> by a value of type
+<code>int</code>; for arrays or constant strings, constant indices must also be in range.
+If both indices are constant, they must satisfy <code>low <= high</code>.
+If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<p>
+Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
+the result of the slice operation is a non-constant value of the same type as the operand.
+For untyped string operands the result is a non-constant value of type <code>string</code>.
+If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
+and the result of the slice operation is a slice with the same element type as the array.
+</p>
+
+<p>
+If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
+is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
+array with the operand.
+</p>
+
+<pre>
+var a [10]int
+s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
+s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
+s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
+</pre>
+
+
+<h4>Full slice expressions</h4>
+
+<p>
+The primary expression
+</p>
+
+<pre>
+a[low : high : max]
+</pre>
+
+<p>
+constructs a slice of the same type, and with the same length and elements as the simple slice
+expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
+by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
+The <a href="#Core_types">core type</a> of <code>a</code> must be an array, pointer to array,
+or slice (but not a string).
+After slicing the array <code>a</code>
+</p>
+
+<pre>
+a := [5]int{1, 2, 3, 4, 5}
+t := a[1:3:5]
+</pre>
+
+<p>
+the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
+</p>
+
+<pre>
+t[0] == 2
+t[1] == 3
+</pre>
+
+<p>
+As for simple slice expressions, if <code>a</code> is a pointer to an array,
+<code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
+If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
+</p>
+
+<p>
+The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
+otherwise they are <i>out of range</i>.
+A <a href="#Constants">constant</a> index must be non-negative and
+<a href="#Representability">representable</a> by a value of type
+<code>int</code>; for arrays, constant indices must also be in range.
+If multiple indices are constant, the constants that are present must be in range relative to each
+other.
+If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<h3 id="Type_assertions">Type assertions</h3>
+
+<p>
+For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
+but not a <a href="#Type_parameter_declarations">type parameter</a>, and a type <code>T</code>,
+the primary expression
+</p>
+
+<pre>
+x.(T)
+</pre>
+
+<p>
+asserts that <code>x</code> is not <code>nil</code>
+and that the value stored in <code>x</code> is of type <code>T</code>.
+The notation <code>x.(T)</code> is called a <i>type assertion</i>.
+</p>
+<p>
+More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
+that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
+to the type <code>T</code>.
+In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
+otherwise the type assertion is invalid since it is not possible for <code>x</code>
+to store a value of type <code>T</code>.
+If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
+of <code>x</code> <a href="#Implementing_an_interface">implements</a> the interface <code>T</code>.
+</p>
+<p>
+If the type assertion holds, the value of the expression is the value
+stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+In other words, even though the dynamic type of <code>x</code>
+is known only at run time, the type of <code>x.(T)</code> is
+known to be <code>T</code> in a correct program.
+</p>
+
+<pre>
+var x interface{} = 7 // x has dynamic type int and value 7
+i := x.(int) // i has type int and value 7
+
+type I interface { m() }
+
+func f(y I) {
+ s := y.(string) // illegal: string does not implement I (missing method m)
+ r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
+ …
+}
+</pre>
+
+<p>
+A type assertion used in an <a href="#Assignments">assignment</a> or initialization of the special form
+</p>
+
+<pre>
+v, ok = x.(T)
+v, ok := x.(T)
+var v, ok = x.(T)
+var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
+</pre>
+
+<p>
+yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
+if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
+the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
+No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
+</p>
+
+
+<h3 id="Calls">Calls</h3>
+
+<p>
+Given an expression <code>f</code> with a <a href="#Core_types">core type</a>
+<code>F</code> of <a href="#Function_types">function type</a>,
+</p>
+
+<pre>
+f(a1, a2, … an)
+</pre>
+
+<p>
+calls <code>f</code> with arguments <code>a1, a2, … an</code>.
+Except for one special case, arguments must be single-valued expressions
+<a href="#Assignability">assignable</a> to the parameter types of
+<code>F</code> and are evaluated before the function is called.
+The type of the expression is the result type
+of <code>F</code>.
+A method invocation is similar but the method itself
+is specified as a selector upon a value of the receiver type for
+the method.
+</p>
+
+<pre>
+math.Atan2(x, y) // function call
+var pt *Point
+pt.Scale(3.5) // method call with receiver pt
+</pre>
+
+<p>
+If <code>f</code> denotes a generic function, it must be
+<a href="#Instantiations">instantiated</a> before it can be called
+or used as a function value.
+</p>
+
+<p>
+In a function call, the function value and arguments are evaluated in
+<a href="#Order_of_evaluation">the usual order</a>.
+After they are evaluated, the parameters of the call are passed by value to the function
+and the called function begins execution.
+The return parameters of the function are passed by value
+back to the caller when the function returns.
+</p>
+
+<p>
+Calling a <code>nil</code> function value
+causes a <a href="#Run_time_panics">run-time panic</a>.
+</p>
+
+<p>
+As a special case, if the return values of a function or method
+<code>g</code> are equal in number and individually
+assignable to the parameters of another function or method
+<code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
+will invoke <code>f</code> after binding the return values of
+<code>g</code> to the parameters of <code>f</code> in order. The call
+of <code>f</code> must contain no parameters other than the call of <code>g</code>,
+and <code>g</code> must have at least one return value.
+If <code>f</code> has a final <code>...</code> parameter, it is
+assigned the return values of <code>g</code> that remain after
+assignment of regular parameters.
+</p>
+
+<pre>
+func Split(s string, pos int) (string, string) {
+ return s[0:pos], s[pos:]
+}
+
+func Join(s, t string) string {
+ return s + t
+}
+
+if Join(Split(value, len(value)/2)) != value {
+ log.Panic("test fails")
+}
+</pre>
+
+<p>
+A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
+of (the type of) <code>x</code> contains <code>m</code> and the
+argument list can be assigned to the parameter list of <code>m</code>.
+If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
+set contains <code>m</code>, <code>x.m()</code> is shorthand
+for <code>(&x).m()</code>:
+</p>
+
+<pre>
+var p Point
+p.Scale(3.5)
+</pre>
+
+<p>
+There is no distinct method type and there are no method literals.
+</p>
+
+<h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
+
+<p>
+If <code>f</code> is <a href="#Function_types">variadic</a> with a final
+parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
+the type of <code>p</code> is equivalent to type <code>[]T</code>.
+If <code>f</code> is invoked with no actual arguments for <code>p</code>,
+the value passed to <code>p</code> is <code>nil</code>.
+Otherwise, the value passed is a new slice
+of type <code>[]T</code> with a new underlying array whose successive elements
+are the actual arguments, which all must be <a href="#Assignability">assignable</a>
+to <code>T</code>. The length and capacity of the slice is therefore
+the number of arguments bound to <code>p</code> and may differ for each
+call site.
+</p>
+
+<p>
+Given the function and calls
+</p>
+<pre>
+func Greeting(prefix string, who ...string)
+Greeting("nobody")
+Greeting("hello:", "Joe", "Anna", "Eileen")
+</pre>
+
+<p>
+within <code>Greeting</code>, <code>who</code> will have the value
+<code>nil</code> in the first call, and
+<code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
+</p>
+
+<p>
+If the final argument is assignable to a slice type <code>[]T</code> and
+is followed by <code>...</code>, it is passed unchanged as the value
+for a <code>...T</code> parameter. In this case no new slice is created.
+</p>
+
+<p>
+Given the slice <code>s</code> and call
+</p>
+
+<pre>
+s := []string{"James", "Jasmine"}
+Greeting("goodbye:", s...)
+</pre>
+
+<p>
+within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
+with the same underlying array.
+</p>
+
+<h3 id="Instantiations">Instantiations</h3>
+
+<p>
+A generic function or type is <i>instantiated</i> by substituting <i>type arguments</i>
+for the type parameters.
+Instantiation proceeds in two steps:
+</p>
+
+<ol>
+<li>
+Each type argument is substituted for its corresponding type parameter in the generic
+declaration.
+This substitution happens across the entire function or type declaration,
+including the type parameter list itself and any types in that list.
+</li>
+
+<li>
+After substitution, each type argument must <a href="#Interface_types">implement</a>
+the <a href="#Type_parameter_declarations">constraint</a> (instantiated, if necessary)
+of the corresponding type parameter. Otherwise instantiation fails.
+</li>
+</ol>
+
+<p>
+Instantiating a type results in a new non-generic <a href="#Types">named type</a>;
+instantiating a function produces a new non-generic function.
+</p>
+
+<pre>
+type parameter list type arguments after substitution
+
+[P any] int int implements any
+[S ~[]E, E any] []int, int []int implements ~[]int, int implements any
+[P io.Writer] string illegal: string doesn't implement io.Writer
+</pre>
+
+<p>
+For a generic function, type arguments may be provided explicitly, or they
+may be partially or completely <a href="#Type_inference">inferred</a>.
+A generic function that is is <i>not</i> <a href="#Calls">called</a> requires a
+type argument list for instantiation; if the list is partial, all
+remaining type arguments must be inferrable.
+A generic function that is called may provide a (possibly partial) type
+argument list, or may omit it entirely if the omitted type arguments are
+inferrable from the ordinary (non-type) function arguments.
+</p>
+
+<pre>
+func min[T ~int|~float64](x, y T) T { … }
+
+f := min // illegal: min must be instantiated with type arguments when used without being called
+minInt := min[int] // minInt has type func(x, y int) int
+a := minInt(2, 3) // a has value 2 of type int
+b := min[float64](2.0, 3) // b has value 2.0 of type float64
+c := min(b, -1) // c has value -1.0 of type float64
+</pre>
+
+<p>
+A partial type argument list cannot be empty; at least the first argument must be present.
+The list is a prefix of the full list of type arguments, leaving the remaining arguments
+to be inferred. Loosely speaking, type arguments may be omitted from "right to left".
+</p>
+
+<pre>
+func apply[S ~[]E, E any](s S, f(E) E) S { … }
+
+f0 := apply[] // illegal: type argument list cannot be empty
+f1 := apply[[]int] // type argument for S explicitly provided, type argument for E inferred
+f2 := apply[[]string, string] // both type arguments explicitly provided
+
+var bytes []byte
+r := apply(bytes, func(byte) byte { … }) // both type arguments inferred from the function arguments
+</pre>
+
+<p>
+For a generic type, all type arguments must always be provided explicitly.
+</p>
+
+<h3 id="Type_inference">Type inference</h3>
+
+<p>
+Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
+Each step attempts to use known information to infer additional type arguments.
+Type inference stops as soon as all type arguments are known.
+After type inference is complete, it is still necessary to substitute all type arguments
+for type parameters and verify that each type argument
+<a href="#Implementing_an_interface">implements</a> the relevant constraint;
+it is possible for an inferred type argument to fail to implement a constraint, in which
+case instantiation fails.
+</p>
+
+<p>
+Type inference is based on
+</p>
+
+<ul>
+<li>
+ a <a href="#Type_parameter_declarations">type parameter list</a>
+</li>
+<li>
+ a substitution map <i>M</i> initialized with the known type arguments, if any
+</li>
+<li>
+ a (possibly empty) list of ordinary function arguments (in case of a function call only)
+</li>
+</ul>
+
+<p>
+and then proceeds with the following steps:
+</p>
+
+<ol>
+<li>
+ apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
+ to all <i>typed</i> ordinary function arguments
+</li>
+<li>
+ apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
+</li>
+<li>
+ apply function argument type inference to all <i>untyped</i> ordinary function arguments
+ using the default type for each of the untyped function arguments
+</li>
+<li>
+ apply constraint type inference
+</li>
+</ol>
+
+<p>
+If there are no ordinary or untyped function arguments, the respective steps are skipped.
+Constraint type inference is skipped if the previous step didn't infer any new type arguments,
+but it is run at least once if there are missing type arguments.
+</p>
+
+<p>
+The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
+The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
+If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
+</p>
+
+<h4 id="Type_unification">Type unification</h4>
+
+<p>
+Type inference is based on <i>type unification</i>. A single unification step
+applies to a <a href="#Type_inference">substitution map</a> and two types, either
+or both of which may be or contain type parameters. The substitution map tracks
+the known (explicitly provided or already inferred) type arguments: the map
+contains an entry <code>P</code> → <code>A</code> for each type
+parameter <code>P</code> and corresponding known type argument <code>A</code>.
+During unification, known type arguments take the place of their corresponding type
+parameters when comparing types. Unification is the process of finding substitution
+map entries that make the two types equivalent.
+</p>
+
+<p>
+For unification, two types that don't contain any type parameters from the current type
+parameter list are <i>equivalent</i>
+if they are identical, or if they are channel types that are identical ignoring channel
+direction, or if their underlying types are equivalent.
+</p>
+
+<p>
+Unification works by comparing the structure of pairs of types: their structure
+disregarding type parameters must be identical, and types other than type parameters
+must be equivalent.
+A type parameter in one type may match any complete subtype in the other type;
+each successful match causes an entry to be added to the substitution map.
+If the structure differs, or types other than type parameters are not equivalent,
+unification fails.
+</p>
+
+<!--
+TODO(gri) Somewhere we need to describe the process of adding an entry to the
+ substitution map: if the entry is already present, the type argument
+ values are themselves unified.
+-->
+
+<p>
+For example, if <code>T1</code> and <code>T2</code> are type parameters,
+<code>[]map[int]bool</code> can be unified with any of the following:
+</p>
+
+<pre>
+[]map[int]bool // types are identical
+T1 // adds T1 → []map[int]bool to substitution map
+[]T1 // adds T1 → map[int]bool to substitution map
+[]map[T1]T2 // adds T1 → int and T2 → bool to substitution map
+</pre>
+
+<p>
+On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
+</p>
+
+<pre>
+int // int is not a slice
+struct{} // a struct is not a slice
+[]struct{} // a struct is not a map
+[]map[T1]string // map element types don't match
+</pre>
+
+<p>
+As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
+<code>D</code> and a type literal <code>L</code> are never equivalent,
+unification compares the underlying type of <code>D</code> with <code>L</code> instead.
+For example, given the defined type
+</p>
+
+<pre>
+type Vector []float64
+</pre>
+
+<p>
+and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
+<code>[]E</code> and adds an entry <code>E</code> → <code>float64</code> to
+the substitution map.
+</p>
+
+<h4 id="Function_argument_type_inference">Function argument type inference</h4>
+
+<!-- In this section and the section on constraint type inference we start with examples
+rather than have the examples follow the rules as is customary elsewhere in spec.
+Hopefully this helps building an intuition and makes the rules easier to follow. -->
+
+<p>
+Function argument type inference infers type arguments from function arguments:
+if a function parameter is declared with a type <code>T</code> that uses
+type parameters,
+<a href="#Type_unification">unifying</a> the type of the corresponding
+function argument with <code>T</code> may infer type arguments for the type
+parameters used by <code>T</code>.
+</p>
+
+<p>
+For instance, given the generic function
+</p>
+
+<pre>
+func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
+</pre>
+
+<p>
+and the call
+</p>
+
+<pre>
+var vector []float64
+scaledVector := scale(vector, 42)
+</pre>
+
+<p>
+the type argument for <code>Number</code> can be inferred from the function argument
+<code>vector</code> by unifying the type of <code>vector</code> with the corresponding
+parameter type: <code>[]float64</code> and <code>[]Number</code>
+match in structure and <code>float64</code> matches with <code>Number</code>.
+This adds the entry <code>Number</code> → <code>float64</code> to the
+<a href="#Type_unification">substitution map</a>.
+Untyped arguments, such as the second function argument <code>42</code> here, are ignored
+in the first round of function argument type inference and only considered if there are
+unresolved type parameters left.
+</p>
+
+<p>
+Inference happens in two separate phases; each phase operates on a specific list of
+(parameter, argument) pairs:
+</p>
+
+<ol>
+<li>
+ The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
+ type uses type parameters and where the function argument is <i>typed</i>.
+</li>
+<li>
+ The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
+ type parameter. In this list, the respective function arguments are untyped.
+</li>
+</ol>
+
+<p>
+Any other (parameter, argument) pair is ignored.
+</p>
+
+<p>
+By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
+(or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
+of untyped values are always predeclared non-composite types, they can never match against
+a composite type, so it is sufficient to only consider parameter types that are single type
+parameters.
+</p>
+
+<p>
+Each list is processed in a separate phase:
+</p>
+
+<ol>
+<li>
+ In the first phase, the parameter and argument types of each pair in <i>Lt</i>
+ are unified. If unification succeeds for a pair, it may yield new entries that
+ are added to the substitution map <i>M</i>. If unification fails, type inference
+ fails.
+</li>
+<li>
+ The second phase considers the entries of list <i>Lu</i>. Type parameters for
+ which the type argument has already been determined are ignored in this phase.
+ For each remaining pair, the parameter type (which is a single type parameter) and
+ the <a href="#Constants">default type</a> of the corresponding untyped argument is
+ unified. If unification fails, type inference fails.
+</li>
+</ol>
+
+<p>
+While unification is successful, processing of each list continues until all list elements
+are considered, even if all type arguments are inferred before the last list element has
+been processed.
+</p>
+
+<p>
+Example:
+</p>
+
+<pre>
+func min[T ~int|~float64](x, y T) T
+
+var x int
+min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
+min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
+min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
+</pre>
+
+<p>
+In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
+yields the substitution map entry <code>T</code> → <code>float64</code>. Because
+processing continues until all untyped arguments are considered, an error is reported. This
+ensures that type inference does not depend on the order of the untyped arguments.
+</p>
+
+<h4 id="Constraint_type_inference">Constraint type inference</h4>
+
+<p>
+Constraint type inference infers type arguments by considering type constraints.
+If a type parameter <code>P</code> has a constraint with a
+<a href="#Core_types">core type</a> <code>C</code>,
+<a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
+may infer additional type arguments, either the type argument for <code>P</code>,
+or if that is already known, possibly the type arguments for type parameters
+used in <code>C</code>.
+</p>
+
+<p>
+For instance, consider the type parameter list with type parameters <code>List</code> and
+<code>Elem</code>:
+</p>
+
+<pre>
+[List ~[]Elem, Elem any]
+</pre>
+
+<p>
+Constraint type inference can deduce the type of <code>Elem</code> from the type argument
+for <code>List</code> because <code>Elem</code> is a type parameter in the core type
+<code>[]Elem</code> of <code>List</code>.
+If the type argument is <code>Bytes</code>:
+</p>
+
+<pre>
+type Bytes []byte
+</pre>
+
+<p>
+unifying the underlying type of <code>Bytes</code> with the core type means
+unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
+the <a href="#Type_unification">substitution map</a> entry
+<code>Elem</code> → <code>byte</code>.
+Thus, in this example, constraint type inference can infer the second type argument from the
+first one.
+</p>
+
+<p>
+Using the core type of a constraint may lose some information: In the (unlikely) case that
+the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
+<code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
+<code>N</code> itself. In this case, constraint type inference may succeed but instantiation
+will fail because the inferred type is not in the type set of the constraint.
+Thus, constraint type inference uses the <i>adjusted core type</i> of
+a constraint: if the type set contains a single type, use that type; otherwise use the
+constraint's core type.
+</p>
+
+<p>
+Generally, constraint type inference proceeds in two phases: Starting with a given
+substitution map <i>M</i>
+</p>
+
+<ol>
+<li>
+For all type parameters with an adjusted core type, unify the type parameter with that
+type. If any unification fails, constraint type inference fails.
+</li>
+
+<li>
+At this point, some entries in <i>M</i> may map type parameters to other
+type parameters or to types containing type parameters. For each entry
+<code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
+contains type parameters <code>Q</code> for which there exist entries
+<code>Q</code> → <code>B</code> in <i>M</i>, substitute those
+<code>Q</code> with the respective <code>B</code> in <code>A</code>.
+Stop when no further substitution is possible.
+</li>
+</ol>
+
+<p>
+The result of constraint type inference is the final substitution map <i>M</i> from type
+parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
+appears in any of the <code>A</code>.
+</p>
+
+<p>
+For instance, given the type parameter list
+</p>
+
+<pre>
+[A any, B []C, C *A]
+</pre>
+
+<p>
+and the single provided type argument <code>int</code> for type parameter <code>A</code>,
+the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
+</p>
+
+<p>
+In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
+with the core type of their respective constraints. This adds the entries
+<code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
+to <i>M</i>.
+
+<p>
+At this point there are two entries in <i>M</i> where the right-hand side
+is or contains type parameters for which there exists other entries in <i>M</i>:
+<code>[]C</code> and <code>*A</code>.
+In the second phase, these type parameters are replaced with their respective
+types. It doesn't matter in which order this happens. Starting with the state
+of <i>M</i> after the first phase:
+</p>
+
+<p>
+<code>A</code> → <code>int</code>,
+<code>B</code> → <code>[]C</code>,
+<code>C</code> → <code>*A</code>
+</p>
+
+<p>
+Replace <code>A</code> on the right-hand side of → with <code>int</code>:
+</p>
+
+<p>
+<code>A</code> → <code>int</code>,
+<code>B</code> → <code>[]C</code>,
+<code>C</code> → <code>*int</code>
+</p>
+
+<p>
+Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
+</p>
+
+<p>
+<code>A</code> → <code>int</code>,
+<code>B</code> → <code>[]*int</code>,
+<code>C</code> → <code>*int</code>
+</p>
+
+<p>
+At this point no further substitution is possible and the map is full.
+Therefore, <code>M</code> represents the final map of type parameters
+to type arguments for the given type parameter list.
+</p>
+
+<h3 id="Operators">Operators</h3>
+
+<p>
+Operators combine operands into expressions.
+</p>
+
+<pre class="ebnf">
+Expression = UnaryExpr | Expression binary_op Expression .
+UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
+
+binary_op = "||" | "&&" | rel_op | add_op | mul_op .
+rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
+add_op = "+" | "-" | "|" | "^" .
+mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
+
+unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
+</pre>
+
+<p>
+Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
+For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
+unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
+For operations involving constants only, see the section on
+<a href="#Constant_expressions">constant expressions</a>.
+</p>
+
+<p>
+Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
+and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
+to the type of the other operand.
+</p>
+
+<p>
+The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
+or be an untyped constant <a href="#Representability">representable</a> by a
+value of type <code>uint</code>.
+If the left operand of a non-constant shift expression is an untyped constant,
+it is first implicitly converted to the type it would assume if the shift expression were
+replaced by its left operand alone.
+</p>
+
+<pre>
+var a [1024]byte
+var s uint = 33
+
+// The results of the following examples are given for 64-bit ints.
+var i = 1<<s // 1 has type int
+var j int32 = 1<<s // 1 has type int32; j == 0
+var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
+var m int = 1.0<<s // 1.0 has type int; m == 1<<33
+var n = 1.0<<s == j // 1.0 has type int32; n == true
+var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
+var p = 1<<s == 1<<33 // 1 has type int; p == true
+var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
+var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
+var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
+var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift
+var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift
+var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
+var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
+var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
+
+// The results of the following examples are given for 32-bit ints,
+// which means the shifts will overflow.
+var mm int = 1.0<<s // 1.0 has type int; mm == 0
+var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
+var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
+var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
+var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
+</pre>
+
+<h4 id="Operator_precedence">Operator precedence</h4>
+<p>
+Unary operators have the highest precedence.
+As the <code>++</code> and <code>--</code> operators form
+statements, not expressions, they fall
+outside the operator hierarchy.
+As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
+<p>
+There are five precedence levels for binary operators.
+Multiplication operators bind strongest, followed by addition
+operators, comparison operators, <code>&&</code> (logical AND),
+and finally <code>||</code> (logical OR):
+</p>
+
+<pre class="grammar">
+Precedence Operator
+ 5 * / % << >> & &^
+ 4 + - | ^
+ 3 == != < <= > >=
+ 2 &&
+ 1 ||
+</pre>
+
+<p>
+Binary operators of the same precedence associate from left to right.
+For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
+</p>
+
+<pre>
++x
+23 + 3*x[i]
+x <= f()
+^a >> b
+f() || g()
+x == y+1 && <-chanInt > 0
+</pre>
+
+
+<h3 id="Arithmetic_operators">Arithmetic operators</h3>
+<p>
+Arithmetic operators apply to numeric values and yield a result of the same
+type as the first operand. The four standard arithmetic operators (<code>+</code>,
+<code>-</code>, <code>*</code>, <code>/</code>) apply to
+<a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
+<a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>.
+The bitwise logical and shift operators apply to integers only.
+</p>
+
+<pre class="grammar">
++ sum integers, floats, complex values, strings
+- difference integers, floats, complex values
+* product integers, floats, complex values
+/ quotient integers, floats, complex values
+% remainder integers
+
+& bitwise AND integers
+| bitwise OR integers
+^ bitwise XOR integers
+&^ bit clear (AND NOT) integers
+
+<< left shift integer << integer >= 0
+>> right shift integer >> integer >= 0
+</pre>
+
+<p>
+If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>,
+the operator must apply to each type in that type set.
+The operands are represented as values of the type argument that the type parameter
+is <a href="#Instantiations">instantiated</a> with, and the operation is computed
+with the precision of that type argument. For example, given the function:
+</p>
+
+<pre>
+func dotProduct[F ~float32|~float64](v1, v2 []F) F {
+ var s F
+ for i, x := range v1 {
+ y := v2[i]
+ s += x * y
+ }
+ return s
+}
+</pre>
+
+<p>
+the product <code>x * y</code> and the addition <code>s += x * y</code>
+are computed with <code>float32</code> or <code>float64</code> precision,
+respectively, depending on the type argument for <code>F</code>.
+</p>
+
+<h4 id="Integer_operators">Integer operators</h4>
+
+<p>
+For two integer values <code>x</code> and <code>y</code>, the integer quotient
+<code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
+relationships:
+</p>
+
+<pre>
+x = q*y + r and |r| < |y|
+</pre>
+
+<p>
+with <code>x / y</code> truncated towards zero
+(<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
+</p>
+
+<pre>
+ x y x / y x % y
+ 5 3 1 2
+-5 3 -1 -2
+ 5 -3 -1 2
+-5 -3 1 -2
+</pre>
+
+<p>
+The one exception to this rule is that if the dividend <code>x</code> is
+the most negative value for the int type of <code>x</code>, the quotient
+<code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
+due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
+</p>
+
+<pre>
+ x, q
+int8 -128
+int16 -32768
+int32 -2147483648
+int64 -9223372036854775808
+</pre>
+
+<p>
+If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
+If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
+If the dividend is non-negative and the divisor is a constant power of 2,
+the division may be replaced by a right shift, and computing the remainder may
+be replaced by a bitwise AND operation:
+</p>
+
+<pre>
+ x x / 4 x % 4 x >> 2 x & 3
+ 11 2 3 2 3
+-11 -2 -3 -3 1
+</pre>
+
+<p>
+The shift operators shift the left operand by the shift count specified by the
+right operand, which must be non-negative. If the shift count is negative at run time,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+The shift operators implement arithmetic shifts if the left operand is a signed
+integer and logical shifts if it is an unsigned integer.
+There is no upper limit on the shift count. Shifts behave
+as if the left operand is shifted <code>n</code> times by 1 for a shift
+count of <code>n</code>.
+As a result, <code>x << 1</code> is the same as <code>x*2</code>
+and <code>x >> 1</code> is the same as
+<code>x/2</code> but truncated towards negative infinity.
+</p>
+
+<p>
+For integer operands, the unary operators
+<code>+</code>, <code>-</code>, and <code>^</code> are defined as
+follows:
+</p>
+
+<pre class="grammar">
++x is 0 + x
+-x negation is 0 - x
+^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
+ and m = -1 for signed x
+</pre>
+
+
+<h4 id="Integer_overflow">Integer overflow</h4>
+
+<p>
+For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>,
+<code>-</code>, <code>*</code>, and <code><<</code> are
+computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
+the unsigned integer's type.
+Loosely speaking, these unsigned integer operations
+discard high bits upon overflow, and programs may rely on "wrap around".
+</p>
+
+<p>
+For signed integers, the operations <code>+</code>,
+<code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
+overflow and the resulting value exists and is deterministically defined
+by the signed integer representation, the operation, and its operands.
+Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
+A compiler may not optimize code under the assumption that overflow does
+not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
+</p>
+
+<h4 id="Floating_point_operators">Floating-point operators</h4>
+
+<p>
+For floating-point and complex numbers,
+<code>+x</code> is the same as <code>x</code>,
+while <code>-x</code> is the negation of <code>x</code>.
+The result of a floating-point or complex division by zero is not specified beyond the
+IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
+occurs is implementation-specific.
+</p>
+
+<p>
+An implementation may combine multiple floating-point operations into a single
+fused operation, possibly across statements, and produce a result that differs
+from the value obtained by executing and rounding the instructions individually.
+An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to
+the precision of the target type, preventing fusion that would discard that rounding.
+</p>
+
+<p>
+For instance, some architectures provide a "fused multiply and add" (FMA) instruction
+that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
+These examples show when a Go implementation can use that instruction:
+</p>
+
+<pre>
+// FMA allowed for computing r, because x*y is not explicitly rounded:
+r = x*y + z
+r = z; r += x*y
+t = x*y; r = t + z
+*p = x*y; r = *p + z
+r = x*y + float64(z)
+
+// FMA disallowed for computing r, because it would omit rounding of x*y:
+r = float64(x*y) + z
+r = z; r += float64(x*y)
+t = float64(x*y); r = t + z
+</pre>
+
+<h4 id="String_concatenation">String concatenation</h4>
+
+<p>
+Strings can be concatenated using the <code>+</code> operator
+or the <code>+=</code> assignment operator:
+</p>
+
+<pre>
+s := "hi" + string(c)
+s += " and good bye"
+</pre>
+
+<p>
+String addition creates a new string by concatenating the operands.
+</p>
+
+<h3 id="Comparison_operators">Comparison operators</h3>
+
+<p>
+Comparison operators compare two operands and yield an untyped boolean value.
+</p>
+
+<pre class="grammar">
+== equal
+!= not equal
+< less
+<= less or equal
+> greater
+>= greater or equal
+</pre>
+
+<p>
+In any comparison, the first operand
+must be <a href="#Assignability">assignable</a>
+to the type of the second operand, or vice versa.
+</p>
+<p>
+The equality operators <code>==</code> and <code>!=</code> apply
+to operands that are <i>comparable</i>.
+The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
+apply to operands that are <i>ordered</i>.
+These terms and the result of the comparisons are defined as follows:
+</p>
+
+<ul>
+ <li>
+ Boolean values are comparable.
+ Two boolean values are equal if they are either both
+ <code>true</code> or both <code>false</code>.
+ </li>
+
+ <li>
+ Integer values are comparable and ordered, in the usual way.
+ </li>
+
+ <li>
+ Floating-point values are comparable and ordered,
+ as defined by the IEEE-754 standard.
+ </li>
+
+ <li>
+ Complex values are comparable.
+ Two complex values <code>u</code> and <code>v</code> are
+ equal if both <code>real(u) == real(v)</code> and
+ <code>imag(u) == imag(v)</code>.
+ </li>
+
+ <li>
+ String values are comparable and ordered, lexically byte-wise.
+ </li>
+
+ <li>
+ Pointer values are comparable.
+ Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
+ Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
+ </li>
+
+ <li>
+ Channel values are comparable.
+ Two channel values are equal if they were created by the same call to
+ <a href="#Making_slices_maps_and_channels"><code>make</code></a>
+ or if both have value <code>nil</code>.
+ </li>
+
+ <li>
+ Interface values are comparable.
+ Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
+ and equal dynamic values or if both have value <code>nil</code>.
+ </li>
+
+ <li>
+ A value <code>x</code> of non-interface type <code>X</code> and
+ a value <code>t</code> of interface type <code>T</code> are comparable when values
+ of type <code>X</code> are comparable and
+ <code>X</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
+ They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
+ and <code>t</code>'s dynamic value is equal to <code>x</code>.
+ </li>
+
+ <li>
+ Struct values are comparable if all their fields are comparable.
+ Two struct values are equal if their corresponding
+ non-<a href="#Blank_identifier">blank</a> fields are equal.
+ </li>
+
+ <li>
+ Array values are comparable if values of the array element type are comparable.
+ Two array values are equal if their corresponding elements are equal.
+ </li>
+</ul>
+
+<p>
+A comparison of two interface values with identical dynamic types
+causes a <a href="#Run_time_panics">run-time panic</a> if values
+of that type are not comparable. This behavior applies not only to direct interface
+value comparisons but also when comparing arrays of interface values
+or structs with interface-valued fields.
+</p>
+
+<p>
+Slice, map, and function values are not comparable.
+However, as a special case, a slice, map, or function value may
+be compared to the predeclared identifier <code>nil</code>.
+Comparison of pointer, channel, and interface values to <code>nil</code>
+is also allowed and follows from the general rules above.
+</p>
+
+<pre>
+const c = 3 < 4 // c is the untyped boolean constant true
+
+type MyBool bool
+var x, y int
+var (
+ // The result of a comparison is an untyped boolean.
+ // The usual assignment rules apply.
+ b3 = x == y // b3 has type bool
+ b4 bool = x == y // b4 has type bool
+ b5 MyBool = x == y // b5 has type MyBool
+)
+</pre>
+
+<h3 id="Logical_operators">Logical operators</h3>
+
+<p>
+Logical operators apply to <a href="#Boolean_types">boolean</a> values
+and yield a result of the same type as the operands.
+The right operand is evaluated conditionally.
+</p>
+
+<pre class="grammar">
+&& conditional AND p && q is "if p then q else false"
+|| conditional OR p || q is "if p then true else q"
+! NOT !p is "not p"
+</pre>
+
+
+<h3 id="Address_operators">Address operators</h3>
+
+<p>
+For an operand <code>x</code> of type <code>T</code>, the address operation
+<code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
+The operand must be <i>addressable</i>,
+that is, either a variable, pointer indirection, or slice indexing
+operation; or a field selector of an addressable struct operand;
+or an array indexing operation of an addressable array.
+As an exception to the addressability requirement, <code>x</code> may also be a
+(possibly parenthesized)
+<a href="#Composite_literals">composite literal</a>.
+If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
+then the evaluation of <code>&x</code> does too.
+</p>
+
+<p>
+For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
+indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
+to by <code>x</code>.
+If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
+will cause a <a href="#Run_time_panics">run-time panic</a>.
+</p>
+
+<pre>
+&x
+&a[f(2)]
+&Point{2, 3}
+*p
+*pf(x)
+
+var x *int = nil
+*x // causes a run-time panic
+&*x // causes a run-time panic
+</pre>
+
+
+<h3 id="Receive_operator">Receive operator</h3>
+
+<p>
+For an operand <code>ch</code> whose <a href="#Core_types">core type</a> is a
+<a href="#Channel_types">channel</a>,
+the value of the receive operation <code><-ch</code> is the value received
+from the channel <code>ch</code>. The channel direction must permit receive operations,
+and the type of the receive operation is the element type of the channel.
+The expression blocks until a value is available.
+Receiving from a <code>nil</code> channel blocks forever.
+A receive operation on a <a href="#Close">closed</a> channel can always proceed
+immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
+after any previously sent values have been received.
+</p>
+
+<pre>
+v1 := <-ch
+v2 = <-ch
+f(<-ch)
+<-strobe // wait until clock pulse and discard received value
+</pre>
+
+<p>
+A receive expression used in an <a href="#Assignments">assignment</a> or initialization of the special form
+</p>
+
+<pre>
+x, ok = <-ch
+x, ok := <-ch
+var x, ok = <-ch
+var x, ok T = <-ch
+</pre>
+
+<p>
+yields an additional untyped boolean result reporting whether the
+communication succeeded. The value of <code>ok</code> is <code>true</code>
+if the value received was delivered by a successful send operation to the
+channel, or <code>false</code> if it is a zero value generated because the
+channel is closed and empty.
+</p>
+
+
+<h3 id="Conversions">Conversions</h3>
+
+<p>
+A conversion changes the <a href="#Types">type</a> of an expression
+to the type specified by the conversion.
+A conversion may appear literally in the source, or it may be <i>implied</i>
+by the context in which an expression appears.
+</p>
+
+<p>
+An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
+where <code>T</code> is a type and <code>x</code> is an expression
+that can be converted to type <code>T</code>.
+</p>
+
+<pre class="ebnf">
+Conversion = Type "(" Expression [ "," ] ")" .
+</pre>
+
+<p>
+If the type starts with the operator <code>*</code> or <code><-</code>,
+or if the type starts with the keyword <code>func</code>
+and has no result list, it must be parenthesized when
+necessary to avoid ambiguity:
+</p>
+
+<pre>
+*Point(p) // same as *(Point(p))
+(*Point)(p) // p is converted to *Point
+<-chan int(c) // same as <-(chan int(c))
+(<-chan int)(c) // c is converted to <-chan int
+func()(x) // function signature func() x
+(func())(x) // x is converted to func()
+(func() int)(x) // x is converted to func() int
+func() int(x) // x is converted to func() int (unambiguous)
+</pre>
+
+<p>
+A <a href="#Constants">constant</a> value <code>x</code> can be converted to
+type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
+by a value of <code>T</code>.
+As a special case, an integer constant <code>x</code> can be explicitly converted to a
+<a href="#String_types">string type</a> using the
+<a href="#Conversions_to_and_from_a_string_type">same rule</a>
+as for non-constant <code>x</code>.
+</p>
+
+<p>
+Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a>
+yields a typed constant.
+</p>
+
+<pre>
+uint(iota) // iota value of type uint
+float32(2.718281828) // 2.718281828 of type float32
+complex128(1) // 1.0 + 0.0i of type complex128
+float32(0.49999999) // 0.5 of type float32
+float64(-1e-1000) // 0.0 of type float64
+string('x') // "x" of type string
+string(0x266c) // "♬" of type string
+MyString("foo" + "bar") // "foobar" of type MyString
+string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
+(*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
+int(1.2) // illegal: 1.2 cannot be represented as an int
+string(65.0) // illegal: 65.0 is not an integer constant
+</pre>
+
+<p>
+Converting a constant to a type parameter yields a <i>non-constant</i> value of that type,
+with the value represented as a value of the type argument that the type parameter
+is <a href="#Instantiations">instantiated</a> with.
+For example, given the function:
+</p>
+
+<pre>
+func f[P ~float32|~float64]() {
+ … P(1.1) …
+}
+</pre>
+
+<p>
+the conversion <code>P(1.1)</code> results in a non-constant value of type <code>P</code>
+and the value <code>1.1</code> is represented as a <code>float32</code> or a <code>float64</code>
+depending on the type argument for <code>f</code>.
+Accordingly, if <code>f</code> is instantiated with a <code>float32</code> type,
+the numeric value of the expression <code>P(1.1) + 1.2</code> will be computed
+with the same precision as the corresponding non-constant <code>float32</code>
+addition.
+</p>
+
+<p>
+A non-constant value <code>x</code> can be converted to type <code>T</code>
+in any of these cases:
+</p>
+
+<ul>
+ <li>
+ <code>x</code> is <a href="#Assignability">assignable</a>
+ to <code>T</code>.
+ </li>
+ <li>
+ ignoring struct tags (see below),
+ <code>x</code>'s type and <code>T</code> are not
+ <a href="#Type_parameter_declarations">type parameters</a> but have
+ <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
+ </li>
+ <li>
+ ignoring struct tags (see below),
+ <code>x</code>'s type and <code>T</code> are pointer types
+ that are not <a href="#Types">named types</a>,
+ and their pointer base types are not type parameters but
+ have identical underlying types.
+ </li>
+ <li>
+ <code>x</code>'s type and <code>T</code> are both integer or floating
+ point types.
+ </li>
+ <li>
+ <code>x</code>'s type and <code>T</code> are both complex types.
+ </li>
+ <li>
+ <code>x</code> is an integer or a slice of bytes or runes
+ and <code>T</code> is a string type.
+ </li>
+ <li>
+ <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
+ </li>
+ <li>
+ <code>x</code> is a slice, <code>T</code> is a pointer to an array,
+ and the slice and array types have <a href="#Type_identity">identical</a> element types.
+ </li>
+</ul>
+
+<p>
+Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type
+parameters, <code>x</code>
+can also be converted to type <code>T</code> if one of the following conditions applies:
+</p>
+
+<ul>
+<li>
+Both <code>V</code> and <code>T</code> are type parameters and a value of each
+type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s
+type set.
+</li>
+<li>
+Only <code>V</code> is a type parameter and a value of each
+type in <code>V</code>'s type set can be converted to <code>T</code>.
+</li>
+<li>
+Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
+type in <code>T</code>'s type set.
+</li>
+</ul>
+
+<p>
+<a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
+for identity for the purpose of conversion:
+</p>
+
+<pre>
+type Person struct {
+ Name string
+ Address *struct {
+ Street string
+ City string
+ }
+}
+
+var data *struct {
+ Name string `json:"name"`
+ Address *struct {
+ Street string `json:"street"`
+ City string `json:"city"`
+ } `json:"address"`
+}
+
+var person = (*Person)(data) // ignoring tags, the underlying types are identical
+</pre>
+
+<p>
+Specific rules apply to (non-constant) conversions between numeric types or
+to and from a string type.
+These conversions may change the representation of <code>x</code>
+and incur a run-time cost.
+All other conversions only change the type but not the representation
+of <code>x</code>.
+</p>
+
+<p>
+There is no linguistic mechanism to convert between pointers and integers.
+The package <a href="#Package_unsafe"><code>unsafe</code></a>
+implements this functionality under restricted circumstances.
+</p>
+
+<h4>Conversions between numeric types</h4>
+
+<p>
+For the conversion of non-constant numeric values, the following rules apply:
+</p>
+
+<ol>
+<li>
+When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is
+sign extended to implicit infinite precision; otherwise it is zero extended.
+It is then truncated to fit in the result type's size.
+For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
+The conversion always yields a valid value; there is no indication of overflow.
+</li>
+<li>
+When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded
+(truncation towards zero).
+</li>
+<li>
+When converting an integer or floating-point number to a floating-point type,
+or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded
+to the precision specified by the destination type.
+For instance, the value of a variable <code>x</code> of type <code>float32</code>
+may be stored using additional precision beyond that of an IEEE-754 32-bit number,
+but float32(x) represents the result of rounding <code>x</code>'s value to
+32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
+of precision, but <code>float32(x + 0.1)</code> does not.
+</li>
+</ol>
+
+<p>
+In all non-constant conversions involving floating-point or complex values,
+if the result type cannot represent the value the conversion
+succeeds but the result value is implementation-dependent.
+</p>
+
+<h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
+
+<ol>
+<li>
+Converting a signed or unsigned integer value to a string type yields a
+string containing the UTF-8 representation of the integer. Values outside
+the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
+
+<pre>
+string('a') // "a"
+string(-1) // "\ufffd" == "\xef\xbf\xbd"
+string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
+type MyString string
+MyString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
+</pre>
+</li>
+
+<li>
+Converting a slice of bytes to a string type yields
+a string whose successive bytes are the elements of the slice.
+
+<pre>
+string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
+string([]byte{}) // ""
+string([]byte(nil)) // ""
+
+type MyBytes []byte
+string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
+</pre>
+</li>
+
+<li>
+Converting a slice of runes to a string type yields
+a string that is the concatenation of the individual rune values
+converted to strings.
+
+<pre>
+string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
+string([]rune{}) // ""
+string([]rune(nil)) // ""
+
+type MyRunes []rune
+string(MyRunes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
+</pre>
+</li>
+
+<li>
+Converting a value of a string type to a slice of bytes type
+yields a slice whose successive elements are the bytes of the string.
+
+<pre>
+[]byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+[]byte("") // []byte{}
+
+MyBytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
+</pre>
+</li>
+
+<li>
+Converting a value of a string type to a slice of runes type
+yields a slice containing the individual Unicode code points of the string.
+
+<pre>
+[]rune(MyString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
+[]rune("") // []rune{}
+
+MyRunes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
+</pre>
+</li>
+</ol>
+
+<h4 id="Conversions_from_slice_to_array_pointer">Conversions from slice to array pointer</h4>
+
+<p>
+Converting a slice to an array pointer yields a pointer to the underlying array of the slice.
+If the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<pre>
+s := make([]byte, 2, 4)
+s0 := (*[0]byte)(s) // s0 != nil
+s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
+s2 := (*[2]byte)(s) // &s2[0] == &s[0]
+s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
+
+var t []string
+t0 := (*[0]string)(t) // t0 == nil
+t1 := (*[1]string)(t) // panics: len([1]string) > len(t)
+
+u := make([]byte, 0)
+u0 := (*[0]byte)(u) // u0 != nil
+</pre>
+
+<h3 id="Constant_expressions">Constant expressions</h3>
+
+<p>
+Constant expressions may contain only <a href="#Constants">constant</a>
+operands and are evaluated at compile time.
+</p>
+
+<p>
+Untyped boolean, numeric, and string constants may be used as operands
+wherever it is legal to use an operand of boolean, numeric, or string type,
+respectively.
+</p>
+
+<p>
+A constant <a href="#Comparison_operators">comparison</a> always yields
+an untyped boolean constant. If the left operand of a constant
+<a href="#Operators">shift expression</a> is an untyped constant, the
+result is an integer constant; otherwise it is a constant of the same
+type as the left operand, which must be of
+<a href="#Numeric_types">integer type</a>.
+</p>
+
+<p>
+Any other operation on untyped constants results in an untyped constant of the
+same kind; that is, a boolean, integer, floating-point, complex, or string
+constant.
+If the untyped operands of a binary operation (other than a shift) are of
+different kinds, the result is of the operand's kind that appears later in this
+list: integer, rune, floating-point, complex.
+For example, an untyped integer constant divided by an
+untyped complex constant yields an untyped complex constant.
+</p>
+
+<pre>
+const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
+const b = 15 / 4 // b == 3 (untyped integer constant)
+const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
+const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
+const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
+const d = 1 << 3.0 // d == 8 (untyped integer constant)
+const e = 1.0 << 3 // e == 8 (untyped integer constant)
+const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
+const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
+const h = "foo" > "bar" // h == true (untyped boolean constant)
+const j = true // j == true (untyped boolean constant)
+const k = 'w' + 1 // k == 'x' (untyped rune constant)
+const l = "hi" // l == "hi" (untyped string constant)
+const m = string(k) // m == "x" (type string)
+const Σ = 1 - 0.707i // (untyped complex constant)
+const Δ = Σ + 2.0e-4 // (untyped complex constant)
+const Φ = iota*1i - 1/1i // (untyped complex constant)
+</pre>
+
+<p>
+Applying the built-in function <code>complex</code> to untyped
+integer, rune, or floating-point constants yields
+an untyped complex constant.
+</p>
+
+<pre>
+const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
+const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
+</pre>
+
+<p>
+Constant expressions are always evaluated exactly; intermediate values and the
+constants themselves may require precision significantly larger than supported
+by any predeclared type in the language. The following are legal declarations:
+</p>
+
+<pre>
+const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
+const Four int8 = Huge >> 98 // Four == 4 (type int8)
+</pre>
+
+<p>
+The divisor of a constant division or remainder operation must not be zero:
+</p>
+
+<pre>
+3.14 / 0.0 // illegal: division by zero
+</pre>
+
+<p>
+The values of <i>typed</i> constants must always be accurately
+<a href="#Representability">representable</a> by values
+of the constant type. The following constant expressions are illegal:
+</p>
+
+<pre>
+uint(-1) // -1 cannot be represented as a uint
+int(3.14) // 3.14 cannot be represented as an int
+int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
+Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
+Four * 100 // product 400 cannot be represented as an int8 (type of Four)
+</pre>
+
+<p>
+The mask used by the unary bitwise complement operator <code>^</code> matches
+the rule for non-constants: the mask is all 1s for unsigned constants
+and -1 for signed and untyped constants.
+</p>
+
+<pre>
+^1 // untyped integer constant, equal to -2
+uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
+^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
+int8(^1) // same as int8(-2)
+^int8(1) // same as -1 ^ int8(1) = -2
+</pre>
+
+<p>
+Implementation restriction: A compiler may use rounding while
+computing untyped floating-point or complex constant expressions; see
+the implementation restriction in the section
+on <a href="#Constants">constants</a>. This rounding may cause a
+floating-point constant expression to be invalid in an integer
+context, even if it would be integral when calculated using infinite
+precision, and vice versa.
+</p>
+
+
+<h3 id="Order_of_evaluation">Order of evaluation</h3>
+
+<p>
+At package level, <a href="#Package_initialization">initialization dependencies</a>
+determine the evaluation order of individual initialization expressions in
+<a href="#Variable_declarations">variable declarations</a>.
+Otherwise, when evaluating the <a href="#Operands">operands</a> of an
+expression, assignment, or
+<a href="#Return_statements">return statement</a>,
+all function calls, method calls, and
+communication operations are evaluated in lexical left-to-right
+order.
+</p>
+
+<p>
+For example, in the (function-local) assignment
+</p>
+<pre>
+y[f()], ok = g(h(), i()+x[j()], <-c), k()
+</pre>
+<p>
+the function calls and communication happen in the order
+<code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
+<code><-c</code>, <code>g()</code>, and <code>k()</code>.
+However, the order of those events compared to the evaluation
+and indexing of <code>x</code> and the evaluation
+of <code>y</code> is not specified.
+</p>
+
+<pre>
+a := 1
+f := func() int { a++; return a }
+x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
+m := map[int]int{a: 1, a: 2} // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified
+n := map[int]int{a: f()} // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified
+</pre>
+
+<p>
+At package level, initialization dependencies override the left-to-right rule
+for individual initialization expressions, but not for operands within each
+expression:
+</p>
+
+<pre>
+var a, b, c = f() + v(), g(), sqr(u()) + v()
+
+func f() int { return c }
+func g() int { return a }
+func sqr(x int) int { return x*x }
+
+// functions u and v are independent of all other variables and functions
+</pre>
+
+<p>
+The function calls happen in the order
+<code>u()</code>, <code>sqr()</code>, <code>v()</code>,
+<code>f()</code>, <code>v()</code>, and <code>g()</code>.
+</p>
+
+<p>
+Floating-point operations within a single expression are evaluated according to
+the associativity of the operators. Explicit parentheses affect the evaluation
+by overriding the default associativity.
+In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
+is performed before adding <code>x</code>.
+</p>
+
+<h2 id="Statements">Statements</h2>
+
+<p>
+Statements control execution.
+</p>
+
+<pre class="ebnf">
+Statement =
+ Declaration | LabeledStmt | SimpleStmt |
+ GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
+ FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
+ DeferStmt .
+
+SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
+</pre>
+
+<h3 id="Terminating_statements">Terminating statements</h3>
+
+<p>
+A <i>terminating statement</i> interrupts the regular flow of control in
+a <a href="#Blocks">block</a>. The following statements are terminating:
+</p>
+
+<ol>
+<li>
+ A <a href="#Return_statements">"return"</a> or
+ <a href="#Goto_statements">"goto"</a> statement.
+ <!-- ul below only for regular layout -->
+ <ul> </ul>
+</li>
+
+<li>
+ A call to the built-in function
+ <a href="#Handling_panics"><code>panic</code></a>.
+ <!-- ul below only for regular layout -->
+ <ul> </ul>
+</li>
+
+<li>
+ A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
+ <!-- ul below only for regular layout -->
+ <ul> </ul>
+</li>
+
+<li>
+ An <a href="#If_statements">"if" statement</a> in which:
+ <ul>
+ <li>the "else" branch is present, and</li>
+ <li>both branches are terminating statements.</li>
+ </ul>
+</li>
+
+<li>
+ A <a href="#For_statements">"for" statement</a> in which:
+ <ul>
+ <li>there are no "break" statements referring to the "for" statement, and</li>
+ <li>the loop condition is absent, and</li>
+ <li>the "for" statement does not use a range clause.</li>
+ </ul>
+</li>
+
+<li>
+ A <a href="#Switch_statements">"switch" statement</a> in which:
+ <ul>
+ <li>there are no "break" statements referring to the "switch" statement,</li>
+ <li>there is a default case, and</li>
+ <li>the statement lists in each case, including the default, end in a terminating
+ statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
+ statement</a>.</li>
+ </ul>
+</li>
+
+<li>
+ A <a href="#Select_statements">"select" statement</a> in which:
+ <ul>
+ <li>there are no "break" statements referring to the "select" statement, and</li>
+ <li>the statement lists in each case, including the default if present,
+ end in a terminating statement.</li>
+ </ul>
+</li>
+
+<li>
+ A <a href="#Labeled_statements">labeled statement</a> labeling
+ a terminating statement.
+</li>
+</ol>
+
+<p>
+All other statements are not terminating.
+</p>
+
+<p>
+A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
+is not empty and its final non-empty statement is terminating.
+</p>
+
+
+<h3 id="Empty_statements">Empty statements</h3>
+
+<p>
+The empty statement does nothing.
+</p>
+
+<pre class="ebnf">
+EmptyStmt = .
+</pre>
+
+
+<h3 id="Labeled_statements">Labeled statements</h3>
+
+<p>
+A labeled statement may be the target of a <code>goto</code>,
+<code>break</code> or <code>continue</code> statement.
+</p>
+
+<pre class="ebnf">
+LabeledStmt = Label ":" Statement .
+Label = identifier .
+</pre>
+
+<pre>
+Error: log.Panic("error encountered")
+</pre>
+
+
+<h3 id="Expression_statements">Expression statements</h3>
+
+<p>
+With the exception of specific built-in functions,
+function and method <a href="#Calls">calls</a> and
+<a href="#Receive_operator">receive operations</a>
+can appear in statement context. Such statements may be parenthesized.
+</p>
+
+<pre class="ebnf">
+ExpressionStmt = Expression .
+</pre>
+
+<p>
+The following built-in functions are not permitted in statement context:
+</p>
+
+<pre>
+append cap complex imag len make new real
+unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice
+</pre>
+
+<pre>
+h(x+y)
+f.Close()
+<-ch
+(<-ch)
+len("foo") // illegal if len is the built-in function
+</pre>
+
+
+<h3 id="Send_statements">Send statements</h3>
+
+<p>
+A send statement sends a value on a channel.
+The channel expression's <a href="#Core_types">core type</a>
+must be a <a href="#Channel_types">channel</a>,
+the channel direction must permit send operations,
+and the type of the value to be sent must be <a href="#Assignability">assignable</a>
+to the channel's element type.
+</p>
+
+<pre class="ebnf">
+SendStmt = Channel "<-" Expression .
+Channel = Expression .
+</pre>
+
+<p>
+Both the channel and the value expression are evaluated before communication
+begins. Communication blocks until the send can proceed.
+A send on an unbuffered channel can proceed if a receiver is ready.
+A send on a buffered channel can proceed if there is room in the buffer.
+A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
+A send on a <code>nil</code> channel blocks forever.
+</p>
+
+<pre>
+ch <- 3 // send value 3 to channel ch
+</pre>
+
+
+<h3 id="IncDec_statements">IncDec statements</h3>
+
+<p>
+The "++" and "--" statements increment or decrement their operands
+by the untyped <a href="#Constants">constant</a> <code>1</code>.
+As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
+or a map index expression.
+</p>
+
+<pre class="ebnf">
+IncDecStmt = Expression ( "++" | "--" ) .
+</pre>
+
+<p>
+The following <a href="#Assignments">assignment statements</a> are semantically
+equivalent:
+</p>
+
+<pre class="grammar">
+IncDec statement Assignment
+x++ x += 1
+x-- x -= 1
+</pre>
+
+
+<h3 id="Assignments">Assignments</h3>
+
+<pre class="ebnf">
+Assignment = ExpressionList assign_op ExpressionList .
+
+assign_op = [ add_op | mul_op ] "=" .
+</pre>
+
+<p>
+Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
+a map index expression, or (for <code>=</code> assignments only) the
+<a href="#Blank_identifier">blank identifier</a>.
+Operands may be parenthesized.
+</p>
+
+<pre>
+x = 1
+*p = f()
+a[i] = 23
+(k) = <-ch // same as: k = <-ch
+</pre>
+
+<p>
+An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
+<code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
+is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
+<code>(y)</code> but evaluates <code>x</code>
+only once. The <i>op</i><code>=</code> construct is a single token.
+In assignment operations, both the left- and right-hand expression lists
+must contain exactly one single-valued expression, and the left-hand
+expression must not be the blank identifier.
+</p>
+
+<pre>
+a[i] <<= 2
+i &^= 1<<n
+</pre>
+
+<p>
+A tuple assignment assigns the individual elements of a multi-valued
+operation to a list of variables. There are two forms. In the
+first, the right hand operand is a single multi-valued expression
+such as a function call, a <a href="#Channel_types">channel</a> or
+<a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
+The number of operands on the left
+hand side must match the number of values. For instance, if
+<code>f</code> is a function returning two values,
+</p>
+
+<pre>
+x, y = f()
+</pre>
+
+<p>
+assigns the first value to <code>x</code> and the second to <code>y</code>.
+In the second form, the number of operands on the left must equal the number
+of expressions on the right, each of which must be single-valued, and the
+<i>n</i>th expression on the right is assigned to the <i>n</i>th
+operand on the left:
+</p>
+
+<pre>
+one, two, three = '一', '二', '三'
+</pre>
+
+<p>
+The <a href="#Blank_identifier">blank identifier</a> provides a way to
+ignore right-hand side values in an assignment:
+</p>
+
+<pre>
+_ = x // evaluate x but ignore it
+x, _ = f() // evaluate f() but ignore second result value
+</pre>
+
+<p>
+The assignment proceeds in two phases.
+First, the operands of <a href="#Index_expressions">index expressions</a>
+and <a href="#Address_operators">pointer indirections</a>
+(including implicit pointer indirections in <a href="#Selectors">selectors</a>)
+on the left and the expressions on the right are all
+<a href="#Order_of_evaluation">evaluated in the usual order</a>.
+Second, the assignments are carried out in left-to-right order.
+</p>
+
+<pre>
+a, b = b, a // exchange a and b
+
+x := []int{1, 2, 3}
+i := 0
+i, x[i] = 1, 2 // set i = 1, x[0] = 2
+
+i = 0
+x[i], i = 2, 1 // set x[0] = 2, i = 1
+
+x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
+
+x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
+
+type Point struct { x, y int }
+var p *Point
+x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
+
+i = 2
+x = []int{3, 5, 7}
+for i, x[i] = range x { // set i, x[2] = 0, x[0]
+ break
+}
+// after this loop, i == 0 and x == []int{3, 5, 3}
+</pre>
+
+<p>
+In assignments, each value must be <a href="#Assignability">assignable</a>
+to the type of the operand to which it is assigned, with the following special cases:
+</p>
+
+<ol>
+<li>
+ Any typed value may be assigned to the blank identifier.
+</li>
+
+<li>
+ If an untyped constant
+ is assigned to a variable of interface type or the blank identifier,
+ the constant is first implicitly <a href="#Conversions">converted</a> to its
+ <a href="#Constants">default type</a>.
+</li>
+
+<li>
+ If an untyped boolean value is assigned to a variable of interface type or
+ the blank identifier, it is first implicitly converted to type <code>bool</code>.
+</li>
+</ol>
+
+<h3 id="If_statements">If statements</h3>
+
+<p>
+"If" statements specify the conditional execution of two branches
+according to the value of a boolean expression. If the expression
+evaluates to true, the "if" branch is executed, otherwise, if
+present, the "else" branch is executed.
+</p>
+
+<pre class="ebnf">
+IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
+</pre>
+
+<pre>
+if x > max {
+ x = max
+}
+</pre>
+
+<p>
+The expression may be preceded by a simple statement, which
+executes before the expression is evaluated.
+</p>
+
+<pre>
+if x := f(); x < y {
+ return x
+} else if x > z {
+ return z
+} else {
+ return y
+}
+</pre>
+
+
+<h3 id="Switch_statements">Switch statements</h3>
+
+<p>
+"Switch" statements provide multi-way execution.
+An expression or type is compared to the "cases"
+inside the "switch" to determine which branch
+to execute.
+</p>
+
+<pre class="ebnf">
+SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
+</pre>
+
+<p>
+There are two forms: expression switches and type switches.
+In an expression switch, the cases contain expressions that are compared
+against the value of the switch expression.
+In a type switch, the cases contain types that are compared against the
+type of a specially annotated switch expression.
+The switch expression is evaluated exactly once in a switch statement.
+</p>
+
+<h4 id="Expression_switches">Expression switches</h4>
+
+<p>
+In an expression switch,
+the switch expression is evaluated and
+the case expressions, which need not be constants,
+are evaluated left-to-right and top-to-bottom; the first one that equals the
+switch expression
+triggers execution of the statements of the associated case;
+the other cases are skipped.
+If no case matches and there is a "default" case,
+its statements are executed.
+There can be at most one default case and it may appear anywhere in the
+"switch" statement.
+A missing switch expression is equivalent to the boolean value
+<code>true</code>.
+</p>
+
+<pre class="ebnf">
+ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
+ExprCaseClause = ExprSwitchCase ":" StatementList .
+ExprSwitchCase = "case" ExpressionList | "default" .
+</pre>
+
+<p>
+If the switch expression evaluates to an untyped constant, it is first implicitly
+<a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
+The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
+The switch expression type must be <a href="#Comparison_operators">comparable</a>.
+</p>
+
+<p>
+If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
+to the type of the switch expression.
+For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
+of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
+</p>
+
+<p>
+In other words, the switch expression is treated as if it were used to declare and
+initialize a temporary variable <code>t</code> without explicit type; it is that
+value of <code>t</code> against which each case expression <code>x</code> is tested
+for equality.
+</p>
+
+<p>
+In a case or default clause, the last non-empty statement
+may be a (possibly <a href="#Labeled_statements">labeled</a>)
+<a href="#Fallthrough_statements">"fallthrough" statement</a> to
+indicate that control should flow from the end of this clause to
+the first statement of the next clause.
+Otherwise control flows to the end of the "switch" statement.
+A "fallthrough" statement may appear as the last statement of all
+but the last clause of an expression switch.
+</p>
+
+<p>
+The switch expression may be preceded by a simple statement, which
+executes before the expression is evaluated.
+</p>
+
+<pre>
+switch tag {
+default: s3()
+case 0, 1, 2, 3: s1()
+case 4, 5, 6, 7: s2()
+}
+
+switch x := f(); { // missing switch expression means "true"
+case x < 0: return -x
+default: return x
+}
+
+switch {
+case x < y: f1()
+case x < z: f2()
+case x == 4: f3()
+}
+</pre>
+
+<p>
+Implementation restriction: A compiler may disallow multiple case
+expressions evaluating to the same constant.
+For instance, the current compilers disallow duplicate integer,
+floating point, or string constants in case expressions.
+</p>
+
+<h4 id="Type_switches">Type switches</h4>
+
+<p>
+A type switch compares types rather than values. It is otherwise similar
+to an expression switch. It is marked by a special switch expression that
+has the form of a <a href="#Type_assertions">type assertion</a>
+using the keyword <code>type</code> rather than an actual type:
+</p>
+
+<pre>
+switch x.(type) {
+// cases
+}
+</pre>
+
+<p>
+Cases then match actual types <code>T</code> against the dynamic type of the
+expression <code>x</code>. As with type assertions, <code>x</code> must be of
+<a href="#Interface_types">interface type</a>, but not a
+<a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type
+<code>T</code> listed in a case must implement the type of <code>x</code>.
+The types listed in the cases of a type switch must all be
+<a href="#Type_identity">different</a>.
+</p>
+
+<pre class="ebnf">
+TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
+TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
+TypeCaseClause = TypeSwitchCase ":" StatementList .
+TypeSwitchCase = "case" TypeList | "default" .
+</pre>
+
+<p>
+The TypeSwitchGuard may include a
+<a href="#Short_variable_declarations">short variable declaration</a>.
+When that form is used, the variable is declared at the end of the
+TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
+In clauses with a case listing exactly one type, the variable
+has that type; otherwise, the variable has the type of the expression
+in the TypeSwitchGuard.
+</p>
+
+<p>
+Instead of a type, a case may use the predeclared identifier
+<a href="#Predeclared_identifiers"><code>nil</code></a>;
+that case is selected when the expression in the TypeSwitchGuard
+is a <code>nil</code> interface value.
+There may be at most one <code>nil</code> case.
+</p>
+
+<p>
+Given an expression <code>x</code> of type <code>interface{}</code>,
+the following type switch:
+</p>
+
+<pre>
+switch i := x.(type) {
+case nil:
+ printString("x is nil") // type of i is type of x (interface{})
+case int:
+ printInt(i) // type of i is int
+case float64:
+ printFloat64(i) // type of i is float64
+case func(int) float64:
+ printFunction(i) // type of i is func(int) float64
+case bool, string:
+ printString("type is bool or string") // type of i is type of x (interface{})
+default:
+ printString("don't know the type") // type of i is type of x (interface{})
+}
+</pre>
+
+<p>
+could be rewritten:
+</p>
+
+<pre>
+v := x // x is evaluated exactly once
+if v == nil {
+ i := v // type of i is type of x (interface{})
+ printString("x is nil")
+} else if i, isInt := v.(int); isInt {
+ printInt(i) // type of i is int
+} else if i, isFloat64 := v.(float64); isFloat64 {
+ printFloat64(i) // type of i is float64
+} else if i, isFunc := v.(func(int) float64); isFunc {
+ printFunction(i) // type of i is func(int) float64
+} else {
+ _, isBool := v.(bool)
+ _, isString := v.(string)
+ if isBool || isString {
+ i := v // type of i is type of x (interface{})
+ printString("type is bool or string")
+ } else {
+ i := v // type of i is type of x (interface{})
+ printString("don't know the type")
+ }
+}
+</pre>
+
+<p>
+A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a>
+may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns
+out to duplicate another entry in the switch, the first matching case is chosen.
+</p>
+
+<pre>
+func f[P any](x any) int {
+ switch x.(type) {
+ case P:
+ return 0
+ case string:
+ return 1
+ case []P:
+ return 2
+ case []byte:
+ return 3
+ default:
+ return 4
+ }
+}
+
+var v1 = f[string]("foo") // v1 == 0
+var v2 = f[byte]([]byte{}) // v2 == 2
+</pre>
+
+<p>
+The type switch guard may be preceded by a simple statement, which
+executes before the guard is evaluated.
+</p>
+
+<p>
+The "fallthrough" statement is not permitted in a type switch.
+</p>
+
+<h3 id="For_statements">For statements</h3>
+
+<p>
+A "for" statement specifies repeated execution of a block. There are three forms:
+The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
+</p>
+
+<pre class="ebnf">
+ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
+Condition = Expression .
+</pre>
+
+<h4 id="For_condition">For statements with single condition</h4>
+
+<p>
+In its simplest form, a "for" statement specifies the repeated execution of
+a block as long as a boolean condition evaluates to true.
+The condition is evaluated before each iteration.
+If the condition is absent, it is equivalent to the boolean value
+<code>true</code>.
+</p>
+
+<pre>
+for a < b {
+ a *= 2
+}
+</pre>
+
+<h4 id="For_clause">For statements with <code>for</code> clause</h4>
+
+<p>
+A "for" statement with a ForClause is also controlled by its condition, but
+additionally it may specify an <i>init</i>
+and a <i>post</i> statement, such as an assignment,
+an increment or decrement statement. The init statement may be a
+<a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
+Variables declared by the init statement are re-used in each iteration.
+</p>
+
+<pre class="ebnf">
+ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
+InitStmt = SimpleStmt .
+PostStmt = SimpleStmt .
+</pre>
+
+<pre>
+for i := 0; i < 10; i++ {
+ f(i)
+}
+</pre>
+
+<p>
+If non-empty, the init statement is executed once before evaluating the
+condition for the first iteration;
+the post statement is executed after each execution of the block (and
+only if the block was executed).
+Any element of the ForClause may be empty but the
+<a href="#Semicolons">semicolons</a> are
+required unless there is only a condition.
+If the condition is absent, it is equivalent to the boolean value
+<code>true</code>.
+</p>
+
+<pre>
+for cond { S() } is the same as for ; cond ; { S() }
+for { S() } is the same as for true { S() }
+</pre>
+
+<h4 id="For_range">For statements with <code>range</code> clause</h4>
+
+<p>
+A "for" statement with a "range" clause
+iterates through all entries of an array, slice, string or map,
+or values received on a channel. For each entry it assigns <i>iteration values</i>
+to corresponding <i>iteration variables</i> if present and then executes the block.
+</p>
+
+<pre class="ebnf">
+RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
+</pre>
+
+<p>
+The expression on the right in the "range" clause is called the <i>range expression</i>,
+its <a href="#Core_types">core type</a> must be
+an array, pointer to an array, slice, string, map, or channel permitting
+<a href="#Receive_operator">receive operations</a>.
+As with an assignment, if present the operands on the left must be
+<a href="#Address_operators">addressable</a> or map index expressions; they
+denote the iteration variables. If the range expression is a channel, at most
+one iteration variable is permitted, otherwise there may be up to two.
+If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
+the range clause is equivalent to the same clause without that identifier.
+</p>
+
+<p>
+The range expression <code>x</code> is evaluated once before beginning the loop,
+with one exception: if at most one iteration variable is present and
+<code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
+the range expression is not evaluated.
+</p>
+
+<p>
+Function calls on the left are evaluated once per iteration.
+For each iteration, iteration values are produced as follows
+if the respective iteration variables are present:
+</p>
+
+<pre class="grammar">
+Range expression 1st value 2nd value
+
+array or slice a [n]E, *[n]E, or []E index i int a[i] E
+string s string type index i int see below rune
+map m map[K]V key k K m[k] V
+channel c chan E, <-chan E element e E
+</pre>
+
+<ol>
+<li>
+For an array, pointer to array, or slice value <code>a</code>, the index iteration
+values are produced in increasing order, starting at element index 0.
+If at most one iteration variable is present, the range loop produces
+iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
+or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
+</li>
+
+<li>
+For a string value, the "range" clause iterates over the Unicode code points
+in the string starting at byte index 0. On successive iterations, the index value will be the
+index of the first byte of successive UTF-8-encoded code points in the string,
+and the second value, of type <code>rune</code>, will be the value of
+the corresponding code point. If the iteration encounters an invalid
+UTF-8 sequence, the second value will be <code>0xFFFD</code>,
+the Unicode replacement character, and the next iteration will advance
+a single byte in the string.
+</li>
+
+<li>
+The iteration order over maps is not specified
+and is not guaranteed to be the same from one iteration to the next.
+If a map entry that has not yet been reached is removed during iteration,
+the corresponding iteration value will not be produced. If a map entry is
+created during iteration, that entry may be produced during the iteration or
+may be skipped. The choice may vary for each entry created and from one
+iteration to the next.
+If the map is <code>nil</code>, the number of iterations is 0.
+</li>
+
+<li>
+For channels, the iteration values produced are the successive values sent on
+the channel until the channel is <a href="#Close">closed</a>. If the channel
+is <code>nil</code>, the range expression blocks forever.
+</li>
+</ol>
+
+<p>
+The iteration values are assigned to the respective
+iteration variables as in an <a href="#Assignments">assignment statement</a>.
+</p>
+
+<p>
+The iteration variables may be declared by the "range" clause using a form of
+<a href="#Short_variable_declarations">short variable declaration</a>
+(<code>:=</code>).
+In this case their types are set to the types of the respective iteration values
+and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
+statement; they are re-used in each iteration.
+If the iteration variables are declared outside the "for" statement,
+after execution their values will be those of the last iteration.
+</p>
+
+<pre>
+var testdata *struct {
+ a *[7]int
+}
+for i, _ := range testdata.a {
+ // testdata.a is never evaluated; len(testdata.a) is constant
+ // i ranges from 0 to 6
+ f(i)
+}
+
+var a [10]string
+for i, s := range a {
+ // type of i is int
+ // type of s is string
+ // s == a[i]
+ g(i, s)
+}
+
+var key string
+var val interface{} // element type of m is assignable to val
+m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
+for key, val = range m {
+ h(key, val)
+}
+// key == last map key encountered in iteration
+// val == map[key]
+
+var ch chan Work = producer()
+for w := range ch {
+ doWork(w)
+}
+
+// empty a channel
+for range ch {}
+</pre>
+
+
+<h3 id="Go_statements">Go statements</h3>
+
+<p>
+A "go" statement starts the execution of a function call
+as an independent concurrent thread of control, or <i>goroutine</i>,
+within the same address space.
+</p>
+
+<pre class="ebnf">
+GoStmt = "go" Expression .
+</pre>
+
+<p>
+The expression must be a function or method call; it cannot be parenthesized.
+Calls of built-in functions are restricted as for
+<a href="#Expression_statements">expression statements</a>.
+</p>
+
+<p>
+The function value and parameters are
+<a href="#Calls">evaluated as usual</a>
+in the calling goroutine, but
+unlike with a regular call, program execution does not wait
+for the invoked function to complete.
+Instead, the function begins executing independently
+in a new goroutine.
+When the function terminates, its goroutine also terminates.
+If the function has any return values, they are discarded when the
+function completes.
+</p>
+
+<pre>
+go Server()
+go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
+</pre>
+
+
+<h3 id="Select_statements">Select statements</h3>
+
+<p>
+A "select" statement chooses which of a set of possible
+<a href="#Send_statements">send</a> or
+<a href="#Receive_operator">receive</a>
+operations will proceed.
+It looks similar to a
+<a href="#Switch_statements">"switch"</a> statement but with the
+cases all referring to communication operations.
+</p>
+
+<pre class="ebnf">
+SelectStmt = "select" "{" { CommClause } "}" .
+CommClause = CommCase ":" StatementList .
+CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
+RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
+RecvExpr = Expression .
+</pre>
+
+<p>
+A case with a RecvStmt may assign the result of a RecvExpr to one or
+two variables, which may be declared using a
+<a href="#Short_variable_declarations">short variable declaration</a>.
+The RecvExpr must be a (possibly parenthesized) receive operation.
+There can be at most one default case and it may appear anywhere
+in the list of cases.
+</p>
+
+<p>
+Execution of a "select" statement proceeds in several steps:
+</p>
+
+<ol>
+<li>
+For all the cases in the statement, the channel operands of receive operations
+and the channel and right-hand-side expressions of send statements are
+evaluated exactly once, in source order, upon entering the "select" statement.
+The result is a set of channels to receive from or send to,
+and the corresponding values to send.
+Any side effects in that evaluation will occur irrespective of which (if any)
+communication operation is selected to proceed.
+Expressions on the left-hand side of a RecvStmt with a short variable declaration
+or assignment are not yet evaluated.
+</li>
+
+<li>
+If one or more of the communications can proceed,
+a single one that can proceed is chosen via a uniform pseudo-random selection.
+Otherwise, if there is a default case, that case is chosen.
+If there is no default case, the "select" statement blocks until
+at least one of the communications can proceed.
+</li>
+
+<li>
+Unless the selected case is the default case, the respective communication
+operation is executed.
+</li>
+
+<li>
+If the selected case is a RecvStmt with a short variable declaration or
+an assignment, the left-hand side expressions are evaluated and the
+received value (or values) are assigned.
+</li>
+
+<li>
+The statement list of the selected case is executed.
+</li>
+</ol>
+
+<p>
+Since communication on <code>nil</code> channels can never proceed,
+a select with only <code>nil</code> channels and no default case blocks forever.
+</p>
+
+<pre>
+var a []int
+var c, c1, c2, c3, c4 chan int
+var i1, i2 int
+select {
+case i1 = <-c1:
+ print("received ", i1, " from c1\n")
+case c2 <- i2:
+ print("sent ", i2, " to c2\n")
+case i3, ok := (<-c3): // same as: i3, ok := <-c3
+ if ok {
+ print("received ", i3, " from c3\n")
+ } else {
+ print("c3 is closed\n")
+ }
+case a[f()] = <-c4:
+ // same as:
+ // case t := <-c4
+ // a[f()] = t
+default:
+ print("no communication\n")
+}
+
+for { // send random sequence of bits to c
+ select {
+ case c <- 0: // note: no statement, no fallthrough, no folding of cases
+ case c <- 1:
+ }
+}
+
+select {} // block forever
+</pre>
+
+
+<h3 id="Return_statements">Return statements</h3>
+
+<p>
+A "return" statement in a function <code>F</code> terminates the execution
+of <code>F</code>, and optionally provides one or more result values.
+Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
+are executed before <code>F</code> returns to its caller.
+</p>
+
+<pre class="ebnf">
+ReturnStmt = "return" [ ExpressionList ] .
+</pre>
+
+<p>
+In a function without a result type, a "return" statement must not
+specify any result values.
+</p>
+<pre>
+func noResult() {
+ return
+}
+</pre>
+
+<p>
+There are three ways to return values from a function with a result
+type:
+</p>
+
+<ol>
+ <li>The return value or values may be explicitly listed
+ in the "return" statement. Each expression must be single-valued
+ and <a href="#Assignability">assignable</a>
+ to the corresponding element of the function's result type.
+<pre>
+func simpleF() int {
+ return 2
+}
+
+func complexF1() (re float64, im float64) {
+ return -7.0, -4.0
+}
+</pre>
+ </li>
+ <li>The expression list in the "return" statement may be a single
+ call to a multi-valued function. The effect is as if each value
+ returned from that function were assigned to a temporary
+ variable with the type of the respective value, followed by a
+ "return" statement listing these variables, at which point the
+ rules of the previous case apply.
+<pre>
+func complexF2() (re float64, im float64) {
+ return complexF1()
+}
+</pre>
+ </li>
+ <li>The expression list may be empty if the function's result
+ type specifies names for its <a href="#Function_types">result parameters</a>.
+ The result parameters act as ordinary local variables
+ and the function may assign values to them as necessary.
+ The "return" statement returns the values of these variables.
+<pre>
+func complexF3() (re float64, im float64) {
+ re = 7.0
+ im = 4.0
+ return
+}
+
+func (devnull) Write(p []byte) (n int, _ error) {
+ n = len(p)
+ return
+}
+</pre>
+ </li>
+</ol>
+
+<p>
+Regardless of how they are declared, all the result values are initialized to
+the <a href="#The_zero_value">zero values</a> for their type upon entry to the
+function. A "return" statement that specifies results sets the result parameters before
+any deferred functions are executed.
+</p>
+
+<p>
+Implementation restriction: A compiler may disallow an empty expression list
+in a "return" statement if a different entity (constant, type, or variable)
+with the same name as a result parameter is in
+<a href="#Declarations_and_scope">scope</a> at the place of the return.
+</p>
+
+<pre>
+func f(n int) (res int, err error) {
+ if _, err := f(n-1); err != nil {
+ return // invalid return statement: err is shadowed
+ }
+ return
+}
+</pre>
+
+<h3 id="Break_statements">Break statements</h3>
+
+<p>
+A "break" statement terminates execution of the innermost
+<a href="#For_statements">"for"</a>,
+<a href="#Switch_statements">"switch"</a>, or
+<a href="#Select_statements">"select"</a> statement
+within the same function.
+</p>
+
+<pre class="ebnf">
+BreakStmt = "break" [ Label ] .
+</pre>
+
+<p>
+If there is a label, it must be that of an enclosing
+"for", "switch", or "select" statement,
+and that is the one whose execution terminates.
+</p>
+
+<pre>
+OuterLoop:
+ for i = 0; i < n; i++ {
+ for j = 0; j < m; j++ {
+ switch a[i][j] {
+ case nil:
+ state = Error
+ break OuterLoop
+ case item:
+ state = Found
+ break OuterLoop
+ }
+ }
+ }
+</pre>
+
+<h3 id="Continue_statements">Continue statements</h3>
+
+<p>
+A "continue" statement begins the next iteration of the
+innermost <a href="#For_statements">"for" loop</a> at its post statement.
+The "for" loop must be within the same function.
+</p>
+
+<pre class="ebnf">
+ContinueStmt = "continue" [ Label ] .
+</pre>
+
+<p>
+If there is a label, it must be that of an enclosing
+"for" statement, and that is the one whose execution
+advances.
+</p>
+
+<pre>
+RowLoop:
+ for y, row := range rows {
+ for x, data := range row {
+ if data == endOfRow {
+ continue RowLoop
+ }
+ row[x] = data + bias(x, y)
+ }
+ }
+</pre>
+
+<h3 id="Goto_statements">Goto statements</h3>
+
+<p>
+A "goto" statement transfers control to the statement with the corresponding label
+within the same function.
+</p>
+
+<pre class="ebnf">
+GotoStmt = "goto" Label .
+</pre>
+
+<pre>
+goto Error
+</pre>
+
+<p>
+Executing the "goto" statement must not cause any variables to come into
+<a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
+For instance, this example:
+</p>
+
+<pre>
+ goto L // BAD
+ v := 3
+L:
+</pre>
+
+<p>
+is erroneous because the jump to label <code>L</code> skips
+the creation of <code>v</code>.
+</p>
+
+<p>
+A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
+For instance, this example:
+</p>
+
+<pre>
+if n%2 == 1 {
+ goto L1
+}
+for n > 0 {
+ f()
+ n--
+L1:
+ f()
+ n--
+}
+</pre>
+
+<p>
+is erroneous because the label <code>L1</code> is inside
+the "for" statement's block but the <code>goto</code> is not.
+</p>
+
+<h3 id="Fallthrough_statements">Fallthrough statements</h3>
+
+<p>
+A "fallthrough" statement transfers control to the first statement of the
+next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
+It may be used only as the final non-empty statement in such a clause.
+</p>
+
+<pre class="ebnf">
+FallthroughStmt = "fallthrough" .
+</pre>
+
+
+<h3 id="Defer_statements">Defer statements</h3>
+
+<p>
+A "defer" statement invokes a function whose execution is deferred
+to the moment the surrounding function returns, either because the
+surrounding function executed a <a href="#Return_statements">return statement</a>,
+reached the end of its <a href="#Function_declarations">function body</a>,
+or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
+</p>
+
+<pre class="ebnf">
+DeferStmt = "defer" Expression .
+</pre>
+
+<p>
+The expression must be a function or method call; it cannot be parenthesized.
+Calls of built-in functions are restricted as for
+<a href="#Expression_statements">expression statements</a>.
+</p>
+
+<p>
+Each time a "defer" statement
+executes, the function value and parameters to the call are
+<a href="#Calls">evaluated as usual</a>
+and saved anew but the actual function is not invoked.
+Instead, deferred functions are invoked immediately before
+the surrounding function returns, in the reverse order
+they were deferred. That is, if the surrounding function
+returns through an explicit <a href="#Return_statements">return statement</a>,
+deferred functions are executed <i>after</i> any result parameters are set
+by that return statement but <i>before</i> the function returns to its caller.
+If a deferred function value evaluates
+to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
+when the function is invoked, not when the "defer" statement is executed.
+</p>
+
+<p>
+For instance, if the deferred function is
+a <a href="#Function_literals">function literal</a> and the surrounding
+function has <a href="#Function_types">named result parameters</a> that
+are in scope within the literal, the deferred function may access and modify
+the result parameters before they are returned.
+If the deferred function has any return values, they are discarded when
+the function completes.
+(See also the section on <a href="#Handling_panics">handling panics</a>.)
+</p>
+
+<pre>
+lock(l)
+defer unlock(l) // unlocking happens before surrounding function returns
+
+// prints 3 2 1 0 before surrounding function returns
+for i := 0; i <= 3; i++ {
+ defer fmt.Print(i)
+}
+
+// f returns 42
+func f() (result int) {
+ defer func() {
+ // result is accessed after it was set to 6 by the return statement
+ result *= 7
+ }()
+ return 6
+}
+</pre>
+
+<h2 id="Built-in_functions">Built-in functions</h2>
+
+<p>
+Built-in functions are
+<a href="#Predeclared_identifiers">predeclared</a>.
+They are called like any other function but some of them
+accept a type instead of an expression as the first argument.
+</p>
+
+<p>
+The built-in functions do not have standard Go types,
+so they can only appear in <a href="#Calls">call expressions</a>;
+they cannot be used as function values.
+</p>
+
+<h3 id="Close">Close</h3>
+
+<p>
+For an argument <code>ch</code> with a <a href="#Core_types">core type</a>
+that is a <a href="#Channel_types">channel</a>, the built-in function <code>close</code>
+records that no more values will be sent on the channel.
+It is an error if <code>ch</code> is a receive-only channel.
+Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
+Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
+After calling <code>close</code>, and after any previously
+sent values have been received, receive operations will return
+the zero value for the channel's type without blocking.
+The multi-valued <a href="#Receive_operator">receive operation</a>
+returns a received value along with an indication of whether the channel is closed.
+</p>
+
+<h3 id="Length_and_capacity">Length and capacity</h3>
+
+<p>
+The built-in functions <code>len</code> and <code>cap</code> take arguments
+of various types and return a result of type <code>int</code>.
+The implementation guarantees that the result always fits into an <code>int</code>.
+</p>
+
+<pre class="grammar">
+Call Argument type Result
+
+len(s) string type string length in bytes
+ [n]T, *[n]T array length (== n)
+ []T slice length
+ map[K]T map length (number of defined keys)
+ chan T number of elements queued in channel buffer
+ type parameter see below
+
+cap(s) [n]T, *[n]T array length (== n)
+ []T slice capacity
+ chan T channel buffer capacity
+ type parameter see below
+</pre>
+
+<p>
+If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>,
+the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
+each type in <code>P</code>'s type set.
+The result is the length (or capacity, respectively) of the argument whose type
+corresponds to the type argument with which <code>P</code> was
+<a href="#Instantiations">instantiated</a>.
+</p>
+
+<p>
+The capacity of a slice is the number of elements for which there is
+space allocated in the underlying array.
+At any time the following relationship holds:
+</p>
+
+<pre>
+0 <= len(s) <= cap(s)
+</pre>
+
+<p>
+The length of a <code>nil</code> slice, map or channel is 0.
+The capacity of a <code>nil</code> slice or channel is 0.
+</p>
+
+<p>
+The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
+<code>s</code> is a string constant. The expressions <code>len(s)</code> and
+<code>cap(s)</code> are constants if the type of <code>s</code> is an array
+or pointer to an array and the expression <code>s</code> does not contain
+<a href="#Receive_operator">channel receives</a> or (non-constant)
+<a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
+Otherwise, invocations of <code>len</code> and <code>cap</code> are not
+constant and <code>s</code> is evaluated.
+</p>
+
+<pre>
+const (
+ c1 = imag(2i) // imag(2i) = 2.0 is a constant
+ c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
+ c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
+ c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
+ c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
+)
+var z complex128
+</pre>
+
+<h3 id="Allocation">Allocation</h3>
+
+<p>
+The built-in function <code>new</code> takes a type <code>T</code>,
+allocates storage for a <a href="#Variables">variable</a> of that type
+at run time, and returns a value of type <code>*T</code>
+<a href="#Pointer_types">pointing</a> to it.
+The variable is initialized as described in the section on
+<a href="#The_zero_value">initial values</a>.
+</p>
+
+<pre class="grammar">
+new(T)
+</pre>
+
+<p>
+For instance
+</p>
+
+<pre>
+type S struct { a int; b float64 }
+new(S)
+</pre>
+
+<p>
+allocates storage for a variable of type <code>S</code>,
+initializes it (<code>a=0</code>, <code>b=0.0</code>),
+and returns a value of type <code>*S</code> containing the address
+of the location.
+</p>
+
+<h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
+
+<p>
+The built-in function <code>make</code> takes a type <code>T</code>,
+optionally followed by a type-specific list of expressions.
+The <a href="#Core_types">core type</a> of <code>T</code> must
+be a slice, map or channel.
+It returns a value of type <code>T</code> (not <code>*T</code>).
+The memory is initialized as described in the section on
+<a href="#The_zero_value">initial values</a>.
+</p>
+
+<pre class="grammar">
+Call Core type Result
+
+make(T, n) slice slice of type T with length n and capacity n
+make(T, n, m) slice slice of type T with length n and capacity m
+
+make(T) map map of type T
+make(T, n) map map of type T with initial space for approximately n elements
+
+make(T) channel unbuffered channel of type T
+make(T, n) channel buffered channel of type T, buffer size n
+</pre>
+
+
+<p>
+Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>,
+have a <a href="#Interface_types">type set</a> containing only integer types,
+or be an untyped <a href="#Constants">constant</a>.
+A constant size argument must be non-negative and <a href="#Representability">representable</a>
+by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
+If both <code>n</code> and <code>m</code> are provided and are constant, then
+<code>n</code> must be no larger than <code>m</code>.
+If <code>n</code> is negative or larger than <code>m</code> at run time,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<pre>
+s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
+s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
+s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
+s := make([]int, 10, 0) // illegal: len(s) > cap(s)
+c := make(chan int, 10) // channel with a buffer size of 10
+m := make(map[string]int, 100) // map with initial space for approximately 100 elements
+</pre>
+
+<p>
+Calling <code>make</code> with a map type and size hint <code>n</code> will
+create a map with initial space to hold <code>n</code> map elements.
+The precise behavior is implementation-dependent.
+</p>
+
+
+<h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
+
+<p>
+The built-in functions <code>append</code> and <code>copy</code> assist in
+common slice operations.
+For both functions, the result is independent of whether the memory referenced
+by the arguments overlaps.
+</p>
+
+<p>
+The <a href="#Function_types">variadic</a> function <code>append</code>
+appends zero or more values <code>x</code> to a slice <code>s</code>
+and returns the resulting slice of the same type as <code>s</code>.
+The <a href="#Core_types">core type</a> of <code>s</code> must be a slice
+of type <code>[]E</code>.
+The values <code>x</code> are passed to a parameter of type <code>...E</code>
+and the respective <a href="#Passing_arguments_to_..._parameters">parameter
+passing rules</a> apply.
+As a special case, if the core type of <code>s</code> is <code>[]byte</code>,
+<code>append</code> also accepts a second argument with core type <code>string</code>
+followed by <code>...</code>. This form appends the bytes of the string.
+</p>
+
+<pre class="grammar">
+append(s S, x ...E) S // core type of S is []E
+</pre>
+
+<p>
+If the capacity of <code>s</code> is not large enough to fit the additional
+values, <code>append</code> allocates a new, sufficiently large underlying
+array that fits both the existing slice elements and the additional values.
+Otherwise, <code>append</code> re-uses the underlying array.
+</p>
+
+<pre>
+s0 := []int{0, 0}
+s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
+s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
+s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
+s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
+
+var t []interface{}
+t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"}
+
+var b []byte
+b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
+</pre>
+
+<p>
+The function <code>copy</code> copies slice elements from
+a source <code>src</code> to a destination <code>dst</code> and returns the
+number of elements copied.
+The <a href="#Core_types">core types</a> of both arguments must be slices
+with <a href="#Type_identity">identical</a> element type.
+The number of elements copied is the minimum of
+<code>len(src)</code> and <code>len(dst)</code>.
+As a special case, if the destination's core type is <code>[]byte</code>,
+<code>copy</code> also accepts a source argument with core type <code>string</code>.
+This form copies the bytes from the string into the byte slice.
+</p>
+
+<pre class="grammar">
+copy(dst, src []T) int
+copy(dst []byte, src string) int
+</pre>
+
+<p>
+Examples:
+</p>
+
+<pre>
+var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
+var s = make([]int, 6)
+var b = make([]byte, 5)
+n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
+n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
+n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
+</pre>
+
+
+<h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
+
+<p>
+The built-in function <code>delete</code> removes the element with key
+<code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
+value <code>k</code> must be <a href="#Assignability">assignable</a>
+to the key type of <code>m</code>.
+</p>
+
+<pre class="grammar">
+delete(m, k) // remove element m[k] from map m
+</pre>
+
+<p>
+If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>,
+all types in that type set must be maps, and they must all have identical key types.
+</p>
+
+<p>
+If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
+does not exist, <code>delete</code> is a no-op.
+</p>
+
+
+<h3 id="Complex_numbers">Manipulating complex numbers</h3>
+
+<p>
+Three functions assemble and disassemble complex numbers.
+The built-in function <code>complex</code> constructs a complex
+value from a floating-point real and imaginary part, while
+<code>real</code> and <code>imag</code>
+extract the real and imaginary parts of a complex value.
+</p>
+
+<pre class="grammar">
+complex(realPart, imaginaryPart floatT) complexT
+real(complexT) floatT
+imag(complexT) floatT
+</pre>
+
+<p>
+The type of the arguments and return value correspond.
+For <code>complex</code>, the two arguments must be of the same
+<a href="#Numeric_types">floating-point type</a> and the return type is the
+<a href="#Numeric_types">complex type</a>
+with the corresponding floating-point constituents:
+<code>complex64</code> for <code>float32</code> arguments, and
+<code>complex128</code> for <code>float64</code> arguments.
+If one of the arguments evaluates to an untyped constant, it is first implicitly
+<a href="#Conversions">converted</a> to the type of the other argument.
+If both arguments evaluate to untyped constants, they must be non-complex
+numbers or their imaginary parts must be zero, and the return value of
+the function is an untyped complex constant.
+</p>
+
+<p>
+For <code>real</code> and <code>imag</code>, the argument must be
+of complex type, and the return type is the corresponding floating-point
+type: <code>float32</code> for a <code>complex64</code> argument, and
+<code>float64</code> for a <code>complex128</code> argument.
+If the argument evaluates to an untyped constant, it must be a number,
+and the return value of the function is an untyped floating-point constant.
+</p>
+
+<p>
+The <code>real</code> and <code>imag</code> functions together form the inverse of
+<code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
+<code>z == Z(complex(real(z), imag(z)))</code>.
+</p>
+
+<p>
+If the operands of these functions are all constants, the return
+value is a constant.
+</p>
+
+<pre>
+var a = complex(2, -2) // complex128
+const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
+x := float32(math.Cos(math.Pi/2)) // float32
+var c64 = complex(5, -x) // complex64
+var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
+_ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
+var rl = real(c64) // float32
+var im = imag(a) // float64
+const c = imag(b) // untyped constant -1.4
+_ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
+</pre>
+
+<p>
+Arguments of type parameter type are not permitted.
+</p>
+
+<h3 id="Handling_panics">Handling panics</h3>
+
+<p> Two built-in functions, <code>panic</code> and <code>recover</code>,
+assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
+and program-defined error conditions.
+</p>
+
+<pre class="grammar">
+func panic(interface{})
+func recover() interface{}
+</pre>
+
+<p>
+While executing a function <code>F</code>,
+an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
+terminates the execution of <code>F</code>.
+Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
+are then executed as usual.
+Next, any deferred functions run by <code>F</code>'s caller are run,
+and so on up to any deferred by the top-level function in the executing goroutine.
+At that point, the program is terminated and the error
+condition is reported, including the value of the argument to <code>panic</code>.
+This termination sequence is called <i>panicking</i>.
+</p>
+
+<pre>
+panic(42)
+panic("unreachable")
+panic(Error("cannot parse"))
+</pre>
+
+<p>
+The <code>recover</code> function allows a program to manage behavior
+of a panicking goroutine.
+Suppose a function <code>G</code> defers a function <code>D</code> that calls
+<code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
+is executing.
+When the running of deferred functions reaches <code>D</code>,
+the return value of <code>D</code>'s call to <code>recover</code> will be the value passed to the call of <code>panic</code>.
+If <code>D</code> returns normally, without starting a new
+<code>panic</code>, the panicking sequence stops. In that case,
+the state of functions called between <code>G</code> and the call to <code>panic</code>
+is discarded, and normal execution resumes.
+Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
+execution terminates by returning to its caller.
+</p>
+
+<p>
+The return value of <code>recover</code> is <code>nil</code> if any of the following conditions holds:
+</p>
+<ul>
+<li>
+<code>panic</code>'s argument was <code>nil</code>;
+</li>
+<li>
+the goroutine is not panicking;
+</li>
+<li>
+<code>recover</code> was not called directly by a deferred function.
+</li>
+</ul>
+
+<p>
+The <code>protect</code> function in the example below invokes
+the function argument <code>g</code> and protects callers from
+run-time panics raised by <code>g</code>.
+</p>
+
+<pre>
+func protect(g func()) {
+ defer func() {
+ log.Println("done") // Println executes normally even if there is a panic
+ if x := recover(); x != nil {
+ log.Printf("run time panic: %v", x)
+ }
+ }()
+ log.Println("start")
+ g()
+}
+</pre>
+
+
+<h3 id="Bootstrapping">Bootstrapping</h3>
+
+<p>
+Current implementations provide several built-in functions useful during
+bootstrapping. These functions are documented for completeness but are not
+guaranteed to stay in the language. They do not return a result.
+</p>
+
+<pre class="grammar">
+Function Behavior
+
+print prints all arguments; formatting of arguments is implementation-specific
+println like print but prints spaces between arguments and a newline at the end
+</pre>
+
+<p>
+Implementation restriction: <code>print</code> and <code>println</code> need not
+accept arbitrary argument types, but printing of boolean, numeric, and string
+<a href="#Types">types</a> must be supported.
+</p>
+
+<h2 id="Packages">Packages</h2>
+
+<p>
+Go programs are constructed by linking together <i>packages</i>.
+A package in turn is constructed from one or more source files
+that together declare constants, types, variables and functions
+belonging to the package and which are accessible in all files
+of the same package. Those elements may be
+<a href="#Exported_identifiers">exported</a> and used in another package.
+</p>
+
+<h3 id="Source_file_organization">Source file organization</h3>
+
+<p>
+Each source file consists of a package clause defining the package
+to which it belongs, followed by a possibly empty set of import
+declarations that declare packages whose contents it wishes to use,
+followed by a possibly empty set of declarations of functions,
+types, variables, and constants.
+</p>
+
+<pre class="ebnf">
+SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
+</pre>
+
+<h3 id="Package_clause">Package clause</h3>
+
+<p>
+A package clause begins each source file and defines the package
+to which the file belongs.
+</p>
+
+<pre class="ebnf">
+PackageClause = "package" PackageName .
+PackageName = identifier .
+</pre>
+
+<p>
+The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
+</p>
+
+<pre>
+package math
+</pre>
+
+<p>
+A set of files sharing the same PackageName form the implementation of a package.
+An implementation may require that all source files for a package inhabit the same directory.
+</p>
+
+<h3 id="Import_declarations">Import declarations</h3>
+
+<p>
+An import declaration states that the source file containing the declaration
+depends on functionality of the <i>imported</i> package
+(<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
+and enables access to <a href="#Exported_identifiers">exported</a> identifiers
+of that package.
+The import names an identifier (PackageName) to be used for access and an ImportPath
+that specifies the package to be imported.
+</p>
+
+<pre class="ebnf">
+ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
+ImportSpec = [ "." | PackageName ] ImportPath .
+ImportPath = string_lit .
+</pre>
+
+<p>
+The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
+to access exported identifiers of the package within the importing source file.
+It is declared in the <a href="#Blocks">file block</a>.
+If the PackageName is omitted, it defaults to the identifier specified in the
+<a href="#Package_clause">package clause</a> of the imported package.
+If an explicit period (<code>.</code>) appears instead of a name, all the
+package's exported identifiers declared in that package's
+<a href="#Blocks">package block</a> will be declared in the importing source
+file's file block and must be accessed without a qualifier.
+</p>
+
+<p>
+The interpretation of the ImportPath is implementation-dependent but
+it is typically a substring of the full file name of the compiled
+package and may be relative to a repository of installed packages.
+</p>
+
+<p>
+Implementation restriction: A compiler may restrict ImportPaths to
+non-empty strings using only characters belonging to
+<a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
+L, M, N, P, and S general categories (the Graphic characters without
+spaces) and may also exclude the characters
+<code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
+and the Unicode replacement character U+FFFD.
+</p>
+
+<p>
+Assume we have compiled a package containing the package clause
+<code>package math</code>, which exports function <code>Sin</code>, and
+installed the compiled package in the file identified by
+<code>"lib/math"</code>.
+This table illustrates how <code>Sin</code> is accessed in files
+that import the package after the
+various types of import declaration.
+</p>
+
+<pre class="grammar">
+Import declaration Local name of Sin
+
+import "lib/math" math.Sin
+import m "lib/math" m.Sin
+import . "lib/math" Sin
+</pre>
+
+<p>
+An import declaration declares a dependency relation between
+the importing and imported package.
+It is illegal for a package to import itself, directly or indirectly,
+or to directly import a package without
+referring to any of its exported identifiers. To import a package solely for
+its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
+identifier as explicit package name:
+</p>
+
+<pre>
+import _ "lib/math"
+</pre>
+
+
+<h3 id="An_example_package">An example package</h3>
+
+<p>
+Here is a complete Go package that implements a concurrent prime sieve.
+</p>
+
+<pre>
+package main
+
+import "fmt"
+
+// Send the sequence 2, 3, 4, … to channel 'ch'.
+func generate(ch chan<- int) {
+ for i := 2; ; i++ {
+ ch <- i // Send 'i' to channel 'ch'.
+ }
+}
+
+// Copy the values from channel 'src' to channel 'dst',
+// removing those divisible by 'prime'.
+func filter(src <-chan int, dst chan<- int, prime int) {
+ for i := range src { // Loop over values received from 'src'.
+ if i%prime != 0 {
+ dst <- i // Send 'i' to channel 'dst'.
+ }
+ }
+}
+
+// The prime sieve: Daisy-chain filter processes together.
+func sieve() {
+ ch := make(chan int) // Create a new channel.
+ go generate(ch) // Start generate() as a subprocess.
+ for {
+ prime := <-ch
+ fmt.Print(prime, "\n")
+ ch1 := make(chan int)
+ go filter(ch, ch1, prime)
+ ch = ch1
+ }
+}
+
+func main() {
+ sieve()
+}
+</pre>
+
+<h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
+
+<h3 id="The_zero_value">The zero value</h3>
+<p>
+When storage is allocated for a <a href="#Variables">variable</a>,
+either through a declaration or a call of <code>new</code>, or when
+a new value is created, either through a composite literal or a call
+of <code>make</code>,
+and no explicit initialization is provided, the variable or value is
+given a default value. Each element of such a variable or value is
+set to the <i>zero value</i> for its type: <code>false</code> for booleans,
+<code>0</code> for numeric types, <code>""</code>
+for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
+This initialization is done recursively, so for instance each element of an
+array of structs will have its fields zeroed if no value is specified.
+</p>
+<p>
+These two simple declarations are equivalent:
+</p>
+
+<pre>
+var i int
+var i int = 0
+</pre>
+
+<p>
+After
+</p>
+
+<pre>
+type T struct { i int; f float64; next *T }
+t := new(T)
+</pre>
+
+<p>
+the following holds:
+</p>
+
+<pre>
+t.i == 0
+t.f == 0.0
+t.next == nil
+</pre>
+
+<p>
+The same would also be true after
+</p>
+
+<pre>
+var t T
+</pre>
+
+<h3 id="Package_initialization">Package initialization</h3>
+
+<p>
+Within a package, package-level variable initialization proceeds stepwise,
+with each step selecting the variable earliest in <i>declaration order</i>
+which has no dependencies on uninitialized variables.
+</p>
+
+<p>
+More precisely, a package-level variable is considered <i>ready for
+initialization</i> if it is not yet initialized and either has
+no <a href="#Variable_declarations">initialization expression</a> or
+its initialization expression has no <i>dependencies</i> on uninitialized variables.
+Initialization proceeds by repeatedly initializing the next package-level
+variable that is earliest in declaration order and ready for initialization,
+until there are no variables ready for initialization.
+</p>
+
+<p>
+If any variables are still uninitialized when this
+process ends, those variables are part of one or more initialization cycles,
+and the program is not valid.
+</p>
+
+<p>
+Multiple variables on the left-hand side of a variable declaration initialized
+by single (multi-valued) expression on the right-hand side are initialized
+together: If any of the variables on the left-hand side is initialized, all
+those variables are initialized in the same step.
+</p>
+
+<pre>
+var x = a
+var a, b = f() // a and b are initialized together, before x is initialized
+</pre>
+
+<p>
+For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
+variables are treated like any other variables in declarations.
+</p>
+
+<p>
+The declaration order of variables declared in multiple files is determined
+by the order in which the files are presented to the compiler: Variables
+declared in the first file are declared before any of the variables declared
+in the second file, and so on.
+</p>
+
+<p>
+Dependency analysis does not rely on the actual values of the
+variables, only on lexical <i>references</i> to them in the source,
+analyzed transitively. For instance, if a variable <code>x</code>'s
+initialization expression refers to a function whose body refers to
+variable <code>y</code> then <code>x</code> depends on <code>y</code>.
+Specifically:
+</p>
+
+<ul>
+<li>
+A reference to a variable or function is an identifier denoting that
+variable or function.
+</li>
+
+<li>
+A reference to a method <code>m</code> is a
+<a href="#Method_values">method value</a> or
+<a href="#Method_expressions">method expression</a> of the form
+<code>t.m</code>, where the (static) type of <code>t</code> is
+not an interface type, and the method <code>m</code> is in the
+<a href="#Method_sets">method set</a> of <code>t</code>.
+It is immaterial whether the resulting function value
+<code>t.m</code> is invoked.
+</li>
+
+<li>
+A variable, function, or method <code>x</code> depends on a variable
+<code>y</code> if <code>x</code>'s initialization expression or body
+(for functions and methods) contains a reference to <code>y</code>
+or to a function or method that depends on <code>y</code>.
+</li>
+</ul>
+
+<p>
+For example, given the declarations
+</p>
+
+<pre>
+var (
+ a = c + b // == 9
+ b = f() // == 4
+ c = f() // == 5
+ d = 3 // == 5 after initialization has finished
+)
+
+func f() int {
+ d++
+ return d
+}
+</pre>
+
+<p>
+the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
+Note that the order of subexpressions in initialization expressions is irrelevant:
+<code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
+order in this example.
+</p>
+
+<p>
+Dependency analysis is performed per package; only references referring
+to variables, functions, and (non-interface) methods declared in the current
+package are considered. If other, hidden, data dependencies exists between
+variables, the initialization order between those variables is unspecified.
+</p>
+
+<p>
+For instance, given the declarations
+</p>
+
+<pre>
+var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
+var _ = sideEffect() // unrelated to x, a, or b
+var a = b
+var b = 42
+
+type I interface { ab() []int }
+type T struct{}
+func (T) ab() []int { return []int{a, b} }
+</pre>
+
+<p>
+the variable <code>a</code> will be initialized after <code>b</code> but
+whether <code>x</code> is initialized before <code>b</code>, between
+<code>b</code> and <code>a</code>, or after <code>a</code>, and
+thus also the moment at which <code>sideEffect()</code> is called (before
+or after <code>x</code> is initialized) is not specified.
+</p>
+
+<p>
+Variables may also be initialized using functions named <code>init</code>
+declared in the package block, with no arguments and no result parameters.
+</p>
+
+<pre>
+func init() { … }
+</pre>
+
+<p>
+Multiple such functions may be defined per package, even within a single
+source file. In the package block, the <code>init</code> identifier can
+be used only to declare <code>init</code> functions, yet the identifier
+itself is not <a href="#Declarations_and_scope">declared</a>. Thus
+<code>init</code> functions cannot be referred to from anywhere
+in a program.
+</p>
+
+<p>
+A package with no imports is initialized by assigning initial values
+to all its package-level variables followed by calling all <code>init</code>
+functions in the order they appear in the source, possibly in multiple files,
+as presented to the compiler.
+If a package has imports, the imported packages are initialized
+before initializing the package itself. If multiple packages import
+a package, the imported package will be initialized only once.
+The importing of packages, by construction, guarantees that there
+can be no cyclic initialization dependencies.
+</p>
+
+<p>
+Package initialization—variable initialization and the invocation of
+<code>init</code> functions—happens in a single goroutine,
+sequentially, one package at a time.
+An <code>init</code> function may launch other goroutines, which can run
+concurrently with the initialization code. However, initialization
+always sequences
+the <code>init</code> functions: it will not invoke the next one
+until the previous one has returned.
+</p>
+
+<p>
+To ensure reproducible initialization behavior, build systems are encouraged
+to present multiple files belonging to the same package in lexical file name
+order to a compiler.
+</p>
+
+
+<h3 id="Program_execution">Program execution</h3>
+<p>
+A complete program is created by linking a single, unimported package
+called the <i>main package</i> with all the packages it imports, transitively.
+The main package must
+have package name <code>main</code> and
+declare a function <code>main</code> that takes no
+arguments and returns no value.
+</p>
+
+<pre>
+func main() { … }
+</pre>
+
+<p>
+Program execution begins by initializing the main package and then
+invoking the function <code>main</code>.
+When that function invocation returns, the program exits.
+It does not wait for other (non-<code>main</code>) goroutines to complete.
+</p>
+
+<h2 id="Errors">Errors</h2>
+
+<p>
+The predeclared type <code>error</code> is defined as
+</p>
+
+<pre>
+type error interface {
+ Error() string
+}
+</pre>
+
+<p>
+It is the conventional interface for representing an error condition,
+with the nil value representing no error.
+For instance, a function to read data from a file might be defined:
+</p>
+
+<pre>
+func Read(f *File, b []byte) (n int, err error)
+</pre>
+
+<h2 id="Run_time_panics">Run-time panics</h2>
+
+<p>
+Execution errors such as attempting to index an array out
+of bounds trigger a <i>run-time panic</i> equivalent to a call of
+the built-in function <a href="#Handling_panics"><code>panic</code></a>
+with a value of the implementation-defined interface type <code>runtime.Error</code>.
+That type satisfies the predeclared interface type
+<a href="#Errors"><code>error</code></a>.
+The exact error values that
+represent distinct run-time error conditions are unspecified.
+</p>
+
+<pre>
+package runtime
+
+type Error interface {
+ error
+ // and perhaps other methods
+}
+</pre>
+
+<h2 id="System_considerations">System considerations</h2>
+
+<h3 id="Package_unsafe">Package <code>unsafe</code></h3>
+
+<p>
+The built-in package <code>unsafe</code>, known to the compiler
+and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
+provides facilities for low-level programming including operations
+that violate the type system. A package using <code>unsafe</code>
+must be vetted manually for type safety and may not be portable.
+The package provides the following interface:
+</p>
+
+<pre class="grammar">
+package unsafe
+
+type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
+type Pointer *ArbitraryType
+
+func Alignof(variable ArbitraryType) uintptr
+func Offsetof(selector ArbitraryType) uintptr
+func Sizeof(variable ArbitraryType) uintptr
+
+type IntegerType int // shorthand for an integer type; it is not a real type
+func Add(ptr Pointer, len IntegerType) Pointer
+func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
+</pre>
+
+<!--
+These conversions also apply to type parameters with suitable core types.
+Determine if we can simply use core type insted of underlying type here,
+of if the general conversion rules take care of this.
+-->
+
+<p>
+A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
+value may not be <a href="#Address_operators">dereferenced</a>.
+Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be
+<a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
+The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
+</p>
+
+<pre>
+var f float64
+bits = *(*uint64)(unsafe.Pointer(&f))
+
+type ptr unsafe.Pointer
+bits = *(*uint64)(ptr(&f))
+
+var p ptr = nil
+</pre>
+
+<p>
+The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
+of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
+as if <code>v</code> was declared via <code>var v = x</code>.
+</p>
+<p>
+The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
+<code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
+or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
+If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
+without pointer indirections through fields of the struct.
+For a struct <code>s</code> with field <code>f</code>:
+</p>
+
+<pre>
+uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
+</pre>
+
+<p>
+Computer architectures may require memory addresses to be <i>aligned</i>;
+that is, for addresses of a variable to be a multiple of a factor,
+the variable's type's <i>alignment</i>. The function <code>Alignof</code>
+takes an expression denoting a variable of any type and returns the
+alignment of the (type of the) variable in bytes. For a variable
+<code>x</code>:
+</p>
+
+<pre>
+uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
+</pre>
+
+<p>
+A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
+is a <a href="#Type_parameter_declarations">type parameter</a>, or if it is an
+array or struct type containing elements
+or fields of variable size. Otherwise the size is <i>constant</i>.
+Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
+are compile-time <a href="#Constant_expressions">constant expressions</a> of
+type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
+the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
+of constant size.
+</p>
+
+<p>
+The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
+and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
+The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
+A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
+if it is an untyped constant it is given type <code>int</code>.
+The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
+</p>
+
+<p>
+The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
+and whose length and capacity are <code>len</code>.
+<code>Slice(ptr, len)</code> is equivalent to
+</p>
+
+<pre>
+(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
+</pre>
+
+<p>
+except that, as a special case, if <code>ptr</code>
+is <code>nil</code> and <code>len</code> is zero,
+<code>Slice</code> returns <code>nil</code>.
+</p>
+
+<p>
+The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
+A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
+if it is an untyped constant it is given type <code>int</code>.
+At run time, if <code>len</code> is negative,
+or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
+a <a href="#Run_time_panics">run-time panic</a> occurs.
+</p>
+
+<h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
+
+<p>
+For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
+</p>
+
+<pre class="grammar">
+type size in bytes
+
+byte, uint8, int8 1
+uint16, int16 2
+uint32, int32, float32 4
+uint64, int64, float64, complex64 8
+complex128 16
+</pre>
+
+<p>
+The following minimal alignment properties are guaranteed:
+</p>
+<ol>
+<li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
+</li>
+
+<li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
+ all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
+</li>
+
+<li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
+ the alignment of a variable of the array's element type.
+</li>
+</ol>
+
+<p>
+A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.
+</p>
diff --git a/cmd/golangorg/testdata/web.txt b/cmd/golangorg/testdata/web.txt
index 4bd5cc9..cc78c63 100644
--- a/cmd/golangorg/testdata/web.txt
+++ b/cmd/golangorg/testdata/web.txt
@@ -342,6 +342,14 @@
GET https://go.dev/ref/spec
body contains <a id="assign_op">assign_op</a>
+# Issue 51686.
+GET https://go.dev/ref/spec
+body contains Version of March 10, 2022
+
+# Issue 51686.
+GET https://go.dev/doc/go1.17_spec
+body contains Version of Oct 15, 2021
+
GET https://golang.org/robots.txt
redirect == https://go.dev/robots.txt
