blob: 29109b6b9e06a0b3369b791c0904eed8d9caeaed [file] [log] [blame]
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of June 29, 2022",
"Path": "/ref/spec"
}-->
<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 syntax 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 a
<a href="https://en.wikipedia.org/wiki/Wirth_syntax_notation">variant</a>
of Extended Backus-Naur Form (EBNF):
</p>
<pre class="grammar">
Syntax = { Production } .
Production = production_name "=" [ Expression ] "." .
Expression = Term { "|" Term } .
Term = Factor { Factor } .
Factor = 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>
Lowercase production names are used to identify lexical (terminal) 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, uppercase and lowercase 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 categories:
</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 categorized as "Letter" */ .
unicode_digit = /* a Unicode code point categorized 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 lowercase 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 syntax 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="#Assignment_statements">assignment operators</a>) and punctuation:
</p>
<pre class="grammar">
+ &amp; += &amp;= &amp;&amp; == != ( )
- | -= |= || &lt; &lt;= [ ]
* ^ *= ^= &lt;- &gt; &gt;= { }
/ &lt;&lt; /= &lt;&lt;= ++ = := , ;
% &gt;&gt; %= &gt;&gt;= -- ! ... . :
&amp;^ &amp;^= ~
</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 lowercase 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>
An unrecognized character following a backslash in a rune literal is illegal.
</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
'\k' // illegal: k is not recognized after a backslash
'\xa' // illegal: too few hexadecimal digits
'\0' // illegal: too few octal digits
'\400' // illegal: octal value over 255
'\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>&quot;bar&quot;</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="#Assignment_statements">assignment statement</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="#Assignment_statements">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>&mdash;array, struct, pointer, function,
interface, slice, map, and channel types&mdash;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>&amp;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 [ TypeArgs ] .
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="#Types">named 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 (non-interface) 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 all non-interface 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 all 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>
The quantification "the set of all non-interface types" refers not just to all (non-interface)
types declared in the program at hand, but all possible types in all possible programs, and
hence is infinite.
Similarly, given the set of all non-interface types that implement a particular method, the
intersection of the method sets of those types will contain exactly that method, even if all
types in the program at hand always pair that method with another method.
</p>
<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>
The type <code>T</code> in a term of the form <code>T</code> or <code>~T</code> 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="#Assignment_statements">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" "&lt;-" | "&lt;-" "chan" ) ElementType .
</pre>
<p>
The optional <code>&lt;-</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="#Assignment_statements">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&lt;- float64 // can only be used to send float64s
&lt;-chan int // can only be used to receive ints
</pre>
<p>
The <code>&lt;-</code> operator associates with the leftmost <code>chan</code>
possible:
</p>
<pre>
chan&lt;- chan int // same as chan&lt;- (chan int)
chan&lt;- &lt;-chan int // same as chan&lt;- (&lt;-chan int)
&lt;-chan &lt;-chan int // same as &lt;-chan (&lt;-chan int)
chan (&lt;-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&lt;- E</code> or <code>&lt;-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&lt;- int } // chan&lt;- 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&lt;- string } // channels have different element types
interface{ &lt;-chan int | chan&lt;- int } // directional channels have different directions
</pre>
<p>
Some operations (<a href="#Slice_expressions">slice expressions</a>,
<a href="#Appending_and_copying_slices"><code>append</code> and <code>copy</code></a>)
rely on a slightly more loose form of core types which accept byte slices and strings.
Specifically, if there are exactly two types, <code>[]byte</code> and <code>string</code>,
which are the underlying types of all types in the type set of interface <code>T</code>,
the core type of <code>T</code> is called <code>bytestring</code>.
</p>
<p>
Examples of interfaces with <code>bytestring</code> core types:
</p>
<pre>
interface{ int } // int (same as ordinary core type)
interface{ []byte | string } // bytestring
interface{ ~[]byte | myString } // bytestring
</pre>
<p>
Note that <code>bytestring</code> is not a real type; it cannot be used to declare
variables are compose other types. It exists solely to describe the behavior of some
operations that read from a sequence of bytes, which may be a byte slice or a string.
</p>
<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 *B5 } and struct{ a, b *B5 }
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>
but are not type parameters 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 begins after the name of the function
and ends at the end of the function body.</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="#Assignment_statements">assignment statements</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 uppercase
letter (Unicode character category 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 &lt;&lt; iota // a == 1 (iota == 0)
b = 1 &lt;&lt; iota // b == 2 (iota == 1)
c = 3 // c == 3 (iota == 2, unused)
d = 1 &lt;&lt; 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 &lt;&lt; iota, 1&lt;&lt;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="#Assignment_statements">assignment statements</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.
The non-blank variable names on the left side of <code>:=</code>
must be <a href="#Uniqueness_of_identifiers">unique</a>.
</p>
<pre>
field1, offset := nextField(str, 0)
field2, offset := nextField(str, offset) // redeclares offset
x, y, x := 1, 2, 3 // illegal: x repeated on left side of :=
</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 &lt; 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="#Assignment_statements">assignment statement</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 [ TypeArgs ] .
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 syntax 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 = &amp;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 := &amp;[]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>&amp;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{&amp;Point{1.5, -3.5}, &amp;Point{}}
[2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&amp;Point{1.5, -3.5}), PPoint(&amp;Point{})}
</pre>