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<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of June 28, 2017",
"Path": "/ref/spec"
}-->
<h2 id="Introduction">Introduction</h2>
<p>
This is a reference manual for the Go programming language. For
more information and other documents, see <a href="/">golang.org</a>.
</p>
<p>
Go is a general-purpose language designed with systems programming
in mind. It is strongly typed and garbage-collected and has explicit
support for concurrent programming. Programs are constructed from
<i>packages</i>, whose properties allow efficient management of
dependencies. The existing implementations use a traditional
compile/link model to generate executable binaries.
</p>
<p>
The grammar is compact and regular, allowing for easy analysis by
automatic tools such as integrated development environments.
</p>
<h2 id="Notation">Notation</h2>
<p>
The syntax is specified using Extended Backus-Naur Form (EBNF):
</p>
<pre class="grammar">
Production = production_name "=" [ Expression ] "." .
Expression = Alternative { "|" Alternative } .
Alternative = Term { Term } .
Term = production_name | token [ "…" token ] | Group | Option | Repetition .
Group = "(" Expression ")" .
Option = "[" Expression "]" .
Repetition = "{" Expression "}" .
</pre>
<p>
Productions are expressions constructed from terms and the following
operators, in increasing precedence:
</p>
<pre class="grammar">
| alternation
() grouping
[] option (0 or 1 times)
{} repetition (0 to n times)
</pre>
<p>
Lower-case production names are used to identify lexical tokens.
Non-terminals are in CamelCase. Lexical tokens are enclosed in
double quotes <code>""</code> or back quotes <code>``</code>.
</p>
<p>
The form <code>a … b</code> represents the set of characters from
<code>a</code> through <code>b</code> as alternatives. The horizontal
ellipsis <code></code> is also used elsewhere in the spec to informally denote various
enumerations or code snippets that are not further specified. The character <code></code>
(as opposed to the three characters <code>...</code>) is not a token of the Go
language.
</p>
<h2 id="Source_code_representation">Source code representation</h2>
<p>
Source code is Unicode text encoded in
<a href="http://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
canonicalized, so a single accented code point is distinct from the
same character constructed from combining an accent and a letter;
those are treated as two code points. For simplicity, this document
will use the unqualified term <i>character</i> to refer to a Unicode code point
in the source text.
</p>
<p>
Each code point is distinct; for instance, upper and lower case letters
are different characters.
</p>
<p>
Implementation restriction: For compatibility with other tools, a
compiler may disallow the NUL character (U+0000) in the source text.
</p>
<p>
Implementation restriction: For compatibility with other tools, a
compiler may ignore a UTF-8-encoded byte order mark
(U+FEFF) if it is the first Unicode code point in the source text.
A byte order mark may be disallowed anywhere else in the source.
</p>
<h3 id="Characters">Characters</h3>
<p>
The following terms are used to denote specific Unicode character classes:
</p>
<pre class="ebnf">
newline = /* the Unicode code point U+000A */ .
unicode_char = /* an arbitrary Unicode code point except newline */ .
unicode_letter = /* a Unicode code point classified as "Letter" */ .
unicode_digit = /* a Unicode code point classified as "Number, decimal digit" */ .
</pre>
<p>
In <a href="http://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
Section 4.5 "General Category" defines a set of character categories.
Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
as Unicode letters, and those in the Number category Nd as Unicode digits.
</p>
<h3 id="Letters_and_digits">Letters and digits</h3>
<p>
The underscore character <code>_</code> (U+005F) is considered a letter.
</p>
<pre class="ebnf">
letter = unicode_letter | "_" .
decimal_digit = "0" … "9" .
octal_digit = "0" … "7" .
hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
</pre>
<h2 id="Lexical_elements">Lexical elements</h2>
<h3 id="Comments">Comments</h3>
<p>
Comments serve as program documentation. There are two forms:
</p>
<ol>
<li>
<i>Line comments</i> start with the character sequence <code>//</code>
and stop at the end of the line.
</li>
<li>
<i>General comments</i> start with the character sequence <code>/*</code>
and stop with the first subsequent character sequence <code>*/</code>.
</li>
</ol>
<p>
A comment cannot start inside a <a href="#Rune_literals">rune</a> or
<a href="#String_literals">string literal</a>, or inside a comment.
A general comment containing no newlines acts like a space.
Any other comment acts like a newline.
</p>
<h3 id="Tokens">Tokens</h3>
<p>
Tokens form the vocabulary of the Go language.
There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators
and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from
spaces (U+0020), horizontal tabs (U+0009),
carriage returns (U+000D), and newlines (U+000A),
is ignored except as it separates tokens
that would otherwise combine into a single token. Also, a newline or end of file
may trigger the insertion of a <a href="#Semicolons">semicolon</a>.
While breaking the input into tokens,
the next token is the longest sequence of characters that form a
valid token.
</p>
<h3 id="Semicolons">Semicolons</h3>
<p>
The formal grammar uses semicolons <code>";"</code> as terminators in
a number of productions. Go programs may omit most of these semicolons
using the following two rules:
</p>
<ol>
<li>
When the input is broken into tokens, a semicolon is automatically inserted
into the token stream immediately after a line's final token if that token is
<ul>
<li>an
<a href="#Identifiers">identifier</a>
</li>
<li>an
<a href="#Integer_literals">integer</a>,
<a href="#Floating-point_literals">floating-point</a>,
<a href="#Imaginary_literals">imaginary</a>,
<a href="#Rune_literals">rune</a>, or
<a href="#String_literals">string</a> literal
</li>
<li>one of the <a href="#Keywords">keywords</a>
<code>break</code>,
<code>continue</code>,
<code>fallthrough</code>, or
<code>return</code>
</li>
<li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a>
<code>++</code>,
<code>--</code>,
<code>)</code>,
<code>]</code>, or
<code>}</code>
</li>
</ul>
</li>
<li>
To allow complex statements to occupy a single line, a semicolon
may be omitted before a closing <code>")"</code> or <code>"}"</code>.
</li>
</ol>
<p>
To reflect idiomatic use, code examples in this document elide semicolons
using these rules.
</p>
<h3 id="Identifiers">Identifiers</h3>
<p>
Identifiers name program entities such as variables and types.
An identifier is a sequence of one or more letters and digits.
The first character in an identifier must be a letter.
</p>
<pre class="ebnf">
identifier = letter { letter | unicode_digit } .
</pre>
<pre>
a
_x9
ThisVariableIsExported
αβ
</pre>
<p>
Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>.
</p>
<h3 id="Keywords">Keywords</h3>
<p>
The following keywords are reserved and may not be used as identifiers.
</p>
<pre class="grammar">
break default func interface select
case defer go map struct
chan else goto package switch
const fallthrough if range type
continue for import return var
</pre>
<h3 id="Operators_and_punctuation">Operators and punctuation</h3>
<p>
The following character sequences represent <a href="#Operators">operators</a>
(including <a href="#assign_op">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>0</code> for octal, <code>0x</code> or
<code>0X</code> for hexadecimal. In hexadecimal literals, letters
<code>a-f</code> and <code>A-F</code> represent values 10 through 15.
</p>
<pre class="ebnf">
int_lit = decimal_lit | octal_lit | hex_lit .
decimal_lit = ( "1" … "9" ) { decimal_digit } .
octal_lit = "0" { octal_digit } .
hex_lit = "0" ( "x" | "X" ) hex_digit { hex_digit } .
</pre>
<pre>
42
0600
0xBadFace
170141183460469231731687303715884105727
</pre>
<h3 id="Floating-point_literals">Floating-point literals</h3>
<p>
A floating-point literal is a decimal representation of a
<a href="#Constants">floating-point constant</a>.
It has an integer part, a decimal point, a fractional part,
and an exponent part. The integer and fractional part comprise
decimal digits; the exponent part is an <code>e</code> or <code>E</code>
followed by an optionally signed decimal exponent. One of the
integer part or the fractional part may be elided; one of the decimal
point or the exponent may be elided.
</p>
<pre class="ebnf">
float_lit = decimals "." [ decimals ] [ exponent ] |
decimals exponent |
"." decimals [ exponent ] .
decimals = decimal_digit { decimal_digit } .
exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .
</pre>
<pre>
0.
72.40
072.40 // == 72.40
2.71828
1.e+0
6.67428e-11
1E6
.25
.12345E+5
</pre>
<h3 id="Imaginary_literals">Imaginary literals</h3>
<p>
An imaginary literal is a decimal representation of the imaginary part of a
<a href="#Constants">complex constant</a>.
It consists of a
<a href="#Floating-point_literals">floating-point literal</a>
or decimal integer followed
by the lower-case letter <code>i</code>.
</p>
<pre class="ebnf">
imaginary_lit = (decimals | float_lit) "i" .
</pre>
<pre>
0i
011i // == 11i
0.i
2.71828i
1.e+0i
6.67428e-11i
1E6i
.25i
.12345E+5i
</pre>
<h3 id="Rune_literals">Rune literals</h3>
<p>
A rune literal represents a <a href="#Constants">rune constant</a>,
an integer value identifying a Unicode code point.
A rune literal is expressed as one or more characters enclosed in single quotes,
as in <code>'x'</code> or <code>'\n'</code>.
Within the quotes, any character may appear except newline and unescaped single
quote. A single quoted character represents the Unicode value
of the character itself,
while multi-character sequences beginning with a backslash encode
values in various formats.
</p>
<p>
The simplest form represents the single character within the quotes;
since Go source text is Unicode characters encoded in UTF-8, multiple
UTF-8-encoded bytes may represent a single integer value. For
instance, the literal <code>'a'</code> holds a single byte representing
a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
<code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
</p>
<p>
Several backslash escapes allow arbitrary values to be encoded as
ASCII text. There are four ways to represent the integer value
as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
digits; <code>\u</code> followed by exactly four hexadecimal digits;
<code>\U</code> followed by exactly eight hexadecimal digits, and a
plain backslash <code>\</code> followed by exactly three octal digits.
In each case the value of the literal is the value represented by
the digits in the corresponding base.
</p>
<p>
Although these representations all result in an integer, they have
different valid ranges. Octal escapes must represent a value between
0 and 255 inclusive. Hexadecimal escapes satisfy this condition
by construction. The escapes <code>\u</code> and <code>\U</code>
represent Unicode code points so within them some values are illegal,
in particular those above <code>0x10FFFF</code> and surrogate halves.
</p>
<p>
After a backslash, certain single-character escapes represent special values:
</p>
<pre class="grammar">
\a U+0007 alert or bell
\b U+0008 backspace
\f U+000C form feed
\n U+000A line feed or newline
\r U+000D carriage return
\t U+0009 horizontal tab
\v U+000b vertical tab
\\ U+005c backslash
\' U+0027 single quote (valid escape only within rune literals)
\" U+0022 double quote (valid escape only within string literals)
</pre>
<p>
All other sequences starting with a backslash are illegal inside rune literals.
</p>
<pre class="ebnf">
rune_lit = "'" ( unicode_value | byte_value ) "'" .
unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
byte_value = octal_byte_value | hex_byte_value .
octal_byte_value = `\` octal_digit octal_digit octal_digit .
hex_byte_value = `\` "x" hex_digit hex_digit .
little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
hex_digit hex_digit hex_digit hex_digit .
escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
</pre>
<pre>
'a'
'ä'
'本'
'\t'
'\000'
'\007'
'\377'
'\x07'
'\xff'
'\u12e4'
'\U00101234'
'\'' // rune literal containing single quote character
'aa' // illegal: too many characters
'\xa' // illegal: too few hexadecimal digits
'\0' // illegal: too few octal digits
'\uDFFF' // illegal: surrogate half
'\U00110000' // illegal: invalid Unicode code point
</pre>
<h3 id="String_literals">String literals</h3>
<p>
A string literal represents a <a href="#Constants">string constant</a>
obtained from concatenating a sequence of characters. There are two forms:
raw string literals and interpreted string literals.
</p>
<p>
Raw string literals are character sequences between back quotes, as in
<code>`foo`</code>. Within the quotes, any character may appear except
back quote. The value of a raw string literal is the
string composed of the uninterpreted (implicitly UTF-8-encoded) characters
between the quotes;
in particular, backslashes have no special meaning and the string may
contain newlines.
Carriage return characters ('\r') inside raw string literals
are discarded from the raw string value.
</p>
<p>
Interpreted string literals are character sequences between double
quotes, as in <code>&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 any value,
<code>cap</code> or <code>len</code> applied to
<a href="#Length_and_capacity">some expressions</a>,
<code>real</code> and <code>imag</code> applied to a complex constant
and <code>complex</code> applied to numeric constants.
The boolean truth values are represented by the predeclared constants
<code>true</code> and <code>false</code>. The predeclared identifier
<a href="#Iota">iota</a> denotes an integer constant.
</p>
<p>
In general, complex constants are a form of
<a href="#Constant_expressions">constant expression</a>
and are discussed in that section.
</p>
<p>
Numeric constants represent exact values of arbitrary precision and do not overflow.
Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
and not-a-number values.
</p>
<p>
Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
and certain <a href="#Constant_expressions">constant expressions</a>
containing only untyped constant operands are untyped.
</p>
<p>
A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
or <a href="#Conversions">conversion</a>, or implicitly when used in a
<a href="#Variable_declarations">variable declaration</a> or an
<a href="#Assignments">assignment</a> or as an
operand in an <a href="#Expressions">expression</a>.
It is an error if the constant value
cannot be represented as a value of the respective type.
For instance, <code>3.0</code> can be given any integer or any
floating-point type, while <code>2147483648.0</code> (equal to <code>1&lt;&lt;31</code>)
can be given the types <code>float32</code>, <code>float64</code>, or <code>uint32</code> but
not <code>int32</code> or <code>string</code>.
</p>
<p>
An untyped constant has a <i>default type</i> which is the type to which the
constant is implicitly converted in contexts where a typed value is required,
for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
such as <code>i := 0</code> where there is no explicit type.
The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
<code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
respectively, depending on whether it is a boolean, rune, integer, floating-point,
complex, or string constant.
</p>
<p>
Implementation restriction: Although numeric constants have arbitrary
precision in the language, a compiler may implement them using an
internal representation with limited precision. That said, every
implementation must:
</p>
<ul>
<li>Represent integer constants with at least 256 bits.</li>
<li>Represent floating-point constants, including the parts of
a complex constant, with a mantissa of at least 256 bits
and a signed binary exponent of at least 16 bits.</li>
<li>Give an error if unable to represent an integer constant
precisely.</li>
<li>Give an error if unable to represent a floating-point or
complex constant due to overflow.</li>
<li>Round to the nearest representable constant if unable to
represent a floating-point or complex constant due to limits
on precision.</li>
</ul>
<p>
These requirements apply both to literal constants and to the result
of evaluating <a href="#Constant_expressions">constant
expressions</a>.
</p>
<h2 id="Variables">Variables</h2>
<p>
A variable is a storage location for holding a <i>value</i>.
The set of permissible values is determined by the
variable's <i><a href="#Types">type</a></i>.
</p>
<p>
A <a href="#Variable_declarations">variable declaration</a>
or, for function parameters and results, the signature
of a <a href="#Function_declarations">function declaration</a>
or <a href="#Function_literals">function literal</a> reserves
storage for a named variable.
Calling the built-in function <a href="#Allocation"><code>new</code></a>
or taking the address of a <a href="#Composite_literals">composite literal</a>
allocates storage for a variable at run time.
Such an anonymous variable is referred to via a (possibly implicit)
<a href="#Address_operators">pointer indirection</a>.
</p>
<p>
<i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
and <a href="#Struct_types">struct</a> types have elements and fields that may
be <a href="#Address_operators">addressed</a> individually. Each such element
acts like a variable.
</p>
<p>
The <i>static type</i> (or just <i>type</i>) of a variable is the
type given in its declaration, the type provided in the
<code>new</code> call or composite literal, or the type of
an element of a structured variable.
Variables of interface type also have a distinct <i>dynamic type</i>,
which is the concrete type of the value assigned to the variable at run time
(unless the value is the predeclared identifier <code>nil</code>,
which has no type).
The dynamic type may vary during execution but values stored in interface
variables are always <a href="#Assignability">assignable</a>
to the static type of the variable.
</p>
<pre>
var x interface{} // x is nil and has static type interface{}
var v *T // v has value nil, static type *T
x = 42 // x has value 42 and dynamic type int
x = v // x has value (*T)(nil) and dynamic type *T
</pre>
<p>
A variable's value is retrieved by referring to the variable in an
<a href="#Expressions">expression</a>; it is the most recent value
<a href="#Assignments">assigned</a> to the variable.
If a variable has not yet been assigned a value, its value is the
<a href="#The_zero_value">zero value</a> for its type.
</p>
<h2 id="Types">Types</h2>
<p>
A type determines a set of values together with operations and methods specific
to those values. A type may be denoted by a <i>type name</i>, if it has one,
or specified using a <i>type literal</i>, which composes a type from existing types.
</p>
<pre class="ebnf">
Type = TypeName | TypeLit | "(" Type ")" .
TypeName = identifier | QualifiedIdent .
TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
SliceType | MapType | ChannelType .
</pre>
<p>
Named instances of the boolean, numeric, and string types are
<a href="#Predeclared_identifiers">predeclared</a>.
Other named types are introduced with <a href="#Type_declarations">type declarations</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>
Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
is one of the predeclared boolean, numeric, or string types, or a type literal,
the corresponding underlying
type is <code>T</code> itself. Otherwise, <code>T</code>'s underlying type
is the underlying type of the type to which <code>T</code> refers in its
<a href="#Type_declarations">type declaration</a>.
</p>
<pre>
type (
A1 = string
A2 = A1
)
type (
B1 string
B2 B1
B3 []B1
B4 B3
)
</pre>
<p>
The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
and <code>B2</code> is <code>string</code>.
The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
</p>
<h3 id="Method_sets">Method sets</h3>
<p>
A type may have a <i>method set</i> associated with it.
The method set of an <a href="#Interface_types">interface type</a> is its interface.
The method set of any other type <code>T</code> consists of all
<a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
The method set of the corresponding <a href="#Pointer_types">pointer type</a> <code>*T</code>
is the set of all methods declared with receiver <code>*T</code> or <code>T</code>
(that is, it also contains the method set of <code>T</code>).
Further rules apply to structs containing embedded fields, as described
in the section on <a href="#Struct_types">struct types</a>.
Any other type has an empty method set.
In a method set, each method must have a
<a href="#Uniqueness_of_identifiers">unique</a>
non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
</p>
<p>
The method set of a type determines the interfaces that the
type <a href="#Interface_types">implements</a>
and the methods that can be <a href="#Calls">called</a>
using a receiver of that type.
</p>
<h3 id="Boolean_types">Boolean types</h3>
<p>
A <i>boolean type</i> represents the set of Boolean truth values
denoted by the predeclared constants <code>true</code>
and <code>false</code>. The predeclared boolean type is <code>bool</code>.
</p>
<h3 id="Numeric_types">Numeric types</h3>
<p>
A <i>numeric type</i> represents sets of integer or floating-point values.
The predeclared architecture-independent numeric types are:
</p>
<pre class="grammar">
uint8 the set of all unsigned 8-bit integers (0 to 255)
uint16 the set of all unsigned 16-bit integers (0 to 65535)
uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
int8 the set of all signed 8-bit integers (-128 to 127)
int16 the set of all signed 16-bit integers (-32768 to 32767)
int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
float32 the set of all IEEE-754 32-bit floating-point numbers
float64 the set of all IEEE-754 64-bit floating-point numbers
complex64 the set of all complex numbers with float32 real and imaginary parts
complex128 the set of all complex numbers with float64 real and imaginary parts
byte alias for uint8
rune alias for int32
</pre>
<p>
The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
<a href="http://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
</p>
<p>
There is also a set of predeclared numeric types with implementation-specific sizes:
</p>
<pre class="grammar">
uint either 32 or 64 bits
int same size as uint
uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
</pre>
<p>
To avoid portability issues all numeric types are distinct except
<code>byte</code>, which is an alias for <code>uint8</code>, and
<code>rune</code>, which is an alias for <code>int32</code>.
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.
Strings are immutable: once created,
it is impossible to change the contents of a string.
The predeclared string type is <code>string</code>.
</p>
<p>
The length of a string <code>s</code> (its size in bytes) 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 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> representable 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 value of an uninitialized slice is <code>nil</code>.
</p>
<pre class="ebnf">
SliceType = "[" "]" ElementType .
</pre>
<p>
Like arrays, slices are indexable and have a length. The length of a
slice <code>s</code> can be discovered by the built-in function
<a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
0 through <code>len(s)-1</code>. The slice index of a
given element may be less than the index of the same element in the
underlying array.
</p>
<p>
A slice, once initialized, is always associated with an underlying
array that holds its elements. A slice therefore shares storage
with its array and with other slices of the same array; by contrast,
distinct arrays always represent distinct storage.
</p>
<p>
The array underlying a slice may extend past the end of the slice.
The <i>capacity</i> is a measure of that extent: it is the sum of
the length of the slice and the length of the array beyond the slice;
a slice of length up to that capacity can be created by
<a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
The capacity of a slice <code>a</code> can be discovered using the
built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
</p>
<p>
A new, initialized slice value for a given element type <code>T</code> is
made using the built-in function
<a href="#Making_slices_maps_and_channels"><code>make</code></a>,
which takes a slice type
and parameters specifying the length and optionally the capacity.
A slice created with <code>make</code> always allocates a new, hidden array
to which the returned slice value refers. That is, executing
</p>
<pre>
make([]T, length, capacity)
</pre>
<p>
produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
it, so these two expressions are equivalent:
</p>
<pre>
make([]int, 50, 100)
new([100]int)[0:50]
</pre>
<p>
Like arrays, slices are always one-dimensional but may be composed to construct
higher-dimensional objects.
With arrays of arrays, the inner arrays are, by construction, always the same length;
however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
Moreover, the inner slices must be initialized individually.
</p>
<h3 id="Struct_types">Struct types</h3>
<p>
A struct is a sequence of named elements, called fields, each of which has a
name and a type. Field names may be specified explicitly (IdentifierList) or
implicitly (EmbeddedField).
Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
be <a href="#Uniqueness_of_identifiers">unique</a>.
</p>
<pre class="ebnf">
StructType = "struct" "{" { FieldDecl ";" } "}" .
FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
EmbeddedField = [ "*" ] TypeName .
Tag = string_lit .
</pre>
<pre>
// An empty struct.
struct {}
// A struct with 6 fields.
struct {
x, y int
u float32
_ float32 // padding
A *[]int
F func()
}
</pre>
<p>
A field declared with a type but no explicit field name is called an <i>embedded field</i>.
An embedded field must be specified as
a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
and <code>T</code> itself may not be
a pointer type. The unqualified type name acts as the field name.
</p>
<pre>
// A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
struct {
T1 // field name is T1
*T2 // field name is T2
P.T3 // field name is T3
*P.T4 // field name is T4
x, y int // field names are x and y
}
</pre>
<p>
The following declaration is illegal because field names must be unique
in a struct type:
</p>
<pre>
struct {
T // conflicts with embedded field *T and *P.T
*T // conflicts with embedded field T and *P.T
*P.T // conflicts with embedded field T and *T
}
</pre>
<p>
A field or <a href="#Method_declarations">method</a> <code>f</code> of an
embedded field in a struct <code>x</code> is called <i>promoted</i> if
<code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
that field or method <code>f</code>.
</p>
<p>
Promoted fields act like ordinary fields
of a struct except that they cannot be used as field names in
<a href="#Composite_literals">composite literals</a> of the struct.
</p>
<p>
Given a struct type <code>S</code> and a type named <code>T</code>,
promoted methods are included in the method set of the struct as follows:
</p>
<ul>
<li>
If <code>S</code> contains an embedded field <code>T</code>,
the <a href="#Method_sets">method sets</a> of <code>S</code>
and <code>*S</code> both include promoted methods with receiver
<code>T</code>. The method set of <code>*S</code> also
includes promoted methods with receiver <code>*T</code>.
</li>
<li>
If <code>S</code> contains an embedded field <code>*T</code>,
the method sets of <code>S</code> and <code>*S</code> both
include promoted methods with receiver <code>T</code> or
<code>*T</code>.
</li>
</ul>
<p>
A field declaration may be followed by an optional string literal <i>tag</i>,
which becomes an attribute for all the fields in the corresponding
field declaration. An empty tag string is equivalent to an absent tag.
The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
and take part in <a href="#Type_identity">type identity</a> for structs
but are otherwise ignored.
</p>
<pre>
struct {
x, y float64 "" // an empty tag string is like an absent tag
name string "any string is permitted as a tag"
_ [4]byte "ceci n'est pas un champ de structure"
}
// A struct corresponding to a TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers;
// they follow the convention outlined by the reflect package.
struct {
microsec uint64 `protobuf:"1"`
serverIP6 uint64 `protobuf:"2"`
}
</pre>
<h3 id="Pointer_types">Pointer types</h3>
<p>
A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
type, called the <i>base type</i> of the pointer.
The value of an uninitialized pointer is <code>nil</code>.
</p>
<pre class="ebnf">
PointerType = "*" BaseType .
BaseType = Type .
</pre>
<pre>
*Point
*[4]int
</pre>
<h3 id="Function_types">Function types</h3>
<p>
A function type denotes the set of all functions with the same parameter
and result types. The value of an uninitialized variable of function type
is <code>nil</code>.
</p>
<pre class="ebnf">
FunctionType = "func" Signature .
Signature = Parameters [ Result ] .
Result = Parameters | Type .
Parameters = "(" [ ParameterList [ "," ] ] ")" .
ParameterList = ParameterDecl { "," ParameterDecl } .
ParameterDecl = [ IdentifierList ] [ "..." ] Type .
</pre>
<p>
Within a list of parameters or results, the names (IdentifierList)
must either all be present or all be absent. If present, each name
stands for one item (parameter or result) of the specified type and
all non-<a href="#Blank_identifier">blank</a> names in the signature
must be <a href="#Uniqueness_of_identifiers">unique</a>.
If absent, each type stands for one item of that type.
Parameter and result
lists are always parenthesized except that if there is exactly
one unnamed result it may be written as an unparenthesized type.
</p>
<p>
The final incoming parameter in a function signature may have
a type prefixed with <code>...</code>.
A function with such a parameter is called <i>variadic</i> and
may be invoked with zero or more arguments for that parameter.
</p>
<pre>
func()
func(x int) int
func(a, _ int, z float32) bool
func(a, b int, z float32) (bool)
func(prefix string, values ...int)
func(a, b int, z float64, opt ...interface{}) (success bool)
func(int, int, float64) (float64, *[]int)
func(n int) func(p *T)
</pre>
<h3 id="Interface_types">Interface types</h3>
<p>
An interface type specifies a <a href="#Method_sets">method set</a> called its <i>interface</i>.
A variable of interface type can store a value of any type with a method set
that is any superset of the interface. Such a type is said to
<i>implement the interface</i>.
The value of an uninitialized variable of interface type is <code>nil</code>.
</p>
<pre class="ebnf">
InterfaceType = "interface" "{" { MethodSpec ";" } "}" .
MethodSpec = MethodName Signature | InterfaceTypeName .
MethodName = identifier .
InterfaceTypeName = TypeName .
</pre>
<p>
As with all method sets, in an interface type, each method must have a
<a href="#Uniqueness_of_identifiers">unique</a>
non-<a href="#Blank_identifier">blank</a> name.
</p>
<pre>
// A simple File interface
interface {
Read(b Buffer) bool
Write(b Buffer) bool
Close()
}
</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(b Buffer) bool { return … }
func (p T) Write(b Buffer) bool { return … }
func (p T) Close() { … }
</pre>
<p>
(where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
then the <code>File</code> interface is implemented by both <code>S1</code> and
<code>S2</code>, regardless of what other methods
<code>S1</code> and <code>S2</code> may have or share.
</p>
<p>
A type implements any interface comprising any subset of its methods
and may therefore implement several distinct interfaces. For
instance, all types implement the <i>empty interface</i>:
</p>
<pre>
interface{}
</pre>
<p>
Similarly, consider this interface specification,
which appears within a <a href="#Type_declarations">type declaration</a>
to define an interface called <code>Locker</code>:
</p>
<pre>
type Locker interface {
Lock()
Unlock()
}
</pre>
<p>
If <code>S1</code> and <code>S2</code> also implement
</p>
<pre>
func (p T) Lock() { … }
func (p T) Unlock() { … }
</pre>
<p>
they implement the <code>Locker</code> interface as well
as the <code>File</code> interface.
</p>
<p>
An interface <code>T</code> may use a (possibly qualified) interface type
name <code>E</code> in place of a method specification. This is called
<i>embedding</i> interface <code>E</code> in <code>T</code>; it adds
all (exported and non-exported) methods of <code>E</code> to the interface
<code>T</code>.
</p>
<pre>
type ReadWriter interface {
Read(b Buffer) bool
Write(b Buffer) bool
}
type File interface {
ReadWriter // same as adding the methods of ReadWriter
Locker // same as adding the methods of Locker
Close()
}
type LockedFile interface {
Locker
File // illegal: Lock, Unlock not unique
Lock() // illegal: Lock not unique
}
</pre>
<p>
An interface type <code>T</code> may not embed itself
or any interface type that embeds <code>T</code>, recursively.
</p>
<pre>
// illegal: Bad cannot embed itself
type Bad interface {
Bad
}
// illegal: Bad1 cannot embed itself using Bad2
type Bad1 interface {
Bad2
}
type Bad2 interface {
Bad1
}
</pre>
<h3 id="Map_types">Map types</h3>
<p>
A map is an unordered group of elements of one type, called the
element type, indexed by a set of unique <i>keys</i> of another type,
called the key type.
The value of an uninitialized map is <code>nil</code>.
</p>
<pre class="ebnf">
MapType = "map" "[" KeyType "]" ElementType .
KeyType = Type .
</pre>
<p>
The <a href="#Comparison_operators">comparison operators</a>
<code>==</code> and <code>!=</code> must be fully defined
for operands of the key type; thus the key type must not be a function, map, or
slice.
If the key type is an interface type, these
comparison operators must be defined for the dynamic key values;
failure will cause a <a href="#Run_time_panics">run-time panic</a>.
</p>
<pre>
map[string]int
map[*T]struct{ x, y float64 }
map[string]interface{}
</pre>
<p>
The number of map elements is called its length.
For a map <code>m</code>, it can be discovered using the
built-in function <a href="#Length_and_capacity"><code>len</code></a>
and may change during execution. Elements may be added during execution
using <a href="#Assignments">assignments</a> and retrieved with
<a href="#Index_expressions">index expressions</a>; they may be removed with the
<a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
</p>
<p>
A new, empty map value is made using the built-in
function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
which takes the map type and an optional capacity hint as arguments:
</p>
<pre>
make(map[string]int)
make(map[string]int, 100)
</pre>
<p>
The initial capacity does not bound its size:
maps grow to accommodate the number of items
stored in them, with the exception of <code>nil</code> maps.
A <code>nil</code> map is equivalent to an empty map except that no elements
may be added.
<h3 id="Channel_types">Channel types</h3>
<p>
A channel provides a mechanism for
<a href="#Go_statements">concurrently executing functions</a>
to communicate by
<a href="#Send_statements">sending</a> and
<a href="#Receive_operator">receiving</a>
values of a specified element type.
The value of an uninitialized channel is <code>nil</code>.
</p>
<pre class="ebnf">
ChannelType = ( "chan" | "chan" "&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 no direction is given, the channel is
<i>bidirectional</i>.
A channel may be constrained only to send or only to receive by
<a href="#Conversions">conversion</a> or <a href="#Assignments">assignment</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="Type_identity">Type identity</h3>
<p>
Two types are either <i>identical</i> or <i>different</i>.
</p>
<p>
A <a href="#Type_definitions">defined type</a> is always different from any other type.
Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
structurally equivalent; that is, they have the same literal structure and corresponding
components have identical types. In detail:
</p>
<ul>
<li>Two array types are identical if they have identical element types and
the same array length.</li>
<li>Two slice types are identical if they have identical element types.</li>
<li>Two struct types are identical if they have the same sequence of fields,
and if corresponding fields have the same names, and identical types,
and identical tags.
<a href="#Exported_identifiers">Non-exported</a> field names from different
packages are always different.</li>
<li>Two pointer types are identical if they have identical base types.</li>
<li>Two function types are identical if they have the same number of parameters
and result values, corresponding parameter and result types are
identical, and either both functions are variadic or neither is.
Parameter and result names are not required to match.</li>
<li>Two interface types are identical if they have the same set of methods
with the same names and identical function types.
<a href="#Exported_identifiers">Non-exported</a> method names from different
packages are always different. The order of the methods is irrelevant.</li>
<li>Two map types are identical if they have identical key and value types.</li>
<li>Two channel types are identical if they have identical value types and
the same direction.</li>
</ul>
<p>
Given the declarations
</p>
<pre>
type (
A0 = []string
A1 = A0
A2 = struct{ a, b int }
A3 = int
A4 = func(A3, float64) *A0
A5 = func(x int, _ float64) *[]string
)
type (
B0 A0
B1 []string
B2 struct{ a, b int }
B3 struct{ a, c int }
B4 func(int, float64) *B0
B5 func(x int, y float64) *A1
)
type C0 = B0
</pre>
<p>
these types are identical:
</p>
<pre>
A0, A1, and []string
A2 and struct{ a, b int }
A3 and int
A4, func(int, float64) *[]string, and A5
B0, B0, and C0
[]int and []int
struct{ a, b *T5 } and struct{ a, b *T5 }
func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
</pre>
<p>
<code>B0</code> and <code>B1</code> are different because they are new types
created by distinct <a href="#Type_definitions">type definitions</a>;
<code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
are different because <code>B0</code> is different from <code>[]string</code>.
</p>
<h3 id="Assignability">Assignability</h3>
<p>
A value <code>x</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
("<code>x</code> is assignable to <code>T</code>") in any of these cases:
</p>
<ul>
<li>
<code>x</code>'s type is identical to <code>T</code>.
</li>
<li>
<code>x</code>'s type <code>V</code> and <code>T</code> have identical
<a href="#Types">underlying types</a> and at least one of <code>V</code>
or <code>T</code> is not a <a href="#Type_definitions">defined</a> type.
</li>
<li>
<code>T</code> is an interface type and
<code>x</code> <a href="#Interface_types">implements</a> <code>T</code>.
</li>
<li>
<code>x</code> is a bidirectional channel value, <code>T</code> is a channel type,
<code>x</code>'s type <code>V</code> and <code>T</code> have identical element types,
and at least one of <code>V</code> or <code>T</code> is not a defined type.
</li>
<li>
<code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
is a pointer, function, slice, map, channel, or interface type.
</li>
<li>
<code>x</code> is an untyped <a href="#Constants">constant</a> representable
by a value of type <code>T</code>.
</li>
</ul>
<h2 id="Blocks">Blocks</h2>
<p>
A <i>block</i> is a possibly empty sequence of declarations and statements
within matching brace brackets.
</p>
<pre class="ebnf">
Block = "{" StatementList "}" .
StatementList = { Statement ";" } .
</pre>
<p>
In addition to explicit blocks in the source code, there are implicit blocks:
</p>
<ol>
<li>The <i>universe block</i> encompasses all Go source text.</li>
<li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
Go source text for that package.</li>
<li>Each file has a <i>file block</i> containing all Go source text
in that file.</li>
<li>Each <a href="#If_statements">"if"</a>,
<a href="#For_statements">"for"</a>, and
<a href="#Switch_statements">"switch"</a>
statement is considered to be in its own implicit block.</li>
<li>Each clause in a <a href="#Switch_statements">"switch"</a>
or <a href="#Select_statements">"select"</a> statement
acts as an implicit block.</li>
</ol>
<p>
Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
</p>
<h2 id="Declarations_and_scope">Declarations and scope</h2>
<p>
A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
<a href="#Constant_declarations">constant</a>,
<a href="#Type_declarations">type</a>,
<a href="#Variable_declarations">variable</a>,
<a href="#Function_declarations">function</a>,
<a href="#Labeled_statements">label</a>, or
<a href="#Import_declarations">package</a>.
Every identifier in a program must be declared.
No identifier may be declared twice in the same block, and
no identifier may be declared in both the file and package block.
</p>
<p>
The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
in a declaration, but it does not introduce a binding and thus is not declared.
In the package block, the identifier <code>init</code> may only be used for
<a href="#Package_initialization"><code>init</code> function</a> declarations,
and like the blank identifier it does not introduce a new binding.
</p>
<pre class="ebnf">
Declaration = ConstDecl | TypeDecl | VarDecl .
TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
</pre>
<p>
The <i>scope</i> of a declared identifier is the extent of source text in which
the identifier denotes the specified constant, type, variable, function, label, or package.
</p>
<p>
Go is lexically scoped using <a href="#Blocks">blocks</a>:
</p>
<ol>
<li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
<li>The scope of an identifier denoting a constant, type, variable,
or function (but not method) declared at top level (outside any
function) is the package block.</li>
<li>The scope of the package name of an imported package is the file block
of the file containing the import declaration.</li>
<li>The scope of an identifier denoting a method receiver, function parameter,
or result variable is the function body.</li>
<li>The scope of a constant or variable identifier declared
inside a function begins at the end of the ConstSpec or VarSpec
(ShortVarDecl for short variable declarations)
and ends at the end of the innermost containing block.</li>
<li>The scope of a type identifier declared inside a function
begins at the identifier in the TypeSpec
and ends at the end of the innermost containing block.</li>
</ol>
<p>
An identifier declared in a block may be redeclared in an inner block.
While the identifier of the inner declaration is in scope, it denotes
the entity declared by the inner declaration.
</p>
<p>
The <a href="#Package_clause">package clause</a> is not a declaration; the package name
does not appear in any scope. Its purpose is to identify the files belonging
to the same <a href="#Packages">package</a> and to specify the default package name for import
declarations.
</p>
<h3 id="Label_scopes">Label scopes</h3>
<p>
Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
used in the <a href="#Break_statements">"break"</a>,
<a href="#Continue_statements">"continue"</a>, and
<a href="#Goto_statements">"goto"</a> statements.
It is illegal to define a label that is never used.
In contrast to other identifiers, labels are not block scoped and do
not conflict with identifiers that are not labels. The scope of a label
is the body of the function in which it is declared and excludes
the body of any nested function.
</p>
<h3 id="Blank_identifier">Blank identifier</h3>
<p>
The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
It serves as an anonymous placeholder instead of a regular (non-blank)
identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>.
</p>
<h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
<p>
The following identifiers are implicitly declared in the
<a href="#Blocks">universe block</a>:
</p>
<pre class="grammar">
Types:
bool byte complex64 complex128 error float32 float64
int int8 int16 int32 int64 rune string
uint uint8 uint16 uint32 uint64 uintptr
Constants:
true false iota
Zero value:
nil
Functions:
append cap close complex copy delete imag len
make new panic print println real recover
</pre>
<h3 id="Exported_identifiers">Exported identifiers</h3>
<p>
An identifier may be <i>exported</i> to permit access to it from another package.
An identifier is exported if both:
</p>
<ol>
<li>the first character of the identifier's name is a Unicode upper case
letter (Unicode class "Lu"); and</li>
<li>the identifier is declared in the <a href="#Blocks">package block</a>
or it is a <a href="#Struct_types">field name</a> or
<a href="#MethodName">method name</a>.</li>
</ol>
<p>
All other identifiers are not exported.
</p>
<h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
<p>
Given a set of identifiers, an identifier is called <i>unique</i> if it is
<i>different</i> from every other in the set.
Two identifiers are different if they are spelled differently, or if they
appear in different <a href="#Packages">packages</a> and are not
<a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
</p>
<h3 id="Constant_declarations">Constant declarations</h3>
<p>
A constant declaration binds a list of identifiers (the names of
the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
The number of identifiers must be equal
to the number of expressions, and the <i>n</i>th identifier on
the left is bound to the value of the <i>n</i>th expression on the
right.
</p>
<pre class="ebnf">
ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
</pre>
<p>
If the type is present, all constants take the type specified, and
the expressions must be <a href="#Assignability">assignable</a> to that type.
If the type is omitted, the constants take the
individual types of the corresponding expressions.
If the expression values are untyped <a href="#Constants">constants</a>,
the declared constants remain untyped and the constant identifiers
denote the constant values. For instance, if the expression is a
floating-point literal, the constant identifier denotes a floating-point
constant, even if the literal's fractional part is zero.
</p>
<pre>
const Pi float64 = 3.14159265358979323846
const zero = 0.0 // untyped floating-point constant
const (
size int64 = 1024
eof = -1 // untyped integer constant
)
const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
const u, v float32 = 0, 3 // u = 0.0, v = 3.0
</pre>
<p>
Within a parenthesized <code>const</code> declaration list the
expression list may be omitted from any but the first declaration.
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>. It is reset to 0 whenever the reserved word <code>const</code>
appears in the source and increments after each <a href="#ConstSpec">ConstSpec</a>.
It can be used to construct a set of related constants:
</p>
<pre>
const ( // iota is reset to 0
c0 = iota // c0 == 0
c1 = iota // c1 == 1
c2 = iota // c2 == 2
)
const ( // iota is reset to 0
a = 1 &lt;&lt; iota // a == 1
b = 1 &lt;&lt; iota // b == 2
c = 3 // c == 3 (iota is not used but still incremented)
d = 1 &lt;&lt; iota // d == 8
)
const ( // iota is reset to 0
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 (iota has been reset)
const y = iota // y == 0 (iota has been reset)
</pre>
<p>
Within an ExpressionList, the value of each <code>iota</code> is the same because
it is only incremented after each ConstSpec:
</p>
<pre>
const (
bit0, mask0 = 1 &lt;&lt; iota, 1&lt;&lt;iota - 1 // bit0 == 1, mask0 == 0
bit1, mask1 // bit1 == 2, mask1 == 1
_, _ // skips iota == 2
bit3, mask3 // bit3 == 8, mask3 == 7
)
</pre>
<p>
This last example exploits the implicit repetition of the
last non-empty expression list.
</p>
<h3 id="Type_declarations">Type declarations</h3>
<p>
A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
Type declarations come in two forms: alias declarations and type definitions.
<p>
<pre class="ebnf">
TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec = AliasDecl | TypeDef .
</pre>
<h4 id="Alias_declarations">Alias declarations</h4>
<p>
An alias declaration binds an identifier to the given type.
</p>
<pre class="ebnf">
AliasDecl = identifier "=" Type .
</pre>
<p>
Within the <a href="#Declarations_and_scope">scope</a> of
the identifier, it serves as an <i>alias</i> for the type.
</p>
<pre>
type (
nodeList = []*Node // nodeList and []*Node are identical types
Polar = polar // Polar and polar denote identical types
)
</pre>
<h4 id="Type_definitions">Type definitions</h4>
<p>
A type definition creates a new, distinct type with the same
<a href="#Types">underlying type</a> and operations as the given type,
and binds an identifier to it.
</p>
<pre class="ebnf">
TypeDef = identifier Type .
</pre>
<p>
The new type is called a <i>defined type</i>.
It is <a href="#Type_identity">different</a> from any other type,
including the type it is created from.
</p>
<pre>
type (
Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
polar Point // polar and Point denote different types
)
type TreeNode struct {
left, right *TreeNode
value *Comparable
}
type Block interface {
BlockSize() int
Encrypt(src, dst []byte)
Decrypt(src, dst []byte)
}
</pre>
<p>
A defined type may have <a href="#Method_declarations">methods</a> associated with it.
It does not inherit any methods bound to the given type,
but the <a href="#Method_sets">method set</a>
of an interface type or of elements of a composite type remains unchanged:
</p>
<pre>
// A Mutex is a data type with two methods, Lock and Unlock.
type Mutex struct { /* Mutex fields */ }
func (m *Mutex) Lock() { /* Lock implementation */ }
func (m *Mutex) Unlock() { /* Unlock implementation */ }
// NewMutex has the same composition as Mutex but its method set is empty.
type NewMutex Mutex
// The method set of the <a href="#Pointer_types">base type</a> of PtrMutex remains unchanged,
// but the method set of PtrMutex is empty.
type PtrMutex *Mutex
// The method set of *PrintableMutex contains the methods
// Lock and Unlock bound to its embedded field Mutex.
type PrintableMutex struct {
Mutex
}
// MyBlock is an interface type that has the same method set as Block.
type MyBlock Block
</pre>
<p>
Type definitions may be used to define different boolean, numeric,
or string types and associate methods with them:
</p>
<pre>
type TimeZone int
const (
EST TimeZone = -(5 + iota)
CST
MST
PST
)
func (tz TimeZone) String() string {
return fmt.Sprintf("GMT%+dh", tz)
}
</pre>
<h3 id="Variable_declarations">Variable declarations</h3>
<p>
A variable declaration creates one or more <a href="#Variables">variables</a>,
binds corresponding identifiers to them, and gives each a type and an initial value.
</p>
<pre class="ebnf">
VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
</pre>
<pre>
var i int
var U, V, W float64
var k = 0
var x, y float32 = -1, -2
var (
i int
u, v, s = 2.0, 3.0, "bar"
)
var re, im = complexSqrt(-1)
var _, found = entries[name] // map lookup; only interested in "found"
</pre>
<p>
If a list of expressions is given, the variables are initialized
with the expressions following the rules for <a href="#Assignments">assignments</a>.
Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
</p>
<p>
If a type is present, each variable is given that type.
Otherwise, each variable is given the type of the corresponding
initialization value in the assignment.
If that value is an untyped constant, it is first
<a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
if it is an untyped boolean value, it is first 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(fd) // os.Pipe() returns two values
_, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
</pre>
<p>
Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
variables provided they were originally declared earlier in the same block
(or the parameter lists if the block is the function body) with the same type,
and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
As a consequence, redeclaration can only appear in a multi-variable short declaration.
Redeclaration does not introduce a new variable; it just assigns a new value to the original.
</p>
<pre>
field1, offset := nextField(str, 0)
field2, offset := nextField(str, offset) // redeclares offset
a, a := 1, 2 // illegal: double declaration of a or no new variable if a was declared elsewhere
</pre>
<p>
Short variable declarations may appear only inside functions.
In some contexts such as the initializers for
<a href="#If_statements">"if"</a>,
<a href="#For_statements">"for"</a>, or
<a href="#Switch_statements">"switch"</a> statements,
they can be used to declare local temporary variables.
</p>
<h3 id="Function_declarations">Function declarations</h3>
<p>
A function declaration binds an identifier, the <i>function name</i>,
to a function.
</p>
<pre class="ebnf">
FunctionDecl = "func" FunctionName ( Function | Signature ) .
FunctionName = identifier .
Function = Signature FunctionBody .
FunctionBody = Block .
</pre>
<p>
If the function's <a href="#Function_types">signature</a> declares
result parameters, the function body's statement list must end in
a <a href="#Terminating_statements">terminating statement</a>.
</p>
<pre>
func IndexRune(s string, r rune) int {
for i, c := range s {
if c == r {
return i
}
}
// invalid: missing return statement
}
</pre>
<p>
A function declaration may omit the body. Such a declaration provides the
signature for a function implemented outside Go, such as an assembly routine.
</p>
<pre>
func min(x int, y int) int {
if x &lt; y {
return x
}
return y
}
func flushICache(begin, end uintptr) // implemented externally
</pre>
<h3 id="Method_declarations">Method declarations</h3>
<p>
A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
A method declaration binds an identifier, the <i>method name</i>, to a method,
and associates the method with the receiver's <i>base type</i>.
</p>
<pre class="ebnf">
MethodDecl = "func" Receiver MethodName ( Function | Signature ) .
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 of the form <code>T</code> or <code>*T</code> (possibly using
parentheses) where <code>T</code> is a type name. The type denoted by <code>T</code> is called
the receiver <i>base type</i>; it must not be a pointer or interface type and
it must be <a href="#Type_definitions">defined</a> in the same package as the method.
The method is said to be <i>bound</i> to the 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 type <code>Point</code>, the declarations
</p>
<pre>
func (p *Point) Length() float64 {
return math.Sqrt(p.x * p.x + p.y * p.y)
}
func (p *Point) Scale(factor float64) {
p.x *= factor
p.y *= factor
}
</pre>
<p>
bind the methods <code>Length</code> and <code>Scale</code>,
with receiver type <code>*Point</code>,
to the base type <code>Point</code>.
</p>
<p>
The type of a method is the type of a function with the receiver as first
argument. For instance, the method <code>Scale</code> has type
</p>
<pre>
func(p *Point, factor float64)
</pre>
<p>
However, a function declared this way is not a method.
</p>
<h2 id="Expressions">Expressions</h2>
<p>
An expression specifies the computation of a value by applying
operators and functions to operands.
</p>
<h3 id="Operands">Operands</h3>
<p>
Operands denote the elementary values in an expression. An operand may be a
literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
non-<a href="#Blank_identifier">blank</a> identifier denoting a
<a href="#Constant_declarations">constant</a>,
<a href="#Variable_declarations">variable</a>, or
<a href="#Function_declarations">function</a>,
a <a href="#Method_expressions">method expression</a> yielding a function,
or a parenthesized expression.
</p>
<p>
The <a href="#Blank_identifier">blank identifier</a> may appear as an
operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
</p>
<pre class="ebnf">
Operand = Literal | OperandName | MethodExpr | "(" Expression ")" .
Literal = BasicLit | CompositeLit | FunctionLit .
BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
OperandName = identifier | QualifiedIdent.
</pre>
<h3 id="Qualified_identifiers">Qualified identifiers</h3>
<p>
A qualified identifier is an identifier qualified with a package name prefix.
Both the package name and the identifier must not be
<a href="#Blank_identifier">blank</a>.
</p>
<pre class="ebnf">
QualifiedIdent = PackageName "." identifier .
</pre>
<p>
A qualified identifier accesses an identifier in a different package, which
must be <a href="#Import_declarations">imported</a>.
The identifier must be <a href="#Exported_identifiers">exported</a> and
declared in the <a href="#Blocks">package block</a> of that package.
</p>
<pre>
math.Sin // denotes the Sin function in package math
</pre>
<h3 id="Composite_literals">Composite literals</h3>
<p>
Composite literals construct values for structs, arrays, slices, and maps
and create a new value each time they are evaluated.
They consist of the type of the literal followed by a brace-bound list of elements.
Each element may optionally be preceded by a corresponding key.
</p>
<pre class="ebnf">
CompositeLit = LiteralType LiteralValue .
LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
SliceType | MapType | TypeName .
LiteralValue = "{" [ ElementList [ "," ] ] "}" .
ElementList = KeyedElement { "," KeyedElement } .
KeyedElement = [ Key ":" ] Element .
Key = FieldName | Expression | LiteralValue .
FieldName = identifier .
Element = Expression | LiteralValue .
</pre>
<p>
The LiteralType's underlying type must be a struct, array, slice, or map type
(the grammar enforces this constraint except when the type is given
as a TypeName).
The types of the elements and keys must be <a href="#Assignability">assignable</a>
to the respective field, element, and key types of the literal type;
there is no additional conversion.
The key is interpreted as a field name for struct literals,
an index for array and slice literals, and a key for map literals.
For map literals, all elements must have a key. It is an error
to specify multiple elements with the same field name or
constant key value. For non-constant map keys, see the section on
<a href="#Order_of_evaluation">evaluation order</a>.
</p>
<p>
For struct literals the following rules apply:
</p>
<ul>
<li>A key must be a field name declared in the struct type.
</li>
<li>An element list that does not contain any keys must
list an element for each struct field in the
order in which the fields are declared.
</li>
<li>If any element has a key, every element must have a key.
</li>
<li>An element list that contains keys does not need to
have an element for each struct field. Omitted fields
get the zero value for that field.
</li>
<li>A literal may omit the element list; such a literal evaluates
to the zero value for its type.
</li>
<li>It is an error to specify an element for a non-exported
field of a struct belonging to a different package.
</li>
</ul>
<p>
Given the declarations
</p>
<pre>
type Point3D struct { x, y, z float64 }
type Line struct { p, q Point3D }
</pre>
<p>
one may write
</p>
<pre>
origin := Point3D{} // zero value for Point3D
line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
</pre>
<p>
For array and slice literals the following rules apply:
</p>
<ul>
<li>Each element has an associated integer index marking
its position in the array.
</li>
<li>An element with a key uses the key as its index. The
key must be a non-negative constant representable by
a value of type <code>int</code>; and if it is typed
it must be of integer type.
</li>
<li>An element without a key uses the previous element's index plus one.
If the first element has no key, its index is zero.
</li>
</ul>
<p>
<a href="#Address_operators">Taking the address</a> of a composite literal
generates a pointer to a unique <a href="#Variables">variable</a> initialized
with the literal's value.
</p>
<pre>
var pointer *Point3D = &amp;Point3D{y: 1000}
</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>
<p>
A parsing ambiguity arises when a composite literal using the
TypeName form of the LiteralType appears as an operand between the
<a href="#Keywords">keyword</a> and the opening brace of the block
of an "if", "for", or "switch" statement, and the composite literal
is not enclosed in parentheses, square brackets, or curly braces.
In this rare case, the opening brace of the literal is erroneously parsed
as the one introducing the block of statements. To resolve the ambiguity,
the composite literal must appear within parentheses.
</p>
<pre>
if x == (T{a,b,c}[i]) { … }
if (x == T{a,b,c}[i]) { … }
</pre>
<p>
Examples of valid array, slice, and map literals:
</p>
<pre>
// list of prime numbers
primes := []int{2, 3, 5, 7, 9, 2147483647}
// vowels[ch] is true if ch is a vowel
vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
// the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
// frequencies in Hz for equal-tempered scale (A4 = 440Hz)
noteFrequency := map[string]float32{
"C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
"G0": 24.50, "A0": 27.50, "B0": 30.87,
}
</pre>
<h3 id="Function_literals">Function literals</h3>
<p>
A function literal represents an anonymous <a href="#Function_declarations">function</a>.
</p>
<pre class="ebnf">
FunctionLit = "func" Function .
</pre>
<pre>
func(a, b int, z float64) bool { return a*b &lt; int(z) }
</pre>
<p>
A function literal can be assigned to a variable or invoked directly.
</p>
<pre>
f := func(x, y int) int { return x + y }
func(ch chan int) { ch &lt;- ACK }(replyChan)
</pre>
<p>
Function literals are <i>closures</i>: they may refer to variables
defined in a surrounding function. Those variables are then shared between
the surrounding function and the function literal, and they survive as long
as they are accessible.
</p>
<h3 id="Primary_expressions">Primary expressions</h3>
<p>
Primary expressions are the operands for unary and binary expressions.
</p>
<pre class="ebnf">
PrimaryExpr =
Operand |
Conversion |
PrimaryExpr Selector |
PrimaryExpr Index |
PrimaryExpr Slice |
PrimaryExpr TypeAssertion |
PrimaryExpr Arguments .
Selector = "." identifier .
Index = "[" Expression "]" .
Slice = "[" [ Expression ] ":" [ Expression ] "]" |
"[" [ Expression ] ":" Expression ":" Expression "]" .
TypeAssertion = "." "(" Type ")" .
Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
</pre>
<pre>
x
2
(s + ".txt")
f(3.1415, true)
Point{1, 2}
m["foo"]
s[i : j + 1]
obj.color
f.p[i].x()
</pre>
<h3 id="Selectors">Selectors</h3>
<p>
For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
that is not a <a href="#Package_clause">package name</a>, the
<i>selector expression</i>
</p>
<pre>
x.f
</pre>
<p>
denotes the field or method <code>f</code> of the value <code>x</code>
(or sometimes <code>*x</code>; see below).
The identifier <code>f</code> is called the (field or method) <i>selector</i>;
it must not be the <a href="#Blank_identifier">blank identifier</a>.
The type of the selector expression is the type of <code>f</code>.
If <code>x</code> is a package name, see the section on
<a href="#Qualified_identifiers">qualified identifiers</a>.
</p>
<p>
A selector <code>f</code> may denote a field or method <code>f</code> of
a type <code>T</code>, or it may refer
to a field or method <code>f</code> of a nested
<a href="#Struct_types">embedded field</a> of <code>T</code>.
The number of embedded fields traversed
to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
The depth of a field or method <code>f</code>
declared in <code>T</code> is zero.
The depth of a field or method <code>f</code> declared in
an embedded field <code>A</code> in <code>T</code> is the
depth of <code>f</code> in <code>A</code> plus one.
</p>
<p>
The following rules apply to selectors:
</p>
<ol>
<li>
For a value <code>x</code> of type <code>T</code> or <code>*T</code>
where <code>T</code> is not a pointer or interface type,
<code>x.f</code> denotes the field or method at the shallowest depth
in <code>T</code> where there
is such an <code>f</code>.
If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
with shallowest depth, the selector expression is illegal.
</li>
<li>
For a value <code>x</code> of type <code>I</code> where <code>I</code>
is an interface type, <code>x.f</code> denotes the actual method with name
<code>f</code> of the dynamic value of <code>x</code>.
If there is no method with name <code>f</code> in the
<a href="#Method_sets">method set</a> of <code>I</code>, the selector
expression is illegal.
</li>
<li>
As an exception, if the type of <code>x</code> is a named pointer type
and <code>(*x).f</code> is a valid selector expression denoting a field
(but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
</li>
<li>
In all other cases, <code>x.f</code> is illegal.
</li>
<li>
If <code>x</code> is of pointer type and has the value
<code>nil</code> and <code>x.f</code> denotes a struct field,
assigning to or evaluating <code>x.f</code>
causes a <a href="#Run_time_panics">run-time panic</a>.
</li>
<li>
If <code>x</code> is of interface type and has the value
<code>nil</code>, <a href="#Calls">calling</a> or
<a href="#Method_values">evaluating</a> the method <code>x.f</code>
causes a <a href="#Run_time_panics">run-time panic</a>.
</li>
</ol>
<p>
For example, given the declarations:
</p>
<pre>
type T0 struct {
x int
}
func (*T0) M0()
type T1 struct {
y int
}
func (T1) M1()
type T2 struct {
z int
T1
*T0
}
func (*T2) M2()
type Q *T2
var t T2 // with t.T0 != nil
var p *T2 // with p != nil and (*p).T0 != nil
var q Q = p
</pre>
<p>
one may write:
</p>
<pre>
t.z // t.z
t.y // t.T1.y
t.x // (*t.T0).x
p.z // (*p).z
p.y // (*p).T1.y
p.x // (*(*p).T0).x
q.x // (*(*q).T0).x (*q).x is a valid field selector
p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
p.M1() // ((*p).T1).M1() M1 expects T1 receiver
p.M2() // p.M2() M2 expects *T2 receiver
t.M2() // (&amp;t).M2() M2 expects *T2 receiver, see section on Calls
</pre>
<p>
but the following is invalid:
</p>
<pre>
q.M0() // (*q).M0 is valid but not a field selector
</pre>
<h3 id="Method_expressions">Method expressions</h3>
<p>
If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
<code>T.M</code> is a function that is callable as a regular function
with the same arguments as <code>M</code> prefixed by an additional
argument that is the receiver of the method.
</p>
<pre class="ebnf">
MethodExpr = ReceiverType "." MethodName .
ReceiverType = TypeName | "(" "*" TypeName ")" | "(" ReceiverType ")" .
</pre>
<p>
Consider a struct type <code>T</code> with two methods,
<code>Mv</code>, whose receiver is of type <code>T</code>, and
<code>Mp</code>, whose receiver is of type <code>*T</code>.
</p>
<pre>
type T struct {
a int
}
func (tv T) Mv(a int) int { return 0 } // value receiver
func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
var t T
</pre>
<p>
The expression
</p>
<pre>
T.Mv
</pre>
<p>
yields a function equivalent to <code>Mv</code> but
with an explicit receiver as its first argument; it has signature
</p>
<pre>
func(tv T, a int) int
</pre>
<p>
That function may be called normally with an explicit receiver, so
these five invocations are equivalent:
</p>
<pre>
t.Mv(7)
T.Mv(t, 7)
(T).Mv(t, 7)
f1 := T.Mv; f1(t, 7)
f2 := (T).Mv; f2(t, 7)
</pre>
<p>
Similarly, the expression
</p>
<pre>
(*T).Mp
</pre>
<p>
yields a function value representing <code>Mp</code> with signature
</p>
<pre>
func(tp *T, f float32) float32
</pre>
<p>
For a method with a value receiver, one can derive a function
with an explicit pointer receiver, so
</p>
<pre>
(*T).Mv
</pre>
<p>
yields a function value representing <code>Mv</code> with signature
</p>
<pre>
func(tv *T, a int) int
</pre>
<p>
Such a function indirects through the receiver to create a value
to pass as the receiver to the underlying method;
the method does not overwrite the value whose address is passed in
the function call.
</p>
<p>
The final case, a value-receiver function for a pointer-receiver method,
is illegal because pointer-receiver methods are not in the method set
of the value type.
</p>
<p>
Function values derived from methods are called with function call syntax;
the receiver is provided as the first argument to the call.
That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
as <code>f(t, 7)</code> not <code>t.f(7)</code>.
To construct a function that binds the receiver, use a
<a href="#Function_literals">function literal</a> or
<a href="#Method_values">method value</a>.
</p>
<p>
It is legal to derive a function value from a method of an interface type.
The resulting function takes an explicit receiver of that interface type.
</p>
<h3 id="Method_values">Method values</h3>
<p>
If the expression <code>x</code> has static type <code>T</code> and
<code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
<code>x.M</code> is called a <i>method value</i>.
The method value <code>x.M</code> is a function value that is callable
with the same arguments as a method call of <code>x.M</code>.
The expression <code>x</code> is evaluated and saved during the evaluation of the
method value; the saved copy is then used as the receiver in any calls,
which may be executed later.
</p>
<p>
The type <code>T</code> may be an interface or non-interface type.
</p>
<p>
As in the discussion of <a href="#Method_expressions">method expressions</a> above,
consider a struct type <code>T</code> with two methods,
<code>Mv</code>, whose receiver is of type <code>T</code>, and
<code>Mp</code>, whose receiver is of type <code>*T</code>.
</p>
<pre>
type T struct {
a int
}
func (tv T) Mv(a int) int { return 0 } // value receiver
func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
var t T
var pt *T
func makeT() T
</pre>
<p>
The expression
</p>
<pre>
t.Mv
</pre>
<p>
yields a function value of type
</p>
<pre>
func(int) int
</pre>
<p>
These two invocations are equivalent:
</p>
<pre>
t.Mv(7)
f := t.Mv; f(7)
</pre>
<p>
Similarly, the expression
</p>
<pre>
pt.Mp
</pre>
<p>
yields a function value of type
</p>
<pre>
func(float32) float32
</pre>
<p>
As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
</p>
<p>
As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&amp;t).Mp</code>.
</p>
<pre>
f := t.Mv; f(7) // like t.Mv(7)
f := pt.Mp; f(7) // like pt.Mp(7)
f := pt.Mv; f(7) // like (*pt).Mv(7)
f := t.Mp; f(7) // like (&amp;t).Mp(7)
f := makeT().Mp // invalid: result of makeT() is not addressable
</pre>
<p>
Although the examples above use non-interface types, it is also legal to create a method value
from a value of interface type.
</p>
<pre>
var i interface { M(int) } = myVal
f := i.M; f(7) // like i.M(7)
</pre>
<h3 id="Index_expressions">Index expressions</h3>
<p>
A primary expression of the form
</p>
<pre>
a[x]
</pre>
<p>
denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
The following rules apply:
</p>
<p>
If <code>a</code> is not a map:
</p>
<ul>
<li>the index <code>x</code> must be of integer type or untyped;
it is <i>in range</i> if <code>0 &lt;= x &lt; len(a)</code>,
otherwise it is <i>out of range</i></li>
<li>a <a href="#Constants">constant</a> index must be non-negative
and representable by a value of type <code>int</code>
</ul>
<p>
For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
</p>
<ul>
<li>a <a href="#Constants">constant</a> index must be in range</li>
<li>if <code>x</code> is out of range at run time,
a <a href="#Run_time_panics">run-time panic</a> occurs</li>
<li><code>a[x]</code> is the array element at index <code>x</code> and the type of
<code>a[x]</code> is the element type of <code>A</code></li>
</ul>
<p>
For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
</p>
<ul>
<li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
</ul>
<p>
For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
</p>
<ul>
<li>if <code>x</code> is out of range at run time,
a <a href="#Run_time_panics">run-time panic</a> occurs</li>
<li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
<code>a[x]</code> is the element type of <code>S</code></li>
</ul>
<p>
For <code>a</code> of <a href="#String_types">string type</a>:
</p>
<ul>
<li>a <a href="#Constants">constant</a> index must be in range
if the string <code>a</code> is also constant</li>
<li>if <code>x</code> is out of range at run time,
a <a href="#Run_time_panics">run-time panic</a> occurs</li>
<li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
<code>a[x]</code> is <code>byte</code></li>
<li><code>a[x]</code> may not be assigned to</li>
</ul>
<p>
For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
</p>
<ul>
<li><code>x</code>'s type must be
<a href="#Assignability">assignable</a>
to the key type of <code>M</code></li>
<li>if the map contains an entry with key <code>x</code>,
<code>a[x]</code> is the map value with key <code>x</code>
and the type of <code>a[x]</code> is the value type of <code>M</code></li>
<li>if the map is <code>nil</code> or does not contain such an entry,
<code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
for the value type of <code>M</code></li>
</ul>
<p>
Otherwise <code>a[x]</code> is illegal.
</p>
<p>
An index expression on a map <code>a</code> of type <code>map[K]V</code>
used in an <a href="#Assignments">assignment</a> or initialization of the special form
</p>
<pre>
v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]
var v, ok T = a[x]
</pre>
<p>
yields an additional untyped boolean value. The value of <code>ok</code> is
<code>true</code> if the key <code>x</code> is present in the map, and
<code>false</code> otherwise.
</p>
<p>
Assigning to an element of a <code>nil</code> map causes a
<a href="#Run_time_panics">run-time panic</a>.
</p>
<h3 id="Slice_expressions">Slice expressions</h3>
<p>
Slice expressions construct a substring or slice from a string, array, pointer
to array, or slice. There are two variants: a simple form that specifies a low
and high bound, and a full form that also specifies a bound on the capacity.
</p>
<h4>Simple slice expressions</h4>
<p>
For a string, array, pointer to array, or slice <code>a</code>, the primary expression
</p>
<pre>
a[low : high]
</pre>
<p>
constructs a substring or slice. The <i>indices</i> <code>low</code> and
<code>high</code> select which elements of operand <code>a</code> appear
in the result. The result has indices starting at 0 and length equal to
<code>high</code>&nbsp;-&nbsp;<code>low</code>.
After slicing the array <code>a</code>
</p>
<pre>
a := [5]int{1, 2, 3, 4, 5}
s := a[1:4]
</pre>
<p>
the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
</p>
<pre>
s[0] == 2
s[1] == 3
s[2] == 4
</pre>
<p>
For convenience, any of the indices may be omitted. A missing <code>low</code>
index defaults to zero; a missing <code>high</code> index defaults to the length of the
sliced operand:
</p>
<pre>
a[2:] // same as a[2 : len(a)]
a[:3] // same as a[0 : 3]
a[:] // same as a[0 : len(a)]
</pre>
<p>
If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
<code>(*a)[low : high]</code>.
</p>
<p>
For arrays or strings, the indices are <i>in range</i> if
<code>0</code> &lt;= <code>low</code> &lt;= <code>high</code> &lt;= <code>len(a)</code>,
otherwise they are <i>out of range</i>.
For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
A <a href="#Constants">constant</a> index must be non-negative and representable by a value of type
<code>int</code>; for arrays or constant strings, constant indices must also be in range.
If both indices are constant, they must satisfy <code>low &lt;= high</code>.
If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
</p>
<p>
Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
the result of the slice operation is a non-constant value of the same type as the operand.
For untyped string operands the result is a non-constant value of type <code>string</code>.
If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
and the result of the slice operation is a slice with the same element type as the array.
</p>
<p>
If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
is a <code>nil</code> slice. Otherwise, the result shares its underlying array with the
operand.
</p>
<h4>Full slice expressions</h4>
<p>
For an array, pointer to array, or slice <code>a</code> (but not a string), the primary expression
</p>
<pre>
a[low : high : max]
</pre>
<p>
constructs a slice of the same type, and with the same length and elements as the simple slice
expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
After slicing the array <code>a</code>
</p>
<pre>
a := [5]int{1, 2, 3, 4, 5}
t := a[1:3:5]
</pre>
<p>
the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
</p>
<pre>
t[0] == 2
t[1] == 3
</pre>
<p>
As for simple slice expressions, if <code>a</code> is a pointer to an array,
<code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
</p>
<p>
The indices are <i>in range</i> if <code>0 &lt;= low &lt;= high &lt;= max &lt;= cap(a)</code>,
otherwise they are <i>out of range</i>.
A <a href="#Constants">constant</a> index must be non-negative and representable by a value of type
<code>int</code>; for arrays, constant indices must also be in range.
If multiple indices are constant, the constants that are present must be in range relative to each
other.
If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
</p>
<h3 id="Type_assertions">Type assertions</h3>
<p>
For an expression <code>x</code> of <a href="#Interface_types">interface type</a>
and a type <code>T</code>, the primary expression
</p>
<pre>
x.(T)
</pre>
<p>
asserts that <code>x</code> is not <code>nil</code>
and that the value stored in <code>x</code> is of type <code>T</code>.
The notation <code>x.(T)</code> is called a <i>type assertion</i>.
</p>
<p>
More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
to the type <code>T</code>.
In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
otherwise the type assertion is invalid since it is not possible for <code>x</code>
to store a value of type <code>T</code>.
If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
of <code>x</code> implements the interface <code>T</code>.
</p>
<p>
If the type assertion holds, the value of the expression is the value
stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
a <a href="#Run_time_panics">run-time panic</a> occurs.
In other words, even though the dynamic type of <code>x</code>
is known only at run time, the type of <code>x.(T)</code> is
known to be <code>T</code> in a correct program.
</p>
<pre>
var x interface{} = 7 // x has dynamic type int and value 7
i := x.(int) // i has type int and value 7
type I interface { m() }
func f(y I) {
s := y.(string) // illegal: string does not implement I (missing method m)
r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
}
</pre>
<p>
A type assertion used in an <a href="#Assignments">assignment</a> or initialization of the special form
</p>
<pre>
v, ok = x.(T)
v, ok := x.(T)
var v, ok = x.(T)
var v, ok T1 = x.(T)
</pre>
<p>
yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
No run-time panic occurs in this case.
</p>
<h3 id="Calls">Calls</h3>
<p>
Given an expression <code>f</code> of function type
<code>F</code>,
</p>
<pre>
f(a1, a2, … an)
</pre>
<p>
calls <code>f</code> with arguments <code>a1, a2, … an</code>.
Except for one special case, arguments must be single-valued expressions
<a href="#Assignability">assignable</a> to the parameter types of
<code>F</code> and are evaluated before the function is called.
The type of the expression is the result type
of <code>F</code>.
A method invocation is similar but the method itself
is specified as a selector upon a value of the receiver type for
the method.
</p>
<pre>
math.Atan2(x, y) // function call
var pt *Point
pt.Scale(3.5) // method call with receiver pt
</pre>
<p>
In a function call, the function value and arguments are evaluated in
<a href="#Order_of_evaluation">the usual order</a>.
After they are evaluated, the parameters of the call are passed by value to the function
and the called function begins execution.
The return parameters of the function are passed by value
back to the calling function when the function returns.
</p>
<p>
Calling a <code>nil</code> function value
causes a <a href="#Run_time_panics">run-time panic</a>.
</p>
<p>
As a special case, if the return values of a function or method
<code>g</code> are equal in number and individually
assignable to the parameters of another function or method
<code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
will invoke <code>f</code> after binding the return values of
<code>g</code> to the parameters of <code>f</code> in order. The call
of <code>f</code> must contain no parameters other than the call of <code>g</code>,
and <code>g</code> must have at least one return value.
If <code>f</code> has a final <code>...</code> parameter, it is
assigned the return values of <code>g</code> that remain after
assignment of regular parameters.
</p>
<pre>
func Split(s string, pos int) (string, string) {
return s[0:pos], s[pos:]
}
func Join(s, t string) string {
return s + t
}
if Join(Split(value, len(value)/2)) != value {
log.Panic("test fails")
}
</pre>
<p>
A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
of (the type of) <code>x</code> contains <code>m</code> and the
argument list can be assigned to the parameter list of <code>m</code>.
If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&amp;x</code>'s method
set contains <code>m</code>, <code>x.m()</code> is shorthand
for <code>(&amp;x).m()</code>:
</p>
<pre>
var p Point
p.Scale(3.5)
</pre>
<p>
There is no distinct method type and there are no method literals.
</p>
<h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
<p>
If <code>f</code> is <a href="#Function_types">variadic</a> with a final
parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
the type of <code>p</code> is equivalent to type <code>[]T</code>.
If <code>f</code> is invoked with no actual arguments for <code>p</code>,
the value passed to <code>p</code> is <code>nil</code>.
Otherwise, the value passed is a new slice
of type <code>[]T</code> with a new underlying array whose successive elements
are the actual arguments, which all must be <a href="#Assignability">assignable</a>
to <code>T</code>. The length and capacity of the slice is therefore
the number of arguments bound to <code>p</code> and may differ for each
call site.
</p>
<p>
Given the function and calls
</p>
<pre>
func Greeting(prefix string, who ...string)
Greeting("nobody")
Greeting("hello:", "Joe", "Anna", "Eileen")
</pre>
<p>
within <code>Greeting</code>, <code>who</code> will have the value
<code>nil</code> in the first call, and
<code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
</p>
<p>
If the final argument is assignable to a slice type <code>[]T</code>, it may be
passed unchanged as the value for a <code>...T</code> parameter if the argument
is followed by <code>...</code>. In this case no new slice is created.
</p>
<p>
Given the slice <code>s</code> and call
</p>
<pre>
s := []string{"James", "Jasmine"}
Greeting("goodbye:", s...)
</pre>
<p>
within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
with the same underlying array.
</p>
<h3 id="Operators">Operators</h3>
<p>
Operators combine operands into expressions.
</p>
<pre class="ebnf">
Expression = UnaryExpr | Expression binary_op Expression .
UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
binary_op = "||" | "&amp;&amp;" | rel_op | add_op | mul_op .
rel_op = "==" | "!=" | "&lt;" | "&lt;=" | ">" | ">=" .
add_op = "+" | "-" | "|" | "^" .
mul_op = "*" | "/" | "%" | "&lt;&lt;" | "&gt;&gt;" | "&amp;" | "&amp;^" .
unary_op = "+" | "-" | "!" | "^" | "*" | "&amp;" | "&lt;-" .
</pre>
<p>
Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
For operations involving constants only, see the section on
<a href="#Constant_expressions">constant expressions</a>.
</p>
<p>
Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
and the other operand is not, the constant is <a href="#Conversions">converted</a>
to the type of the other operand.
</p>
<p>
The right operand in a shift expression must have unsigned integer type
or be an untyped constant representable by a value of type <code>uint</code>.
If the left operand of a non-constant shift expression is an untyped constant,
it is first converted to the type it would assume if the shift expression were
replaced by its left operand alone.
</p>
<pre>
var s uint = 33
var i = 1&lt;&lt;s // 1 has type int
var j int32 = 1&lt;&lt;s // 1 has type int32; j == 0
var k = uint64(1&lt;&lt;s) // 1 has type uint64; k == 1&lt;&lt;33
var m int = 1.0&lt;&lt;s // 1.0 has type int; m == 0 if ints are 32bits in size
var n = 1.0&lt;&lt;s == j // 1.0 has type int32; n == true
var o = 1&lt;&lt;s == 2&lt;&lt;s // 1 and 2 have type int; o == true if ints are 32bits in size
var p = 1&lt;&lt;s == 1&lt;&lt;33 // illegal if ints are 32bits in size: 1 has type int, but 1&lt;&lt;33 overflows int
var u = 1.0&lt;&lt;s // illegal: 1.0 has type float64, cannot shift
var u1 = 1.0&lt;&lt;s != 0 // illegal: 1.0 has type float64, cannot shift
var u2 = 1&lt;&lt;s != 1.0 // illegal: 1 has type float64, cannot shift
var v float32 = 1&lt;&lt;s // illegal: 1 has type float32, cannot shift
var w int64 = 1.0&lt;&lt;33 // 1.0&lt;&lt;33 is a constant shift expression
</pre>
<h4 id="Operator_precedence">Operator precedence</h4>
<p>
Unary operators have the highest precedence.
As the <code>++</code> and <code>--</code> operators form
statements, not expressions, they fall
outside the operator hierarchy.
As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
<p>
There are five precedence levels for binary operators.
Multiplication operators bind strongest, followed by addition
operators, comparison operators, <code>&amp;&amp;</code> (logical AND),
and finally <code>||</code> (logical OR):
</p>
<pre class="grammar">
Precedence Operator
5 * / % &lt;&lt; &gt;&gt; &amp; &amp;^
4 + - | ^
3 == != &lt; &lt;= &gt; &gt;=
2 &amp;&amp;
1 ||
</pre>
<p>
Binary operators of the same precedence associate from left to right.
For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
</p>
<pre>
+x
23 + 3*x[i]
x &lt;= f()
^a &gt;&gt; b
f() || g()
x == y+1 &amp;&amp; &lt;-chanPtr &gt; 0
</pre>
<h3 id="Arithmetic_operators">Arithmetic operators</h3>
<p>
Arithmetic operators apply to numeric values and yield a result of the same
type as the first operand. The four standard arithmetic operators (<code>+</code>,
<code>-</code>, <code>*</code>, <code>/</code>) apply to integer,
floating-point, and complex types; <code>+</code> also applies to strings.
The bitwise logical and shift operators apply to integers only.
</p>
<pre class="grammar">
+ sum integers, floats, complex values, strings
- difference integers, floats, complex values
* product integers, floats, complex values
/ quotient integers, floats, complex values
% remainder integers
&amp; bitwise AND integers
| bitwise OR integers
^ bitwise XOR integers
&amp;^ bit clear (AND NOT) integers
&lt;&lt; left shift integer &lt;&lt; unsigned integer
&gt;&gt; right shift integer &gt;&gt; unsigned integer
</pre>
<h4 id="Integer_operators">Integer operators</h4>
<p>
For two integer values <code>x</code> and <code>y</code>, the integer quotient
<code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
relationships:
</p>
<pre>
x = q*y + r and |r| &lt; |y|
</pre>
<p>
with <code>x / y</code> truncated towards zero
(<a href="http://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
</p>
<pre>
x y x / y x % y
5 3 1 2
-5 3 -1 -2
5 -3 -1 2
-5 -3 1 -2
</pre>
<p>
As an exception to this rule, if the dividend <code>x</code> is the most
negative value for the int type of <code>x</code>, the quotient
<code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>).
</p>
<pre>
x, q
int8 -128
int16 -32768
int32 -2147483648
int64 -9223372036854775808
</pre>
<p>
If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
If the dividend is non-negative and the divisor is a constant power of 2,
the division may be replaced by a right shift, and computing the remainder may
be replaced by a bitwise AND operation:
</p>
<pre>
x x / 4 x % 4 x &gt;&gt; 2 x &amp; 3
11 2 3 2 3
-11 -2 -3 -3 1
</pre>
<p>
The shift operators shift the left operand by the shift count specified by the
right operand. They implement arithmetic shifts if the left operand is a signed
integer and logical shifts if it is an unsigned integer.
There is no upper limit on the shift count. Shifts behave
as if the left operand is shifted <code>n</code> times by 1 for a shift
count of <code>n</code>.
As a result, <code>x &lt;&lt; 1</code> is the same as <code>x*2</code>
and <code>x &gt;&gt; 1</code> is the same as
<code>x/2</code> but truncated towards negative infinity.
</p>
<p>
For integer operands, the unary operators
<code>+</code>, <code>-</code>, and <code>^</code> are defined as
follows:
</p>
<pre class="grammar">
+x is 0 + x
-x negation is 0 - x
^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
and m = -1 for signed x
</pre>
<h4 id="Integer_overflow">Integer overflow</h4>
<p>
For unsigned integer values, the operations <code>+</code>,
<code>-</code>, <code>*</code>, and <code>&lt;&lt;</code> are
computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
the <a href="#Numeric_types">unsigned integer</a>'s type.
Loosely speaking, these unsigned integer operations
discard high bits upon overflow, and programs may rely on ``wrap around''.
</p>
<p>
For signed integers, the operations <code>+</code>,
<code>-</code>, <code>*</code>, and <code>&lt;&lt;</code> may legally
overflow and the resulting value exists and is deterministically defined
by the signed integer representation, the operation, and its operands.
No exception is raised as a result of overflow. A
compiler may not optimize code under the assumption that overflow does
not occur. For instance, it may not assume that <code>x &lt; x + 1</code> is always true.
</p>
<h4 id="Floating_point_operators">Floating-point operators</h4>
<p>
For floating-point and complex numbers,
<code>+x</code> is the same as <code>x</code>,
while <code>-x</code> is the negation of <code>x</code>.
The result of a floating-point or complex division by zero is not specified beyond the
IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
occurs is implementation-specific.
</p>
<p>
An implementation may combine multiple floating-point operations into a single
fused operation, possibly across statements, and produce a result that differs
from the value obtained by executing and rounding the instructions individually.
A floating-point type <a href="#Conversions">conversion</a> explicitly rounds to
the precision of the target type, preventing fusion that would discard that rounding.
</p>
<p>
For instance, some architectures provide a "fused multiply and add" (FMA) instruction
that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
These examples show when a Go implementation can use that instruction:
</p>
<pre>
// FMA allowed for computing r, because x*y is not explicitly rounded:
r = x*y + z
r = z; r += x*y
t = x*y; r = t + z
*p = x*y; r = *p + z
r = x*y + float64(z)
// FMA disallowed for computing r, because it would omit rounding of x*y:
r = float64(x*y) + z
r = z; r += float64(x*y)
t = float64(x*y); r = t + z
</pre>
<h4 id="String_concatenation">String concatenation</h4>
<p>
Strings can be concatenated using the <code>+</code> operator
or the <code>+=</code> assignment operator:
</p>
<pre>
s := "hi" + string(c)
s += " and good bye"
</pre>
<p>
String addition creates a new string by concatenating the operands.
</p>
<h3 id="Comparison_operators">Comparison operators</h3>
<p>
Comparison operators compare two operands and yield an untyped boolean value.
</p>
<pre class="grammar">
== equal
!= not equal
&lt; less
&lt;= less or equal
&gt; greater
&gt;= greater or equal
</pre>
<p>
In any comparison, the first operand
must be <a href="#Assignability">assignable</a>
to the type of the second operand, or vice versa.
</p>
<p>
The equality operators <code>==</code> and <code>!=</code> apply
to operands that are <i>comparable</i>.
The ordering operators <code>&lt;</code>, <code>&lt;=</code>, <code>&gt;</code>, and <code>&gt;=</code>
apply to operands that are <i>ordered</i>.
These terms and the result of the comparisons are defined as follows:
</p>
<ul>
<li>
Boolean values are comparable.
Two boolean values are equal if they are either both
<code>true</code> or both <code>false