@go-len/go-len-runtime-linux/go/doc/go_spec.html

Embedded Go runtime

<!--{
	"Title": "The Go Programming Language Specification",
	"Subtitle": "Version of November 16, 2018",
	"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.
</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="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
canonicalized, so a single accented code point is distinct from the
same character constructed from combining an accent and a letter;
those are treated as two code points.  For simplicity, this document
will use the unqualified term <i>character</i> to refer to a Unicode code point
in the source text.
</p>
<p>
Each code point is distinct; for instance, upper and lower case letters
are different characters.
</p>
<p>
Implementation restriction: For compatibility with other tools, a
compiler may disallow the NUL character (U+0000) in the source text.
</p>
<p>
Implementation restriction: For compatibility with other tools, a
compiler may ignore a UTF-8-encoded byte order mark
(U+FEFF) if it is the first Unicode code point in the source text.
A byte order mark may be disallowed anywhere else in the source.
</p>

<h3 id="Characters">Characters</h3>

<p>
The following terms are used to denote specific Unicode character classes:
</p>
<pre class="ebnf">
newline        = /* the Unicode code point U+000A */ .
unicode_char   = /* an arbitrary Unicode code point except newline */ .
unicode_letter = /* a Unicode code point classified as "Letter" */ .
unicode_digit  = /* a Unicode code point classified as "Number, decimal digit" */ .
</pre>

<p>
In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
Section 4.5 "General Category" defines a set of character categories.
Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
as Unicode letters, and those in the Number category Nd as Unicode digits.
</p>

<h3 id="Letters_and_digits">Letters and digits</h3>

<p>
The underscore character <code>_</code> (U+005F) is considered a letter.
</p>
<pre class="ebnf">
letter        = unicode_letter | "_" .
decimal_digit = "0" … "9" .
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 <a href="#Representability">represented</a> as a value of the respective type.
</p>

<p>
An untyped constant has a <i>default type</i> which is the type to which the
constant is implicitly converted in contexts where a typed value is required,
for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
such as <code>i := 0</code> where there is no explicit type.
The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
<code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
respectively, depending on whether it is a boolean, rune, integer, floating-point,
complex, or string constant.
</p>

<p>
Implementation restriction: Although numeric constants have arbitrary
precision in the language, a compiler may implement them using an
internal representation with limited precision.  That said, every
implementation must:
</p>
<ul>
	<li>Represent integer constants with at least 256 bits.</li>

	<li>Represent floating-point constants, including the parts of
	    a complex constant, with a mantissa of at least 256 bits
	    and a signed binary exponent of at least 16 bits.</li>

	<li>Give an error if unable to represent an integer constant
	    precisely.</li>

	<li>Give an error if unable to represent a floating-point or
	    complex constant due to overflow.</li>

	<li>Round to the nearest representable constant if unable to
	    represent a floating-point or complex constant due to limits
	    on precision.</li>
</ul>
<p>
These requirements apply both to literal constants and to the result
of evaluating <a href="#Constant_expressions">constant
expressions</a>.
</p>

<h2 id="Variables">Variables</h2>

<p>
A variable is a storage location for holding a <i>value</i>.
The set of permissible values is determined by the
variable's <i><a href="#Types">type</a></i>.
</p>

<p>
A <a href="#Variable_declarations">variable declaration</a>
or, for function parameters and results, the signature
of a <a href="#Function_declarations">function declaration</a>
or <a href="#Function_literals">function literal</a> reserves
storage for a named variable.

Calling the built-in function <a href="#Allocation"><code>new</code></a>
or taking the address of a <a href="#Composite_literals">composite literal</a>
allocates storage for a variable at run time.
Such an anonymous variable is referred to via a (possibly implicit)
<a href="#Address_operators">pointer indirection</a>.
</p>

<p>
<i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
and <a href="#Struct_types">struct</a> types have elements and fields that may
be <a href="#Address_operators">addressed</a> individually. Each such element
acts like a variable.
</p>

<p>
The <i>static type</i> (or just <i>type</i>) of a variable is the
type given in its declaration, the type provided in the
<code>new</code> call or composite literal, or the type of
an element of a structured variable.
Variables of interface type also have a distinct <i>dynamic type</i>,
which is the concrete type of the value assigned to the variable at run time
(unless the value is the predeclared identifier <code>nil</code>,
which has no type).
The dynamic type may vary during execution but values stored in interface
variables are always <a href="#Assignability">assignable</a>
to the static type of the variable.
</p>

<pre>
var x interface{}  // x is nil and has static type interface{}
var v *T           // v has value nil, static type *T
x = 42             // x has value 42 and dynamic type int
x = v              // x has value (*T)(nil) and dynamic type *T
</pre>

<p>
A variable's value is retrieved by referring to the variable in an
<a href="#Expressions">expression</a>; it is the most recent value
<a href="#Assignments">assigned</a> to the variable.
If a variable has not yet been assigned a value, its value is the
<a href="#The_zero_value">zero value</a> for its type.
</p>


<h2 id="Types">Types</h2>

<p>
A type determines a set of values together with operations and methods specific
to those values. A type may be denoted by a <i>type name</i>, if it has one,
or specified using a <i>type literal</i>, which composes a type from existing types.
</p>

<pre class="ebnf">
Type      = TypeName | TypeLit | "(" Type ")" .
TypeName  = identifier | QualifiedIdent .
TypeLit   = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
	    SliceType | MapType | ChannelType .
</pre>

<p>
The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
Others are introduced with <a href="#Type_declarations">type declarations</a>.
<i>Composite types</i>&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>;
it is a <a href="#Type_definitions">defined type</a>.
</p>

<h3 id="Numeric_types">Numeric types</h3>

<p>
A <i>numeric type</i> represents sets of integer or floating-point values.
The predeclared architecture-independent numeric types are:
</p>

<pre class="grammar">
uint8       the set of all unsigned  8-bit integers (0 to 255)
uint16      the set of all unsigned 16-bit integers (0 to 65535)
uint32      the set of all unsigned 32-bit integers (0 to 4294967295)
uint64      the set of all unsigned 64-bit integers (0 to 18446744073709551615)

int8        the set of all signed  8-bit integers (-128 to 127)
int16       the set of all signed 16-bit integers (-32768 to 32767)
int32       the set of all signed 32-bit integers (-2147483648 to 2147483647)
int64       the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)

float32     the set of all IEEE-754 32-bit floating-point numbers
float64     the set of all IEEE-754 64-bit floating-point numbers

complex64   the set of all complex numbers with float32 real and imaginary parts
complex128  the set of all complex numbers with float64 real and imaginary parts

byte        alias for uint8
rune        alias for int32
</pre>

<p>
The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
<a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
</p>

<p>
There is also a set of predeclared numeric types with implementation-specific sizes:
</p>

<pre class="grammar">
uint     either 32 or 64 bits
int      same size as uint
uintptr  an unsigned integer large enough to store the uninterpreted bits of a pointer value
</pre>

<p>
To avoid portability issues all numeric types are <a href="#Type_definitions">defined
types</a> and thus distinct except
<code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
<code>rune</code>, which is an alias for <code>int32</code>.
Explicit conversions
are required when different numeric types are mixed in an expression
or assignment. For instance, <code>int32</code> and <code>int</code>
are not the same type even though they may have the same size on a
particular architecture.


<h3 id="String_types">String types</h3>

<p>
A <i>string type</i> represents the set of string values.
A string value is a (possibly empty) sequence of bytes.
The number of bytes is called the length of the string and is never negative.
Strings are immutable: once created,
it is impossible to change the contents of a string.
The predeclared string type is <code>string</code>;
it is a <a href="#Type_definitions">defined type</a>.
</p>

<p>
The length of a string <code>s</code> can be discovered using
the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
The length is a compile-time constant if the string is a constant.
A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
0 through <code>len(s)-1</code>.
It is illegal to take the address of such an element; if
<code>s[i]</code> is the <code>i</code>'th byte of a
string, <code>&amp;s[i]</code> is invalid.
</p>


<h3 id="Array_types">Array types</h3>

<p>
An array is a numbered sequence of elements of a single
type, called the element type.
The number of elements is called the length of the array and is never negative.
</p>

<pre class="ebnf">
ArrayType   = "[" ArrayLength "]" ElementType .
ArrayLength = Expression .
ElementType = Type .
</pre>

<p>
The length is part of the array's type; it must evaluate to a
non-negative <a href="#Constants">constant</a>
<a href="#Representability">representable</a> by a value
of type <code>int</code>.
The length of array <code>a</code> can be discovered
using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
The elements can be addressed by integer <a href="#Index_expressions">indices</a>
0 through <code>len(a)-1</code>.
Array types are always one-dimensional but may be composed to form
multi-dimensional types.
</p>

<pre>
[32]byte
[2*N] struct { x, y int32 }
[1000]*float64
[3][5]int
[2][2][2]float64  // same as [2]([2]([2]float64))
</pre>

<h3 id="Slice_types">Slice types</h3>

<p>
A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
provides access to a numbered sequence of elements from that array.
A slice type denotes the set of all slices of arrays of its element type.
The number of elements is called the length of the slice and is never negative.
The value of an uninitialized slice is <code>nil</code>.
</p>

<pre class="ebnf">
SliceType = "[" "]" ElementType .
</pre>

<p>
The length of a slice <code>s</code> can be discovered by the built-in function
<a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
execution.  The elements can be addressed by integer <a href="#Index_expressions">indices</a>
0 through <code>len(s)-1</code>.  The slice index of a
given element may be less than the index of the same element in the
underlying array.
</p>
<p>
A slice, once initialized, is always associated with an underlying
array that holds its elements.  A slice therefore shares storage
with its array and with other slices of the same array; by contrast,
distinct arrays always represent distinct storage.
</p>
<p>
The array underlying a slice may extend past the end of the slice.
The <i>capacity</i> is a measure of that extent: it is the sum of
the length of the slice and the length of the array beyond the slice;
a slice of length up to that capacity can be created by
<a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
The capacity of a slice <code>a</code> can be discovered using the
built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
</p>

<p>
A new, initialized slice value for a given element type <code>T</code> is
made using the built-in function
<a href="#Making_slices_maps_and_channels"><code>make</code></a>,
which takes a slice type
and parameters specifying the length and optionally the capacity.
A slice created with <code>make</code> always allocates a new, hidden array
to which the returned slice value refers. That is, executing
</p>

<pre>
make([]T, length, capacity)
</pre>

<p>
produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
it, so these two expressions are equivalent:
</p>

<pre>
make([]int, 50, 100)
new([100]int)[0:50]
</pre>

<p>
Like arrays, slices are always one-dimensional but may be composed to construct
higher-dimensional objects.
With arrays of arrays, the inner arrays are, by construction, always the same length;
however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
Moreover, the inner slices must be initialized individually.
</p>

<h3 id="Struct_types">Struct types</h3>

<p>
A struct is a sequence of named elements, called fields, each of which has a
name and a type. Field names may be specified explicitly (IdentifierList) or
implicitly (EmbeddedField).
Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
be <a href="#Uniqueness_of_identifiers">unique</a>.
</p>

<pre class="ebnf">
StructType    = "struct" "{" { FieldDecl ";" } "}" .
FieldDecl     = (IdentifierList Type | EmbeddedField) [ Tag ] .
EmbeddedField = [ "*" ] TypeName .
Tag           = string_lit .
</pre>

<pre>
// An empty struct.
struct {}

// A struct with 6 fields.
struct {
	x, y int
	u float32
	_ float32  // padding
	A *[]int
	F func()
}
</pre>

<p>
A field declared with a type but no explicit field name is called an <i>embedded field</i>.
An embedded field must be specified as
a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
and <code>T</code> itself may not be
a pointer type. The unqualified type name acts as the field name.
</p>

<pre>
// A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
struct {
	T1        // field name is T1
	*T2       // field name is T2
	P.T3      // field name is T3
	*P.T4     // field name is T4
	x, y int  // field names are x and y
}
</pre>

<p>
The following declaration is illegal because field names must be unique
in a struct type:
</p>

<pre>
struct {
	T     // conflicts with embedded field *T and *P.T
	*T    // conflicts with embedded field T and *P.T
	*P.T  // conflicts with embedded field T and *T
}
</pre>

<p>
A field or <a href="#Method_declarations">method</a> <code>f</code> of an
embedded field in a struct <code>x</code> is called <i>promoted</i> if
<code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
that field or method <code>f</code>.
</p>

<p>
Promoted fields act like ordinary fields
of a struct except that they cannot be used as field names in
<a href="#Composite_literals">composite literals</a> of the struct.
</p>

<p>
Given a struct type <code>S</code> and a <a href="#Type_definitions">defined type</a>
<code>T</code>, promoted methods are included in the method set of the struct as follows:
</p>
<ul>
	<li>
	If <code>S</code> contains an embedded field <code>T</code>,
	the <a href="#Method_sets">method sets</a> of <code>S</code>
	and <code>*S</code> both include promoted methods with receiver
	<code>T</code>. The method set of <code>*S</code> also
	includes promoted methods with receiver <code>*T</code>.
	</li>

	<li>
	If <code>S</code> contains an embedded field <code>*T</code>,
	the method sets of <code>S</code> and <code>*S</code> both
	include promoted methods with receiver <code>T</code> or
	<code>*T</code>.
	</li>
</ul>

<p>
A field declaration may be followed by an optional string literal <i>tag</i>,
which becomes an attribute for all the fields in the corresponding
field declaration. An empty tag string is equivalent to an absent tag.
The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
and take part in <a href="#Type_identity">type identity</a> for structs
but are otherwise ignored.
</p>

<pre>
struct {
	x, y float64 ""  // an empty tag string is like an absent tag
	name string  "any string is permitted as a tag"
	_    [4]byte "ceci n'est pas un champ de structure"
}

// A struct corresponding to a TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers;
// they follow the convention outlined by the reflect package.
struct {
	microsec  uint64 `protobuf:"1"`
	serverIP6 uint64 `protobuf:"2"`
}
</pre>

<h3 id="Pointer_types">Pointer types</h3>

<p>
A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
type, called the <i>base type</i> of the pointer.
The value of an uninitialized pointer is <code>nil</code>.
</p>

<pre class="ebnf">
PointerType = "*" BaseType .
BaseType    = Type .
</pre>

<pre>
*Point
*[4]int
</pre>

<h3 id="Function_types">Function types</h3>

<p>
A function type denotes the set of all functions with the same parameter
and result types. The value of an uninitialized variable of function type
is <code>nil</code>.
</p>

<pre class="ebnf">
FunctionType   = "func" Signature .
Signature      = Parameters [ Result ] .
Result         = Parameters | Type .
Parameters     = "(" [ ParameterList [ "," ] ] ")" .
ParameterList  = ParameterDecl { "," ParameterDecl } .
ParameterDecl  = [ IdentifierList ] [ "..." ] Type .
</pre>

<p>
Within a list of parameters or results, the names (IdentifierList)
must either all be present or all be absent. If present, each name
stands for one item (parameter or result) of the specified type and
all non-<a href="#Blank_identifier">blank</a> names in the signature
must be <a href="#Uniqueness_of_identifiers">unique</a>.
If absent, each type stands for one item of that type.
Parameter and result
lists are always parenthesized except that if there is exactly
one unnamed result it may be written as an unparenthesized type.
</p>

<p>
The final incoming parameter in a function signature may have
a type prefixed with <code>...</code>.
A function with such a parameter is called <i>variadic</i> and
may be invoked with zero or more arguments for that parameter.
</p>

<pre>
func()
func(x int) int
func(a, _ int, z float32) bool
func(a, b int, z float32) (bool)
func(prefix string, values ...int)
func(a, b int, z float64, opt ...interface{}) (success bool)
func(int, int, float64) (float64, *[]int)
func(n int) func(p *T)
</pre>


<h3 id="Interface_types">Interface types</h3>

<p>
An interface type specifies a <a href="#Method_sets">method set</a> called its <i>interface</i>.
A variable of interface type can store a value of any type with a method set
that is any superset of the interface. Such a type is said to
<i>implement the interface</i>.
The value of an uninitialized variable of interface type is <code>nil</code>.
</p>

<pre class="ebnf">
InterfaceType      = "interface" "{" { MethodSpec ";" } "}" .
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="#Assignments">assignment</a> or
explicit <a href="#Conversions">conversion</a>.
</p>

<pre>
chan T          // can be used to send and receive values of type T
chan&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 element types.</li>

	<li>Two channel types are identical if they have identical element types and
	    the same direction.</li>
</ul>

<p>
Given the declarations
</p>

<pre>
type (
	A0 = []string
	A1 = A0
	A2 = struct{ a, b int }
	A3 = int
	A4 = func(A3, float64) *A0
	A5 = func(x int, _ float64) *[]string
)

type (
	B0 A0
	B1 []string
	B2 struct{ a, b int }
	B3 struct{ a, c int }
	B4 func(int, float64) *B0
	B5 func(x int, y float64) *A1
)

type	C0 = B0
</pre>

<p>
these types are identical:
</p>

<pre>
A0, A1, and []string
A2 and struct{ a, b int }
A3 and int
A4, func(int, float64) *[]string, and A5

B0 and C0
[]int and []int
struct{ a, b *T5 } and struct{ a, b *T5 }
func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
</pre>

<p>
<code>B0</code> and <code>B1</code> are different because they are new types
created by distinct <a href="#Type_definitions">type definitions</a>;
<code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
are different because <code>B0</code> is different from <code>[]string</code>.
</p>


<h3 id="Assignability">Assignability</h3>

<p>
A value <code>x</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
</p>

<ul>
<li>
<code>x</code>'s type is identical to <code>T</code>.
</li>
<li>
<code>x</code>'s type <code>V</code> and <code>T</code> have identical
<a href="#Types">underlying types</a> and at least one of <code>V</code>
or <code>T</code> is not a <a href="#Type_definitions">defined</a> type.
</li>
<li>
<code>T</code> is an interface type and
<code>x</code> <a href="#Interface_types">implements</a> <code>T</code>.
</li>
<li>
<code>x</code> is a bidirectional channel value, <code>T</code> is a channel type,
<code>x</code>'s type <code>V</code> and <code>T</code> have identical element types,
and at least one of <code>V</code> or <code>T</code> is not a defined type.
</li>
<li>
<code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
is