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<!-- The Go Programming Language Specification -->
<!--
Todo
[ ] clarify: two equal lowercase identifiers from different packages denote different objects
[ ] need language about function/method calls and parameter passing rules
[ ] need to say something about "scope" of selectors?
[ ] clarify what a field name is in struct declarations
(struct{T} vs struct {T T} vs struct {t T})
[ ] need explicit language about the result type of operations
[ ] may want to have some examples for the types of shift operations
[ ] should string(1<<s) and float(1<<s) be valid?
[ ] should probably write something about evaluation order of statements even
though obvious
[ ] specify iteration direction for range clause
[ ] review language on implicit dereferencing
-->
<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="http://golang.org/">http://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 symbols 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.
</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 term <i>character</i> to refer to a Unicode code point.
</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>
<h3 id="Characters">Characters</h3>
<p>
The following terms are used to denote specific Unicode character classes:
</p>
<pre class="ebnf">
unicode_char = /* an arbitrary Unicode code point */ .
unicode_letter = /* a Unicode code point classified as "Letter" */ .
unicode_digit = /* a Unicode code point classified as "Digit" */ .
</pre>
<p>
In <a href="http://www.unicode.org/versions/Unicode5.2.0/">The Unicode Standard 5.2</a>,
Section 4.5 General Category-Normative
defines a set of character categories. Go treats
those characters in category Lu, Ll, Lt, Lm, or Lo as Unicode letters,
and those in 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>
There are two forms of comments:
</p>
<ol>
<li>
<i>Line comments</i> start with the character sequence <code>//</code>
and continue through the next newline. A line comment acts like a newline.
</li>
<li>
<i>General comments</i> start with the character sequence <code>/*</code>
and continue through the character sequence <code>*/</code>. A general
comment that spans multiple lines acts like a newline, otherwise it acts
like a space.
</li>
</ol>
<p>
Comments do not nest.
</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 delimiters</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
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>
<p>
When the input is broken into tokens, a semicolon is automatically inserted
into the token stream at the end of a non-blank line if the line's final
token is
</p>
<ul>
<li>an identifier
<li>an integer, floating-point, character, or string literal
<li>one of the keywords
<code>break</code>, <code>continue</code>, <code>fallthrough</code>,
or <code>return</code>
</li>
<li>one of the operators and delimiters
<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_Delimiters">Operators and Delimiters</h3>
<p>
The following character sequences represent <a href="#Operators">operators</a>, delimiters, and other special tokens:
</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.
2.71828
1.e+0
6.67428e-11
1E6
.25
.12345E+5
</pre>
<h3 id="Character_literals">Character literals</h3>
<p>
A character literal represents an <a href="#Constants">integer constant</a>,
typically a Unicode code point, as one or more characters enclosed in single
quotes. Within the quotes, any character may appear except single
quote and newline. A single quoted character represents 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 represented
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 character literals)
\" U+0022 double quote (valid escape only within string literals)
</pre>
<p>
All other sequences starting with a backslash are illegal inside character literals.
</p>
<pre class="ebnf">
char_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'
</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
<code>``</code>. Within the quotes, any character is legal except
back quote. The value of a raw string literal is the
string composed of the uninterpreted characters between the quotes;
in particular, backslashes have no special meaning and the string may
span multiple lines.
</p>
<p>
Interpreted string literals are character sequences between double
quotes <code>&quot;&quot;</code>. The text between the quotes,
which may not span multiple lines, forms the
value of the literal, with backslash escapes interpreted as they
are in character literals (except that <code>\'</code> is illegal and
<code>\"</code> is legal). 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 } "`" .
interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
</pre>
<pre>
`abc` // same as "abc"
`\n
\n` // same as "\\n\n\\n"
"\n"
""
"Hello, world!\n"
"日本語"
"\u65e5本\U00008a9e"
"\xff\u00FF"
</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 character 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>integer constants</i>, <i>floating-point constants</i>,
and <i>string constants</i>. Integer and floating-point constants are
collectively called <i>numeric constants</i>.
</p>
<p>
A constant value is represented by an
<a href="#Integer_literals">integer</a>,
<a href="#Floating-point_literals">floating-point</a>,
<a href="#Character_literals">character</a>, or
<a href="#String_literals">string</a> literal,
an identifier denoting a constant,
a <a href="#Constant_expressions">constant expression</a>, or
the result value of some built-in functions such as <code>unsafe.Sizeof</code>
and <code>cap</code> or <code>len</code> applied to an array,
or <code>len</code> applied to a string constant.
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>
Numeric constants represent values of arbitrary precision and do not overflow.
</p>
<p>
Constants may be <a href="#Types">typed</a> or untyped.
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 accurately 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>
There are no constants denoting the IEEE-754 infinity and not-a-number values,
but the <a href="/pkg/math/"><code>math</code> package</a>'s
<a href="/pkg/math/#Inf">Inf</a>,
<a href="/pkg/math/#NaN">NaN</a>,
<a href="/pkg/math/#IsInf">IsInf</a>, and
<a href="/pkg/math/#IsNaN">IsNaN</a>
functions return and test for those values at run time.
</p>
<p>
Implementation restriction: A compiler may implement numeric constants by choosing
an internal representation with at least twice as many bits as any machine type;
for floating-point values, both the mantissa and exponent must be twice as large.
</p>
<h2 id="Types">Types</h2>
<p>
A type determines the set of values and operations specific to values of that
type. A type may be specified by a (possibly qualified) <i>type name</i>
<a href="#Qualified_identifier">Qualified identifier</a>, §<a href="#Type_declarations">Type declarations</a>) or a <i>type literal</i>,
which composes a new type from previously declared types.
</p>
<pre class="ebnf">
Type = TypeName | TypeLit | "(" Type ")" .
TypeName = 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>.
<i>Composite types</i>&mdash;array, struct, pointer, function,
interface, slice, map, and channel types&mdash;may be constructed using
type literals.
</p>
<p>
A type may have a <i>method set</i> associated with it
<a href="#Interface_types">Interface types</a>, §<a href="#Method_declarations">Method declarations</a>).
The method set of an <a href="#Interface_types">interface type</a> is its interface.
The method set of any other named type <code>T</code>
consists of all methods with receiver type <code>T</code>.
The method set of the corresponding pointer type <code>*T</code>
is the set of all methods with receiver <code>*T</code> or <code>T</code>
(that is, it also contains the method set of <code>T</code>).
Any other type has an empty method set.
In a method set, each method must have a unique name.
</p>
<p>
The <i>static type</i> (or just <i>type</i>) of a variable is the
type defined by its declaration. Variables of interface type
also have a distinct <i>dynamic type</i>, which
is the actual type of the value stored in the variable at run-time.
The dynamic type may vary during execution but is always assignment compatible
to the static type of the interface variable. For non-interface
types, the dynamic type is always the static type.
</p>
<h3 id="Boolean_types">Boolean types</h3>
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>.
<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
byte familiar alias for uint8
</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 either 32 or 64 bits
float either 32 or 64 bits
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>.
Conversions
are required when incompatible 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.
Strings behave like arrays of bytes but are immutable: once created,
it is impossible to change the contents of a string.
The predeclared string type is <code>string</code>.
<p>
The elements of strings have type <code>byte</code> and may be
accessed using the usual <a href="#Indexes">indexing operations</a>. It is
illegal to take the address of such an element; if
<code>s[i]</code> is the <i>i</i>th byte of a
string, <code>&amp;s[i]</code> is invalid. The length of string
<code>s</code> can be discovered using the built-in function
<code>len</code>. The length is a compile-time constant if <code>s</code>
is a string literal.
</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 and must be a
<a href="#Constant_expressions">constant expression</a> that evaluates to a non-negative
integer value. The length of array <code>a</code> can be discovered
using the built-in function <code>len(a)</code>, which is a
compile-time constant. The elements can be indexed by integer
indices 0 through the <code>len(a)-1</code><a href="#Indexes">Indexes</a>).
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 reference to a contiguous segment of an array and
contains a numbered sequence of elements from that array. A slice
type denotes the set of all slices of arrays of its element type.
A slice value may be <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
<code>len(s)</code>; unlike with arrays it may change during
execution. The elements can be addressed by integer indices 0
through <code>len(s)-1</code><a href="#Indexes">Indexes</a>). 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 `slicing' a new
one from the original slice (§<a href="#Slices">Slices</a>).
The capacity of a slice <code>a</code> can be discovered using the
built-in function <code>cap(a)</code> and the relationship between
<code>len()</code> and <code>cap()</code> is:
</p>
<pre>
0 <= len(a) <= cap(a)
</pre>
<p>
The value of an uninitialized slice is <code>nil</code>.
The length and capacity of a <code>nil</code> slice
are 0. A new, initialized slice value for a given element type <code>T</code> is
made using the built-in function <code>make</code>, which takes a slice type
and parameters specifying the length and optionally the capacity:
</p>
<pre>
make([]T, length)
make([]T, length, capacity)
</pre>
<p>
The <code>make()</code> call 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 slicing it, so these two examples
result in the same slice:
</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 lengths may vary dynamically.
Moreover, the inner slices must be allocated individually (with <code>make</code>).
</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 (AnonymousField).
Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
be unique.
</p>
<pre class="ebnf">
StructType = "struct" "{" { FieldDecl ";" } "}" .
FieldDecl = (IdentifierList Type | AnonymousField) [ Tag ] .
AnonymousField = [ "*" ] TypeName .
Tag = string_lit .
</pre>
<pre>
// An empty struct.
struct {}
// A struct with 6 fields.
struct {
x, y int
u float
_ float // padding
A *[]int
F func()
}
</pre>
<p>
A field declared with a type but no explicit field name is an <i>anonymous field</i>.
Such a field type must be specified as
a type name <code>T</code> or as a pointer to a 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 anonymous fields of type 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 anonymous field *T and *P.T
*T // conflicts with anonymous field T and *P.T
*P.T // conflicts with anonymous field T and *T
}
</pre>
<p>
Fields and methods (§<a href="#Method_declarations">Method declarations</a>) of an anonymous field are
promoted to be ordinary fields and methods of the struct (§<a href="#Selectors">Selectors</a>).
The following rules apply for a struct type named <code>S</code> and
a type named <code>T</code>:
</p>
<ul>
<li>If <code>S</code> contains an anonymous field <code>T</code>, the
method set of <code>S</code> includes the method set of <code>T</code>.
</li>
<li>If <code>S</code> contains an anonymous field <code>*T</code>, the
method set of <code>S</code> includes the method set of <code>*T</code>
(which itself includes the method set of <code>T</code>).
</li>
<li>If <code>S</code> contains an anonymous field <code>T</code> or
<code>*T</code>, the method set of <code>*S</code> includes the
method set of <code>*T</code> (which itself includes the method
set of <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. The tags are made
visible through a <a href="#Package_unsafe">reflection interface</a>
but are otherwise ignored.
</p>
<pre>
// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
microsec uint64 "field 1"
serverIP6 uint64 "field 2"
process string "field 3"
}
</pre>
<h3 id="Pointer_types">Pointer types</h3>
<p>
A pointer type denotes the set of all pointers to variables of a given
type, called the <i>base type</i> of the pointer.
A pointer value may be <code>nil</code>.
</p>
<pre class="ebnf">
PointerType = "*" BaseType .
BaseType = Type .
</pre>
<pre>
*int
*map[string] *chan 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.
A function value may be <code>nil</code>.
</p>
<pre class="ebnf">
FunctionType = "func" Signature .
Signature = Parameters [ Result ] .
Result = Parameters | Type .
Parameters = "(" [ ParameterList [ "," ] ] ")" .
ParameterList = ParameterDecl { "," ParameterDecl } .
ParameterDecl = [ IdentifierList ] ( Type | "..." [ 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; 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 written as an unparenthesized type.
</p>
<p>
For the last parameter only, instead of a type one may write
<code>...</code> or <code>... T</code> to indicate that the function
may be invoked with zero or more additional arguments. If the type
<code>T</code> is present in the parameter declaration, the additional
arguments must all be
<a href="#Assignment_compatibility">assignment compatible</a>
with type <code>T</code>; otherwise they may be of any type.
</p>
<pre>
func()
func(x int)
func() int
func(string, float, ...)
func(prefix string, values ... int)
func(a, b int, z float) bool
func(a, b int, z float) (bool)
func(a, b int, z float, opt ...) (success bool)
func(int, int, float) (float, *[]int)
func(n int) func(p *T)
</pre>
<h3 id="Interface_types">Interface types</h3>
<p>
An interface type specifies a <a href="#Types">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>. An interface value may be <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 unique 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>Lock</code>:
</p>
<pre>
type Lock 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>Lock</code> interface as well
as the <code>File</code> interface.
</p>
<p>
An interface may contain an interface type name <code>T</code>
in place of a method specification.
The effect is equivalent to enumerating the methods of <code>T</code> explicitly
in the interface.
</p>
<pre>
type ReadWrite interface {
Read(b Buffer) bool
Write(b Buffer) bool
}
type File interface {
ReadWrite // same as enumerating the methods in ReadWrite
Lock // same as enumerating the methods in Lock
Close()
}
</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.
A map value may be <code>nil</code>.
</p>
<pre class="ebnf">
MapType = "map" "[" KeyType "]" ElementType .
KeyType = Type .
</pre>
<p>
The comparison operators <code>==</code> and <code>!=</code>
<a href="#Comparison_operators">Comparison operators</a>) must be fully defined for operands of the
key type; thus the key type must be a boolean, numeric, string, pointer, function, interface,
map, or channel type. If the key type is an interface type, these
comparison operators must be defined for the dynamic key values;
failure will cause a run-time error.
</p>
<pre>
map [string] int
map [*T] struct { x, y float }
map [string] interface {}
</pre>
<p>
The number of elements is called the length and is never negative.
The length of a map <code>m</code> can be discovered using the
built-in function <code>len(m)</code> and may change during execution.
Values may be added and removed
during execution using special forms of <a href="#Assignments">assignment</a>.
</p>
<p>
The value of an uninitialized map is <code>nil</code>.
A new, empty map value is made using the built-in
function <code>make</code>, 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.
</p>
<h3 id="Channel_types">Channel types</h3>
<p>
A channel provides a mechanism for two concurrently executing functions
to synchronize execution and communicate by passing a value of a
specified element type.
A value of channel type may be <code>nil</code>.
</p>
<pre class="ebnf">
ChannelType = Channel | SendChannel | RecvChannel .
Channel = "chan" ElementType .
SendChannel = "chan" "&lt;-" ElementType .
RecvChannel = "&lt;-" "chan" ElementType .
</pre>
<p>
To avoid a parsing ambiguity in cases such as <code>chan&lt;- chan int</code>,
the Channel production's ElementType cannot be a RecvChannel.
To construct such a type, parenthesize the RecvChannel first.
</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>
Upon creation, a channel can be used both to send and to receive values.
By conversion or assignment, a channel may be constrained only to send or
to receive. This constraint is called a channel's <i>direction</i>; either
<i>send</i>, <i>receive</i>, or <i>bi-directional</i> (unconstrained).
</p>
<pre>
chan T // can be used to send and receive values of type T
chan&lt;- float // can only be used to send floats
&lt;-chan int // can only be used to receive ints
</pre>
<p>
The value of an uninitialized channel is <code>nil</code>. A new, initialized channel
value can be made using the built-in function <code>make</code>,
which takes the channel type and an optional capacity 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 greater than zero, the channel is asynchronous: provided the
buffer is not full, sends can succeed without blocking. If the capacity is zero
or absent, the communication succeeds only when both a sender and receiver are ready.
</p>
<p>
A channel may be closed and tested for closure with the built-in functions
<a href="#Close_and_closed"><code>close</code> and <code>closed</code></a>.
</p>
<h2 id="Properties_of_types_and_values">Properties of types and values</h2>
<p>
Two types are either <i>identical</i> or <i>different</i>, and they are
either <i>compatible</i> or <i>incompatible</i>.
Identical types are always compatible, but compatible types need not be identical.
</p>
<h3 id="Type_identity_and_compatibility">Type identity and compatibility</h3>
<h4 id="Type_identity">Type identity</h4>
<p>
Two named types are identical if their type names originate in the same
type declaration (§<a href="#Declarations_and_scope">Declarations and scope</a>). A named and an unnamed type
are never identical. Two unnamed types are identical if the corresponding
type literals 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.
Two anonymous fields are considered to have the same name.</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 and if corresponding parameter and result types are
identical. All "..." parameters without a specified type are defined to have
identical type. All "..." parameters with specified identical type
<code>T</code> are defined to have identical type.
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. 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>
<h4 id="Type_compatibility">Type compatibility</h4>
<p>
Type compatibility is less stringent than type identity: a named and an unnamed
type are compatible if the respective type literals are compatible.
In all other respects, the definition of type compatibility is the
same as for type identity listed above but with ``compatible''
substituted for ``identical''.
</p>
<p>
Given the declarations
</p>
<pre>
type (
T0 []string
T1 []string
T2 struct { a, b int }
T3 struct { a, c int }
T4 func(int, float) *T0
T5 func(x int, y float) *[]string
)
</pre>
<p>
these types are identical:
</p>
<pre>
T0 and T0
[]int and []int
struct { a, b *T5 } and struct { a, b *T5 }
func(x int, y float) *[]string and func(int, float) (result *[]string)
</pre>
<p>
<code>T0</code> and <code>T1</code> are neither identical nor compatible
because they are named types with distinct declarations.
</p>
<p>
These types are compatible:
</p>
<pre>
T0 and T0
T0 and []string
T3 and struct { a int; c int }
T4 and func(x int, y float) *[]string
</pre>
<p>
<code>T2</code> and <code>struct { a, c int }</code> are incompatible because
they have different field names.
</p>
<h3 id="Assignment_compatibility">Assignment compatibility</h3>
<p>
A value <code>v</code> of static type <code>V</code> is <i>assignment compatible</i>
with a type <code>T</code> if one or more of the following conditions applies:
</p>
<ul>
<li>
<code>V</code> is compatible with <code>T</code>.
</li>
<li>
<code>T</code> is an interface type and
<code>V</code> <a href="#Interface_types">implements</a> <code>T</code>.
</li>
<li>
<code>V</code> is a pointer to an array and <code>T</code> is a slice type
with compatible element type and at least one of <code>V</code> or <code>T</code> is unnamed.
After assignment, the slice variable refers to the original array; the elements are not
copied.
</li>
<li>
<code>V</code> is a bidirectional channel and <code>T</code> is a channel type
with compatible element type and at least one of <code>V</code> or <code>T</code> is unnamed.
</li>
</ul>
<p>
If <code>T</code> is a struct type, either all fields of <code>T</code>
must be <a href="#Exported_identifiers">exported</a>, or the assignment must be in
the same package in which <code>T</code> is declared.
In other words, a struct value can be assigned to a struct variable only if
every field of the struct may be legally assigned individually by the program.
</p>
<p>
An untyped <a href="#Constants">constant</a> <code>v</code>
is assignment compatible with type <code>T</code> if <code>v</code>
can be represented accurately as a value of type <code>T</code>.
</p>
<p>
The predeclared identifier <code>nil</code> is assignment compatible with any
pointer, function, slice, map, channel, or interface type and
represents the <a href="#The_zero_value">zero value</a> for that type.
</p>
<p>
Any value may be assigned to the <a href="#Blank_identifier">blank identifier</a>.
</p>
<h3 id="Comparison_compatibility">Comparison compatibility</h3>
<p>
Except as noted, values of any type may be compared to other values of
<a href="#Type_compatibility">compatible static type</a>.
Values of numeric and string type may be compared using the
full range of <a href="#Comparison_operators;">comparison operators</a>;
booleans may be compared only for equality or inequality.
</p>
<p>
Values of composite type may be
compared for equality or inequality using the <code>==</code> and
<code>!=</code> operators, with the following provisos:
</p>
<ul>
<li>
Arrays and structs may not be compared to anything.
</li>
<li>
A slice value may only be compared explicitly against <code>nil</code>.
A slice value is equal to <code>nil</code> if it has been assigned the explicit
value <code>nil</code>, if it is uninitialized, or if it has
been assigned another slice value equal to <code>nil</code>·
</li>
<li>
An interface value is equal to <code>nil</code> if it has
been assigned the explicit value <code>nil</code>, if it is uninitialized,
or if it has been assigned another interface value equal to <code>nil</code>.
</li>
<li>
For types that can be compared to <code>nil</code>,
two values of the same type are equal if they both equal <code>nil</code>,
unequal if one equals <code>nil</code> and one does not.
</li>
<li>
Pointer values are equal if they point to the same location.
</li>
<li>
Function values are equal if they refer to the same function.
</li>
<li>
Channel and map values are equal if they were created by the same call to <code>make</code>
<a href="#Making_slices">Making slices</a>, maps, and channels).
When comparing two values of channel type, the channel value types
must be compatible but the channel direction is ignored.
</li>
<li>
Interface values may be compared if they have compatible static types.
They will be equal only if they have the same dynamic type and the underlying values are equal.
</li>
</ul>
<h2 id="Blocks">Blocks</h2>
<p>
A <i>block</i> is a sequence of declarations and statements within matching
brace brackets.
</p>
<pre class="ebnf">
Block = "{" { 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 <code>if</code>, <code>for</code>, and <code>switch</code>
statement is considered to be in its own implicit block.</li>
<li>Each clause in a <code>switch</code> or <code>select</code> 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 declaration binds a non-<a href="#Blank_identifier">blank</a>
identifier to a constant, type, variable, function, or package.
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>
<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, or package.
</p>
<p>
Go is lexically scoped using blocks:
</p>
<ol>
<li>The scope of a predeclared identifier is the universe block.</li>
<li>The scope of an identifier denoting a constant, type, variable,
or function declared at top level (outside any function) is the
package block.</li>
<li>The scope of an imported package identifier is the file block
of the file containing the import declaration.</li>
<li>The scope of an identifier denoting a 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
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 <code>break</code>, <code>continue</code>, and <code>goto</code>
statements (§<a href="#Break_statements">Break statements</a>, §<a href="#Continue_statements">Continue statements</a>, §<a href="#Goto_statements">Goto statements</a>).
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="Predeclared_identifiers">Predeclared identifiers</h3>
<p>
The following identifiers are implicitly declared in the universe block:
</p>
<pre class="grammar">
Basic types:
bool byte float32 float64 int8 int16 int32 int64
string uint8 uint16 uint32 uint64
Architecture-specific convenience types:
float int uint uintptr
Constants:
true false iota
Zero value:
nil
Functions:
cap close closed copy len make new panic panicln print println
</pre>
<h3 id="Exported_identifiers">Exported identifiers</h3>
<p>
An identifier may be <i>exported</i> to permit access to it from another package
using a <a href="#Qualified_identifiers">qualified identifier</a>. 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>the identifier is declared in the <a href="#Blocks">package block</a> or denotes a field or method of a type
declared in that block.
</ol>
<p>
All other identifiers are not exported.
</p>
<h3 id="Blank_identifier">Blank identifier</h3>
<p>
The <i>blank identifier</i>, represented by the underscore character <code>_</code>, may be used in a declaration like
any other identifier but the declaration does not introduce a new binding.
</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="#Assignment_compatibility">assignment compatible</a> with 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 float = 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 constant declaration, 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 with each
<a href="#Semicolons">semicolon</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 (
a = 1 &lt;&lt; iota // a == 1 (iota has been reset)
b = 1 &lt;&lt; iota // b == 2
c = 1 &lt;&lt; iota // c == 4
)
const (
u = iota * 42 // u == 0 (untyped integer constant)
v float = iota * 42 // v == 42.0 (float 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 at a semicolon:
</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 new type
that has the same definition (element, fields, channel direction, etc.) as
an existing type. The new type is
<a href="#Properties_of_types_and_values">compatible</a> with, but
<a href="#Properties_of_types_and_values">different</a> from, the existing type.
</p>
<pre class="ebnf">
TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec = identifier Type .
</pre>
<pre>
type IntArray [16]int
type (
Point struct { x, y float }
Polar Point
)
type TreeNode struct {
left, right *TreeNode
value *Comparable
}
type Cipher interface {
BlockSize() int
Encrypt(src, dst []byte)
Decrypt(src, dst []byte)
}
</pre>
<p>
The declared type does not inherit any <a href="#Method_declarations">methods</a>
bound to the existing type, but the <a href="#Types">method set</a>
of elements of a composite type is not changed:
</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
// PrintableMutex's method set contains the methods
// Lock and Unlock bound to its anonymous field Mutex.
type PrintableMutex struct {
Mutex
}
</pre>
<p>
A type declaration may be used to define a different boolean, numeric, or string
type and attach methods to it:
</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 a variable, binds an identifier to it and
gives it a type and optionally 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 float
var k = 0
var x, y float = -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
by assigning the expressions to the variables (§<a href="#Assignments">Assignments</a>)
in order; all expressions must be consumed and all variables initialized from them.
Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
</p>
<p>
If the type is present, each variable is given that type.
Otherwise, the types are deduced from the assignment
of the expression list.
</p>
<p>
If the type is absent and the corresponding expression evaluates to an
untyped <a href="#Constants">constant</a>, the type of the declared variable
is <code>bool</code>, <code>int</code>, <code>float</code>, or <code>string</code>
respectively, depending on whether the value is a boolean, integer,
floating-point, or string constant:
</p>
<pre>
var b = true // t has type bool
var i = 0 // i has type int
var f = 3.0 // f has type float
var s = "OMDB" // s has type string
</pre>
<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 a shorthand for a regular variable declaration 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 redeclare variables provided they
were originally declared in the same block 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
</pre>
<p>
Short variable declarations may appear only inside functions.
In some contexts such as the initializers for <code>if</code>,
<code>for</code>, or <code>switch</code> statements,
they can be used to declare local temporary variables (§<a href="#Statements">Statements</a>).
</p>
<h3 id="Function_declarations">Function declarations</h3>
<p>
A function declaration binds an identifier to a function (§<a href="#Function_types">Function types</a>).
</p>
<pre class="ebnf">
FunctionDecl = "func" identifier Signature [ Body ] .
Body = Block.
</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 function with a <i>receiver</i>.
A method declaration binds an identifier to a method.
</p>
<pre class="ebnf">
MethodDecl = "func" Receiver MethodName Signature [ Body ] .
Receiver = "(" [ identifier ] [ "*" ] BaseTypeName ")" .
BaseTypeName = identifier .
</pre>
<p>
The receiver type must be of the form <code>T</code> or <code>*T</code> where
<code>T</code> is a type name. <code>T</code> is called the
<i>receiver base type</i> or just <i>base type</i>.
The base type must not be a pointer or interface type and must be
declared in the same package as the method.
The method is said to be <i>bound</i> to the base type
and is visible only within selectors for that type
<a href="#Type_declarations">Type declarations</a>, §<a href="#Selectors">Selectors</a>).
</p>
<p>
Given type <code>Point</code>, the declarations
</p>
<pre>
func (p *Point) Length() float {
return Math.sqrt(p.x * p.x + p.y * p.y)
}
func (p *Point) Scale(factor float) {
p.x = p.x * factor
p.y = p.y * factor
}
</pre>
<p>
bind the methods <code>Length</code> and <code>Scale</code>,
with receiver type <code>*Point</code>,
to the base type <code>Point</code>.
</p>
<p>
If the receiver'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>
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>
(p *Point, factor float)
</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.
</p>
<pre class="ebnf">
Operand = Literal | QualifiedIdent | MethodExpr | "(" Expression ")" .
Literal = BasicLit | CompositeLit | FunctionLit .
BasicLit = int_lit | float_lit | char_lit | string_lit .
</pre>
<h3 id="Qualified_identifiers">Qualified identifiers</h3>
<p>
A qualified identifier is a non-<a href="#Blank_identifier">blank</a> identifier qualified by a package name prefix.
</p>
<pre class="ebnf">
QualifiedIdent = [ PackageName "." ] identifier .
</pre>
<p>
A qualified identifier accesses an identifier in a separate package.
The identifier must be <a href="#Exported_identifiers">exported</a> by that
package, which means that it must begin with a Unicode upper case letter.
</p>
<pre>
math.Sin
</pre>
<!---
<p>
<span class="alert">TODO: Unify this section with Selectors - it's the same syntax.</span>
</p>
--->
<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 value
followed by a brace-bound list of composite elements. An element may be
a single expression or a key-value pair.
</p>
<pre class="ebnf">
CompositeLit = LiteralType "{" [ ElementList [ "," ] ] "}" .
LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
SliceType | MapType | TypeName | "(" LiteralType ")" .
ElementList = Element { "," Element } .
Element = [ Key ":" ] Value .
Key = FieldName | ElementIndex .
FieldName = identifier .
ElementIndex = Expression .
Value = Expression .
</pre>
<p>
The LiteralType 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 expressions must be <a href="#Assignment_compatibility">assignment compatible</a> with
the respective field, element, and key types of the LiteralType;
there is no additional conversion.
The key is interpreted as a field name for struct literals,
an index expression 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.
</p>
<p>
For struct literals the following rules apply:
</p>
<ul>
<li>A key must be a field name declared in the LiteralType.
</li>
<li>A literal 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>A literal 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 Point struct { x, y, z float }
type Line struct { p, q Point }
</pre>
<p>
one may write
</p>
<pre>
origin := Point{} // zero value for Point
line := Line{origin, Point{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 constant integer expression.
</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>
Taking the address of a composite literal (§<a href="#Address_operators">Address operators</a>)
generates a unique pointer to an instance of the literal's value.
</p>
<pre>
var pointer *Point = &amp;Point{y: 1000}
</pre>
<p>
The length of an array literal is the length specified in the LiteralType.
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 a shortcut for a slice operation applied to an array literal:
</p>
<pre>
[n]T{x1, x2, ... xn}[0 : n]
</pre>
<p>
A parsing ambiguity arises when a composite literal using the
TypeName form of the LiteralType appears in the condition of an
"if", "for", or "switch" statement, because the braces surrounding
the expressions in the literal are confused with those introducing
a block of statements. To resolve the ambiguity in this rare case,
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, 11, 13, 17, 19, 991}
// 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]float{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
filter := [10]float{-1, 4: -0.1, -0.1, 9: -1}
// frequencies in Hz for equal-tempered scale (A4 = 440Hz)
noteFrequency := map[string]float{
"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 function.
It consists of a specification of the function type and a function body.
</p>
<pre class="ebnf">
FunctionLit = FunctionType Body .
</pre>
<pre>
func(a, b int, z float) 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 } (reply_chan)
</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 |
BuiltinCall |
PrimaryExpr Selector |
PrimaryExpr Index |
PrimaryExpr Slice |
PrimaryExpr TypeAssertion |
PrimaryExpr Call .
Selector = "." identifier .
Index = "[" Expression "]" .
Slice = "[" Expression ":" [ Expression ] "]" .
TypeAssertion = "." "(" Type ")" .
Call = "(" [ ExpressionList [ "," ] ] ")" .
</pre>
<pre>
x
2
(s + ".txt")
f(3.1415, true)
Point{1, 2}
m["foo"]
s[i : j + 1]
obj.color
Math.sin
f.p[i].x()
</pre>
<h3 id="Selectors">Selectors</h3>
<p>
A primary expression of the form
</p>
<pre>
x.f
</pre>
<p>
denotes the field or method <code>f</code> of the value denoted by <code>x</code>
(or of <code>*x</code> if
<code>x</code> is of pointer type). 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 expression is the type of <code>f</code>.
</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 anonymous field of
<code>T</code>.
The number of anonymous 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 anonymous 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 an 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 one <code>f</code> with shallowest depth, the selector
expression is illegal.
</li>
<li>
For a variable <code>x</code> of type <code>I</code> or <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 value assigned
to <code>x</code> if there is such a method.
If no value or <code>nil</code> was assigned to <code>x</code>, <code>x.f</code> is illegal.
</li>
<li>
In all other cases, <code>x.f</code> is illegal.
</ol>
<p>
Selectors automatically dereference pointers.
If <code>x</code> is of pointer type, <code>x.y</code>
is shorthand for <code>(*x).y</code>; if <code>y</code>
is also of pointer type, <code>x.y.z</code> is shorthand
for <code>(*(*x).y).z</code>, and so on.
If <code>*x</code> is of pointer type, dereferencing
must be explicit;
only one level of automatic dereferencing is provided.
For an <code>x</code> of type <code>T</code> containing an
anonymous field declared as <code>*A</code>,
<code>x.f</code> is a shortcut for <code>(*x.A).f</code>.
</p>
<p>
For example, given the declarations:
</p>
<pre>
type T0 struct {
x int
}
func (recv *T0) M0()
type T1 struct {
y int
}
func (recv T1) M1()
type T2 struct {
z int
T1
*T0
}
func (recv *T2) M2()
var p *T2 // with p != nil and p.T1 != nil
</pre>
<p>
one may write:
</p>
<pre>
p.z // (*p).z
p.y // ((*p).T1).y
p.x // (*(*p).T0).x
p.M2 // (*p).M2
p.M1 // ((*p).T1).M1
p.M0 // ((*p).T0).M0
</pre>
<!---
<span class="alert">
TODO: Specify what happens to receivers.
</span>
--->
<h3 id="Indexes">Indexes</h3>
<p>
A primary expression of the form
</p>
<pre>
a[x]
</pre>
<p>
denotes the element of the 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>
For <code>a</code> of type <code>A</code> or <code>*A</code>
where <code>A</code> is an <a href="#Array_types">array type</a>,
or for <code>a</code> of type <code>S</code> where <code>S</code> is a <a href="#Slice_types">slice type</a>:
</p>
<ul>
<li><code>x</code> must be an integer value and <code>0 &lt;= x &lt; len(a)</code>
<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>
</ul>
<p>
For <code>a</code> of type <code>T</code>
where <code>T</code> is a <a href="#String_types">string type</a>:
</p>
<ul>
<li><code>x</code> must be an integer value and <code>0 &lt;= x &lt; len(a)</code>
<li><code>a[x]</code> is the byte at index <code>x</code> and the type of
<code>a[x]</code> is <code>byte</code>
<li><code>a[x]</code> may not be assigned to
</ul>
<p>
For <code>a</code> of type <code>M</code>
where <code>M</code> is a <a href="#Map_types">map type</a>:
</p>
<ul>
<li><code>x</code>'s type must be compatible with the key type of <code>M</code>
and the map must contain an entry with key <code>x</code> (but see special forms below)
<li><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>
</ul>
<p>
Otherwise <code>a[x]</code> is illegal. If the index or key is out of range evaluating
an otherwise legal index expression, a run-time exception occurs.
</p>
<p>
However, if an index expression on a map <code>a</code> of type <code>map[K] V</code>
is used in an assignment or initialization of the form
</p>
<pre>
r, ok = a[x]
r, ok := a[x]
var r, ok = a[x]
</pre>
<p>
the result of the index expression is a pair of values with types
<code>(V, bool)</code>.
If the key is present in the map,
the expression returns the pair <code>(a[x], true)</code>;
otherwise it returns <code>(Z, false)</code> where <code>Z</code> is
the <a href="#The_zero_value">zero value</a> for <code>V</code>.
No run-time exception occurs in this case.
The index expression in this construct thus acts like a function call
returning a value and a boolean indicating success. (§<a href="#Assignments">Assignments</a>)
</p>
<p>
Similarly, if an assignment to a map has the special form
</p>
<pre>
a[x] = r, ok
</pre>
<p>
and boolean <code>ok</code> has the value <code>false</code>,
the entry for key <code>x</code> is deleted from the map; if
<code>ok</code> is <code>true</code>, the construct acts like
a regular assignment to an element of the map.
</p>
<h3 id="Slices">Slices</h3>
<p>
For a string, array, or slice <code>a</code>, the primary expression
</p>
<pre>
a[lo : hi]
</pre>
<p>
constructs a substring or slice. The index expressions <code>lo</code> and
<code>hi</code> select which elements appear in the result. The result has
indexes starting at 0 and length equal to
<code>hi</code>&nbsp;-&nbsp;<code>lo</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, the <code>hi</code> expression may be omitted; the notation
<code>a[lo :]</code> is shorthand for <code>a[lo : len(a)]</code>.
For arrays or strings, the indexes
<code>lo</code> and <code>hi</code> must satisfy
0 &lt;= <code>lo</code> &lt;= <code>hi</code> &lt;= length;
for slices, the upper bound is the capacity rather than the length.
</p>
<p>
If the sliced operand is a string or slice, the result of the slice operation
is a string or slice of the same type.
If the sliced operand is an array, the result of the slice operation is a slice
with the same element type as the array.
</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 identical to the type <code>T</code>
<a href="#Type_identity_and_compatibility">Type identity and compatibility</a>).
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><a href="#Interface_types">Interface types</a>).
</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 run-time
exception 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>
<p>
If a type assertion is used in an assignment or initialization of the form
</p>
<pre>
v, ok = x.(T)
v, ok := x.(T)
var v, ok = x.(T)
</pre>
<p>
the result of the assertion is a pair of values with types <code>(T, bool)</code>.
If the assertion holds, the expression returns the pair <code>(x.(T), true)</code>;
otherwise, the expression returns <code>(Z, false)</code> where <code>Z</code>
is the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
No run-time exception occurs in this case.
The type assertion in this construct thus acts like a function call
returning a value and a boolean indicating success. (§<a href="#Assignments">Assignments</a>)
</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="#Assignment_compatibility">assignment compatible</a> with 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>
As a special case, if the return parameters of a function or method
<code>g</code> are equal in number and individually assignment
compatible with 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>.
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.Crash("test fails")
}
</pre>
<p>
A method call <code>x.m()</code> is valid if the method set of
(the type of) <code>x</code> contains <code>m</code> and the
argument list is compatible with 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>
When a function <code>f</code> has a <code>...</code> parameter,
it is always the last formal parameter. Within calls to <code>f</code>,
the arguments before the <code>...</code> are treated normally.
After those, an arbitrary number (including zero) of trailing
arguments may appear in the call and are bound to the <code>...</code>
parameter.
</p>
<p>
Within <code>f</code>, a <code>...</code> parameter with no
specified type has static type <code>interface{}</code> (the empty
interface). For each call, its dynamic type is a structure whose
sequential fields are the trailing arguments of the call. That is,
the actual arguments provided for a <code>...</code> parameter are
wrapped into a struct that is passed to the function instead of the
actual arguments. Using the <a href="#Package_unsafe">reflection</a>
interface, <code>f</code> may unpack the elements of the dynamic
type to recover the actual arguments.
</p>
<p>
Given the function and call
</p>
<pre>
func Fprintf(f io.Writer, format string, args ...)
Fprintf(os.Stdout, "%s %d", "hello", 23)
</pre>
<p>
Within <code>Fprintf</code>, the dynamic type of <code>args</code> for this
call will be, schematically,
<code> struct { string; int }</code>.
</p>
<p>
If the final parameter of <code>f</code> has type <code>... T</code>,
within the function it is equivalent to a parameter of type
<code>[]T</code>. At each call of <code>f</code>, the actual
arguments provided for the <code>... T</code> parameter are placed
into a new slice of type <code>[]T</code> whose successive elements are
the actual arguments. The length of the slice is therefore the
number of arguments bound to the <code>... T</code> parameter and
may differ for each call site.
</p>
<p>
Given the function and call
</p>
<pre>
func Greeting(prefix string, who ... string)
Greeting("hello:", "Joe", "Anna", "Eileen")
</pre>
<p>
Within <code>Greeting</code>, <code>who</code> will have value
<code>[]string{"Joe", "Anna", "Eileen")</code>
</p>
<p>
As a special case, if a function passes its own <code>...</code> parameter,
with or without specified type, as the argument
for a <code>...</code> in a call to another function with a <code>...</code> parameter
of identical type,
the parameter is not wrapped again but passed directly. In short, a formal <code>...</code>
parameter is passed unchanged as an actual <code>...</code> parameter provided the
types match.
</p>
<h3 id="Operators">Operators</h3>
<p>
Operators combine operands into expressions.
</p>
<pre class="ebnf">
Expression = UnaryExpr | Expression binary_op UnaryExpr .
UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
binary_op = log_op | com_op | rel_op | add_op | mul_op .
log_op = "||" | "&amp;&amp;" .
com_op = "&lt;-" .
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 identical
<a href="#Properties_of_types_and_values">Properties of types and values</a>)
unless the operation involves channels, 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>
In a channel send, the first operand is always a channel and the second
must be a value <a href="#Assignment_compatibility">assignment compatible</a>
with the channel's element type.
</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 operation must have unsigned integer type
or be an untyped constant that can be converted to unsigned integer type.
</p>
<p>
If the left operand of a non-constant shift operation is an untyped constant,
the type of constant is what it would be if the shift operation were replaced by
the 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 u = uint64(1&lt;&lt;s) // 1 has type uint64; u == 1&lt;&lt;33
var f = float(1&lt;&lt;s) // illegal: 1 has type float, cannot shift
var g = float(1&lt;&lt;33) // legal; 1&lt;&lt;33 is a constant shift operation; g == 1&lt;&lt;33
</pre>
<h3 id="Operator_precedence">Operator precedence</h3>
<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 six precedence levels for binary operators.
Multiplication operators bind strongest, followed by addition
operators, comparison operators, <code>&lt;-</code> (channel send),
<code>&amp;&amp;</code> (logical and), and finally <code>||</code> (logical or):
</p>
<pre class="grammar">
Precedence Operator
6 * / % &lt;&lt; &gt;&gt; &amp; &amp;^
5 + - | ^
4 == != &lt; &lt;= > >=
3 &lt;-
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;-chan_ptr > 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 and
floating-point types; <code>+</code> also applies
to strings. All other arithmetic operators apply to integers only.
</p>
<pre class="grammar">
+ sum integers, floats, strings
- difference integers, floats
* product integers, floats
/ quotient integers, floats
% 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>
<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>
<p>
For integer values, <code>/</code> and <code>%</code> satisfy the following relationship:
</p>
<pre>
(a / b) * b + a % b == a
</pre>
<p>
with <code>(a / b)</code> truncated towards zero.
Examples:
</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>
If the dividend is positive 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. The shift count must
be 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>
<p>
For floating-point numbers,
<code>+x</code> is the same as <code>x</code>,
while <code>-x</code> is the negation of <code>x</code>.
</p>
<h3 id="Integer_overflow">Integer overflow</h3>
<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 unsigned integer's type
<a href="#Numeric_types">Numeric types</a>). 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>
<h3 id="Comparison_operators">Comparison operators</h3>
<p>
Comparison operators yield a value of type <code>bool</code>.
The operators <code>==</code> and <code>!=</code> apply, at least in some cases,
to operands of all types except arrays and structs.
All other comparison operators apply only to numeric and string values.
</p>
<pre class="grammar">
== equal
!= not equal
< less
<= less or equal
> greater
>= greater or equal
</pre>
<p>
Operands of numeric type are compared in the usual way.
</p>
<p>
Operands of string type are compared byte-wise (lexically).
</p>
<p>
Operands of boolean type are equal if they are either both <code>true</code>
or both <code>false</code>.
</p>
<p>
The rules for comparison of composite types are described in the
section on §<a href="#Comparison_compatibility">Comparison compatibility</a>.
</p>
<h3 id="Logical_operators">Logical operators</h3>
<p>
Logical operators apply to <a href="#Boolean_types">boolean</a> values
and yield a result of the same type as the operands.
The right operand is evaluated conditionally.
</p>
<pre class="grammar">
&amp;&amp; conditional and p &amp;&amp; q is "if p then q else false"
|| conditional or p || q is "if p then true else q"
! not !p is "not p"
</pre>
<h3 id="Address_operators">Address operators</h3>
<p>
The address-of operator <code>&amp;</code> generates the address of its operand,
which must be <i>addressable</i>,
that is, either a variable, pointer indirection, array or slice indexing
operation,
or a field selector of an addressable struct operand.
A function result variable is not addressable.
<!--- (<span class="alert">TODO: remove this restriction.</span>) --->
Given an operand of pointer type, the pointer indirection
operator <code>*</code> retrieves the value pointed
to by the operand.
</p>
<pre>
&amp;x
&amp;a[f(2)]
*p
*pf(x)
</pre>
<h3 id="Communication_operators">Communication operators</h3>
<p>
The term <i>channel</i> means "value of <a href="#Channel_types">channel type</a>".
</p>
<p>
The send operation uses the binary operator "&lt;-", which operates on
a channel and a value (expression):
</p>
<pre>
ch &lt;- 3
</pre>
<p>
The send operation sends the value on the channel. Both the channel
and the expression are evaluated before communication begins.
Communication blocks until the send can proceed, at which point the
value is transmitted on the channel.
A send on an unbuffered channel can proceed if a receiver is ready.
A send on a buffered channel can proceed if there is room in the buffer.
</p>
<p>
If the send operation appears in an expression context, the value
of the expression is a boolean and the operation is non-blocking.
The value of the boolean reports true if the communication succeeded,
false if it did not. (The channel and
the expression to be sent are evaluated regardless.)
These two examples are equivalent:
</p>
<pre>
ok := ch &lt;- 3
if ok { print("sent") } else { print("not sent") }
if ch &lt;- 3 { print("sent") } else { print("not sent") }
</pre>
<p>
In other words, if the program tests the value of a send operation,
the send is non-blocking and the value of the expression is the
success of the operation. If the program does not test the value,
the operation blocks until it succeeds.
</p>
<p>
The receive operation uses the prefix unary operator "&lt;-".
The value of the expression is the value received, whose type
is the element type of the channel.
</p>
<pre>
&lt;-ch
</pre>
<p>
The expression blocks until a value is available, which then can
be assigned to a variable or used like any other expression.
If the receive expression does not save the value, the value is
discarded.
</p>
<pre>
v1 := &lt;-ch
v2 = &lt;-ch
f(&lt;-ch)
&lt;-strobe // wait until clock pulse
</pre>
<p>
If a receive expression is used in an assignment or initialization of the form
</p>
<pre>
x, ok = &lt;-ch
x, ok := &lt;-ch
var x, ok = &lt;-ch
</pre>
<p>
the receive operation becomes non-blocking.
If the operation can proceed, the boolean variable
<code>ok</code> will be set to <code>true</code>
and the value stored in <code>x</code>; otherwise
<code>ok</code> is set
to <code>false</code> and <code>x</code> is set to the
zero value for its type (§<a href="#The_zero_value">The zero value</a>).
</p>
<!---
<p>
<span class="alert">TODO: Probably in a separate section, communication semantics
need to be presented regarding send, receive, select, and goroutines.</span>
</p>
--->
<h3 id="Method_expressions">Method expressions</h3>
<p>
If <code>M</code> is in the method set 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 ")" .
</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 float) float { 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 three invocations are equivalent:
</p>
<pre>
t.Mv(7)
T.Mv(t, 7)
f := T.Mv; f(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 float) float
</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">closure</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="Conversions">Conversions</h3>
<p>
Conversions are expressions of the form <code>T(x)</code>
where <code>T</code> is a type and <code>x</code> is an expression
that can be converted to type <code>T</code>.
</p>
<pre class="ebnf">
Conversion = LiteralType "(" Expression ")" .
</pre>
<p>
In general, a conversion succeeds if the value of <code>x</code> is
<a href="#Assignment_compatibility">assignment compatible</a> with type <code>T</code>,
or if the value would be assignment compatible with type <code>T</code> if the
value's type, or <code>T</code>, or any of their component types were unnamed.
Usually, such a conversion changes the type but not the representation of the value
of <code>x</code> and thus has no run-time cost.
</p>
<p>
Specific rules apply to conversions where <code>T</code> is a numeric or string type.
These conversions may change the representation of a value and incur a run-time cost.
</p>
<h4>Conversions between integer types</h4>
<p>
If the value is a signed quantity, it is
sign extended to implicit infinite precision; otherwise it is zero
extended. It is then truncated to fit in the result type's size.
For example, if <code>x := uint16(0x10F0)</code>, then <code>uint32(int8(x)) == 0xFFFFFFF0</code>.
The conversion always yields a valid value; there is no indication of overflow.
</p>
<h4>Conversions involving floating point types</h4>
<ol>
<li>
When converting a floating-point number to an integer, the fraction is discarded
(truncation towards zero).
</li>
<li>
When converting a number to a floating-point type, the result value is rounded
to the precision specified by the floating point type.
For instance, the value of a variable <code>x</code> of type <code>float32</code>
may be stored using additional precision beyond that of an IEEE-754 32-bit number,
but float32(x) represents the result of rounding <code>x</code>'s value to
32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
of precision, <code>but float32(x + 0.1)</code> does not.
</li>
</ol>
<p>
In all conversions involving floating-point values, if the result type cannot
represent the value the conversion succeeds but the result value is
implementation-dependent.
</p>
<h4>Conversions to and from a string type</h4>
<ol>
<li>
Converting a signed or unsigned integer value to a string type yields a
string containing the UTF-8 representation of the integer.
Negative values are converted to <code>"\uFFFD"</code>.
<pre>
string('a') // "a"
string(-1) // "\ufffd" == "\xef\xbf\xbd "
string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
type MyString string
MyString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
</pre>
</li>
<li>
Converting a value of type <code>[]byte</code> (or
the equivalent <code>[]uint8</code>) to a string type yields a
string whose successive bytes are the elements of the slice. If
the slice value is <code>nil</code>, the result is the empty string.
<pre>
string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
</pre>
</li>
<li>
Converting a value of type <code>[]int</code> to a string type yields
a string that is the concatenation of the individual integers
converted to strings. If the slice value is <code>nil</code>, the
result is the empty string.
<pre>
string([]int{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
</pre>
</li>
<li>
Converting a value of a string type to <code>[]byte</code> (or <code>[]uint8</code>)
yields a slice whose successive elements are the bytes of the string.
If the string is empty, the result is <code>[]byte(nil)</code>.
<pre>
[]byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
</pre>
</li>
<li>
Converting a value of a string type to <code>[]int</code> yields a
slice containing the individual Unicode code points of the string.
If the string is empty, the result is <code>[]int(nil)</code>.
<pre>
[]int(MyString("白鵬翔")) // []int{0x767d, 0x9d6c, 0x7fd4}
</pre>
</li>
</ol>
<p>
There is no linguistic mechanism to convert between pointers and integers.
The package <a href="#Package_unsafe"><code>unsafe</code></a>
implements this functionality under
restricted circumstances.
</p>
<h3 id="Constant_expressions">Constant expressions</h3>
<p>
Constant expressions may contain only <a href="#Constants">constant</a>
operands and are evaluated at compile-time.
</p>
<p>
Untyped boolean, numeric, and string constants may be used as operands
wherever it is legal to use an operand of boolean, numeric, or string type,
respectively. Except for shift operations, if the operands of a binary operation
are an untyped integer constant and an untyped floating-point constant,
the integer constant is converted to an untyped floating-point constant
(relevant for <code>/</code> and <code>%</code>).
</p>
<p>
Applying an operator to untyped constants results in an untyped
constant of the same kind (that is, a boolean, integer, floating-point, or