This is the reference manual for the Go programming language as it was for language version 1.17, in October 2021, before the introduction of generics. It is provided for historical interest. The current reference manual can be found here. For more information and other documents, see go.dev.
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 packages, whose properties allow efficient management of dependencies.
The grammar is compact and simple to parse, allowing for easy analysis by automatic tools such as integrated development environments.
The syntax is specified using Extended Backus-Naur Form (EBNF):
Production = production_name "=" [ Expression ] "." .
Expression = Alternative { "|" Alternative } .
Alternative = Term { Term } .
Term = production_name | token [ "…" token ] | Group | Option | Repetition .
Group = "(" Expression ")" .
Option = "[" Expression "]" .
Repetition = "{" Expression "}" .
Productions are expressions constructed from terms and the following operators, in increasing precedence:
| alternation
() grouping
[] option (0 or 1 times)
{} repetition (0 to n times)
Lower-case production names are used to identify lexical tokens.
Non-terminals are in CamelCase. Lexical tokens are enclosed in
double quotes "" or back quotes ``.
The form a … b represents the set of characters from
a through b as alternatives. The horizontal
ellipsis … is also used elsewhere in the spec to informally denote various
enumerations or code snippets that are not further specified. The character …
(as opposed to the three characters ...) is not a token of the Go
language.
Source code is Unicode text encoded in UTF-8. The text is not canonicalized, so a single accented code point is distinct from the same character constructed from combining an accent and a letter; those are treated as two code points. For simplicity, this document will use the unqualified term character to refer to a Unicode code point in the source text.
Each code point is distinct; for instance, upper and lower case letters are different characters.
Implementation restriction: For compatibility with other tools, a compiler may disallow the NUL character (U+0000) in the source text.
Implementation restriction: For compatibility with other tools, a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF) if it is the first Unicode code point in the source text. A byte order mark may be disallowed anywhere else in the source.
The following terms are used to denote specific Unicode character classes:
newline = /* the Unicode code point U+000A */ . unicode_char = /* an arbitrary Unicode code point except newline */ . unicode_letter = /* a Unicode code point classified as "Letter" */ . unicode_digit = /* a Unicode code point classified as "Number, decimal digit" */ .
In The Unicode Standard 8.0, Section 4.5 "General Category" defines a set of character categories. Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo as Unicode letters, and those in the Number category Nd as Unicode digits.
The underscore character _ (U+005F) is considered a letter.
letter = unicode_letter | "_" . decimal_digit = "0" … "9" . binary_digit = "0" | "1" . octal_digit = "0" … "7" . hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
Comments serve as program documentation. There are two forms:
//
and stop at the end of the line.
/*
and stop with the first subsequent character sequence */.
A comment cannot start inside a rune or string literal, or inside a comment. A general comment containing no newlines acts like a space. Any other comment acts like a newline.
Tokens form the vocabulary of the Go language. There are four classes: identifiers, keywords, operators and punctuation, and literals. White space, formed from spaces (U+0020), horizontal tabs (U+0009), carriage returns (U+000D), and newlines (U+000A), is ignored except as it separates tokens that would otherwise combine into a single token. Also, a newline or end of file may trigger the insertion of a semicolon. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.
The formal grammar uses semicolons ";" as terminators in
a number of productions. Go programs may omit most of these semicolons
using the following two rules:
break,
continue,
fallthrough, or
return
++,
--,
),
], or
}
")" or "}".
To reflect idiomatic use, code examples in this document elide semicolons using these rules.
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.
identifier = letter { letter | unicode_digit } .
a _x9 ThisVariableIsExported αβ
Some identifiers are predeclared.
The following keywords are reserved and may not be used as identifiers.
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
The following character sequences represent operators (including assignment operators) and punctuation:
+ & += &= && == != ( )
- | -= |= || < <= [ ]
* ^ *= ^= <- > >= { }
/ << /= <<= ++ = := , ;
% >> %= >>= -- ! ... . :
&^ &^=
An integer literal is a sequence of digits representing an
integer constant.
An optional prefix sets a non-decimal base: 0b or 0B
for binary, 0, 0o, or 0O for octal,
and 0x or 0X for hexadecimal.
A single 0 is considered a decimal zero.
In hexadecimal literals, letters a through f
and A through F represent values 10 through 15.
For readability, an underscore character _ may appear after
a base prefix or between successive digits; such underscores do not change
the literal's value.
int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
binary_digits = binary_digit { [ "_" ] binary_digit } .
octal_digits = octal_digit { [ "_" ] octal_digit } .
hex_digits = hex_digit { [ "_" ] hex_digit } .
42 4_2 0600 0_600 0o600 0O600 // second character is capital letter 'O' 0xBadFace 0xBad_Face 0x_67_7a_2f_cc_40_c6 170141183460469231731687303715884105727 170_141183_460469_231731_687303_715884_105727 _42 // an identifier, not an integer literal 42_ // invalid: _ must separate successive digits 4__2 // invalid: only one _ at a time 0_xBadFace // invalid: _ must separate successive digits
A floating-point literal is a decimal or hexadecimal representation of a floating-point constant.
A decimal floating-point literal consists of an integer part (decimal digits),
a decimal point, a fractional part (decimal digits), and an exponent part
(e or E followed by an optional sign and decimal digits).
One of the integer part or the fractional part may be elided; one of the decimal point
or the exponent part may be elided.
An exponent value exp scales the mantissa (integer and fractional part) by 10exp.
A hexadecimal floating-point literal consists of a 0x or 0X
prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
and an exponent part (p or P followed by an optional sign and decimal digits).
One of the integer part or the fractional part may be elided; the radix point may be elided as well,
but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
An exponent value exp scales the mantissa (integer and fractional part) by 2exp.
For readability, an underscore character _ may appear after
a base prefix or between successive digits; such underscores do not change
the literal value.
float_lit = decimal_float_lit | hex_float_lit .
decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
decimal_digits decimal_exponent |
"." decimal_digits [ decimal_exponent ] .
decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
[ "_" ] hex_digits |
"." hex_digits .
hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
0. 72.40 072.40 // == 72.40 2.71828 1.e+0 6.67428e-11 1E6 .25 .12345E+5 1_5. // == 15.0 0.15e+0_2 // == 15.0 0x1p-2 // == 0.25 0x2.p10 // == 2048.0 0x1.Fp+0 // == 1.9375 0X.8p-0 // == 0.5 0X_1FFFP-16 // == 0.1249847412109375 0x15e-2 // == 0x15e - 2 (integer subtraction) 0x.p1 // invalid: mantissa has no digits 1p-2 // invalid: p exponent requires hexadecimal mantissa 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent 1_.5 // invalid: _ must separate successive digits 1._5 // invalid: _ must separate successive digits 1.5_e1 // invalid: _ must separate successive digits 1.5e_1 // invalid: _ must separate successive digits 1.5e1_ // invalid: _ must separate successive digits
An imaginary literal represents the imaginary part of a
complex constant.
It consists of an integer or
floating-point literal
followed by the lower-case letter i.
The value of an imaginary literal is the value of the respective
integer or floating-point literal multiplied by the imaginary unit i.
imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
For backward compatibility, an imaginary literal's integer part consisting
entirely of decimal digits (and possibly underscores) is considered a decimal
integer, even if it starts with a leading 0.
0i 0123i // == 123i for backward-compatibility 0o123i // == 0o123 * 1i == 83i 0xabci // == 0xabc * 1i == 2748i 0.i 2.71828i 1.e+0i 6.67428e-11i 1E6i .25i .12345E+5i 0x1p-2i // == 0x1p-2 * 1i == 0.25i
A rune literal represents a rune constant,
an integer value identifying a Unicode code point.
A rune literal is expressed as one or more characters enclosed in single quotes,
as in 'x' or '\n'.
Within the quotes, any character may appear except newline and unescaped single
quote. A single quoted character represents the Unicode value
of the character itself,
while multi-character sequences beginning with a backslash encode
values in various formats.
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 'a' holds a single byte representing
a literal a, Unicode U+0061, value 0x61, while
'ä' holds two bytes (0xc3 0xa4) representing
a literal a-dieresis, U+00E4, value 0xe4.
Several backslash escapes allow arbitrary values to be encoded as
ASCII text. There are four ways to represent the integer value
as a numeric constant: \x followed by exactly two hexadecimal
digits; \u followed by exactly four hexadecimal digits;
\U followed by exactly eight hexadecimal digits, and a
plain backslash \ 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.
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 \u and \U
represent Unicode code points so within them some values are illegal,
in particular those above 0x10FFFF and surrogate halves.
After a backslash, certain single-character escapes represent special values:
\a U+0007 alert or bell \b U+0008 backspace \f U+000C form feed \n U+000A line feed or newline \r U+000D carriage return \t U+0009 horizontal tab \v U+000B vertical tab \\ U+005C backslash \' U+0027 single quote (valid escape only within rune literals) \" U+0022 double quote (valid escape only within string literals)
All other sequences starting with a backslash are illegal inside rune literals.
rune_lit = "'" ( unicode_value | byte_value ) "'" .
unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
byte_value = octal_byte_value | hex_byte_value .
octal_byte_value = `\` octal_digit octal_digit octal_digit .
hex_byte_value = `\` "x" hex_digit hex_digit .
little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
hex_digit hex_digit hex_digit hex_digit .
escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
'a' 'ä' '本' '\t' '\000' '\007' '\377' '\x07' '\xff' '\u12e4' '\U00101234' '\'' // rune literal containing single quote character 'aa' // illegal: too many characters '\xa' // illegal: too few hexadecimal digits '\0' // illegal: too few octal digits '\uDFFF' // illegal: surrogate half '\U00110000' // illegal: invalid Unicode code point
A string literal represents a string constant obtained from concatenating a sequence of characters. There are two forms: raw string literals and interpreted string literals.
Raw string literals are character sequences between back quotes, as in
`foo`. Within the quotes, any character may appear except
back quote. The value of a raw string literal is the
string composed of the uninterpreted (implicitly UTF-8-encoded) characters
between the quotes;
in particular, backslashes have no special meaning and the string may
contain newlines.
Carriage return characters ('\r') inside raw string literals
are discarded from the raw string value.
Interpreted string literals are character sequences between double
quotes, as in "bar".
Within the quotes, any character may appear except newline and unescaped double quote.
The text between the quotes forms the
value of the literal, with backslash escapes interpreted as they
are in rune literals (except that \' is illegal and
\" is legal), with the same restrictions.
The three-digit octal (\nnn)
and two-digit hexadecimal (\xnn) escapes represent individual
bytes of the resulting string; all other escapes represent
the (possibly multi-byte) UTF-8 encoding of individual characters.
Thus inside a string literal \377 and \xFF represent
a single byte of value 0xFF=255, while ÿ,
\u00FF, \U000000FF and \xc3\xbf represent
the two bytes 0xc3 0xbf of the UTF-8 encoding of character
U+00FF.
string_lit = raw_string_lit | interpreted_string_lit .
raw_string_lit = "`" { unicode_char | newline } "`" .
interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
`abc` // same as "abc" `\n \n` // same as "\\n\n\\n" "\n" "\"" // same as `"` "Hello, world!\n" "日本語" "\u65e5本\U00008a9e" "\xff\u00FF" "\uD800" // illegal: surrogate half "\U00110000" // illegal: invalid Unicode code point
These examples all represent the same string:
"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
If the source code represents a character as two code points, such as a combining form involving an accent and a letter, the result will be an error if placed in a rune literal (it is not a single code point), and will appear as two code points if placed in a string literal.
There are boolean constants, rune constants, integer constants, floating-point constants, complex constants, and string constants. Rune, integer, floating-point, and complex constants are collectively called numeric constants.
A constant value is represented by a
rune,
integer,
floating-point,
imaginary,
or
string literal,
an identifier denoting a constant,
a constant expression,
a conversion with a result that is a constant, or
the result value of some built-in functions such as
unsafe.Sizeof applied to any value,
cap or len applied to
some expressions,
real and imag applied to a complex constant
and complex applied to numeric constants.
The boolean truth values are represented by the predeclared constants
true and false. The predeclared identifier
iota denotes an integer constant.
In general, complex constants are a form of constant expression and are discussed in that section.
Numeric constants represent exact values of arbitrary precision and do not overflow. Consequently, there are no constants denoting the IEEE 754 negative zero, infinity, and not-a-number values.
Constants may be typed or untyped.
Literal constants, true, false, iota,
and certain constant expressions
containing only untyped constant operands are untyped.
A constant may be given a type explicitly by a constant declaration or conversion, or implicitly when used in a variable declaration or an assignment or as an operand in an expression. It is an error if the constant value cannot be represented as a value of the respective type.
An untyped constant has a default type which is the type to which the
constant is implicitly converted in contexts where a typed value is required,
for instance, in a short variable declaration
such as i := 0 where there is no explicit type.
The default type of an untyped constant is bool, rune,
int, float64, complex128 or string
respectively, depending on whether it is a boolean, rune, integer, floating-point,
complex, or string constant.
Implementation restriction: Although numeric constants have arbitrary precision in the language, a compiler may implement them using an internal representation with limited precision. That said, every implementation must:
These requirements apply both to literal constants and to the result of evaluating constant expressions.
A variable is a storage location for holding a value. The set of permissible values is determined by the variable's type.
A variable declaration
or, for function parameters and results, the signature
of a function declaration
or function literal reserves
storage for a named variable.
Calling the built-in function new
or taking the address of a composite literal
allocates storage for a variable at run time.
Such an anonymous variable is referred to via a (possibly implicit)
pointer indirection.
Structured variables of array, slice, and struct types have elements and fields that may be addressed individually. Each such element acts like a variable.
The static type (or just type) of a variable is the
type given in its declaration, the type provided in the
new call or composite literal, or the type of
an element of a structured variable.
Variables of interface type also have a distinct dynamic type,
which is the concrete type of the value assigned to the variable at run time
(unless the value is the predeclared identifier nil,
which has no type).
The dynamic type may vary during execution but values stored in interface
variables are always assignable
to the static type of the variable.
var x interface{} // x is nil and has static type interface{}
var v *T // v has value nil, static type *T
x = 42 // x has value 42 and dynamic type int
x = v // x has value (*T)(nil) and dynamic type *T
A variable's value is retrieved by referring to the variable in an expression; it is the most recent value assigned to the variable. If a variable has not yet been assigned a value, its value is the zero value for its type.
A type determines a set of values together with operations and methods specific to those values. A type may be denoted by a type name, if it has one, or specified using a type literal, which composes a type from existing types.
Type = TypeName | TypeLit | "(" Type ")" .
TypeName = identifier | QualifiedIdent .
TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
SliceType | MapType | ChannelType .
The language predeclares certain type names. Others are introduced with type declarations. Composite types—array, struct, pointer, function, interface, slice, map, and channel types—may be constructed using type literals.
Each type T has an underlying type: If T
is one of the predeclared boolean, numeric, or string types, or a type literal,
the corresponding underlying
type is T itself. Otherwise, T's underlying type
is the underlying type of the type to which T refers in its
type declaration.
type ( A1 = string A2 = A1 ) type ( B1 string B2 B1 B3 []B1 B4 B3 )
The underlying type of string, A1, A2, B1,
and B2 is string.
The underlying type of []B1, B3, and B4 is []B1.
A type has a (possibly empty) method set associated with it.
The method set of an interface type is its interface.
The method set of any other type T consists of all
methods declared with receiver type T.
The method set of the corresponding pointer type *T
is the set of all methods declared with receiver *T or T
(that is, it also contains the method set of T).
Further rules apply to structs containing embedded fields, as described
in the section on struct types.
Any other type has an empty method set.
In a method set, each method must have a
unique
non-blank method name.
The method set of a type determines the interfaces that the type implements and the methods that can be called using a receiver of that type.
A boolean type represents the set of Boolean truth values
denoted by the predeclared constants true
and false. The predeclared boolean type is bool;
it is a defined type.
A numeric type represents sets of integer or floating-point values. The predeclared architecture-independent numeric types are:
uint8 the set of all unsigned 8-bit integers (0 to 255) uint16 the set of all unsigned 16-bit integers (0 to 65535) uint32 the set of all unsigned 32-bit integers (0 to 4294967295) uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615) int8 the set of all signed 8-bit integers (-128 to 127) int16 the set of all signed 16-bit integers (-32768 to 32767) int32 the set of all signed 32-bit integers (-2147483648 to 2147483647) int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807) float32 the set of all IEEE 754 32-bit floating-point numbers float64 the set of all IEEE 754 64-bit floating-point numbers complex64 the set of all complex numbers with float32 real and imaginary parts complex128 the set of all complex numbers with float64 real and imaginary parts byte alias for uint8 rune alias for int32
The value of an n-bit integer is n bits wide and represented using two's complement arithmetic.
There is also a set of predeclared numeric types with implementation-specific sizes:
uint either 32 or 64 bits int same size as uint uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
To avoid portability issues all numeric types are defined
types and thus distinct except
byte, which is an alias for uint8, and
rune, which is an alias for int32.
Explicit conversions
are required when different numeric types are mixed in an expression
or assignment. For instance, int32 and int
are not the same type even though they may have the same size on a
particular architecture.
A string type represents the set of string values.
A string value is a (possibly empty) sequence of bytes.
The number of bytes is called the length of the string and is never negative.
Strings are immutable: once created,
it is impossible to change the contents of a string.
The predeclared string type is string;
it is a defined type.
The length of a string s can be discovered using
the built-in function len.
The length is a compile-time constant if the string is a constant.
A string's bytes can be accessed by integer indices
0 through len(s)-1.
It is illegal to take the address of such an element; if
s[i] is the i'th byte of a
string, &s[i] is invalid.
An array is a numbered sequence of elements of a single type, called the element type. The number of elements is called the length of the array and is never negative.
ArrayType = "[" ArrayLength "]" ElementType . ArrayLength = Expression . ElementType = Type .
The length is part of the array's type; it must evaluate to a
non-negative constant
representable by a value
of type int.
The length of array a can be discovered
using the built-in function len.
The elements can be addressed by integer indices
0 through len(a)-1.
Array types are always one-dimensional but may be composed to form
multi-dimensional types.
[32]byte
[2*N] struct { x, y int32 }
[1000]*float64
[3][5]int
[2][2][2]float64 // same as [2]([2]([2]float64))
A slice is a descriptor for a contiguous segment of an underlying array and
provides access to a numbered sequence of elements from that array.
A slice type denotes the set of all slices of arrays of its element type.
The number of elements is called the length of the slice and is never negative.
The value of an uninitialized slice is nil.
SliceType = "[" "]" ElementType .
The length of a slice s can be discovered by the built-in function
len; unlike with arrays it may change during
execution. The elements can be addressed by integer indices
0 through len(s)-1. The slice index of a
given element may be less than the index of the same element in the
underlying array.
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.
The array underlying a slice may extend past the end of the slice.
The capacity 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.
The capacity of a slice a can be discovered using the
built-in function cap(a).
A new, initialized slice value for a given element type T is
made using the built-in function
make,
which takes a slice type
and parameters specifying the length and optionally the capacity.
A slice created with make always allocates a new, hidden array
to which the returned slice value refers. That is, executing
make([]T, length, capacity)
produces the same slice as allocating an array and slicing it, so these two expressions are equivalent:
make([]int, 50, 100) new([100]int)[0:50]
Like arrays, slices are always one-dimensional but may be composed to construct higher-dimensional objects. With arrays of arrays, the inner arrays are, by construction, always the same length; however with slices of slices (or arrays of slices), the inner lengths may vary dynamically. Moreover, the inner slices must be initialized individually.
A struct is a sequence of named elements, called fields, each of which has a name and a type. Field names may be specified explicitly (IdentifierList) or implicitly (EmbeddedField). Within a struct, non-blank field names must be unique.
StructType = "struct" "{" { FieldDecl ";" } "}" .
FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
EmbeddedField = [ "*" ] TypeName .
Tag = string_lit .
// An empty struct.
struct {}
// A struct with 6 fields.
struct {
x, y int
u float32
_ float32 // padding
A *[]int
F func()
}
A field declared with a type but no explicit field name is called an embedded field.
An embedded field must be specified as
a type name T or as a pointer to a non-interface type name *T,
and T itself may not be
a pointer type. The unqualified type name acts as the field name.
// A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
struct {
T1 // field name is T1
*T2 // field name is T2
P.T3 // field name is T3
*P.T4 // field name is T4
x, y int // field names are x and y
}
The following declaration is illegal because field names must be unique in a struct type:
struct {
T // conflicts with embedded field *T and *P.T
*T // conflicts with embedded field T and *P.T
*P.T // conflicts with embedded field T and *T
}
A field or method f of an
embedded field in a struct x is called promoted if
x.f is a legal selector that denotes
that field or method f.
Promoted fields act like ordinary fields of a struct except that they cannot be used as field names in composite literals of the struct.
Given a struct type S and a defined type
T, promoted methods are included in the method set of the struct as follows:
S contains an embedded field T,
the method sets of S
and *S both include promoted methods with receiver
T. The method set of *S also
includes promoted methods with receiver *T.
S contains an embedded field *T,
the method sets of S and *S both
include promoted methods with receiver T or
*T.
A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. An empty tag string is equivalent to an absent tag. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.
struct {
x, y float64 "" // an empty tag string is like an absent tag
name string "any string is permitted as a tag"
_ [4]byte "ceci n'est pas un champ de structure"
}
// A struct corresponding to a TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers;
// they follow the convention outlined by the reflect package.
struct {
microsec uint64 `protobuf:"1"`
serverIP6 uint64 `protobuf:"2"`
}
A pointer type denotes the set of all pointers to variables of a given
type, called the base type of the pointer.
The value of an uninitialized pointer is nil.
PointerType = "*" BaseType . BaseType = Type .
*Point *[4]int
A function type denotes the set of all functions with the same parameter
and result types. The value of an uninitialized variable of function type
is nil.
FunctionType = "func" Signature .
Signature = Parameters [ Result ] .
Result = Parameters | Type .
Parameters = "(" [ ParameterList [ "," ] ] ")" .
ParameterList = ParameterDecl { "," ParameterDecl } .
ParameterDecl = [ IdentifierList ] [ "..." ] Type .
Within a list of parameters or results, the names (IdentifierList) must either all be present or all be absent. If present, each name stands for one item (parameter or result) of the specified type and all non-blank names in the signature must be unique. If absent, each type stands for one item of that type. Parameter and result lists are always parenthesized except that if there is exactly one unnamed result it may be written as an unparenthesized type.
The final incoming parameter in a function signature may have
a type prefixed with ....
A function with such a parameter is called variadic and
may be invoked with zero or more arguments for that parameter.
func()
func(x int) int
func(a, _ int, z float32) bool
func(a, b int, z float32) (bool)
func(prefix string, values ...int)
func(a, b int, z float64, opt ...interface{}) (success bool)
func(int, int, float64) (float64, *[]int)
func(n int) func(p *T)
An interface type specifies a method set called its interface.
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
implement the interface.
The value of an uninitialized variable of interface type is nil.
InterfaceType = "interface" "{" { ( MethodSpec | InterfaceTypeName ) ";" } "}" .
MethodSpec = MethodName Signature .
MethodName = identifier .
InterfaceTypeName = TypeName .
An interface type may specify methods explicitly through method specifications, or it may embed methods of other interfaces through interface type names.
// A simple File interface.
interface {
Read([]byte) (int, error)
Write([]byte) (int, error)
Close() error
}
The name of each explicitly specified method must be unique and not blank.
interface {
String() string
String() string // illegal: String not unique
_(x int) // illegal: method must have non-blank name
}
More than one type may implement an interface.
For instance, if two types S1 and S2
have the method set
func (p T) Read(p []byte) (n int, err error) func (p T) Write(p []byte) (n int, err error) func (p T) Close() error
(where T stands for either S1 or S2)
then the File interface is implemented by both S1 and
S2, regardless of what other methods
S1 and S2 may have or share.
A type implements any interface comprising any subset of its methods and may therefore implement several distinct interfaces. For instance, all types implement the empty interface:
interface{}
Similarly, consider this interface specification,
which appears within a type declaration
to define an interface called Locker:
type Locker interface {
Lock()
Unlock()
}
If S1 and S2 also implement
func (p T) Lock() { … }
func (p T) Unlock() { … }
they implement the Locker interface as well
as the File interface.
An interface T may use a (possibly qualified) interface type
name E in place of a method specification. This is called
embedding interface E in T.
The method set of T is the union
of the method sets of T’s explicitly declared methods and of
T’s embedded interfaces.
type Reader interface {
Read(p []byte) (n int, err error)
Close() error
}
type Writer interface {
Write(p []byte) (n int, err error)
Close() error
}
// ReadWriter's methods are Read, Write, and Close.
type ReadWriter interface {
Reader // includes methods of Reader in ReadWriter's method set
Writer // includes methods of Writer in ReadWriter's method set
}
A union of method sets contains the (exported and non-exported) methods of each method set exactly once, and methods with the same names must have identical signatures.
type ReadCloser interface {
Reader // includes methods of Reader in ReadCloser's method set
Close() // illegal: signatures of Reader.Close and Close are different
}
An interface type T may not embed itself
or any interface type that embeds T, recursively.
// illegal: Bad cannot embed itself
type Bad interface {
Bad
}
// illegal: Bad1 cannot embed itself using Bad2
type Bad1 interface {
Bad2
}
type Bad2 interface {
Bad1
}
A map is an unordered group of elements of one type, called the
element type, indexed by a set of unique keys of another type,
called the key type.
The value of an uninitialized map is nil.
MapType = "map" "[" KeyType "]" ElementType . KeyType = Type .
The comparison operators
== and != must be fully defined
for operands of the key type; thus the key type must not be a function, map, or
slice.
If the key type is an interface type, these
comparison operators must be defined for the dynamic key values;
failure will cause a run-time panic.
map[string]int
map[*T]struct{ x, y float64 }
map[string]interface{}
The number of map elements is called its length.
For a map m, it can be discovered using the
built-in function len
and may change during execution. Elements may be added during execution
using assignments and retrieved with
index expressions; they may be removed with the
delete built-in function.
A new, empty map value is made using the built-in
function make,
which takes the map type and an optional capacity hint as arguments:
make(map[string]int) make(map[string]int, 100)
The initial capacity does not bound its size:
maps grow to accommodate the number of items
stored in them, with the exception of nil maps.
A nil map is equivalent to an empty map except that no elements
may be added.
A channel provides a mechanism for
concurrently executing functions
to communicate by
sending and
receiving
values of a specified element type.
The value of an uninitialized channel is nil.
ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
The optional <- operator specifies the channel direction,
send or receive. If no direction is given, the channel is
bidirectional.
A channel may be constrained only to send or only to receive by
assignment or
explicit conversion.
chan T // can be used to send and receive values of type T chan<- float64 // can only be used to send float64s <-chan int // can only be used to receive ints
The <- operator associates with the leftmost chan
possible:
chan<- chan int // same as chan<- (chan int) chan<- <-chan int // same as chan<- (<-chan int) <-chan <-chan int // same as <-chan (<-chan int) chan (<-chan int)
A new, initialized channel
value can be made using the built-in function
make,
which takes the channel type and an optional capacity as arguments:
make(chan int, 100)
The capacity, in number of elements, sets the size of the buffer in the channel.
If the capacity is zero or absent, the channel is unbuffered and communication
succeeds only when both a sender and receiver are ready. Otherwise, the channel
is buffered and communication succeeds without blocking if the buffer
is not full (sends) or not empty (receives).
A nil channel is never ready for communication.
A channel may be closed with the built-in function
close.
The multi-valued assignment form of the
receive operator
reports whether a received value was sent before
the channel was closed.
A single channel may be used in
send statements,
receive operations,
and calls to the built-in functions
cap and
len
by any number of goroutines without further synchronization.
Channels act as first-in-first-out queues.
For example, if one goroutine sends values on a channel
and a second goroutine receives them, the values are
received in the order sent.
Two types are either identical or different.
A defined type is always different from any other type. Otherwise, two types are identical if their underlying type literals are structurally equivalent; that is, they have the same literal structure and corresponding components have identical types. In detail:
Given the declarations
type (
A0 = []string
A1 = A0
A2 = struct{ a, b int }
A3 = int
A4 = func(A3, float64) *A0
A5 = func(x int, _ float64) *[]string
)
type (
B0 A0
B1 []string
B2 struct{ a, b int }
B3 struct{ a, c int }
B4 func(int, float64) *B0
B5 func(x int, y float64) *A1
)
type C0 = B0
these types are identical:
A0, A1, and []string
A2 and struct{ a, b int }
A3 and int
A4, func(int, float64) *[]string, and A5
B0 and C0
[]int and []int
struct{ a, b *T5 } and struct{ a, b *T5 }
func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
B0 and B1 are different because they are new types
created by distinct type definitions;
func(int, float64) *B0 and func(x int, y float64) *[]string
are different because B0 is different from []string.
A value x is assignable to a variable of type T
("x is assignable to T") if one of the following conditions applies:
x's type is identical to T.
x's type V and T have identical
underlying types and at least one of V
or T is not a defined type.
T is an interface type and
x implements T.
x is a bidirectional channel value, T is a channel type,
x's type V and T have identical element types,
and at least one of V or T is not a defined type.
x is the predeclared identifier nil and T
is a pointer, function, slice, map, channel, or interface type.
x is an untyped constant
representable
by a value of type T.
A constant x is representable
by a value of type T if one of the following conditions applies:
x is in the set of values determined by T.
T is a floating-point type and x can be rounded to T's
precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
negative zero further simplified to an unsigned zero. Note that constant values never result
in an IEEE negative zero, NaN, or infinity.
T is a complex type, and x's
components real(x) and imag(x)
are representable by values of T's component type (float32 or
float64).
x T x is representable by a value of T because 'a' byte 97 is in the set of byte values 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers "foo" string "foo" is in the set of string values 1024 int16 1024 is in the set of 16-bit integers 42.0 byte 42 is in the set of unsigned 8-bit integers 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0 0i int 0 is an integer value (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
x T x is not representable by a value of T because 0 bool 0 is not in the set of boolean values 'a' string 'a' is a rune, it is not in the set of string values 1024 byte 1024 is not in the set of unsigned 8-bit integers -1 uint16 -1 is not in the set of unsigned 16-bit integers 1.1 int 1.1 is not an integer value 42i float32 (0 + 42i) is not in the set of float32 values 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
A block is a possibly empty sequence of declarations and statements within matching brace brackets.
Block = "{" StatementList "}" .
StatementList = { Statement ";" } .
In addition to explicit blocks in the source code, there are implicit blocks:
Blocks nest and influence scoping.
A declaration binds a non-blank identifier to a constant, type, variable, function, label, 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.
The blank identifier may be used like any other identifier
in a declaration, but it does not introduce a binding and thus is not declared.
In the package block, the identifier init may only be used for
init function declarations,
and like the blank identifier it does not introduce a new binding.
Declaration = ConstDecl | TypeDecl | VarDecl . TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
The scope of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, label, or package.
Go is lexically scoped using blocks:
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.
The package clause is not a declaration; the package name does not appear in any scope. Its purpose is to identify the files belonging to the same package and to specify the default package name for import declarations.
Labels are declared by labeled statements and are used in the "break", "continue", and "goto" statements. It is illegal to define a label that is never used. In contrast to other identifiers, labels are not block scoped and do not conflict with identifiers that are not labels. The scope of a label is the body of the function in which it is declared and excludes the body of any nested function.
The blank identifier is represented by the underscore character _.
It serves as an anonymous placeholder instead of a regular (non-blank)
identifier and has special meaning in declarations,
as an operand, and in assignments.
The following identifiers are implicitly declared in the universe block:
Types: bool byte complex64 complex128 error float32 float64 int int8 int16 int32 int64 rune string uint uint8 uint16 uint32 uint64 uintptr Constants: true false iota Zero value: nil Functions: append cap close complex copy delete imag len make new panic print println real recover
An identifier may be exported to permit access to it from another package. An identifier is exported if both:
All other identifiers are not exported.
Given a set of identifiers, an identifier is called unique if it is different from every other in the set. Two identifiers are different if they are spelled differently, or if they appear in different packages and are not exported. Otherwise, they are the same.
A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of constant expressions. The number of identifiers must be equal to the number of expressions, and the nth identifier on the left is bound to the value of the nth expression on the right.
ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
If the type is present, all constants take the type specified, and the expressions must be assignable to that type. If the type is omitted, the constants take the individual types of the corresponding expressions. If the expression values are untyped constants, 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.
const Pi float64 = 3.14159265358979323846 const zero = 0.0 // untyped floating-point constant const ( size int64 = 1024 eof = -1 // untyped integer constant ) const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants const u, v float32 = 0, 3 // u = 0.0, v = 3.0
Within a parenthesized const declaration list the
expression list may be omitted from any but the first ConstSpec.
Such an empty list is equivalent to the textual substitution of the
first preceding non-empty expression list and its type if any.
Omitting the list of expressions is therefore equivalent to
repeating the previous list. The number of identifiers must be equal
to the number of expressions in the previous list.
Together with the iota constant generator
this mechanism permits light-weight declaration of sequential values:
const ( Sunday = iota Monday Tuesday Wednesday Thursday Friday Partyday numberOfDays // this constant is not exported )
Within a constant declaration, the predeclared identifier
iota represents successive untyped integer
constants. Its value is the index of the respective ConstSpec
in that constant declaration, starting at zero.
It can be used to construct a set of related constants:
const ( c0 = iota // c0 == 0 c1 = iota // c1 == 1 c2 = iota // c2 == 2 ) const ( a = 1 << iota // a == 1 (iota == 0) b = 1 << iota // b == 2 (iota == 1) c = 3 // c == 3 (iota == 2, unused) d = 1 << iota // d == 8 (iota == 3) ) const ( u = iota * 42 // u == 0 (untyped integer constant) v float64 = iota * 42 // v == 42.0 (float64 constant) w = iota * 42 // w == 84 (untyped integer constant) ) const x = iota // x == 0 const y = iota // y == 0
By definition, multiple uses of iota in the same ConstSpec all have the same value:
const ( bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0) bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1) _, _ // (iota == 2, unused) bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3) )
This last example exploits the implicit repetition of the last non-empty expression list.
A type declaration binds an identifier, the type name, to a type. Type declarations come in two forms: alias declarations and type definitions.
TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec = AliasDecl | TypeDef .
An alias declaration binds an identifier to the given type.
AliasDecl = identifier "=" Type .
Within the scope of the identifier, it serves as an alias for the type.
type ( nodeList = []*Node // nodeList and []*Node are identical types Polar = polar // Polar and polar denote identical types )
A type definition creates a new, distinct type with the same underlying type and operations as the given type, and binds an identifier to it.
TypeDef = identifier Type .
The new type is called a defined type. It is different from any other type, including the type it is created from.
type (
Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
polar Point // polar and Point denote different types
)
type TreeNode struct {
left, right *TreeNode
value *Comparable
}
type Block interface {
BlockSize() int
Encrypt(src, dst []byte)
Decrypt(src, dst []byte)
}
A defined type may have methods associated with it. It does not inherit any methods bound to the given type, but the method set of an interface type or of elements of a composite type remains unchanged:
// A Mutex is a data type with two methods, Lock and Unlock.
type Mutex struct { /* Mutex fields */ }
func (m *Mutex) Lock() { /* Lock implementation */ }
func (m *Mutex) Unlock() { /* Unlock implementation */ }
// NewMutex has the same composition as Mutex but its method set is empty.
type NewMutex Mutex
// The method set of PtrMutex's underlying type *Mutex remains unchanged,
// but the method set of PtrMutex is empty.
type PtrMutex *Mutex
// The method set of *PrintableMutex contains the methods
// Lock and Unlock bound to its embedded field Mutex.
type PrintableMutex struct {
Mutex
}
// MyBlock is an interface type that has the same method set as Block.
type MyBlock Block
Type definitions may be used to define different boolean, numeric, or string types and associate methods with them:
type TimeZone int
const (
EST TimeZone = -(5 + iota)
CST
MST
PST
)
func (tz TimeZone) String() string {
return fmt.Sprintf("GMT%+dh", tz)
}
A variable declaration creates one or more variables, binds corresponding identifiers to them, and gives each a type and an initial value.
VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
var i int var U, V, W float64 var k = 0 var x, y float32 = -1, -2 var ( i int u, v, s = 2.0, 3.0, "bar" ) var re, im = complexSqrt(-1) var _, found = entries[name] // map lookup; only interested in "found"
If a list of expressions is given, the variables are initialized with the expressions following the rules for assignments. Otherwise, each variable is initialized to its zero value.
If a type is present, each variable is given that type.
Otherwise, each variable is given the type of the corresponding
initialization value in the assignment.
If that value is an untyped constant, it is first implicitly
converted to its default type;
if it is an untyped boolean value, it is first implicitly converted to type bool.
The predeclared value nil cannot be used to initialize a variable
with no explicit type.
var d = math.Sin(0.5) // d is float64 var i = 42 // i is int var t, ok = x.(T) // t is T, ok is bool var n = nil // illegal
Implementation restriction: A compiler may make it illegal to declare a variable inside a function body if the variable is never used.
A short variable declaration uses the syntax:
ShortVarDecl = IdentifierList ":=" ExpressionList .
It is shorthand for a regular variable declaration with initializer expressions but no types:
"var" IdentifierList = ExpressionList .
i, j := 0, 10
f := func() int { return 7 }
ch := make(chan int)
r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
_, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-blank 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.
field1, offset := nextField(str, 0) field2, offset := nextField(str, offset) // redeclares offset a, a := 1, 2 // illegal: double declaration of a or no new variable if a was declared elsewhere
Short variable declarations may appear only inside functions. In some contexts such as the initializers for "if", "for", or "switch" statements, they can be used to declare local temporary variables.
A function declaration binds an identifier, the function name, to a function.
FunctionDecl = "func" FunctionName Signature [ FunctionBody ] . FunctionName = identifier . FunctionBody = Block .
If the function's signature declares result parameters, the function body's statement list must end in a terminating statement.
func IndexRune(s string, r rune) int {
for i, c := range s {
if c == r {
return i
}
}
// invalid: missing return statement
}
A function declaration may omit the body. Such a declaration provides the signature for a function implemented outside Go, such as an assembly routine.
func min(x int, y int) int {
if x < y {
return x
}
return y
}
func flushICache(begin, end uintptr) // implemented externally
A method is a function with a receiver. A method declaration binds an identifier, the method name, to a method, and associates the method with the receiver's base type.
MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] . Receiver = Parameters .
The receiver is specified via an extra parameter section preceding the method
name. That parameter section must declare a single non-variadic parameter, the receiver.
Its type must be a defined type T or a
pointer to a defined type T. T is called the receiver
base type. A receiver base type cannot be a pointer or interface type and
it must be defined in the same package as the method.
The method is said to be bound to its receiver base type and the method name
is visible only within selectors for type T
or *T.
A non-blank receiver identifier must be unique in the method signature. If the receiver's value is not referenced inside the body of the method, its identifier may be omitted in the declaration. The same applies in general to parameters of functions and methods.
For a base type, the non-blank names of methods bound to it must be unique. If the base type is a struct type, the non-blank method and field names must be distinct.
Given defined type Point, the declarations
func (p *Point) Length() float64 {
return math.Sqrt(p.x * p.x + p.y * p.y)
}
func (p *Point) Scale(factor float64) {
p.x *= factor
p.y *= factor
}
bind the methods Length and Scale,
with receiver type *Point,
to the base type Point.
The type of a method is the type of a function with the receiver as first
argument. For instance, the method Scale has type
func(p *Point, factor float64)
However, a function declared this way is not a method.
An expression specifies the computation of a value by applying operators and functions to operands.
Operands denote the elementary values in an expression. An operand may be a literal, a (possibly qualified) non-blank identifier denoting a constant, variable, or function, or a parenthesized expression.
The blank identifier may appear as an operand only on the left-hand side of an assignment.
Operand = Literal | OperandName | "(" Expression ")" .
Literal = BasicLit | CompositeLit | FunctionLit .
BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
OperandName = identifier | QualifiedIdent .
A qualified identifier is an identifier qualified with a package name prefix. Both the package name and the identifier must not be blank.
QualifiedIdent = PackageName "." identifier .
A qualified identifier accesses an identifier in a different package, which must be imported. The identifier must be exported and declared in the package block of that package.
math.Sin // denotes the Sin function in package math
Composite literals construct values for structs, arrays, slices, and maps and create a new value each time they are evaluated. They consist of the type of the literal followed by a brace-bound list of elements. Each element may optionally be preceded by a corresponding key.
CompositeLit = LiteralType LiteralValue .
LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
SliceType | MapType | TypeName .
LiteralValue = "{" [ ElementList [ "," ] ] "}" .
ElementList = KeyedElement { "," KeyedElement } .
KeyedElement = [ Key ":" ] Element .
Key = FieldName | Expression | LiteralValue .
FieldName = identifier .
Element = Expression | LiteralValue .
The LiteralType's underlying type must be a struct, array, slice, or map type (the grammar enforces this constraint except when the type is given as a TypeName). The types of the elements and keys must be assignable to the respective field, element, and key types of the literal type; there is no additional conversion. The key is interpreted as a field name for struct literals, an index for array and slice literals, and a key for map literals. For map literals, all elements must have a key. It is an error to specify multiple elements with the same field name or constant key value. For non-constant map keys, see the section on evaluation order.
For struct literals the following rules apply:
Given the declarations
type Point3D struct { x, y, z float64 }
type Line struct { p, q Point3D }
one may write
origin := Point3D{} // zero value for Point3D
line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
For array and slice literals the following rules apply:
int; and if it is typed
it must be of integer type.
Taking the address of a composite literal generates a pointer to a unique variable initialized with the literal's value.
var pointer *Point3D = &Point3D{y: 1000}
Note that the zero value for a slice or map type is not the same as an initialized but empty value of the same type. Consequently, taking the address of an empty slice or map composite literal does not have the same effect as allocating a new slice or map value with new.
p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
The length of an array literal is the length specified in the literal type.
If fewer elements than the length are provided in the literal, the missing
elements are set to the zero value for the array element type.
It is an error to provide elements with index values outside the index range
of the array. The notation ... specifies an array length equal
to the maximum element index plus one.
buffer := [10]string{} // len(buffer) == 10
intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
days := [...]string{"Sat", "Sun"} // len(days) == 2
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
[]T{x1, x2, … xn}
and is shorthand for a slice operation applied to an array:
tmp := [n]T{x1, x2, … xn}
tmp[0 : n]
Within a composite literal of array, slice, or map type T,
elements or map keys that are themselves composite literals may elide the respective
literal type if it is identical to the element or key type of T.
Similarly, elements or keys that are addresses of composite literals may elide
the &T when the element or key type is *T.
[...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
[][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
[][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
type PPoint *Point
[2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
[2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the keyword and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening b