This is a reference manual for the Go programming language. For more information and other documents, see http://golang.org.
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 existing implementations use a traditional compile/link model to generate executable binaries.
The grammar is compact and regular, 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 symbols 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
ellipis … 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 term character to refer to a Unicode code point.
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.
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 "Decimal Digit" */ .
In The Unicode Standard 6.0, Section 4.5 "General Category" 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.
The underscore character _ (U+005F) is considered a letter.
letter = unicode_letter | "_" . decimal_digit = "0" … "9" . octal_digit = "0" … "7" . hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
There are two forms of comments:
//
and stop at the end of the line. A line comment acts like a newline.
/*
and continue through the character sequence */. A general
comment containing one or more newlines acts like a newline, otherwise it acts
like a space.
Comments do not nest.
Tokens form the vocabulary of the Go language. There are four classes: identifiers, keywords, operators and delimiters, 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:
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
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, delimiters, and other special tokens:
+ & += &= && == != ( )
- | -= |= || < <= [ ]
* ^ *= ^= <- > >= { }
/ << /= <<= ++ = := , ;
% >> %= >>= -- ! ... . :
&^ &^=
An integer literal is a sequence of digits representing an
integer constant.
An optional prefix sets a non-decimal base: 0 for octal, 0x or
0X for hexadecimal. In hexadecimal literals, letters
a-f and A-F represent values 10 through 15.
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 } .
42 0600 0xBadFace 170141183460469231731687303715884105727
A floating-point literal is a decimal representation of a
floating-point constant.
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 e or E
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.
float_lit = decimals "." [ decimals ] [ exponent ] |
decimals exponent |
"." decimals [ exponent ] .
decimals = decimal_digit { decimal_digit } .
exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .
0. 72.40 072.40 // == 72.40 2.71828 1.e+0 6.67428e-11 1E6 .25 .12345E+5
An imaginary literal is a decimal representation of the imaginary part of a
complex constant.
It consists of a
floating-point literal
or decimal integer followed
by the lower-case letter i.
imaginary_lit = (decimals | float_lit) "i" .
0i 011i // == 11i 0.i 2.71828i 1.e+0i 6.67428e-11i 1E6i .25i .12345E+5i
A character literal represents a character constant, 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.
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 represented
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 character literals) \" U+0022 double quote (valid escape only within string literals)
All other sequences starting with a backslash are illegal inside character literals.
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" | `\` | "'" | `"` ) .
'a' 'ä' '本' '\t' '\000' '\007' '\377' '\x07' '\xff' '\u12e4' '\U00101234'
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
``. 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
contain newlines.
Carriage returns inside raw string literals
are discarded from the raw string value.
Interpreted string literals are character sequences between double
quotes "". The text between the quotes,
which may not contain newlines, forms the
value of the literal, with backslash escapes interpreted as they
are in character literals (except that \' is illegal and
\" is legal). 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" "" "Hello, world!\n" "日本語" "\u65e5本\U00008a9e" "\xff\u00FF"
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 character 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, character constants, integer constants, floating-point constants, complex constants, and string constants. Character, integer, floating-point, and complex constants are collectively called numeric constants.
A constant value is represented by a
character,
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 values of arbitrary precision and do not overflow.
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.
For instance, 3.0 can be given any integer or any
floating-point type, while 2147483648.0 (equal to 1<<31)
can be given the types float32, float64, or uint32 but
not int32 or string.
There are no constants denoting the IEEE-754 infinity and not-a-number values,
but the math package's
Inf,
NaN,
IsInf, and
IsNaN
functions return and test for those values at run time.
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.
A type determines the set of values and operations specific to values of that type. A type may be specified by a (possibly qualified) type name (§Qualified identifier, §Type declarations) or a type literal, which composes a new type from previously declared types.
Type = TypeName | TypeLit | "(" Type ")" .
TypeName = QualifiedIdent .
TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
SliceType | MapType | ChannelType .
Named instances of the boolean, numeric, and string types are predeclared. Composite types—array, struct, pointer, function, interface, slice, map, and channel types—may be constructed using type literals.
The static type (or just type) of a variable is the type defined by its declaration. Variables of interface type also have a distinct dynamic type, 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 assignable to the static type of the interface variable. For non-interface types, the dynamic type is always the static type.
Each type T has an underlying type: If T
is a predeclared type 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 T1 string type T2 T1 type T3 []T1 type T4 T3
The underlying type of string, T1, and T2
is string. The underlying type of []T1, T3,
and T4 is []T1.
A type may have a method set associated with it
(§Interface types, §Method declarations).
The method set of an interface type is its interface.
The method set of any other named type T
consists of all methods with receiver type T.
The method set of the corresponding pointer type *T
is the set of all methods with receiver *T or T
(that is, it also contains the method set of T).
Any other type has an empty method set.
In a method set, each method must have a unique 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.
true
and false. The predeclared boolean type is bool.
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 distinct except
byte, which is an alias for uint8, and
rune, which is an alias for int32.
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.
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 string.
The elements of strings have type byte and may be
accessed using the usual indexing operations. It is
illegal to take the address of such an element; if
s[i] is the ith byte of a
string, &s[i] is invalid. The length of string
s can be discovered using the built-in function
len. The length is a compile-time constant if s
is a string literal.
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.
ArrayType = "[" ArrayLength "]" ElementType . ArrayLength = Expression . ElementType = Type .
The length is part of the array's type and must be a
constant expression that evaluates to a non-negative
integer value. The length of array a can be discovered
using the built-in function len(a).
The elements can be indexed by integer
indices 0 through len(a)-1 (§Indexes).
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 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.
The value of an uninitialized slice is nil.
SliceType = "[" "]" ElementType .
Like arrays, slices are indexable and have a length. The length of a
slice s can be discovered by the built-in function
len(s); unlike with arrays it may change during
execution. The elements can be addressed by integer indices 0
through len(s)-1 (§Indexes). 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 (§Slices).
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:
make([]T, length) make([]T, length, capacity)
A call to make 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 examples result in the same slice:
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 lengths may vary dynamically.
Moreover, the inner slices must be allocated individually (with make).
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-blank field names must be unique.
StructType = "struct" "{" { FieldDecl ";" } "}" .
FieldDecl = (IdentifierList Type | AnonymousField) [ Tag ] .
AnonymousField = [ "*" ] 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 an anonymous field,
also called an embedded field or an embedding of the type in the struct.
An embedded type 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 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
}
The following declaration is illegal because field names must be unique in a struct type:
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
}
Fields and methods (§Method declarations) of an anonymous field are
promoted to be ordinary fields and methods of the struct (§Selectors).
The following rules apply for a struct type named S and
a type named T:
S contains an anonymous field T, the
method set of S includes the
method set of T.
S contains an anonymous field *T, the
method set of S includes the method set of *T
(which itself includes the method set of T).
S contains an anonymous field T or
*T, the method set of *S includes the
method set of *T (which itself includes the method
set of 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. The tags are made visible through a reflection interface but are otherwise ignored.
// 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"
}
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 .
*int *map[string]*chan 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; 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 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)
func() int
func(prefix string, values ...int)
func(a, b int, z float32) bool
func(a, b int, z float32) (bool)
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 ";" } "}" .
MethodSpec = MethodName Signature | InterfaceTypeName .
MethodName = identifier .
InterfaceTypeName = TypeName .
As with all method sets, in an interface type, each method must have a unique name.
// A simple File interface
interface {
Read(b Buffer) bool
Write(b Buffer) bool
Close()
}
More than one type may implement an interface.
For instance, if two types S1 and S2
have the method set
func (p T) Read(b Buffer) bool { return … }
func (p T) Write(b Buffer) bool { return … }
func (p T) Close() { … }
(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 Lock:
type Lock interface {
Lock()
Unlock()
}
If S1 and S2 also implement
func (p T) Lock() { … }
func (p T) Unlock() { … }
they implement the Lock interface as well
as the File interface.
An interface may use an interface type name T
in place of a method specification.
The effect, called embedding an interface,
is equivalent to enumerating the methods of T explicitly
in the interface.
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()
}
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 !=
(§Comparison operators) 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(m)
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 two concurrently executing functions
to synchronize execution and communicate by passing a value of a
specified element type.
The value of an uninitialized channel is nil.
ChannelType = ( "chan" [ "<-" ] | "<-" "chan" ) ElementType .
The <- operator specifies the channel direction,
send or receive. If no direction is given, the channel is
bi-directional.
A channel may be constrained only to send or only to receive by
conversion or assignment.
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 greater than zero, the channel is asynchronous: communication operations
succeed without blocking if the buffer is not full (sends) or not empty (receives),
and elements are received in the order they are sent.
If the capacity is zero or absent, the communication succeeds only when both a sender and
receiver are ready.
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
tests whether a channel has been closed.
Two types are either identical or different.
Two named types are identical if their type names originate in the same type declaration. A named and an unnamed type are always different. Two unnamed types are identical if the corresponding type literals are identical, that is, if they have the same literal structure and corresponding components have identical types. In detail:
Given the declarations
type (
T0 []string
T1 []string
T2 struct{ a, b int }
T3 struct{ a, c int }
T4 func(int, float64) *T0
T5 func(x int, y float64) *[]string
)
these types are identical:
T0 and T0
[]int and []int
struct{ a, b *T5 } and struct{ a, b *T5 }
func(x int, y float64) *[]string and func(int, float64) (result *[]string)
T0 and T1 are different because they are named types
with distinct declarations; func(int, float64) *T0 and
func(x int, y float64) *[]string are different because T0
is different from []string.
A value x is assignable to a variable of type T
("x is assignable to T") in any of these cases:
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 named 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 named 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.
Any value may be assigned to the blank identifier.
A block is a sequence of declarations and statements within matching brace brackets.
Block = "{" { Statement ";" } "}" .
In addition to explicit blocks in the source code, there are implicit blocks:
if, for, and switch
statement is considered to be in its own implicit block.switch or select statement
acts as an implicit block.Blocks nest and influence scoping.
A declaration binds a non-blank 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.
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, 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 (§Break statements, §Continue statements, §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 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 using a qualified identifier. An identifier is exported if both:
All other identifiers are not exported.
The blank identifier, represented by the underscore character _, may be used in a declaration like
any other identifier but the declaration does not introduce a new binding.
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 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 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. It is reset to 0 whenever the reserved word const
appears in the source and increments after each ConstSpec.
It can be used to construct a set of related constants:
const ( // iota is reset to 0 c0 = iota // c0 == 0 c1 = iota // c1 == 1 c2 = iota // c2 == 2 ) const ( a = 1 << iota // a == 1 (iota has been reset) b = 1 << iota // b == 2 c = 1 << iota // c == 4 ) 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 (iota has been reset) const y = iota // y == 0 (iota has been reset)
Within an ExpressionList, the value of each iota is the same because
it is only incremented after each ConstSpec:
const ( bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 bit1, mask1 // bit1 == 2, mask1 == 1 _, _ // skips iota == 2 bit3, mask3 // bit3 == 8, mask3 == 7 )
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 new type that has the same underlying type as an existing type. The new type is different from the existing type.
TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec = identifier Type .
type IntArray [16]int
type (
Point struct{ x, y float64 }
Polar Point
)
type TreeNode struct {
left, right *TreeNode
value *Comparable
}
type Block interface {
BlockSize() int
Encrypt(src, dst []byte)
Decrypt(src, dst []byte)
}
The declared type does not inherit any methods bound to the existing 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 the base type of PtrMutex remains unchanged,
// but the method set of PtrMutex is empty.
type PtrMutex *Mutex
// The method set of *PrintableMutex contains the methods
// Lock and Unlock bound to its anonymous field Mutex.
type PrintableMutex struct {
Mutex
}
// MyBlock is an interface type that has the same method set as Block.
type MyBlock Block
A type declaration may be used to define a different boolean, numeric, or string type and attach methods to it:
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 a variable, binds an identifier to it and gives it a type and optionally 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 by assigning the expressions to the variables (§Assignments) in order; all expressions must be consumed and all variables initialized from them. Otherwise, each variable is initialized to its zero value.
If the type is present, each variable is given that type. Otherwise, the types are deduced from the assignment of the expression list.
If the type is absent and the corresponding expression evaluates to an untyped constant, the type of the declared variable is as described in §Assignments.
A short variable declaration uses the syntax:
ShortVarDecl = IdentifierList ":=" ExpressionList .
It is a 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(fd) // os.Pipe() returns two values
_, 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 in the same block 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
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 (§Statements).
A function declaration binds an identifier to a function (§Function types).
FunctionDecl = "func" identifier Signature [ Body ] . Body = Block .
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 to a method.
MethodDecl = "func" Receiver MethodName Signature [ Body ] .
Receiver = "(" [ identifier ] [ "*" ] BaseTypeName ")" .
BaseTypeName = identifier .
The receiver type must be of the form T or *T where
T is a type name. T is called the
receiver base type or just base type.
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 bound to the base type
and is visible only within selectors for that type
(§Type declarations, §Selectors).
Given 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.
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.
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.
Operand = Literal | QualifiedIdent | MethodExpr | "(" Expression ")" .
Literal = BasicLit | CompositeLit | FunctionLit .
BasicLit = int_lit | float_lit | imaginary_lit | char_lit | string_lit .
A qualified identifier is a non-blank identifier qualified by a package name prefix.
QualifiedIdent = [ PackageName "." ] identifier .
A qualified identifier accesses an identifier in a separate package. The identifier must be exported by that package, which means that it must begin with a Unicode upper case letter.
math.Sin
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.
CompositeLit = LiteralType LiteralValue .
LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
SliceType | MapType | TypeName .
LiteralValue = "{" [ ElementList [ "," ] ] "}" .
ElementList = Element { "," Element } .
Element = [ Key ":" ] Value .
Key = FieldName | ElementIndex .
FieldName = identifier .
ElementIndex = Expression .
Value = Expression | LiteralValue .
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 assignable to 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.
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:
Taking the address of a composite literal (§Address operators) generates a pointer to a unique instance of the literal's value.
var pointer *Point3D = &Point3D{y: 1000}
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 ... 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 a shortcut 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 that are themselves composite literals may elide the respective
literal type if it is identical to the element type of T.
Similarly, elements that are addresses of composite literals may elide
the &T when the the element 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{{1.5, -3.5}, {0, 0}} // same as [...]*Point{&Point{1.5, -3.5}, &Point{0, 0}}
A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears between the keyword and the opening brace of the block of an "if", "for", or "switch" statement, because the braces surrounding the expressions in the literal are confused with those introducing the block of statements. To resolve the ambiguity in this rare case, the composite literal must appear within parentheses.
if x == (T{a,b,c}[i]) { … }
if (x == T{a,b,c}[i]) { … }
Examples of valid array, slice, and map literals:
// list of prime numbers
primes := []int{2, 3, 5, 7, 9, 2147483647}
// vowels[ch] is true if ch is a vowel
vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
// the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
// frequencies in Hz for equal-tempered scale (A4 = 440Hz)
noteFrequency := map[string]float32{
"C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
"G0": 24.50, "A0": 27.50, "B0": 30.87,
}
A function literal represents an anonymous function. It consists of a specification of the function type and a function body.
FunctionLit = FunctionType Body .
func(a, b int, z float64) bool { return a*b < int(z) }
A function literal can be assigned to a variable or invoked directly.
f := func(x, y int) int { return x + y }
func(ch chan int) { ch <- ACK } (reply_chan)
Function literals are closures: 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.
Primary expressions are the operands for unary and binary expressions.
PrimaryExpr =
Operand |
Conversion |
BuiltinCall |
PrimaryExpr Selector |
PrimaryExpr Index |
PrimaryExpr Slice |
PrimaryExpr TypeAssertion |
PrimaryExpr Call .
Selector = "." identifier .
Index = "[" Expression "]" .
Slice = "[" [ Expression ] ":" [ Expression ] "]" .
TypeAssertion = "." "(" Type ")" .
Call = "(" [ ArgumentList [ "," ] ] ")" .
ArgumentList = ExpressionList [ "..." ] .
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()
A primary expression of the form
x.f
denotes the field or method f of the value denoted by x
(or sometimes *x; see below). The identifier f
is called the (field or method)
selector; it must not be the blank identifier.
The type of the expression is the type of f.
A selector f may denote a field or method f of
a type T, or it may refer
to a field or method f of a nested anonymous field of
T.
The number of anonymous fields traversed
to reach f is called its depth in T.
The depth of a field or method f
declared in T is zero.
The depth of a field or method f declared in
an anonymous field A in T is the
depth of f in A plus one.
The following rules apply to selectors:
x of type T or *T
where T is not an interface type,
x.f denotes the field or method at the shallowest depth
in T where there
is such an f.
If there is not exactly one f with shallowest depth, the selector
expression is illegal.
x of type I
where I is an interface type,
x.f denotes the actual method with name f of the value assigned
to x if there is such a method.
If no value or nil was assigned to x, x.f is illegal.
x.f is illegal.
Selectors automatically dereference pointers to structs.
If x is a pointer to a struct, x.y
is shorthand for (*x).y; if the field y
is also a pointer to a struct, x.y.z is shorthand
for (*(*x).y).z, and so on.
If x contains an anonymous field of type *A,
where A is also a struct type,
x.f is a shortcut for (*x.A).f.
For example, given the declarations:
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
one may write:
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
A primary expression of the form
a[x]
denotes the element of the array, slice, string or map a indexed by x.
The value x is called the
index or map key, respectively. The following
rules apply:
For a of type A or *A
where A is an array type,
or for a of type S where S is a slice type:
x must be an integer value and 0 <= x < len(a)a[x] is the array element at index x and the type of
a[x] is the element type of Aa is nil or if the index x is out of range,
a run-time panic occurs
For a of type T
where T is a string type:
x must be an integer value and 0 <= x < len(a)a[x] is the byte at index x and the type of
a[x] is bytea[x] may not be assigned tox is out of range,
a run-time panic occurs
For a of type M
where M is a map type:
x's type must be
assignable
to the key type of Mx,
a[x] is the map value with key x
and the type of a[x] is the value type of Mnil or does not contain such an entry,
a[x] is the zero value
for the value type of M
Otherwise a[x] is illegal.
An index expression on a map a of type map[K]V
may be used in an assignment or initialization of the special form
v, ok = a[x] v, ok := a[x] var v, ok = a[x]
where the result of the index expression is a pair of values with types
(V, bool). In this form, the value of ok is
true if the key x is present in the map, and
false otherwise. The value of v is the value
a[x] as in the single-result form.
Assigning to an element of a nil map causes a
run-time panic.
For a string, array, pointer to array, or slice a, the primary expression
a[low : high]
constructs a substring or slice. The index expressions low and
high select which elements appear in the result. The result has
indexes starting at 0 and length equal to
high - low.
After slicing the array a
a := [5]int{1, 2, 3, 4, 5}
s := a[1:4]
the slice s has type []int, length 3, capacity 4, and elements
s[0] == 2 s[1] == 3 s[2] == 4
For convenience, any of the index expressions may be omitted. A missing low
index defaults to zero; a missing high index defaults to the length of the
sliced operand:
a[2:] // same a[2 : len(a)] a[:3] // same as a[0 : 3] a[:] // same as a[0 : len(a)]
For arrays or strings, the indexes low and high must
satisfy 0 <= low <= high <= length; for
slices, the upper bound is the capacity rather than the length.
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, it must be addressable and the result of the slice operation is a slice with the same element type as the array.
For an expression x of interface type
and a type T, the primary expression
x.(T)
asserts that x is not nil
and that the value stored in x is of type T.
The notation x.(T) is called a type assertion.
More precisely, if T is not an interface type, x.(T) asserts
that the dynamic type of x is identical
to the type T.
If T is an interface type, x.(T) asserts that the dynamic type
of x implements the interface T (§Interface types).
If the type assertion holds, the value of the expression is the value
stored in x and its type is T. If the type assertion is false,
a run-time panic occurs.
In other words, even though the dynamic type of x
is known only at run-time, the type of x.(T) is
known to be T in a correct program.
If a type assertion is used in an assignment or initialization of the form
v, ok = x.(T) v, ok := x.(T) var v, ok = x.(T)
the result of the assertion is a pair of values with types (T, bool).
If the assertion holds, the expression returns the pair (x.(T), true);
otherwise, the expression returns (Z, false) where Z
is the zero value for type T.
No run-time panic occurs in this case.
The type assertion in this construct thus acts like a function call
returning a value and a boolean indicating success. (§Assignments)
Given an expression f of function type
F,
f(a1, a2, … an)
calls f with arguments a1, a2, … an.
Except for one special case, arguments must be single-valued expressions
assignable to the parameter types of
F and are evaluated before the function is called.
The type of the expression is the result type
of F.
A method invocation is similar but the method itself
is specified as a selector upon a value of the receiver type for
the method.
math.Atan2(x, y) // function call var pt *Point pt.Scale(3.5) // method call with receiver pt
In a function call, the function value and arguments are evaluated in the usual order. After they are evaluated, the parameters of the call are passed by value to the function and the called function begins execution. The return parameters of the function are passed by value back to the calling function when the function returns.
Calling a nil function value
causes a run-time panic.
As a special case, if the return parameters of a function or method
g are equal in number and individually
assignable to the parameters of another function or method
f, then the call f(g(parameters_of_g))
will invoke f after binding the return values of
g to the parameters of f in order. The call
of f must contain no parameters other than the call of g.
If f has a final ... parameter, it is
assigned the return values of g that remain after
assignment of regular parameters.
func Split(s string, pos int) (string, string) {
return s[0:pos], s[pos:]
}
func Join(s, t string) string {
return s + t
}
if Join(Split(value, len(value)/2)) != value {
log.Panic("test fails")
}
A method call x.m() is valid if the method set
of (the type of) x contains m and the
argument list can be assigned to the parameter list of m.
If x is addressable and &x's method
set contains m, x.m() is shorthand
for (&x).m():
var p Point p.Scale(3.5)
There is no distinct method type and there are no method literals.
... parameters
If f is variadic with final parameter type ...T,
then within the function the argument is equivalent to a parameter of type
[]T. At each call of f, the argument
passed to the final parameter is
a new slice of type []T whose successive elements are
the actual arguments, which all must be assignable
to the type T. The length of the slice is therefore the number of
arguments bound to the final parameter and may differ for each call site.
Given the function and call
func Greeting(prefix string, who ...string)
Greeting("hello:", "Joe", "Anna", "Eileen")
within Greeting, who will have the value
[]string{"Joe", "Anna", "Eileen"}
If the final argument is assignable to a slice type []T, it may be
passed unchanged as the value for a ...T parameter if the argument
is followed by .... In this case no new slice is created.
Given the slice s and call
s := []string{"James", "Jasmine"}
Greeting("goodbye:", s...)
within Greeting, who will have the same value as s
with the same underlying array.
Operators combine operands into expressions.
Expression = UnaryExpr | Expression binary_op UnaryExpr . UnaryExpr = PrimaryExpr | unary_op UnaryExpr . binary_op = "||" | "&&" | rel_op | add_op | mul_op . rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" . add_op = "+" | "-" | "|" | "^" . mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" . unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
Comparisons are discussed elsewhere. For other binary operators, the operand types must be identical unless the operation involves shifts or untyped constants. For operations involving constants only, see the section on constant expressions.
Except for shift operations, if one operand is an untyped constant and the other operand is not, the constant is converted to the type of the other operand.
The right operand in a shift expression must have unsigned integer type
or be an untyped constant that can be converted to unsigned integer type.
If the left operand of a non-constant shift expression is an untyped constant,
the type of the constant is what it would be if the shift expression were
replaced by its left operand alone; the type is int if it cannot
be determined from the context (for instance, if the shift expression is an
operand in a comparison against an untyped constant).
var s uint = 33 var i = 1<<s // 1 has type int var j int32 = 1<<s // 1 has type int32; j == 0 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33 var m int = 1.0<<s // legal: 1.0 has type int var n = 1.0<<s != 0 // legal: 1.0 has type int; n == false if ints are 32bits in size var o = 1<<s == 2<<s // legal: 1 and 2 have type int; o == true if ints are 32bits in size var p = 1<<s == 1<<33 // illegal if ints are 32bits in size: 1 has type int, but 1<<33 overflows int var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift var v float32 = 1<<s // illegal: 1 has type float32, cannot shift var w int64 = 1.0<<33 // legal: 1.0<<33 is a constant shift expression
Unary operators have the highest precedence.
As the ++ and -- operators form
statements, not expressions, they fall
outside the operator hierarchy.
As a consequence, statement *p++ is the same as (*p)++.
There are five precedence levels for binary operators.
Multiplication operators bind strongest, followed by addition
operators, comparison operators, && (logical and),
and finally || (logical or):
Precedence Operator
5 * / % << >> & &^
4 + - | ^
3 == != < <= > >=
2 &&
1 ||
Binary operators of the same precedence associate from left to right.
For instance, x / y * z is the same as (x / y) * z.
+x 23 + 3*x[i] x <= f() ^a >> b f() || g() x == y+1 && <-chan_ptr > 0
Arithmetic operators apply to numeric values and yield a result of the same
type as the first operand. The four standard arithmetic operators (+,
-, *, /) apply to integer,
floating-point, and complex types; + also applies
to strings. All other arithmetic operators apply to integers only.
+ sum integers, floats, complex values, strings - difference integers, floats, complex values * product integers, floats, complex values / quotient integers, floats, complex values % remainder integers & bitwise and integers | bitwise or integers ^ bitwise xor integers &^ bit clear (and not) integers << left shift integer << unsigned integer >> right shift integer >> unsigned integer
Strings can be concatenated using the + operator
or the += assignment operator:
s := "hi" + string(c) s += " and good bye"
String addition creates a new string by concatenating the operands.
For two integer values x and y, the integer quotient
q = x / y and remainder r = x % y satisfy the following
relationships:
x = q*y + r and |r| < |y|
with x / y truncated towards zero
("truncated division").
x y x / y x % y 5 3 1 2 -5 3 -1 -2 5 -3 -1 2 -5 -3 1 -2
As an exception to this rule, if the dividend x is the most
negative value for the int type of x, the quotient
q = x / -1 is equal to x (and r = 0).
x, q int8 -128 int16 -32768 int32 -2147483648 int64 -9223372036854775808
If the divisor is zero, a run-time panic occurs. 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:
x x / 4 x % 4 x >> 2 x & 3 11 2 3 2 3 -11 -2 -3 -3 1
The shift operators shift the left operand by the shift count specified by the
right operand. They implement arithmetic shifts if the left operand is a signed
integer and logical shifts if it is an unsigned integer.
There is no upper limit on the shift count. Shifts behave
as if the left operand is shifted n times by 1 for a shift
count of n.
As a result, x << 1 is the same as x*2
and x >> 1 is the same as
x/2 but truncated towards negative infinity.
For integer operands, the unary operators
+, -, and ^ are defined as
follows:
+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
For floating-point numbers,
+x is the same as x,
while -x is the negation of x.
The result of a floating-point division by zero is not specified beyond the
IEEE-754 standard; whether a run-time panic
occurs is implementation-specific.
For unsigned integer values, the operations +,
-, *, and << are
computed modulo 2n, where n is the bit width of
the unsigned integer's type
(§Numeric types). Loosely speaking, these unsigned integer operations
discard high bits upon overflow, and programs may rely on ``wrap around''.
For signed integers, the operations +,
-, *, and << 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 x < x + 1 is always true.
Comparison operators compare two operands and yield a value of type bool.
== equal != not equal < less <= less or equal > greater >= greater or equal
In any comparison, the first operand must be assignable to the type of the second operand, or vice versa.
The equality operators == and != apply
to operands that are comparable.
The ordering operators <, <=, >, and >=
apply to operands that are ordered.
These terms and the result of the comparisons are defined as follows:
true or both false.
u and v are
equal if both real(u) == real(v) and
imag(u) == imag(v).
nil.
Pointers to distinct zero-size variables may or may not be equal.
make
(§Making slices, maps, and channels)
or if both have value nil.
nil.
x of non-interface type X and
a value t of interface type T are comparable when values
of type X are comparable and
X implements T.
They are equal if t's dynamic type is identical to X
and t's dynamic value is equal to x.
A comparison of two interface values with identical dynamic types causes a run-time panic if values of that type are not comparable. This behavior applies not only to direct interface value comparisons but also when comparing arrays of interface values or structs with interface-valued fields.
Slice, map, and function values are not comparable.
However, as a special case, a slice, map, or function value may
be compared to the predeclared identifier nil.
Comparison of pointer, channel, and interface values to nil
is also allowed and follows from the general rules above.
Logical operators apply to boolean values and yield a result of the same type as the operands. The right operand is evaluated conditionally.
&& conditional and p && q is "if p then q else false" || conditional or p || q is "if p then true else q" ! not !p is "not p"
For an operand x of type T, the address operation
&x generates a pointer of type *T to x.
The operand must be addressable,
that is, either a variable, pointer indirection, or slice indexing
operation; or a field selector of an addressable struct operand;
or an array indexing operation of an addressable array.
As an exception to the addressability requirement, x may also be a
composite literal.
For an operand x of pointer type *T, the pointer
indirection *x denotes the value of type T pointed
to by x.
If x is nil, an attempt to evaluate *x
will cause a run-time panic.
&x &a[f(2)] *p *pf(x)
For an operand ch of channel type,
the value of the receive operation <-ch is the value received
from the channel ch. The type of the value is the element type of
the channel. The expression blocks until a value is available.
Receiving from a nil channel blocks forever.
v1 := <-ch v2 = <-ch f(<-ch) <-strobe // wait until clock pulse and discard received value
A receive expression used in an assignment or initialization of the form
x, ok = <-ch x, ok := <-ch var x, ok = <-ch
yields an additional result.
The boolean variable ok indicates whether
the received value was sent on the channel (true)
or is a zero value returned
because the channel is closed and empty (false).
If M is in the method set of type T,
T.M is a function that is callable as a regular function
with the same arguments as M prefixed by an additional
argument that is the receiver of the method.
MethodExpr = ReceiverType "." MethodName .
ReceiverType = TypeName | "(" "*" TypeName ")" .
Consider a struct type T with two methods,
Mv, whose receiver is of type T, and
Mp, whose receiver is of type *T.
type T struct {
a int
}
func (tv T) Mv(a int) int { return 0 } // value receiver
func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
var t T
The expression
T.Mv
yields a function equivalent to Mv but
with an explicit receiver as its first argument; it has signature
func(tv T, a int) int
That function may be called normally with an explicit receiver, so these three invocations are equivalent:
t.Mv(7) T.Mv(t, 7) f := T.Mv; f(t, 7)
Similarly, the expression
(*T).Mp
yields a function value representing Mp with signature
func(tp *T, f float32) float32
For a method with a value receiver, one can derive a function with an explicit pointer receiver, so
(*T).Mv
yields a function value representing Mv with signature
func(tv *T, a int) int
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.
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.
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 f := T.Mv, f is invoked
as f(t, 7) not t.f(7).
To construct a function that binds the receiver, use a
closure.
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.
Conversions are expressions of the form T(x)
where T is a type and x is an expression
that can be converted to type T.
Conversion = Type "(" Expression ")" .
If the type starts with an operator it must be parenthesized:
*Point(p) // same as *(Point(p)) (*Point)(p) // p is converted to (*Point) <-chan int(c) // same as <-(chan int(c)) (<-chan int)(c) // c is converted to (<-chan int)
A constant value x can be converted to
type T in any of these cases:
x is representable by a value of type T.
x is an integer constant and T is a
string type.
The same rule as for non-constant x applies in this case
(§Conversions to and from a string type).
Converting a constant yields a typed constant as result.
uint(iota) // iota value of type uint
float32(2.718281828) // 2.718281828 of type float32
complex128(1) // 1.0 + 0.0i of type complex128
string('x') // "x" of type string
string(0x266c) // "♬" of type string
MyString("foo" + "bar") // "foobar" of type MyString
string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
(*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
int(1.2) // illegal: 1.2 cannot be represented as an int
string(65.0) // illegal: 65.0 is not an integer constant
A non-constant value x can be converted to type T
in any of these cases:
x is assignable
to T.
x's type and T have identical
underlying types.
x's type and T are unnamed pointer types
and their pointer base types have identical underlying types.
x's type and T are both integer or floating
point types.
x's type and T are both complex types.
x is an integer or has type []byte or
[]rune and T is a string type.
x is a string and T is []byte or
[]rune.
Specific rules apply to (non-constant) conversions between numeric types or
to and from a string type.
These conversions may change the representation of x
and incur a run-time cost.
All other conversions only change the type but not the representation
of x.
There is no linguistic mechanism to convert between pointers and integers.
The package unsafe
implements this functionality under
restricted circumstances.
For the conversion of non-constant numeric values, the following rules apply:
v := uint16(0x10F0), then uint32(int8(v)) == 0xFFFFFFF0.
The conversion always yields a valid value; there is no indication of overflow.
x of type float32
may be stored using additional precision beyond that of an IEEE-754 32-bit number,
but float32(x) represents the result of rounding x's value to
32-bit precision. Similarly, x + 0.1 may use more than 32 bits
of precision, but float32(x + 0.1) does not.
In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent.
"\uFFFD".
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"
nil, the result is the empty string.
string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
type MyBytes []byte
string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
nil, the
result is the empty string.
string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
type MyRunes []rune
string(MyRunes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
[]byte(nil).
[]byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
MyBytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
[]rune(nil).
[]rune(MyString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
MyRunes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
Constant expressions may contain only constant operands and are evaluated at compile-time.
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 different kinds of untyped constants, the operation and result use the kind that appears later in this list: integer, character, floating-point, complex. For example, an untyped integer constant divided by an untyped complex constant yields an untyped complex constant.
A constant comparison always yields an untyped boolean constant. If the left operand of a constant shift expression is an untyped constant, the result is an integer constant; otherwise it is a constant of the same type as the left operand, which must be of integer type (§Arithmetic operators). Applying all other operators to untyped constants results in an untyped constant of the same kind (that is, a boolean, integer, floating-point, complex, or string constant).
const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant) const b = 15 / 4 // b == 3 (untyped integer constant) const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant) const Θ float64 = 3/2 // Θ == 1.5 (type float64) const d = 1 << 3.0 // d == 8 (untyped integer constant) const e = 1.0 << 3 // e == 8 (untyped integer constant) const f = int32(1) << 33 // f == 0 (type int32) const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant) const h = "foo" > "bar" // h == true (untyped boolean constant) const j = true // j == true (untyped boolean constant) const k = 'w' + 1 // k == 'x' (untyped character constant) const l = "hi" // l == "hi" (untyped string constant) const m = string(k) // m == "x" (type string) const Σ = 1 - 0.707i // (untyped complex constant) const Δ = Σ + 2.0e-4 // (untyped complex constant) const Φ = iota*1i - 1/1i // (untyped complex constant)
Applying the built-in function complex to untyped
integer, character, or floating-point constants yields
an untyped complex constant.
const ic = complex(0, c) // ic == 3.75i (untyped complex constant) const iΘ = complex(0, Θ) // iΘ == 1.5i (type complex128)
Constant expressions are always evaluated exactly; intermediate values and the constants themselves may require precision significantly larger than supported by any predeclared type in the language. The following are legal declarations:
const Huge = 1 << 100 const Four int8 = Huge >> 98
The values of typed constants must always be accurately representable as values of the constant type. The following constant expressions are illegal:
uint(-1) // -1 cannot be represented as a uint int(3.14) // 3.14 cannot be represented as an int int64(Huge) // 1<<100 cannot be represented as an int64 Four * 300 // 300 cannot be represented as an int8 Four * 100 // 400 cannot be represented as an int8
The mask used by the unary bitwise complement operator ^ matches
the rule for non-constants: the mask is all 1s for unsigned constants
and -1 for signed and untyped constants.
^1 // untyped integer constant, equal to -2 uint8(^1) // error, same as uint8(-2), out of range ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE) int8(^1) // same as int8(-2) ^int8(1) // same as -1 ^ int8(1) = -2
When evaluating the elements of an assignment or expression, all function calls, method calls and communication operations are evaluated in lexical left-to-right order.
For example, in the assignment
y[f()], ok = g(h(), i()+x[j()], <-c), k()
the function calls and communication happen in the order
f(), h(), i(), j(),
<-c, g(), and k().
However, the order of those events compared to the evaluation
and indexing of x and the evaluation
of y is not specified.
Floating-point operations within a single expression are evaluated according to
the associativity of the operators. Explicit parentheses affect the evaluation
by overriding the default associativity.
In the expression x + (y + z) the addition y + z
is performed before adding x.
Statements control execution.
Statement = Declaration | LabeledStmt | SimpleStmt | GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt | FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt | DeferStmt . SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
The empty statement does nothing.
EmptyStmt = .
A labeled statement may be the target of a goto,
break or continue statement.
LabeledStmt = Label ":" Statement . Label = identifier .
Error: log.Panic("error encountered")
Function calls, method calls, and receive operations can appear in statement context. Such statements may be parenthesized.
ExpressionStmt = Expression .
h(x+y) f.Close() <-ch (<-ch)
A send statement sends a value on a channel. The channel expression must be of channel type and the type of the value must be assignable to the channel's element type.
SendStmt = Channel "<-" Expression . Channel = Expression .
Both the channel and the value expression are evaluated before communication
begins. Communication blocks until the send can proceed.
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.
A send on a closed channel proceeds by causing a run-time panic.
A send on a nil channel blocks forever.
ch <- 3
The "++" and "--" statements increment or decrement their operands
by the untyped constant 1.
As with an assignment, the operand must be addressable
or a map index expression.
IncDecStmt = Expression ( "++" | "--" ) .
The following assignment statements are semantically equivalent:
IncDec statement Assignment x++ x += 1 x-- x -= 1
Assignment = ExpressionList assign_op ExpressionList . assign_op = [ add_op | mul_op ] "=" .
Each left-hand side operand must be addressable, a map index expression, or the blank identifier. Operands may be parenthesized.
x = 1 *p = f() a[i] = 23 (k) = <-ch // same as: k = <-ch
An assignment operation x op=
y where op is a binary arithmetic operation is equivalent
to x = x op
y but evaluates x
only once. The op= construct is a single token.
In assignment operations, both the left- and right-hand expression lists
must contain exactly one single-valued expression.
a[i] <<= 2 i &^= 1<<n
A tuple assignment assigns the individual elements of a multi-valued
operation to a list of variables. There are two forms. In the
first, the right hand operand is a single multi-valued expression
such as a function evaluation or channel or
map operation or a type assertion.
The number of operands on the left
hand side must match the number of values. For instance, if
f is a function returning two values,
x, y = f()
assigns the first value to x and the second to y.
The blank identifier provides a
way to ignore values returned by a multi-valued expression:
x, _ = f() // ignore second value returned by f()
In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the nth expression on the right is assigned to the nth operand on the left. The assignment proceeds in two phases. First, the operands of index expressions and pointer indirections (including implicit pointer indirections in selectors) on the left and the expressions on the right are all evaluated in the usual order. Second, the assignments are carried out in left-to-right order.
a, b = b, a // exchange a and b
x := []int{1, 2, 3}
i := 0
i, x[i] = 1, 2 // set i = 1, x[0] = 2
i = 0
x[i], i = 2, 1 // set x[0] = 2, i = 1
x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] = 2 at end)
x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
type Point struct { x, y int }
var p *Point
x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
In assignments, each value must be
assignable to the type of the
operand to which it is assigned. If an untyped constant
is assigned to a variable of interface type, the constant is converted
to type bool, rune, int, float64,
complex128 or string
respectively, depending on whether the value is a boolean,
character, integer, floating-point, complex, or string constant.
"If" statements specify the conditional execution of two branches according to the value of a boolean expression. If the expression evaluates to true, the "if" branch is executed, otherwise, if present, the "else" branch is executed.
IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
if x > max {
x = max
}
The expression may be preceded by a simple statement, which executes before the expression is evaluated.
if x := f(); x < y {
return x
} else if x > z {
return z
} else {
return y
}
"Switch" statements provide multi-way execution. An expression or type specifier is compared to the "cases" inside the "switch" to determine which branch to execute.
SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
There are two forms: expression switches and type switches. In an expression switch, the cases contain expressions that are compared against the value of the switch expression. In a type switch, the cases contain types that are compared against the type of a specially annotated switch expression.
In an expression switch,
the switch expression is evaluated and
the case expressions, which need not be constants,
are evaluated left-to-right and top-to-bottom; the first one that equals the
switch expression
triggers execution of the statements of the associated case;
the other cases are skipped.
If no case matches and there is a "default" case,
its statements are executed.
There can be at most one default case and it may appear anywhere in the
"switch" statement.
A missing switch expression is equivalent to
the expression true.
ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
ExprCaseClause = ExprSwitchCase ":" { Statement ";" } .
ExprSwitchCase = "case" ExpressionList | "default" .
In a case or default clause, the last statement only may be a "fallthrough" statement (§Fallthrough statement) to indicate that control should flow from the end of this clause to the first statement of the next clause. Otherwise control flows to the end of the "switch" statement.
The expression may be preceded by a simple statement, which executes before the expression is evaluated.
switch tag {
default: s3()
case 0, 1, 2, 3: s1()
case 4, 5, 6, 7: s2()
}
switch x := f(); { // missing switch expression means "true"
case x < 0: return -x
default: return x
}
switch {
case x < y: f1()
case x < z: f2()
case x == 4: f3()
}
A type switch compares types rather than values. It is otherwise similar
to an expression switch. It is marked by a special switch expression that
has the form of a type assertion
using the reserved word type rather than an actual type.
Cases then match literal types against the dynamic type of the expression
in the type assertion.
TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
TypeCaseClause = TypeSwitchCase ":" { Statement ";" } .
TypeSwitchCase = "case" TypeList | "default" .
TypeList = Type { "," Type } .
The TypeSwitchGuard may include a short variable declaration. When that form is used, the variable is declared in each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard.
The type in a case may be nil
(§Predeclared identifiers);
that case is used when the expression in the TypeSwitchGuard
is a nil interface value.
Given an expression x of type interface{},
the following type switch:
switch i := x.(type) {
case nil:
printString("x is nil")
case int:
printInt(i) // i is an int
case float64:
printFloat64(i) // i is a float64
case func(int) float64:
printFunction(i) // i is a function
case bool, string:
printString("type is bool or string") // i is an interface{}
default:
printString("don't know the type")
}
could be rewritten:
v := x // x is evaluated exactly once
if v == nil {
printString("x is nil")
} else if i, is_int := v.(int); is_int {
printInt(i) // i is an int
} else if i, is_float64 := v.(float64); is_float64 {
printFloat64(i) // i is a float64
} else if i, is_func := v.(func(int) float64); is_func {
printFunction(i) // i is a function
} else {
i1, is_bool := v.(bool)
i2, is_string := v.(string)
if is_bool || is_string {
i := v
printString("type is bool or string") // i is an interface{}
} else {
i := v
printString("don't know the type") // i is an interface{}
}
}
The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated.
The "fallthrough" statement is not permitted in a type switch.
A "for" statement specifies repeated execution of a block. The iteration is controlled by a condition, a "for" clause, or a "range" clause.
ForStmt = "for" [ Condition | ForClause | RangeClause ] Block . Condition = Expression .
In its simplest form, a "for" statement specifies the repeated execution of
a block as long as a boolean condition evaluates to true.
The condition is evaluated before each iteration.
If the condition is absent, it is equivalent to true.
for a < b {
a *= 2
}
A "for" statement with a ForClause is also controlled by its condition, but additionally it may specify an init and a post statement, such as an assignment, an increment or decrement statement. The init statement may be a short variable declaration, but the post statement must not.
ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] . InitStmt = SimpleStmt . PostStmt = SimpleStmt .
for i := 0; i < 10; i++ {
f(i)
}
If non-empty, the init statement is executed once before evaluating the
condition for the first iteration;
the post statement is executed after each execution of the block (and
only if the block was executed).
Any element of the ForClause may be empty but the
semicolons are
required unless there is only a condition.
If the condition is absent, it is equivalent to true.
for cond { S() } is the same as for ; cond ; { S() }
for { S() } is the same as for true { S() }
A "for" statement with a "range" clause iterates through all entries of an array, slice, string or map, or values received on a channel. For each entry it assigns iteration values to corresponding iteration variables and then executes the block.
RangeClause = Expression [ "," Expression ] ( "=" | ":=" ) "range" Expression .
The expression on the right in the "range" clause is called the range expression, which may be an array, pointer to an array, slice, string, map, or channel. As with an assignment, the operands on the left must be addressable or map index expressions; they denote the iteration variables. If the range expression is a channel, only one iteration variable is permitted, otherwise there may be one or two. If the second iteration variable is the blank identifier, the range clause is equivalent to the same clause with only the first variable present.
The range expression is evaluated once before beginning the loop except if the expression is an array, in which case, depending on the expression, it might not be evaluated (see below). Function calls on the left are evaluated once per iteration. For each iteration, iteration values are produced as follows:
Range expression 1st value 2nd value (if 2nd variable is present) array or slice a [n]E, *[n]E, or []E index i int a[i] E string s string type index i int see below rune map m map[K]V key k K m[k] V channel c chan E element e E
a, the index iteration
values are produced in increasing order, starting at element index 0. As a special
case, if only the first iteration variable is present, the range loop produces
iteration values from 0 up to len(a) and does not index into the array
or slice itself. For a nil slice, the number of iterations is 0.
rune, will be the value of
the corresponding code point. If the iteration encounters an invalid
UTF-8 sequence, the second value will be 0xFFFD,
the Unicode replacement character, and the next iteration will advance
a single byte in the string.
nil, the number of iterations is 0.
nil, the range expression blocks forever.
The iteration values are assigned to the respective iteration variables as in an assignment statement.
The iteration variables may be declared by the "range" clause (:=).
In this case their types are set to the types of the respective iteration values
and their scope ends at the end of the "for"
statement; they are re-used in each iteration.
If the iteration variables are declared outside the "for" statement,
after execution their values will be those of the last iteration.
var testdata *struct {
a *[7]int
}
for i, _ := range testdata.a {
// testdata.a is never evaluated; len(testdata.a) is constant
// i ranges from 0 to 6
f(i)
}
var a [10]string
m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
for i, s := range a {
// type of i is int
// type of s is string
// s == a[i]
g(i, s)
}
var key string
var val interface {} // value type of m is assignable to val
for key, val = range m {
h(key, val)
}
// key == last map key encountered in iteration
// val == map[key]
var ch chan Work = producer()
for w := range ch {
doWork(w)
}
A "go" statement starts the execution of a function or method call as an independent concurrent thread of control, or goroutine, within the same address space.
GoStmt = "go" Expression .
The expression must be a call. The function value and parameters are evaluated as usual in the calling goroutine, but unlike with a regular call, program execution does not wait for the invoked function to complete. Instead, the function begins executing independently in a new goroutine. When the function terminates, its goroutine also terminates. If the function has any return values, they are discarded when the function completes.
go Server()
go func(ch chan<- bool) { for { sleep(10); ch <- true; }} (c)
A "select" statement chooses which of a set of possible communications will proceed. It looks similar to a "switch" statement but with the cases all referring to communication operations.
SelectStmt = "select" "{" { CommClause } "}" .
CommClause = CommCase ":" { Statement ";" } .
CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
RecvStmt = [ Expression [ "," Expression ] ( "=" | ":=" ) ] RecvExpr .
RecvExpr = Expression .
RecvExpr must be a receive operation.
For all the cases in the "select"
statement, the channel expressions are evaluated in top-to-bottom order, along with
any expressions that appear on the right hand side of send statements.
A channel may be nil,
which is equivalent to that case not
being present in the select statement
except, if a send, its expression is still evaluated.
If any of the resulting operations can proceed, one of those is
chosen and the corresponding communication and statements are
evaluated. Otherwise, if there is a default case, that executes;
if there is no default case, the statement blocks until one of the communications can
complete.
If there are no cases with non-nil channels,
the statement blocks forever.
Even if the statement blocks,
the channel and send expressions are evaluated only once,
upon entering the select statement.
Since all the channels and send expressions are evaluated, any side effects in that evaluation will occur for all the communications in the "select" statement.
If multiple cases can proceed, a uniform pseudo-random choice is made to decide which single communication will execute.
The receive case may declare one or two new variables using a short variable declaration.
var c, c1, c2, c3 chan int
var i1, i2 int
select {
case i1 = <-c1:
print("received ", i1, " from c1\n")
case c2 <- i2:
print("sent ", i2, " to c2\n")
case i3, ok := (<-c3): // same as: i3, ok := <-c3
if ok {
print("received ", i3, " from c3\n")
} else {
print("c3 is closed\n")
}
default:
print("no communication\n")
}
for { // send random sequence of bits to c
select {
case c <- 0: // note: no statement, no fallthrough, no folding of cases
case c <- 1:
}
}
select {} // block forever
A "return" statement terminates execution of the containing function and optionally provides a result value or values to the caller.
ReturnStmt = "return" [ ExpressionList ] .
In a function without a result type, a "return" statement must not specify any result values.
func no_result() {
return
}
There are three ways to return values from a function with a result type:
func simple_f() int {
return 2
}
func complex_f1() (re float64, im float64) {
return -7.0, -4.0
}
func complex_f2() (re float64, im float64) {
return complex_f1()
}
func complex_f3() (re float64, im float64) {
re = 7.0
im = 4.0
return
}
func (devnull) Write(p []byte) (n int, _ error) {
n = len(p)
return
}
Regardless of how they are declared, all the result values are initialized to the zero values for their type (§The zero value) upon entry to the function.
A "break" statement terminates execution of the innermost "for", "switch" or "select" statement.
BreakStmt = "break" [ Label ] .
If there is a label, it must be that of an enclosing "for", "switch" or "select" statement, and that is the one whose execution terminates (§For statements, §Switch statements, §Select statements).
L:
for i < n {
switch i {
case 5:
break L
}
}
A "continue" statement begins the next iteration of the innermost "for" loop at its post statement (§For statements).
ContinueStmt = "continue" [ Label ] .
If there is a label, it must be that of an enclosing "for" statement, and that is the one whose execution advances (§For statements).
A "goto" statement transfers control to the statement with the corresponding label.
GotoStmt = "goto" Label .
goto Error
Executing the "goto" statement must not cause any variables to come into scope that were not already in scope at the point of the goto. For instance, this example:
goto L // BAD v := 3 L:
is erroneous because the jump to label L skips
the creation of v.
A "goto" statement outside a block cannot jump to a label inside that block. For instance, this example:
if n%2 == 1 {
goto L1
}
for n > 0 {
f()
n--
L1:
f()
n--
}
is erroneous because the label L1 is inside
the "for" statement's block but the goto is not.
A "fallthrough" statement transfers control to the first statement of the next case clause in a expression "switch" statement (§Expression switches). It may be used only as the final non-empty statement in a case or default clause in an expression "switch" statement.
FallthroughStmt = "fallthrough" .
A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns.
DeferStmt = "defer" Expression .
The expression must be a function or method call. Each time the "defer" statement executes, the function value and parameters to the call are evaluated as usual and saved anew but the actual function is not invoked. Instead, deferred calls are executed in LIFO order immediately before the surrounding function returns, after the return values, if any, have been evaluated, but before they are returned to the caller. For instance, if the deferred function is a function literal and the surrounding function has named result parameters that are in scope within the literal, the deferred function may access and modify the result parameters before they are returned. If the deferred function has any return values, they are discarded when the function completes.
lock(l)
defer unlock(l) // unlocking happens before surrounding function returns
// prints 3 2 1 0 before surrounding function returns
for i := 0; i <= 3; i++ {
defer fmt.Print(i)
}
// f returns 1
func f() (result int) {
defer func() {
result++
}()
return 0
}
Built-in functions are predeclared. They are called like any other function but some of them accept a type instead of an expression as the first argument.
The built-in functions do not have standard Go types, so they can only appear in call expressions; they cannot be used as function values.
BuiltinCall = identifier "(" [ BuiltinArgs [ "," ] ] ")" .
BuiltinArgs = Type [ "," ExpressionList ] | ExpressionList .
For a channel c, the built-in function close(c)
records that no more values will be sent on the channel.
It is an error if c is a receive-only channel.
Sending to or closing a closed channel causes a run-time panic.
Closing the nil channel also causes a run-time panic.
After calling close, and after any previously
sent values have been received, receive operations will return
the zero value for the channel's type without blocking.
The multi-valued receive operation
returns a received value along with an indication of whether the channel is closed.
The built-in functions len and cap take arguments
of various types and return a result of type int.
The implementation guarantees that the result always fits into an int.
Call Argument type Result
len(s) string type string length in bytes
[n]T, *[n]T array length (== n)
[]T slice length
map[K]T map length (number of defined keys)
chan T number of elements queued in channel buffer
cap(s) [n]T, *[n]T array length (== n)
[]T slice capacity
chan T channel buffer capacity
The capacity of a slice is the number of elements for which there is space allocated in the underlying array. At any time the following relationship holds:
0 <= len(s) <= cap(s)
The length and capacity of a nil slice, map, or channel are 0.
The expression len(s) is constant if
s is a string constant. The expressions len(s) and
cap(s) are constants if the type of s is an array
or pointer to an array and the expression s does not contain
channel receives or
function calls; in this case s is not evaluated.
Otherwise, invocations of len and cap are not
constant and s is evaluated.
The built-in function new takes a type T and
returns a value of type *T.
The memory is initialized as described in the section on initial values
(§The zero value).
new(T)
For instance
type S struct { a int; b float64 }
new(S)
dynamically allocates memory for a variable of type S,
initializes it (a=0, b=0.0),
and returns a value of type *S containing the address
of the memory.
Slices, maps and channels are reference types that do not require the
extra indirection of an allocation with new.
The built-in function make takes a type T,
which must be a slice, map or channel type,
optionally followed by a type-specific list of expressions.
It returns a value of type T (not *T).
The memory is initialized as described in the section on initial values
(§The zero value).
Call Type T Result make(T, n) slice slice of type T with length n and capacity n make(T, n, m) slice slice of type T with length n and capacity m make(T) map map of type T make(T, n) map map of type T with initial space for n elements make(T) channel synchronous channel of type T make(T, n) channel asynchronous channel of type T, buffer size n
The arguments n and m must be of integer type.
A run-time panic occurs if n
is negative or larger than m, or if n or
m cannot be represented by an int.
s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100 s := make([]int, 10) // slice with len(s) == cap(s) == 10 c := make(chan int, 10) // channel with a buffer size of 10 m := make(map[string]int, 100) // map with initial space for 100 elements
Two built-in functions assist in common slice operations.
The variadic function append
appends zero or more values x
to s of type S, which must be a slice type, and
returns the resulting slice, also of type S.
The values x are passed to a parameter of type ...T
where T is the element type of
S and the respective
parameter passing rules apply.
As a special case, append also accepts a first argument
assignable to type []byte with a second argument of
string type followed by .... This form appends the
bytes of the string.
append(s S, x ...T) S // T is the element type of S
If the capacity of s is not large enough to fit the additional
values, append allocates a new, sufficiently large slice that fits
both the existing slice elements and the additional values. Thus, the returned
slice may refer to a different underlying array.
s0 := []int{0, 0}
s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
var t []interface{}
t = append(t, 42, 3.1415, "foo") t == []interface{}{42, 3.1415, "foo"}
var b []byte
b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
The function copy copies slice elements from
a source src to a destination dst and returns the
number of elements copied. Source and destination may overlap.
Both arguments must have identical element type T and must be
assignable to a slice of type []T.
The number of elements copied is the minimum of
len(src) and len(dst).
As a special case, copy also accepts a destination argument assignable
to type []byte with a source argument of a string type.
This form copies the bytes from the string into the byte slice.
copy(dst, src []T) int copy(dst []byte, src string) int
Examples:
var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
var s = make([]int, 6)
var b = make([]byte, 5)
n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
The built-in function delete removes the element with key
k from a map m. The
type of k must be assignable
to the key type of m.
delete(m, k) // remove element m[k] from map m
If the element m[k] does not exist, delete is
a no-op. Calling delete with a nil map causes a
run-time panic.
Three functions assemble and disassemble complex numbers.
The built-in function complex constructs a complex
value from a floating-point real and imaginary part, while
real and imag
extract the real and imaginary parts of a complex value.
complex(realPart, imaginaryPart floatT) complexT real(complexT) floatT imag(complexT) floatT
The type of the arguments and return value correspond.
For complex, the two arguments must be of the same
floating-point type and the return type is the complex type
with the corresponding floating-point constituents:
complex64 for float32,
complex128 for float64.
The real and imag functions
together form the inverse, so for a complex value z,
z == complex(real(z), imag(z)).
If the operands of these functions are all constants, the return value is a constant.
var a = complex(2, -2) // complex128 var b = complex(1.0, -1.4) // complex128 x := float32(math.Cos(math.Pi/2)) // float32 var c64 = complex(5, -x) // complex64 var im = imag(b) // float64 var rl = real(c64) // float32
Two built-in functions, panic and recover,
assist in reporting and handling run-time panics
and program-defined error conditions.
func panic(interface{})
func recover() interface{}
When a function F calls panic, normal
execution of F stops immediately. Any functions whose
execution was deferred by the
invocation of F are run in the usual way, and then
F returns to its caller. To the caller, F
then behaves like a call to panic, terminating its own
execution and running deferred functions. This continues until all
functions in the goroutine have ceased execution, in reverse order.
At that point, the program is
terminated and the error condition is reported, including the value of
the argument to panic. This termination sequence is
called panicking.
panic(42)
panic("unreachable")
panic(Error("cannot parse"))
The recover function allows a program to manage behavior
of a panicking goroutine. Executing a recover call
inside a deferred function (but not any function called by it) stops
the panicking sequence by restoring normal execution, and retrieves
the error value passed to the call of panic. If
recover is called outside the deferred function it will
not stop a panicking sequence. In this case, or when the goroutine
is not panicking, or if the argument supplied to panic
was nil, recover returns nil.
The protect function in the example below invokes
the function argument g and protects callers from
run-time panics raised by g.
func protect(g func()) {
defer func() {
log.Println("done") // Println executes normally even if there is a panic
if x := recover(); x != nil {
log.Printf("run time panic: %v", x)
}
}()
log.Println("start")
g()
}
Current implementations provide several built-in functions useful during bootstrapping. These functions are documented for completeness but are not guaranteed to stay in the language. They do not return a result.
Function Behavior print prints all arguments; formatting of arguments is implementation-specific println like print but prints spaces between arguments and a newline at the end
Go programs are constructed by linking together packages. A package in turn is constructed from one or more source files that together declare constants, types, variables and functions belonging to the package and which are accessible in all files of the same package. Those elements may be exported and used in another package.
Each source file consists of a package clause defining the package to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations of functions, types, variables, and constants.
SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
A package clause begins each source file and defines the package to which the file belongs.
PackageClause = "package" PackageName . PackageName = identifier .
The PackageName must not be the blank identifier.
package math
A set of files sharing the same PackageName form the implementation of a package. An implementation may require that all source files for a package inhabit the same directory.
An import declaration states that the source file containing the declaration uses identifiers exported by the imported package and enables access to them. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported.
ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
ImportSpec = [ "." | PackageName ] ImportPath .
ImportPath = string_lit .
The PackageName is used in qualified identifiers
to access the exported identifiers of the package within the importing source file.
It is declared in the file block.
If the PackageName is omitted, it defaults to the identifier specified in the
package clause of the imported package.
If an explicit period (.) appears instead of a name, all the
package's exported identifiers will be declared in the current file's
file block and can be accessed without a qualifier.
The interpretation of the ImportPath is implementation-dependent but it is typically a substring of the full file name of the compiled package and may be relative to a repository of installed packages.
Assume we have compiled a package containing the package clause
package math, which exports function Sin, and
installed the compiled package in the file identified by
"lib/math".
This table illustrates how Sin may be accessed in files
that import the package after the
various types of import declaration.
Import declaration Local name of Sin import "lib/math" math.Sin import M "lib/math" M.Sin import . "lib/math" Sin
An import declaration declares a dependency relation between the importing and imported package. It is illegal for a package to import itself or to import a package without referring to any of its exported identifiers. To import a package solely for its side-effects (initialization), use the blank identifier as explicit package name:
import _ "lib/math"
Here is a complete Go package that implements a concurrent prime sieve.
package main
import "fmt"
// Send the sequence 2, 3, 4, … to channel 'ch'.
func generate(ch chan<- int) {
for i := 2; ; i++ {
ch <- i // Send 'i' to channel 'ch'.
}
}
// Copy the values from channel 'src' to channel 'dst',
// removing those divisible by 'prime'.
func filter(src <-chan int, dst chan<- int, prime int) {
for i := range src { // Loop over values received from 'src'.
if i%prime != 0 {
dst <- i // Send 'i' to channel 'dst'.
}
}
}
// The prime sieve: Daisy-chain filter processes together.
func sieve() {
ch := make(chan int) // Create a new channel.
go generate(ch) // Start generate() as a subprocess.
for {
prime := <-ch
fmt.Print(prime, "\n")
ch1 := make(chan int)
go filter(ch, ch1, prime)
ch = ch1
}
}
func main() {
sieve()
}
When memory is allocated to store a value, either through a declaration
or a call of make or new,
and no explicit initialization is provided, the memory is
given a default initialization. Each element of such a value is
set to the zero value for its type: false for booleans,
0 for integers, 0.0 for floats, ""
for strings, and nil for pointers, functions, interfaces, slices, channels, and maps.
This initialization is done recursively, so for instance each element of an
array of structs will have its fields zeroed if no value is specified.
These two simple declarations are equivalent:
var i int var i int = 0
After
type T struct { i int; f float64; next *T }
t := new(T)
the following holds:
t.i == 0 t.f == 0.0 t.next == nil
The same would also be true after
var t T
A package with no imports is initialized by assigning initial values to all its package-level variables and then calling any package-level function with the name and signature of
func init()
defined in its source.
A package may contain multiple
init functions, even
within a single source file; they execute
in unspecified order.
Within a package, package-level variables are initialized,
and constant values are determined, in
data-dependent order: if the initializer of A
depends on the value of B, A
will be set after B.
It is an error if such dependencies form a cycle.
Dependency analysis is done lexically: A
depends on B if the value of A
contains a mention of B, contains a value
whose initializer
mentions B, or mentions a function that
mentions B, recursively.
If two items are not interdependent, they will be initialized
in the order they appear in the source.
Since the dependency analysis is done per package, it can produce
unspecified results if A's initializer calls a function defined
in another package that refers to B.
An init function cannot be referred to from anywhere
in a program. In particular, init cannot be called explicitly,
nor can a pointer to init be assigned to a function variable.
If a package has imports, the imported packages are initialized
before initializing the package itself. If multiple packages import
a package P, P will be initialized only once.
The importing of packages, by construction, guarantees that there can be no cyclic dependencies in initialization.
A complete program is created by linking a single, unimported package
called the main package with all the packages it imports, transitively.
The main package must
have package name main and
declare a function main that takes no
arguments and returns no value.
func main() { … }
Program execution begins by initializing the main package and then
invoking the function main.
When the function main returns, the program exits.
It does not wait for other (non-main) goroutines to complete.
Package initialization—variable initialization and the invocation of
init functions—happens in a single goroutine,
sequentially, one package at a time.
An init function may launch other goroutines, which can run
concurrently with the initialization code. However, initialization
always sequences
the init functions: it will not start the next
init until
the previous one has returned.
The predeclared type error is defined as
type error interface {
Error() string
}
It is the conventional interface for representing an error condition, with the nil value representing no error. For instance, a function to read data from a file might be defined:
func Read(f *File, b []byte) (n int, err error)
Execution errors such as attempting to index an array out
of bounds trigger a run-time panic equivalent to a call of
the built-in function panic
with a value of the implementation-defined interface type runtime.Error.
That type satisfies the predeclared interface type
error.
The exact error values that
represent distinct run-time error conditions are unspecified.
package runtime
type Error interface {
error
// and perhaps other methods
}
unsafe
The built-in package unsafe, known to the compiler,
provides facilities for low-level programming including operations
that violate the type system. A package using unsafe
must be vetted manually for type safety. The package provides the
following interface:
package unsafe
type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
type Pointer *ArbitraryType
func Alignof(variable ArbitraryType) uintptr
func Offsetof(selector ArbitraryType) uinptr
func Sizeof(variable ArbitraryType) uintptr
func Reflect(val interface{}) (typ runtime.Type, addr uintptr)
func Typeof(val interface{}) (typ interface{})
func Unreflect(typ runtime.Type, addr uintptr) interface{}
Any pointer or value of underlying type uintptr can be converted into
a Pointer and vice versa.
The function Sizeof takes an expression denoting a
variable of any type and returns the size of the variable in bytes.
The function Offsetof takes a selector (§Selectors) denoting a struct
field of any type and returns the field offset in bytes relative to the
struct's address.
For a struct s with field f:
uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
Computer architectures may require memory addresses to be aligned;
that is, for addresses of a variable to be a multiple of a factor,
the variable's type's alignment. The function Alignof
takes an expression denoting a variable of any type and returns the
alignment of the (type of the) variable in bytes. For a variable
x:
uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
Calls to Alignof, Offsetof, and
Sizeof are compile-time constant expressions of type uintptr.
The functions unsafe.Typeof,
unsafe.Reflect,
and unsafe.Unreflect allow access at run time to the dynamic
types and values stored in interfaces.
Typeof returns a representation of
val's
dynamic type as a runtime.Type.
Reflect allocates a copy of
val's dynamic
value and returns both the type and the address of the copy.
Unreflect inverts Reflect,
creating an
interface value from a type and address.
The reflect package built on these primitives
provides a safe, more convenient way to inspect interface values.
For the numeric types (§Numeric types), the following sizes are guaranteed:
type size in bytes byte, uint8, int8 1 uint16, int16 2 uint32, int32, float32 4 uint64, int64, float64, complex64 8 complex128 16
The following minimal alignment properties are guaranteed:
x of any type: unsafe.Alignof(x) is at least 1.
x of struct type: unsafe.Alignof(x) is the largest of
all the values unsafe.Alignof(x.f) for each field f of x, but at least 1.
x of array type: unsafe.Alignof(x) is the same as
unsafe.Alignof(x[0]), but at least 1.
A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.
len(x) is only a constant if x is a (qualified) identifier denoting an array or pointer to an array.