This is a reference manual for the Go programming language. For more information and other documents, see the Go home page.
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 "" or back quotes ``.
The form a ... b represents the set of characters from
a through b as alternatives.
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.
The following terms are used to denote specific Unicode character classes:
unicode_char = /* an arbitrary Unicode code point */ . unicode_letter = /* a Unicode code point classified as "Letter" */ . unicode_digit = /* a Unicode code point classified as "Digit" */ .(The Unicode Standard, Section 4.5 General Category - Normative.)
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. The first starts at the character
sequence // and continues through the next newline. The
second starts at the character sequence /* and continues
through the character sequence */. 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 blanks, tabs, and newlines, is ignored except as it separates tokens that would otherwise combine into a single token. Comments behave as white space. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.
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 αβSome identifiers are predeclared (§Predeclared identifiers).
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 one or more digits in the
corresponding base, which may be 8, 10, or 16. 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
number. 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. 2.71828 1.e+0 6.67428e-11 1E6 .25 .12345E+5
Integer literals represent values of arbitrary precision, or ideal integers. Similarly, floating-point literals represent values of arbitrary precision, or ideal floats. These ideal numbers have no size or named type and cannot overflow. However, when (used in an expression) assigned to a variable or typed constant, the destination must be able to represent the assigned value.
Implementation restriction: A compiler may implement ideal numbers by choosing an internal representation with at least twice the precision of any machine type.
A character literal represents an integer value, 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 `Unicode' 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 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'
The value of a character literal is an ideal integer, just as with integer literals.
String literals represent ideal string values. Ideal strings don't
have a named type but they are compatible with type string
(§Type identity and compatibility).
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
span multiple lines.
Interpreted string literals are character sequences between double
quotes "". The text between the quotes 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 (\000)
and two-digit hexadecimal (\x00) 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.
A sequence of string literals is concatenated to form a single string.
StringLit = string_lit { string_lit } .
string_lit = raw_string_lit | interpreted_string_lit .
raw_string_lit = "`" { unicode_char } "`" .
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" "Alea iacta est." "Alea " /* The die */ `iacta est` /* is cast */ "." // same as "Alea iacta est."
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.
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 .
Basic types such as int are predeclared (§Predeclared identifiers).
Other types may be constructed from these, recursively,
including arrays, structs, pointers, functions, interfaces, slices, maps, and
channels.
At any point in the source code, a type may be complete or incomplete. An incomplete type is one whose size is not yet known, such as a struct whose fields are not yet fully defined or a forward declared type (§Forward declarations). Most types are always complete; for instance, a pointer type is always complete even if it points to an incomplete type because the size of the pointer itself is always known. (TODO: Need to figure out how forward declarations of interface fit in here.)
A type may have a method set associated with it
(§Interface types, §Method declarations).
The method set of an interface type (§Interface types) 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.
The static type (or just type) of a variable is the type defined by its declaration. Variables of interface type (§Interface types) 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 compatible with the static type of the interface variable. For non-interface types, the dynamic type is always the static type.
Basic types include traditional numeric types, booleans, and strings. All are predeclared.
The 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 valid IEEE-754 32-bit floating point numbers float64 the set of all valid IEEE-754 64-bit floating point numbers byte familiar alias for uint8
Integer types are represented in the usual binary format; the value of an n-bit integer is n bits wide. A negative signed integer is represented as the two's complement of its absolute value.
There is also a set of numeric types with implementation-specific sizes:
uint either 32 or 64 bits int either 32 or 64 bits float either 32 or 64 bits uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
To avoid portability issues all numeric types are distinct except
byte, which is an alias for uint8.
Conversions
are required when incompatible 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.
bool comprises the Boolean truth values
represented by the predeclared constants true
and false.
The 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 elements of strings have type byte and may be
accessed using the usual indexing operations (§Indexes). It is
illegal to take the address of such an element, that is, even 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(s). It 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, which must be complete (§Types). The number of elements is called the length and is never negative.
ArrayType = "[" ArrayLength "]" ElementType . ArrayLength = Expression . ElementType = CompleteType .
The length is part of the array's type and must must be a constant
expression (§Constant expressions) that evaluates to a non-negative
integer value. The length of array a can be discovered
using the built-in function len(a), which is a
compile-time constant. The elements can be indexed by integer
indices 0 through the len(a)-1 (§Indexes).
[32]byte
[2*N] struct { x, y int32 }
[1000]*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.
A slice value may be 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 therfore 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) and the relationship between
len() and cap() is:
0 <= len(a) <= cap(a)
The value of an uninitialized slice is nil.
The length and capacity of a nil slice
are 0. 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)
The make() call allocates a new, hidden array to which the returned
slice value refers. That is, calling make
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]
A struct is a sequence of named elements, called fields, with various types. A struct type declares an identifier and type for each field. Within a struct, field identifiers must be unique and field types must be complete (§Types).
StructType = "struct" "{" [ FieldDeclList ] "}" .
FieldDeclList = FieldDecl { ";" FieldDecl } [ ";" ] .
FieldDecl = (IdentifierList CompleteType | [ "*" ] TypeName) [ Tag ] .
Tag = StringLit .
// An empty struct.
struct {}
// A struct with 5 fields.
struct {
x, y int;
u float;
A *[]int;
F func();
}
A field declared with a type but no field identifier is an anonymous field.
Such a field type must be specified as
a type name T or as a pointer to a type name *T,
and T itself, may not be
a pointer or interface type. The unqualified type name acts as the field identifier.
// A struct with four anonymous fields of type T1, *T2, P.T3 and *P.T4
struct {
T1; // the field name is T1
*T2; // the field name is T2
P.T3; // the field name is T3
*P.T4; // the field name is T4
x, y int;
}
The unqualified type name of an anonymous field must not conflict with the field identifier (or unqualified type name for an anonymous field) of any other field within the struct. The following declaration is illegal:
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 identifiers in the corresponding field declaration. The tags are made visible through a reflection library (TODO: reference?) but are otherwise ignored.
// A struct corresponding to the EventIdMessage protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
time_usec uint64 "field 1";
server_ip uint32 "field 2";
process_id uint32 "field 3";
}
A pointer type denotes the set of all pointers to variables of a given
type, called the base type of the pointer.
A pointer value may be 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.
A function value may be nil.
FunctionType = "func" Signature .
Signature = Parameters [ Result ] .
Result = Parameters | CompleteType .
Parameters = "(" [ ParameterList ] ")" .
ParameterList = ParameterDecl { "," ParameterDecl } .
ParameterDecl = [ IdentifierList ] ( CompleteType | "..." ) .
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 that is not a function type it may writen as an unparenthesized type. The types of parameters and results must be complete. (TODO: is completeness necessary?)
For the last parameter only, instead of a type one may write
... to indicate that the function may be invoked with
zero or more additional arguments of any
type. If parameters of such a function are named, the final identifier
list must be a single name, that of the ... parameter.
func () func (x int) func () int func (string, float, ...) func (a, b int, z float) bool func (a, b int, z float) (bool) func (a, b int, z float, opt ...) (success bool) func (int, int, float) (float, *[]int) func (n int) (func (p* T))
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. An interface value may be nil.
InterfaceType = "interface" "{" [ MethodSpecList ] "}" .
MethodSpecList = MethodSpec { ";" MethodSpec } [ ";" ] .
MethodSpec = IdentifierList Signature | InterfaceTypeName .
InterfaceTypeName = TypeName .
// A simple File interface
interface {
Read, 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 (§Type declarations)
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 contain an interface type name T
in place of a method specification.
In this notation, T must denote a different, complete interface type
and the effect is equivalent to enumerating the methods of T explicitly
in the interface.
type ReadWrite interface {
Read, Write (b Buffer) bool;
}
type File interface {
ReadWrite; // same as enumerating the methods in ReadWrite
Lock; // same as enumerating the methods in Lock
Close();
}
A map is an unordered group of elements of one type, called the
value type, indexed by a set of unique keys of another type,
called the key type. Both key and value types must be complete.
(§Types).
(TODO: is completeness necessary here?)
A map value may be nil.
MapType = "map" "[" KeyType "]" ValueType . KeyType = CompleteType . ValueType = CompleteType .
The comparison operators == and !=
(§Comparison operators) must be fully defined for operands of the
key type; thus the key type must be a basic, pointer, interface,
map, or channel type. If the key type is an interface type, these
comparison operators must be defined for the dynamic key values;
failure will cause a run-time error.
map [string] int
map [*T] struct { x, y float }
map [string] interface {}
The number of elements is called the length and is never negative.
The length of a map m can be discovered using the
built-in function len(m) and may change during execution.
The value of an uninitialized map is nil.
Upon creation, a map is empty. Values may be added and removed
during execution using special forms of assignment (§Assignments).
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.
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 element type must be complete (§Types).
(TODO: is completeness necessary here?)
A value of channel type may be nil.
ChannelType = Channel | SendChannel | RecvChannel . Channel = "chan" ValueType . SendChannel = "chan" "<-" ValueType . RecvChannel = "<-" "chan" ValueType .
Upon creation, a channel can be used both to send and to receive values. By conversion or assignment, a channel may be constrained only to send or to receive. This constraint is called a channel's direction; either send, receive, or bi-directional (unconstrained).
chan T // can be used to send and receive values of type T chan <- float // can only be used to send floats <-chan int // can only be used to receive ints
The value of an uninitialized channel is nil. A new, initialized channel
value is 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 and, provided the buffer is not full, sends can succeed without blocking. If the capacity is zero or absent, the communication succeeds only when both a sender and receiver are ready.
For a channel c, the predefined function close(c)
marks the channel as unable to accept more
values through a send operation. After any previously
sent values have been received, receives will return
the zero value for the channel's type. After at least one such zero value has been
received, closed(c) returns true.
Two types may be identical, compatible, or incompatible. Two identical types are always compatible, but two compatible types may not be identical. Go is type safe: a value of one type cannot be assigned to a variable of an incompatible type, and two values of incompatible types cannot be mixed in binary operations.
Two named types are identical if their type names originate in the same type declaration (§Declarations and Scope). A named and an unnamed type are never identical. Two unnamed types are identical if the corresponding type literals have the same literal structure and corresponding components have identical types. In detail:
Type compatibility is less stringent than type identity: a named and an unnamed type are compatible if the respective type literals are compatible. In all other respects, the definition of type compatibility is the same as for type identity listed above but with ``compatible'' substituted for ``identical''.
Given the declarations
type (
T0 []string;
T1 []string
T2 struct { a, b int };
T3 struct { a, c int };
T4 func (int, float) *T0
T5 func (x int, y float) *[]string
)
these types are identical:
T0 and T0
[]int and []int
struct { a, b *T5 } and struct { a, b *T5 }
func (x int, y float) *[]string and func (int, float) (result *[]string)
T0 and T1 are neither identical nor compatible
because they are named types with distinct declarations.
These types are compatible:
T0 and T0
T0 and []string
T3 and struct { a int; c int }
T4 and func (x int, y float) *[]string
T2 and struct { a, c int } are incompatible because
they have different field names.
Values of any type may always be assigned to variables of compatible static type. Some types and values have conditions under which they may be assigned to otherwise incompatible types:
nil can be assigned to any
pointer, function, slice, map, channel, or interface variable.
p to an array can be assigned to a slice variable
v with compatible element type
if the type of p or v is unnamed.
The slice variable then refers to the original array; the data is not copied.
c can be assigned to a channel variable
v with compatible channel value type
if the type of c or v is unnamed.
Values of any type may be compared to other values of compatible static type. Values of numeric and string type may be compared using the full range of comparison operators as described in §Comparison operators; booleans may be compared only for equality or inequality.
Values of composite type may be
compared for equality or inequality using the == and
!= operators, with the following provisos:
nil.
A slice value is equal to nil if it has been assigned the explicit
value nil or if it is a variable (or array element,
field, etc.) that has not been modified since it was created
uninitialized.
nil if it has
been assigned the explicit value nil or if it is a
variable (or array element, field, etc.) that has not been modified
since it was created uninitialized.
nil,
two values of the same type are equal if they both equal nil,
unequal if one equals nil and one does not.
make
(§Making slices, maps, and channels).
A declaration binds an identifier to a language entity such as a variable or function and specifies properties such as its type. Every identifier in a program must be declared.
Declaration = ConstDecl | TypeDecl | VarDecl | FunctionDecl | MethodDecl .
The scope of an identifier is the extent of source text within which the identifier denotes the bound entity. No identifier may be declared twice in a single scope, but inner blocks can declare a new entity with the same identifier, in which case the scope created by the outer declaration excludes that created by the inner.
There are levels of scoping in effect before each source file is compiled. In order from outermost to innermost:
The scope of an identifier depends on the entity declared:
if, for,
or switch statement, the
innermost surrounding block is the block associated
with that statement.The following identifiers are implicitly declared in the outermost scope:
Basic types: bool byte float32 float64 int8 int16 int32 int64 string uint8 uint16 uint32 uint64 Architecture-specific convenience types: float int uint uintptr Constants: true false iota nil Functions: cap len make new panic panicln print println Packages: unsafe
By default, identifiers are visible only within the package in which they are declared. Some identifiers are exported and can be referenced using qualified identifiers in other packages (§Qualified identifiers). If an identifier satisfies these two conditions:
it will be exported.
A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of constant expressions (§Constant expressions). The number of identifiers must be equal to the number of expressions, and the nth identifier on the left is bound to value of the nth expression on the right.
ConstDecl = "const" ( ConstSpec | "(" [ ConstSpecList ] ")" ) .
ConstSpecList = ConstSpec { ";" ConstSpec } [ ";" ] .
ConstSpec = IdentifierList [ [ CompleteType ] "=" ExpressionList ] .
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
CompleteType = Type .
If the type (CompleteType) is omitted, the constants take the individual types of the corresponding expressions, which may be ideal integer or ideal float (§Ideal number). If the type is present, all constants take the type specified, and the types of all the expressions must be assignment-compatible with that type.
const Pi float64 = 3.14159265358979323846 const E = 2.718281828 const ( size int64 = 1024; eof = -1; ) const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo" const u, v float = 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
(§Iota) 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 pseudo-constant
iota represents successive integers. It is reset to 0
whenever the reserved word const appears in the source
and increments with each semicolon. 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 (ideal integer) v float = iota * 42; // v == 42.0 (float) w = iota * 42; // w == 84 (ideal integer) ) 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 at a semicolon:
const ( bit0, mask0 = 1 << iota, 1 << iota - 1; // bit0 == 1, mask0 == 0 bit1, mask1; // bit1 == 2, mask1 == 1 bit2, mask2; // bit2 == 4, mask2 == 3 )
This last example exploits the implicit repetition of the last non-empty expression list.
A type declaration binds an identifier, the type name, to a new type. TODO: what exactly is a "new type"?
TypeDecl = "type" ( TypeSpec | "(" [ TypeSpecList ] ")" ) .
TypeSpecList = TypeSpec { ";" TypeSpec } [ ";" ] .
TypeSpec = identifier ( Type | "struct" | "interface" ) .
type IntArray [16] int
type (
Point struct { x, y float };
Polar Point
)
type Comparable interface
type TreeNode struct {
left, right *TreeNode;
value *Comparable;
}
type Comparable interface {
cmp(Comparable) int
}
A variable declaration creates a variable, binds an identifier to it and gives it a type and optionally an initial value. The type must be complete (§Types).
VarDecl = "var" ( VarSpec | "(" [ VarSpecList ] ")" ) .
VarSpecList = VarSpec { ";" VarSpec } [ ";" ] .
VarSpec = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) .
var i int var U, V, W float var k = 0 var x, y float = -1.0, -2.0 var ( i int; u, v, s = 2.0, 3.0, "bar" )
If there are expressions, their number must be equal to the number of identifiers, and the nth variable is initialized to the value of the nth expression. Otherwise, each variable is initialized to the zero of the type (§The zero value). The expressions can be general expressions; they need not be constants.
Either the type or the expression list must be present. If the type is present, it sets the type of each variable and the expressions (if any) must be assignment-compatible to that type. If the type is absent, the variables take the types of the corresponding expressions.
If the type is absent and the corresponding expression is a constant
expression of ideal integer or ideal float type, the type of the
declared variable is int or float
respectively:
var i = 0 // i has type int var f = 3.1415 // f has type float
SimpleVarDecl = IdentifierList ":=" ExpressionList .and is shorthand for the declaration syntax
"var" IdentifierList = ExpressionList .
i, j := 0, 10;
f := func() int { return 7; }
ch := make(chan int);
Unlike regular variable declarations, short variable declarations can be used, by analogy with tuple assignment (§Assignments), to receive the individual elements of a multi-valued expression such as a call to a multi-valued function. In this form, the ExpressionList must be a single such multi-valued expression, the number of identifiers must equal the number of values, and the declared variables will be assigned the corresponding values.
r, w := os.Pipe(fd); // os.Pipe() returns two values
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 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 [ Block ] .
func min(x int, y int) int {
if x < y {
return x;
}
return y;
}
A function must be declared or forward-declared before it can be invoked (§Forward declarations). Implementation restriction: Functions can only be declared at the package level.
A method declaration binds an identifier to a method, which is a function with a receiver.
MethodDecl = "func" Receiver identifier Signature [ Block ] .
Receiver = "(" [ identifier ] [ "*" ] TypeName ")" .
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 source file 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() float {
return Math.sqrt(p.x * p.x + p.y * p.y);
}
func (p *Point) Scale(factor float) {
p.x = p.x * factor;
p.y = p.y * factor;
}
bind the methods Length and Scale
to the base type Point.
If the receiver's value is not referenced inside the the body of the method, its identifier may be omitted in the declaration. The same applies in general to parameters of functions and methods.
Methods can be declared only after their base type is declared or forward-declared, and invoked only after their own declaration or forward-declaration (§Forward declarations). Implementation restriction: They can only be declared at package level.
The type of a method is the type of a function with the receiver as first
argument. For instance, the method Scale has type
(p *Point, factor float)
However, a function declared this way is not a method.
Mutually-recursive types require that one be forward declared so that it may be named in the other. A forward declaration of a type omits the block containing the fields or methods of the type.
type List struct // forward declaration of List
type Item struct {
value int;
next *List;
}
type List struct {
head, tail *Item
}
A forward-declared type is incomplete (§Types) until it is fully declared. The full declaration must follow before the end of the block containing the forward declaration; it cannot be contained in an inner block.
Functions and methods may similarly be forward-declared by omitting their body.
func F(a int) int // forward declaration of F
func G(a, b int) int {
return F(a) + F(b)
}
func F(a int) int {
if a <= 0 { return 0 }
return G(a-1, b+1)
}
An expression specifies the computation of a value by applying operators and functions to operands. An expression has a value and a type.
Operand = Literal | QualifiedIdent | "(" Expression ")" .
Literal = BasicLit | CompositeLit | FunctionLit .
BasicLit = int_lit | float_lit | char_lit | StringLit .
A constant is a literal of a basic type
(including the predeclared constants true, false
and nil
and values denoted by iota)
or a constant expression (§Constant expressions).
Constants have values that are known at compile time.
A qualified identifier is an identifier qualified by a package name prefix.
QualifiedIdent = [ ( LocalPackageName | PackageName ) "." ] identifier . LocalPackageName = identifier . 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 (§Exported identifiers).
The LocalPackageName is that of the package in which the qualified identifier appears and is only necessary to access names hidden by intervening declarations of a package-level identifier.
Math.Sin mypackage.hiddenName mypackage.Math.Sin // if Math is declared in an intervening scopeTODO: 6g does not implement LocalPackageName. Is this new? Is it needed?
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 "{" [ ElementList ] "}" .
LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
SliceType | MapType | TypeName .
ElementList = Element { "," Element } [ "," ] .
Element = [ Key ":" ] Value .
Key = Expression .
Value = Expression .
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 assignment compatible 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 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 Point struct { x, y, z float }
type Line struct { p, q Point }
one may write
origin := Point{}; // zero value for Point
line := Line{origin, Point{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 unique pointer to an instance of the literal's value.
var pointer *Point = &Point{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 is 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 literal:
[n]T{x1, x2, ... xn}[0 : n]
A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears in the condition of an "if", "for", or "switch" statement, because the braces surrounding the expressions in the literal are confused with those introducing a block of statements. To resolve the ambiguity in this rare case, the composite literal must appear within parentheses.
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, 11, 13, 17, 19, 991};
// vowels[ch] is true if ch is a vowel
vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true};
// the array [10]float{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1};
filter := [10]float{-1, 4: -0.1, -0.1, 9: -1};
// frequencies in Hz for equal-tempered scale (A4 = 440Hz)
noteFrequency := map[string]float{
"C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
"G0": 24.50, "A0": 27.50, "B0": 30.87,
}
A function literal represents an anonymous function. It consists of a specification of the function type and a function body.
FunctionLit = FunctionType Block .
Block = "{" StatementList "}" .
func (a, b int, z float) 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.
PrimaryExpr =
Operand |
PrimaryExpr Selector |
PrimaryExpr Index |
PrimaryExpr Slice |
PrimaryExpr TypeAssertion |
PrimaryExpr Call .
Selector = "." identifier .
Index = "[" Expression "]" .
Slice = "[" Expression ":" Expression "]" .
TypeAssertion = "." "(" Type ")" .
Call = "(" [ 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 of *x if
x is of pointer type). The identifier f
is called the (field or method)
selector.
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 or *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 as necessary.
If x is of pointer type, x.y
is shorthand for (*x).y; if y
is also of pointer type, x.y.z is shorthand
for (*(*x).y).z, and so on.
If *x is of pointer type, dereferencing
must be explicit;
only one level of automatic dereferencing is provided.
For an x of type T containing an
anonymous field declared as *A,
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).M0TODO: Specify what happens to receivers.
A primary expression of the form
a[x]
denotes the array or map element of a indexed by x.
The value x is called the
array index or map key, respectively. The following
rules apply:
For a of type A or *A
where A is an array type (§Array types),
or for a of type S where S is a slice type (§Slice types):
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 A
For a of type T
where T is a string type (§Strings):
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 byte
a[x] may not be assigned to
For a of type M
where M is a map type (§Map types):
x's type must be compatible with the key type of M
and the map must contain an entry with key x (but see special forms below)
a[x] is the map value with key x
and the type of a[x] is the value type of M
Otherwise a[x] is illegal. If the index or key is out of range evaluating
an otherwise legal index expression, a run-time exception occurs.
However, if an index expression on a map a of type map[K] V
is used in an assignment of one of the special forms
r, ok = a[x] r, ok := a[x]
the result of the index expression is a pair of values with types
(K, bool).
If the key is present in the map,
the expression returns the pair (a[x], true);
otherwise it returns (Z, false) where Z is
the zero value for V (§The zero value).
No run-time exception occurs in this case.
The index expression in this construct thus acts like a function call
returning a value and a boolean indicating success. (§Assignments)
Similarly, if an assignment to a map has the special form
a[x] = r, ok
and boolean ok has the value false,
the entry for key x is deleted from the map; if
ok is true, the construct acts like
a regular assignment to an element of the map.
Strings, arrays, and slices can be sliced to construct substrings or descriptors
of subarrays. The index expressions in the slice select which elements appear
in the result. The result has indexes starting at 0 and length equal to the
difference in the index values in the slice. After slicing the array a
a := [4]int{1, 2, 3, 4};
s := a[1:3];
the slice s has type []int, length 2, capacity 3, and elements
s[0] == 2 s[1] == 3
The slice length must be non-negative.
For arrays or strings, the indexes
lo and hi must satisfy
0 <= lo <= hi <= length;
for slices, the upper bound is the capacity rather than the length.
If the sliced operand is a string, the result of the slice operation is another, new string (§Strings). If the sliced operand is an array or slice, the result of the slice operation is a slice (§Slice types).
For an expression x and a type T, the primary expression
x.(T)
asserts that x is not the zero interface value
and that the value stored in x is of type T.
The notation x.(T) is called a type assertion.
The type of x must be an interface type.
More precisely, if T is not an interface type, x.(T) asserts
that the dynamic type of x is identical to the type T
(§Type identity and compatibility).
If T is an interface type, x.(T) asserts that the dynamic type
of T 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
exception 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 of one of the special forms,
v, ok = x.(T) 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 (§The zero value).
No run-time exception occurs in this case.
The type assertion in this construct thus acts like a function call
returning a value and a boolean indicating success. (§Assignments)
Given an expression f of function type
F,
f(a1, a2, ... an)
calls f with arguments a1, a2, ... an.
The arguments must be single-valued expressions
assignment compatible with the parameters 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.
Atan2(x, y) // function call var pt *Point; pt.Scale(3.5) // method call with receiver pt
A method call x.m() is valid if the method set of
(the type of) x contains m (and the
argument list is compatible with 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
When a function f has a ... parameter,
it is always the last formal parameter. Within calls to f,
the arguments before the ... are treated normally.
After those, an arbitrary number (including zero) of trailing
arguments may appear in the call and are bound to the ...
parameter.
Within f, the ... parameter has static
type interface{} (the empty interface). For each call,
its dynamic type is a structure whose sequential fields are the
trailing arguments of the call. That is, the actual arguments
provided for a ... parameter are wrapped into a struct
that is passed to the function instead of the actual arguments.
Using the reflection library (TODO: reference), f may
unpack the elements of the dynamic type to recover the actual
arguments.
Given the function and call
func Fprintf(f io.Writer, format string, args ...) Fprintf(os.Stdout, "%s %d", "hello", 23);
Within Fprintf, the dynamic type of args for this
call will be, schematically,
struct { string; int }.
As a special case, if a function passes its own ... parameter as the argument
for a ... in a call to another function with a ... parameter,
the parameter is not wrapped again but passed directly. In short, a formal ...
parameter is passed unchanged as an actual ... parameter.
Operators combine operands into expressions.
Expression = UnaryExpr | Expression binary_op UnaryExpr . UnaryExpr = PrimaryExpr | unary_op UnaryExpr . binary_op = log_op | com_op | rel_op | add_op | mul_op . log_op = "||" | "&&" . com_op = "<-" . rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" . add_op = "+" | "-" | "|" | "^" . mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" . unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
The operand types in binary operations must be compatible, with the following exceptions:
/ and %).
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 six precedence levels for binary operators.
Multiplication operators bind strongest, followed by addition
operators, comparison operators, communication operators,
&& (logical and), and finally || (logical or):
Precedence Operator
6 * / % << >> & &^
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.
Examples:
+x 23 + 3*x[i] x <= f() ^a >> b f() || g() x == y + 1 && <-chan_ptr > 0
Arithmetic operators apply to numeric types and yield a result of the same
type as the first operand. The four standard arithmetic operators (+,
-, *, /) apply both to integer and
floating point types, while + applies also
to strings; all other arithmetic operators apply to integers only.
+ sum integers, floats, strings - difference integers, floats * product integers, floats / quotient integers, floats % 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 integer values, / and % satisfy the following relationship:
(a / b) * b + a % b == a
with (a / b) truncated towards zero.
Examples:
x y x / y x % y 5 3 1 2 -5 3 -1 -2 5 -3 -1 2 -5 -3 1 -2
If the dividend is positive and the divisor is a constant power of 2, the division may be replaced by a left 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. The shift count must
be an unsigned integer. There is no upper limit on the shift count. Shifts behave
as if the left operand is shifted 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 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.
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 yield a boolean result. All comparison operators apply
to basic types except bools.
The operators == and != apply, at least in some cases,
to all types except arrays and structs.
== equal != not equal < less <= less or equal > greater >= greater or equal
Numeric basic types are compared in the usual way.
Strings are compared byte-wise (lexically).
Booleans are equal if they are either both "true" or both "false".
The rules for comparison of composite types are described in the section on §Comparison compatibility.
Logical operators apply to boolean operands and yield a boolean result. 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"
The unary prefix address-of operator & generates the address of its operand, which must be a variable,
pointer indirection, field selector, or array or slice indexing operation. It is illegal to take the address of a function
result variable.
Given an operand of pointer type, the unary prefix pointer indirection operator * retrieves the value pointed
to by the operand.
&x &a[f(2)] *p *pf(x)
TODO: This text needs to be cleaned up and go elsewhere, there are no address operators involved.
Methods are a form of function and a method ``value'' has a function type. Consider the type T with method M:
type T struct {
a int;
}
func (tp *T) M(a int) int;
var t *T;
To construct the value of method M, one writes
t.M
using the variable t (not the type T). TODO: It makes perfect sense to be able to say T.M (in fact, it makes more sense then t.M, since only the type T is needed to find the method M, i.e., its address). TBD.
The expression t.M is a function value with type
func (t *T, a int) int
and may be invoked only as a function, not as a method:
var f func (t *T, a int) int; f = t.M; x := f(t, 7);
Note that one does not write t.f(7); taking the value of a method demotes it to a function.
In general, given type T with method M and variable t of type T, the method invocation
t.M(args)
is equivalent to the function call
(t.M)(t, args)
TODO: should probably describe the effect of (t.m) under §Expressions if t.m denotes a method: Effect is as described above, converts into function.
If T is an interface type, the expression t.M does not determine which underlying type's M is called until the point of the call itself. Thus given T1 and T2, both implementing interface I with method M, the sequence
var t1 *T1; var t2 *T2; var i I = t1; m := i.M; m(t2, 7);
will invoke t2.M() even though m was constructed with an expression involving t1. Effectively, the value of m is a function literal
func (recv I, a int) {
recv.M(a);
}
that is automatically created.
TODO: Document implementation restriction: It is illegal to take the address of a result parameter (e.g.: func f() (x int, p *int) { return 2, &x }). (TBD: is it an implementation restriction or fact?)
The term channel means "variable of channel type" (§Channel types).
The send operation uses the binary operator "<-", which operates on a channel and a value (expression):
ch <- 3
The send operation sends the value on the channel. Both the channel and the expression are evaluated before communication begins. Communication blocks until the send can proceed, at which point the value is transmitted on the channel. A send can proceed if the channel is asynchronous and there is room in its buffer or the channel is synchronous and a receiver is ready.
If the send operation appears in an expression context, the value of the expression is a boolean and the operation is non-blocking. The value of the boolean reports true if the communication succeeded, false if it did not. (The channel and the expression to be sent are evaluated regardless.) These two examples are equivalent:
ok := ch <- 3;
if ok { print("sent") } else { print("not sent") }
if ch <- 3 { print("sent") } else { print("not sent") }
In other words, if the program tests the value of a send operation, the send is non-blocking and the value of the expression is the success of the operation. If the program does not test the value, the operation blocks until it succeeds.
The receive operation uses the prefix unary operator "<-". The value of the expression is the value received, whose type is the element type of the channel.
<-ch
The expression blocks until a value is available, which then can be assigned to a variable or used like any other expression. If the receive expression does not save the value, the value is discarded.
v1 := <-ch v2 = <-ch f(<-ch) <-strobe // wait until clock pulse
If a receive expression is used in a tuple assignment of the form
x, ok = <-ch; // or: x, ok := <-ch
the receive operation becomes non-blocking.
If the operation can proceeed, the boolean variable
ok will be set to true
and the value stored in x; otherwise
ok is set
to false and x is set to the
zero value for its type (§The zero value).
TODO: Probably in a separate section, communication semantices need to be presented regarding send, receive, select, and goroutines.
Constant expressions may contain only constants, iota,
numeric literals, string literals, and
some constant-valued built-in functions such as unsafe.Sizeof
and len applied to an array.
In practice, constant expressions are those that can be evaluated at compile time.
The type of a constant expression is determined by the type of its elements. If it contains only numeric literals, its type is ideal integer or ideal float (§Ideal number). Whether a literal is an integer or float depends on the syntax of the literals (123 vs. 123.0). The nature of the arithmetic operations within the expression depends, elementwise, on the values; for example, 3/2 is an integer division yielding 1, while 3./2. is a floating point division yielding 1.5. Thus
const x = 3./2. + 3/2;
yields a floating point constant of ideal float value 2.5 (1.5 + 1); its constituent expressions are evaluated using distinct rules for division.
Intermediate values and the constants themselves may require precision significantly larger than any concrete type in the language. The following are legal declarations:
const Huge = 1 << 100; const Four int8 = Huge >> 98;
A constant expression may appear in any context, such as assignment
to a variable of any numeric type, as long as the value of the
expression can be represented accurately in that context.
It is erroneous to assign a value with a non-zero fractional part
to an integer, or if the assignment would overflow or underflow,
or in general if the value cannot be represented by the type of
the variable.
For
instance, 3 can be assigned to any integer variable but also to any
floating point variable, while -1e12 can be assigned to a
float32, float64, or even int64
but not uint64 or string.
If a typed constant expression evaluates to a value that is not representable by that type, the compiler reports an error.
uint8(-1) // error, out of range uint8(100) * 100 // error, out of range
The mask used by the unary bitwise complement operator matches the rule for non-constants: the mask is the all 1s for unsigned constants and -1 for signed and ideal constants.
^1 // ideal 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
TODO: perhaps ^ should be disallowed on non-uints instead of assuming twos complement. Also it may be possible to make typed constants more like variables, at the cost of fewer overflow etc. errors being caught.
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. Otherwise, the order of evaluation is unspecified.
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.
Statements control execution.
Statement =
Declaration | EmptyStmt | LabeledStmt |
SimpleStmt | GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
DeferStmt .
SimpleStmt = ExpressionStmt | IncDecStmt | Assignment | SimpleVarDecl .
StatementList = Statement { Separator Statement } .
Separator = [ ";" ] .
Elements of a list of statements are separated by semicolons, which may be omitted only if the previous statement:
The empty statement does nothing.
EmptyStmt = .
A statement list can always in effect be terminated with a semicolon by adding an empty statement.
A labeled statement may be the target of a goto,
break or continue statement.
LabeledStmt = Label ":" Statement . Label = identifier .
Error: log.Fatal("error encountered")
Function calls, method calls, and channel operations can appear in statement context.
ExpressionStmt = Expression .
f(x+y) <-ch
The "++" and "--" statements increment or decrement their operands by the ideal numeric value 1. As with an assignment, the operand must be a variable, pointer indirection, field selector or index expression.
IncDecStmt = Expression ( "++" | "--" ) .
The following assignment statements (§Assignments) 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 a variable, pointer indirection, field selector, or index expression.
x = 1 *p = f() a[i] = 23 k = <-ch i &^= 1<<n
An assignment operation x op=
y where op is a binary arithmetic operation is equivalent
to x = x op
y but evalutates x
only once. The op= construct is a single token.
a[i] <<= 2
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 (§Channel
operations, §Map operations) or a type assertion (§Type assertions).
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.
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. The expressions on the right are evaluated before assigning to any of the operands on the left, but otherwise the evaluation order is unspecified.
a, b = b, a // exchange a and b
In assignments, the type of each value must be assignment compatible (§Assignment compatibility) with the type of the operand to which it is assigned.
"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. A missing condition
is equivalent to true.
IfStmt = "if" [ [ SimpleStmt ] ";" ] [ Expression ] Block [ "else" Statement ] .
if x > 0 {
return true;
}
An "if" statement may include a simple statement before the expression. The scope of any variables declared by that statement extends to the end of the "if" statement and the variables are initialized once before the statement is entered.
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 expression is equivalent to
the expression true.
ExprSwitchStmt = "switch" [ [ SimpleStmt ] ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
ExprCaseClause = ExprSwitchCase ":" [ StatementList ] .
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.
Each case clause acts as a block for scoping purposes (§Declarations and scope rules).
A "switch" statement may include a simple statement before the expression. The scope of any variables declared by that statement extends to the end of the "switch" statement and the variables are initialized once before the statement is entered.
switch tag {
default: s3()
case 0, 1, 2, 3: s1()
case 4, 5, 6, 7: s2()
}
switch x := f(); {
case x < 0: return -x
default: return x
}
switch { // missing expression means "true"
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 introduced by special
notation in the form of a simple declaration whose right hand side
has the form of a type assertion (§Type assertions)
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 ":=" Expression "." "(" "type" ")" .
TypeCaseClause = TypeSwitchCase ":" [ StatementList ] .
TypeSwitchCase = "case" Type | "default" .
As a special case, the type in the type switch case may be an
identifier denoting the predeclared constant nil
(§Predeclared identifiers).
If the interface value equals nil,
only an explict nil case or "default"
case will execute.
Given a function f
that returns a value of interface type,
the following type switch:
switch i := f().(type) {
case nil:
printString("f() returns nil");
case int:
printInt(i); // i is an int
case float:
printFloat(i); // i is a float
case func(int) float:
printFunction(i); // i is a function
default:
printString("don't know the type");
}
could be rewritten:
v := f();
if v == nil {
printString("f() returns nil");
} else if i, is_int := v.(int); is_int {
printInt(i); // i is an int
} else if i, is_float := v.(float); is_float {
printFloat(i); // i is a float
} else if i, is_func := v.(func(int) float); is_func {
printFunction(i); // i is a function
} else {
printString("don't know the type");
}
In a type switch, the guard is mandatory, there can be only one type per "case", and the "fallthrough" statement is not allowed.
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 "for" clause 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 (but not the post statement) may also be a short variable declaration; the scope of the variables it declares ends at the end of the statement (§Declarations and scope rules).
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 "for" clause 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 true { S() } is the same as for { 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 first assigns the current index or key to an iteration variable - or the current (index, element) or (key, value) pair to a pair of iteration variables - and then executes the block.
RangeClause = ExpressionList ( "=" | ":=" ) "range" Expression .
The type of the right-hand expression in the "range" clause must be an
array, slice, string or map, or a pointer to an array, slice, string or map;
or it may be a channel.
Except for channels,
the identifier list must contain one or two expressions
(as in assignments, these must be a
variable, pointer indirection, field selector, or index expression)
denoting the
iteration variables. On each iteration,
the first variable is set to the string, array or slice index or
map key, and the second variable, if present, is set to the corresponding
string or array element or map value.
The types of the array or slice index (always int)
and element, or of the map key and value respectively,
must be assignment compatible to the iteration variables.
For strings, the "range" clause iterates over the Unicode code points
in the string. On successive iterations, the index variable will be the
index of successive UTF-8-encoded code points in the string, and
the second variable, of type int, will be the value of
the corresponding code point. If the iteration encounters an invalid
UTF-8 sequence, the second variable will be 0xFFFD,
the Unicode replacement character, and the next iteration will advance
a single byte in the string.
For channels, the identifier list must contain one identifier. The iteration receives values sent on the channel until the channel is closed; it does not process the zero value sent before the channel is closed.
The iteration variables may be declared by the "range" clause (":="), in which
case their scope ends at the end of the "for" statement (§Declarations and
scope rules). In this case their types are set to
int and the array element type, or the map key and value types, respectively.
If the iteration variables are declared outside the "for" statement,
after execution their values will be those of the last iteration.
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 assignment-compatible to val
for key, value = range m {
h(key, value)
}
// key == last map key encountered in iteration
// val == map[key]
If map entries that have not yet been processed are deleted during iteration, they will not be processed. If map entries are inserted during iteration, the behavior is implementation-dependent, but each entry will be processed at most once.
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, and unlike with a regular call, program execution does not wait for the invoked function to complete.
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 ":" StatementList .
CommCase = "case" ( SendExpr | RecvExpr) | "default" .
SendExpr = Expression "<-" Expression .
RecvExpr = [ Expression ( "=" | ":=" ) ] "<-" Expression .
Each communication clause acts as a block for the purpose of scoping (§Declarations and scope rules).
For all the send and receive expressions in the "select"
statement, the channel expression is evaluated. Any expressions
that appear on the right hand side of send expressions are also
evaluated. If any of the resulting channels can proceed, one is
chosen and the corresponding communication and statements are
evaluated. Otherwise, if there is a default case, that executes;
if not, the statement blocks until one of the communications can
complete. The channels and send expressions are not re-evaluated.
A channel pointer 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.
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 fair choice is made to decide which single communication will execute.
The receive case may declare a new variable using a short variable declaration (§Short variable declarations). The scope of such variables continues to the end of the respective case's statements.
var c, c1, c2 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");
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:
}
}
TODO: Make semantics more precise.
A "return" statement terminates execution of the containing function and optionally provides a result value or values to the caller.
ReturnStmt = "return" [ ExpressionList ] .
func procedure() {
return
}
There are two ways to return values from a function with a result type. The first is to explicitly list the return value or values in the "return" statement. Normally, the expressions must be single-valued and assignment-compatible to the elements of the result type of the function.
func simple_f() int {
return 2
}
func complex_f1() (re float, im float) {
return -7.0, -4.0
}
However, if the expression list in the "return" statement is a single call to a multi-valued function, the values returned from the called function will be returned from this one. The result types of the current function and the called function must match.
func complex_f2() (re float, im float) {
return complex_f1()
}
The second way to return values is to use the elements of the result list of the function as variables. When the function begins execution, these variables are initialized to the zero values for their type (§The zero value). The function can assign them as necessary; if the "return" provides no values, those of the variables will be returned to the caller.
func complex_f3() (re float, im float) {
re = 7.0;
im = 4.0;
return;
}
TODO: Define when return is required.
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 the post statement (§For statements).
ContinueStmt = "continue" [ Label ].
The optional label is analogous to that of a "break" statement.
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.
(TODO: Eliminate in favor of used and not set errors?)
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 parameters to the function call are evaluated and saved anew but the function is not invoked. Immediately before the innermost function surrounding the "defer" statement returns, but after its return value (if any) is evaluated, each deferred function is executed with its saved parameters. Deferred functions are executed in LIFO order.
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);
}
Call Argument type Result
len(s) string, *string string length (in bytes)
[n]T, *[n]T array length (== n)
[]T, *[]T slice length
map[K]T, *map[K]T map length
chan T number of elements in channel buffer
cap(s) []T, *[]T capacity of s
map[K]T, *map[K]T capacity of s
chan T channel buffer capacity
The type of the result is always int and the
implementation guarantees that
the result always fits into an int.
The capacity of a slice or map is the number of elements for which there is
space allocated in the underlying array (for a slice) or map. For a slice
s, at any time the following relationship holds:
0 <= len(s) <= cap(s)
Conversions look like function calls of the form
T(value)
where T is a type
and value is an expression
that can be converted to a value
of result type T.
The following conversion rules apply:
uint32(int8(0xFF)) is 0xFFFFFFFF.
The conversion always yields a valid value; there is no signal for overflow.
string(0x65e5) // "\u65e5"
nil, the result is the empty string.
string([]int{0x65e5, 0x672c, 0x8a9e}) // "\u65e5\u672c\u8a9e"
nil,
the result is the empty string.
string([]byte{'h', 'e', 'l', 'l', 'o'}) // "hello"
There is no linguistic mechanism to convert between pointers and integers.
The unsafe package
implements this functionality under
restricted circumstances (§Package unsafe).
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 float }
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).
make(T [, optional list of expressions])
For instance
make(map[string] int)
creates a new map value and initializes it to an empty map.
The parameters affect sizes for allocating slices, maps, and buffered channels:
s := make([]int, 10, 100); # slice with len(s) == 10, cap(s) == 100 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
Go programs are constructed by linking together packages. A package is in turn constructed from one or more source files that together provide access to a set of types, constants, functions, and variables. Those elements may be imported 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. The source text following the package clause acts as a block for scoping (§Declarations and scope rules).
SourceFile = PackageClause { ImportDecl [ ";" ] } { Declaration [ ";" ] } .
A package clause begins each source file and defines the package to which the file belongs.
PackageClause = "package" PackageName .
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.
A source file gains access to exported identifiers (§Exported identifiers) from another package through an import declaration. In the general form, an import declaration provides an identifier that code in the source file may use to access the imported package's contents and a file name referring to the (compiled) implementation of the package. The file name may be relative to a repository of installed packages.
ImportDecl = "import" ( ImportSpec | "(" [ ImportSpecList ] ")" ) .
ImportSpecList = ImportSpec { ";" ImportSpec } [ ";" ] .
ImportSpec = [ "." | PackageName ] PackageFileName .
PackageFileName = StringLit .
After an import, in the usual case an exported name N from the imported
package P may be accessed by the qualified identifier
P.N (§Qualified identifiers). The actual
name P depends on the form of the import declaration. If
an explicit package name p1 is provided, the qualified
identifer will have the form p1.N. If no name
is provided in the import declaration, P will be the package
name declared within the source files of the imported package.
Finally, if the import declaration uses an explicit period
(.) for the package name, N will appear
in the package-level scope of the current file and the qualified name is
unnecessary and erroneous. In this form, it is an error if the import introduces
a name conflict.
In this table, assume we have compiled a package named
math, which exports function Sin, and
installed the compiled package in file
"lib/math".
Import syntax Local name of Sin import M "lib/math" M.Sin import "lib/math" math.Sin import . "lib/math" Sin
If a package is constructed from multiple source files, all names at package-level scope, not just exported names, are visible to all the files in the package. An import declaration is still necessary to declare intention to use the names, but the imported names do not need a qualified identifer to be accessed.
The compilation of a multi-file package may require that the files be compiled and installed in an order that satisfies the resolution of names imported within the package.
If source file math1.go contains
package math
const twoPi = 6.283185307179586
function Sin(x float) float { return ... }
and file "math2.go" begins
package math import "lib/math"
then, provided "math1.go" is compiled first and
installed in "lib/math", math2.go
may refer directly to Sin and twoPi
without a qualified identifier.
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 'in' to channel 'out',
// 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 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 and interfaces.
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 float; 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 in declaration order and then calling any package-level function with the name and signature of
func init()
defined in its source. Since a package may contain more
than one source file, there may be more than one
init() function in a package, but
only one per source file.
Initialization code may contain "go" statements, but the functions they invoke do not begin execution until initialization of the entire program is complete. Therefore, all initialization code is run in a single goroutine.
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, possibly created by linking multiple packages,
must have one package called main, with a function
func main() { ... }
defined.
The function main.main() takes no arguments and returns no value.
Program execution begins by initializing the main package and then
invoking main.main().
When main.main() returns, the program exits.
Implementation restriction: The compiler assumes package main
is created by a single source file and that it is not imported by any other package.
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) int
func Offsetof(selector ArbitraryType) int
func Reflect(i interface {}) (value uint64, typestring string, indir bool)
func Sizeof(variable ArbitraryType) int
func Unreflect(value uint64, typestring string, indir bool) interface {}
Any pointer or value of type uintptr can be converted into
a Pointer and vice versa.
The function Sizeof takes an expression denoting a
variable of any (complete) 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)) + uintptr(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)) % uintptr(unsafe.Alignof(x)) == 0
Calls to Alignof, Offsetof, and
Sizeof are constant expressions of type int.
TODO describe Reflect, Unreflect
type size in bytes byte, uint8, int8 1 uint16, int16 2 uint32, int32, float32 4 uint64, int64, float64 8
The following minimal alignment properties are guaranteed:
x of any type: 1 <= unsafe.Alignof(x) <= unsafe.Maxalign.
x of numeric type: unsafe.Alignof(x) is the smaller
of unsafe.Sizeof(x) and unsafe.Maxalign, but 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.
Implementation accepts only ASCII digits for digits; doc says Unicode.
Implementation does not honor the restriction on goto statements and targets (no intervening declarations).
cap() does not work on maps or chans.
len() does not work on chans.