Go+ has a recommended best practice syntax set, which we call the Go+ Mini Specification. It is simple but Turing-complete and can elegantly implement any business requirements.
The syntax is specified using a variant of Extended Backus-Naur Form (EBNF):
Syntax = { Production } .
Production = production_name "=" [ Expression ] "." .
Expression = Term { "|" Term } .
Term = Factor { Factor } .
Factor = 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)Lowercase production names are used to identify lexical (terminal) tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes "" or back quotes ``.
The form a … b represents the set of characters from a through b as alternatives. The horizontal ellipsis … is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character … (as opposed to the three characters ...) is not a token of the Go+ language.
Source code is Unicode text encoded in UTF-8. The text is not canonicalized, so a single accented code point is distinct from the same character constructed from combining an accent and a letter; those are treated as two code points. For simplicity, this document will use the unqualified term character to refer to a Unicode code point in the source text.
Each code point is distinct; for instance, uppercase and lowercase letters are different characters.
Implementation restriction: For compatibility with other tools, a compiler may disallow the NUL character (U+0000) in the source text.
Implementation restriction: For compatibility with other tools, a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF) if it is the first Unicode code point in the source text. A byte order mark may be disallowed anywhere else in the source.
The following terms are used to denote specific Unicode character categories:
newline = /* the Unicode code point U+000A */ .
unicode_char = /* an arbitrary Unicode code point except newline */ .
unicode_letter = /* a Unicode code point categorized as "Letter" */ .
unicode_digit = /* a Unicode code point categorized as "Number, decimal digit" */ .In The Unicode Standard 8.0, Section 4.5 "General Category" defines a set of character categories. Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo as Unicode letters, and those in the Number category Nd as Unicode digits.
The underscore character _ (U+005F) is considered a lowercase letter.
letter = unicode_letter | "_" .
decimal_digit = "0" … "9" .
binary_digit = "0" | "1" .
octal_digit = "0" … "7" .
hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .Comments serve as program documentation. There are three forms:
- Line comments start with the character sequence
//and stop at the end of the line. - Line comments start with the character sequence
#and stop at the end of the line. - General comments start with the character sequence
/*and stop with the first subsequent character sequence*/.
A general comment containing no newlines acts like a space. Any other comment acts like a newline.
# this is a line comment
// this is another line comment
/* this is a general comment */
Tokens form the vocabulary of the Go+ language. There are four classes: identifiers, keywords, operators and punctuation, and literals. White space, formed from spaces (U+0020), horizontal tabs (U+0009), carriage returns (U+000D), and newlines (U+000A), is ignored except as it separates tokens that would otherwise combine into a single token. Also, a newline or end of file may trigger the insertion of a semicolon. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.
The formal syntax uses semicolons ";" as terminators in a number of productions. Go+ programs may omit most of these semicolons using the following two rules:
- When the input is broken into tokens, a semicolon is automatically inserted into the token stream immediately after a line's final token if that token is
- an identifier
- an integer, floating-point, imaginary, rune, or string literal
- one of the keywords
break,continue,fallthrough, orreturn - one of the operators and punctuation
++,--,),], or}
- To allow complex statements to occupy a single line, a semicolon may be omitted before a closing
")"or"}".
To reflect idiomatic use, code examples in this document elide semicolons using these rules.
Identifiers name program entities such as variables and types. An identifier is a sequence of one or more letters and digits. The first character in an identifier must be a letter.
identifier = letter { letter | unicode_digit } .a
_x9
ThisVariableIsExported
αβSome identifiers are predeclared.
The following keywords are reserved and may not be used as identifiers (TODO: some keywords are allowed 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 varThe following character sequences represent operators (including assignment operators) and punctuation:
+ & += &= && == != ( )
- | -= |= || < <= [ ]
* ^ *= ^= <- > >= { }
/ << /= <<= ++ = := , ;
% >> %= >>= -- ! ... . :
&^ &^= ~
An integer literal is a sequence of digits representing an integer constant. An optional prefix sets a non-decimal base: 0b or 0B for binary, 0, 0o, or 0O for octal, and 0x or 0X for hexadecimal. A single 0 is considered a decimal zero. In hexadecimal literals, letters a through f and A through F represent values 10 through 15.
For readability, an underscore character _ may appear after a base prefix or between successive digits; such underscores do not change the literal's value.
int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
binary_digits = binary_digit { [ "_" ] binary_digit } .
octal_digits = octal_digit { [ "_" ] octal_digit } .
hex_digits = hex_digit { [ "_" ] hex_digit } .42
4_2
0600
0_600
0o600
0O600 // second character is capital letter 'O'
0xBadFace
0xBad_Face
0x_67_7a_2f_cc_40_c6
170141183460469231731687303715884105727
170_141183_460469_231731_687303_715884_105727
_42 // an identifier, not an integer literal
42_ // invalid: _ must separate successive digits
4__2 // invalid: only one _ at a time
0_xBadFace // invalid: _ must separate successive digitsA floating-point literal is a decimal or hexadecimal representation of a floating-point constant.
A decimal floating-point literal consists of an integer part (decimal digits), a decimal point, a fractional part (decimal digits), and an exponent part (e or E followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; one of the decimal point or the exponent part may be elided. An exponent value exp scales the mantissa (integer and fractional part) by 10exp.
A hexadecimal floating-point literal consists of a 0x or 0X prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits), and an exponent part (p or P followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; the radix point may be elided as well, but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.) An exponent value exp scales the mantissa (integer and fractional part) by 2exp.
For readability, an underscore character _ may appear after a base prefix or between successive digits; such underscores do not change the literal value.
float_lit = decimal_float_lit | hex_float_lit .
decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
decimal_digits decimal_exponent |
"." decimal_digits [ decimal_exponent ] .
decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
[ "_" ] hex_digits |
"." hex_digits .
hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .0.
72.40
072.40 // == 72.40
2.71828
1.e+0
6.67428e-11
1E6
.25
.12345E+5
1_5. // == 15.0
0.15e+0_2 // == 15.0
0x1p-2 // == 0.25
0x2.p10 // == 2048.0
0x1.Fp+0 // == 1.9375
0X.8p-0 // == 0.5
0X_1FFFP-16 // == 0.1249847412109375
0x15e-2 // == 0x15e - 2 (integer subtraction)
0x.p1 // invalid: mantissa has no digits
1p-2 // invalid: p exponent requires hexadecimal mantissa
0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent
1_.5 // invalid: _ must separate successive digits
1._5 // invalid: _ must separate successive digits
1.5_e1 // invalid: _ must separate successive digits
1.5e_1 // invalid: _ must separate successive digits
1.5e1_ // invalid: _ must separate successive digitsTODO
1r # bigint 1
2/3r # bigrat 2/3An imaginary literal represents the imaginary part of a complex constant. It consists of an integer or floating-point literal followed by the lowercase letter i. The value of an imaginary literal is the value of the respective integer or floating-point literal multiplied by the imaginary unit i.
imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .For backward compatibility, an imaginary literal's integer part consisting entirely of decimal digits (and possibly underscores) is considered a decimal integer, even if it starts with a leading 0.
0i
0123i // == 123i for backward-compatibility
0o123i // == 0o123 * 1i == 83i
0xabci // == 0xabc * 1i == 2748i
0.i
2.71828i
1.e+0i
6.67428e-11i
1E6i
.25i
.12345E+5i
0x1p-2i // == 0x1p-2 * 1i == 0.25iThe boolean truth values are represented by the predeclared constants true and false.
true
falseA rune literal represents a rune constant, an integer value identifying a Unicode code point. A rune literal is expressed as one or more characters enclosed in single quotes, as in 'x' or '\n'. Within the quotes, any character may appear except newline and unescaped single quote. A single quoted character represents the Unicode value of the character itself, while multi-character sequences beginning with a backslash encode values in various formats.
The simplest form represents the single character within the quotes; since Go+ source text is Unicode characters encoded in UTF-8, multiple UTF-8-encoded bytes may represent a single integer value. For instance, the literal 'a' holds a single byte representing a literal a, Unicode U+0061, value 0x61, while 'ä' holds two bytes (0xc3 0xa4) representing a literal a-dieresis, U+00E4, value 0xe4.
Several backslash escapes allow arbitrary values to be encoded as ASCII text. There are four ways to represent the integer value as a numeric constant: \x followed by exactly two hexadecimal digits; \u followed by exactly four hexadecimal digits; \U followed by exactly eight hexadecimal digits, and a plain backslash \ followed by exactly three octal digits. In each case the value of the literal is the value represented by the digits in the corresponding base.
Although these representations all result in an integer, they have different valid ranges. Octal escapes must represent a value between 0 and 255 inclusive. Hexadecimal escapes satisfy this condition by construction. The escapes \u and \U represent Unicode code points so within them some values are illegal, in particular those above 0x10FFFF and surrogate halves.
After a backslash, certain single-character escapes represent special values:
\a U+0007 alert or bell
\b U+0008 backspace
\f U+000C form feed
\n U+000A line feed or newline
\r U+000D carriage return
\t U+0009 horizontal tab
\v U+000B vertical tab
\\ U+005C backslash
\' U+0027 single quote (valid escape only within rune literals)
\" U+0022 double quote (valid escape only within string literals)
An unrecognized character following a backslash in a rune literal is illegal.
rune_lit = "'" ( unicode_value | byte_value ) "'" .
unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
byte_value = octal_byte_value | hex_byte_value .
octal_byte_value = `\` octal_digit octal_digit octal_digit .
hex_byte_value = `\` "x" hex_digit hex_digit .
little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
hex_digit hex_digit hex_digit hex_digit .
escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .'a'
'ä'
'本'
'\t'
'\000'
'\007'
'\377'
'\x07'
'\xff'
'\u12e4'
'\U00101234'
'\'' // rune literal containing single quote character
'aa' // illegal: too many characters
'\k' // illegal: k is not recognized after a backslash
'\xa' // illegal: too few hexadecimal digits
'\0' // illegal: too few octal digits
'\400' // illegal: octal value over 255
'\uDFFF' // illegal: surrogate half
'\U00110000' // illegal: invalid Unicode code pointA string literal represents a string constant obtained from concatenating a sequence of characters. There are two forms: raw string literals and interpreted string literals.
Raw string literals are character sequences between back quotes, as in `foo`. Within the quotes, any character may appear except back quote. The value of a raw string literal is the string composed of the uninterpreted (implicitly UTF-8-encoded) characters between the quotes; in particular, backslashes have no special meaning and the string may contain newlines. Carriage return characters ('\r') inside raw string literals are discarded from the raw string value.
Interpreted string literals are character sequences between double quotes, as in "bar". Within the quotes, any character may appear except newline and unescaped double quote. The text between the quotes forms the value of the literal, with backslash escapes interpreted as they are in rune literals (except that \' is illegal and \" is legal), with the same restrictions. The three-digit octal (\nnn) and two-digit hexadecimal (\xnn) escapes represent individual bytes of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual characters. Thus inside a string literal \377 and \xFF represent a single byte of value 0xFF=255, while ÿ, \u00FF, \U000000FF and \xc3\xbf represent the two bytes 0xc3 0xbf of the UTF-8 encoding of character U+00FF.
string_lit = raw_string_lit | interpreted_string_lit .
raw_string_lit = "`" { unicode_char | newline } "`" .
interpreted_string_lit = `"` { unicode_value | byte_value } `"` .`abc` // same as "abc"
`\n
\n` // same as "\\n\n\\n"
"\n"
"\"" // same as `"`
"Hello, world!\n"
"日本語"
"\u65e5本\U00008a9e"
"\xff\u00FF"
"\uD800" // illegal: surrogate half
"\U00110000" // illegal: invalid Unicode code pointThese 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 bytesIf the source code represents a character as two code points, such as a combining form involving an accent and a letter, the result will be an error if placed in a rune literal (it is not a single code point), and will appear as two code points if placed in a string literal.
TODO
nil
iotaThere are boolean constants, rune constants, integer constants, floating-point constants, complex constants, and string constants. Rune, integer, floating-point, and complex constants are collectively called numeric constants.
A constant value is represented by a rune, integer, floating-point, imaginary, boolean or string literal, an identifier denoting a constant, a constant expression, a conversion with a result that is a constant, or the result value of some built-in functions such as min or max applied to constant arguments, unsafe.Sizeof applied to certain values, cap or len applied to some expressions, real and imag applied to a complex constant and complex applied to numeric constants. The boolean truth values are represented by the predeclared constants true and false. The predeclared identifier iota denotes an integer constant.
Not all literals are constants. For example (TODO: check this):
nil
1r
2/3rIn general, complex constants are a form of constant expression and are discussed in that section.
Numeric constants represent exact values of arbitrary precision and do not overflow. Consequently, there are no constants denoting the IEEE-754 negative zero, infinity, and not-a-number values.
Constants may be typed or untyped. Literal constants (including true, false, iota), and certain constant expressions containing only untyped constant operands are untyped.
A constant may be given a type explicitly by a constant declaration or conversion, or implicitly when used in a variable declaration or an assignment statement or as an operand in an expression. It is an error if the constant value cannot be represented as a value of the respective type.
An untyped constant has a default type which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a short variable declaration such as i := 0 where there is no explicit type. The default type of an untyped constant is bool, rune, int, float64, complex128, or string respectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant.
A variable is a storage location for holding a value. The set of permissible values is determined by the variable's type.
A variable declaration or, for function parameters and results, the signature of a function declaration or function literal reserves storage for a named variable. Calling the built-in function new or taking the address of a composite literal allocates storage for a variable at run time. Such an anonymous variable is referred to via a (possibly implicit) pointer indirection.
Structured variables of array, slice, and class types have elements and fields that may be addressed individually. Each such element acts like a variable.
The static type (or just type) of a variable is the type given in its declaration, the type provided in the new call or composite literal, or the type of an element of a class variable. Variables of interface type also have a distinct dynamic type, which is the (non-interface) type of the value assigned to the variable at run time (unless the value is the predeclared identifier nil, which has no type). The dynamic type may vary during execution but values stored in interface variables are always assignable to the static type of the variable.
var x any // x is nil and has static type any
var v *T // v has value nil, static type *T
x = 42 // x has value 42 and dynamic type int
x = v // x has value (*T)(nil) and dynamic type *TA variable's value is retrieved by referring to the variable in an expression; it is the most recent value assigned to the variable. If a variable has not yet been assigned a value, its value is the zero value for its type.
A boolean type represents the set of Boolean truth values denoted by the predeclared constants true and false. The predeclared boolean type is bool; it is a defined type.
boolAn integer, floating-point, rational or complex type represents the set of integer, floating-point, or complex values, respectively. They are collectively called numeric types. The predeclared architecture-independent numeric types are:
uint8 // the set of all unsigned 8-bit integers (0 to 255)
uint16 // the set of all unsigned 16-bit integers (0 to 65535)
uint32 // the set of all unsigned 32-bit integers (0 to 4294967295)
uint64 // the set of all unsigned 64-bit integers (0 to 18446744073709551615)
int8 // the set of all signed 8-bit integers (-128 to 127)
int16 // the set of all signed 16-bit integers (-32768 to 32767)
int32 // the set of all signed 32-bit integers (-2147483648 to 2147483647)
int64 // the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
float32 // the set of all IEEE-754 32-bit floating-point numbers
float64 // the set of all IEEE-754 64-bit floating-point numbers
complex64 // the set of all complex numbers with float32 real and imaginary parts
complex128 // the set of all complex numbers with float64 real and imaginary parts
byte // alias for uint8
rune // alias for int32The value of an n-bit integer is n bits wide and represented using two's complement arithmetic.
There is also a set of predeclared integer types with implementation-specific sizes:
uint // either 32 or 64 bits
int // same size as uint
uintptr // an unsigned integer large enough to store the uninterpreted bits of a pointer valueTo avoid portability issues all numeric types are defined types and thus distinct except byte, which is an alias for uint8, and rune, which is an alias for int32. Explicit conversions are required when different numeric types are mixed in an expression or assignment. For instance, int32 and int are not the same type even though they may have the same size on a particular architecture.
TODO:
bigint // TODO
bigrat // TODOA string type represents the set of string values. A string value is a (possibly empty) sequence of bytes. The number of bytes is called the length of the string and is never negative. Strings are immutable: once created, it is impossible to change the contents of a string. The predeclared string type is string; it is a defined type.
stringThe length of a string s can be discovered using the built-in function len. The length is a compile-time constant if the string is a constant. A string's bytes can be accessed by integer indices 0 through len(s)-1. It is illegal to take the address of such an element; if s[i] is the i'th byte of a string, &s[i] is invalid.
An array is a numbered sequence of elements of a single type, called the element type. The number of elements is called the length of the array and is never negative.
[N]TThe length is part of the array's type; it must evaluate to a non-negative constant representable by a value of type int. The length of array a can be discovered using the built-in function len. The elements can be addressed by integer indices 0 through len(a)-1. Array types are always one-dimensional but may be composed to form multi-dimensional types.
[32]byte
[1000]*float64
[3][5]int
[2][2][2]float64 // same as [2]([2]([2]float64))A pointer type denotes the set of all pointers to variables of a given type, called the base type of the pointer. The value of an uninitialized pointer is nil.
*TFor example:
*Point
*[4]intA slice is a descriptor for a contiguous segment of an underlying array and provides access to a numbered sequence of elements from that array. A slice type denotes the set of all slices of arrays of its element type. The number of elements is called the length of the slice and is never negative. The value of an uninitialized slice is nil.
[]TThe length of a slice s can be discovered by the built-in function len; unlike with arrays it may change during execution. The elements can be addressed by integer indices 0 through len(s)-1. The slice index of a given element may be less than the index of the same element in the underlying array.
A slice, once initialized, is always associated with an underlying array that holds its elements. A slice therefore shares storage with its array and with other slices of the same array; by contrast, distinct arrays always represent distinct storage.
The array underlying a slice may extend past the end of the slice. The capacity is a measure of that extent: it is the sum of the length of the slice and the length of the array beyond the slice; a slice of length up to that capacity can be created by slicing a new one from the original slice. The capacity of a slice a can be discovered using the built-in function cap(a).
A new, initialized slice value for a given element type T may be made using the built-in function make, which takes a slice type and parameters specifying the length and optionally the capacity. A slice created with make always allocates a new, hidden array to which the returned slice value refers. That is, executing
make([]T, length, capacity)produces the same slice as allocating an array and slicing it, so these two expressions are equivalent:
make([]int, 50, 100)
new([100]int)[0:50]
Like arrays, slices are always one-dimensional but may be composed to construct higher-dimensional objects. With arrays of arrays, the inner arrays are, by construction, always the same length; however with slices of slices (or arrays of slices), the inner lengths may vary dynamically. Moreover, the inner slices must be initialized individually.
A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keys of another type, called the key type. The value of an uninitialized map is nil.
map[KeyT]ElemTThe comparison operators == and != must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice. If the key type is an interface type, these comparison operators must be defined for the dynamic key values; failure will cause a run-time panic.
map[string]int
map[*T]string
map[string]anyThe number of map elements is called its length. For a map m, it can be discovered using the built-in function len and may change during execution. Elements may be added during execution using assignments and retrieved with index expressions; they may be removed with the delete and clear built-in function.
A new, empty map value is made using the built-in function make, which takes the map type and an optional capacity hint as arguments:
make(map[string]int)
make(map[string]int, 100)The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of nil maps. A nil map is equivalent to an empty map except that no elements may be added.
A function type denotes the set of all functions with the same parameter and result types. The value of an uninitialized variable of function type is nil.
func(parameters) resultsWithin a list of parameters or results, the names (IdentifierList) must either all be present or all be absent. If present, each name stands for one item (parameter or result) of the specified type and all non-blank names in the signature must be unique. If absent, each type stands for one item of that type. Parameter and result lists are always parenthesized except that if there is exactly one unnamed result it may be written as an unparenthesized type.
The final incoming parameter in a function signature may have a type prefixed with .... A function with such a parameter is called variadic and may be invoked with zero or more arguments for that parameter.
func()
func(x int) int
func(a, _ int, z float32) bool
func(a, b int, z float32) (bool)
func(prefix string, values ...int)
func(a, b int, z float64, opt ...any) (success bool)
func(int, int, float64) (float64, *[]int)
func(n int) func(p *T)TODO:
error
anyThe predeclared type error is defined as
type error interface {
Error() string
}It is the conventional interface for representing an error condition, with the nil value representing no error. For instance, a function to read data from a file might be defined:
func Read(f *File, b []byte) (n int, err error)TODO (classfile)
TODO
echo "Hello world"
echo("Hello world")TODO
Operators combine operands into expressions.
Binary operators:
|| && == != < <= > >=
+ - * / %
| & ^ &^ << >>Unary operators:
+ - ! ^ * &Unary operators have the highest precedence. As the ++ and -- operators form statements, not expressions, they fall outside the operator hierarchy. As a consequence, statement *p++ is the same as (*p)++.
There are five precedence levels for binary operators. Multiplication operators bind strongest, followed by addition operators, comparison operators, && (logical AND), and finally || (logical OR):
Precedence Operator
5 * / % << >> & &^
4 + - | ^
3 == != < <= > >=
2 &&
1 ||
Binary operators of the same precedence associate from left to right. For instance, x / y * z is the same as (x / y) * z.
+x // x
42 + a - b // (42 + a) - b
23 + 3*x[i] // 23 + (3 * x[i])
x <= f() // x <= f()
^a >> b // (^a) >> b
f() || g() // f() || g()
x == y+1 && <-chanInt > 0 // (x == (y+1)) && ((<-chanInt) > 0)Arithmetic operators apply to numeric values and yield a result of the same type as the first operand. The four standard arithmetic operators (+, -, *, /) apply to integer, floating-point, rational and complex types; + also applies to strings. The bitwise logical and shift operators apply to integers only.
+ sum integers (including bigint), floats, bigrat, complex values, strings
- difference integers (including bigint), floats, bigrat, complex values
* product integers (including bigint), floats, bigrat, complex values
/ quotient integers (including bigint), floats, bigrat, complex values
% remainder integers (including bigint)
& bitwise AND integers (including bigint)
| bitwise OR integers (including bigint)
^ bitwise XOR integers (including bigint)
&^ bit clear (AND NOT) integers (including bigint)
<< left shift integer << integer >= 0
>> right shift integer >> integer >= 0
TODO
Comparison operators compare two operands and yield an untyped boolean value.
== equal
!= not equal
< less
<= less or equal
> greater
>= greater or equalIn any comparison, the first operand must be assignable to the type of the second operand, or vice versa.
The equality operators == and != apply to operands of comparable types. The ordering operators <, <=, >, and >= apply to operands of ordered types. These terms and the result of the comparisons are defined as follows:
- Boolean types are comparable. Two boolean values are equal if they are either both true or both false.
- Integer types are comparable and ordered. Two integer values are compared in the usual way.
- Floating-point types are comparable and ordered. Two floating-point values are compared as defined by the IEEE-754 standard.
- Complex types are comparable. Two complex values u and v are equal if both real(u) == real(v) and imag(u) == imag(v).
- String types are comparable and ordered. Two string values are compared lexically byte-wise.
- Pointer types are comparable. Two pointer values are equal if they point to the same variable or if both have value
nil. Pointers to distinct zero-size variables may or may not be equal. - Interface types are comparable. Two interface values are equal if they have identical dynamic types and equal dynamic values or if both have value
nil. - A value x of non-interface type X and a value t of interface type T can be compared if type X is comparable and X implements T. They are equal if t's dynamic type is identical to X and t's dynamic value is equal to x.
- Array types are comparable if their array element types are comparable. Two array values are equal if their corresponding element values are equal. The elements are compared in ascending index order, and comparison stops as soon as two element values differ (or all elements have been compared).
A comparison of two interface values with identical dynamic types causes a run-time panic if that type is not comparable. This behavior applies not only to direct interface value comparisons but also when comparing arrays of interface values or structs with interface-valued fields.
Slice, map, and function types are not comparable. However, as a special case, a slice, map, or function value may be compared to the predeclared identifier nil. Comparison of pointer, channel, and interface values to nil is also allowed and follows from the general rules above.
Logical operators apply to boolean values and yield a result of the same type as the operands. The left operand is evaluated, and then the right if the condition requires it.
&& conditional AND p && q is "if p then q else false"
|| conditional OR p || q is "if p then true else q"
! NOT !p is "not p"
For an operand x of type T, the address operation &x generates a pointer of type *T to x. The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, x may also be a (possibly parenthesized) composite literal. If the evaluation of x would cause a run-time panic, then the evaluation of &x does too.
For an operand x of pointer type *T, the pointer indirection *x denotes the variable of type T pointed to by x. If x is nil, an attempt to evaluate *x will cause a run-time panic.
&x
&a[f(2)]
&Point{2, 3}
*p
*pf(x)
var x *int = nil
*x // causes a run-time panic
&*x // causes a run-time panicA conversion changes the type of an expression to the type specified by the conversion. A conversion may appear literally in the source, or it may be implied by the context in which an expression appears.
An explicit conversion is an expression of the form T(x) where T is a type and x is an expression that can be converted to type T.
T(x)If the type starts with the operator * or <-, or if the type starts with the keyword func and has no result list, it must be parenthesized when necessary to avoid ambiguity:
*Point(p) // same as *(Point(p))
(*Point)(p) // p is converted to *Point
func()(x) // function signature func() x
(func())(x) // x is converted to func()
(func() int)(x) // x is converted to func() int
func() int(x) // x is converted to func() int (unambiguous)A constant value x can be converted to type T if x is representable by a value of T. As a special case, an integer constant x can be explicitly converted to a string type using the same rule as for non-constant x.
Converting a constant to a type yields a typed constant.
uint(iota) // iota value of type uint
float32(2.718281828) // 2.718281828 of type float32
complex128(1) // 1.0 + 0.0i of type complex128
float32(0.49999999) // 0.5 of type float32
float64(-1e-1000) // 0.0 of type float64
string('x') // "x" of type string
string(0x266c) // "♬" of type string
myString("foo" + "bar") // "foobar" of type myString
string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
(*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
int(1.2) // illegal: 1.2 cannot be represented as an int
string(65.0) // illegal: 65.0 is not an integer constantFor the conversion of non-constant numeric values, the following rules apply:
- When converting between integer types, if the value is a signed integer, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type's size. For example, if v := uint16(0x10F0), then uint32(int8(v)) == 0xFFFFFFF0. The conversion always yields a valid value; there is no indication of overflow.
- When converting a floating-point number to an integer, the fraction is discarded (truncation towards zero).
- When converting an integer or floating-point number to a floating-point type, or a complex number to another complex type, the result value is rounded to the precision specified by the destination type. For instance, the value of a variable x of type float32 may be stored using additional precision beyond that of an IEEE-754 32-bit number, but float32(x) represents the result of rounding x's value to 32-bit precision. Similarly, x + 0.1 may use more than 32 bits of precision, but float32(x + 0.1) does not.
In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent.
TODO
TODO
Constant expressions may contain only constant operands and are evaluated at compile time.
Untyped boolean, numeric, and string constants may be used as operands wherever it is legal to use an operand of boolean, numeric, or string type, respectively.
A constant comparison always yields an untyped boolean constant. If the left operand of a constant shift expression is an untyped constant, the result is an integer constant; otherwise it is a constant of the same type as the left operand, which must be of integer type.
Any other operation on untyped constants results in an untyped constant of the same kind; that is, a boolean, integer, floating-point, complex, or string constant. If the untyped operands of a binary operation (other than a shift) are of different kinds, the result is of the operand's kind that appears later in this list: integer, rune, floating-point, complex. For example, an untyped integer constant divided by an untyped complex constant yields an untyped complex constant.
const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
const b = 15 / 4 // b == 3 (untyped integer constant)
const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
const d = 1 << 3.0 // d == 8 (untyped integer constant)
const e = 1.0 << 3 // e == 8 (untyped integer constant)
const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
const h = "foo" > "bar" // h == true (untyped boolean constant)
const j = true // j == true (untyped boolean constant)
const k = 'w' + 1 // k == 'x' (untyped rune constant)
const l = "hi" // l == "hi" (untyped string constant)
const m = string(k) // m == "x" (type string)
const Σ = 1 - 0.707i // (untyped complex constant)
const Δ = Σ + 2.0e-4 // (untyped complex constant)
const Φ = iota*1i - 1/1i // (untyped complex constant)Applying the built-in function complex to untyped integer, rune, or floating-point constants yields an untyped complex constant.
const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)Constant expressions are always evaluated exactly; intermediate values and the constants themselves may require precision significantly larger than supported by any predeclared type in the language. The following are legal declarations:
const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
const Four int8 = Huge >> 98 // Four == 4 (type int8)The divisor of a constant division or remainder operation must not be zero:
3.14 / 0.0 // illegal: division by zeroThe values of typed constants must always be accurately representable by values of the constant type. The following constant expressions are illegal:
uint(-1) // -1 cannot be represented as a uint
int(3.14) // 3.14 cannot be represented as an int
int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
Four * 100 // product 400 cannot be represented as an int8 (type of Four)The mask used by the unary bitwise complement operator ^ matches the rule for non-constants: the mask is all 1s for unsigned constants and -1 for signed and untyped constants.
^1 // untyped integer constant, equal to -2
uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
^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) = -2A short variable declaration uses the syntax:
ShortVarDecl = IdentifierList ":=" ExpressionList .It is shorthand for a regular variable declaration with initializer expressions but no types:
"var" IdentifierList "=" ExpressionList .i, j := 0, 10
f := func() int { return 7 }
ints := make([]int)
r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
_, y, _ := coord(p) // coord() returns three values; only interested in y coordinateUnlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-blank variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original. The non-blank variable names on the left side of := must be unique.
field1, offset := nextField(str, 0)
field2, offset := nextField(str, offset) // redeclares offset
x, y, x := 1, 2, 3 // illegal: x repeated on left side of :=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.
TODO
[expression1, ...]For example:
[] // []any
[1, 2, 3] // []int
[10, 3.14, 200] // []float64
["Hello", "world"] // []string
["Hello", 100, true] // []anyThe type of slice literals can be inferred from the context:
func echoF32s(vals []float32) {
echo vals
}
echo [10, 3.14, 200] // []float64
echoF32s [10, 3.14, 200] // []float32
var a []any = [10, 3.14, 200] // []any
echo aTODO
{key1: value1, ...}For example:
{} // map[string]any
{"Monday": 1, "Sunday": 7} // map[string]int
{1: 100, 3: 3.14, 5: 10} // map[int]float64The type of map literals can be inferred from the context:
func echoS2f32(vals map[string]float32) {
echo vals
}
echo {"Monday": 1, "Sunday": 7}
echoS2f32 {"Monday": 1, "Sunday": 7}
var a map[string]any = {"Monday": 1, "Sunday": 7}
echo aAt package level, initialization dependencies determine the evaluation order of individual initialization expressions in variable declarations. Otherwise, when evaluating the operands of an expression, assignment, or return statement, all function calls, method calls, receive operations, and binary logical operations are evaluated in lexical left-to-right order.
For example, in the (function-local) assignment
y[f()], ok = g(z || h(), i()+x[j()], <-c), k()the function calls and communication happen in the order f(), h() (if z evaluates to false), 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 and z is not specified, except as required lexically. For instance, g cannot be called before its arguments are evaluated.
a := 1
f := func() int { a++; return a }
x := [a, f()] // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
m := {a: 1, a: 2} // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified
n := {a: f()} // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specifiedAt package level, initialization dependencies override the left-to-right rule for individual initialization expressions, but not for operands within each expression:
var a, b, c = f() + v(), g(), sqr(u()) + v()
func f() int { return c }
func g() int { return a }
func sqr(x int) int { return x*x }
// functions u and v are independent of all other variables and functionsThe function calls happen in the order u(), sqr(), v(), f(), v(), and g().
Floating-point operations within a single expression are evaluated according to the associativity of the operators. Explicit parentheses affect the evaluation by overriding the default associativity. In the expression x + (y + z) the addition y + z is performed before adding x.
Statements control execution.
Statement =
Declaration | SimpleStmt | IfStmt | ForStmt | SwitchStmt |
LabeledStmt | BreakStmt | ContinueStmt | FallthroughStmt | GotoStmt |
ReturnStmt | DeferStmt | Block .
SimpleStmt = EmptyStmt | ExpressionStmt | IncDecStmt | Assignment | ShortVarDecl .The empty statement does nothing.
EmptyStmt = .With the exception of specific built-in functions, function and method calls and receive operations can appear in statement context. Such statements may be parenthesized.
ExpressionStmt = Expression .The following built-in functions are not permitted in statement context:
append cap complex imag len make new real
unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice unsafe.SliceData unsafe.String unsafe.StringDatah(x+y)
f.Close()
<-ch
(<-ch)
len("foo") // illegal if len is the built-in functionThe "++" and "--" statements increment or decrement their operands by the untyped constant 1. As with an assignment, the operand must be addressable or a map index expression.
IncDecStmt = Expression ( "++" | "--" ) .The following assignment statements are semantically equivalent:
IncDec statement Assignment
x++ x += 1
x-- x -= 1An assignment replaces the current value stored in a variable with a new value specified by an expression. An assignment statement may assign a single value to a single variable, or multiple values to a matching number of variables.
Assignment = ExpressionList assign_op ExpressionList .
ExpressionList = Expression { "," Expression } .Here assign_op can be:
= += -= |= ^= *= /= %= <<= >>= &= &^=Each left-hand side operand must be addressable, a map index expression, or (for = assignments only) the blank identifier. Operands may be parenthesized.
x = 1
*p = f()
a[i] = 23
(k) = <-ch // same as: k = <-chAn assignment operation x op= y where op is a binary arithmetic operator is equivalent to x = x op (y) but evaluates x only once. The op= construct is a single token. In assignment operations, both the left- and right-hand expression lists must contain exactly one single-valued expression, and the left-hand expression must not be the blank identifier.
a[i] <<= 2
i &^= 1<<nA 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 call, a channel or map operation, or a type assertion. The number of operands on the left hand side must match the number of values. For instance, if f is a function returning two values,
x, y = f()assigns the first value to x and the second to y. In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the nth expression on the right is assigned to the nth operand on the left:
one, two, three = '一', '二', '三'The blank identifier provides a way to ignore right-hand side values in an assignment:
_ = x // evaluate x but ignore it
x, _ = f() // evaluate f() but ignore second result valueThe assignment proceeds in two phases. First, the operands of index expressions and pointer indirections (including implicit pointer indirections in selectors) on the left and the expressions on the right are all evaluated in the usual order. Second, the assignments are carried out in left-to-right order.
a, b = b, a // exchange a and b
x := [1, 2, 3]
i := 0
i, x[i] = 1, 2 // set i = 1, x[0] = 2
i = 0
x[i], i = 2, 1 // set x[0] = 2, i = 1
x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
i = 2
x = [3, 5, 7]
for i, x[i] <- x { // set i, x[2] = 0, x[0]
break
}
// after this loop, i == 0 and x is [3, 5, 3]In assignments, each value must be assignable to the type of the operand to which it is assigned, with the following special cases:
- Any typed value may be assigned to the blank identifier.
- If an untyped constant is assigned to a variable of interface type or the blank identifier, the constant is first implicitly converted to its default type.
- If an untyped boolean value is assigned to a variable of interface type or the blank identifier, it is first implicitly converted to type bool.
"If" statements specify the conditional execution of two branches according to the value of a boolean expression. If the expression evaluates to true, the "if" branch is executed, otherwise, if present, the "else" branch is executed.
IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .if x > 1 {
x = 1
}The expression may be preceded by a simple statement, which executes before the expression is evaluated.
if x := f(); x < y {
return x
} else if x > z {
return z
} else {
return y
}A "for" statement specifies repeated execution of a block. There are three forms: The iteration may be controlled by a single 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 the boolean value true.
for a < b {
a *= 2
}A "for" statement with a ForClause is also controlled by its condition, but additionally it may specify an init and a post statement, such as an assignment, an increment or decrement statement. The init statement may be a short variable declaration, but the post statement must not.
ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
InitStmt = SimpleStmt .
PostStmt = SimpleStmt .for i := 0; i < 10; i++ {
f(i)
}If non-empty, the init statement is executed once before evaluating the condition for the first iteration; the post statement is executed after each execution of the block (and only if the block was executed). Any element of the ForClause may be empty but the semicolons are required unless there is only a condition. If the condition is absent, it is equivalent to the boolean value true.
for cond { S() } is the same as for ; cond ; { S() }
for { S() } is the same as for true { S() }Each iteration has its own separate declared variable (or variables) Go 1.22. The variable used by the first iteration is declared by the init statement. The variable used by each subsequent iteration is declared implicitly before executing the post statement and initialized to the value of the previous iteration's variable at that moment.
var prints []func()
for i := 0; i < 5; i++ {
prints = append(prints, func() { println(i) })
i++
}
for _, p := range prints {
p()
}prints
1
3
5
Prior to [Go 1.22], iterations share one set of variables instead of having their own separate variables. In that case, the example above prints
6
6
6
TODO
"Switch" statements provide multi-way execution. An expression or type 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. The switch expression is evaluated exactly once in a switch statement.
In an expression switch, the switch expression is evaluated and the case expressions, which need not be constants, are evaluated left-to-right and top-to-bottom; the first one that equals the switch expression triggers execution of the statements of the associated case; the other cases are skipped. If no case matches and there is a "default" case, its statements are executed. There can be at most one default case and it may appear anywhere in the "switch" statement. A missing switch expression is equivalent to the boolean value true.
ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
ExprCaseClause = ExprSwitchCase ":" StatementList .
ExprSwitchCase = "case" ExpressionList | "default" .If the switch expression evaluates to an untyped constant, it is first implicitly converted to its default type. The predeclared untyped value nil cannot be used as a switch expression. The switch expression type must be comparable.
If a case expression is untyped, it is first implicitly converted to the type of the switch expression. For each (possibly converted) case expression x and the value t of the switch expression, x == t must be a valid comparison.
In other words, the switch expression is treated as if it were used to declare and initialize a temporary variable t without explicit type; it is that value of t against which each case expression x is tested for equality.
In a case or default clause, the last non-empty statement may be a (possibly labeled) "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. A "fallthrough" statement may appear as the last statement of all but the last clause of an expression switch.
The switch expression may be preceded by a simple statement, which executes before the expression is evaluated.
switch tag {
default: s3()
case 0, 1, 2, 3: s1()
case 4, 5, 6, 7: s2()
}
switch x := f(); { // missing switch expression means "true"
case x < 0: return -x
default: return x
}
switch {
case x < y: f1()
case x < z: f2()
case x == 4: f3()
}Implementation restriction: A compiler may disallow multiple case expressions evaluating to the same constant. For instance, the current compilers disallow duplicate integer, floating point, or string constants in case expressions.
A type switch compares types rather than values. It is otherwise similar to an expression switch. It is marked by a special switch expression that has the form of a type assertion using the keyword type rather than an actual type:
switch x.(type) {
// cases
}Cases then match actual types T against the dynamic type of the expression x. As with type assertions, x must be of interface type, but not a type parameter, and each non-interface type T listed in a case must implement the type of x. The types listed in the cases of a type switch must all be different.
TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
TypeCaseClause = TypeSwitchCase ":" StatementList .
TypeSwitchCase = "case" TypeList | "default" .The TypeSwitchGuard may include a short variable declaration. When that form is used, the variable is declared at the end of the TypeSwitchCase in the implicit block of each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard.
Instead of a type, a case may use the predeclared identifier nil; that case is selected when the expression in the TypeSwitchGuard is a nil interface value. There may be at most one nil case.
Given an expression x of type any, the following type switch:
switch i := x.(type) {
case nil:
printString("x is nil") // type of i is type of x (any)
case int:
printInt(i) // type of i is int
case float64:
printFloat64(i) // type of i is float64
case func(int) float64:
printFunction(i) // type of i is func(int) float64
case bool, string:
printString("type is bool or string") // type of i is type of x (any)
default:
printString("don't know the type") // type of i is type of x (any)
}
could be rewritten:
v := x // x is evaluated exactly once
if v == nil {
i := v // type of i is type of x (any)
printString("x is nil")
} else if i, isInt := v.(int); isInt {
printInt(i) // type of i is int
} else if i, isFloat64 := v.(float64); isFloat64 {
printFloat64(i) // type of i is float64
} else if i, isFunc := v.(func(int) float64); isFunc {
printFunction(i) // type of i is func(int) float64
} else {
_, isBool := v.(bool)
_, isString := v.(string)
if isBool || isString {
i := v // type of i is type of x (any)
printString("type is bool or string")
} else {
i := v // type of i is type of x (any)
printString("don't know the type")
}
}The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated.
The "fallthrough" statement is not permitted in a type switch.
A labeled statement may be the target of a goto, break or continue statement.
LabeledStmt = Label ":" Statement .
Label = identifier .Error:
log.Panic("error encountered")A "break" statement terminates execution of the innermost "for" or "switch" statement within the same function.
BreakStmt = "break" [ Label ] .If there is a label, it must be that of an enclosing "for" or "switch" statement, and that is the one whose execution terminates.
OuterLoop:
for i = 0; i < n; i++ {
for j = 0; j < m; j++ {
switch a[i][j] {
case nil:
state = Error
break OuterLoop
case item:
state = Found
break OuterLoop
}
}
}A "continue" statement begins the next iteration of the innermost enclosing "for" loop by advancing control to the end of the loop block. The "for" loop must be within the same function.
ContinueStmt = "continue" [ Label ] .If there is a label, it must be that of an enclosing "for" statement, and that is the one whose execution advances.
RowLoop:
for y, row := range rows {
for x, data := range row {
if data == endOfRow {
continue RowLoop
}
row[x] = data + bias(x, y)
}
}A "fallthrough" statement transfers control to the first statement of the next case clause in an expression "switch" statement. It may be used only as the final non-empty statement in such a clause.
FallthroughStmt = "fallthrough" .A "goto" statement transfers control to the statement with the corresponding label within the same function.
GotoStmt = "goto" Label .goto ErrorExecuting the "goto" statement must not cause any variables to come into scope that were not already in scope at the point of the goto. For instance, this example:
goto L // BAD
v := 3
L:is erroneous because the jump to label L skips the creation of v.
A "goto" statement outside a block cannot jump to a label inside that block. For instance, this example:
if n%2 == 1 {
goto L1 // BAD
}
for n > 0 {
f()
n--
L1:
f()
n--
}is erroneous because the label L1 is inside the "for" statement's block but the goto is not.
A "return" statement in a function F terminates the execution of F, and optionally provides one or more result values. Any functions deferred by F are executed before F returns to its caller.
ReturnStmt = "return" [ ExpressionList ] .In a function without a result type, a "return" statement must not specify any result values.
func noResult() {
return
}There are three ways to return values from a function with a result type:
- The return value or values may be explicitly listed in the "return" statement. Each expression must be single-valued and assignable to the corresponding element of the function's result type.
func simpleF() int {
return 2
}
func complexF1() (re float64, im float64) {
return -7.0, -4.0
}- The expression list in the "return" statement may be a single call to a multi-valued function. The effect is as if each value returned from that function were assigned to a temporary variable with the type of the respective value, followed by a "return" statement listing these variables, at which point the rules of the previous case apply.
func complexF2() (re float64, im float64) {
return complexF1()
}- The expression list may be empty if the function's result type specifies names for its result parameters. The result parameters act as ordinary local variables and the function may assign values to them as necessary. The "return" statement returns the values of these variables.
func complexF3() (re float64, im float64) {
re = 7.0
im = 4.0
return
}
func (devnull) Write(p []byte) (n int, _ error) {
n = len(p)
return
}Regardless of how they are declared, all the result values are initialized to the zero values for their type upon entry to the function. A "return" statement that specifies results sets the result parameters before any deferred functions are executed.
Implementation restriction: A compiler may disallow an empty expression list in a "return" statement if a different entity (constant, type, or variable) with the same name as a result parameter is in scope at the place of the return.
func f(n int) (res int, err error) {
if _, err := f(n-1); err != nil {
return // invalid return statement: err is shadowed
}
return
}A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.
DeferStmt = "defer" Expression .The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for expression statements.
Each time a "defer" statement executes, the function value and parameters to the call are evaluated as usual and saved anew but the actual function is not invoked. Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. That is, if the surrounding function returns through an explicit return statement, deferred functions are executed after any result parameters are set by that return statement but before the function returns to its caller. If a deferred function value evaluates to nil, execution panics when the function is invoked, not when the "defer" statement is executed.
For instance, if the deferred function is a function literal and the surrounding function has named result parameters that are in scope within the literal, the deferred function may access and modify the result parameters before they are returned. If the deferred function has any return values, they are discarded when the function completes. (See also the section on handling panics.)
lock(l)
defer unlock(l) // unlocking happens before surrounding function returns
// f returns 42
func f() (result int) {
defer func() {
// result is accessed after it was set to 6 by the return statement
result *= 7
}()
return 6
}TODO
Built-in functions are predeclared. They are called like any other function but some of them accept a type instead of an expression as the first argument.
The built-in functions do not have standard Go types, so they can only appear in call expressions; they cannot be used as function values.
The built-in functions append and copy assist in common slice operations. For both functions, the result is independent of whether the memory referenced by the arguments overlaps.
The variadic function append appends zero or more values x to a slice s and returns the resulting slice of the same type as s. The core type of s must be a slice of type []E. The values x are passed to a parameter of type ...E and the respective parameter passing rules apply. As a special case, if the core type of s is []byte, append also accepts a second argument with core type bytestring followed by .... This form appends the bytes of the byte slice or string.
append(s S, x ...E) S // core type of S is []EIf the capacity of s is not large enough to fit the additional values, append allocates a new, sufficiently large underlying array that fits both the existing slice elements and the additional values. Otherwise, append re-uses the underlying array.
s0 := [0, 0]
s1 := append(s0, 2) // append a single element s1 is [0, 0, 2]
s2 := append(s1, 3, 5, 7) // append multiple elements s2 is [0, 0, 2, 3, 5, 7]
s3 := append(s2, s0...) // append a slice s3 is [0, 0, 2, 3, 5, 7, 0, 0]
s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 is [3, 5, 7, 2, 3, 5, 7, 0, 0]
var t []any
t = append(t, 42, 3.1415, "foo") // t is [42, 3.1415, "foo"]
var b []byte
b = append(b, "bar"...) // append string contents b is []byte("bar")The function copy copies slice elements from a source src to a destination dst and returns the number of elements copied. The core types of both arguments must be slices with identical element type. The number of elements copied is the minimum of len(src) and len(dst). As a special case, if the destination's core type is []byte, copy also accepts a source argument with core type bytestring. This form copies the bytes from the byte slice or string into the byte slice.
copy(dst, src []T) int
copy(dst []byte, src string) intExamples:
a := [0, 1, 2, 3, 4, 5, 6, 7]
s := make([]int, 6)
b := make([]byte, 5)
n1 := copy(s, a) // n1 == 6, s is []int{0, 1, 2, 3, 4, 5}
n2 := copy(s, s[2:]) // n2 == 4, s is []int{2, 3, 4, 5, 4, 5}
n3 := copy(b, "Hello, World!") // n3 == 5, b is []byte("Hello")The built-in function clear takes an argument of map or slice and deletes or zeroes out all elements.
Call Argument type Result
clear(m) map[K]T deletes all entries, resulting in an
empty map (len(m) == 0)
clear(s) []T sets all elements up to the length of
s to the zero value of T
If the map or slice is nil, clear is a no-op.
Three functions assemble and disassemble complex numbers. The built-in function complex constructs a complex value from a floating-point real and imaginary part, while real and imag extract the real and imaginary parts of a complex value.
complex(realPart, imaginaryPart floatT) complexT
real(complexT) floatT
imag(complexT) floatTThe type of the arguments and return value correspond. For complex, the two arguments must be of the same floating-point type and the return type is the complex type with the corresponding floating-point constituents: complex64 for float32 arguments, and complex128 for float64 arguments. If one of the arguments evaluates to an untyped constant, it is first implicitly converted to the type of the other argument. If both arguments evaluate to untyped constants, they must be non-complex numbers or their imaginary parts must be zero, and the return value of the function is an untyped complex constant.
For real and imag, the argument must be of complex type, and the return type is the corresponding floating-point type: float32 for a complex64 argument, and float64 for a complex128 argument. If the argument evaluates to an untyped constant, it must be a number, and the return value of the function is an untyped floating-point constant.
The real and imag functions together form the inverse of complex, so for a value z of a complex type Z, z == Z(complex(real(z), imag(z))).
If the operands of these functions are all constants, the return value is a constant.
var a = complex(2, -2) // complex128
const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
x := float32(math.Cos(math.Pi/2)) // float32
var c64 = complex(5, -x) // complex64
var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
_ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
var rl = real(c64) // float32
var im = imag(a) // float64
const c = imag(b) // untyped constant -1.4
_ = imag(3 << s) // illegal: 3 assumes complex type, cannot shiftThe built-in function delete removes the element with key k from a map m. The value k must be assignable to the key type of m.
delete(m, k) // remove element m[k] from map mIf the map m is nil or the element m[k] does not exist, delete is a no-op.
The built-in functions len and cap take arguments of various types and return a result of type int. The implementation guarantees that the result always fits into an int.
Call Argument type Result
len(s) string type string length in bytes
[n]T, *[n]T array length (== n)
[]T slice length
map[K]T map length (number of defined keys)
chan T number of elements queued in channel buffer
type parameter see below
cap(s) [n]T, *[n]T array length (== n)
[]T slice capacity
chan T channel buffer capacity
type parameter see belowThe capacity of a slice is the number of elements for which there is space allocated in the underlying array. At any time the following relationship holds:
0 <= len(s) <= cap(s)The length of a nil slice, map or channel is 0. The capacity of a nil slice or channel is 0.
The expression len(s) is constant if s is a string constant. The expressions len(s) and cap(s) are constants if the type of s is an array or pointer to an array and the expression s does not contain (non-constant) function calls; in this case s is not evaluated. Otherwise, invocations of len and cap are not constant and s is evaluated.
const (
c1 = imag(2i) // imag(2i) = 2.0 is a constant
c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
)
var z complex128Making slices and maps
The built-in function make takes a type T, optionally followed by a type-specific list of expressions. The core type of T must be a slice or map. It returns a value of type T (not *T). The memory is initialized as described in the section on initial values.
Call Core type Result
make(T, n) slice slice of type T with length n and capacity n
make(T, n, m) slice slice of type T with length n and capacity m
make(T) map map of type T
make(T, n) map map of type T with initial space for approximately n elementsEach of the size arguments n and m must be of integer type, have a type set containing only integer types, or be an untyped constant. A constant size argument must be non-negative and representable by a value of type int; if it is an untyped constant it is given type int. If both n and m are provided and are constant, then n must be no larger than m. For slices, if n is negative or larger than m at run time, a run-time panic occurs.
s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
s := make([]int, 10, 0) // illegal: len(s) > cap(s)
m := make(map[string]int, 100) // map with initial space for approximately 100 elementsCalling make with a map type and size hint n will create a map with initial space to hold n map elements. The precise behavior is implementation-dependent.
The built-in function new takes a type T, allocates storage for a variable of that type at run time, and returns a value of type *T pointing to it. The variable is initialized as described in the section on initial values.
new(T)For instance
new(int)allocates storage for a variable of type int, initializes it 0, and returns a value of type *int containing the address of the location.
The built-in functions min and max compute the smallest—or largest, respectively—value of a fixed number of arguments of ordered types. There must be at least one argument.
The same type rules as for operators apply: for ordered arguments x and y, min(x, y) is valid if x + y is valid, and the type of min(x, y) is the type of x + y (and similarly for max). If all arguments are constant, the result is constant.
var x, y int
m := min(x) // m == x
m := min(x, y) // m is the smaller of x and y
m := max(x, y, 10) // m is the larger of x and y but at least 10
c := max(1, 2.0, 10) // c == 10.0 (floating-point kind)
f := max(0, float32(x)) // type of f is float32
var s []string
_ = min(s...) // invalid: slice arguments are not permitted
t := max("", "foo", "bar") // t == "foo" (string kind)For numeric arguments, assuming all NaNs are equal, min and max are commutative and associative:
min(x, y) == min(y, x)
min(x, y, z) == min(min(x, y), z) == min(x, min(y, z))
For floating-point arguments negative zero, NaN, and infinity the following rules apply:
x y min(x, y) max(x, y)
-0.0 0.0 -0.0 0.0 // negative zero is smaller than (non-negative) zero
-Inf y -Inf y // negative infinity is smaller than any other number
+Inf y y +Inf // positive infinity is larger than any other number
NaN y NaN NaN // if any argument is a NaN, the result is a NaNFor string arguments the result for min is the first argument with the smallest (or for max, largest) value, compared lexically byte-wise:
min(x, y) == if x <= y then x else y
min(x, y, z) == min(min(x, y), z)Two built-in functions, panic and recover, assist in reporting and handling run-time panics and program-defined error conditions.
func panic(any)
func recover() anyWhile executing a function F, an explicit call to panic or a run-time panic terminates the execution of F. Any functions deferred by F are then executed as usual. Next, any deferred functions run by F's caller are run, and so on up to any deferred by the top-level function in the executing goroutine. At that point, the program is terminated and the error condition is reported, including the value of the argument to panic. This termination sequence is called panicking.
panic(42)
panic("unreachable")
panic(Error("cannot parse"))The recover function allows a program to manage behavior of a panicking goroutine. Suppose a function G defers a function D that calls recover and a panic occurs in a function on the same goroutine in which G is executing. When the running of deferred functions reaches D, the return value of D's call to recover will be the value passed to the call of panic. If D returns normally, without starting a new panic, the panicking sequence stops. In that case, the state of functions called between G and the call to panic is discarded, and normal execution resumes. Any functions deferred by G before D are then run and G's execution terminates by returning to its caller.
The return value of recover is nil when the goroutine is not panicking or recover was not called directly by a deferred function. Conversely, if a goroutine is panicking and recover was called directly by a deferred function, the return value of recover is guaranteed not to be nil. To ensure this, calling panic with a nil interface value (or an untyped nil) causes a run-time panic.
The protect function in the example below invokes the function argument g and protects callers from run-time panics raised by g.
func protect(g func()) {
defer func() {
log.Println("done") // Println executes normally even if there is a panic
if x := recover(); x != nil {
log.Printf("run time panic: %v", x)
}
}()
log.Println("start")
g()
}print
printf
println
...A declaration binds a non-blank identifier to a constant, type, variable, function, label, or package. Every identifier in a program must be declared. No identifier may be declared twice in the same block, and no identifier may be declared in both the file and package block.
The blank identifier may be used like any other identifier in a declaration, but it does not introduce a binding and thus is not declared. In the package block, the identifier init may only be used for init function declarations, and like the blank identifier it does not introduce a new binding.
Declaration = ConstDecl | TypeDecl | VarDecl .
TopLevelDecl = Declaration | FunctionDecl .The scope of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, label, or package.
Go+ is lexically scoped using blocks:
- The scope of a predeclared identifier is the universe block.
- The scope of an identifier denoting a constant, type, variable, or function declared at top level (outside any function) is the package block.
- The scope of the package name of an imported package is the file block of the file containing the import declaration.
- The scope of an identifier denoting function parameter, or result variable is the function body.
- The scope of a constant or variable identifier declared inside a function begins at the end of the
ConstSpecorVarSpec(ShortVarDeclfor short variable declarations) and ends at the end of the innermost containing block.
An identifier declared in a block may be redeclared in an inner block. While the identifier of the inner declaration is in scope, it denotes the entity declared by the inner declaration.
The package clause is not a declaration; the package name does not appear in any scope. Its purpose is to identify the files belonging to the same package and to specify the default package name for import declarations.
Labels are declared by labeled statements and are used in the "break", "continue", and "goto" statements. It is illegal to define a label that is never used. In contrast to other identifiers, labels are not block scoped and do not conflict with identifiers that are not labels. The scope of a label is the body of the function in which it is declared and excludes the body of any nested function.
The blank identifier is represented by the underscore character _. It serves as an anonymous placeholder instead of a regular (non-blank) identifier and has special meaning in declarations, as an operand, and in assignment statements.
The following identifiers are implicitly declared in the universe block:
Types:
any bigint bigrat bool byte comparable
complex64 complex128 error float32 float64
int int8 int16 int32 int64 rune string
uint uint8 uint16 uint32 uint64 uintptr
Constants:
true false iota
Zero value:
nil
Functions:
append cap clear close complex copy delete echo imag len
make max min new panic print printf println real recover // TODO(xsw): moreAn identifier may be exported to permit access to it from another package. An identifier is exported if both:
- the first character of the identifier's name is a Unicode uppercase letter (Unicode character category Lu); and
- the identifier is declared in the package block or it is a field name or method name.
All other identifiers are not exported.
Given a set of identifiers, an identifier is called unique if it is different from every other in the set. Two identifiers are different if they are spelled differently, or if they appear in different packages and are not exported. Otherwise, they are the same.
A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of constant expressions. The number of identifiers must be equal to the number of expressions, and the nth identifier on the left is bound to the value of the nth expression on the right.
ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .If the type is present, all constants take the type specified, and the expressions must be assignable to that type, which must not be a type parameter. If the type is omitted, the constants take the individual types of the corresponding expressions. If the expression values are untyped constants, the declared constants remain untyped and the constant identifiers denote the constant values. For instance, if the expression is a floating-point literal, the constant identifier denotes a floating-point constant, even if the literal's fractional part is zero.
const Pi float64 = 3.14159265358979323846
const zero = 0.0 // untyped floating-point constant
const (
size int64 = 1024
eof = -1 // untyped integer constant
)
const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
const u, v float32 = 0, 3 // u = 0.0, v = 3.0Within a parenthesized const declaration list the expression list may be omitted from any but the first ConstSpec. Such an empty list is equivalent to the textual substitution of the first preceding non-empty expression list and its type if any. Omitting the list of expressions is therefore equivalent to repeating the previous list. The number of identifiers must be equal to the number of expressions in the previous list. Together with the iota constant generator this mechanism permits light-weight declaration of sequential values:
const (
Sunday = iota
Monday
Tuesday
Wednesday
Thursday
Friday
Partyday
numberOfDays // this constant is not exported
)Within a constant declaration, the predeclared identifier iota represents successive untyped integer constants. Its value is the index of the respective ConstSpec in that constant declaration, starting at zero. It can be used to construct a set of related constants:
const (
c0 = iota // c0 == 0
c1 = iota // c1 == 1
c2 = iota // c2 == 2
)
const (
a = 1 << iota // a == 1 (iota == 0)
b = 1 << iota // b == 2 (iota == 1)
c = 3 // c == 3 (iota == 2, unused)
d = 1 << iota // d == 8 (iota == 3)
)
const (
u = iota * 42 // u == 0 (untyped integer constant)
v float64 = iota * 42 // v == 42.0 (float64 constant)
w = iota * 42 // w == 84 (untyped integer constant)
)
const x = iota // x == 0
const y = iota // y == 0By definition, multiple uses of iota in the same ConstSpec all have the same value:
const (
bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
_, _ // (iota == 2, unused)
bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
)This last example exploits the implicit repetition of the last non-empty expression list.
A type declaration binds an identifier, the type name, to a type. Type declarations come in two forms: alias declarations and type definitions.
TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec = AliasDecl | TypeDef .An alias declaration binds an identifier to the given type.
AliasDecl = identifier "=" Type .Within the scope of the identifier, it serves as an alias for the type.
type (
nodeList = []*Node // nodeList and []*Node are identical types
Polar = polar // Polar and polar denote identical types
)A type definition creates a new, distinct type with the same underlying type and operations as the given type and binds an identifier, the type name, to it.
TypeDef = identifier Type .The new type is called a defined type. It is different from any other type, including the type it is created from.
type (
Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
polar Point // polar and Point denote different types
)
type TreeNode struct {
left, right *TreeNode
value any
}
type Block interface {
BlockSize() int
Encrypt(src, dst []byte)
Decrypt(src, dst []byte)
}Type definitions may be used to define different boolean, numeric, or string types:
type TimeZone int
const (
EST TimeZone = -(5 + iota)
CST
MST
PST
)A variable declaration creates one or more variables, binds corresponding identifiers to them, and gives each a type and an initial value.
VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .var i int
var U, V, W float64
var k = 0
var x, y float32 = -1, -2
var (
i int
u, v, s = 2.0, 3.0, "bar"
)
var re, im = complexSqrt(-1)
var _, found = entries[name] // map lookup; only interested in "found"If a list of expressions is given, the variables are initialized with the expressions following the rules for assignment statements. Otherwise, each variable is initialized to its zero value.
If a type is present, each variable is given that type. Otherwise, each variable is given the type of the corresponding initialization value in the assignment. If that value is an untyped constant, it is first implicitly converted to its default type; if it is an untyped boolean value, it is first implicitly converted to type bool. The predeclared value nil cannot be used to initialize a variable with no explicit type.
var d = math.Sin(0.5) // d is float64
var i = 42 // i is int
var t, ok = x.(T) // t is T, ok is bool
var n = nil // illegalImplementation restriction: A compiler may make it illegal to declare a variable inside a function body if the variable is never used.
In a function body, variables do not need to be explicitly defined.
d := math.Sin(0.5) // d is float64
i := 42 // i is int
t, ok := x.(T) // t is T, ok is boolSee short variable declarations.
A function declaration binds an identifier, the function name, to a function.
FunctionDecl = "func" FunctionName Signature [ FunctionBody ] .
FunctionName = identifier .
FunctionBody = Block .If the function's signature declares result parameters, the function body's statement list must end in a terminating statement.
func IndexRune(s string, r rune) int {
for i, c := range s {
if c == r {
return i
}
}
// invalid: missing return statement
}Go+ programs are constructed by linking together packages. A package in turn is constructed from one or more source files that together declare constants, types, variables and functions belonging to the package and which are accessible in all files of the same package. Those elements may be exported and used in another package.
Each source file consists of a package clause defining the package to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations of functions, types, variables, and constants.
SourceFile = [ PackageClause ";" ] { ImportDecl ";" } { TopLevelDecl ";" } .A package clause begins each source file and defines the package to which the file belongs.
PackageClause = "package" PackageName .
PackageName = identifier .The PackageName must not be the blank identifier.
package mathAn import declaration states that the source file containing the declaration depends on functionality of the imported package (Program initialization and execution) and enables access to exported identifiers of that package. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported.
ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
ImportSpec = [ "." | PackageName ] ImportPath .
ImportPath = string_lit .The PackageName is used in qualified identifiers to access exported identifiers of the package within the importing source file. It is declared in the file block. If the PackageName is omitted, it defaults to the identifier specified in the package clause of the imported package. If an explicit period (.) appears instead of a name, all the package's exported identifiers declared in that package's package block will be declared in the importing source file's file block and must be accessed without a qualifier.
The interpretation of the ImportPath is implementation-dependent but it is typically a substring of the full file name of the compiled package and may be relative to a repository of installed packages.
Implementation restriction: A compiler may restrict ImportPaths to non-empty strings using only characters belonging to Unicode's L, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters !"#$%&'()*,:;<=>?[]^`{|} and the Unicode replacement character U+FFFD.
Consider a compiled a package containing the package clause package math, which exports function Sin, and installed the compiled package in the file identified by "lib/math". This table illustrates how Sin is accessed in files that import the package after the various types of import declaration.
Import declaration Local name of Sin
import "lib/math" math.Sin
import m "lib/math" m.Sin
import . "lib/math" SinAn import declaration declares a dependency relation between the importing and imported package. It is illegal for a package to import itself, directly or indirectly, or to directly import a package without referring to any of its exported identifiers. To import a package solely for its side-effects (initialization), use the blank identifier as explicit package name:
import _ "lib/math"Here is a complete Go+ package that implements XXX.
TODOWhen storage is allocated for a variable, either through a declaration or a call of new, or when a new value is created, either through a composite literal or a call of make, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the zero value for its type: false for booleans, 0 for numeric types, "" for strings, and nil for pointers, functions, interfaces, slices, and maps. This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified.
These two simple declarations are equivalent:
var i int
var i int = 0TODO
Within a package, package-level variable initialization proceeds stepwise, with each step selecting the variable earliest in declaration order which has no dependencies on uninitialized variables.
More precisely, a package-level variable is considered ready for initialization if it is not yet initialized and either has no initialization expression or its initialization expression has no dependencies on uninitialized variables. Initialization proceeds by repeatedly initializing the next package-level variable that is earliest in declaration order and ready for initialization, until there are no variables ready for initialization.
If any variables are still uninitialized when this process ends, those variables are part of one or more initialization cycles, and the program is not valid.
Multiple variables on the left-hand side of a variable declaration initialized by single (multi-valued) expression on the right-hand side are initialized together: If any of the variables on the left-hand side is initialized, all those variables are initialized in the same step.
var x = a
var a, b = f() // a and b are initialized together, before x is initializedFor the purpose of package initialization, blank variables are treated like any other variables in declarations.
The declaration order of variables declared in multiple files is determined by the order in which the files are presented to the compiler: Variables declared in the first file are declared before any of the variables declared in the second file, and so on. To ensure reproducible initialization behavior, build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler.
Dependency analysis does not rely on the actual values of the variables, only on lexical references to them in the source, analyzed transitively. For instance, if a variable x's initialization expression refers to a function whose body refers to variable y then x depends on y. Specifically:
- A reference to a variable or function is an identifier denoting that variable or function.
- A reference to a method m is a method value or method expression of the form
t.m, where the (static) type of t is not an interface type, and the methodmis in the method set oft. It is immaterial whether the resulting function valuet.mis invoked. - A variable, function, or method x depends on a variable y if x's initialization expression or body (for functions and methods) contains a reference to y or to a function or method that depends on y.
For example, given the declarations
var (
a = c + b // == 9
b = f() // == 4
c = f() // == 5
d = 3 // == 5 after initialization has finished
)
func f() int {
d++
return d
}the initialization order is d, b, c, a. Note that the order of subexpressions in initialization expressions is irrelevant: a = c + b and a = b + c result in the same initialization order in this example.
Dependency analysis is performed per package; only references referring to variables, functions, and (non-interface) methods declared in the current package are considered. If other, hidden, data dependencies exists between variables, the initialization order between those variables is unspecified.
For instance, given the declarations
var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
var _ = sideEffect() // unrelated to x, a, or b
var a = b
var b = 42
type I interface { ab() []int }
type T struct{}
func (T) ab() []int { return []int{a, b} }the variable a will be initialized after b but whether x is initialized before b, between b and a, or after a, and thus also the moment at which sideEffect() is called (before or after x is initialized) is not specified.
Variables may also be initialized using functions named init declared in the package block, with no arguments and no result parameters.
func init() { … }Multiple such functions may be defined per package, even within a single source file. In the package block, the init identifier can be used only to declare init functions, yet the identifier itself is not declared. Thus init functions cannot be referred to from anywhere in a program.
The entire package is initialized by assigning initial values to all its package-level variables followed by calling all init functions in the order they appear in the source, possibly in multiple files, as presented to the compiler.
The packages of a complete program are initialized stepwise, one package at a time. If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package, the imported package will be initialized only once. The importing of packages, by construction, guarantees that there can be no cyclic initialization dependencies. More precisely:
Given the list of all packages, sorted by import path, in each step the first uninitialized package in the list for which all imported packages (if any) are already initialized is initialized. This step is repeated until all packages are initialized.
Package initialization—variable initialization and the invocation of init functions—happens in a single goroutine, sequentially, one package at a time. An init function may launch other goroutines, which can run concurrently with the initialization code. However, initialization always sequences the init functions: it will not invoke the next one until the previous one has returned.
A complete program is created by linking a single, unimported package called the main package with all the packages it imports, transitively. The main package must have package name main and declare a function main that takes no arguments and returns no value.
func main() { … }Program execution begins by initializing the program and then invoking the function main in package main. When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.
Execution errors such as attempting to index an array out of bounds trigger a run-time panic equivalent to a call of the built-in function panic with a value of the implementation-defined interface type runtime.Error. That type satisfies the predeclared interface type error. The exact error values that represent distinct run-time error conditions are unspecified.
package runtime
type Error interface {
error
// and perhaps other methods
}The built-in package unsafe, known to the compiler and accessible through the import path "unsafe", provides facilities for low-level programming including operations that violate the type system. A package using unsafe must be vetted manually for type safety and may not be portable. The package provides the following interface:
package unsafe
type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
type Pointer *ArbitraryType
func Alignof(variable ArbitraryType) uintptr
func Offsetof(selector ArbitraryType) uintptr
func Sizeof(variable ArbitraryType) uintptr
type IntegerType int // shorthand for an integer type; it is not a real type
func Add(ptr Pointer, len IntegerType) Pointer
func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
func SliceData(slice []ArbitraryType) *ArbitraryType
func String(ptr *byte, len IntegerType) string
func StringData(str string) *byteA Pointer is a pointer type but a Pointer value may not be dereferenced. Any pointer or value of core type uintptr can be converted to a type of core type Pointer and vice versa. The effect of converting between Pointer and uintptr is implementation-defined.
var f float64
bits = *(*uint64)(unsafe.Pointer(&f))
type ptr unsafe.Pointer
bits = *(*uint64)(ptr(&f))
func f[P ~*B, B any](p P) uintptr {
return uintptr(unsafe.Pointer(p))
}
var p ptr = nilThe functions Alignof and Sizeof take an expression x of any type and return the alignment or size, respectively, of a hypothetical variable v as if v was declared via var v = x.
The function Offsetof takes a (possibly parenthesized) selector s.f, denoting a field f of the struct denoted by s or *s, and returns the field offset in bytes relative to the struct's address. If f is an embedded field, it must be reachable without pointer indirections through fields of the struct. For a struct s with field f:
uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))Computer architectures may require memory addresses to be aligned; that is, for addresses of a variable to be a multiple of a factor, the variable's type's alignment. The function Alignof takes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variable x:
uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0A (variable of) type T has variable size if T is a type parameter, or if it is an array or struct type containing elements or fields of variable size. Otherwise the size is constant. Calls to Alignof, Offsetof, and Sizeof are compile-time constant expressions of type uintptr if their arguments (or the struct s in the selector expression s.f for Offsetof) are types of constant size.
The function Add adds len to ptr and returns the updated pointer unsafe.Pointer(uintptr(ptr) + uintptr(len)). The len argument must be of integer type or an untyped constant. A constant len argument must be representable by a value of type int; if it is an untyped constant it is given type int. The rules for valid uses of Pointer still apply.
The function Slice returns a slice whose underlying array starts at ptr and whose length and capacity are len. Slice(ptr, len) is equivalent to
(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]except that, as a special case, if ptr is nil and len is zero, Slice returns nil.
The len argument must be of integer type or an untyped constant. A constant len argument must be non-negative and representable by a value of type int; if it is an untyped constant it is given type int. At run time, if len is negative, or if ptr is nil and len is not zero, a run-time panic occurs.
The function SliceData returns a pointer to the underlying array of the slice argument. If the slice's capacity cap(slice) is not zero, that pointer is &slice[:1][0]. If slice is nil, the result is nil. Otherwise it is a non-nil pointer to an unspecified memory address.
The function String returns a string value whose underlying bytes start at ptr and whose length is len. The same requirements apply to the ptr and len argument as in the function Slice. If len is zero, the result is the empty string "". Since Go+ strings are immutable, the bytes passed to String must not be modified afterwards.
The function StringData returns a pointer to the underlying bytes of the str argument. For an empty string the return value is unspecified, and may be nil. Since Go+ strings are immutable, the bytes returned by StringData must not be modified.
For the numeric types, the following sizes are guaranteed:
type size in bytes
byte, uint8, int8 1
uint16, int16 2
uint32, int32, float32 4
uint64, int64, float64, complex64 8
complex128 16The following minimal alignment properties are guaranteed:
- For a variable
xof any type:unsafe.Alignof(x)is at least 1. - For a variable
xof struct type:unsafe.Alignof(x)is the largest of all the valuesunsafe.Alignof(x.f)for each fieldfofx, but at least 1. - For a variable
xof array type:unsafe.Alignof(x)is the same as the alignment of a variable of the array's element type.
A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.
TODO
The type unification rules describe if and how two types unify. The precise details are relevant for Go+ implementations, affect the specifics of error messages (such as whether a compiler reports a type inference or other error), and may explain why type inference fails in unusual code situations. But by and large these rules can be ignored when writing Go code: type inference is designed to mostly "work as expected", and the unification rules are fine-tuned accordingly.
Type unification is controlled by a matching mode, which may be exact or loose. As unification recursively descends a composite type structure, the matching mode used for elements of the type, the element matching mode, remains the same as the matching mode except when two types are unified for assignability (≡A): in this case, the matching mode is loose at the top level but then changes to exact for element types, reflecting the fact that types don't have to be identical to be assignable.
Two types that are not bound type parameters unify exactly if any of following conditions is true:
- Both types are identical.
- Both types have identical structure and their element types unify exactly.
- Exactly one type is an unbound type parameter with a core type, and that core type unifies with the other type per the unification rules for ≡A (loose unification at the top level and exact unification for element types).
If both types are bound type parameters, they unify per the given matching modes if:
- Both type parameters are identical.
- At most one of the type parameters has a known type argument. In this case, the type parameters are
joined: they both stand for the same type argument. If neither type parameter has a known type argument yet, a future type argument inferred for one the type parameters is simultaneously inferred for both of them. - Both type parameters have a known type argument and the type arguments unify per the given matching modes.
A single bound type parameter P and another type T unify per the given matching modes if:
- P doesn't have a known type argument. In this case, T is inferred as the type argument for P.
- P does have a known type argument A, A and T unify per the given matching modes, and one of the following conditions is true:
- Both A and T are interface types: In this case, if both A and T are also defined types, they must be identical. Otherwise, if neither of them is a defined type, they must have the same number of methods (unification of A and T already established that the methods match).
- Neither A nor T are interface types: In this case, if T is a defined type, T replaces A as the inferred type argument for P.
Finally, two types that are not bound type parameters unify loosely (and per the element matching mode) if:
- Both types unify exactly.
- One type is a defined type, the other type is a type literal, but not an interface, and their underlying types unify per the element matching mode.
- Both types are interfaces (but not type parameters) with identical type terms, both or neither embed the predeclared type comparable, corresponding method types unify exactly, and the method set of one of the interfaces is a subset of the method set of the other interface.
- Only one type is an interface (but not a type parameter), corresponding methods of the two types unify per the element matching mode, and the method set of the interface is a subset of the method set of the other type.
- Both types have the same structure and their element types unify per the element matching mode.