2 "Title": "The Go Programming Language Specification",
3 "Subtitle": "Version of January 19, 2022",
7 <h2 id="Introduction">Introduction</h2>
10 This is the reference manual for the Go programming language.
11 The pre-Go1.18 version, without generics, can be found
12 <a href="/doc/go1.17_spec.html">here</a>.
13 For more information and other documents, see <a href="/">golang.org</a>.
17 Go is a general-purpose language designed with systems programming
18 in mind. It is strongly typed and garbage-collected and has explicit
19 support for concurrent programming. Programs are constructed from
20 <i>packages</i>, whose properties allow efficient management of
25 The syntax is compact and simple to parse, allowing for easy analysis
26 by automatic tools such as integrated development environments.
29 <h2 id="Notation">Notation</h2>
31 The syntax is specified using a
32 <a href="https://en.wikipedia.org/wiki/Wirth_syntax_notation">variant</a>
33 of Extended Backus-Naur Form (EBNF):
37 Syntax = { Production } .
38 Production = production_name "=" [ Expression ] "." .
39 Expression = Term { "|" Term } .
40 Term = Factor { Factor } .
41 Factor = production_name | token [ "…" token ] | Group | Option | Repetition .
42 Group = "(" Expression ")" .
43 Option = "[" Expression "]" .
44 Repetition = "{" Expression "}" .
48 Productions are expressions constructed from terms and the following
49 operators, in increasing precedence:
54 [] option (0 or 1 times)
55 {} repetition (0 to n times)
59 Lowercase production names are used to identify lexical (terminal) tokens.
60 Non-terminals are in CamelCase. Lexical tokens are enclosed in
61 double quotes <code>""</code> or back quotes <code>``</code>.
65 The form <code>a … b</code> represents the set of characters from
66 <code>a</code> through <code>b</code> as alternatives. The horizontal
67 ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various
68 enumerations or code snippets that are not further specified. The character <code>…</code>
69 (as opposed to the three characters <code>...</code>) is not a token of the Go
73 <h2 id="Source_code_representation">Source code representation</h2>
76 Source code is Unicode text encoded in
77 <a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
78 canonicalized, so a single accented code point is distinct from the
79 same character constructed from combining an accent and a letter;
80 those are treated as two code points. For simplicity, this document
81 will use the unqualified term <i>character</i> to refer to a Unicode code point
85 Each code point is distinct; for instance, uppercase and lowercase letters
86 are different characters.
89 Implementation restriction: For compatibility with other tools, a
90 compiler may disallow the NUL character (U+0000) in the source text.
93 Implementation restriction: For compatibility with other tools, a
94 compiler may ignore a UTF-8-encoded byte order mark
95 (U+FEFF) if it is the first Unicode code point in the source text.
96 A byte order mark may be disallowed anywhere else in the source.
99 <h3 id="Characters">Characters</h3>
102 The following terms are used to denote specific Unicode character categories:
105 newline = /* the Unicode code point U+000A */ .
106 unicode_char = /* an arbitrary Unicode code point except newline */ .
107 unicode_letter = /* a Unicode code point categorized as "Letter" */ .
108 unicode_digit = /* a Unicode code point categorized as "Number, decimal digit" */ .
112 In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
113 Section 4.5 "General Category" defines a set of character categories.
114 Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
115 as Unicode letters, and those in the Number category Nd as Unicode digits.
118 <h3 id="Letters_and_digits">Letters and digits</h3>
121 The underscore character <code>_</code> (U+005F) is considered a lowercase letter.
124 letter = unicode_letter | "_" .
125 decimal_digit = "0" … "9" .
126 binary_digit = "0" | "1" .
127 octal_digit = "0" … "7" .
128 hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
131 <h2 id="Lexical_elements">Lexical elements</h2>
133 <h3 id="Comments">Comments</h3>
136 Comments serve as program documentation. There are two forms:
141 <i>Line comments</i> start with the character sequence <code>//</code>
142 and stop at the end of the line.
145 <i>General comments</i> start with the character sequence <code>/*</code>
146 and stop with the first subsequent character sequence <code>*/</code>.
151 A comment cannot start inside a <a href="#Rune_literals">rune</a> or
152 <a href="#String_literals">string literal</a>, or inside a comment.
153 A general comment containing no newlines acts like a space.
154 Any other comment acts like a newline.
157 <h3 id="Tokens">Tokens</h3>
160 Tokens form the vocabulary of the Go language.
161 There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators
162 and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from
163 spaces (U+0020), horizontal tabs (U+0009),
164 carriage returns (U+000D), and newlines (U+000A),
165 is ignored except as it separates tokens
166 that would otherwise combine into a single token. Also, a newline or end of file
167 may trigger the insertion of a <a href="#Semicolons">semicolon</a>.
168 While breaking the input into tokens,
169 the next token is the longest sequence of characters that form a
173 <h3 id="Semicolons">Semicolons</h3>
176 The formal syntax uses semicolons <code>";"</code> as terminators in
177 a number of productions. Go programs may omit most of these semicolons
178 using the following two rules:
183 When the input is broken into tokens, a semicolon is automatically inserted
184 into the token stream immediately after a line's final token if that token is
187 <a href="#Identifiers">identifier</a>
191 <a href="#Integer_literals">integer</a>,
192 <a href="#Floating-point_literals">floating-point</a>,
193 <a href="#Imaginary_literals">imaginary</a>,
194 <a href="#Rune_literals">rune</a>, or
195 <a href="#String_literals">string</a> literal
198 <li>one of the <a href="#Keywords">keywords</a>
200 <code>continue</code>,
201 <code>fallthrough</code>, or
205 <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a>
216 To allow complex statements to occupy a single line, a semicolon
217 may be omitted before a closing <code>")"</code> or <code>"}"</code>.
222 To reflect idiomatic use, code examples in this document elide semicolons
227 <h3 id="Identifiers">Identifiers</h3>
230 Identifiers name program entities such as variables and types.
231 An identifier is a sequence of one or more letters and digits.
232 The first character in an identifier must be a letter.
235 identifier = letter { letter | unicode_digit } .
240 ThisVariableIsExported
245 Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>.
249 <h3 id="Keywords">Keywords</h3>
252 The following keywords are reserved and may not be used as identifiers.
254 <pre class="grammar">
255 break default func interface select
256 case defer go map struct
257 chan else goto package switch
258 const fallthrough if range type
259 continue for import return var
262 <h3 id="Operators_and_punctuation">Operators and punctuation</h3>
265 The following character sequences represent <a href="#Operators">operators</a>
266 (including <a href="#Assignment_statements">assignment operators</a>) and punctuation:
268 <pre class="grammar">
269 + & += &= && == != ( )
270 - | -= |= || < <= [ ]
271 * ^ *= ^= <- > >= { }
272 / << /= <<= ++ = := , ;
273 % >> %= >>= -- ! ... . :
277 <h3 id="Integer_literals">Integer literals</h3>
280 An integer literal is a sequence of digits representing an
281 <a href="#Constants">integer constant</a>.
282 An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code>
283 for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal,
284 and <code>0x</code> or <code>0X</code> for hexadecimal.
285 A single <code>0</code> is considered a decimal zero.
286 In hexadecimal literals, letters <code>a</code> through <code>f</code>
287 and <code>A</code> through <code>F</code> represent values 10 through 15.
291 For readability, an underscore character <code>_</code> may appear after
292 a base prefix or between successive digits; such underscores do not change
296 int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
297 decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
298 binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
299 octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
300 hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
302 decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
303 binary_digits = binary_digit { [ "_" ] binary_digit } .
304 octal_digits = octal_digit { [ "_" ] octal_digit } .
305 hex_digits = hex_digit { [ "_" ] hex_digit } .
314 0O600 // second character is capital letter 'O'
318 170141183460469231731687303715884105727
319 170_141183_460469_231731_687303_715884_105727
321 _42 // an identifier, not an integer literal
322 42_ // invalid: _ must separate successive digits
323 4__2 // invalid: only one _ at a time
324 0_xBadFace // invalid: _ must separate successive digits
328 <h3 id="Floating-point_literals">Floating-point literals</h3>
331 A floating-point literal is a decimal or hexadecimal representation of a
332 <a href="#Constants">floating-point constant</a>.
336 A decimal floating-point literal consists of an integer part (decimal digits),
337 a decimal point, a fractional part (decimal digits), and an exponent part
338 (<code>e</code> or <code>E</code> followed by an optional sign and decimal digits).
339 One of the integer part or the fractional part may be elided; one of the decimal point
340 or the exponent part may be elided.
341 An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.
345 A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code>
346 prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
347 and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits).
348 One of the integer part or the fractional part may be elided; the radix point may be elided as well,
349 but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
350 An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.
354 For readability, an underscore character <code>_</code> may appear after
355 a base prefix or between successive digits; such underscores do not change
360 float_lit = decimal_float_lit | hex_float_lit .
362 decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
363 decimal_digits decimal_exponent |
364 "." decimal_digits [ decimal_exponent ] .
365 decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
367 hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
368 hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
371 hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
389 0x1.Fp+0 // == 1.9375
391 0X_1FFFP-16 // == 0.1249847412109375
392 0x15e-2 // == 0x15e - 2 (integer subtraction)
394 0x.p1 // invalid: mantissa has no digits
395 1p-2 // invalid: p exponent requires hexadecimal mantissa
396 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent
397 1_.5 // invalid: _ must separate successive digits
398 1._5 // invalid: _ must separate successive digits
399 1.5_e1 // invalid: _ must separate successive digits
400 1.5e_1 // invalid: _ must separate successive digits
401 1.5e1_ // invalid: _ must separate successive digits
405 <h3 id="Imaginary_literals">Imaginary literals</h3>
408 An imaginary literal represents the imaginary part of a
409 <a href="#Constants">complex constant</a>.
410 It consists of an <a href="#Integer_literals">integer</a> or
411 <a href="#Floating-point_literals">floating-point</a> literal
412 followed by the lowercase letter <code>i</code>.
413 The value of an imaginary literal is the value of the respective
414 integer or floating-point literal multiplied by the imaginary unit <i>i</i>.
418 imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
422 For backward compatibility, an imaginary literal's integer part consisting
423 entirely of decimal digits (and possibly underscores) is considered a decimal
424 integer, even if it starts with a leading <code>0</code>.
429 0123i // == 123i for backward-compatibility
430 0o123i // == 0o123 * 1i == 83i
431 0xabci // == 0xabc * 1i == 2748i
439 0x1p-2i // == 0x1p-2 * 1i == 0.25i
443 <h3 id="Rune_literals">Rune literals</h3>
446 A rune literal represents a <a href="#Constants">rune constant</a>,
447 an integer value identifying a Unicode code point.
448 A rune literal is expressed as one or more characters enclosed in single quotes,
449 as in <code>'x'</code> or <code>'\n'</code>.
450 Within the quotes, any character may appear except newline and unescaped single
451 quote. A single quoted character represents the Unicode value
452 of the character itself,
453 while multi-character sequences beginning with a backslash encode
454 values in various formats.
458 The simplest form represents the single character within the quotes;
459 since Go source text is Unicode characters encoded in UTF-8, multiple
460 UTF-8-encoded bytes may represent a single integer value. For
461 instance, the literal <code>'a'</code> holds a single byte representing
462 a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
463 <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
464 a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
468 Several backslash escapes allow arbitrary values to be encoded as
469 ASCII text. There are four ways to represent the integer value
470 as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
471 digits; <code>\u</code> followed by exactly four hexadecimal digits;
472 <code>\U</code> followed by exactly eight hexadecimal digits, and a
473 plain backslash <code>\</code> followed by exactly three octal digits.
474 In each case the value of the literal is the value represented by
475 the digits in the corresponding base.
479 Although these representations all result in an integer, they have
480 different valid ranges. Octal escapes must represent a value between
481 0 and 255 inclusive. Hexadecimal escapes satisfy this condition
482 by construction. The escapes <code>\u</code> and <code>\U</code>
483 represent Unicode code points so within them some values are illegal,
484 in particular those above <code>0x10FFFF</code> and surrogate halves.
488 After a backslash, certain single-character escapes represent special values:
491 <pre class="grammar">
492 \a U+0007 alert or bell
495 \n U+000A line feed or newline
496 \r U+000D carriage return
497 \t U+0009 horizontal tab
498 \v U+000B vertical tab
500 \' U+0027 single quote (valid escape only within rune literals)
501 \" U+0022 double quote (valid escape only within string literals)
505 An unrecognized character following a backslash in a rune literal is illegal.
509 rune_lit = "'" ( unicode_value | byte_value ) "'" .
510 unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
511 byte_value = octal_byte_value | hex_byte_value .
512 octal_byte_value = `\` octal_digit octal_digit octal_digit .
513 hex_byte_value = `\` "x" hex_digit hex_digit .
514 little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
515 big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
516 hex_digit hex_digit hex_digit hex_digit .
517 escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
532 '\'' // rune literal containing single quote character
533 'aa' // illegal: too many characters
534 '\k' // illegal: k is not recognized after a backslash
535 '\xa' // illegal: too few hexadecimal digits
536 '\0' // illegal: too few octal digits
537 '\400' // illegal: octal value over 255
538 '\uDFFF' // illegal: surrogate half
539 '\U00110000' // illegal: invalid Unicode code point
543 <h3 id="String_literals">String literals</h3>
546 A string literal represents a <a href="#Constants">string constant</a>
547 obtained from concatenating a sequence of characters. There are two forms:
548 raw string literals and interpreted string literals.
552 Raw string literals are character sequences between back quotes, as in
553 <code>`foo`</code>. Within the quotes, any character may appear except
554 back quote. The value of a raw string literal is the
555 string composed of the uninterpreted (implicitly UTF-8-encoded) characters
557 in particular, backslashes have no special meaning and the string may
559 Carriage return characters ('\r') inside raw string literals
560 are discarded from the raw string value.
564 Interpreted string literals are character sequences between double
565 quotes, as in <code>"bar"</code>.
566 Within the quotes, any character may appear except newline and unescaped double quote.
567 The text between the quotes forms the
568 value of the literal, with backslash escapes interpreted as they
569 are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and
570 <code>\"</code> is legal), with the same restrictions.
571 The three-digit octal (<code>\</code><i>nnn</i>)
572 and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual
573 <i>bytes</i> of the resulting string; all other escapes represent
574 the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.
575 Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent
576 a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>,
577 <code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent
578 the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character
583 string_lit = raw_string_lit | interpreted_string_lit .
584 raw_string_lit = "`" { unicode_char | newline } "`" .
585 interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
589 `abc` // same as "abc"
591 \n` // same as "\\n\n\\n"
598 "\uD800" // illegal: surrogate half
599 "\U00110000" // illegal: invalid Unicode code point
603 These examples all represent the same string:
607 "日本語" // UTF-8 input text
608 `日本語` // UTF-8 input text as a raw literal
609 "\u65e5\u672c\u8a9e" // the explicit Unicode code points
610 "\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points
611 "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes
615 If the source code represents a character as two code points, such as
616 a combining form involving an accent and a letter, the result will be
617 an error if placed in a rune literal (it is not a single code
618 point), and will appear as two code points if placed in a string
623 <h2 id="Constants">Constants</h2>
625 <p>There are <i>boolean constants</i>,
626 <i>rune constants</i>,
627 <i>integer constants</i>,
628 <i>floating-point constants</i>, <i>complex constants</i>,
629 and <i>string constants</i>. Rune, integer, floating-point,
630 and complex constants are
631 collectively called <i>numeric constants</i>.
635 A constant value is represented by a
636 <a href="#Rune_literals">rune</a>,
637 <a href="#Integer_literals">integer</a>,
638 <a href="#Floating-point_literals">floating-point</a>,
639 <a href="#Imaginary_literals">imaginary</a>,
641 <a href="#String_literals">string</a> literal,
642 an identifier denoting a constant,
643 a <a href="#Constant_expressions">constant expression</a>,
644 a <a href="#Conversions">conversion</a> with a result that is a constant, or
645 the result value of some built-in functions such as
646 <code>unsafe.Sizeof</code> applied to <a href="#Package_unsafe">certain values</a>,
647 <code>cap</code> or <code>len</code> applied to
648 <a href="#Length_and_capacity">some expressions</a>,
649 <code>real</code> and <code>imag</code> applied to a complex constant
650 and <code>complex</code> applied to numeric constants.
651 The boolean truth values are represented by the predeclared constants
652 <code>true</code> and <code>false</code>. The predeclared identifier
653 <a href="#Iota">iota</a> denotes an integer constant.
657 In general, complex constants are a form of
658 <a href="#Constant_expressions">constant expression</a>
659 and are discussed in that section.
663 Numeric constants represent exact values of arbitrary precision and do not overflow.
664 Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
665 and not-a-number values.
669 Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
670 Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
671 and certain <a href="#Constant_expressions">constant expressions</a>
672 containing only untyped constant operands are untyped.
676 A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
677 or <a href="#Conversions">conversion</a>, or implicitly when used in a
678 <a href="#Variable_declarations">variable declaration</a> or an
679 <a href="#Assignment_statements">assignment statement</a> or as an
680 operand in an <a href="#Expressions">expression</a>.
681 It is an error if the constant value
682 cannot be <a href="#Representability">represented</a> as a value of the respective type.
683 If the type is a type parameter, the constant is converted into a non-constant
684 value of the type parameter.
688 An untyped constant has a <i>default type</i> which is the type to which the
689 constant is implicitly converted in contexts where a typed value is required,
690 for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
691 such as <code>i := 0</code> where there is no explicit type.
692 The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
693 <code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
694 respectively, depending on whether it is a boolean, rune, integer, floating-point,
695 complex, or string constant.
699 Implementation restriction: Although numeric constants have arbitrary
700 precision in the language, a compiler may implement them using an
701 internal representation with limited precision. That said, every
706 <li>Represent integer constants with at least 256 bits.</li>
708 <li>Represent floating-point constants, including the parts of
709 a complex constant, with a mantissa of at least 256 bits
710 and a signed binary exponent of at least 16 bits.</li>
712 <li>Give an error if unable to represent an integer constant
715 <li>Give an error if unable to represent a floating-point or
716 complex constant due to overflow.</li>
718 <li>Round to the nearest representable constant if unable to
719 represent a floating-point or complex constant due to limits
724 These requirements apply both to literal constants and to the result
725 of evaluating <a href="#Constant_expressions">constant
730 <h2 id="Variables">Variables</h2>
733 A variable is a storage location for holding a <i>value</i>.
734 The set of permissible values is determined by the
735 variable's <i><a href="#Types">type</a></i>.
739 A <a href="#Variable_declarations">variable declaration</a>
740 or, for function parameters and results, the signature
741 of a <a href="#Function_declarations">function declaration</a>
742 or <a href="#Function_literals">function literal</a> reserves
743 storage for a named variable.
745 Calling the built-in function <a href="#Allocation"><code>new</code></a>
746 or taking the address of a <a href="#Composite_literals">composite literal</a>
747 allocates storage for a variable at run time.
748 Such an anonymous variable is referred to via a (possibly implicit)
749 <a href="#Address_operators">pointer indirection</a>.
753 <i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
754 and <a href="#Struct_types">struct</a> types have elements and fields that may
755 be <a href="#Address_operators">addressed</a> individually. Each such element
756 acts like a variable.
760 The <i>static type</i> (or just <i>type</i>) of a variable is the
761 type given in its declaration, the type provided in the
762 <code>new</code> call or composite literal, or the type of
763 an element of a structured variable.
764 Variables of interface type also have a distinct <i>dynamic type</i>,
765 which is the (non-interface) type of the value assigned to the variable at run time
766 (unless the value is the predeclared identifier <code>nil</code>,
768 The dynamic type may vary during execution but values stored in interface
769 variables are always <a href="#Assignability">assignable</a>
770 to the static type of the variable.
774 var x interface{} // x is nil and has static type interface{}
775 var v *T // v has value nil, static type *T
776 x = 42 // x has value 42 and dynamic type int
777 x = v // x has value (*T)(nil) and dynamic type *T
781 A variable's value is retrieved by referring to the variable in an
782 <a href="#Expressions">expression</a>; it is the most recent value
783 <a href="#Assignment_statements">assigned</a> to the variable.
784 If a variable has not yet been assigned a value, its value is the
785 <a href="#The_zero_value">zero value</a> for its type.
789 <h2 id="Types">Types</h2>
792 A type determines a set of values together with operations and methods specific
793 to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be
794 followed by <a href="#Instantiations">type arguments</a> if the type is generic.
795 A type may also be specified using a <i>type literal</i>, which composes a type
800 Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
801 TypeName = identifier | QualifiedIdent .
802 TypeArgs = "[" TypeList [ "," ] "]" .
803 TypeList = Type { "," Type } .
804 TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
805 SliceType | MapType | ChannelType .
809 The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
810 Others are introduced with <a href="#Type_declarations">type declarations</a>
811 or <a href="#Type_parameter_declarations">type parameter lists</a>.
812 <i>Composite types</i>—array, struct, pointer, function,
813 interface, slice, map, and channel types—may be constructed using
818 Predeclared types, defined types, and type parameters are called <i>named types</i>.
819 An alias denotes a named type if the type given in the alias declaration is a named type.
822 <h3 id="Boolean_types">Boolean types</h3>
825 A <i>boolean type</i> represents the set of Boolean truth values
826 denoted by the predeclared constants <code>true</code>
827 and <code>false</code>. The predeclared boolean type is <code>bool</code>;
828 it is a <a href="#Type_definitions">defined type</a>.
831 <h3 id="Numeric_types">Numeric types</h3>
834 An <i>integer</i>, <i>floating-point</i>, or <i>complex</i> type
835 represents the set of integer, floating-point, or complex values, respectively.
836 They are collectively called <i>numeric types</i>.
837 The predeclared architecture-independent numeric types are:
840 <pre class="grammar">
841 uint8 the set of all unsigned 8-bit integers (0 to 255)
842 uint16 the set of all unsigned 16-bit integers (0 to 65535)
843 uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
844 uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
846 int8 the set of all signed 8-bit integers (-128 to 127)
847 int16 the set of all signed 16-bit integers (-32768 to 32767)
848 int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
849 int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
851 float32 the set of all IEEE-754 32-bit floating-point numbers
852 float64 the set of all IEEE-754 64-bit floating-point numbers
854 complex64 the set of all complex numbers with float32 real and imaginary parts
855 complex128 the set of all complex numbers with float64 real and imaginary parts
862 The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
863 <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
867 There is also a set of predeclared integer types with implementation-specific sizes:
870 <pre class="grammar">
871 uint either 32 or 64 bits
872 int same size as uint
873 uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
877 To avoid portability issues all numeric types are <a href="#Type_definitions">defined
878 types</a> and thus distinct except
879 <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
880 <code>rune</code>, which is an alias for <code>int32</code>.
882 are required when different numeric types are mixed in an expression
883 or assignment. For instance, <code>int32</code> and <code>int</code>
884 are not the same type even though they may have the same size on a
885 particular architecture.
888 <h3 id="String_types">String types</h3>
891 A <i>string type</i> represents the set of string values.
892 A string value is a (possibly empty) sequence of bytes.
893 The number of bytes is called the length of the string and is never negative.
894 Strings are immutable: once created,
895 it is impossible to change the contents of a string.
896 The predeclared string type is <code>string</code>;
897 it is a <a href="#Type_definitions">defined type</a>.
901 The length of a string <code>s</code> can be discovered using
902 the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
903 The length is a compile-time constant if the string is a constant.
904 A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
905 0 through <code>len(s)-1</code>.
906 It is illegal to take the address of such an element; if
907 <code>s[i]</code> is the <code>i</code>'th byte of a
908 string, <code>&s[i]</code> is invalid.
912 <h3 id="Array_types">Array types</h3>
915 An array is a numbered sequence of elements of a single
916 type, called the element type.
917 The number of elements is called the length of the array and is never negative.
921 ArrayType = "[" ArrayLength "]" ElementType .
922 ArrayLength = Expression .
927 The length is part of the array's type; it must evaluate to a
928 non-negative <a href="#Constants">constant</a>
929 <a href="#Representability">representable</a> by a value
930 of type <code>int</code>.
931 The length of array <code>a</code> can be discovered
932 using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
933 The elements can be addressed by integer <a href="#Index_expressions">indices</a>
934 0 through <code>len(a)-1</code>.
935 Array types are always one-dimensional but may be composed to form
936 multi-dimensional types.
941 [2*N] struct { x, y int32 }
944 [2][2][2]float64 // same as [2]([2]([2]float64))
948 An array type <code>T</code> may not have an element of type <code>T</code>,
949 or of a type containing <code>T</code> as a component, directly or indirectly,
950 if those containing types are only array or struct types.
954 // invalid array types
956 T1 [10]T1 // element type of T1 is T1
957 T2 [10]struct{ f T2 } // T2 contains T2 as component of a struct
958 T3 [10]T4 // T3 contains T3 as component of a struct in T4
959 T4 struct{ f T3 } // T4 contains T4 as component of array T3 in a struct
964 T5 [10]*T5 // T5 contains T5 as component of a pointer
965 T6 [10]func() T6 // T6 contains T6 as component of a function type
966 T7 [10]struct{ f []T7 } // T7 contains T7 as component of a slice in a struct
970 <h3 id="Slice_types">Slice types</h3>
973 A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
974 provides access to a numbered sequence of elements from that array.
975 A slice type denotes the set of all slices of arrays of its element type.
976 The number of elements is called the length of the slice and is never negative.
977 The value of an uninitialized slice is <code>nil</code>.
981 SliceType = "[" "]" ElementType .
985 The length of a slice <code>s</code> can be discovered by the built-in function
986 <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
987 execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
988 0 through <code>len(s)-1</code>. The slice index of a
989 given element may be less than the index of the same element in the
993 A slice, once initialized, is always associated with an underlying
994 array that holds its elements. A slice therefore shares storage
995 with its array and with other slices of the same array; by contrast,
996 distinct arrays always represent distinct storage.
999 The array underlying a slice may extend past the end of the slice.
1000 The <i>capacity</i> is a measure of that extent: it is the sum of
1001 the length of the slice and the length of the array beyond the slice;
1002 a slice of length up to that capacity can be created by
1003 <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
1004 The capacity of a slice <code>a</code> can be discovered using the
1005 built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
1009 A new, initialized slice value for a given element type <code>T</code> may be
1010 made using the built-in function
1011 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1012 which takes a slice type
1013 and parameters specifying the length and optionally the capacity.
1014 A slice created with <code>make</code> always allocates a new, hidden array
1015 to which the returned slice value refers. That is, executing
1019 make([]T, length, capacity)
1023 produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
1024 it, so these two expressions are equivalent:
1028 make([]int, 50, 100)
1033 Like arrays, slices are always one-dimensional but may be composed to construct
1034 higher-dimensional objects.
1035 With arrays of arrays, the inner arrays are, by construction, always the same length;
1036 however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
1037 Moreover, the inner slices must be initialized individually.
1040 <h3 id="Struct_types">Struct types</h3>
1043 A struct is a sequence of named elements, called fields, each of which has a
1044 name and a type. Field names may be specified explicitly (IdentifierList) or
1045 implicitly (EmbeddedField).
1046 Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
1047 be <a href="#Uniqueness_of_identifiers">unique</a>.
1051 StructType = "struct" "{" { FieldDecl ";" } "}" .
1052 FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
1053 EmbeddedField = [ "*" ] TypeName [ TypeArgs ] .
1061 // A struct with 6 fields.
1065 _ float32 // padding
1072 A field declared with a type but no explicit field name is called an <i>embedded field</i>.
1073 An embedded field must be specified as
1074 a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
1075 and <code>T</code> itself may not be
1076 a pointer type. The unqualified type name acts as the field name.
1080 // A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
1082 T1 // field name is T1
1083 *T2 // field name is T2
1084 P.T3 // field name is T3
1085 *P.T4 // field name is T4
1086 x, y int // field names are x and y
1091 The following declaration is illegal because field names must be unique
1097 T // conflicts with embedded field *T and *P.T
1098 *T // conflicts with embedded field T and *P.T
1099 *P.T // conflicts with embedded field T and *T
1104 A field or <a href="#Method_declarations">method</a> <code>f</code> of an
1105 embedded field in a struct <code>x</code> is called <i>promoted</i> if
1106 <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
1107 that field or method <code>f</code>.
1111 Promoted fields act like ordinary fields
1112 of a struct except that they cannot be used as field names in
1113 <a href="#Composite_literals">composite literals</a> of the struct.
1117 Given a struct type <code>S</code> and a <a href="#Types">named type</a>
1118 <code>T</code>, promoted methods are included in the method set of the struct as follows:
1122 If <code>S</code> contains an embedded field <code>T</code>,
1123 the <a href="#Method_sets">method sets</a> of <code>S</code>
1124 and <code>*S</code> both include promoted methods with receiver
1125 <code>T</code>. The method set of <code>*S</code> also
1126 includes promoted methods with receiver <code>*T</code>.
1130 If <code>S</code> contains an embedded field <code>*T</code>,
1131 the method sets of <code>S</code> and <code>*S</code> both
1132 include promoted methods with receiver <code>T</code> or
1138 A field declaration may be followed by an optional string literal <i>tag</i>,
1139 which becomes an attribute for all the fields in the corresponding
1140 field declaration. An empty tag string is equivalent to an absent tag.
1141 The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
1142 and take part in <a href="#Type_identity">type identity</a> for structs
1143 but are otherwise ignored.
1148 x, y float64 "" // an empty tag string is like an absent tag
1149 name string "any string is permitted as a tag"
1150 _ [4]byte "ceci n'est pas un champ de structure"
1153 // A struct corresponding to a TimeStamp protocol buffer.
1154 // The tag strings define the protocol buffer field numbers;
1155 // they follow the convention outlined by the reflect package.
1157 microsec uint64 `protobuf:"1"`
1158 serverIP6 uint64 `protobuf:"2"`
1163 A struct type <code>T</code> may not contain a field of type <code>T</code>,
1164 or of a type containing <code>T</code> as a component, directly or indirectly,
1165 if those containing types are only array or struct types.
1169 // invalid struct types
1171 T1 struct{ T1 } // T1 contains a field of T1
1172 T2 struct{ f [10]T2 } // T2 contains T2 as component of an array
1173 T3 struct{ T4 } // T3 contains T3 as component of an array in struct T4
1174 T4 struct{ f [10]T3 } // T4 contains T4 as component of struct T3 in an array
1177 // valid struct types
1179 T5 struct{ f *T5 } // T5 contains T5 as component of a pointer
1180 T6 struct{ f func() T6 } // T6 contains T6 as component of a function type
1181 T7 struct{ f [10][]T7 } // T7 contains T7 as component of a slice in an array
1185 <h3 id="Pointer_types">Pointer types</h3>
1188 A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
1189 type, called the <i>base type</i> of the pointer.
1190 The value of an uninitialized pointer is <code>nil</code>.
1194 PointerType = "*" BaseType .
1203 <h3 id="Function_types">Function types</h3>
1206 A function type denotes the set of all functions with the same parameter
1207 and result types. The value of an uninitialized variable of function type
1208 is <code>nil</code>.
1212 FunctionType = "func" Signature .
1213 Signature = Parameters [ Result ] .
1214 Result = Parameters | Type .
1215 Parameters = "(" [ ParameterList [ "," ] ] ")" .
1216 ParameterList = ParameterDecl { "," ParameterDecl } .
1217 ParameterDecl = [ IdentifierList ] [ "..." ] Type .
1221 Within a list of parameters or results, the names (IdentifierList)
1222 must either all be present or all be absent. If present, each name
1223 stands for one item (parameter or result) of the specified type and
1224 all non-<a href="#Blank_identifier">blank</a> names in the signature
1225 must be <a href="#Uniqueness_of_identifiers">unique</a>.
1226 If absent, each type stands for one item of that type.
1227 Parameter and result
1228 lists are always parenthesized except that if there is exactly
1229 one unnamed result it may be written as an unparenthesized type.
1233 The final incoming parameter in a function signature may have
1234 a type prefixed with <code>...</code>.
1235 A function with such a parameter is called <i>variadic</i> and
1236 may be invoked with zero or more arguments for that parameter.
1242 func(a, _ int, z float32) bool
1243 func(a, b int, z float32) (bool)
1244 func(prefix string, values ...int)
1245 func(a, b int, z float64, opt ...interface{}) (success bool)
1246 func(int, int, float64) (float64, *[]int)
1247 func(n int) func(p *T)
1250 <h3 id="Interface_types">Interface types</h3>
1253 An interface type defines a <i>type set</i>.
1254 A variable of interface type can store a value of any type that is in the type
1255 set of the interface. Such a type is said to
1256 <a href="#Implementing_an_interface">implement the interface</a>.
1257 The value of an uninitialized variable of interface type is <code>nil</code>.
1261 InterfaceType = "interface" "{" { InterfaceElem ";" } "}" .
1262 InterfaceElem = MethodElem | TypeElem .
1263 MethodElem = MethodName Signature .
1264 MethodName = identifier .
1265 TypeElem = TypeTerm { "|" TypeTerm } .
1266 TypeTerm = Type | UnderlyingType .
1267 UnderlyingType = "~" Type .
1271 An interface type is specified by a list of <i>interface elements</i>.
1272 An interface element is either a <i>method</i> or a <i>type element</i>,
1273 where a type element is a union of one or more <i>type terms</i>.
1274 A type term is either a single type or a single underlying type.
1277 <h4 id="Basic_interfaces">Basic interfaces</h4>
1280 In its most basic form an interface specifies a (possibly empty) list of methods.
1281 The type set defined by such an interface is the set of types which implement all of
1282 those methods, and the corresponding <a href="#Method_sets">method set</a> consists
1283 exactly of the methods specified by the interface.
1284 Interfaces whose type sets can be defined entirely by a list of methods are called
1285 <i>basic interfaces.</i>
1289 // A simple File interface.
1291 Read([]byte) (int, error)
1292 Write([]byte) (int, error)
1298 The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
1299 and not <a href="#Blank_identifier">blank</a>.
1305 String() string // illegal: String not unique
1306 _(x int) // illegal: method must have non-blank name
1311 More than one type may implement an interface.
1312 For instance, if two types <code>S1</code> and <code>S2</code>
1317 func (p T) Read(p []byte) (n int, err error)
1318 func (p T) Write(p []byte) (n int, err error)
1319 func (p T) Close() error
1323 (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
1324 then the <code>File</code> interface is implemented by both <code>S1</code> and
1325 <code>S2</code>, regardless of what other methods
1326 <code>S1</code> and <code>S2</code> may have or share.
1330 Every type that is a member of the type set of an interface implements that interface.
1331 Any given type may implement several distinct interfaces.
1332 For instance, all types implement the <i>empty interface</i> which stands for the set
1333 of all (non-interface) types:
1341 For convenience, the predeclared type <code>any</code> is an alias for the empty interface.
1345 Similarly, consider this interface specification,
1346 which appears within a <a href="#Type_declarations">type declaration</a>
1347 to define an interface called <code>Locker</code>:
1351 type Locker interface {
1358 If <code>S1</code> and <code>S2</code> also implement
1362 func (p T) Lock() { … }
1363 func (p T) Unlock() { … }
1367 they implement the <code>Locker</code> interface as well
1368 as the <code>File</code> interface.
1371 <h4 id="Embedded_interfaces">Embedded interfaces</h4>
1374 In a slightly more general form
1375 an interface <code>T</code> may use a (possibly qualified) interface type
1376 name <code>E</code> as an interface element. This is called
1377 <i>embedding</i> interface <code>E</code> in <code>T</code>.
1378 The type set of <code>T</code> is the <i>intersection</i> of the type sets
1379 defined by <code>T</code>'s explicitly declared methods and the type sets
1380 of <code>T</code>’s embedded interfaces.
1381 In other words, the type set of <code>T</code> is the set of all types that implement all the
1382 explicitly declared methods of <code>T</code> and also all the methods of
1387 type Reader interface {
1388 Read(p []byte) (n int, err error)
1392 type Writer interface {
1393 Write(p []byte) (n int, err error)
1397 // ReadWriter's methods are Read, Write, and Close.
1398 type ReadWriter interface {
1399 Reader // includes methods of Reader in ReadWriter's method set
1400 Writer // includes methods of Writer in ReadWriter's method set
1405 When embedding interfaces, methods with the
1406 <a href="#Uniqueness_of_identifiers">same</a> names must
1407 have <a href="#Type_identity">identical</a> signatures.
1411 type ReadCloser interface {
1412 Reader // includes methods of Reader in ReadCloser's method set
1413 Close() // illegal: signatures of Reader.Close and Close are different
1417 <h4 id="General_interfaces">General interfaces</h4>
1420 In their most general form, an interface element may also be an arbitrary type term
1421 <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
1422 or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
1423 Together with method specifications, these elements enable the precise
1424 definition of an interface's type set as follows:
1428 <li>The type set of the empty interface is the set of all non-interface types.
1431 <li>The type set of a non-empty interface is the intersection of the type sets
1432 of its interface elements.
1435 <li>The type set of a method specification is the set of all non-interface types
1436 whose method sets include that method.
1439 <li>The type set of a non-interface type term is the set consisting
1443 <li>The type set of a term of the form <code>~T</code>
1444 is the set of all types whose underlying type is <code>T</code>.
1447 <li>The type set of a <i>union</i> of terms
1448 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
1449 is the union of the type sets of the terms.
1454 The quantification "the set of all non-interface types" refers not just to all (non-interface)
1455 types declared in the program at hand, but all possible types in all possible programs, and
1457 Similarly, given the set of all non-interface types that implement a particular method, the
1458 intersection of the method sets of those types will contain exactly that method, even if all
1459 types in the program at hand always pair that method with another method.
1463 By construction, an interface's type set never contains an interface type.
1467 // An interface representing only the type int.
1472 // An interface representing all types with underlying type int.
1477 // An interface representing all types with underlying type int that implement the String method.
1483 // An interface representing an empty type set: there is no type that is both an int and a string.
1491 In a term of the form <code>~T</code>, the underlying type of <code>T</code>
1492 must be itself, and <code>T</code> cannot be an interface.
1499 ~[]byte // the underlying type of []byte is itself
1500 ~MyInt // illegal: the underlying type of MyInt is not MyInt
1501 ~error // illegal: error is an interface
1506 Union elements denote unions of type sets:
1510 // The Float interface represents all floating-point types
1511 // (including any named types whose underlying types are
1512 // either float32 or float64).
1513 type Float interface {
1519 The type <code>T</code> in a term of the form <code>T</code> or <code>~T</code> cannot
1520 be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all
1521 non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
1522 Given a type parameter <code>P</code>:
1527 P // illegal: P is a type parameter
1528 int | ~P // illegal: P is a type parameter
1529 ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
1530 float32 | Float // overlapping type sets but Float is an interface
1535 Implementation restriction:
1536 A union (with more than one term) cannot contain the
1537 <a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code>
1538 or interfaces that specify methods, or embed <code>comparable</code> or interfaces
1539 that specify methods.
1543 Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type
1544 constraints, or as elements of other interfaces used as constraints.
1545 They cannot be the types of values or variables, or components of other,
1546 non-interface types.
1550 var x Float // illegal: Float is not a basic interface
1552 var x interface{} = Float(nil) // illegal
1554 type Floatish struct {
1560 An interface type <code>T</code> may not embed a type element
1561 that is, contains, or embeds <code>T</code>, directly or indirectly.
1565 // illegal: Bad may not embed itself
1566 type Bad interface {
1570 // illegal: Bad1 may not embed itself using Bad2
1571 type Bad1 interface {
1574 type Bad2 interface {
1578 // illegal: Bad3 may not embed a union containing Bad3
1579 type Bad3 interface {
1580 ~int | ~string | Bad3
1583 // illegal: Bad4 may not embed an array containing Bad4 as element type
1584 type Bad4 interface {
1589 <h4 id="Implementing_an_interface">Implementing an interface</h4>
1592 A type <code>T</code> implements an interface <code>I</code> if
1597 <code>T</code> is not an interface and is an element of the type set of <code>I</code>; or
1600 <code>T</code> is an interface and the type set of <code>T</code> is a subset of the
1601 type set of <code>I</code>.
1606 A value of type <code>T</code> implements an interface if <code>T</code>
1607 implements the interface.
1610 <h3 id="Map_types">Map types</h3>
1613 A map is an unordered group of elements of one type, called the
1614 element type, indexed by a set of unique <i>keys</i> of another type,
1615 called the key type.
1616 The value of an uninitialized map is <code>nil</code>.
1620 MapType = "map" "[" KeyType "]" ElementType .
1625 The <a href="#Comparison_operators">comparison operators</a>
1626 <code>==</code> and <code>!=</code> must be fully defined
1627 for operands of the key type; thus the key type must not be a function, map, or
1629 If the key type is an interface type, these
1630 comparison operators must be defined for the dynamic key values;
1631 failure will cause a <a href="#Run_time_panics">run-time panic</a>.
1636 map[*T]struct{ x, y float64 }
1637 map[string]interface{}
1641 The number of map elements is called its length.
1642 For a map <code>m</code>, it can be discovered using the
1643 built-in function <a href="#Length_and_capacity"><code>len</code></a>
1644 and may change during execution. Elements may be added during execution
1645 using <a href="#Assignment_statements">assignments</a> and retrieved with
1646 <a href="#Index_expressions">index expressions</a>; they may be removed with the
1647 <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
1650 A new, empty map value is made using the built-in
1651 function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1652 which takes the map type and an optional capacity hint as arguments:
1656 make(map[string]int)
1657 make(map[string]int, 100)
1661 The initial capacity does not bound its size:
1662 maps grow to accommodate the number of items
1663 stored in them, with the exception of <code>nil</code> maps.
1664 A <code>nil</code> map is equivalent to an empty map except that no elements
1667 <h3 id="Channel_types">Channel types</h3>
1670 A channel provides a mechanism for
1671 <a href="#Go_statements">concurrently executing functions</a>
1673 <a href="#Send_statements">sending</a> and
1674 <a href="#Receive_operator">receiving</a>
1675 values of a specified element type.
1676 The value of an uninitialized channel is <code>nil</code>.
1680 ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
1684 The optional <code><-</code> operator specifies the channel <i>direction</i>,
1685 <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
1686 otherwise it is <i>bidirectional</i>.
1687 A channel may be constrained only to send or only to receive by
1688 <a href="#Assignment_statements">assignment</a> or
1689 explicit <a href="#Conversions">conversion</a>.
1693 chan T // can be used to send and receive values of type T
1694 chan<- float64 // can only be used to send float64s
1695 <-chan int // can only be used to receive ints
1699 The <code><-</code> operator associates with the leftmost <code>chan</code>
1704 chan<- chan int // same as chan<- (chan int)
1705 chan<- <-chan int // same as chan<- (<-chan int)
1706 <-chan <-chan int // same as <-chan (<-chan int)
1707 chan (<-chan int)
1711 A new, initialized channel
1712 value can be made using the built-in function
1713 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1714 which takes the channel type and an optional <i>capacity</i> as arguments:
1722 The capacity, in number of elements, sets the size of the buffer in the channel.
1723 If the capacity is zero or absent, the channel is unbuffered and communication
1724 succeeds only when both a sender and receiver are ready. Otherwise, the channel
1725 is buffered and communication succeeds without blocking if the buffer
1726 is not full (sends) or not empty (receives).
1727 A <code>nil</code> channel is never ready for communication.
1731 A channel may be closed with the built-in function
1732 <a href="#Close"><code>close</code></a>.
1733 The multi-valued assignment form of the
1734 <a href="#Receive_operator">receive operator</a>
1735 reports whether a received value was sent before
1736 the channel was closed.
1740 A single channel may be used in
1741 <a href="#Send_statements">send statements</a>,
1742 <a href="#Receive_operator">receive operations</a>,
1743 and calls to the built-in functions
1744 <a href="#Length_and_capacity"><code>cap</code></a> and
1745 <a href="#Length_and_capacity"><code>len</code></a>
1746 by any number of goroutines without further synchronization.
1747 Channels act as first-in-first-out queues.
1748 For example, if one goroutine sends values on a channel
1749 and a second goroutine receives them, the values are
1750 received in the order sent.
1753 <h2 id="Properties_of_types_and_values">Properties of types and values</h2>
1755 <h3 id="Underlying_types">Underlying types</h3>
1758 Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
1759 is one of the predeclared boolean, numeric, or string types, or a type literal,
1760 the corresponding underlying type is <code>T</code> itself.
1761 Otherwise, <code>T</code>'s underlying type is the underlying type of the
1762 type to which <code>T</code> refers in its declaration.
1763 For a type parameter that is the underlying type of its
1764 <a href="#Type_constraints">type constraint</a>, which is always an interface.
1780 func f[P any](x P) { … }
1784 The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
1785 and <code>B2</code> is <code>string</code>.
1786 The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
1787 The underlying type of <code>P</code> is <code>interface{}</code>.
1790 <h3 id="Core_types">Core types</h3>
1793 Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the
1794 <a href="#Underlying_types">underlying type</a> of <code>T</code>.
1798 An interface <code>T</code> has a core type if one of the following
1799 conditions is satisfied:
1804 There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a>
1805 of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or
1808 the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a>
1809 with identical element type <code>E</code>, and all directional channels have the same
1815 No other interfaces have a core type.
1819 The core type of an interface is, depending on the condition that is satisfied, either:
1824 the type <code>U</code>; or
1827 the type <code>chan E</code> if <code>T</code> contains only bidirectional
1828 channels, or the type <code>chan<- E</code> or <code><-chan E</code>
1829 depending on the direction of the directional channels present.
1834 By definition, a core type is never a <a href="#Type_definitions">defined type</a>,
1835 <a href="#Type_parameter_declarations">type parameter</a>, or
1836 <a href="#Interface_types">interface type</a>.
1840 Examples of interfaces with core types:
1844 type Celsius float32
1847 interface{ int } // int
1848 interface{ Celsius|Kelvin } // float32
1849 interface{ ~chan int } // chan int
1850 interface{ ~chan int|~chan<- int } // chan<- int
1851 interface{ ~[]*data; String() string } // []*data
1855 Examples of interfaces without core types:
1859 interface{} // no single underlying type
1860 interface{ Celsius|float64 } // no single underlying type
1861 interface{ chan int | chan<- string } // channels have different element types
1862 interface{ <-chan int | chan<- int } // directional channels have different directions
1866 Some operations (<a href="#Slice_expressions">slice expressions</a>,
1867 <a href="#Appending_and_copying_slices"><code>append</code> and <code>copy</code></a>)
1868 rely on a slightly more loose form of core types which accept byte slices and strings.
1869 Specifically, if there are exactly two types, <code>[]byte</code> and <code>string</code>,
1870 which are the underlying types of all types in the type set of interface <code>T</code>,
1871 the core type of <code>T</code> is called <code>bytestring</code>.
1875 Examples of interfaces with <code>bytestring</code> core types:
1879 interface{ int } // int (same as ordinary core type)
1880 interface{ []byte | string } // bytestring
1881 interface{ ~[]byte | myString } // bytestring
1885 Note that <code>bytestring</code> is not a real type; it cannot be used to declare
1886 variables are compose other types. It exists solely to describe the behavior of some
1887 operations that read from a sequence of bytes, which may be a byte slice or a string.
1890 <h3 id="Type_identity">Type identity</h3>
1893 Two types are either <i>identical</i> or <i>different</i>.
1897 A <a href="#Types">named type</a> is always different from any other type.
1898 Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
1899 structurally equivalent; that is, they have the same literal structure and corresponding
1900 components have identical types. In detail:
1904 <li>Two array types are identical if they have identical element types and
1905 the same array length.</li>
1907 <li>Two slice types are identical if they have identical element types.</li>
1909 <li>Two struct types are identical if they have the same sequence of fields,
1910 and if corresponding fields have the same names, and identical types,
1912 <a href="#Exported_identifiers">Non-exported</a> field names from different
1913 packages are always different.</li>
1915 <li>Two pointer types are identical if they have identical base types.</li>
1917 <li>Two function types are identical if they have the same number of parameters
1918 and result values, corresponding parameter and result types are
1919 identical, and either both functions are variadic or neither is.
1920 Parameter and result names are not required to match.</li>
1922 <li>Two interface types are identical if they define the same type set.
1925 <li>Two map types are identical if they have identical key and element types.</li>
1927 <li>Two channel types are identical if they have identical element types and
1928 the same direction.</li>
1930 <li>Two <a href="#Instantiations">instantiated</a> types are identical if
1931 their defined types and all type arguments are identical.
1936 Given the declarations
1943 A2 = struct{ a, b int }
1945 A4 = func(A3, float64) *A0
1946 A5 = func(x int, _ float64) *[]string
1950 B2 struct{ a, b int }
1951 B3 struct{ a, c int }
1952 B4 func(int, float64) *B0
1953 B5 func(x int, y float64) *A1
1956 D0[P1, P2 any] struct{ x P1; y P2 }
1957 E0 = D0[int, string]
1962 these types are identical:
1966 A0, A1, and []string
1967 A2 and struct{ a, b int }
1969 A4, func(int, float64) *[]string, and A5
1972 D0[int, string] and E0
1974 struct{ a, b *B5 } and struct{ a, b *B5 }
1975 func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
1979 <code>B0</code> and <code>B1</code> are different because they are new types
1980 created by distinct <a href="#Type_definitions">type definitions</a>;
1981 <code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
1982 are different because <code>B0</code> is different from <code>[]string</code>;
1983 and <code>P1</code> and <code>P2</code> are different because they are different
1985 <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are
1986 different because the former is an <a href="#Instantiations">instantiated</a>
1987 defined type while the latter is a type literal
1988 (but they are still <a href="#Assignability">assignable</a>).
1991 <h3 id="Assignability">Assignability</h3>
1994 A value <code>x</code> of type <code>V</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
1995 ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
2000 <code>V</code> and <code>T</code> are identical.
2003 <code>V</code> and <code>T</code> have identical
2004 <a href="#Underlying_types">underlying types</a>
2005 but are not type parameters and at least one of <code>V</code>
2006 or <code>T</code> is not a <a href="#Types">named type</a>.
2009 <code>V</code> and <code>T</code> are channel types with
2010 identical element types, <code>V</code> is a bidirectional channel,
2011 and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
2014 <code>T</code> is an interface type, but not a type parameter, and
2015 <code>x</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
2018 <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
2019 is a pointer, function, slice, map, channel, or interface type,
2020 but not a type parameter.
2023 <code>x</code> is an untyped <a href="#Constants">constant</a>
2024 <a href="#Representability">representable</a>
2025 by a value of type <code>T</code>.
2030 Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code>
2031 is assignable to a variable of type <code>T</code> if one of the following conditions applies:
2036 <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
2037 a type parameter, and <code>x</code> is assignable to each type in
2038 <code>T</code>'s type set.
2041 <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
2042 a type parameter, and <code>x</code> is assignable to each type in
2043 <code>T</code>'s type set.
2046 <code>V</code> is a type parameter and <code>T</code> is not a named type,
2047 and values of each type in <code>V</code>'s type set are assignable
2052 <h3 id="Representability">Representability</h3>
2055 A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
2056 by a value of type <code>T</code>,
2057 where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>,
2058 if one of the following conditions applies:
2063 <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
2067 <code>T</code> is a <a href="#Numeric_types">floating-point type</a> and <code>x</code> can be rounded to <code>T</code>'s
2068 precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
2069 negative zero further simplified to an unsigned zero. Note that constant values never result
2070 in an IEEE negative zero, NaN, or infinity.
2074 <code>T</code> is a complex type, and <code>x</code>'s
2075 <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
2076 are representable by values of <code>T</code>'s component type (<code>float32</code> or
2077 <code>float64</code>).
2082 If <code>T</code> is a type parameter,
2083 <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
2084 by a value of each type in <code>T</code>'s type set.
2088 x T x is representable by a value of T because
2090 'a' byte 97 is in the set of byte values
2091 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
2092 "foo" string "foo" is in the set of string values
2093 1024 int16 1024 is in the set of 16-bit integers
2094 42.0 byte 42 is in the set of unsigned 8-bit integers
2095 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
2096 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
2097 -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
2098 0i int 0 is an integer value
2099 (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
2103 x T x is not representable by a value of T because
2105 0 bool 0 is not in the set of boolean values
2106 'a' string 'a' is a rune, it is not in the set of string values
2107 1024 byte 1024 is not in the set of unsigned 8-bit integers
2108 -1 uint16 -1 is not in the set of unsigned 16-bit integers
2109 1.1 int 1.1 is not an integer value
2110 42i float32 (0 + 42i) is not in the set of float32 values
2111 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
2114 <h3 id="Method_sets">Method sets</h3>
2117 The <i>method set</i> of a type determines the methods that can be
2118 <a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type.
2119 Every type has a (possibly empty) method set associated with it:
2123 <li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all
2124 <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
2128 The method set of a pointer to a defined type <code>T</code>
2129 (where <code>T</code> is neither a pointer nor an interface)
2130 is the set of all methods declared with receiver <code>*T</code> or <code>T</code>.
2133 <li>The method set of an <a href="#Interface_types">interface type</a> is the intersection
2134 of the method sets of each type in the interface's <a href="#Interface_types">type set</a>
2135 (the resulting method set is usually just the set of declared methods in the interface).
2140 Further rules apply to structs (and pointer to structs) containing embedded fields,
2141 as described in the section on <a href="#Struct_types">struct types</a>.
2142 Any other type has an empty method set.
2146 In a method set, each method must have a
2147 <a href="#Uniqueness_of_identifiers">unique</a>
2148 non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
2151 <h2 id="Blocks">Blocks</h2>
2154 A <i>block</i> is a possibly empty sequence of declarations and statements
2155 within matching brace brackets.
2159 Block = "{" StatementList "}" .
2160 StatementList = { Statement ";" } .
2164 In addition to explicit blocks in the source code, there are implicit blocks:
2168 <li>The <i>universe block</i> encompasses all Go source text.</li>
2170 <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
2171 Go source text for that package.</li>
2173 <li>Each file has a <i>file block</i> containing all Go source text
2176 <li>Each <a href="#If_statements">"if"</a>,
2177 <a href="#For_statements">"for"</a>, and
2178 <a href="#Switch_statements">"switch"</a>
2179 statement is considered to be in its own implicit block.</li>
2181 <li>Each clause in a <a href="#Switch_statements">"switch"</a>
2182 or <a href="#Select_statements">"select"</a> statement
2183 acts as an implicit block.</li>
2187 Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
2191 <h2 id="Declarations_and_scope">Declarations and scope</h2>
2194 A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
2195 <a href="#Constant_declarations">constant</a>,
2196 <a href="#Type_declarations">type</a>,
2197 <a href="#Type_parameter_declarations">type parameter</a>,
2198 <a href="#Variable_declarations">variable</a>,
2199 <a href="#Function_declarations">function</a>,
2200 <a href="#Labeled_statements">label</a>, or
2201 <a href="#Import_declarations">package</a>.
2202 Every identifier in a program must be declared.
2203 No identifier may be declared twice in the same block, and
2204 no identifier may be declared in both the file and package block.
2208 The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
2209 in a declaration, but it does not introduce a binding and thus is not declared.
2210 In the package block, the identifier <code>init</code> may only be used for
2211 <a href="#Package_initialization"><code>init</code> function</a> declarations,
2212 and like the blank identifier it does not introduce a new binding.
2216 Declaration = ConstDecl | TypeDecl | VarDecl .
2217 TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
2221 The <i>scope</i> of a declared identifier is the extent of source text in which
2222 the identifier denotes the specified constant, type, variable, function, label, or package.
2226 Go is lexically scoped using <a href="#Blocks">blocks</a>:
2230 <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
2232 <li>The scope of an identifier denoting a constant, type, variable,
2233 or function (but not method) declared at top level (outside any
2234 function) is the package block.</li>
2236 <li>The scope of the package name of an imported package is the file block
2237 of the file containing the import declaration.</li>
2239 <li>The scope of an identifier denoting a method receiver, function parameter,
2240 or result variable is the function body.</li>
2242 <li>The scope of an identifier denoting a type parameter of a function
2243 or declared by a method receiver begins after the name of the function
2244 and ends at the end of the function body.</li>
2246 <li>The scope of an identifier denoting a type parameter of a type
2247 begins after the name of the type and ends at the end
2248 of the TypeSpec.</li>
2250 <li>The scope of a constant or variable identifier declared
2251 inside a function begins at the end of the ConstSpec or VarSpec
2252 (ShortVarDecl for short variable declarations)
2253 and ends at the end of the innermost containing block.</li>
2255 <li>The scope of a type identifier declared inside a function
2256 begins at the identifier in the TypeSpec
2257 and ends at the end of the innermost containing block.</li>
2261 An identifier declared in a block may be redeclared in an inner block.
2262 While the identifier of the inner declaration is in scope, it denotes
2263 the entity declared by the inner declaration.
2267 The <a href="#Package_clause">package clause</a> is not a declaration; the package name
2268 does not appear in any scope. Its purpose is to identify the files belonging
2269 to the same <a href="#Packages">package</a> and to specify the default package name for import
2274 <h3 id="Label_scopes">Label scopes</h3>
2277 Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
2278 used in the <a href="#Break_statements">"break"</a>,
2279 <a href="#Continue_statements">"continue"</a>, and
2280 <a href="#Goto_statements">"goto"</a> statements.
2281 It is illegal to define a label that is never used.
2282 In contrast to other identifiers, labels are not block scoped and do
2283 not conflict with identifiers that are not labels. The scope of a label
2284 is the body of the function in which it is declared and excludes
2285 the body of any nested function.
2289 <h3 id="Blank_identifier">Blank identifier</h3>
2292 The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
2293 It serves as an anonymous placeholder instead of a regular (non-blank)
2294 identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
2295 as an <a href="#Operands">operand</a>, and in <a href="#Assignment_statements">assignment statements</a>.
2299 <h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
2302 The following identifiers are implicitly declared in the
2303 <a href="#Blocks">universe block</a>:
2305 <pre class="grammar">
2307 any bool byte comparable
2308 complex64 complex128 error float32 float64
2309 int int8 int16 int32 int64 rune string
2310 uint uint8 uint16 uint32 uint64 uintptr
2319 append cap close complex copy delete imag len
2320 make new panic print println real recover
2323 <h3 id="Exported_identifiers">Exported identifiers</h3>
2326 An identifier may be <i>exported</i> to permit access to it from another package.
2327 An identifier is exported if both:
2330 <li>the first character of the identifier's name is a Unicode uppercase
2331 letter (Unicode character category Lu); and</li>
2332 <li>the identifier is declared in the <a href="#Blocks">package block</a>
2333 or it is a <a href="#Struct_types">field name</a> or
2334 <a href="#MethodName">method name</a>.</li>
2337 All other identifiers are not exported.
2340 <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
2343 Given a set of identifiers, an identifier is called <i>unique</i> if it is
2344 <i>different</i> from every other in the set.
2345 Two identifiers are different if they are spelled differently, or if they
2346 appear in different <a href="#Packages">packages</a> and are not
2347 <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
2350 <h3 id="Constant_declarations">Constant declarations</h3>
2353 A constant declaration binds a list of identifiers (the names of
2354 the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
2355 The number of identifiers must be equal
2356 to the number of expressions, and the <i>n</i>th identifier on
2357 the left is bound to the value of the <i>n</i>th expression on the
2362 ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
2363 ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
2365 IdentifierList = identifier { "," identifier } .
2366 ExpressionList = Expression { "," Expression } .
2370 If the type is present, all constants take the type specified, and
2371 the expressions must be <a href="#Assignability">assignable</a> to that type,
2372 which must not be a type parameter.
2373 If the type is omitted, the constants take the
2374 individual types of the corresponding expressions.
2375 If the expression values are untyped <a href="#Constants">constants</a>,
2376 the declared constants remain untyped and the constant identifiers
2377 denote the constant values. For instance, if the expression is a
2378 floating-point literal, the constant identifier denotes a floating-point
2379 constant, even if the literal's fractional part is zero.
2383 const Pi float64 = 3.14159265358979323846
2384 const zero = 0.0 // untyped floating-point constant
2387 eof = -1 // untyped integer constant
2389 const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
2390 const u, v float32 = 0, 3 // u = 0.0, v = 3.0
2394 Within a parenthesized <code>const</code> declaration list the
2395 expression list may be omitted from any but the first ConstSpec.
2396 Such an empty list is equivalent to the textual substitution of the
2397 first preceding non-empty expression list and its type if any.
2398 Omitting the list of expressions is therefore equivalent to
2399 repeating the previous list. The number of identifiers must be equal
2400 to the number of expressions in the previous list.
2401 Together with the <a href="#Iota"><code>iota</code> constant generator</a>
2402 this mechanism permits light-weight declaration of sequential values:
2414 numberOfDays // this constant is not exported
2419 <h3 id="Iota">Iota</h3>
2422 Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
2423 <code>iota</code> represents successive untyped integer <a href="#Constants">
2424 constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
2425 in that constant declaration, starting at zero.
2426 It can be used to construct a set of related constants:
2431 c0 = iota // c0 == 0
2432 c1 = iota // c1 == 1
2433 c2 = iota // c2 == 2
2437 a = 1 << iota // a == 1 (iota == 0)
2438 b = 1 << iota // b == 2 (iota == 1)
2439 c = 3 // c == 3 (iota == 2, unused)
2440 d = 1 << iota // d == 8 (iota == 3)
2444 u = iota * 42 // u == 0 (untyped integer constant)
2445 v float64 = iota * 42 // v == 42.0 (float64 constant)
2446 w = iota * 42 // w == 84 (untyped integer constant)
2449 const x = iota // x == 0
2450 const y = iota // y == 0
2454 By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
2459 bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
2460 bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
2461 _, _ // (iota == 2, unused)
2462 bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
2467 This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
2468 of the last non-empty expression list.
2472 <h3 id="Type_declarations">Type declarations</h3>
2475 A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
2476 Type declarations come in two forms: alias declarations and type definitions.
2480 TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
2481 TypeSpec = AliasDecl | TypeDef .
2484 <h4 id="Alias_declarations">Alias declarations</h4>
2487 An alias declaration binds an identifier to the given type.
2491 AliasDecl = identifier "=" Type .
2495 Within the <a href="#Declarations_and_scope">scope</a> of
2496 the identifier, it serves as an <i>alias</i> for the type.
2501 nodeList = []*Node // nodeList and []*Node are identical types
2502 Polar = polar // Polar and polar denote identical types
2507 <h4 id="Type_definitions">Type definitions</h4>
2510 A type definition creates a new, distinct type with the same
2511 <a href="#Types">underlying type</a> and operations as the given type
2512 and binds an identifier, the <i>type name</i>, to it.
2516 TypeDef = identifier [ TypeParameters ] Type .
2520 The new type is called a <i>defined type</i>.
2521 It is <a href="#Type_identity">different</a> from any other type,
2522 including the type it is created from.
2527 Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
2528 polar Point // polar and Point denote different types
2531 type TreeNode struct {
2532 left, right *TreeNode
2536 type Block interface {
2538 Encrypt(src, dst []byte)
2539 Decrypt(src, dst []byte)
2544 A defined type may have <a href="#Method_declarations">methods</a> associated with it.
2545 It does not inherit any methods bound to the given type,
2546 but the <a href="#Method_sets">method set</a>
2547 of an interface type or of elements of a composite type remains unchanged:
2551 // A Mutex is a data type with two methods, Lock and Unlock.
2552 type Mutex struct { /* Mutex fields */ }
2553 func (m *Mutex) Lock() { /* Lock implementation */ }
2554 func (m *Mutex) Unlock() { /* Unlock implementation */ }
2556 // NewMutex has the same composition as Mutex but its method set is empty.
2559 // The method set of PtrMutex's underlying type *Mutex remains unchanged,
2560 // but the method set of PtrMutex is empty.
2561 type PtrMutex *Mutex
2563 // The method set of *PrintableMutex contains the methods
2564 // Lock and Unlock bound to its embedded field Mutex.
2565 type PrintableMutex struct {
2569 // MyBlock is an interface type that has the same method set as Block.
2574 Type definitions may be used to define different boolean, numeric,
2575 or string types and associate methods with them:
2582 EST TimeZone = -(5 + iota)
2588 func (tz TimeZone) String() string {
2589 return fmt.Sprintf("GMT%+dh", tz)
2594 If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>,
2595 the type name denotes a <i>generic type</i>.
2596 Generic types must be <a href="#Instantiations">instantiated</a> when they
2601 type List[T any] struct {
2608 In a type definition the given type cannot be a type parameter.
2612 type T[P any] P // illegal: P is a type parameter
2615 type L T // illegal: T is a type parameter declared by the enclosing function
2620 A generic type may also have <a href="#Method_declarations">methods</a> associated with it.
2621 In this case, the method receivers must declare the same number of type parameters as
2622 present in the generic type definition.
2626 // The method Len returns the number of elements in the linked list l.
2627 func (l *List[T]) Len() int { … }
2630 <h3 id="Type_parameter_declarations">Type parameter declarations</h3>
2633 A type parameter list declares the <i>type parameters</i> of a generic function or type declaration.
2634 The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a>
2635 except that the type parameter names must all be present and the list is enclosed
2636 in square brackets rather than parentheses.
2640 TypeParameters = "[" TypeParamList [ "," ] "]" .
2641 TypeParamList = TypeParamDecl { "," TypeParamDecl } .
2642 TypeParamDecl = IdentifierList TypeConstraint .
2646 All non-blank names in the list must be unique.
2647 Each name declares a type parameter, which is a new and different <a href="#Types">named type</a>
2648 that acts as a place holder for an (as of yet) unknown type in the declaration.
2649 The type parameter is replaced with a <i>type argument</i> upon
2650 <a href="#Instantiations">instantiation</a> of the generic function or type.
2655 [S interface{ ~[]byte|string }]
2662 Just as each ordinary function parameter has a parameter type, each type parameter
2663 has a corresponding (meta-)type which is called its
2664 <a href="#Type_constraints"><i>type constraint</i></a>.
2668 A parsing ambiguity arises when the type parameter list for a generic type
2669 declares a single type parameter <code>P</code> with a constraint <code>C</code>
2670 such that the text <code>P C</code> forms a valid expression:
2681 In these rare cases, the type parameter list is indistinguishable from an
2682 expression and the type declaration is parsed as an array type declaration.
2683 To resolve the ambiguity, embed the constraint in an
2684 <a href="#Interface_types">interface</a> or use a trailing comma:
2688 type T[P interface{*C}] …
2693 Type parameters may also be declared by the receiver specification
2694 of a <a href="#Method_declarations">method declaration</a> associated
2695 with a generic type.
2699 Within a type parameter list of a generic type <code>T</code>, a type constraint
2700 may not (directly, or indirectly through the type parameter list of another
2701 generic type) refer to <code>T</code>.
2705 type T1[P T1[P]] … // illegal: T1 refers to itself
2706 type T2[P interface{ T2[int] }] … // illegal: T2 refers to itself
2707 type T3[P interface{ m(T3[int])}] … // illegal: T3 refers to itself
2708 type T4[P T5[P]] … // illegal: T4 refers to T5 and
2709 type T5[P T4[P]] … // T5 refers to T4
2711 type T6[P int] struct{ f *T6[P] } // ok: reference to T6 is not in type parameter list
2714 <h4 id="Type_constraints">Type constraints</h4>
2717 A <i>type constraint</i> is an <a href="#Interface_types">interface</a> that defines the
2718 set of permissible type arguments for the respective type parameter and controls the
2719 operations supported by values of that type parameter.
2723 TypeConstraint = TypeElem .
2727 If the constraint is an interface literal of the form <code>interface{E}</code> where
2728 <code>E</code> is an embedded <a href="#Interface_types">type element</a> (not a method), in a type parameter list
2729 the enclosing <code>interface{ … }</code> may be omitted for convenience:
2733 [T []P] // = [T interface{[]P}]
2734 [T ~int] // = [T interface{~int}]
2735 [T int|string] // = [T interface{int|string}]
2736 type Constraint ~int // illegal: ~int is not in a type parameter list
2740 We should be able to simplify the rules for comparable or delegate some of them
2741 elsewhere since we have a section that clearly defines how interfaces implement
2742 other interfaces based on their type sets. But this should get us going for now.
2746 The <a href="#Predeclared_identifiers">predeclared</a>
2747 <a href="#Interface_types">interface type</a> <code>comparable</code>
2748 denotes the set of all non-interface types that are
2749 <a href="#Comparison_operators">strictly comparable</a>.
2753 Even though interfaces that are not type parameters are <a href="#Comparison_operators">comparable</a>,
2754 they are not strictly comparable and therefore they do not implement <code>comparable</code>.
2755 However, they <a href="#Satisfying_a_type_constraint">satisfy</a> <code>comparable</code>.
2759 int // implements comparable (int is strictly comparable)
2760 []byte // does not implement comparable (slices cannot be compared)
2761 interface{} // does not implement comparable (see above)
2762 interface{ ~int | ~string } // type parameter only: implements comparable (int, string types are strictly comparable)
2763 interface{ comparable } // type parameter only: implements comparable (comparable implements itself)
2764 interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (slices are not comparable)
2765 interface{ ~struct{ any } } // type parameter only: does not implement comparable (field any is not strictly comparable)
2769 The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
2770 <code>comparable</code> may only be used as type constraints. They cannot be the types of
2771 values or variables, or components of other, non-interface types.
2774 <h4 id="Satisfying_a_type_constraint">Satisfying a type constraint</h4>
2777 A type argument <code>T</code><i> satisfies</i> a type constraint <code>C</code>
2778 if <code>T</code> is an element of the type set defined by <code>C</code>; i.e.,
2779 if <code>T</code> <a href="#Implementing_an_interface">implements</a> <code>C</code>.
2780 As an exception, a <a href="#Comparison_operators">strictly comparable</a>
2781 type constraint may also be satisfied by a <a href="#Comparison_operators">comparable</a>
2782 (not necessarily strictly comparable) type argument.
2787 A type T <i>satisfies</i> a constraint <code>C</code> if
2792 <code>T</code> <a href="#Implementing_an_interface">implements</a> <code>C</code>; or
2795 <code>C</code> can be written in the form <code>interface{ comparable; E }</code>,
2796 where <code>E</code> is a <a href="#Basic_interfaces">basic interface</a> and
2797 <code>T</code> is <a href="#Comparison_operators">comparable</a> and implements <code>E</code>.
2802 type argument type constraint // constraint satisfaction
2804 int interface{ ~int } // satisfied: int implements interface{ ~int }
2805 string comparable // satisfied: string implements comparable (string is strictly comparable)
2806 []byte comparable // not satisfied: slices are not comparable
2807 any interface{ comparable; int } // not satisfied: any does not implement interface{ int }
2808 any comparable // satisfied: any is comparable and implements the basic interface any
2809 struct{f any} comparable // satisfied: struct{f any} is comparable and implements the basic interface any
2810 any interface{ comparable; m() } // not satisfied: any does not implement the basic interface interface{ m() }
2811 interface{ m() } interface{ comparable; m() } // satisfied: interface{ m() } is comparable and implements the basic interface interface{ m() }
2815 Because of the exception in the constraint satisfaction rule, comparing operands of type parameter type
2816 may panic at run-time (even though comparable type parameters are always strictly comparable).
2819 <h3 id="Variable_declarations">Variable declarations</h3>
2822 A variable declaration creates one or more <a href="#Variables">variables</a>,
2823 binds corresponding identifiers to them, and gives each a type and an initial value.
2827 VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
2828 VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
2835 var x, y float32 = -1, -2
2838 u, v, s = 2.0, 3.0, "bar"
2840 var re, im = complexSqrt(-1)
2841 var _, found = entries[name] // map lookup; only interested in "found"
2845 If a list of expressions is given, the variables are initialized
2846 with the expressions following the rules for <a href="#Assignment_statements">assignment statements</a>.
2847 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
2851 If a type is present, each variable is given that type.
2852 Otherwise, each variable is given the type of the corresponding
2853 initialization value in the assignment.
2854 If that value is an untyped constant, it is first implicitly
2855 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
2856 if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
2857 The predeclared value <code>nil</code> cannot be used to initialize a variable
2858 with no explicit type.
2862 var d = math.Sin(0.5) // d is float64
2863 var i = 42 // i is int
2864 var t, ok = x.(T) // t is T, ok is bool
2865 var n = nil // illegal
2869 Implementation restriction: A compiler may make it illegal to declare a variable
2870 inside a <a href="#Function_declarations">function body</a> if the variable is
2874 <h3 id="Short_variable_declarations">Short variable declarations</h3>
2877 A <i>short variable declaration</i> uses the syntax:
2881 ShortVarDecl = IdentifierList ":=" ExpressionList .
2885 It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
2886 with initializer expressions but no types:
2889 <pre class="grammar">
2890 "var" IdentifierList "=" ExpressionList .
2895 f := func() int { return 7 }
2896 ch := make(chan int)
2897 r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
2898 _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
2902 Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
2903 variables provided they were originally declared earlier in the same block
2904 (or the parameter lists if the block is the function body) with the same type,
2905 and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
2906 As a consequence, redeclaration can only appear in a multi-variable short declaration.
2907 Redeclaration does not introduce a new variable; it just assigns a new value to the original.
2908 The non-blank variable names on the left side of <code>:=</code>
2909 must be <a href="#Uniqueness_of_identifiers">unique</a>.
2913 field1, offset := nextField(str, 0)
2914 field2, offset := nextField(str, offset) // redeclares offset
2915 x, y, x := 1, 2, 3 // illegal: x repeated on left side of :=
2919 Short variable declarations may appear only inside functions.
2920 In some contexts such as the initializers for
2921 <a href="#If_statements">"if"</a>,
2922 <a href="#For_statements">"for"</a>, or
2923 <a href="#Switch_statements">"switch"</a> statements,
2924 they can be used to declare local temporary variables.
2927 <h3 id="Function_declarations">Function declarations</h3>
2930 Given the importance of functions, this section has always
2931 been woefully underdeveloped. Would be nice to expand this
2936 A function declaration binds an identifier, the <i>function name</i>,
2941 FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
2942 FunctionName = identifier .
2943 FunctionBody = Block .
2947 If the function's <a href="#Function_types">signature</a> declares
2948 result parameters, the function body's statement list must end in
2949 a <a href="#Terminating_statements">terminating statement</a>.
2953 func IndexRune(s string, r rune) int {
2954 for i, c := range s {
2959 // invalid: missing return statement
2964 If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>,
2965 the function name denotes a <i>generic function</i>.
2966 A generic function must be <a href="#Instantiations">instantiated</a> before it can be
2967 called or used as a value.
2971 func min[T ~int|~float64](x, y T) T {
2980 A function declaration without type parameters may omit the body.
2981 Such a declaration provides the signature for a function implemented outside Go,
2982 such as an assembly routine.
2986 func flushICache(begin, end uintptr) // implemented externally
2989 <h3 id="Method_declarations">Method declarations</h3>
2992 A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
2993 A method declaration binds an identifier, the <i>method name</i>, to a method,
2994 and associates the method with the receiver's <i>base type</i>.
2998 MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
2999 Receiver = Parameters .
3003 The receiver is specified via an extra parameter section preceding the method
3004 name. That parameter section must declare a single non-variadic parameter, the receiver.
3005 Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
3006 pointer to a defined type <code>T</code>, possibly followed by a list of type parameter
3007 names <code>[P1, P2, …]</code> enclosed in square brackets.
3008 <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be
3009 a pointer or interface type and it must be defined in the same package as the method.
3010 The method is said to be <i>bound</i> to its receiver base type and the method name
3011 is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
3016 A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
3017 <a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
3018 If the receiver's value is not referenced inside the body of the method,
3019 its identifier may be omitted in the declaration. The same applies in
3020 general to parameters of functions and methods.
3024 For a base type, the non-blank names of methods bound to it must be unique.
3025 If the base type is a <a href="#Struct_types">struct type</a>,
3026 the non-blank method and field names must be distinct.
3030 Given defined type <code>Point</code> the declarations
3034 func (p *Point) Length() float64 {
3035 return math.Sqrt(p.x * p.x + p.y * p.y)
3038 func (p *Point) Scale(factor float64) {
3045 bind the methods <code>Length</code> and <code>Scale</code>,
3046 with receiver type <code>*Point</code>,
3047 to the base type <code>Point</code>.
3051 If the receiver base type is a <a href="#Type_declarations">generic type</a>, the
3052 receiver specification must declare corresponding type parameters for the method
3053 to use. This makes the receiver type parameters available to the method.
3054 Syntactically, this type parameter declaration looks like an
3055 <a href="#Instantiations">instantiation</a> of the receiver base type: the type
3056 arguments must be identifiers denoting the type parameters being declared, one
3057 for each type parameter of the receiver base type.
3058 The type parameter names do not need to match their corresponding parameter names in the
3059 receiver base type definition, and all non-blank parameter names must be unique in the
3060 receiver parameter section and the method signature.
3061 The receiver type parameter constraints are implied by the receiver base type definition:
3062 corresponding type parameters have corresponding constraints.
3066 type Pair[A, B any] struct {
3071 func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B
3072 func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair
3075 <h2 id="Expressions">Expressions</h2>
3078 An expression specifies the computation of a value by applying
3079 operators and functions to operands.
3082 <h3 id="Operands">Operands</h3>
3085 Operands denote the elementary values in an expression. An operand may be a
3086 literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
3087 non-<a href="#Blank_identifier">blank</a> identifier denoting a
3088 <a href="#Constant_declarations">constant</a>,
3089 <a href="#Variable_declarations">variable</a>, or
3090 <a href="#Function_declarations">function</a>,
3091 or a parenthesized expression.
3095 Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
3096 Literal = BasicLit | CompositeLit | FunctionLit .
3097 BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
3098 OperandName = identifier | QualifiedIdent .
3102 An operand name denoting a <a href="#Function_declarations">generic function</a>
3103 may be followed by a list of <a href="#Instantiations">type arguments</a>; the
3104 resulting operand is an <a href="#Instantiations">instantiated</a> function.
3108 The <a href="#Blank_identifier">blank identifier</a> may appear as an
3109 operand only on the left-hand side of an <a href="#Assignment_statements">assignment statement</a>.
3113 Implementation restriction: A compiler need not report an error if an operand's
3114 type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty
3115 <a href="#Interface_types">type set</a>. Functions with such type parameters
3116 cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead
3117 to an error at the instantiation site.
3120 <h3 id="Qualified_identifiers">Qualified identifiers</h3>
3123 A <i>qualified identifier</i> is an identifier qualified with a package name prefix.
3124 Both the package name and the identifier must not be
3125 <a href="#Blank_identifier">blank</a>.
3129 QualifiedIdent = PackageName "." identifier .
3133 A qualified identifier accesses an identifier in a different package, which
3134 must be <a href="#Import_declarations">imported</a>.
3135 The identifier must be <a href="#Exported_identifiers">exported</a> and
3136 declared in the <a href="#Blocks">package block</a> of that package.
3140 math.Sin // denotes the Sin function in package math
3143 <h3 id="Composite_literals">Composite literals</h3>
3146 Composite literals construct new composite values each time they are evaluated.
3147 They consist of the type of the literal followed by a brace-bound list of elements.
3148 Each element may optionally be preceded by a corresponding key.
3152 CompositeLit = LiteralType LiteralValue .
3153 LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
3154 SliceType | MapType | TypeName [ TypeArgs ] .
3155 LiteralValue = "{" [ ElementList [ "," ] ] "}" .
3156 ElementList = KeyedElement { "," KeyedElement } .
3157 KeyedElement = [ Key ":" ] Element .
3158 Key = FieldName | Expression | LiteralValue .
3159 FieldName = identifier .
3160 Element = Expression | LiteralValue .
3164 The LiteralType's <a href="#Core_types">core type</a> <code>T</code>
3165 must be a struct, array, slice, or map type
3166 (the syntax enforces this constraint except when the type is given
3168 The types of the elements and keys must be <a href="#Assignability">assignable</a>
3169 to the respective field, element, and key types of type <code>T</code>;
3170 there is no additional conversion.
3171 The key is interpreted as a field name for struct literals,
3172 an index for array and slice literals, and a key for map literals.
3173 For map literals, all elements must have a key. It is an error
3174 to specify multiple elements with the same field name or
3175 constant key value. For non-constant map keys, see the section on
3176 <a href="#Order_of_evaluation">evaluation order</a>.
3180 For struct literals the following rules apply:
3183 <li>A key must be a field name declared in the struct type.
3185 <li>An element list that does not contain any keys must
3186 list an element for each struct field in the
3187 order in which the fields are declared.
3189 <li>If any element has a key, every element must have a key.
3191 <li>An element list that contains keys does not need to
3192 have an element for each struct field. Omitted fields
3193 get the zero value for that field.
3195 <li>A literal may omit the element list; such a literal evaluates
3196 to the zero value for its type.
3198 <li>It is an error to specify an element for a non-exported
3199 field of a struct belonging to a different package.
3204 Given the declarations
3207 type Point3D struct { x, y, z float64 }
3208 type Line struct { p, q Point3D }
3216 origin := Point3D{} // zero value for Point3D
3217 line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
3221 For array and slice literals the following rules apply:
3224 <li>Each element has an associated integer index marking
3225 its position in the array.
3227 <li>An element with a key uses the key as its index. The
3228 key must be a non-negative constant
3229 <a href="#Representability">representable</a> by
3230 a value of type <code>int</code>; and if it is typed
3231 it must be of <a href="#Numeric_types">integer type</a>.
3233 <li>An element without a key uses the previous element's index plus one.
3234 If the first element has no key, its index is zero.
3239 <a href="#Address_operators">Taking the address</a> of a composite literal
3240 generates a pointer to a unique <a href="#Variables">variable</a> initialized
3241 with the literal's value.
3245 var pointer *Point3D = &Point3D{y: 1000}
3249 Note that the <a href="#The_zero_value">zero value</a> for a slice or map
3250 type is not the same as an initialized but empty value of the same type.
3251 Consequently, taking the address of an empty slice or map composite literal
3252 does not have the same effect as allocating a new slice or map value with
3253 <a href="#Allocation">new</a>.
3257 p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
3258 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
3262 The length of an array literal is the length specified in the literal type.
3263 If fewer elements than the length are provided in the literal, the missing
3264 elements are set to the zero value for the array element type.
3265 It is an error to provide elements with index values outside the index range
3266 of the array. The notation <code>...</code> specifies an array length equal
3267 to the maximum element index plus one.
3271 buffer := [10]string{} // len(buffer) == 10
3272 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
3273 days := [...]string{"Sat", "Sun"} // len(days) == 2
3277 A slice literal describes the entire underlying array literal.
3278 Thus the length and capacity of a slice literal are the maximum
3279 element index plus one. A slice literal has the form
3287 and is shorthand for a slice operation applied to an array:
3291 tmp := [n]T{x1, x2, … xn}
3296 Within a composite literal of array, slice, or map type <code>T</code>,
3297 elements or map keys that are themselves composite literals may elide the respective
3298 literal type if it is identical to the element or key type of <code>T</code>.
3299 Similarly, elements or keys that are addresses of composite literals may elide
3300 the <code>&T</code> when the element or key type is <code>*T</code>.
3304 [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
3305 [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
3306 [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
3307 map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
3308 map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
3311 [2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
3312 [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
3316 A parsing ambiguity arises when a composite literal using the
3317 TypeName form of the LiteralType appears as an operand between the
3318 <a href="#Keywords">keyword</a> and the opening brace of the block
3319 of an "if", "for", or "switch" statement, and the composite literal
3320 is not enclosed in parentheses, square brackets, or curly braces.
3321 In this rare case, the opening brace of the literal is erroneously parsed
3322 as the one introducing the block of statements. To resolve the ambiguity,
3323 the composite literal must appear within parentheses.
3327 if x == (T{a,b,c}[i]) { … }
3328 if (x == T{a,b,c}[i]) { … }
3332 Examples of valid array, slice, and map literals:
3336 // list of prime numbers
3337 primes := []int{2, 3, 5, 7, 9, 2147483647}
3339 // vowels[ch] is true if ch is a vowel
3340 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
3342 // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
3343 filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
3345 // frequencies in Hz for equal-tempered scale (A4 = 440Hz)
3346 noteFrequency := map[string]float32{
3347 "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
3348 "G0": 24.50, "A0": 27.50, "B0": 30.87,
3353 <h3 id="Function_literals">Function literals</h3>
3356 A function literal represents an anonymous <a href="#Function_declarations">function</a>.
3357 Function literals cannot declare type parameters.
3361 FunctionLit = "func" Signature FunctionBody .
3365 func(a, b int, z float64) bool { return a*b < int(z) }
3369 A function literal can be assigned to a variable or invoked directly.
3373 f := func(x, y int) int { return x + y }
3374 func(ch chan int) { ch <- ACK }(replyChan)
3378 Function literals are <i>closures</i>: they may refer to variables
3379 defined in a surrounding function. Those variables are then shared between
3380 the surrounding function and the function literal, and they survive as long
3381 as they are accessible.
3385 <h3 id="Primary_expressions">Primary expressions</h3>
3388 Primary expressions are the operands for unary and binary expressions.
3396 PrimaryExpr Selector |
3399 PrimaryExpr TypeAssertion |
3400 PrimaryExpr Arguments .
3402 Selector = "." identifier .
3403 Index = "[" Expression [ "," ] "]" .
3404 Slice = "[" [ Expression ] ":" [ Expression ] "]" |
3405 "[" [ Expression ] ":" Expression ":" Expression "]" .
3406 TypeAssertion = "." "(" Type ")" .
3407 Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
3424 <h3 id="Selectors">Selectors</h3>
3427 For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
3428 that is not a <a href="#Package_clause">package name</a>, the
3429 <i>selector expression</i>
3437 denotes the field or method <code>f</code> of the value <code>x</code>
3438 (or sometimes <code>*x</code>; see below).
3439 The identifier <code>f</code> is called the (field or method) <i>selector</i>;
3440 it must not be the <a href="#Blank_identifier">blank identifier</a>.
3441 The type of the selector expression is the type of <code>f</code>.
3442 If <code>x</code> is a package name, see the section on
3443 <a href="#Qualified_identifiers">qualified identifiers</a>.
3447 A selector <code>f</code> may denote a field or method <code>f</code> of
3448 a type <code>T</code>, or it may refer
3449 to a field or method <code>f</code> of a nested
3450 <a href="#Struct_types">embedded field</a> of <code>T</code>.
3451 The number of embedded fields traversed
3452 to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
3453 The depth of a field or method <code>f</code>
3454 declared in <code>T</code> is zero.
3455 The depth of a field or method <code>f</code> declared in
3456 an embedded field <code>A</code> in <code>T</code> is the
3457 depth of <code>f</code> in <code>A</code> plus one.
3461 The following rules apply to selectors:
3466 For a value <code>x</code> of type <code>T</code> or <code>*T</code>
3467 where <code>T</code> is not a pointer or interface type,
3468 <code>x.f</code> denotes the field or method at the shallowest depth
3469 in <code>T</code> where there is such an <code>f</code>.
3470 If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
3471 with shallowest depth, the selector expression is illegal.
3475 For a value <code>x</code> of type <code>I</code> where <code>I</code>
3476 is an interface type, <code>x.f</code> denotes the actual method with name
3477 <code>f</code> of the dynamic value of <code>x</code>.
3478 If there is no method with name <code>f</code> in the
3479 <a href="#Method_sets">method set</a> of <code>I</code>, the selector
3480 expression is illegal.
3484 As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
3485 pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
3486 (but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
3490 In all other cases, <code>x.f</code> is illegal.
3494 If <code>x</code> is of pointer type and has the value
3495 <code>nil</code> and <code>x.f</code> denotes a struct field,
3496 assigning to or evaluating <code>x.f</code>
3497 causes a <a href="#Run_time_panics">run-time panic</a>.
3501 If <code>x</code> is of interface type and has the value
3502 <code>nil</code>, <a href="#Calls">calling</a> or
3503 <a href="#Method_values">evaluating</a> the method <code>x.f</code>
3504 causes a <a href="#Run_time_panics">run-time panic</a>.
3509 For example, given the declarations:
3535 var t T2 // with t.T0 != nil
3536 var p *T2 // with p != nil and (*p).T0 != nil
3553 q.x // (*(*q).T0).x (*q).x is a valid field selector
3555 p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
3556 p.M1() // ((*p).T1).M1() M1 expects T1 receiver
3557 p.M2() // p.M2() M2 expects *T2 receiver
3558 t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
3562 but the following is invalid:
3566 q.M0() // (*q).M0 is valid but not a field selector
3570 <h3 id="Method_expressions">Method expressions</h3>
3573 If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3574 <code>T.M</code> is a function that is callable as a regular function
3575 with the same arguments as <code>M</code> prefixed by an additional
3576 argument that is the receiver of the method.
3580 MethodExpr = ReceiverType "." MethodName .
3581 ReceiverType = Type .
3585 Consider a struct type <code>T</code> with two methods,
3586 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3587 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3594 func (tv T) Mv(a int) int { return 0 } // value receiver
3595 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3609 yields a function equivalent to <code>Mv</code> but
3610 with an explicit receiver as its first argument; it has signature
3614 func(tv T, a int) int
3618 That function may be called normally with an explicit receiver, so
3619 these five invocations are equivalent:
3626 f1 := T.Mv; f1(t, 7)
3627 f2 := (T).Mv; f2(t, 7)
3631 Similarly, the expression
3639 yields a function value representing <code>Mp</code> with signature
3643 func(tp *T, f float32) float32
3647 For a method with a value receiver, one can derive a function
3648 with an explicit pointer receiver, so
3656 yields a function value representing <code>Mv</code> with signature
3660 func(tv *T, a int) int
3664 Such a function indirects through the receiver to create a value
3665 to pass as the receiver to the underlying method;
3666 the method does not overwrite the value whose address is passed in
3671 The final case, a value-receiver function for a pointer-receiver method,
3672 is illegal because pointer-receiver methods are not in the method set
3677 Function values derived from methods are called with function call syntax;
3678 the receiver is provided as the first argument to the call.
3679 That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
3680 as <code>f(t, 7)</code> not <code>t.f(7)</code>.
3681 To construct a function that binds the receiver, use a
3682 <a href="#Function_literals">function literal</a> or
3683 <a href="#Method_values">method value</a>.
3687 It is legal to derive a function value from a method of an interface type.
3688 The resulting function takes an explicit receiver of that interface type.
3691 <h3 id="Method_values">Method values</h3>
3694 If the expression <code>x</code> has static type <code>T</code> and
3695 <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3696 <code>x.M</code> is called a <i>method value</i>.
3697 The method value <code>x.M</code> is a function value that is callable
3698 with the same arguments as a method call of <code>x.M</code>.
3699 The expression <code>x</code> is evaluated and saved during the evaluation of the
3700 method value; the saved copy is then used as the receiver in any calls,
3701 which may be executed later.
3705 type S struct { *T }
3707 func (t T) M() { print(t) }
3711 f := t.M // receiver *t is evaluated and stored in f
3712 g := s.M // receiver *(s.T) is evaluated and stored in g
3713 *t = 42 // does not affect stored receivers in f and g
3717 The type <code>T</code> may be an interface or non-interface type.
3721 As in the discussion of <a href="#Method_expressions">method expressions</a> above,
3722 consider a struct type <code>T</code> with two methods,
3723 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3724 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3731 func (tv T) Mv(a int) int { return 0 } // value receiver
3732 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3748 yields a function value of type
3756 These two invocations are equivalent:
3765 Similarly, the expression
3773 yields a function value of type
3777 func(float32) float32
3781 As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
3782 using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
3786 As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
3787 using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
3791 f := t.Mv; f(7) // like t.Mv(7)
3792 f := pt.Mp; f(7) // like pt.Mp(7)
3793 f := pt.Mv; f(7) // like (*pt).Mv(7)
3794 f := t.Mp; f(7) // like (&t).Mp(7)
3795 f := makeT().Mp // invalid: result of makeT() is not addressable
3799 Although the examples above use non-interface types, it is also legal to create a method value
3800 from a value of interface type.
3804 var i interface { M(int) } = myVal
3805 f := i.M; f(7) // like i.M(7)
3809 <h3 id="Index_expressions">Index expressions</h3>
3812 A primary expression of the form
3820 denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
3821 The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
3822 The following rules apply:
3826 If <code>a</code> is neither a map nor a type parameter:
3829 <li>the index <code>x</code> must be an untyped constant or its
3830 <a href="#Core_types">core type</a> must be an <a href="#Numeric_types">integer</a></li>
3831 <li>a constant index must be non-negative and
3832 <a href="#Representability">representable</a> by a value of type <code>int</code></li>
3833 <li>a constant index that is untyped is given type <code>int</code></li>
3834 <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
3835 otherwise it is <i>out of range</i></li>
3839 For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
3842 <li>a <a href="#Constants">constant</a> index must be in range</li>
3843 <li>if <code>x</code> is out of range at run time,
3844 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3845 <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
3846 <code>a[x]</code> is the element type of <code>A</code></li>
3850 For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
3853 <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
3857 For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
3860 <li>if <code>x</code> is out of range at run time,
3861 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3862 <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
3863 <code>a[x]</code> is the element type of <code>S</code></li>
3867 For <code>a</code> of <a href="#String_types">string type</a>:
3870 <li>a <a href="#Constants">constant</a> index must be in range
3871 if the string <code>a</code> is also constant</li>
3872 <li>if <code>x</code> is out of range at run time,
3873 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3874 <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
3875 <code>a[x]</code> is <code>byte</code></li>
3876 <li><code>a[x]</code> may not be assigned to</li>
3880 For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
3883 <li><code>x</code>'s type must be
3884 <a href="#Assignability">assignable</a>
3885 to the key type of <code>M</code></li>
3886 <li>if the map contains an entry with key <code>x</code>,
3887 <code>a[x]</code> is the map element with key <code>x</code>
3888 and the type of <code>a[x]</code> is the element type of <code>M</code></li>
3889 <li>if the map is <code>nil</code> or does not contain such an entry,
3890 <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
3891 for the element type of <code>M</code></li>
3895 For <code>a</code> of <a href="#Type_parameter_declarations">type parameter type</a> <code>P</code>:
3898 <li>The index expression <code>a[x]</code> must be valid for values
3899 of all types in <code>P</code>'s type set.</li>
3900 <li>The element types of all types in <code>P</code>'s type set must be identical.
3901 In this context, the element type of a string type is <code>byte</code>.</li>
3902 <li>If there is a map type in the type set of <code>P</code>,
3903 all types in that type set must be map types, and the respective key types
3904 must be all identical.</li>
3905 <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
3906 or the map element with key <code>x</code> of the type argument
3907 that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
3908 the type of the (identical) element types.</li>
3909 <li><code>a[x]</code> may not be assigned to if <code>P</code>'s type set
3910 includes string types.
3914 Otherwise <code>a[x]</code> is illegal.
3918 An index expression on a map <code>a</code> of type <code>map[K]V</code>
3919 used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
3929 yields an additional untyped boolean value. The value of <code>ok</code> is
3930 <code>true</code> if the key <code>x</code> is present in the map, and
3931 <code>false</code> otherwise.
3935 Assigning to an element of a <code>nil</code> map causes a
3936 <a href="#Run_time_panics">run-time panic</a>.
3940 <h3 id="Slice_expressions">Slice expressions</h3>
3943 Slice expressions construct a substring or slice from a string, array, pointer
3944 to array, or slice. There are two variants: a simple form that specifies a low
3945 and high bound, and a full form that also specifies a bound on the capacity.
3948 <h4>Simple slice expressions</h4>
3951 The primary expression
3959 constructs a substring or slice. The <a href="#Core_types">core type</a> of
3960 <code>a</code> must be a string, array, pointer to array, slice, or a
3961 <a href="#Core_types"><code>bytestring</code></a>.
3962 The <i>indices</i> <code>low</code> and
3963 <code>high</code> select which elements of operand <code>a</code> appear
3964 in the result. The result has indices starting at 0 and length equal to
3965 <code>high</code> - <code>low</code>.
3966 After slicing the array <code>a</code>
3970 a := [5]int{1, 2, 3, 4, 5}
3975 the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
3985 For convenience, any of the indices may be omitted. A missing <code>low</code>
3986 index defaults to zero; a missing <code>high</code> index defaults to the length of the
3991 a[2:] // same as a[2 : len(a)]
3992 a[:3] // same as a[0 : 3]
3993 a[:] // same as a[0 : len(a)]
3997 If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
3998 <code>(*a)[low : high]</code>.
4002 For arrays or strings, the indices are <i>in range</i> if
4003 <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
4004 otherwise they are <i>out of range</i>.
4005 For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
4006 A <a href="#Constants">constant</a> index must be non-negative and
4007 <a href="#Representability">representable</a> by a value of type
4008 <code>int</code>; for arrays or constant strings, constant indices must also be in range.
4009 If both indices are constant, they must satisfy <code>low <= high</code>.
4010 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4014 Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
4015 the result of the slice operation is a non-constant value of the same type as the operand.
4016 For untyped string operands the result is a non-constant value of type <code>string</code>.
4017 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
4018 and the result of the slice operation is a slice with the same element type as the array.
4022 If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
4023 is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
4024 array with the operand.
4029 s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
4030 s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
4031 s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
4034 s3 := s[:0] // s3 == nil
4038 <h4>Full slice expressions</h4>
4041 The primary expression
4049 constructs a slice of the same type, and with the same length and elements as the simple slice
4050 expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
4051 by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
4052 The <a href="#Core_types">core type</a> of <code>a</code> must be an array, pointer to array,
4053 or slice (but not a string).
4054 After slicing the array <code>a</code>
4058 a := [5]int{1, 2, 3, 4, 5}
4063 the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
4072 As for simple slice expressions, if <code>a</code> is a pointer to an array,
4073 <code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
4074 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
4078 The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
4079 otherwise they are <i>out of range</i>.
4080 A <a href="#Constants">constant</a> index must be non-negative and
4081 <a href="#Representability">representable</a> by a value of type
4082 <code>int</code>; for arrays, constant indices must also be in range.
4083 If multiple indices are constant, the constants that are present must be in range relative to each
4085 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4088 <h3 id="Type_assertions">Type assertions</h3>
4091 For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
4092 but not a <a href="#Type_parameter_declarations">type parameter</a>, and a type <code>T</code>,
4093 the primary expression
4101 asserts that <code>x</code> is not <code>nil</code>
4102 and that the value stored in <code>x</code> is of type <code>T</code>.
4103 The notation <code>x.(T)</code> is called a <i>type assertion</i>.
4106 More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
4107 that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
4108 to the type <code>T</code>.
4109 In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
4110 otherwise the type assertion is invalid since it is not possible for <code>x</code>
4111 to store a value of type <code>T</code>.
4112 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
4113 of <code>x</code> <a href="#Implementing_an_interface">implements</a> the interface <code>T</code>.
4116 If the type assertion holds, the value of the expression is the value
4117 stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
4118 a <a href="#Run_time_panics">run-time panic</a> occurs.
4119 In other words, even though the dynamic type of <code>x</code>
4120 is known only at run time, the type of <code>x.(T)</code> is
4121 known to be <code>T</code> in a correct program.
4125 var x interface{} = 7 // x has dynamic type int and value 7
4126 i := x.(int) // i has type int and value 7
4128 type I interface { m() }
4131 s := y.(string) // illegal: string does not implement I (missing method m)
4132 r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
4138 A type assertion used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
4145 var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
4149 yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
4150 if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
4151 the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
4152 No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
4156 <h3 id="Calls">Calls</h3>
4159 Given an expression <code>f</code> with a <a href="#Core_types">core type</a>
4160 <code>F</code> of <a href="#Function_types">function type</a>,
4168 calls <code>f</code> with arguments <code>a1, a2, … an</code>.
4169 Except for one special case, arguments must be single-valued expressions
4170 <a href="#Assignability">assignable</a> to the parameter types of
4171 <code>F</code> and are evaluated before the function is called.
4172 The type of the expression is the result type
4174 A method invocation is similar but the method itself
4175 is specified as a selector upon a value of the receiver type for
4180 math.Atan2(x, y) // function call
4182 pt.Scale(3.5) // method call with receiver pt
4186 If <code>f</code> denotes a generic function, it must be
4187 <a href="#Instantiations">instantiated</a> before it can be called
4188 or used as a function value.
4192 In a function call, the function value and arguments are evaluated in
4193 <a href="#Order_of_evaluation">the usual order</a>.
4194 After they are evaluated, the parameters of the call are passed by value to the function
4195 and the called function begins execution.
4196 The return parameters of the function are passed by value
4197 back to the caller when the function returns.
4201 Calling a <code>nil</code> function value
4202 causes a <a href="#Run_time_panics">run-time panic</a>.
4206 As a special case, if the return values of a function or method
4207 <code>g</code> are equal in number and individually
4208 assignable to the parameters of another function or method
4209 <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
4210 will invoke <code>f</code> after binding the return values of
4211 <code>g</code> to the parameters of <code>f</code> in order. The call
4212 of <code>f</code> must contain no parameters other than the call of <code>g</code>,
4213 and <code>g</code> must have at least one return value.
4214 If <code>f</code> has a final <code>...</code> parameter, it is
4215 assigned the return values of <code>g</code> that remain after
4216 assignment of regular parameters.
4220 func Split(s string, pos int) (string, string) {
4221 return s[0:pos], s[pos:]
4224 func Join(s, t string) string {
4228 if Join(Split(value, len(value)/2)) != value {
4229 log.Panic("test fails")
4234 A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
4235 of (the type of) <code>x</code> contains <code>m</code> and the
4236 argument list can be assigned to the parameter list of <code>m</code>.
4237 If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
4238 set contains <code>m</code>, <code>x.m()</code> is shorthand
4239 for <code>(&x).m()</code>:
4248 There is no distinct method type and there are no method literals.
4251 <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
4254 If <code>f</code> is <a href="#Function_types">variadic</a> with a final
4255 parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
4256 the type of <code>p</code> is equivalent to type <code>[]T</code>.
4257 If <code>f</code> is invoked with no actual arguments for <code>p</code>,
4258 the value passed to <code>p</code> is <code>nil</code>.
4259 Otherwise, the value passed is a new slice
4260 of type <code>[]T</code> with a new underlying array whose successive elements
4261 are the actual arguments, which all must be <a href="#Assignability">assignable</a>
4262 to <code>T</code>. The length and capacity of the slice is therefore
4263 the number of arguments bound to <code>p</code> and may differ for each
4268 Given the function and calls
4271 func Greeting(prefix string, who ...string)
4273 Greeting("hello:", "Joe", "Anna", "Eileen")
4277 within <code>Greeting</code>, <code>who</code> will have the value
4278 <code>nil</code> in the first call, and
4279 <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
4283 If the final argument is assignable to a slice type <code>[]T</code> and
4284 is followed by <code>...</code>, it is passed unchanged as the value
4285 for a <code>...T</code> parameter. In this case no new slice is created.
4289 Given the slice <code>s</code> and call
4293 s := []string{"James", "Jasmine"}
4294 Greeting("goodbye:", s...)
4298 within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
4299 with the same underlying array.
4302 <h3 id="Instantiations">Instantiations</h3>
4305 A generic function or type is <i>instantiated</i> by substituting <i>type arguments</i>
4306 for the type parameters.
4307 Instantiation proceeds in two steps:
4312 Each type argument is substituted for its corresponding type parameter in the generic
4314 This substitution happens across the entire function or type declaration,
4315 including the type parameter list itself and any types in that list.
4319 After substitution, each type argument must <a href="#Satisfying_a_type_constraint">satisfy</a>
4320 the <a href="#Type_parameter_declarations">constraint</a> (instantiated, if necessary)
4321 of the corresponding type parameter. Otherwise instantiation fails.
4326 Instantiating a type results in a new non-generic <a href="#Types">named type</a>;
4327 instantiating a function produces a new non-generic function.
4331 type parameter list type arguments after substitution
4333 [P any] int int satisfies any
4334 [S ~[]E, E any] []int, int []int satisfies ~[]int, int satisfies any
4335 [P io.Writer] string illegal: string doesn't satisfy io.Writer
4336 [P comparable] any any satisfies (but does not implement) comparable
4340 For a generic function, type arguments may be provided explicitly, or they
4341 may be partially or completely <a href="#Type_inference">inferred</a>.
4342 A generic function that is <i>not</i> <a href="#Calls">called</a> requires a
4343 type argument list for instantiation; if the list is partial, all
4344 remaining type arguments must be inferrable.
4345 A generic function that is called may provide a (possibly partial) type
4346 argument list, or may omit it entirely if the omitted type arguments are
4347 inferrable from the ordinary (non-type) function arguments.
4351 func min[T ~int|~float64](x, y T) T { … }
4353 f := min // illegal: min must be instantiated with type arguments when used without being called
4354 minInt := min[int] // minInt has type func(x, y int) int
4355 a := minInt(2, 3) // a has value 2 of type int
4356 b := min[float64](2.0, 3) // b has value 2.0 of type float64
4357 c := min(b, -1) // c has value -1.0 of type float64
4361 A partial type argument list cannot be empty; at least the first argument must be present.
4362 The list is a prefix of the full list of type arguments, leaving the remaining arguments
4363 to be inferred. Loosely speaking, type arguments may be omitted from "right to left".
4367 func apply[S ~[]E, E any](s S, f func(E) E) S { … }
4369 f0 := apply[] // illegal: type argument list cannot be empty
4370 f1 := apply[[]int] // type argument for S explicitly provided, type argument for E inferred
4371 f2 := apply[[]string, string] // both type arguments explicitly provided
4374 r := apply(bytes, func(byte) byte { … }) // both type arguments inferred from the function arguments
4378 For a generic type, all type arguments must always be provided explicitly.
4381 <h3 id="Type_inference">Type inference</h3>
4384 Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
4385 Each step attempts to use known information to infer additional type arguments.
4386 Type inference stops as soon as all type arguments are known.
4387 After type inference is complete, it is still necessary to substitute all type arguments
4388 for type parameters and verify that each type argument
4389 <a href="#Implementing_an_interface">implements</a> the relevant constraint;
4390 it is possible for an inferred type argument to fail to implement a constraint, in which
4391 case instantiation fails.
4395 Type inference is based on
4400 a <a href="#Type_parameter_declarations">type parameter list</a>
4403 a substitution map <i>M</i> initialized with the known type arguments, if any
4406 a (possibly empty) list of ordinary function arguments (in case of a function call only)
4411 and then proceeds with the following steps:
4416 apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
4417 to all <i>typed</i> ordinary function arguments
4420 apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
4423 apply function argument type inference to all <i>untyped</i> ordinary function arguments
4424 using the default type for each of the untyped function arguments
4427 apply constraint type inference
4432 If there are no ordinary or untyped function arguments, the respective steps are skipped.
4433 Constraint type inference is skipped if the previous step didn't infer any new type arguments,
4434 but it is run at least once if there are missing type arguments.
4438 The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
4439 The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
4440 If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
4443 <h4 id="Type_unification">Type unification</h4>
4446 Type inference is based on <i>type unification</i>. A single unification step
4447 applies to a <a href="#Type_inference">substitution map</a> and two types, either
4448 or both of which may be or contain type parameters. The substitution map tracks
4449 the known (explicitly provided or already inferred) type arguments: the map
4450 contains an entry <code>P</code> → <code>A</code> for each type
4451 parameter <code>P</code> and corresponding known type argument <code>A</code>.
4452 During unification, known type arguments take the place of their corresponding type
4453 parameters when comparing types. Unification is the process of finding substitution
4454 map entries that make the two types equivalent.
4458 For unification, two types that don't contain any type parameters from the current type
4459 parameter list are <i>equivalent</i>
4460 if they are identical, or if they are channel types that are identical ignoring channel
4461 direction, or if their underlying types are equivalent.
4465 Unification works by comparing the structure of pairs of types: their structure
4466 disregarding type parameters must be identical, and types other than type parameters
4468 A type parameter in one type may match any complete subtype in the other type;
4469 each successful match causes an entry to be added to the substitution map.
4470 If the structure differs, or types other than type parameters are not equivalent,
4475 TODO(gri) Somewhere we need to describe the process of adding an entry to the
4476 substitution map: if the entry is already present, the type argument
4477 values are themselves unified.
4481 For example, if <code>T1</code> and <code>T2</code> are type parameters,
4482 <code>[]map[int]bool</code> can be unified with any of the following:
4486 []map[int]bool // types are identical
4487 T1 // adds T1 → []map[int]bool to substitution map
4488 []T1 // adds T1 → map[int]bool to substitution map
4489 []map[T1]T2 // adds T1 → int and T2 → bool to substitution map
4493 On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
4497 int // int is not a slice
4498 struct{} // a struct is not a slice
4499 []struct{} // a struct is not a map
4500 []map[T1]string // map element types don't match
4504 As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
4505 <code>D</code> and a type literal <code>L</code> are never equivalent,
4506 unification compares the underlying type of <code>D</code> with <code>L</code> instead.
4507 For example, given the defined type
4511 type Vector []float64
4515 and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
4516 <code>[]E</code> and adds an entry <code>E</code> → <code>float64</code> to
4517 the substitution map.
4520 <h4 id="Function_argument_type_inference">Function argument type inference</h4>
4522 <!-- In this section and the section on constraint type inference we start with examples
4523 rather than have the examples follow the rules as is customary elsewhere in spec.
4524 Hopefully this helps building an intuition and makes the rules easier to follow. -->
4527 Function argument type inference infers type arguments from function arguments:
4528 if a function parameter is declared with a type <code>T</code> that uses
4530 <a href="#Type_unification">unifying</a> the type of the corresponding
4531 function argument with <code>T</code> may infer type arguments for the type
4532 parameters used by <code>T</code>.
4536 For instance, given the generic function
4540 func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
4548 var vector []float64
4549 scaledVector := scale(vector, 42)
4553 the type argument for <code>Number</code> can be inferred from the function argument
4554 <code>vector</code> by unifying the type of <code>vector</code> with the corresponding
4555 parameter type: <code>[]float64</code> and <code>[]Number</code>
4556 match in structure and <code>float64</code> matches with <code>Number</code>.
4557 This adds the entry <code>Number</code> → <code>float64</code> to the
4558 <a href="#Type_unification">substitution map</a>.
4559 Untyped arguments, such as the second function argument <code>42</code> here, are ignored
4560 in the first round of function argument type inference and only considered if there are
4561 unresolved type parameters left.
4565 Inference happens in two separate phases; each phase operates on a specific list of
4566 (parameter, argument) pairs:
4571 The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
4572 type uses type parameters and where the function argument is <i>typed</i>.
4575 The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
4576 type parameter. In this list, the respective function arguments are untyped.
4581 Any other (parameter, argument) pair is ignored.
4585 By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
4586 (or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
4587 of untyped values are always predeclared non-composite types, they can never match against
4588 a composite type, so it is sufficient to only consider parameter types that are single type
4593 Each list is processed in a separate phase:
4598 In the first phase, the parameter and argument types of each pair in <i>Lt</i>
4599 are unified. If unification succeeds for a pair, it may yield new entries that
4600 are added to the substitution map <i>M</i>. If unification fails, type inference
4604 The second phase considers the entries of list <i>Lu</i>. Type parameters for
4605 which the type argument has already been determined are ignored in this phase.
4606 For each remaining pair, the parameter type (which is a single type parameter) and
4607 the <a href="#Constants">default type</a> of the corresponding untyped argument is
4608 unified. If unification fails, type inference fails.
4613 While unification is successful, processing of each list continues until all list elements
4614 are considered, even if all type arguments are inferred before the last list element has
4623 func min[T ~int|~float64](x, y T) T
4626 min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
4627 min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
4628 min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
4632 In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
4633 yields the substitution map entry <code>T</code> → <code>float64</code>. Because
4634 processing continues until all untyped arguments are considered, an error is reported. This
4635 ensures that type inference does not depend on the order of the untyped arguments.
4638 <h4 id="Constraint_type_inference">Constraint type inference</h4>
4641 Constraint type inference infers type arguments by considering type constraints.
4642 If a type parameter <code>P</code> has a constraint with a
4643 <a href="#Core_types">core type</a> <code>C</code>,
4644 <a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
4645 may infer additional type arguments, either the type argument for <code>P</code>,
4646 or if that is already known, possibly the type arguments for type parameters
4647 used in <code>C</code>.
4651 For instance, consider the type parameter list with type parameters <code>List</code> and
4656 [List ~[]Elem, Elem any]
4660 Constraint type inference can deduce the type of <code>Elem</code> from the type argument
4661 for <code>List</code> because <code>Elem</code> is a type parameter in the core type
4662 <code>[]Elem</code> of <code>List</code>.
4663 If the type argument is <code>Bytes</code>:
4671 unifying the underlying type of <code>Bytes</code> with the core type means
4672 unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
4673 the <a href="#Type_unification">substitution map</a> entry
4674 <code>Elem</code> → <code>byte</code>.
4675 Thus, in this example, constraint type inference can infer the second type argument from the
4680 Using the core type of a constraint may lose some information: In the (unlikely) case that
4681 the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
4682 <code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
4683 <code>N</code> itself. In this case, constraint type inference may succeed but instantiation
4684 will fail because the inferred type is not in the type set of the constraint.
4685 Thus, constraint type inference uses the <i>adjusted core type</i> of
4686 a constraint: if the type set contains a single type, use that type; otherwise use the
4687 constraint's core type.
4691 Generally, constraint type inference proceeds in two phases: Starting with a given
4692 substitution map <i>M</i>
4697 For all type parameters with an adjusted core type, unify the type parameter with that
4698 type. If any unification fails, constraint type inference fails.
4702 At this point, some entries in <i>M</i> may map type parameters to other
4703 type parameters or to types containing type parameters. For each entry
4704 <code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
4705 contains type parameters <code>Q</code> for which there exist entries
4706 <code>Q</code> → <code>B</code> in <i>M</i>, substitute those
4707 <code>Q</code> with the respective <code>B</code> in <code>A</code>.
4708 Stop when no further substitution is possible.
4713 The result of constraint type inference is the final substitution map <i>M</i> from type
4714 parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
4715 appears in any of the <code>A</code>.
4719 For instance, given the type parameter list
4723 [A any, B []C, C *A]
4727 and the single provided type argument <code>int</code> for type parameter <code>A</code>,
4728 the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
4732 In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
4733 with the core type of their respective constraints. This adds the entries
4734 <code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
4738 At this point there are two entries in <i>M</i> where the right-hand side
4739 is or contains type parameters for which there exists other entries in <i>M</i>:
4740 <code>[]C</code> and <code>*A</code>.
4741 In the second phase, these type parameters are replaced with their respective
4742 types. It doesn't matter in which order this happens. Starting with the state
4743 of <i>M</i> after the first phase:
4747 <code>A</code> → <code>int</code>,
4748 <code>B</code> → <code>[]C</code>,
4749 <code>C</code> → <code>*A</code>
4753 Replace <code>A</code> on the right-hand side of → with <code>int</code>:
4757 <code>A</code> → <code>int</code>,
4758 <code>B</code> → <code>[]C</code>,
4759 <code>C</code> → <code>*int</code>
4763 Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
4767 <code>A</code> → <code>int</code>,
4768 <code>B</code> → <code>[]*int</code>,
4769 <code>C</code> → <code>*int</code>
4773 At this point no further substitution is possible and the map is full.
4774 Therefore, <code>M</code> represents the final map of type parameters
4775 to type arguments for the given type parameter list.
4778 <h3 id="Operators">Operators</h3>
4781 Operators combine operands into expressions.
4785 Expression = UnaryExpr | Expression binary_op Expression .
4786 UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
4788 binary_op = "||" | "&&" | rel_op | add_op | mul_op .
4789 rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
4790 add_op = "+" | "-" | "|" | "^" .
4791 mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
4793 unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
4797 Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
4798 For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
4799 unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
4800 For operations involving constants only, see the section on
4801 <a href="#Constant_expressions">constant expressions</a>.
4805 Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
4806 and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
4807 to the type of the other operand.
4811 The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
4812 or be an untyped constant <a href="#Representability">representable</a> by a
4813 value of type <code>uint</code>.
4814 If the left operand of a non-constant shift expression is an untyped constant,
4815 it is first implicitly converted to the type it would assume if the shift expression were
4816 replaced by its left operand alone.
4823 // The results of the following examples are given for 64-bit ints.
4824 var i = 1<<s // 1 has type int
4825 var j int32 = 1<<s // 1 has type int32; j == 0
4826 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
4827 var m int = 1.0<<s // 1.0 has type int; m == 1<<33
4828 var n = 1.0<<s == j // 1.0 has type int32; n == true
4829 var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
4830 var p = 1<<s == 1<<33 // 1 has type int; p == true
4831 var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
4832 var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
4833 var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
4834 var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift
4835 var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift
4836 var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
4837 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
4838 var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
4840 // The results of the following examples are given for 32-bit ints,
4841 // which means the shifts will overflow.
4842 var mm int = 1.0<<s // 1.0 has type int; mm == 0
4843 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
4844 var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
4845 var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
4846 var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
4849 <h4 id="Operator_precedence">Operator precedence</h4>
4851 Unary operators have the highest precedence.
4852 As the <code>++</code> and <code>--</code> operators form
4853 statements, not expressions, they fall
4854 outside the operator hierarchy.
4855 As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
4857 There are five precedence levels for binary operators.
4858 Multiplication operators bind strongest, followed by addition
4859 operators, comparison operators, <code>&&</code> (logical AND),
4860 and finally <code>||</code> (logical OR):
4863 <pre class="grammar">
4865 5 * / % << >> & &^
4867 3 == != < <= > >=
4873 Binary operators of the same precedence associate from left to right.
4874 For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
4883 x == y+1 && <-chanInt > 0
4887 <h3 id="Arithmetic_operators">Arithmetic operators</h3>
4889 Arithmetic operators apply to numeric values and yield a result of the same
4890 type as the first operand. The four standard arithmetic operators (<code>+</code>,
4891 <code>-</code>, <code>*</code>, <code>/</code>) apply to
4892 <a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
4893 <a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>.
4894 The bitwise logical and shift operators apply to integers only.
4897 <pre class="grammar">
4898 + sum integers, floats, complex values, strings
4899 - difference integers, floats, complex values
4900 * product integers, floats, complex values
4901 / quotient integers, floats, complex values
4902 % remainder integers
4904 & bitwise AND integers
4905 | bitwise OR integers
4906 ^ bitwise XOR integers
4907 &^ bit clear (AND NOT) integers
4909 << left shift integer << integer >= 0
4910 >> right shift integer >> integer >= 0
4914 If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>,
4915 the operator must apply to each type in that type set.
4916 The operands are represented as values of the type argument that the type parameter
4917 is <a href="#Instantiations">instantiated</a> with, and the operation is computed
4918 with the precision of that type argument. For example, given the function:
4922 func dotProduct[F ~float32|~float64](v1, v2 []F) F {
4924 for i, x := range v1 {
4933 the product <code>x * y</code> and the addition <code>s += x * y</code>
4934 are computed with <code>float32</code> or <code>float64</code> precision,
4935 respectively, depending on the type argument for <code>F</code>.
4938 <h4 id="Integer_operators">Integer operators</h4>
4941 For two integer values <code>x</code> and <code>y</code>, the integer quotient
4942 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
4947 x = q*y + r and |r| < |y|
4951 with <code>x / y</code> truncated towards zero
4952 (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
4964 The one exception to this rule is that if the dividend <code>x</code> is
4965 the most negative value for the int type of <code>x</code>, the quotient
4966 <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
4967 due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
4975 int64 -9223372036854775808
4979 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
4980 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4981 If the dividend is non-negative and the divisor is a constant power of 2,
4982 the division may be replaced by a right shift, and computing the remainder may
4983 be replaced by a bitwise AND operation:
4987 x x / 4 x % 4 x >> 2 x & 3
4993 The shift operators shift the left operand by the shift count specified by the
4994 right operand, which must be non-negative. If the shift count is negative at run time,
4995 a <a href="#Run_time_panics">run-time panic</a> occurs.
4996 The shift operators implement arithmetic shifts if the left operand is a signed
4997 integer and logical shifts if it is an unsigned integer.
4998 There is no upper limit on the shift count. Shifts behave
4999 as if the left operand is shifted <code>n</code> times by 1 for a shift
5000 count of <code>n</code>.
5001 As a result, <code>x << 1</code> is the same as <code>x*2</code>
5002 and <code>x >> 1</code> is the same as
5003 <code>x/2</code> but truncated towards negative infinity.
5007 For integer operands, the unary operators
5008 <code>+</code>, <code>-</code>, and <code>^</code> are defined as
5012 <pre class="grammar">
5014 -x negation is 0 - x
5015 ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
5016 and m = -1 for signed x
5020 <h4 id="Integer_overflow">Integer overflow</h4>
5023 For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>,
5024 <code>-</code>, <code>*</code>, and <code><<</code> are
5025 computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
5026 the unsigned integer's type.
5027 Loosely speaking, these unsigned integer operations
5028 discard high bits upon overflow, and programs may rely on "wrap around".
5032 For signed integers, the operations <code>+</code>,
5033 <code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
5034 overflow and the resulting value exists and is deterministically defined
5035 by the signed integer representation, the operation, and its operands.
5036 Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
5037 A compiler may not optimize code under the assumption that overflow does
5038 not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
5041 <h4 id="Floating_point_operators">Floating-point operators</h4>
5044 For floating-point and complex numbers,
5045 <code>+x</code> is the same as <code>x</code>,
5046 while <code>-x</code> is the negation of <code>x</code>.
5047 The result of a floating-point or complex division by zero is not specified beyond the
5048 IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
5049 occurs is implementation-specific.
5053 An implementation may combine multiple floating-point operations into a single
5054 fused operation, possibly across statements, and produce a result that differs
5055 from the value obtained by executing and rounding the instructions individually.
5056 An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to
5057 the precision of the target type, preventing fusion that would discard that rounding.
5061 For instance, some architectures provide a "fused multiply and add" (FMA) instruction
5062 that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
5063 These examples show when a Go implementation can use that instruction:
5067 // FMA allowed for computing r, because x*y is not explicitly rounded:
5071 *p = x*y; r = *p + z
5072 r = x*y + float64(z)
5074 // FMA disallowed for computing r, because it would omit rounding of x*y:
5075 r = float64(x*y) + z
5076 r = z; r += float64(x*y)
5077 t = float64(x*y); r = t + z
5080 <h4 id="String_concatenation">String concatenation</h4>
5083 Strings can be concatenated using the <code>+</code> operator
5084 or the <code>+=</code> assignment operator:
5088 s := "hi" + string(c)
5089 s += " and good bye"
5093 String addition creates a new string by concatenating the operands.
5096 <h3 id="Comparison_operators">Comparison operators</h3>
5099 Comparison operators compare two operands and yield an untyped boolean value.
5102 <pre class="grammar">
5108 >= greater or equal
5112 In any comparison, the first operand
5113 must be <a href="#Assignability">assignable</a>
5114 to the type of the second operand, or vice versa.
5117 The equality operators <code>==</code> and <code>!=</code> apply
5118 to operands of <i>comparable</i> types.
5119 The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
5120 apply to operands of <i>ordered</i> types.
5121 These terms and the result of the comparisons are defined as follows:
5126 Boolean types are comparable.
5127 Two boolean values are equal if they are either both
5128 <code>true</code> or both <code>false</code>.
5132 Integer types are comparable and ordered.
5133 Two integer values are compared in the usual way.
5137 Floating-point types are comparable and ordered.
5138 Two floating-point values are compared as defined by the IEEE-754 standard.
5142 Complex types are comparable.
5143 Two complex values <code>u</code> and <code>v</code> are
5144 equal if both <code>real(u) == real(v)</code> and
5145 <code>imag(u) == imag(v)</code>.
5149 String types are comparable and ordered.
5150 Two string values are compared lexically byte-wise.
5154 Pointer types are comparable.
5155 Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
5156 Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
5160 Channel types are comparable.
5161 Two channel values are equal if they were created by the same call to
5162 <a href="#Making_slices_maps_and_channels"><code>make</code></a>
5163 or if both have value <code>nil</code>.
5167 Interface types that are not type parameters are comparable.
5168 Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
5169 and equal dynamic values or if both have value <code>nil</code>.
5173 A value <code>x</code> of non-interface type <code>X</code> and
5174 a value <code>t</code> of interface type <code>T</code> can be compared
5175 if type <code>X</code> is comparable and
5176 <code>X</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
5177 They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
5178 and <code>t</code>'s dynamic value is equal to <code>x</code>.
5182 Struct types are comparable if all their field types are comparable.
5183 Two struct values are equal if their corresponding
5184 non-<a href="#Blank_identifier">blank</a> field values are equal.
5185 The fields are compared in source order, and comparison stops as
5186 soon as two field values differ (or all fields have been compared).
5190 Array types are comparable if their array element types are comparable.
5191 Two array values are equal if their corresponding element values are equal.
5192 The elements are compared in ascending index order, and comparison stops
5193 as soon as two element values differ (or all elements have been compared).
5197 Type parameters are comparable if they are strictly comparable (see below).
5202 A comparison of two interface values with identical dynamic types
5203 causes a <a href="#Run_time_panics">run-time panic</a> if that type
5204 is not comparable. This behavior applies not only to direct interface
5205 value comparisons but also when comparing arrays of interface values
5206 or structs with interface-valued fields.
5210 Slice, map, and function types are not comparable.
5211 However, as a special case, a slice, map, or function value may
5212 be compared to the predeclared identifier <code>nil</code>.
5213 Comparison of pointer, channel, and interface values to <code>nil</code>
5214 is also allowed and follows from the general rules above.
5218 const c = 3 < 4 // c is the untyped boolean constant true
5223 // The result of a comparison is an untyped boolean.
5224 // The usual assignment rules apply.
5225 b3 = x == y // b3 has type bool
5226 b4 bool = x == y // b4 has type bool
5227 b5 MyBool = x == y // b5 has type MyBool
5232 A type is <i>strictly comparable</i> if it is comparable and not an interface
5233 type nor composed of interface types.
5239 Boolean, numeric, string, pointer, and channel types are strictly comparable.
5243 Struct types are strictly comparable if all their field types are strictly comparable.
5247 Array types are strictly comparable if their array element types are strictly comparable.
5251 Type parameters are strictly comparable if all types in their type set are strictly comparable.
5255 <h3 id="Logical_operators">Logical operators</h3>
5258 Logical operators apply to <a href="#Boolean_types">boolean</a> values
5259 and yield a result of the same type as the operands.
5260 The right operand is evaluated conditionally.
5263 <pre class="grammar">
5264 && conditional AND p && q is "if p then q else false"
5265 || conditional OR p || q is "if p then true else q"
5270 <h3 id="Address_operators">Address operators</h3>
5273 For an operand <code>x</code> of type <code>T</code>, the address operation
5274 <code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
5275 The operand must be <i>addressable</i>,
5276 that is, either a variable, pointer indirection, or slice indexing
5277 operation; or a field selector of an addressable struct operand;
5278 or an array indexing operation of an addressable array.
5279 As an exception to the addressability requirement, <code>x</code> may also be a
5280 (possibly parenthesized)
5281 <a href="#Composite_literals">composite literal</a>.
5282 If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
5283 then the evaluation of <code>&x</code> does too.
5287 For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
5288 indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
5289 to by <code>x</code>.
5290 If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
5291 will cause a <a href="#Run_time_panics">run-time panic</a>.
5302 *x // causes a run-time panic
5303 &*x // causes a run-time panic
5307 <h3 id="Receive_operator">Receive operator</h3>
5310 For an operand <code>ch</code> whose <a href="#Core_types">core type</a> is a
5311 <a href="#Channel_types">channel</a>,
5312 the value of the receive operation <code><-ch</code> is the value received
5313 from the channel <code>ch</code>. The channel direction must permit receive operations,
5314 and the type of the receive operation is the element type of the channel.
5315 The expression blocks until a value is available.
5316 Receiving from a <code>nil</code> channel blocks forever.
5317 A receive operation on a <a href="#Close">closed</a> channel can always proceed
5318 immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
5319 after any previously sent values have been received.
5326 <-strobe // wait until clock pulse and discard received value
5330 A receive expression used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
5337 var x, ok T = <-ch
5341 yields an additional untyped boolean result reporting whether the
5342 communication succeeded. The value of <code>ok</code> is <code>true</code>
5343 if the value received was delivered by a successful send operation to the
5344 channel, or <code>false</code> if it is a zero value generated because the
5345 channel is closed and empty.
5349 <h3 id="Conversions">Conversions</h3>
5352 A conversion changes the <a href="#Types">type</a> of an expression
5353 to the type specified by the conversion.
5354 A conversion may appear literally in the source, or it may be <i>implied</i>
5355 by the context in which an expression appears.
5359 An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
5360 where <code>T</code> is a type and <code>x</code> is an expression
5361 that can be converted to type <code>T</code>.
5365 Conversion = Type "(" Expression [ "," ] ")" .
5369 If the type starts with the operator <code>*</code> or <code><-</code>,
5370 or if the type starts with the keyword <code>func</code>
5371 and has no result list, it must be parenthesized when
5372 necessary to avoid ambiguity:
5376 *Point(p) // same as *(Point(p))
5377 (*Point)(p) // p is converted to *Point
5378 <-chan int(c) // same as <-(chan int(c))
5379 (<-chan int)(c) // c is converted to <-chan int
5380 func()(x) // function signature func() x
5381 (func())(x) // x is converted to func()
5382 (func() int)(x) // x is converted to func() int
5383 func() int(x) // x is converted to func() int (unambiguous)
5387 A <a href="#Constants">constant</a> value <code>x</code> can be converted to
5388 type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
5389 by a value of <code>T</code>.
5390 As a special case, an integer constant <code>x</code> can be explicitly converted to a
5391 <a href="#String_types">string type</a> using the
5392 <a href="#Conversions_to_and_from_a_string_type">same rule</a>
5393 as for non-constant <code>x</code>.
5397 Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a>
5398 yields a typed constant.
5402 uint(iota) // iota value of type uint
5403 float32(2.718281828) // 2.718281828 of type float32
5404 complex128(1) // 1.0 + 0.0i of type complex128
5405 float32(0.49999999) // 0.5 of type float32
5406 float64(-1e-1000) // 0.0 of type float64
5407 string('x') // "x" of type string
5408 string(0x266c) // "♬" of type string
5409 myString("foo" + "bar") // "foobar" of type myString
5410 string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
5411 (*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
5412 int(1.2) // illegal: 1.2 cannot be represented as an int
5413 string(65.0) // illegal: 65.0 is not an integer constant
5417 Converting a constant to a type parameter yields a <i>non-constant</i> value of that type,
5418 with the value represented as a value of the type argument that the type parameter
5419 is <a href="#Instantiations">instantiated</a> with.
5420 For example, given the function:
5424 func f[P ~float32|~float64]() {
5430 the conversion <code>P(1.1)</code> results in a non-constant value of type <code>P</code>
5431 and the value <code>1.1</code> is represented as a <code>float32</code> or a <code>float64</code>
5432 depending on the type argument for <code>f</code>.
5433 Accordingly, if <code>f</code> is instantiated with a <code>float32</code> type,
5434 the numeric value of the expression <code>P(1.1) + 1.2</code> will be computed
5435 with the same precision as the corresponding non-constant <code>float32</code>
5440 A non-constant value <code>x</code> can be converted to type <code>T</code>
5441 in any of these cases:
5446 <code>x</code> is <a href="#Assignability">assignable</a>
5450 ignoring struct tags (see below),
5451 <code>x</code>'s type and <code>T</code> are not
5452 <a href="#Type_parameter_declarations">type parameters</a> but have
5453 <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
5456 ignoring struct tags (see below),
5457 <code>x</code>'s type and <code>T</code> are pointer types
5458 that are not <a href="#Types">named types</a>,
5459 and their pointer base types are not type parameters but
5460 have identical underlying types.
5463 <code>x</code>'s type and <code>T</code> are both integer or floating
5467 <code>x</code>'s type and <code>T</code> are both complex types.
5470 <code>x</code> is an integer or a slice of bytes or runes
5471 and <code>T</code> is a string type.
5474 <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
5477 <code>x</code> is a slice, <code>T</code> is an array or a pointer to an array,
5478 and the slice and array types have <a href="#Type_identity">identical</a> element types.
5483 Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type
5484 parameters, <code>x</code>
5485 can also be converted to type <code>T</code> if one of the following conditions applies:
5490 Both <code>V</code> and <code>T</code> are type parameters and a value of each
5491 type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s
5495 Only <code>V</code> is a type parameter and a value of each
5496 type in <code>V</code>'s type set can be converted to <code>T</code>.
5499 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
5500 type in <code>T</code>'s type set.
5505 <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
5506 for identity for the purpose of conversion:
5510 type Person struct {
5519 Name string `json:"name"`
5521 Street string `json:"street"`
5522 City string `json:"city"`
5526 var person = (*Person)(data) // ignoring tags, the underlying types are identical
5530 Specific rules apply to (non-constant) conversions between numeric types or
5531 to and from a string type.
5532 These conversions may change the representation of <code>x</code>
5533 and incur a run-time cost.
5534 All other conversions only change the type but not the representation
5539 There is no linguistic mechanism to convert between pointers and integers.
5540 The package <a href="#Package_unsafe"><code>unsafe</code></a>
5541 implements this functionality under restricted circumstances.
5544 <h4>Conversions between numeric types</h4>
5547 For the conversion of non-constant numeric values, the following rules apply:
5552 When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is
5553 sign extended to implicit infinite precision; otherwise it is zero extended.
5554 It is then truncated to fit in the result type's size.
5555 For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
5556 The conversion always yields a valid value; there is no indication of overflow.
5559 When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded
5560 (truncation towards zero).
5563 When converting an integer or floating-point number to a floating-point type,
5564 or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded
5565 to the precision specified by the destination type.
5566 For instance, the value of a variable <code>x</code> of type <code>float32</code>
5567 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
5568 but float32(x) represents the result of rounding <code>x</code>'s value to
5569 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
5570 of precision, but <code>float32(x + 0.1)</code> does not.
5575 In all non-constant conversions involving floating-point or complex values,
5576 if the result type cannot represent the value the conversion
5577 succeeds but the result value is implementation-dependent.
5580 <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
5584 Converting a signed or unsigned integer value to a string type yields a
5585 string containing the UTF-8 representation of the integer. Values outside
5586 the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
5590 string(-1) // "\ufffd" == "\xef\xbf\xbd"
5591 string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
5593 type myString string
5594 myString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
5599 Converting a slice of bytes to a string type yields
5600 a string whose successive bytes are the elements of the slice.
5603 string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5604 string([]byte{}) // ""
5605 string([]byte(nil)) // ""
5608 string(bytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5611 string([]myByte{'w', 'o', 'r', 'l', 'd', '!'}) // "world!"
5612 myString([]myByte{'\xf0', '\x9f', '\x8c', '\x8d'}) // "🌍"
5617 Converting a slice of runes to a string type yields
5618 a string that is the concatenation of the individual rune values
5619 converted to strings.
5622 string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5623 string([]rune{}) // ""
5624 string([]rune(nil)) // ""
5627 string(runes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5630 string([]myRune{0x266b, 0x266c}) // "\u266b\u266c" == "♫♬"
5631 myString([]myRune{0x1f30e}) // "\U0001f30e" == "🌎"
5636 Converting a value of a string type to a slice of bytes type
5637 yields a slice whose successive elements are the bytes of the string.
5640 []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5641 []byte("") // []byte{}
5643 bytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5645 []myByte("world!") // []myByte{'w', 'o', 'r', 'l', 'd', '!'}
5646 []myByte(myString("🌏")) // []myByte{'\xf0', '\x9f', '\x8c', '\x8f'}
5651 Converting a value of a string type to a slice of runes type
5652 yields a slice containing the individual Unicode code points of the string.
5655 []rune(myString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
5656 []rune("") // []rune{}
5658 runes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
5660 []myRune("♫♬") // []myRune{0x266b, 0x266c}
5661 []myRune(myString("🌐")) // []myRune{0x1f310}
5666 <h4 id="Conversions_from_slice_to_array_or_array_pointer">Conversions from slice to array or array pointer</h4>
5669 Converting a slice to an array yields an array containing the elements of the underlying array of the slice.
5670 Similarly, converting a slice to an array pointer yields a pointer to the underlying array of the slice.
5671 In both cases, if the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
5672 a <a href="#Run_time_panics">run-time panic</a> occurs.
5676 s := make([]byte, 2, 4)
5679 a1 := [1]byte(s[1:]) // a1[0] == s[1]
5680 a2 := [2]byte(s) // a2[0] == s[0]
5681 a4 := [4]byte(s) // panics: len([4]byte) > len(s)
5683 s0 := (*[0]byte)(s) // s0 != nil
5684 s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
5685 s2 := (*[2]byte)(s) // &s2[0] == &s[0]
5686 s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
5689 t0 := [0]string(t) // ok for nil slice t
5690 t1 := (*[0]string)(t) // t1 == nil
5691 t2 := (*[1]string)(t) // panics: len([1]string) > len(t)
5693 u := make([]byte, 0)
5694 u0 := (*[0]byte)(u) // u0 != nil
5697 <h3 id="Constant_expressions">Constant expressions</h3>
5700 Constant expressions may contain only <a href="#Constants">constant</a>
5701 operands and are evaluated at compile time.
5705 Untyped boolean, numeric, and string constants may be used as operands
5706 wherever it is legal to use an operand of boolean, numeric, or string type,
5711 A constant <a href="#Comparison_operators">comparison</a> always yields
5712 an untyped boolean constant. If the left operand of a constant
5713 <a href="#Operators">shift expression</a> is an untyped constant, the
5714 result is an integer constant; otherwise it is a constant of the same
5715 type as the left operand, which must be of
5716 <a href="#Numeric_types">integer type</a>.
5720 Any other operation on untyped constants results in an untyped constant of the
5721 same kind; that is, a boolean, integer, floating-point, complex, or string
5723 If the untyped operands of a binary operation (other than a shift) are of
5724 different kinds, the result is of the operand's kind that appears later in this
5725 list: integer, rune, floating-point, complex.
5726 For example, an untyped integer constant divided by an
5727 untyped complex constant yields an untyped complex constant.
5731 const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
5732 const b = 15 / 4 // b == 3 (untyped integer constant)
5733 const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
5734 const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
5735 const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
5736 const d = 1 << 3.0 // d == 8 (untyped integer constant)
5737 const e = 1.0 << 3 // e == 8 (untyped integer constant)
5738 const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
5739 const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
5740 const h = "foo" > "bar" // h == true (untyped boolean constant)
5741 const j = true // j == true (untyped boolean constant)
5742 const k = 'w' + 1 // k == 'x' (untyped rune constant)
5743 const l = "hi" // l == "hi" (untyped string constant)
5744 const m = string(k) // m == "x" (type string)
5745 const Σ = 1 - 0.707i // (untyped complex constant)
5746 const Δ = Σ + 2.0e-4 // (untyped complex constant)
5747 const Φ = iota*1i - 1/1i // (untyped complex constant)
5751 Applying the built-in function <code>complex</code> to untyped
5752 integer, rune, or floating-point constants yields
5753 an untyped complex constant.
5757 const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
5758 const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
5762 Constant expressions are always evaluated exactly; intermediate values and the
5763 constants themselves may require precision significantly larger than supported
5764 by any predeclared type in the language. The following are legal declarations:
5768 const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
5769 const Four int8 = Huge >> 98 // Four == 4 (type int8)
5773 The divisor of a constant division or remainder operation must not be zero:
5777 3.14 / 0.0 // illegal: division by zero
5781 The values of <i>typed</i> constants must always be accurately
5782 <a href="#Representability">representable</a> by values
5783 of the constant type. The following constant expressions are illegal:
5787 uint(-1) // -1 cannot be represented as a uint
5788 int(3.14) // 3.14 cannot be represented as an int
5789 int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
5790 Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
5791 Four * 100 // product 400 cannot be represented as an int8 (type of Four)
5795 The mask used by the unary bitwise complement operator <code>^</code> matches
5796 the rule for non-constants: the mask is all 1s for unsigned constants
5797 and -1 for signed and untyped constants.
5801 ^1 // untyped integer constant, equal to -2
5802 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
5803 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
5804 int8(^1) // same as int8(-2)
5805 ^int8(1) // same as -1 ^ int8(1) = -2
5809 Implementation restriction: A compiler may use rounding while
5810 computing untyped floating-point or complex constant expressions; see
5811 the implementation restriction in the section
5812 on <a href="#Constants">constants</a>. This rounding may cause a
5813 floating-point constant expression to be invalid in an integer
5814 context, even if it would be integral when calculated using infinite
5815 precision, and vice versa.
5819 <h3 id="Order_of_evaluation">Order of evaluation</h3>
5822 At package level, <a href="#Package_initialization">initialization dependencies</a>
5823 determine the evaluation order of individual initialization expressions in
5824 <a href="#Variable_declarations">variable declarations</a>.
5825 Otherwise, when evaluating the <a href="#Operands">operands</a> of an
5826 expression, assignment, or
5827 <a href="#Return_statements">return statement</a>,
5828 all function calls, method calls, and
5829 communication operations are evaluated in lexical left-to-right
5834 For example, in the (function-local) assignment
5837 y[f()], ok = g(h(), i()+x[j()], <-c), k()
5840 the function calls and communication happen in the order
5841 <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
5842 <code><-c</code>, <code>g()</code>, and <code>k()</code>.
5843 However, the order of those events compared to the evaluation
5844 and indexing of <code>x</code> and the evaluation
5845 of <code>y</code> is not specified.
5850 f := func() int { a++; return a }
5851 x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
5852 m := map[int]int{a: 1, a: 2} // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified
5853 n := map[int]int{a: f()} // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified
5857 At package level, initialization dependencies override the left-to-right rule
5858 for individual initialization expressions, but not for operands within each
5863 var a, b, c = f() + v(), g(), sqr(u()) + v()
5865 func f() int { return c }
5866 func g() int { return a }
5867 func sqr(x int) int { return x*x }
5869 // functions u and v are independent of all other variables and functions
5873 The function calls happen in the order
5874 <code>u()</code>, <code>sqr()</code>, <code>v()</code>,
5875 <code>f()</code>, <code>v()</code>, and <code>g()</code>.
5879 Floating-point operations within a single expression are evaluated according to
5880 the associativity of the operators. Explicit parentheses affect the evaluation
5881 by overriding the default associativity.
5882 In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
5883 is performed before adding <code>x</code>.
5886 <h2 id="Statements">Statements</h2>
5889 Statements control execution.
5894 Declaration | LabeledStmt | SimpleStmt |
5895 GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
5896 FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
5899 SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
5902 <h3 id="Terminating_statements">Terminating statements</h3>
5905 A <i>terminating statement</i> interrupts the regular flow of control in
5906 a <a href="#Blocks">block</a>. The following statements are terminating:
5911 A <a href="#Return_statements">"return"</a> or
5912 <a href="#Goto_statements">"goto"</a> statement.
5913 <!-- ul below only for regular layout -->
5918 A call to the built-in function
5919 <a href="#Handling_panics"><code>panic</code></a>.
5920 <!-- ul below only for regular layout -->
5925 A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
5926 <!-- ul below only for regular layout -->
5931 An <a href="#If_statements">"if" statement</a> in which:
5933 <li>the "else" branch is present, and</li>
5934 <li>both branches are terminating statements.</li>
5939 A <a href="#For_statements">"for" statement</a> in which:
5941 <li>there are no "break" statements referring to the "for" statement, and</li>
5942 <li>the loop condition is absent, and</li>
5943 <li>the "for" statement does not use a range clause.</li>
5948 A <a href="#Switch_statements">"switch" statement</a> in which:
5950 <li>there are no "break" statements referring to the "switch" statement,</li>
5951 <li>there is a default case, and</li>
5952 <li>the statement lists in each case, including the default, end in a terminating
5953 statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
5959 A <a href="#Select_statements">"select" statement</a> in which:
5961 <li>there are no "break" statements referring to the "select" statement, and</li>
5962 <li>the statement lists in each case, including the default if present,
5963 end in a terminating statement.</li>
5968 A <a href="#Labeled_statements">labeled statement</a> labeling
5969 a terminating statement.
5974 All other statements are not terminating.
5978 A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
5979 is not empty and its final non-empty statement is terminating.
5983 <h3 id="Empty_statements">Empty statements</h3>
5986 The empty statement does nothing.
5994 <h3 id="Labeled_statements">Labeled statements</h3>
5997 A labeled statement may be the target of a <code>goto</code>,
5998 <code>break</code> or <code>continue</code> statement.
6002 LabeledStmt = Label ":" Statement .
6003 Label = identifier .
6007 Error: log.Panic("error encountered")
6011 <h3 id="Expression_statements">Expression statements</h3>
6014 With the exception of specific built-in functions,
6015 function and method <a href="#Calls">calls</a> and
6016 <a href="#Receive_operator">receive operations</a>
6017 can appear in statement context. Such statements may be parenthesized.
6021 ExpressionStmt = Expression .
6025 The following built-in functions are not permitted in statement context:
6029 append cap complex imag len make new real
6030 unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice
6038 len("foo") // illegal if len is the built-in function
6042 <h3 id="Send_statements">Send statements</h3>
6045 A send statement sends a value on a channel.
6046 The channel expression's <a href="#Core_types">core type</a>
6047 must be a <a href="#Channel_types">channel</a>,
6048 the channel direction must permit send operations,
6049 and the type of the value to be sent must be <a href="#Assignability">assignable</a>
6050 to the channel's element type.
6054 SendStmt = Channel "<-" Expression .
6055 Channel = Expression .
6059 Both the channel and the value expression are evaluated before communication
6060 begins. Communication blocks until the send can proceed.
6061 A send on an unbuffered channel can proceed if a receiver is ready.
6062 A send on a buffered channel can proceed if there is room in the buffer.
6063 A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
6064 A send on a <code>nil</code> channel blocks forever.
6068 ch <- 3 // send value 3 to channel ch
6072 <h3 id="IncDec_statements">IncDec statements</h3>
6075 The "++" and "--" statements increment or decrement their operands
6076 by the untyped <a href="#Constants">constant</a> <code>1</code>.
6077 As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
6078 or a map index expression.
6082 IncDecStmt = Expression ( "++" | "--" ) .
6086 The following <a href="#Assignment_statements">assignment statements</a> are semantically
6090 <pre class="grammar">
6091 IncDec statement Assignment
6097 <h3 id="Assignment_statements">Assignment statements</h3>
6100 An <i>assignment</i> replaces the current value stored in a <a href="#Variables">variable</a>
6101 with a new value specified by an <a href="#Expressions">expression</a>.
6102 An assignment statement may assign a single value to a single variable, or multiple values to a
6103 matching number of variables.
6107 Assignment = ExpressionList assign_op ExpressionList .
6109 assign_op = [ add_op | mul_op ] "=" .
6113 Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
6114 a map index expression, or (for <code>=</code> assignments only) the
6115 <a href="#Blank_identifier">blank identifier</a>.
6116 Operands may be parenthesized.
6123 (k) = <-ch // same as: k = <-ch
6127 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
6128 <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
6129 is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
6130 <code>(y)</code> but evaluates <code>x</code>
6131 only once. The <i>op</i><code>=</code> construct is a single token.
6132 In assignment operations, both the left- and right-hand expression lists
6133 must contain exactly one single-valued expression, and the left-hand
6134 expression must not be the blank identifier.
6139 i &^= 1<<n
6143 A tuple assignment assigns the individual elements of a multi-valued
6144 operation to a list of variables. There are two forms. In the
6145 first, the right hand operand is a single multi-valued expression
6146 such as a function call, a <a href="#Channel_types">channel</a> or
6147 <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
6148 The number of operands on the left
6149 hand side must match the number of values. For instance, if
6150 <code>f</code> is a function returning two values,
6158 assigns the first value to <code>x</code> and the second to <code>y</code>.
6159 In the second form, the number of operands on the left must equal the number
6160 of expressions on the right, each of which must be single-valued, and the
6161 <i>n</i>th expression on the right is assigned to the <i>n</i>th
6162 operand on the left:
6166 one, two, three = '一', '二', '三'
6170 The <a href="#Blank_identifier">blank identifier</a> provides a way to
6171 ignore right-hand side values in an assignment:
6175 _ = x // evaluate x but ignore it
6176 x, _ = f() // evaluate f() but ignore second result value
6180 The assignment proceeds in two phases.
6181 First, the operands of <a href="#Index_expressions">index expressions</a>
6182 and <a href="#Address_operators">pointer indirections</a>
6183 (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
6184 on the left and the expressions on the right are all
6185 <a href="#Order_of_evaluation">evaluated in the usual order</a>.
6186 Second, the assignments are carried out in left-to-right order.
6190 a, b = b, a // exchange a and b
6194 i, x[i] = 1, 2 // set i = 1, x[0] = 2
6197 x[i], i = 2, 1 // set x[0] = 2, i = 1
6199 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
6201 x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
6203 type Point struct { x, y int }
6205 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
6209 for i, x[i] = range x { // set i, x[2] = 0, x[0]
6212 // after this loop, i == 0 and x == []int{3, 5, 3}
6216 In assignments, each value must be <a href="#Assignability">assignable</a>
6217 to the type of the operand to which it is assigned, with the following special cases:
6222 Any typed value may be assigned to the blank identifier.
6226 If an untyped constant
6227 is assigned to a variable of interface type or the blank identifier,
6228 the constant is first implicitly <a href="#Conversions">converted</a> to its
6229 <a href="#Constants">default type</a>.
6233 If an untyped boolean value is assigned to a variable of interface type or
6234 the blank identifier, it is first implicitly converted to type <code>bool</code>.
6238 <h3 id="If_statements">If statements</h3>
6241 "If" statements specify the conditional execution of two branches
6242 according to the value of a boolean expression. If the expression
6243 evaluates to true, the "if" branch is executed, otherwise, if
6244 present, the "else" branch is executed.
6248 IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
6258 The expression may be preceded by a simple statement, which
6259 executes before the expression is evaluated.
6263 if x := f(); x < y {
6265 } else if x > z {
6273 <h3 id="Switch_statements">Switch statements</h3>
6276 "Switch" statements provide multi-way execution.
6277 An expression or type is compared to the "cases"
6278 inside the "switch" to determine which branch
6283 SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
6287 There are two forms: expression switches and type switches.
6288 In an expression switch, the cases contain expressions that are compared
6289 against the value of the switch expression.
6290 In a type switch, the cases contain types that are compared against the
6291 type of a specially annotated switch expression.
6292 The switch expression is evaluated exactly once in a switch statement.
6295 <h4 id="Expression_switches">Expression switches</h4>
6298 In an expression switch,
6299 the switch expression is evaluated and
6300 the case expressions, which need not be constants,
6301 are evaluated left-to-right and top-to-bottom; the first one that equals the
6303 triggers execution of the statements of the associated case;
6304 the other cases are skipped.
6305 If no case matches and there is a "default" case,
6306 its statements are executed.
6307 There can be at most one default case and it may appear anywhere in the
6309 A missing switch expression is equivalent to the boolean value
6314 ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
6315 ExprCaseClause = ExprSwitchCase ":" StatementList .
6316 ExprSwitchCase = "case" ExpressionList | "default" .
6320 If the switch expression evaluates to an untyped constant, it is first implicitly
6321 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
6322 The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
6323 The switch expression type must be <a href="#Comparison_operators">comparable</a>.
6327 If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
6328 to the type of the switch expression.
6329 For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
6330 of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
6334 In other words, the switch expression is treated as if it were used to declare and
6335 initialize a temporary variable <code>t</code> without explicit type; it is that
6336 value of <code>t</code> against which each case expression <code>x</code> is tested
6341 In a case or default clause, the last non-empty statement
6342 may be a (possibly <a href="#Labeled_statements">labeled</a>)
6343 <a href="#Fallthrough_statements">"fallthrough" statement</a> to
6344 indicate that control should flow from the end of this clause to
6345 the first statement of the next clause.
6346 Otherwise control flows to the end of the "switch" statement.
6347 A "fallthrough" statement may appear as the last statement of all
6348 but the last clause of an expression switch.
6352 The switch expression may be preceded by a simple statement, which
6353 executes before the expression is evaluated.
6359 case 0, 1, 2, 3: s1()
6360 case 4, 5, 6, 7: s2()
6363 switch x := f(); { // missing switch expression means "true"
6364 case x < 0: return -x
6376 Implementation restriction: A compiler may disallow multiple case
6377 expressions evaluating to the same constant.
6378 For instance, the current compilers disallow duplicate integer,
6379 floating point, or string constants in case expressions.
6382 <h4 id="Type_switches">Type switches</h4>
6385 A type switch compares types rather than values. It is otherwise similar
6386 to an expression switch. It is marked by a special switch expression that
6387 has the form of a <a href="#Type_assertions">type assertion</a>
6388 using the keyword <code>type</code> rather than an actual type:
6398 Cases then match actual types <code>T</code> against the dynamic type of the
6399 expression <code>x</code>. As with type assertions, <code>x</code> must be of
6400 <a href="#Interface_types">interface type</a>, but not a
6401 <a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type
6402 <code>T</code> listed in a case must implement the type of <code>x</code>.
6403 The types listed in the cases of a type switch must all be
6404 <a href="#Type_identity">different</a>.
6408 TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
6409 TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
6410 TypeCaseClause = TypeSwitchCase ":" StatementList .
6411 TypeSwitchCase = "case" TypeList | "default" .
6415 The TypeSwitchGuard may include a
6416 <a href="#Short_variable_declarations">short variable declaration</a>.
6417 When that form is used, the variable is declared at the end of the
6418 TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
6419 In clauses with a case listing exactly one type, the variable
6420 has that type; otherwise, the variable has the type of the expression
6421 in the TypeSwitchGuard.
6425 Instead of a type, a case may use the predeclared identifier
6426 <a href="#Predeclared_identifiers"><code>nil</code></a>;
6427 that case is selected when the expression in the TypeSwitchGuard
6428 is a <code>nil</code> interface value.
6429 There may be at most one <code>nil</code> case.
6433 Given an expression <code>x</code> of type <code>interface{}</code>,
6434 the following type switch:
6438 switch i := x.(type) {
6440 printString("x is nil") // type of i is type of x (interface{})
6442 printInt(i) // type of i is int
6444 printFloat64(i) // type of i is float64
6445 case func(int) float64:
6446 printFunction(i) // type of i is func(int) float64
6448 printString("type is bool or string") // type of i is type of x (interface{})
6450 printString("don't know the type") // type of i is type of x (interface{})
6459 v := x // x is evaluated exactly once
6461 i := v // type of i is type of x (interface{})
6462 printString("x is nil")
6463 } else if i, isInt := v.(int); isInt {
6464 printInt(i) // type of i is int
6465 } else if i, isFloat64 := v.(float64); isFloat64 {
6466 printFloat64(i) // type of i is float64
6467 } else if i, isFunc := v.(func(int) float64); isFunc {
6468 printFunction(i) // type of i is func(int) float64
6470 _, isBool := v.(bool)
6471 _, isString := v.(string)
6472 if isBool || isString {
6473 i := v // type of i is type of x (interface{})
6474 printString("type is bool or string")
6476 i := v // type of i is type of x (interface{})
6477 printString("don't know the type")
6483 A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a>
6484 may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns
6485 out to duplicate another entry in the switch, the first matching case is chosen.
6489 func f[P any](x any) int {
6504 var v1 = f[string]("foo") // v1 == 0
6505 var v2 = f[byte]([]byte{}) // v2 == 2
6509 The type switch guard may be preceded by a simple statement, which
6510 executes before the guard is evaluated.
6514 The "fallthrough" statement is not permitted in a type switch.
6517 <h3 id="For_statements">For statements</h3>
6520 A "for" statement specifies repeated execution of a block. There are three forms:
6521 The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
6525 ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
6526 Condition = Expression .
6529 <h4 id="For_condition">For statements with single condition</h4>
6532 In its simplest form, a "for" statement specifies the repeated execution of
6533 a block as long as a boolean condition evaluates to true.
6534 The condition is evaluated before each iteration.
6535 If the condition is absent, it is equivalent to the boolean value
6545 <h4 id="For_clause">For statements with <code>for</code> clause</h4>
6548 A "for" statement with a ForClause is also controlled by its condition, but
6549 additionally it may specify an <i>init</i>
6550 and a <i>post</i> statement, such as an assignment,
6551 an increment or decrement statement. The init statement may be a
6552 <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
6553 Variables declared by the init statement are re-used in each iteration.
6557 ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
6558 InitStmt = SimpleStmt .
6559 PostStmt = SimpleStmt .
6563 for i := 0; i < 10; i++ {
6569 If non-empty, the init statement is executed once before evaluating the
6570 condition for the first iteration;
6571 the post statement is executed after each execution of the block (and
6572 only if the block was executed).
6573 Any element of the ForClause may be empty but the
6574 <a href="#Semicolons">semicolons</a> are
6575 required unless there is only a condition.
6576 If the condition is absent, it is equivalent to the boolean value
6581 for cond { S() } is the same as for ; cond ; { S() }
6582 for { S() } is the same as for true { S() }
6585 <h4 id="For_range">For statements with <code>range</code> clause</h4>
6588 A "for" statement with a "range" clause
6589 iterates through all entries of an array, slice, string or map,
6590 or values received on a channel. For each entry it assigns <i>iteration values</i>
6591 to corresponding <i>iteration variables</i> if present and then executes the block.
6595 RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
6599 The expression on the right in the "range" clause is called the <i>range expression</i>,
6600 its <a href="#Core_types">core type</a> must be
6601 an array, pointer to an array, slice, string, map, or channel permitting
6602 <a href="#Receive_operator">receive operations</a>.
6603 As with an assignment, if present the operands on the left must be
6604 <a href="#Address_operators">addressable</a> or map index expressions; they
6605 denote the iteration variables. If the range expression is a channel, at most
6606 one iteration variable is permitted, otherwise there may be up to two.
6607 If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
6608 the range clause is equivalent to the same clause without that identifier.
6612 The range expression <code>x</code> is evaluated once before beginning the loop,
6613 with one exception: if at most one iteration variable is present and
6614 <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
6615 the range expression is not evaluated.
6619 Function calls on the left are evaluated once per iteration.
6620 For each iteration, iteration values are produced as follows
6621 if the respective iteration variables are present:
6624 <pre class="grammar">
6625 Range expression 1st value 2nd value
6627 array or slice a [n]E, *[n]E, or []E index i int a[i] E
6628 string s string type index i int see below rune
6629 map m map[K]V key k K m[k] V
6630 channel c chan E, <-chan E element e E
6635 For an array, pointer to array, or slice value <code>a</code>, the index iteration
6636 values are produced in increasing order, starting at element index 0.
6637 If at most one iteration variable is present, the range loop produces
6638 iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
6639 or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
6643 For a string value, the "range" clause iterates over the Unicode code points
6644 in the string starting at byte index 0. On successive iterations, the index value will be the
6645 index of the first byte of successive UTF-8-encoded code points in the string,
6646 and the second value, of type <code>rune</code>, will be the value of
6647 the corresponding code point. If the iteration encounters an invalid
6648 UTF-8 sequence, the second value will be <code>0xFFFD</code>,
6649 the Unicode replacement character, and the next iteration will advance
6650 a single byte in the string.
6654 The iteration order over maps is not specified
6655 and is not guaranteed to be the same from one iteration to the next.
6656 If a map entry that has not yet been reached is removed during iteration,
6657 the corresponding iteration value will not be produced. If a map entry is
6658 created during iteration, that entry may be produced during the iteration or
6659 may be skipped. The choice may vary for each entry created and from one
6660 iteration to the next.
6661 If the map is <code>nil</code>, the number of iterations is 0.
6665 For channels, the iteration values produced are the successive values sent on
6666 the channel until the channel is <a href="#Close">closed</a>. If the channel
6667 is <code>nil</code>, the range expression blocks forever.
6672 The iteration values are assigned to the respective
6673 iteration variables as in an <a href="#Assignment_statements">assignment statement</a>.
6677 The iteration variables may be declared by the "range" clause using a form of
6678 <a href="#Short_variable_declarations">short variable declaration</a>
6680 In this case their types are set to the types of the respective iteration values
6681 and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
6682 statement; they are re-used in each iteration.
6683 If the iteration variables are declared outside the "for" statement,
6684 after execution their values will be those of the last iteration.
6688 var testdata *struct {
6691 for i, _ := range testdata.a {
6692 // testdata.a is never evaluated; len(testdata.a) is constant
6693 // i ranges from 0 to 6
6698 for i, s := range a {
6700 // type of s is string
6706 var val interface{} // element type of m is assignable to val
6707 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
6708 for key, val = range m {
6711 // key == last map key encountered in iteration
6714 var ch chan Work = producer()
6724 <h3 id="Go_statements">Go statements</h3>
6727 A "go" statement starts the execution of a function call
6728 as an independent concurrent thread of control, or <i>goroutine</i>,
6729 within the same address space.
6733 GoStmt = "go" Expression .
6737 The expression must be a function or method call; it cannot be parenthesized.
6738 Calls of built-in functions are restricted as for
6739 <a href="#Expression_statements">expression statements</a>.
6743 The function value and parameters are
6744 <a href="#Calls">evaluated as usual</a>
6745 in the calling goroutine, but
6746 unlike with a regular call, program execution does not wait
6747 for the invoked function to complete.
6748 Instead, the function begins executing independently
6750 When the function terminates, its goroutine also terminates.
6751 If the function has any return values, they are discarded when the
6757 go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
6761 <h3 id="Select_statements">Select statements</h3>
6764 A "select" statement chooses which of a set of possible
6765 <a href="#Send_statements">send</a> or
6766 <a href="#Receive_operator">receive</a>
6767 operations will proceed.
6768 It looks similar to a
6769 <a href="#Switch_statements">"switch"</a> statement but with the
6770 cases all referring to communication operations.
6774 SelectStmt = "select" "{" { CommClause } "}" .
6775 CommClause = CommCase ":" StatementList .
6776 CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
6777 RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
6778 RecvExpr = Expression .
6782 A case with a RecvStmt may assign the result of a RecvExpr to one or
6783 two variables, which may be declared using a
6784 <a href="#Short_variable_declarations">short variable declaration</a>.
6785 The RecvExpr must be a (possibly parenthesized) receive operation.
6786 There can be at most one default case and it may appear anywhere
6787 in the list of cases.
6791 Execution of a "select" statement proceeds in several steps:
6796 For all the cases in the statement, the channel operands of receive operations
6797 and the channel and right-hand-side expressions of send statements are
6798 evaluated exactly once, in source order, upon entering the "select" statement.
6799 The result is a set of channels to receive from or send to,
6800 and the corresponding values to send.
6801 Any side effects in that evaluation will occur irrespective of which (if any)
6802 communication operation is selected to proceed.
6803 Expressions on the left-hand side of a RecvStmt with a short variable declaration
6804 or assignment are not yet evaluated.
6808 If one or more of the communications can proceed,
6809 a single one that can proceed is chosen via a uniform pseudo-random selection.
6810 Otherwise, if there is a default case, that case is chosen.
6811 If there is no default case, the "select" statement blocks until
6812 at least one of the communications can proceed.
6816 Unless the selected case is the default case, the respective communication
6817 operation is executed.
6821 If the selected case is a RecvStmt with a short variable declaration or
6822 an assignment, the left-hand side expressions are evaluated and the
6823 received value (or values) are assigned.
6827 The statement list of the selected case is executed.
6832 Since communication on <code>nil</code> channels can never proceed,
6833 a select with only <code>nil</code> channels and no default case blocks forever.
6838 var c, c1, c2, c3, c4 chan int
6842 print("received ", i1, " from c1\n")
6844 print("sent ", i2, " to c2\n")
6845 case i3, ok := (<-c3): // same as: i3, ok := <-c3
6847 print("received ", i3, " from c3\n")
6849 print("c3 is closed\n")
6851 case a[f()] = <-c4:
6853 // case t := <-c4
6856 print("no communication\n")
6859 for { // send random sequence of bits to c
6861 case c <- 0: // note: no statement, no fallthrough, no folding of cases
6866 select {} // block forever
6870 <h3 id="Return_statements">Return statements</h3>
6873 A "return" statement in a function <code>F</code> terminates the execution
6874 of <code>F</code>, and optionally provides one or more result values.
6875 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6876 are executed before <code>F</code> returns to its caller.
6880 ReturnStmt = "return" [ ExpressionList ] .
6884 In a function without a result type, a "return" statement must not
6885 specify any result values.
6894 There are three ways to return values from a function with a result
6899 <li>The return value or values may be explicitly listed
6900 in the "return" statement. Each expression must be single-valued
6901 and <a href="#Assignability">assignable</a>
6902 to the corresponding element of the function's result type.
6904 func simpleF() int {
6908 func complexF1() (re float64, im float64) {
6913 <li>The expression list in the "return" statement may be a single
6914 call to a multi-valued function. The effect is as if each value
6915 returned from that function were assigned to a temporary
6916 variable with the type of the respective value, followed by a
6917 "return" statement listing these variables, at which point the
6918 rules of the previous case apply.
6920 func complexF2() (re float64, im float64) {
6925 <li>The expression list may be empty if the function's result
6926 type specifies names for its <a href="#Function_types">result parameters</a>.
6927 The result parameters act as ordinary local variables
6928 and the function may assign values to them as necessary.
6929 The "return" statement returns the values of these variables.
6931 func complexF3() (re float64, im float64) {
6937 func (devnull) Write(p []byte) (n int, _ error) {
6946 Regardless of how they are declared, all the result values are initialized to
6947 the <a href="#The_zero_value">zero values</a> for their type upon entry to the
6948 function. A "return" statement that specifies results sets the result parameters before
6949 any deferred functions are executed.
6953 Implementation restriction: A compiler may disallow an empty expression list
6954 in a "return" statement if a different entity (constant, type, or variable)
6955 with the same name as a result parameter is in
6956 <a href="#Declarations_and_scope">scope</a> at the place of the return.
6960 func f(n int) (res int, err error) {
6961 if _, err := f(n-1); err != nil {
6962 return // invalid return statement: err is shadowed
6968 <h3 id="Break_statements">Break statements</h3>
6971 A "break" statement terminates execution of the innermost
6972 <a href="#For_statements">"for"</a>,
6973 <a href="#Switch_statements">"switch"</a>, or
6974 <a href="#Select_statements">"select"</a> statement
6975 within the same function.
6979 BreakStmt = "break" [ Label ] .
6983 If there is a label, it must be that of an enclosing
6984 "for", "switch", or "select" statement,
6985 and that is the one whose execution terminates.
6990 for i = 0; i < n; i++ {
6991 for j = 0; j < m; j++ {
7004 <h3 id="Continue_statements">Continue statements</h3>
7007 A "continue" statement begins the next iteration of the
7008 innermost enclosing <a href="#For_statements">"for" loop</a>
7009 by advancing control to the end of the loop block.
7010 The "for" loop must be within the same function.
7014 ContinueStmt = "continue" [ Label ] .
7018 If there is a label, it must be that of an enclosing
7019 "for" statement, and that is the one whose execution
7025 for y, row := range rows {
7026 for x, data := range row {
7027 if data == endOfRow {
7030 row[x] = data + bias(x, y)
7035 <h3 id="Goto_statements">Goto statements</h3>
7038 A "goto" statement transfers control to the statement with the corresponding label
7039 within the same function.
7043 GotoStmt = "goto" Label .
7051 Executing the "goto" statement must not cause any variables to come into
7052 <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
7053 For instance, this example:
7063 is erroneous because the jump to label <code>L</code> skips
7064 the creation of <code>v</code>.
7068 A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
7069 For instance, this example:
7086 is erroneous because the label <code>L1</code> is inside
7087 the "for" statement's block but the <code>goto</code> is not.
7090 <h3 id="Fallthrough_statements">Fallthrough statements</h3>
7093 A "fallthrough" statement transfers control to the first statement of the
7094 next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
7095 It may be used only as the final non-empty statement in such a clause.
7099 FallthroughStmt = "fallthrough" .
7103 <h3 id="Defer_statements">Defer statements</h3>
7106 A "defer" statement invokes a function whose execution is deferred
7107 to the moment the surrounding function returns, either because the
7108 surrounding function executed a <a href="#Return_statements">return statement</a>,
7109 reached the end of its <a href="#Function_declarations">function body</a>,
7110 or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
7114 DeferStmt = "defer" Expression .
7118 The expression must be a function or method call; it cannot be parenthesized.
7119 Calls of built-in functions are restricted as for
7120 <a href="#Expression_statements">expression statements</a>.
7124 Each time a "defer" statement
7125 executes, the function value and parameters to the call are
7126 <a href="#Calls">evaluated as usual</a>
7127 and saved anew but the actual function is not invoked.
7128 Instead, deferred functions are invoked immediately before
7129 the surrounding function returns, in the reverse order
7130 they were deferred. That is, if the surrounding function
7131 returns through an explicit <a href="#Return_statements">return statement</a>,
7132 deferred functions are executed <i>after</i> any result parameters are set
7133 by that return statement but <i>before</i> the function returns to its caller.
7134 If a deferred function value evaluates
7135 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
7136 when the function is invoked, not when the "defer" statement is executed.
7140 For instance, if the deferred function is
7141 a <a href="#Function_literals">function literal</a> and the surrounding
7142 function has <a href="#Function_types">named result parameters</a> that
7143 are in scope within the literal, the deferred function may access and modify
7144 the result parameters before they are returned.
7145 If the deferred function has any return values, they are discarded when
7146 the function completes.
7147 (See also the section on <a href="#Handling_panics">handling panics</a>.)
7152 defer unlock(l) // unlocking happens before surrounding function returns
7154 // prints 3 2 1 0 before surrounding function returns
7155 for i := 0; i <= 3; i++ {
7160 func f() (result int) {
7162 // result is accessed after it was set to 6 by the return statement
7169 <h2 id="Built-in_functions">Built-in functions</h2>
7172 Built-in functions are
7173 <a href="#Predeclared_identifiers">predeclared</a>.
7174 They are called like any other function but some of them
7175 accept a type instead of an expression as the first argument.
7179 The built-in functions do not have standard Go types,
7180 so they can only appear in <a href="#Calls">call expressions</a>;
7181 they cannot be used as function values.
7184 <h3 id="Close">Close</h3>
7187 For an argument <code>ch</code> with a <a href="#Core_types">core type</a>
7188 that is a <a href="#Channel_types">channel</a>, the built-in function <code>close</code>
7189 records that no more values will be sent on the channel.
7190 It is an error if <code>ch</code> is a receive-only channel.
7191 Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
7192 Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
7193 After calling <code>close</code>, and after any previously
7194 sent values have been received, receive operations will return
7195 the zero value for the channel's type without blocking.
7196 The multi-valued <a href="#Receive_operator">receive operation</a>
7197 returns a received value along with an indication of whether the channel is closed.
7200 <h3 id="Length_and_capacity">Length and capacity</h3>
7203 The built-in functions <code>len</code> and <code>cap</code> take arguments
7204 of various types and return a result of type <code>int</code>.
7205 The implementation guarantees that the result always fits into an <code>int</code>.
7208 <pre class="grammar">
7209 Call Argument type Result
7211 len(s) string type string length in bytes
7212 [n]T, *[n]T array length (== n)
7214 map[K]T map length (number of defined keys)
7215 chan T number of elements queued in channel buffer
7216 type parameter see below
7218 cap(s) [n]T, *[n]T array length (== n)
7220 chan T channel buffer capacity
7221 type parameter see below
7225 If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>,
7226 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
7227 each type in <code>P</code>'s type set.
7228 The result is the length (or capacity, respectively) of the argument whose type
7229 corresponds to the type argument with which <code>P</code> was
7230 <a href="#Instantiations">instantiated</a>.
7234 The capacity of a slice is the number of elements for which there is
7235 space allocated in the underlying array.
7236 At any time the following relationship holds:
7240 0 <= len(s) <= cap(s)
7244 The length of a <code>nil</code> slice, map or channel is 0.
7245 The capacity of a <code>nil</code> slice or channel is 0.
7249 The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
7250 <code>s</code> is a string constant. The expressions <code>len(s)</code> and
7251 <code>cap(s)</code> are constants if the type of <code>s</code> is an array
7252 or pointer to an array and the expression <code>s</code> does not contain
7253 <a href="#Receive_operator">channel receives</a> or (non-constant)
7254 <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
7255 Otherwise, invocations of <code>len</code> and <code>cap</code> are not
7256 constant and <code>s</code> is evaluated.
7261 c1 = imag(2i) // imag(2i) = 2.0 is a constant
7262 c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
7263 c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
7264 c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
7265 c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
7270 <h3 id="Allocation">Allocation</h3>
7273 The built-in function <code>new</code> takes a type <code>T</code>,
7274 allocates storage for a <a href="#Variables">variable</a> of that type
7275 at run time, and returns a value of type <code>*T</code>
7276 <a href="#Pointer_types">pointing</a> to it.
7277 The variable is initialized as described in the section on
7278 <a href="#The_zero_value">initial values</a>.
7281 <pre class="grammar">
7290 type S struct { a int; b float64 }
7295 allocates storage for a variable of type <code>S</code>,
7296 initializes it (<code>a=0</code>, <code>b=0.0</code>),
7297 and returns a value of type <code>*S</code> containing the address
7301 <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
7304 The built-in function <code>make</code> takes a type <code>T</code>,
7305 optionally followed by a type-specific list of expressions.
7306 The <a href="#Core_types">core type</a> of <code>T</code> must
7307 be a slice, map or channel.
7308 It returns a value of type <code>T</code> (not <code>*T</code>).
7309 The memory is initialized as described in the section on
7310 <a href="#The_zero_value">initial values</a>.
7313 <pre class="grammar">
7314 Call Core type Result
7316 make(T, n) slice slice of type T with length n and capacity n
7317 make(T, n, m) slice slice of type T with length n and capacity m
7319 make(T) map map of type T
7320 make(T, n) map map of type T with initial space for approximately n elements
7322 make(T) channel unbuffered channel of type T
7323 make(T, n) channel buffered channel of type T, buffer size n
7328 Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>,
7329 have a <a href="#Interface_types">type set</a> containing only integer types,
7330 or be an untyped <a href="#Constants">constant</a>.
7331 A constant size argument must be non-negative and <a href="#Representability">representable</a>
7332 by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
7333 If both <code>n</code> and <code>m</code> are provided and are constant, then
7334 <code>n</code> must be no larger than <code>m</code>.
7335 For slices and channels, if <code>n</code> is negative or larger than <code>m</code> at run time,
7336 a <a href="#Run_time_panics">run-time panic</a> occurs.
7340 s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
7341 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
7342 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
7343 s := make([]int, 10, 0) // illegal: len(s) > cap(s)
7344 c := make(chan int, 10) // channel with a buffer size of 10
7345 m := make(map[string]int, 100) // map with initial space for approximately 100 elements
7349 Calling <code>make</code> with a map type and size hint <code>n</code> will
7350 create a map with initial space to hold <code>n</code> map elements.
7351 The precise behavior is implementation-dependent.
7355 <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
7358 The built-in functions <code>append</code> and <code>copy</code> assist in
7359 common slice operations.
7360 For both functions, the result is independent of whether the memory referenced
7361 by the arguments overlaps.
7365 The <a href="#Function_types">variadic</a> function <code>append</code>
7366 appends zero or more values <code>x</code> to a slice <code>s</code>
7367 and returns the resulting slice of the same type as <code>s</code>.
7368 The <a href="#Core_types">core type</a> of <code>s</code> must be a slice
7369 of type <code>[]E</code>.
7370 The values <code>x</code> are passed to a parameter of type <code>...E</code>
7371 and the respective <a href="#Passing_arguments_to_..._parameters">parameter
7372 passing rules</a> apply.
7373 As a special case, if the core type of <code>s</code> is <code>[]byte</code>,
7374 <code>append</code> also accepts a second argument with core type
7375 <a href="#Core_types"><code>bytestring</code></a> followed by <code>...</code>.
7376 This form appends the bytes of the byte slice or string.
7379 <pre class="grammar">
7380 append(s S, x ...E) S // core type of S is []E
7384 If the capacity of <code>s</code> is not large enough to fit the additional
7385 values, <code>append</code> <a href="#Allocation">allocates</a> a new, sufficiently large underlying
7386 array that fits both the existing slice elements and the additional values.
7387 Otherwise, <code>append</code> re-uses the underlying array.
7392 s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
7393 s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
7394 s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
7395 s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
7398 t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"}
7401 b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
7405 The function <code>copy</code> copies slice elements from
7406 a source <code>src</code> to a destination <code>dst</code> and returns the
7407 number of elements copied.
7408 The <a href="#Core_types">core types</a> of both arguments must be slices
7409 with <a href="#Type_identity">identical</a> element type.
7410 The number of elements copied is the minimum of
7411 <code>len(src)</code> and <code>len(dst)</code>.
7412 As a special case, if the destination's core type is <code>[]byte</code>,
7413 <code>copy</code> also accepts a source argument with core type
7414 </a> <a href="#Core_types"><code>bytestring</code></a>.
7415 This form copies the bytes from the byte slice or string into the byte slice.
7418 <pre class="grammar">
7419 copy(dst, src []T) int
7420 copy(dst []byte, src string) int
7428 var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
7429 var s = make([]int, 6)
7430 var b = make([]byte, 5)
7431 n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
7432 n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
7433 n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
7437 <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
7440 The built-in function <code>delete</code> removes the element with key
7441 <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
7442 value <code>k</code> must be <a href="#Assignability">assignable</a>
7443 to the key type of <code>m</code>.
7446 <pre class="grammar">
7447 delete(m, k) // remove element m[k] from map m
7451 If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>,
7452 all types in that type set must be maps, and they must all have identical key types.
7456 If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
7457 does not exist, <code>delete</code> is a no-op.
7461 <h3 id="Complex_numbers">Manipulating complex numbers</h3>
7464 Three functions assemble and disassemble complex numbers.
7465 The built-in function <code>complex</code> constructs a complex
7466 value from a floating-point real and imaginary part, while
7467 <code>real</code> and <code>imag</code>
7468 extract the real and imaginary parts of a complex value.
7471 <pre class="grammar">
7472 complex(realPart, imaginaryPart floatT) complexT
7473 real(complexT) floatT
7474 imag(complexT) floatT
7478 The type of the arguments and return value correspond.
7479 For <code>complex</code>, the two arguments must be of the same
7480 <a href="#Numeric_types">floating-point type</a> and the return type is the
7481 <a href="#Numeric_types">complex type</a>
7482 with the corresponding floating-point constituents:
7483 <code>complex64</code> for <code>float32</code> arguments, and
7484 <code>complex128</code> for <code>float64</code> arguments.
7485 If one of the arguments evaluates to an untyped constant, it is first implicitly
7486 <a href="#Conversions">converted</a> to the type of the other argument.
7487 If both arguments evaluate to untyped constants, they must be non-complex
7488 numbers or their imaginary parts must be zero, and the return value of
7489 the function is an untyped complex constant.
7493 For <code>real</code> and <code>imag</code>, the argument must be
7494 of complex type, and the return type is the corresponding floating-point
7495 type: <code>float32</code> for a <code>complex64</code> argument, and
7496 <code>float64</code> for a <code>complex128</code> argument.
7497 If the argument evaluates to an untyped constant, it must be a number,
7498 and the return value of the function is an untyped floating-point constant.
7502 The <code>real</code> and <code>imag</code> functions together form the inverse of
7503 <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
7504 <code>z == Z(complex(real(z), imag(z)))</code>.
7508 If the operands of these functions are all constants, the return
7509 value is a constant.
7513 var a = complex(2, -2) // complex128
7514 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
7515 x := float32(math.Cos(math.Pi/2)) // float32
7516 var c64 = complex(5, -x) // complex64
7517 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
7518 _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
7519 var rl = real(c64) // float32
7520 var im = imag(a) // float64
7521 const c = imag(b) // untyped constant -1.4
7522 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
7526 Arguments of type parameter type are not permitted.
7529 <h3 id="Handling_panics">Handling panics</h3>
7531 <p> Two built-in functions, <code>panic</code> and <code>recover</code>,
7532 assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
7533 and program-defined error conditions.
7536 <pre class="grammar">
7537 func panic(interface{})
7538 func recover() interface{}
7542 While executing a function <code>F</code>,
7543 an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
7544 terminates the execution of <code>F</code>.
7545 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
7546 are then executed as usual.
7547 Next, any deferred functions run by <code>F</code>'s caller are run,
7548 and so on up to any deferred by the top-level function in the executing goroutine.
7549 At that point, the program is terminated and the error
7550 condition is reported, including the value of the argument to <code>panic</code>.
7551 This termination sequence is called <i>panicking</i>.
7556 panic("unreachable")
7557 panic(Error("cannot parse"))
7561 The <code>recover</code> function allows a program to manage behavior
7562 of a panicking goroutine.
7563 Suppose a function <code>G</code> defers a function <code>D</code> that calls
7564 <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
7566 When the running of deferred functions reaches <code>D</code>,
7567 the return value of <code>D</code>'s call to <code>recover</code> will be the value passed to the call of <code>panic</code>.
7568 If <code>D</code> returns normally, without starting a new
7569 <code>panic</code>, the panicking sequence stops. In that case,
7570 the state of functions called between <code>G</code> and the call to <code>panic</code>
7571 is discarded, and normal execution resumes.
7572 Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
7573 execution terminates by returning to its caller.
7577 The return value of <code>recover</code> is <code>nil</code> when the
7578 goroutine is not panicking or <code>recover</code> was not called directly by a deferred function.
7579 Conversely, if a goroutine is panicking and <code>recover</code> was called directly by a deferred function,
7580 the return value of <code>recover</code> is guaranteed not to be <code>nil</code>.
7581 To ensure this, calling <code>panic</code> with a <code>nil</code> interface value (or an untyped <code>nil</code>)
7582 causes a <a href="#Run_time_panics">run-time panic</a>.
7586 The <code>protect</code> function in the example below invokes
7587 the function argument <code>g</code> and protects callers from
7588 run-time panics raised by <code>g</code>.
7592 func protect(g func()) {
7594 log.Println("done") // Println executes normally even if there is a panic
7595 if x := recover(); x != nil {
7596 log.Printf("run time panic: %v", x)
7599 log.Println("start")
7605 <h3 id="Bootstrapping">Bootstrapping</h3>
7608 Current implementations provide several built-in functions useful during
7609 bootstrapping. These functions are documented for completeness but are not
7610 guaranteed to stay in the language. They do not return a result.
7613 <pre class="grammar">
7616 print prints all arguments; formatting of arguments is implementation-specific
7617 println like print but prints spaces between arguments and a newline at the end
7621 Implementation restriction: <code>print</code> and <code>println</code> need not
7622 accept arbitrary argument types, but printing of boolean, numeric, and string
7623 <a href="#Types">types</a> must be supported.
7626 <h2 id="Packages">Packages</h2>
7629 Go programs are constructed by linking together <i>packages</i>.
7630 A package in turn is constructed from one or more source files
7631 that together declare constants, types, variables and functions
7632 belonging to the package and which are accessible in all files
7633 of the same package. Those elements may be
7634 <a href="#Exported_identifiers">exported</a> and used in another package.
7637 <h3 id="Source_file_organization">Source file organization</h3>
7640 Each source file consists of a package clause defining the package
7641 to which it belongs, followed by a possibly empty set of import
7642 declarations that declare packages whose contents it wishes to use,
7643 followed by a possibly empty set of declarations of functions,
7644 types, variables, and constants.
7648 SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
7651 <h3 id="Package_clause">Package clause</h3>
7654 A package clause begins each source file and defines the package
7655 to which the file belongs.
7659 PackageClause = "package" PackageName .
7660 PackageName = identifier .
7664 The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
7672 A set of files sharing the same PackageName form the implementation of a package.
7673 An implementation may require that all source files for a package inhabit the same directory.
7676 <h3 id="Import_declarations">Import declarations</h3>
7679 An import declaration states that the source file containing the declaration
7680 depends on functionality of the <i>imported</i> package
7681 (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
7682 and enables access to <a href="#Exported_identifiers">exported</a> identifiers
7684 The import names an identifier (PackageName) to be used for access and an ImportPath
7685 that specifies the package to be imported.
7689 ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
7690 ImportSpec = [ "." | PackageName ] ImportPath .
7691 ImportPath = string_lit .
7695 The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
7696 to access exported identifiers of the package within the importing source file.
7697 It is declared in the <a href="#Blocks">file block</a>.
7698 If the PackageName is omitted, it defaults to the identifier specified in the
7699 <a href="#Package_clause">package clause</a> of the imported package.
7700 If an explicit period (<code>.</code>) appears instead of a name, all the
7701 package's exported identifiers declared in that package's
7702 <a href="#Blocks">package block</a> will be declared in the importing source
7703 file's file block and must be accessed without a qualifier.
7707 The interpretation of the ImportPath is implementation-dependent but
7708 it is typically a substring of the full file name of the compiled
7709 package and may be relative to a repository of installed packages.
7713 Implementation restriction: A compiler may restrict ImportPaths to
7714 non-empty strings using only characters belonging to
7715 <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
7716 L, M, N, P, and S general categories (the Graphic characters without
7717 spaces) and may also exclude the characters
7718 <code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
7719 and the Unicode replacement character U+FFFD.
7723 Consider a compiled a package containing the package clause
7724 <code>package math</code>, which exports function <code>Sin</code>, and
7725 installed the compiled package in the file identified by
7726 <code>"lib/math"</code>.
7727 This table illustrates how <code>Sin</code> is accessed in files
7728 that import the package after the
7729 various types of import declaration.
7732 <pre class="grammar">
7733 Import declaration Local name of Sin
7735 import "lib/math" math.Sin
7736 import m "lib/math" m.Sin
7737 import . "lib/math" Sin
7741 An import declaration declares a dependency relation between
7742 the importing and imported package.
7743 It is illegal for a package to import itself, directly or indirectly,
7744 or to directly import a package without
7745 referring to any of its exported identifiers. To import a package solely for
7746 its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
7747 identifier as explicit package name:
7755 <h3 id="An_example_package">An example package</h3>
7758 Here is a complete Go package that implements a concurrent prime sieve.
7766 // Send the sequence 2, 3, 4, … to channel 'ch'.
7767 func generate(ch chan<- int) {
7769 ch <- i // Send 'i' to channel 'ch'.
7773 // Copy the values from channel 'src' to channel 'dst',
7774 // removing those divisible by 'prime'.
7775 func filter(src <-chan int, dst chan<- int, prime int) {
7776 for i := range src { // Loop over values received from 'src'.
7778 dst <- i // Send 'i' to channel 'dst'.
7783 // The prime sieve: Daisy-chain filter processes together.
7785 ch := make(chan int) // Create a new channel.
7786 go generate(ch) // Start generate() as a subprocess.
7789 fmt.Print(prime, "\n")
7790 ch1 := make(chan int)
7791 go filter(ch, ch1, prime)
7801 <h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
7803 <h3 id="The_zero_value">The zero value</h3>
7805 When storage is allocated for a <a href="#Variables">variable</a>,
7806 either through a declaration or a call of <code>new</code>, or when
7807 a new value is created, either through a composite literal or a call
7808 of <code>make</code>,
7809 and no explicit initialization is provided, the variable or value is
7810 given a default value. Each element of such a variable or value is
7811 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
7812 <code>0</code> for numeric types, <code>""</code>
7813 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
7814 This initialization is done recursively, so for instance each element of an
7815 array of structs will have its fields zeroed if no value is specified.
7818 These two simple declarations are equivalent:
7831 type T struct { i int; f float64; next *T }
7836 the following holds:
7846 The same would also be true after
7853 <h3 id="Package_initialization">Package initialization</h3>
7856 Within a package, package-level variable initialization proceeds stepwise,
7857 with each step selecting the variable earliest in <i>declaration order</i>
7858 which has no dependencies on uninitialized variables.
7862 More precisely, a package-level variable is considered <i>ready for
7863 initialization</i> if it is not yet initialized and either has
7864 no <a href="#Variable_declarations">initialization expression</a> or
7865 its initialization expression has no <i>dependencies</i> on uninitialized variables.
7866 Initialization proceeds by repeatedly initializing the next package-level
7867 variable that is earliest in declaration order and ready for initialization,
7868 until there are no variables ready for initialization.
7872 If any variables are still uninitialized when this
7873 process ends, those variables are part of one or more initialization cycles,
7874 and the program is not valid.
7878 Multiple variables on the left-hand side of a variable declaration initialized
7879 by single (multi-valued) expression on the right-hand side are initialized
7880 together: If any of the variables on the left-hand side is initialized, all
7881 those variables are initialized in the same step.
7886 var a, b = f() // a and b are initialized together, before x is initialized
7890 For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
7891 variables are treated like any other variables in declarations.
7895 The declaration order of variables declared in multiple files is determined
7896 by the order in which the files are presented to the compiler: Variables
7897 declared in the first file are declared before any of the variables declared
7898 in the second file, and so on.
7902 Dependency analysis does not rely on the actual values of the
7903 variables, only on lexical <i>references</i> to them in the source,
7904 analyzed transitively. For instance, if a variable <code>x</code>'s
7905 initialization expression refers to a function whose body refers to
7906 variable <code>y</code> then <code>x</code> depends on <code>y</code>.
7912 A reference to a variable or function is an identifier denoting that
7913 variable or function.
7917 A reference to a method <code>m</code> is a
7918 <a href="#Method_values">method value</a> or
7919 <a href="#Method_expressions">method expression</a> of the form
7920 <code>t.m</code>, where the (static) type of <code>t</code> is
7921 not an interface type, and the method <code>m</code> is in the
7922 <a href="#Method_sets">method set</a> of <code>t</code>.
7923 It is immaterial whether the resulting function value
7924 <code>t.m</code> is invoked.
7928 A variable, function, or method <code>x</code> depends on a variable
7929 <code>y</code> if <code>x</code>'s initialization expression or body
7930 (for functions and methods) contains a reference to <code>y</code>
7931 or to a function or method that depends on <code>y</code>.
7936 For example, given the declarations
7944 d = 3 // == 5 after initialization has finished
7954 the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
7955 Note that the order of subexpressions in initialization expressions is irrelevant:
7956 <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
7957 order in this example.
7961 Dependency analysis is performed per package; only references referring
7962 to variables, functions, and (non-interface) methods declared in the current
7963 package are considered. If other, hidden, data dependencies exists between
7964 variables, the initialization order between those variables is unspecified.
7968 For instance, given the declarations
7972 var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
7973 var _ = sideEffect() // unrelated to x, a, or b
7977 type I interface { ab() []int }
7979 func (T) ab() []int { return []int{a, b} }
7983 the variable <code>a</code> will be initialized after <code>b</code> but
7984 whether <code>x</code> is initialized before <code>b</code>, between
7985 <code>b</code> and <code>a</code>, or after <code>a</code>, and
7986 thus also the moment at which <code>sideEffect()</code> is called (before
7987 or after <code>x</code> is initialized) is not specified.
7991 Variables may also be initialized using functions named <code>init</code>
7992 declared in the package block, with no arguments and no result parameters.
8000 Multiple such functions may be defined per package, even within a single
8001 source file. In the package block, the <code>init</code> identifier can
8002 be used only to declare <code>init</code> functions, yet the identifier
8003 itself is not <a href="#Declarations_and_scope">declared</a>. Thus
8004 <code>init</code> functions cannot be referred to from anywhere
8009 A package with no imports is initialized by assigning initial values
8010 to all its package-level variables followed by calling all <code>init</code>
8011 functions in the order they appear in the source, possibly in multiple files,
8012 as presented to the compiler.
8013 If a package has imports, the imported packages are initialized
8014 before initializing the package itself. If multiple packages import
8015 a package, the imported package will be initialized only once.
8016 The importing of packages, by construction, guarantees that there
8017 can be no cyclic initialization dependencies.
8021 Package initialization—variable initialization and the invocation of
8022 <code>init</code> functions—happens in a single goroutine,
8023 sequentially, one package at a time.
8024 An <code>init</code> function may launch other goroutines, which can run
8025 concurrently with the initialization code. However, initialization
8027 the <code>init</code> functions: it will not invoke the next one
8028 until the previous one has returned.
8032 To ensure reproducible initialization behavior, build systems are encouraged
8033 to present multiple files belonging to the same package in lexical file name
8034 order to a compiler.
8038 <h3 id="Program_execution">Program execution</h3>
8040 A complete program is created by linking a single, unimported package
8041 called the <i>main package</i> with all the packages it imports, transitively.
8042 The main package must
8043 have package name <code>main</code> and
8044 declare a function <code>main</code> that takes no
8045 arguments and returns no value.
8053 Program execution begins by initializing the main package and then
8054 invoking the function <code>main</code>.
8055 When that function invocation returns, the program exits.
8056 It does not wait for other (non-<code>main</code>) goroutines to complete.
8059 <h2 id="Errors">Errors</h2>
8062 The predeclared type <code>error</code> is defined as
8066 type error interface {
8072 It is the conventional interface for representing an error condition,
8073 with the nil value representing no error.
8074 For instance, a function to read data from a file might be defined:
8078 func Read(f *File, b []byte) (n int, err error)
8081 <h2 id="Run_time_panics">Run-time panics</h2>
8084 Execution errors such as attempting to index an array out
8085 of bounds trigger a <i>run-time panic</i> equivalent to a call of
8086 the built-in function <a href="#Handling_panics"><code>panic</code></a>
8087 with a value of the implementation-defined interface type <code>runtime.Error</code>.
8088 That type satisfies the predeclared interface type
8089 <a href="#Errors"><code>error</code></a>.
8090 The exact error values that
8091 represent distinct run-time error conditions are unspecified.
8097 type Error interface {
8099 // and perhaps other methods
8103 <h2 id="System_considerations">System considerations</h2>
8105 <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
8108 The built-in package <code>unsafe</code>, known to the compiler
8109 and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
8110 provides facilities for low-level programming including operations
8111 that violate the type system. A package using <code>unsafe</code>
8112 must be vetted manually for type safety and may not be portable.
8113 The package provides the following interface:
8116 <pre class="grammar">
8119 type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
8120 type Pointer *ArbitraryType
8122 func Alignof(variable ArbitraryType) uintptr
8123 func Offsetof(selector ArbitraryType) uintptr
8124 func Sizeof(variable ArbitraryType) uintptr
8126 type IntegerType int // shorthand for an integer type; it is not a real type
8127 func Add(ptr Pointer, len IntegerType) Pointer
8128 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
8129 func SliceData(slice []ArbitraryType) *ArbitraryType
8130 func String(ptr *byte, len IntegerType) string
8131 func StringData(str string) *byte
8135 These conversions also apply to type parameters with suitable core types.
8136 Determine if we can simply use core type instead of underlying type here,
8137 of if the general conversion rules take care of this.
8141 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
8142 value may not be <a href="#Address_operators">dereferenced</a>.
8143 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be
8144 <a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
8145 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
8150 bits = *(*uint64)(unsafe.Pointer(&f))
8152 type ptr unsafe.Pointer
8153 bits = *(*uint64)(ptr(&f))
8159 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
8160 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
8161 as if <code>v</code> was declared via <code>var v = x</code>.
8164 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
8165 <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
8166 or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
8167 If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
8168 without pointer indirections through fields of the struct.
8169 For a struct <code>s</code> with field <code>f</code>:
8173 uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
8177 Computer architectures may require memory addresses to be <i>aligned</i>;
8178 that is, for addresses of a variable to be a multiple of a factor,
8179 the variable's type's <i>alignment</i>. The function <code>Alignof</code>
8180 takes an expression denoting a variable of any type and returns the
8181 alignment of the (type of the) variable in bytes. For a variable
8186 uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
8190 A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
8191 is a <a href="#Type_parameter_declarations">type parameter</a>, or if it is an
8192 array or struct type containing elements
8193 or fields of variable size. Otherwise the size is <i>constant</i>.
8194 Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
8195 are compile-time <a href="#Constant_expressions">constant expressions</a> of
8196 type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
8197 the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
8202 The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
8203 and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
8204 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8205 A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
8206 if it is an untyped constant it is given type <code>int</code>.
8207 The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
8211 The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
8212 and whose length and capacity are <code>len</code>.
8213 <code>Slice(ptr, len)</code> is equivalent to
8217 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
8221 except that, as a special case, if <code>ptr</code>
8222 is <code>nil</code> and <code>len</code> is zero,
8223 <code>Slice</code> returns <code>nil</code>.
8227 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8228 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
8229 if it is an untyped constant it is given type <code>int</code>.
8230 At run time, if <code>len</code> is negative,
8231 or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
8232 a <a href="#Run_time_panics">run-time panic</a> occurs.
8236 The function <code>SliceData</code> returns a pointer to the underlying array of the <code>slice</code> argument.
8237 If the slice's capacity <code>cap(slice)</code> is not zero, that pointer is <code>&slice[:1][0]</code>.
8238 If <code>slice</code> is <code>nil</code>, the result is <code>nil</code>.
8239 Otherwise it is a non-<code>nil</code> pointer to an unspecified memory address.
8243 The function <code>String</code> returns a <code>string</code> value whose underlying bytes start at
8244 <code>ptr</code> and whose length is <code>len</code>.
8245 The same requirements apply to the <code>ptr</code> and <code>len</code> argument as in the function
8246 <code>Slice</code>. If <code>len</code> is zero, the result is the empty string <code>""</code>.
8247 Since Go strings are immutable, the bytes passed to <code>String</code> must not be modified afterwards.
8251 The function <code>StringData</code> returns a pointer to the underlying bytes of the <code>str</code> argument.
8252 For an empty string the return value is unspecified, and may be <code>nil</code>.
8253 Since Go strings are immutable, the bytes returned by <code>StringData</code> must not be modified.
8256 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
8259 For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
8262 <pre class="grammar">
8267 uint32, int32, float32 4
8268 uint64, int64, float64, complex64 8
8273 The following minimal alignment properties are guaranteed:
8276 <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
8279 <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
8280 all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
8283 <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
8284 the alignment of a variable of the array's element type.
8289 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.