2 "Title": "The Go Programming Language Specification",
3 "Subtitle": "Version of December 14, 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))
947 <h3 id="Slice_types">Slice types</h3>
950 A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
951 provides access to a numbered sequence of elements from that array.
952 A slice type denotes the set of all slices of arrays of its element type.
953 The number of elements is called the length of the slice and is never negative.
954 The value of an uninitialized slice is <code>nil</code>.
958 SliceType = "[" "]" ElementType .
962 The length of a slice <code>s</code> can be discovered by the built-in function
963 <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
964 execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
965 0 through <code>len(s)-1</code>. The slice index of a
966 given element may be less than the index of the same element in the
970 A slice, once initialized, is always associated with an underlying
971 array that holds its elements. A slice therefore shares storage
972 with its array and with other slices of the same array; by contrast,
973 distinct arrays always represent distinct storage.
976 The array underlying a slice may extend past the end of the slice.
977 The <i>capacity</i> is a measure of that extent: it is the sum of
978 the length of the slice and the length of the array beyond the slice;
979 a slice of length up to that capacity can be created by
980 <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
981 The capacity of a slice <code>a</code> can be discovered using the
982 built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
986 A new, initialized slice value for a given element type <code>T</code> may be
987 made using the built-in function
988 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
989 which takes a slice type
990 and parameters specifying the length and optionally the capacity.
991 A slice created with <code>make</code> always allocates a new, hidden array
992 to which the returned slice value refers. That is, executing
996 make([]T, length, capacity)
1000 produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
1001 it, so these two expressions are equivalent:
1005 make([]int, 50, 100)
1010 Like arrays, slices are always one-dimensional but may be composed to construct
1011 higher-dimensional objects.
1012 With arrays of arrays, the inner arrays are, by construction, always the same length;
1013 however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
1014 Moreover, the inner slices must be initialized individually.
1017 <h3 id="Struct_types">Struct types</h3>
1020 A struct is a sequence of named elements, called fields, each of which has a
1021 name and a type. Field names may be specified explicitly (IdentifierList) or
1022 implicitly (EmbeddedField).
1023 Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
1024 be <a href="#Uniqueness_of_identifiers">unique</a>.
1028 StructType = "struct" "{" { FieldDecl ";" } "}" .
1029 FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
1030 EmbeddedField = [ "*" ] TypeName [ TypeArgs ] .
1038 // A struct with 6 fields.
1042 _ float32 // padding
1049 A field declared with a type but no explicit field name is called an <i>embedded field</i>.
1050 An embedded field must be specified as
1051 a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
1052 and <code>T</code> itself may not be
1053 a pointer type. The unqualified type name acts as the field name.
1057 // A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
1059 T1 // field name is T1
1060 *T2 // field name is T2
1061 P.T3 // field name is T3
1062 *P.T4 // field name is T4
1063 x, y int // field names are x and y
1068 The following declaration is illegal because field names must be unique
1074 T // conflicts with embedded field *T and *P.T
1075 *T // conflicts with embedded field T and *P.T
1076 *P.T // conflicts with embedded field T and *T
1081 A field or <a href="#Method_declarations">method</a> <code>f</code> of an
1082 embedded field in a struct <code>x</code> is called <i>promoted</i> if
1083 <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
1084 that field or method <code>f</code>.
1088 Promoted fields act like ordinary fields
1089 of a struct except that they cannot be used as field names in
1090 <a href="#Composite_literals">composite literals</a> of the struct.
1094 Given a struct type <code>S</code> and a <a href="#Types">named type</a>
1095 <code>T</code>, promoted methods are included in the method set of the struct as follows:
1099 If <code>S</code> contains an embedded field <code>T</code>,
1100 the <a href="#Method_sets">method sets</a> of <code>S</code>
1101 and <code>*S</code> both include promoted methods with receiver
1102 <code>T</code>. The method set of <code>*S</code> also
1103 includes promoted methods with receiver <code>*T</code>.
1107 If <code>S</code> contains an embedded field <code>*T</code>,
1108 the method sets of <code>S</code> and <code>*S</code> both
1109 include promoted methods with receiver <code>T</code> or
1115 A field declaration may be followed by an optional string literal <i>tag</i>,
1116 which becomes an attribute for all the fields in the corresponding
1117 field declaration. An empty tag string is equivalent to an absent tag.
1118 The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
1119 and take part in <a href="#Type_identity">type identity</a> for structs
1120 but are otherwise ignored.
1125 x, y float64 "" // an empty tag string is like an absent tag
1126 name string "any string is permitted as a tag"
1127 _ [4]byte "ceci n'est pas un champ de structure"
1130 // A struct corresponding to a TimeStamp protocol buffer.
1131 // The tag strings define the protocol buffer field numbers;
1132 // they follow the convention outlined by the reflect package.
1134 microsec uint64 `protobuf:"1"`
1135 serverIP6 uint64 `protobuf:"2"`
1139 <h3 id="Pointer_types">Pointer types</h3>
1142 A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
1143 type, called the <i>base type</i> of the pointer.
1144 The value of an uninitialized pointer is <code>nil</code>.
1148 PointerType = "*" BaseType .
1157 <h3 id="Function_types">Function types</h3>
1160 A function type denotes the set of all functions with the same parameter
1161 and result types. The value of an uninitialized variable of function type
1162 is <code>nil</code>.
1166 FunctionType = "func" Signature .
1167 Signature = Parameters [ Result ] .
1168 Result = Parameters | Type .
1169 Parameters = "(" [ ParameterList [ "," ] ] ")" .
1170 ParameterList = ParameterDecl { "," ParameterDecl } .
1171 ParameterDecl = [ IdentifierList ] [ "..." ] Type .
1175 Within a list of parameters or results, the names (IdentifierList)
1176 must either all be present or all be absent. If present, each name
1177 stands for one item (parameter or result) of the specified type and
1178 all non-<a href="#Blank_identifier">blank</a> names in the signature
1179 must be <a href="#Uniqueness_of_identifiers">unique</a>.
1180 If absent, each type stands for one item of that type.
1181 Parameter and result
1182 lists are always parenthesized except that if there is exactly
1183 one unnamed result it may be written as an unparenthesized type.
1187 The final incoming parameter in a function signature may have
1188 a type prefixed with <code>...</code>.
1189 A function with such a parameter is called <i>variadic</i> and
1190 may be invoked with zero or more arguments for that parameter.
1196 func(a, _ int, z float32) bool
1197 func(a, b int, z float32) (bool)
1198 func(prefix string, values ...int)
1199 func(a, b int, z float64, opt ...interface{}) (success bool)
1200 func(int, int, float64) (float64, *[]int)
1201 func(n int) func(p *T)
1204 <h3 id="Interface_types">Interface types</h3>
1207 An interface type defines a <i>type set</i>.
1208 A variable of interface type can store a value of any type that is in the type
1209 set of the interface. Such a type is said to
1210 <a href="#Implementing_an_interface">implement the interface</a>.
1211 The value of an uninitialized variable of interface type is <code>nil</code>.
1215 InterfaceType = "interface" "{" { InterfaceElem ";" } "}" .
1216 InterfaceElem = MethodElem | TypeElem .
1217 MethodElem = MethodName Signature .
1218 MethodName = identifier .
1219 TypeElem = TypeTerm { "|" TypeTerm } .
1220 TypeTerm = Type | UnderlyingType .
1221 UnderlyingType = "~" Type .
1225 An interface type is specified by a list of <i>interface elements</i>.
1226 An interface element is either a <i>method</i> or a <i>type element</i>,
1227 where a type element is a union of one or more <i>type terms</i>.
1228 A type term is either a single type or a single underlying type.
1231 <h4 id="Basic_interfaces">Basic interfaces</h4>
1234 In its most basic form an interface specifies a (possibly empty) list of methods.
1235 The type set defined by such an interface is the set of types which implement all of
1236 those methods, and the corresponding <a href="#Method_sets">method set</a> consists
1237 exactly of the methods specified by the interface.
1238 Interfaces whose type sets can be defined entirely by a list of methods are called
1239 <i>basic interfaces.</i>
1243 // A simple File interface.
1245 Read([]byte) (int, error)
1246 Write([]byte) (int, error)
1252 The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
1253 and not <a href="#Blank_identifier">blank</a>.
1259 String() string // illegal: String not unique
1260 _(x int) // illegal: method must have non-blank name
1265 More than one type may implement an interface.
1266 For instance, if two types <code>S1</code> and <code>S2</code>
1271 func (p T) Read(p []byte) (n int, err error)
1272 func (p T) Write(p []byte) (n int, err error)
1273 func (p T) Close() error
1277 (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
1278 then the <code>File</code> interface is implemented by both <code>S1</code> and
1279 <code>S2</code>, regardless of what other methods
1280 <code>S1</code> and <code>S2</code> may have or share.
1284 Every type that is a member of the type set of an interface implements that interface.
1285 Any given type may implement several distinct interfaces.
1286 For instance, all types implement the <i>empty interface</i> which stands for the set
1287 of all (non-interface) types:
1295 For convenience, the predeclared type <code>any</code> is an alias for the empty interface.
1299 Similarly, consider this interface specification,
1300 which appears within a <a href="#Type_declarations">type declaration</a>
1301 to define an interface called <code>Locker</code>:
1305 type Locker interface {
1312 If <code>S1</code> and <code>S2</code> also implement
1316 func (p T) Lock() { … }
1317 func (p T) Unlock() { … }
1321 they implement the <code>Locker</code> interface as well
1322 as the <code>File</code> interface.
1325 <h4 id="Embedded_interfaces">Embedded interfaces</h4>
1328 In a slightly more general form
1329 an interface <code>T</code> may use a (possibly qualified) interface type
1330 name <code>E</code> as an interface element. This is called
1331 <i>embedding</i> interface <code>E</code> in <code>T</code>.
1332 The type set of <code>T</code> is the <i>intersection</i> of the type sets
1333 defined by <code>T</code>'s explicitly declared methods and the type sets
1334 of <code>T</code>’s embedded interfaces.
1335 In other words, the type set of <code>T</code> is the set of all types that implement all the
1336 explicitly declared methods of <code>T</code> and also all the methods of
1341 type Reader interface {
1342 Read(p []byte) (n int, err error)
1346 type Writer interface {
1347 Write(p []byte) (n int, err error)
1351 // ReadWriter's methods are Read, Write, and Close.
1352 type ReadWriter interface {
1353 Reader // includes methods of Reader in ReadWriter's method set
1354 Writer // includes methods of Writer in ReadWriter's method set
1359 When embedding interfaces, methods with the
1360 <a href="#Uniqueness_of_identifiers">same</a> names must
1361 have <a href="#Type_identity">identical</a> signatures.
1365 type ReadCloser interface {
1366 Reader // includes methods of Reader in ReadCloser's method set
1367 Close() // illegal: signatures of Reader.Close and Close are different
1371 <h4 id="General_interfaces">General interfaces</h4>
1374 In their most general form, an interface element may also be an arbitrary type term
1375 <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
1376 or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
1377 Together with method specifications, these elements enable the precise
1378 definition of an interface's type set as follows:
1382 <li>The type set of the empty interface is the set of all non-interface types.
1385 <li>The type set of a non-empty interface is the intersection of the type sets
1386 of its interface elements.
1389 <li>The type set of a method specification is the set of all non-interface types
1390 whose method sets include that method.
1393 <li>The type set of a non-interface type term is the set consisting
1397 <li>The type set of a term of the form <code>~T</code>
1398 is the set of all types whose underlying type is <code>T</code>.
1401 <li>The type set of a <i>union</i> of terms
1402 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
1403 is the union of the type sets of the terms.
1408 The quantification "the set of all non-interface types" refers not just to all (non-interface)
1409 types declared in the program at hand, but all possible types in all possible programs, and
1411 Similarly, given the set of all non-interface types that implement a particular method, the
1412 intersection of the method sets of those types will contain exactly that method, even if all
1413 types in the program at hand always pair that method with another method.
1417 By construction, an interface's type set never contains an interface type.
1421 // An interface representing only the type int.
1426 // An interface representing all types with underlying type int.
1431 // An interface representing all types with underlying type int that implement the String method.
1437 // An interface representing an empty type set: there is no type that is both an int and a string.
1445 In a term of the form <code>~T</code>, the underlying type of <code>T</code>
1446 must be itself, and <code>T</code> cannot be an interface.
1453 ~[]byte // the underlying type of []byte is itself
1454 ~MyInt // illegal: the underlying type of MyInt is not MyInt
1455 ~error // illegal: error is an interface
1460 Union elements denote unions of type sets:
1464 // The Float interface represents all floating-point types
1465 // (including any named types whose underlying types are
1466 // either float32 or float64).
1467 type Float interface {
1473 The type <code>T</code> in a term of the form <code>T</code> or <code>~T</code> cannot
1474 be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all
1475 non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
1476 Given a type parameter <code>P</code>:
1481 P // illegal: P is a type parameter
1482 int | ~P // illegal: P is a type parameter
1483 ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
1484 float32 | Float // overlapping type sets but Float is an interface
1489 Implementation restriction:
1490 A union (with more than one term) cannot contain the
1491 <a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code>
1492 or interfaces that specify methods, or embed <code>comparable</code> or interfaces
1493 that specify methods.
1497 Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type
1498 constraints, or as elements of other interfaces used as constraints.
1499 They cannot be the types of values or variables, or components of other,
1500 non-interface types.
1504 var x Float // illegal: Float is not a basic interface
1506 var x interface{} = Float(nil) // illegal
1508 type Floatish struct {
1514 An interface type <code>T</code> may not embed any type element
1515 that is, contains, or embeds <code>T</code>, recursively.
1519 // illegal: Bad cannot embed itself
1520 type Bad interface {
1524 // illegal: Bad1 cannot embed itself using Bad2
1525 type Bad1 interface {
1528 type Bad2 interface {
1532 // illegal: Bad3 cannot embed a union containing Bad3
1533 type Bad3 interface {
1534 ~int | ~string | Bad3
1538 <h4 id="Implementing_an_interface">Implementing an interface</h4>
1541 A type <code>T</code> implements an interface <code>I</code> if
1546 <code>T</code> is not an interface and is an element of the type set of <code>I</code>; or
1549 <code>T</code> is an interface and the type set of <code>T</code> is a subset of the
1550 type set of <code>I</code>.
1555 A value of type <code>T</code> implements an interface if <code>T</code>
1556 implements the interface.
1559 <h3 id="Map_types">Map types</h3>
1562 A map is an unordered group of elements of one type, called the
1563 element type, indexed by a set of unique <i>keys</i> of another type,
1564 called the key type.
1565 The value of an uninitialized map is <code>nil</code>.
1569 MapType = "map" "[" KeyType "]" ElementType .
1574 The <a href="#Comparison_operators">comparison operators</a>
1575 <code>==</code> and <code>!=</code> must be fully defined
1576 for operands of the key type; thus the key type must not be a function, map, or
1578 If the key type is an interface type, these
1579 comparison operators must be defined for the dynamic key values;
1580 failure will cause a <a href="#Run_time_panics">run-time panic</a>.
1585 map[*T]struct{ x, y float64 }
1586 map[string]interface{}
1590 The number of map elements is called its length.
1591 For a map <code>m</code>, it can be discovered using the
1592 built-in function <a href="#Length_and_capacity"><code>len</code></a>
1593 and may change during execution. Elements may be added during execution
1594 using <a href="#Assignment_statements">assignments</a> and retrieved with
1595 <a href="#Index_expressions">index expressions</a>; they may be removed with the
1596 <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
1599 A new, empty map value is made using the built-in
1600 function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1601 which takes the map type and an optional capacity hint as arguments:
1605 make(map[string]int)
1606 make(map[string]int, 100)
1610 The initial capacity does not bound its size:
1611 maps grow to accommodate the number of items
1612 stored in them, with the exception of <code>nil</code> maps.
1613 A <code>nil</code> map is equivalent to an empty map except that no elements
1616 <h3 id="Channel_types">Channel types</h3>
1619 A channel provides a mechanism for
1620 <a href="#Go_statements">concurrently executing functions</a>
1622 <a href="#Send_statements">sending</a> and
1623 <a href="#Receive_operator">receiving</a>
1624 values of a specified element type.
1625 The value of an uninitialized channel is <code>nil</code>.
1629 ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
1633 The optional <code><-</code> operator specifies the channel <i>direction</i>,
1634 <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
1635 otherwise it is <i>bidirectional</i>.
1636 A channel may be constrained only to send or only to receive by
1637 <a href="#Assignment_statements">assignment</a> or
1638 explicit <a href="#Conversions">conversion</a>.
1642 chan T // can be used to send and receive values of type T
1643 chan<- float64 // can only be used to send float64s
1644 <-chan int // can only be used to receive ints
1648 The <code><-</code> operator associates with the leftmost <code>chan</code>
1653 chan<- chan int // same as chan<- (chan int)
1654 chan<- <-chan int // same as chan<- (<-chan int)
1655 <-chan <-chan int // same as <-chan (<-chan int)
1656 chan (<-chan int)
1660 A new, initialized channel
1661 value can be made using the built-in function
1662 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1663 which takes the channel type and an optional <i>capacity</i> as arguments:
1671 The capacity, in number of elements, sets the size of the buffer in the channel.
1672 If the capacity is zero or absent, the channel is unbuffered and communication
1673 succeeds only when both a sender and receiver are ready. Otherwise, the channel
1674 is buffered and communication succeeds without blocking if the buffer
1675 is not full (sends) or not empty (receives).
1676 A <code>nil</code> channel is never ready for communication.
1680 A channel may be closed with the built-in function
1681 <a href="#Close"><code>close</code></a>.
1682 The multi-valued assignment form of the
1683 <a href="#Receive_operator">receive operator</a>
1684 reports whether a received value was sent before
1685 the channel was closed.
1689 A single channel may be used in
1690 <a href="#Send_statements">send statements</a>,
1691 <a href="#Receive_operator">receive operations</a>,
1692 and calls to the built-in functions
1693 <a href="#Length_and_capacity"><code>cap</code></a> and
1694 <a href="#Length_and_capacity"><code>len</code></a>
1695 by any number of goroutines without further synchronization.
1696 Channels act as first-in-first-out queues.
1697 For example, if one goroutine sends values on a channel
1698 and a second goroutine receives them, the values are
1699 received in the order sent.
1702 <h2 id="Properties_of_types_and_values">Properties of types and values</h2>
1704 <h3 id="Underlying_types">Underlying types</h3>
1707 Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
1708 is one of the predeclared boolean, numeric, or string types, or a type literal,
1709 the corresponding underlying type is <code>T</code> itself.
1710 Otherwise, <code>T</code>'s underlying type is the underlying type of the
1711 type to which <code>T</code> refers in its declaration.
1712 For a type parameter that is the underlying type of its
1713 <a href="#Type_constraints">type constraint</a>, which is always an interface.
1729 func f[P any](x P) { … }
1733 The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
1734 and <code>B2</code> is <code>string</code>.
1735 The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
1736 The underlying type of <code>P</code> is <code>interface{}</code>.
1739 <h3 id="Core_types">Core types</h3>
1742 Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the
1743 <a href="#Underlying_types">underlying type</a> of <code>T</code>.
1747 An interface <code>T</code> has a core type if one of the following
1748 conditions is satisfied:
1753 There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a>
1754 of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or
1757 the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a>
1758 with identical element type <code>E</code>, and all directional channels have the same
1764 No other interfaces have a core type.
1768 The core type of an interface is, depending on the condition that is satisfied, either:
1773 the type <code>U</code>; or
1776 the type <code>chan E</code> if <code>T</code> contains only bidirectional
1777 channels, or the type <code>chan<- E</code> or <code><-chan E</code>
1778 depending on the direction of the directional channels present.
1783 By definition, a core type is never a <a href="#Type_definitions">defined type</a>,
1784 <a href="#Type_parameter_declarations">type parameter</a>, or
1785 <a href="#Interface_types">interface type</a>.
1789 Examples of interfaces with core types:
1793 type Celsius float32
1796 interface{ int } // int
1797 interface{ Celsius|Kelvin } // float32
1798 interface{ ~chan int } // chan int
1799 interface{ ~chan int|~chan<- int } // chan<- int
1800 interface{ ~[]*data; String() string } // []*data
1804 Examples of interfaces without core types:
1808 interface{} // no single underlying type
1809 interface{ Celsius|float64 } // no single underlying type
1810 interface{ chan int | chan<- string } // channels have different element types
1811 interface{ <-chan int | chan<- int } // directional channels have different directions
1815 Some operations (<a href="#Slice_expressions">slice expressions</a>,
1816 <a href="#Appending_and_copying_slices"><code>append</code> and <code>copy</code></a>)
1817 rely on a slightly more loose form of core types which accept byte slices and strings.
1818 Specifically, if there are exactly two types, <code>[]byte</code> and <code>string</code>,
1819 which are the underlying types of all types in the type set of interface <code>T</code>,
1820 the core type of <code>T</code> is called <code>bytestring</code>.
1824 Examples of interfaces with <code>bytestring</code> core types:
1828 interface{ int } // int (same as ordinary core type)
1829 interface{ []byte | string } // bytestring
1830 interface{ ~[]byte | myString } // bytestring
1834 Note that <code>bytestring</code> is not a real type; it cannot be used to declare
1835 variables are compose other types. It exists solely to describe the behavior of some
1836 operations that read from a sequence of bytes, which may be a byte slice or a string.
1839 <h3 id="Type_identity">Type identity</h3>
1842 Two types are either <i>identical</i> or <i>different</i>.
1846 A <a href="#Types">named type</a> is always different from any other type.
1847 Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
1848 structurally equivalent; that is, they have the same literal structure and corresponding
1849 components have identical types. In detail:
1853 <li>Two array types are identical if they have identical element types and
1854 the same array length.</li>
1856 <li>Two slice types are identical if they have identical element types.</li>
1858 <li>Two struct types are identical if they have the same sequence of fields,
1859 and if corresponding fields have the same names, and identical types,
1861 <a href="#Exported_identifiers">Non-exported</a> field names from different
1862 packages are always different.</li>
1864 <li>Two pointer types are identical if they have identical base types.</li>
1866 <li>Two function types are identical if they have the same number of parameters
1867 and result values, corresponding parameter and result types are
1868 identical, and either both functions are variadic or neither is.
1869 Parameter and result names are not required to match.</li>
1871 <li>Two interface types are identical if they define the same type set.
1874 <li>Two map types are identical if they have identical key and element types.</li>
1876 <li>Two channel types are identical if they have identical element types and
1877 the same direction.</li>
1879 <li>Two <a href="#Instantiations">instantiated</a> types are identical if
1880 their defined types and all type arguments are identical.
1885 Given the declarations
1892 A2 = struct{ a, b int }
1894 A4 = func(A3, float64) *A0
1895 A5 = func(x int, _ float64) *[]string
1899 B2 struct{ a, b int }
1900 B3 struct{ a, c int }
1901 B4 func(int, float64) *B0
1902 B5 func(x int, y float64) *A1
1905 D0[P1, P2 any] struct{ x P1; y P2 }
1906 E0 = D0[int, string]
1911 these types are identical:
1915 A0, A1, and []string
1916 A2 and struct{ a, b int }
1918 A4, func(int, float64) *[]string, and A5
1921 D0[int, string] and E0
1923 struct{ a, b *B5 } and struct{ a, b *B5 }
1924 func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
1928 <code>B0</code> and <code>B1</code> are different because they are new types
1929 created by distinct <a href="#Type_definitions">type definitions</a>;
1930 <code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
1931 are different because <code>B0</code> is different from <code>[]string</code>;
1932 and <code>P1</code> and <code>P2</code> are different because they are different
1934 <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are
1935 different because the former is an <a href="#Instantiations">instantiated</a>
1936 defined type while the latter is a type literal
1937 (but they are still <a href="#Assignability">assignable</a>).
1940 <h3 id="Assignability">Assignability</h3>
1943 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>
1944 ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
1949 <code>V</code> and <code>T</code> are identical.
1952 <code>V</code> and <code>T</code> have identical
1953 <a href="#Underlying_types">underlying types</a>
1954 but are not type parameters and at least one of <code>V</code>
1955 or <code>T</code> is not a <a href="#Types">named type</a>.
1958 <code>V</code> and <code>T</code> are channel types with
1959 identical element types, <code>V</code> is a bidirectional channel,
1960 and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
1963 <code>T</code> is an interface type, but not a type parameter, and
1964 <code>x</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
1967 <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
1968 is a pointer, function, slice, map, channel, or interface type,
1969 but not a type parameter.
1972 <code>x</code> is an untyped <a href="#Constants">constant</a>
1973 <a href="#Representability">representable</a>
1974 by a value of type <code>T</code>.
1979 Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code>
1980 is assignable to a variable of type <code>T</code> if one of the following conditions applies:
1985 <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
1986 a type parameter, and <code>x</code> is assignable to each type in
1987 <code>T</code>'s type set.
1990 <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
1991 a type parameter, and <code>x</code> is assignable to each type in
1992 <code>T</code>'s type set.
1995 <code>V</code> is a type parameter and <code>T</code> is not a named type,
1996 and values of each type in <code>V</code>'s type set are assignable
2001 <h3 id="Representability">Representability</h3>
2004 A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
2005 by a value of type <code>T</code>,
2006 where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>,
2007 if one of the following conditions applies:
2012 <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
2016 <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
2017 precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
2018 negative zero further simplified to an unsigned zero. Note that constant values never result
2019 in an IEEE negative zero, NaN, or infinity.
2023 <code>T</code> is a complex type, and <code>x</code>'s
2024 <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
2025 are representable by values of <code>T</code>'s component type (<code>float32</code> or
2026 <code>float64</code>).
2031 If <code>T</code> is a type parameter,
2032 <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
2033 by a value of each type in <code>T</code>'s type set.
2037 x T x is representable by a value of T because
2039 'a' byte 97 is in the set of byte values
2040 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
2041 "foo" string "foo" is in the set of string values
2042 1024 int16 1024 is in the set of 16-bit integers
2043 42.0 byte 42 is in the set of unsigned 8-bit integers
2044 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
2045 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
2046 -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
2047 0i int 0 is an integer value
2048 (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
2052 x T x is not representable by a value of T because
2054 0 bool 0 is not in the set of boolean values
2055 'a' string 'a' is a rune, it is not in the set of string values
2056 1024 byte 1024 is not in the set of unsigned 8-bit integers
2057 -1 uint16 -1 is not in the set of unsigned 16-bit integers
2058 1.1 int 1.1 is not an integer value
2059 42i float32 (0 + 42i) is not in the set of float32 values
2060 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
2063 <h3 id="Method_sets">Method sets</h3>
2066 The <i>method set</i> of a type determines the methods that can be
2067 <a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type.
2068 Every type has a (possibly empty) method set associated with it:
2072 <li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all
2073 <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
2077 The method set of a pointer to a defined type <code>T</code>
2078 (where <code>T</code> is neither a pointer nor an interface)
2079 is the set of all methods declared with receiver <code>*T</code> or <code>T</code>.
2082 <li>The method set of an <a href="#Interface_types">interface type</a> is the intersection
2083 of the method sets of each type in the interface's <a href="#Interface_types">type set</a>
2084 (the resulting method set is usually just the set of declared methods in the interface).
2089 Further rules apply to structs (and pointer to structs) containing embedded fields,
2090 as described in the section on <a href="#Struct_types">struct types</a>.
2091 Any other type has an empty method set.
2095 In a method set, each method must have a
2096 <a href="#Uniqueness_of_identifiers">unique</a>
2097 non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
2100 <h2 id="Blocks">Blocks</h2>
2103 A <i>block</i> is a possibly empty sequence of declarations and statements
2104 within matching brace brackets.
2108 Block = "{" StatementList "}" .
2109 StatementList = { Statement ";" } .
2113 In addition to explicit blocks in the source code, there are implicit blocks:
2117 <li>The <i>universe block</i> encompasses all Go source text.</li>
2119 <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
2120 Go source text for that package.</li>
2122 <li>Each file has a <i>file block</i> containing all Go source text
2125 <li>Each <a href="#If_statements">"if"</a>,
2126 <a href="#For_statements">"for"</a>, and
2127 <a href="#Switch_statements">"switch"</a>
2128 statement is considered to be in its own implicit block.</li>
2130 <li>Each clause in a <a href="#Switch_statements">"switch"</a>
2131 or <a href="#Select_statements">"select"</a> statement
2132 acts as an implicit block.</li>
2136 Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
2140 <h2 id="Declarations_and_scope">Declarations and scope</h2>
2143 A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
2144 <a href="#Constant_declarations">constant</a>,
2145 <a href="#Type_declarations">type</a>,
2146 <a href="#Type_parameter_declarations">type parameter</a>,
2147 <a href="#Variable_declarations">variable</a>,
2148 <a href="#Function_declarations">function</a>,
2149 <a href="#Labeled_statements">label</a>, or
2150 <a href="#Import_declarations">package</a>.
2151 Every identifier in a program must be declared.
2152 No identifier may be declared twice in the same block, and
2153 no identifier may be declared in both the file and package block.
2157 The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
2158 in a declaration, but it does not introduce a binding and thus is not declared.
2159 In the package block, the identifier <code>init</code> may only be used for
2160 <a href="#Package_initialization"><code>init</code> function</a> declarations,
2161 and like the blank identifier it does not introduce a new binding.
2165 Declaration = ConstDecl | TypeDecl | VarDecl .
2166 TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
2170 The <i>scope</i> of a declared identifier is the extent of source text in which
2171 the identifier denotes the specified constant, type, variable, function, label, or package.
2175 Go is lexically scoped using <a href="#Blocks">blocks</a>:
2179 <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
2181 <li>The scope of an identifier denoting a constant, type, variable,
2182 or function (but not method) declared at top level (outside any
2183 function) is the package block.</li>
2185 <li>The scope of the package name of an imported package is the file block
2186 of the file containing the import declaration.</li>
2188 <li>The scope of an identifier denoting a method receiver, function parameter,
2189 or result variable is the function body.</li>
2191 <li>The scope of an identifier denoting a type parameter of a function
2192 or declared by a method receiver begins after the name of the function
2193 and ends at the end of the function body.</li>
2195 <li>The scope of an identifier denoting a type parameter of a type
2196 begins after the name of the type and ends at the end
2197 of the TypeSpec.</li>
2199 <li>The scope of a constant or variable identifier declared
2200 inside a function begins at the end of the ConstSpec or VarSpec
2201 (ShortVarDecl for short variable declarations)
2202 and ends at the end of the innermost containing block.</li>
2204 <li>The scope of a type identifier declared inside a function
2205 begins at the identifier in the TypeSpec
2206 and ends at the end of the innermost containing block.</li>
2210 An identifier declared in a block may be redeclared in an inner block.
2211 While the identifier of the inner declaration is in scope, it denotes
2212 the entity declared by the inner declaration.
2216 The <a href="#Package_clause">package clause</a> is not a declaration; the package name
2217 does not appear in any scope. Its purpose is to identify the files belonging
2218 to the same <a href="#Packages">package</a> and to specify the default package name for import
2223 <h3 id="Label_scopes">Label scopes</h3>
2226 Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
2227 used in the <a href="#Break_statements">"break"</a>,
2228 <a href="#Continue_statements">"continue"</a>, and
2229 <a href="#Goto_statements">"goto"</a> statements.
2230 It is illegal to define a label that is never used.
2231 In contrast to other identifiers, labels are not block scoped and do
2232 not conflict with identifiers that are not labels. The scope of a label
2233 is the body of the function in which it is declared and excludes
2234 the body of any nested function.
2238 <h3 id="Blank_identifier">Blank identifier</h3>
2241 The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
2242 It serves as an anonymous placeholder instead of a regular (non-blank)
2243 identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
2244 as an <a href="#Operands">operand</a>, and in <a href="#Assignment_statements">assignment statements</a>.
2248 <h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
2251 The following identifiers are implicitly declared in the
2252 <a href="#Blocks">universe block</a>:
2254 <pre class="grammar">
2256 any bool byte comparable
2257 complex64 complex128 error float32 float64
2258 int int8 int16 int32 int64 rune string
2259 uint uint8 uint16 uint32 uint64 uintptr
2268 append cap close complex copy delete imag len
2269 make new panic print println real recover
2272 <h3 id="Exported_identifiers">Exported identifiers</h3>
2275 An identifier may be <i>exported</i> to permit access to it from another package.
2276 An identifier is exported if both:
2279 <li>the first character of the identifier's name is a Unicode uppercase
2280 letter (Unicode character category Lu); and</li>
2281 <li>the identifier is declared in the <a href="#Blocks">package block</a>
2282 or it is a <a href="#Struct_types">field name</a> or
2283 <a href="#MethodName">method name</a>.</li>
2286 All other identifiers are not exported.
2289 <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
2292 Given a set of identifiers, an identifier is called <i>unique</i> if it is
2293 <i>different</i> from every other in the set.
2294 Two identifiers are different if they are spelled differently, or if they
2295 appear in different <a href="#Packages">packages</a> and are not
2296 <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
2299 <h3 id="Constant_declarations">Constant declarations</h3>
2302 A constant declaration binds a list of identifiers (the names of
2303 the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
2304 The number of identifiers must be equal
2305 to the number of expressions, and the <i>n</i>th identifier on
2306 the left is bound to the value of the <i>n</i>th expression on the
2311 ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
2312 ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
2314 IdentifierList = identifier { "," identifier } .
2315 ExpressionList = Expression { "," Expression } .
2319 If the type is present, all constants take the type specified, and
2320 the expressions must be <a href="#Assignability">assignable</a> to that type,
2321 which must not be a type parameter.
2322 If the type is omitted, the constants take the
2323 individual types of the corresponding expressions.
2324 If the expression values are untyped <a href="#Constants">constants</a>,
2325 the declared constants remain untyped and the constant identifiers
2326 denote the constant values. For instance, if the expression is a
2327 floating-point literal, the constant identifier denotes a floating-point
2328 constant, even if the literal's fractional part is zero.
2332 const Pi float64 = 3.14159265358979323846
2333 const zero = 0.0 // untyped floating-point constant
2336 eof = -1 // untyped integer constant
2338 const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
2339 const u, v float32 = 0, 3 // u = 0.0, v = 3.0
2343 Within a parenthesized <code>const</code> declaration list the
2344 expression list may be omitted from any but the first ConstSpec.
2345 Such an empty list is equivalent to the textual substitution of the
2346 first preceding non-empty expression list and its type if any.
2347 Omitting the list of expressions is therefore equivalent to
2348 repeating the previous list. The number of identifiers must be equal
2349 to the number of expressions in the previous list.
2350 Together with the <a href="#Iota"><code>iota</code> constant generator</a>
2351 this mechanism permits light-weight declaration of sequential values:
2363 numberOfDays // this constant is not exported
2368 <h3 id="Iota">Iota</h3>
2371 Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
2372 <code>iota</code> represents successive untyped integer <a href="#Constants">
2373 constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
2374 in that constant declaration, starting at zero.
2375 It can be used to construct a set of related constants:
2380 c0 = iota // c0 == 0
2381 c1 = iota // c1 == 1
2382 c2 = iota // c2 == 2
2386 a = 1 << iota // a == 1 (iota == 0)
2387 b = 1 << iota // b == 2 (iota == 1)
2388 c = 3 // c == 3 (iota == 2, unused)
2389 d = 1 << iota // d == 8 (iota == 3)
2393 u = iota * 42 // u == 0 (untyped integer constant)
2394 v float64 = iota * 42 // v == 42.0 (float64 constant)
2395 w = iota * 42 // w == 84 (untyped integer constant)
2398 const x = iota // x == 0
2399 const y = iota // y == 0
2403 By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
2408 bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
2409 bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
2410 _, _ // (iota == 2, unused)
2411 bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
2416 This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
2417 of the last non-empty expression list.
2421 <h3 id="Type_declarations">Type declarations</h3>
2424 A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
2425 Type declarations come in two forms: alias declarations and type definitions.
2429 TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
2430 TypeSpec = AliasDecl | TypeDef .
2433 <h4 id="Alias_declarations">Alias declarations</h4>
2436 An alias declaration binds an identifier to the given type.
2440 AliasDecl = identifier "=" Type .
2444 Within the <a href="#Declarations_and_scope">scope</a> of
2445 the identifier, it serves as an <i>alias</i> for the type.
2450 nodeList = []*Node // nodeList and []*Node are identical types
2451 Polar = polar // Polar and polar denote identical types
2456 <h4 id="Type_definitions">Type definitions</h4>
2459 A type definition creates a new, distinct type with the same
2460 <a href="#Types">underlying type</a> and operations as the given type
2461 and binds an identifier, the <i>type name</i>, to it.
2465 TypeDef = identifier [ TypeParameters ] Type .
2469 The new type is called a <i>defined type</i>.
2470 It is <a href="#Type_identity">different</a> from any other type,
2471 including the type it is created from.
2476 Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
2477 polar Point // polar and Point denote different types
2480 type TreeNode struct {
2481 left, right *TreeNode
2485 type Block interface {
2487 Encrypt(src, dst []byte)
2488 Decrypt(src, dst []byte)
2493 A defined type may have <a href="#Method_declarations">methods</a> associated with it.
2494 It does not inherit any methods bound to the given type,
2495 but the <a href="#Method_sets">method set</a>
2496 of an interface type or of elements of a composite type remains unchanged:
2500 // A Mutex is a data type with two methods, Lock and Unlock.
2501 type Mutex struct { /* Mutex fields */ }
2502 func (m *Mutex) Lock() { /* Lock implementation */ }
2503 func (m *Mutex) Unlock() { /* Unlock implementation */ }
2505 // NewMutex has the same composition as Mutex but its method set is empty.
2508 // The method set of PtrMutex's underlying type *Mutex remains unchanged,
2509 // but the method set of PtrMutex is empty.
2510 type PtrMutex *Mutex
2512 // The method set of *PrintableMutex contains the methods
2513 // Lock and Unlock bound to its embedded field Mutex.
2514 type PrintableMutex struct {
2518 // MyBlock is an interface type that has the same method set as Block.
2523 Type definitions may be used to define different boolean, numeric,
2524 or string types and associate methods with them:
2531 EST TimeZone = -(5 + iota)
2537 func (tz TimeZone) String() string {
2538 return fmt.Sprintf("GMT%+dh", tz)
2543 If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>,
2544 the type name denotes a <i>generic type</i>.
2545 Generic types must be <a href="#Instantiations">instantiated</a> when they
2550 type List[T any] struct {
2557 In a type definition the given type cannot be a type parameter.
2561 type T[P any] P // illegal: P is a type parameter
2564 type L T // illegal: T is a type parameter declared by the enclosing function
2569 A generic type may also have <a href="#Method_declarations">methods</a> associated with it.
2570 In this case, the method receivers must declare the same number of type parameters as
2571 present in the generic type definition.
2575 // The method Len returns the number of elements in the linked list l.
2576 func (l *List[T]) Len() int { … }
2579 <h3 id="Type_parameter_declarations">Type parameter declarations</h3>
2582 A type parameter list declares the <i>type parameters</i> of a generic function or type declaration.
2583 The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a>
2584 except that the type parameter names must all be present and the list is enclosed
2585 in square brackets rather than parentheses.
2589 TypeParameters = "[" TypeParamList [ "," ] "]" .
2590 TypeParamList = TypeParamDecl { "," TypeParamDecl } .
2591 TypeParamDecl = IdentifierList TypeConstraint .
2595 All non-blank names in the list must be unique.
2596 Each name declares a type parameter, which is a new and different <a href="#Types">named type</a>
2597 that acts as a place holder for an (as of yet) unknown type in the declaration.
2598 The type parameter is replaced with a <i>type argument</i> upon
2599 <a href="#Instantiations">instantiation</a> of the generic function or type.
2604 [S interface{ ~[]byte|string }]
2611 Just as each ordinary function parameter has a parameter type, each type parameter
2612 has a corresponding (meta-)type which is called its
2613 <a href="#Type_constraints"><i>type constraint</i></a>.
2617 A parsing ambiguity arises when the type parameter list for a generic type
2618 declares a single type parameter <code>P</code> with a constraint <code>C</code>
2619 such that the text <code>P C</code> forms a valid expression:
2630 In these rare cases, the type parameter list is indistinguishable from an
2631 expression and the type declaration is parsed as an array type declaration.
2632 To resolve the ambiguity, embed the constraint in an
2633 <a href="#Interface_types">interface</a> or use a trailing comma:
2637 type T[P interface{*C}] …
2642 Type parameters may also be declared by the receiver specification
2643 of a <a href="#Method_declarations">method declaration</a> associated
2644 with a generic type.
2648 Within a type parameter list of a generic type <code>T</code>, a type constraint
2649 may not (directly, or indirectly through the type parameter list of another
2650 generic type) refer to <code>T</code>.
2654 type T1[P T1[P]] … // illegal: T1 refers to itself
2655 type T2[P interface{ T2[int] }] … // illegal: T2 refers to itself
2656 type T3[P interface{ m(T3[int])}] … // illegal: T3 refers to itself
2657 type T4[P T5[P]] … // illegal: T4 refers to T5 and
2658 type T5[P T4[P]] … // T5 refers to T4
2660 type T6[P int] struct{ f *T6[P] } // ok: reference to T6 is not in type parameter list
2663 <h4 id="Type_constraints">Type constraints</h4>
2666 A <i>type constraint</i> is an <a href="#Interface_types">interface</a> that defines the
2667 set of permissible type arguments for the respective type parameter and controls the
2668 operations supported by values of that type parameter.
2672 TypeConstraint = TypeElem .
2676 If the constraint is an interface literal of the form <code>interface{E}</code> where
2677 <code>E</code> is an embedded <a href="#Interface_types">type element</a> (not a method), in a type parameter list
2678 the enclosing <code>interface{ … }</code> may be omitted for convenience:
2682 [T []P] // = [T interface{[]P}]
2683 [T ~int] // = [T interface{~int}]
2684 [T int|string] // = [T interface{int|string}]
2685 type Constraint ~int // illegal: ~int is not in a type parameter list
2689 We should be able to simplify the rules for comparable or delegate some of them
2690 elsewhere since we have a section that clearly defines how interfaces implement
2691 other interfaces based on their type sets. But this should get us going for now.
2695 The <a href="#Predeclared_identifiers">predeclared</a>
2696 <a href="#Interface_types">interface type</a> <code>comparable</code>
2697 denotes the set of all non-interface types that are
2698 <a href="#Comparison_operators">strictly comparable</a>.
2702 Even though interfaces that are not type parameters are <a href="#Comparison_operators">comparable</a>,
2703 they are not strictly comparable and therefore they do not implement <code>comparable</code>.
2704 However, they <a href="#Satisfying_a_type_constraint">satisfy</a> <code>comparable</code>.
2708 int // implements comparable (int is strictly comparable)
2709 []byte // does not implement comparable (slices cannot be compared)
2710 interface{} // does not implement comparable (see above)
2711 interface{ ~int | ~string } // type parameter only: implements comparable (int, string types are stricly comparable)
2712 interface{ comparable } // type parameter only: implements comparable (comparable implements itself)
2713 interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (slices are not comparable)
2714 interface{ ~struct{ any } } // type parameter only: does not implement comparable (field any is not strictly comparable)
2718 The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
2719 <code>comparable</code> may only be used as type constraints. They cannot be the types of
2720 values or variables, or components of other, non-interface types.
2723 <h4 id="Satisfying_a_type_constraint">Satisfying a type constraint</h4>
2726 A type argument <code>T</code><i> satisfies</i> a type constraint <code>C</code>
2727 if <code>T</code> is an element of the type set defined by <code>C</code>; i.e.,
2728 if <code>T</code> <a href="#Implementing_an_interface">implements</a> <code>C</code>.
2729 As an exception, a <a href="#Comparison_operators">strictly comparable</a>
2730 type constraint may also be satisfied by a <a href="#Comparison_operators">comparable</a>
2731 (not necessarily strictly comparable) type argument.
2736 A type T <i>satisfies</i> a constraint <code>C</code> if
2741 <code>T</code> <a href="#Implementing_an_interface">implements</a> <code>C</code>; or
2744 <code>C</code> can be written in the form <code>interface{ comparable; E }</code>,
2745 where <code>E</code> is a <a href="#Basic_interfaces">basic interface</a> and
2746 <code>T</code> is <a href="#Comparison_operators">comparable</a> and implements <code>E</code>.
2751 type argument type constraint // constraint satisfaction
2753 int interface{ ~int } // satisfied: int implements interface{ ~int }
2754 string comparable // satisfied: string implements comparable (string is stricty comparable)
2755 []byte comparable // not satisfied: slices are not comparable
2756 any interface{ comparable; int } // not satisfied: any does not implement interface{ int }
2757 any comparable // satisfied: any is comparable and implements the basic interface any
2758 struct{f any} comparable // satisfied: struct{f any} is comparable and implements the basic interface any
2759 any interface{ comparable; m() } // not satisfied: any does not implement the basic interface interface{ m() }
2760 interface{ m() } interface{ comparable; m() } // satisfied: interface{ m() } is comparable and implements the basic interface interface{ m() }
2764 Because of the exception in the constraint satisfaction rule, comparing operands of type parameter type
2765 may panic at run-time (even though comparable type parameters are always strictly comparable).
2768 <h3 id="Variable_declarations">Variable declarations</h3>
2771 A variable declaration creates one or more <a href="#Variables">variables</a>,
2772 binds corresponding identifiers to them, and gives each a type and an initial value.
2776 VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
2777 VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
2784 var x, y float32 = -1, -2
2787 u, v, s = 2.0, 3.0, "bar"
2789 var re, im = complexSqrt(-1)
2790 var _, found = entries[name] // map lookup; only interested in "found"
2794 If a list of expressions is given, the variables are initialized
2795 with the expressions following the rules for <a href="#Assignment_statements">assignment statements</a>.
2796 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
2800 If a type is present, each variable is given that type.
2801 Otherwise, each variable is given the type of the corresponding
2802 initialization value in the assignment.
2803 If that value is an untyped constant, it is first implicitly
2804 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
2805 if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
2806 The predeclared value <code>nil</code> cannot be used to initialize a variable
2807 with no explicit type.
2811 var d = math.Sin(0.5) // d is float64
2812 var i = 42 // i is int
2813 var t, ok = x.(T) // t is T, ok is bool
2814 var n = nil // illegal
2818 Implementation restriction: A compiler may make it illegal to declare a variable
2819 inside a <a href="#Function_declarations">function body</a> if the variable is
2823 <h3 id="Short_variable_declarations">Short variable declarations</h3>
2826 A <i>short variable declaration</i> uses the syntax:
2830 ShortVarDecl = IdentifierList ":=" ExpressionList .
2834 It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
2835 with initializer expressions but no types:
2838 <pre class="grammar">
2839 "var" IdentifierList "=" ExpressionList .
2844 f := func() int { return 7 }
2845 ch := make(chan int)
2846 r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
2847 _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
2851 Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
2852 variables provided they were originally declared earlier in the same block
2853 (or the parameter lists if the block is the function body) with the same type,
2854 and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
2855 As a consequence, redeclaration can only appear in a multi-variable short declaration.
2856 Redeclaration does not introduce a new variable; it just assigns a new value to the original.
2857 The non-blank variable names on the left side of <code>:=</code>
2858 must be <a href="#Uniqueness_of_identifiers">unique</a>.
2862 field1, offset := nextField(str, 0)
2863 field2, offset := nextField(str, offset) // redeclares offset
2864 x, y, x := 1, 2, 3 // illegal: x repeated on left side of :=
2868 Short variable declarations may appear only inside functions.
2869 In some contexts such as the initializers for
2870 <a href="#If_statements">"if"</a>,
2871 <a href="#For_statements">"for"</a>, or
2872 <a href="#Switch_statements">"switch"</a> statements,
2873 they can be used to declare local temporary variables.
2876 <h3 id="Function_declarations">Function declarations</h3>
2879 Given the importance of functions, this section has always
2880 been woefully underdeveloped. Would be nice to expand this
2885 A function declaration binds an identifier, the <i>function name</i>,
2890 FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
2891 FunctionName = identifier .
2892 FunctionBody = Block .
2896 If the function's <a href="#Function_types">signature</a> declares
2897 result parameters, the function body's statement list must end in
2898 a <a href="#Terminating_statements">terminating statement</a>.
2902 func IndexRune(s string, r rune) int {
2903 for i, c := range s {
2908 // invalid: missing return statement
2913 If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>,
2914 the function name denotes a <i>generic function</i>.
2915 A generic function must be <a href="#Instantiations">instantiated</a> before it can be
2916 called or used as a value.
2920 func min[T ~int|~float64](x, y T) T {
2929 A function declaration without type parameters may omit the body.
2930 Such a declaration provides the signature for a function implemented outside Go,
2931 such as an assembly routine.
2935 func flushICache(begin, end uintptr) // implemented externally
2938 <h3 id="Method_declarations">Method declarations</h3>
2941 A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
2942 A method declaration binds an identifier, the <i>method name</i>, to a method,
2943 and associates the method with the receiver's <i>base type</i>.
2947 MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
2948 Receiver = Parameters .
2952 The receiver is specified via an extra parameter section preceding the method
2953 name. That parameter section must declare a single non-variadic parameter, the receiver.
2954 Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
2955 pointer to a defined type <code>T</code>, possibly followed by a list of type parameter
2956 names <code>[P1, P2, …]</code> enclosed in square brackets.
2957 <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be
2958 a pointer or interface type and it must be defined in the same package as the method.
2959 The method is said to be <i>bound</i> to its receiver base type and the method name
2960 is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
2965 A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
2966 <a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
2967 If the receiver's value is not referenced inside the body of the method,
2968 its identifier may be omitted in the declaration. The same applies in
2969 general to parameters of functions and methods.
2973 For a base type, the non-blank names of methods bound to it must be unique.
2974 If the base type is a <a href="#Struct_types">struct type</a>,
2975 the non-blank method and field names must be distinct.
2979 Given defined type <code>Point</code> the declarations
2983 func (p *Point) Length() float64 {
2984 return math.Sqrt(p.x * p.x + p.y * p.y)
2987 func (p *Point) Scale(factor float64) {
2994 bind the methods <code>Length</code> and <code>Scale</code>,
2995 with receiver type <code>*Point</code>,
2996 to the base type <code>Point</code>.
3000 If the receiver base type is a <a href="#Type_declarations">generic type</a>, the
3001 receiver specification must declare corresponding type parameters for the method
3002 to use. This makes the receiver type parameters available to the method.
3003 Syntactically, this type parameter declaration looks like an
3004 <a href="#Instantiations">instantiation</a> of the receiver base type: the type
3005 arguments must be identifiers denoting the type parameters being declared, one
3006 for each type parameter of the receiver base type.
3007 The type parameter names do not need to match their corresponding parameter names in the
3008 receiver base type definition, and all non-blank parameter names must be unique in the
3009 receiver parameter section and the method signature.
3010 The receiver type parameter constraints are implied by the receiver base type definition:
3011 corresponding type parameters have corresponding constraints.
3015 type Pair[A, B any] struct {
3020 func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B
3021 func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair
3024 <h2 id="Expressions">Expressions</h2>
3027 An expression specifies the computation of a value by applying
3028 operators and functions to operands.
3031 <h3 id="Operands">Operands</h3>
3034 Operands denote the elementary values in an expression. An operand may be a
3035 literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
3036 non-<a href="#Blank_identifier">blank</a> identifier denoting a
3037 <a href="#Constant_declarations">constant</a>,
3038 <a href="#Variable_declarations">variable</a>, or
3039 <a href="#Function_declarations">function</a>,
3040 or a parenthesized expression.
3044 Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
3045 Literal = BasicLit | CompositeLit | FunctionLit .
3046 BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
3047 OperandName = identifier | QualifiedIdent .
3051 An operand name denoting a <a href="#Function_declarations">generic function</a>
3052 may be followed by a list of <a href="#Instantiations">type arguments</a>; the
3053 resulting operand is an <a href="#Instantiations">instantiated</a> function.
3057 The <a href="#Blank_identifier">blank identifier</a> may appear as an
3058 operand only on the left-hand side of an <a href="#Assignment_statements">assignment statement</a>.
3062 Implementation restriction: A compiler need not report an error if an operand's
3063 type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty
3064 <a href="#Interface_types">type set</a>. Functions with such type parameters
3065 cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead
3066 to an error at the instantiation site.
3069 <h3 id="Qualified_identifiers">Qualified identifiers</h3>
3072 A <i>qualified identifier</i> is an identifier qualified with a package name prefix.
3073 Both the package name and the identifier must not be
3074 <a href="#Blank_identifier">blank</a>.
3078 QualifiedIdent = PackageName "." identifier .
3082 A qualified identifier accesses an identifier in a different package, which
3083 must be <a href="#Import_declarations">imported</a>.
3084 The identifier must be <a href="#Exported_identifiers">exported</a> and
3085 declared in the <a href="#Blocks">package block</a> of that package.
3089 math.Sin // denotes the Sin function in package math
3092 <h3 id="Composite_literals">Composite literals</h3>
3095 Composite literals construct new composite values each time they are evaluated.
3096 They consist of the type of the literal followed by a brace-bound list of elements.
3097 Each element may optionally be preceded by a corresponding key.
3101 CompositeLit = LiteralType LiteralValue .
3102 LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
3103 SliceType | MapType | TypeName [ TypeArgs ] .
3104 LiteralValue = "{" [ ElementList [ "," ] ] "}" .
3105 ElementList = KeyedElement { "," KeyedElement } .
3106 KeyedElement = [ Key ":" ] Element .
3107 Key = FieldName | Expression | LiteralValue .
3108 FieldName = identifier .
3109 Element = Expression | LiteralValue .
3113 The LiteralType's <a href="#Core_types">core type</a> <code>T</code>
3114 must be a struct, array, slice, or map type
3115 (the syntax enforces this constraint except when the type is given
3117 The types of the elements and keys must be <a href="#Assignability">assignable</a>
3118 to the respective field, element, and key types of type <code>T</code>;
3119 there is no additional conversion.
3120 The key is interpreted as a field name for struct literals,
3121 an index for array and slice literals, and a key for map literals.
3122 For map literals, all elements must have a key. It is an error
3123 to specify multiple elements with the same field name or
3124 constant key value. For non-constant map keys, see the section on
3125 <a href="#Order_of_evaluation">evaluation order</a>.
3129 For struct literals the following rules apply:
3132 <li>A key must be a field name declared in the struct type.
3134 <li>An element list that does not contain any keys must
3135 list an element for each struct field in the
3136 order in which the fields are declared.
3138 <li>If any element has a key, every element must have a key.
3140 <li>An element list that contains keys does not need to
3141 have an element for each struct field. Omitted fields
3142 get the zero value for that field.
3144 <li>A literal may omit the element list; such a literal evaluates
3145 to the zero value for its type.
3147 <li>It is an error to specify an element for a non-exported
3148 field of a struct belonging to a different package.
3153 Given the declarations
3156 type Point3D struct { x, y, z float64 }
3157 type Line struct { p, q Point3D }
3165 origin := Point3D{} // zero value for Point3D
3166 line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
3170 For array and slice literals the following rules apply:
3173 <li>Each element has an associated integer index marking
3174 its position in the array.
3176 <li>An element with a key uses the key as its index. The
3177 key must be a non-negative constant
3178 <a href="#Representability">representable</a> by
3179 a value of type <code>int</code>; and if it is typed
3180 it must be of <a href="#Numeric_types">integer type</a>.
3182 <li>An element without a key uses the previous element's index plus one.
3183 If the first element has no key, its index is zero.
3188 <a href="#Address_operators">Taking the address</a> of a composite literal
3189 generates a pointer to a unique <a href="#Variables">variable</a> initialized
3190 with the literal's value.
3194 var pointer *Point3D = &Point3D{y: 1000}
3198 Note that the <a href="#The_zero_value">zero value</a> for a slice or map
3199 type is not the same as an initialized but empty value of the same type.
3200 Consequently, taking the address of an empty slice or map composite literal
3201 does not have the same effect as allocating a new slice or map value with
3202 <a href="#Allocation">new</a>.
3206 p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
3207 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
3211 The length of an array literal is the length specified in the literal type.
3212 If fewer elements than the length are provided in the literal, the missing
3213 elements are set to the zero value for the array element type.
3214 It is an error to provide elements with index values outside the index range
3215 of the array. The notation <code>...</code> specifies an array length equal
3216 to the maximum element index plus one.
3220 buffer := [10]string{} // len(buffer) == 10
3221 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
3222 days := [...]string{"Sat", "Sun"} // len(days) == 2
3226 A slice literal describes the entire underlying array literal.
3227 Thus the length and capacity of a slice literal are the maximum
3228 element index plus one. A slice literal has the form
3236 and is shorthand for a slice operation applied to an array:
3240 tmp := [n]T{x1, x2, … xn}
3245 Within a composite literal of array, slice, or map type <code>T</code>,
3246 elements or map keys that are themselves composite literals may elide the respective
3247 literal type if it is identical to the element or key type of <code>T</code>.
3248 Similarly, elements or keys that are addresses of composite literals may elide
3249 the <code>&T</code> when the element or key type is <code>*T</code>.
3253 [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
3254 [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
3255 [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
3256 map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
3257 map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
3260 [2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
3261 [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
3265 A parsing ambiguity arises when a composite literal using the
3266 TypeName form of the LiteralType appears as an operand between the
3267 <a href="#Keywords">keyword</a> and the opening brace of the block
3268 of an "if", "for", or "switch" statement, and the composite literal
3269 is not enclosed in parentheses, square brackets, or curly braces.
3270 In this rare case, the opening brace of the literal is erroneously parsed
3271 as the one introducing the block of statements. To resolve the ambiguity,
3272 the composite literal must appear within parentheses.
3276 if x == (T{a,b,c}[i]) { … }
3277 if (x == T{a,b,c}[i]) { … }
3281 Examples of valid array, slice, and map literals:
3285 // list of prime numbers
3286 primes := []int{2, 3, 5, 7, 9, 2147483647}
3288 // vowels[ch] is true if ch is a vowel
3289 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
3291 // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
3292 filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
3294 // frequencies in Hz for equal-tempered scale (A4 = 440Hz)
3295 noteFrequency := map[string]float32{
3296 "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
3297 "G0": 24.50, "A0": 27.50, "B0": 30.87,
3302 <h3 id="Function_literals">Function literals</h3>
3305 A function literal represents an anonymous <a href="#Function_declarations">function</a>.
3306 Function literals cannot declare type parameters.
3310 FunctionLit = "func" Signature FunctionBody .
3314 func(a, b int, z float64) bool { return a*b < int(z) }
3318 A function literal can be assigned to a variable or invoked directly.
3322 f := func(x, y int) int { return x + y }
3323 func(ch chan int) { ch <- ACK }(replyChan)
3327 Function literals are <i>closures</i>: they may refer to variables
3328 defined in a surrounding function. Those variables are then shared between
3329 the surrounding function and the function literal, and they survive as long
3330 as they are accessible.
3334 <h3 id="Primary_expressions">Primary expressions</h3>
3337 Primary expressions are the operands for unary and binary expressions.
3345 PrimaryExpr Selector |
3348 PrimaryExpr TypeAssertion |
3349 PrimaryExpr Arguments .
3351 Selector = "." identifier .
3352 Index = "[" Expression [ "," ] "]" .
3353 Slice = "[" [ Expression ] ":" [ Expression ] "]" |
3354 "[" [ Expression ] ":" Expression ":" Expression "]" .
3355 TypeAssertion = "." "(" Type ")" .
3356 Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
3373 <h3 id="Selectors">Selectors</h3>
3376 For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
3377 that is not a <a href="#Package_clause">package name</a>, the
3378 <i>selector expression</i>
3386 denotes the field or method <code>f</code> of the value <code>x</code>
3387 (or sometimes <code>*x</code>; see below).
3388 The identifier <code>f</code> is called the (field or method) <i>selector</i>;
3389 it must not be the <a href="#Blank_identifier">blank identifier</a>.
3390 The type of the selector expression is the type of <code>f</code>.
3391 If <code>x</code> is a package name, see the section on
3392 <a href="#Qualified_identifiers">qualified identifiers</a>.
3396 A selector <code>f</code> may denote a field or method <code>f</code> of
3397 a type <code>T</code>, or it may refer
3398 to a field or method <code>f</code> of a nested
3399 <a href="#Struct_types">embedded field</a> of <code>T</code>.
3400 The number of embedded fields traversed
3401 to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
3402 The depth of a field or method <code>f</code>
3403 declared in <code>T</code> is zero.
3404 The depth of a field or method <code>f</code> declared in
3405 an embedded field <code>A</code> in <code>T</code> is the
3406 depth of <code>f</code> in <code>A</code> plus one.
3410 The following rules apply to selectors:
3415 For a value <code>x</code> of type <code>T</code> or <code>*T</code>
3416 where <code>T</code> is not a pointer or interface type,
3417 <code>x.f</code> denotes the field or method at the shallowest depth
3418 in <code>T</code> where there is such an <code>f</code>.
3419 If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
3420 with shallowest depth, the selector expression is illegal.
3424 For a value <code>x</code> of type <code>I</code> where <code>I</code>
3425 is an interface type, <code>x.f</code> denotes the actual method with name
3426 <code>f</code> of the dynamic value of <code>x</code>.
3427 If there is no method with name <code>f</code> in the
3428 <a href="#Method_sets">method set</a> of <code>I</code>, the selector
3429 expression is illegal.
3433 As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
3434 pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
3435 (but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
3439 In all other cases, <code>x.f</code> is illegal.
3443 If <code>x</code> is of pointer type and has the value
3444 <code>nil</code> and <code>x.f</code> denotes a struct field,
3445 assigning to or evaluating <code>x.f</code>
3446 causes a <a href="#Run_time_panics">run-time panic</a>.
3450 If <code>x</code> is of interface type and has the value
3451 <code>nil</code>, <a href="#Calls">calling</a> or
3452 <a href="#Method_values">evaluating</a> the method <code>x.f</code>
3453 causes a <a href="#Run_time_panics">run-time panic</a>.
3458 For example, given the declarations:
3484 var t T2 // with t.T0 != nil
3485 var p *T2 // with p != nil and (*p).T0 != nil
3502 q.x // (*(*q).T0).x (*q).x is a valid field selector
3504 p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
3505 p.M1() // ((*p).T1).M1() M1 expects T1 receiver
3506 p.M2() // p.M2() M2 expects *T2 receiver
3507 t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
3511 but the following is invalid:
3515 q.M0() // (*q).M0 is valid but not a field selector
3519 <h3 id="Method_expressions">Method expressions</h3>
3522 If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3523 <code>T.M</code> is a function that is callable as a regular function
3524 with the same arguments as <code>M</code> prefixed by an additional
3525 argument that is the receiver of the method.
3529 MethodExpr = ReceiverType "." MethodName .
3530 ReceiverType = Type .
3534 Consider a struct type <code>T</code> with two methods,
3535 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3536 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3543 func (tv T) Mv(a int) int { return 0 } // value receiver
3544 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3558 yields a function equivalent to <code>Mv</code> but
3559 with an explicit receiver as its first argument; it has signature
3563 func(tv T, a int) int
3567 That function may be called normally with an explicit receiver, so
3568 these five invocations are equivalent:
3575 f1 := T.Mv; f1(t, 7)
3576 f2 := (T).Mv; f2(t, 7)
3580 Similarly, the expression
3588 yields a function value representing <code>Mp</code> with signature
3592 func(tp *T, f float32) float32
3596 For a method with a value receiver, one can derive a function
3597 with an explicit pointer receiver, so
3605 yields a function value representing <code>Mv</code> with signature
3609 func(tv *T, a int) int
3613 Such a function indirects through the receiver to create a value
3614 to pass as the receiver to the underlying method;
3615 the method does not overwrite the value whose address is passed in
3620 The final case, a value-receiver function for a pointer-receiver method,
3621 is illegal because pointer-receiver methods are not in the method set
3626 Function values derived from methods are called with function call syntax;
3627 the receiver is provided as the first argument to the call.
3628 That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
3629 as <code>f(t, 7)</code> not <code>t.f(7)</code>.
3630 To construct a function that binds the receiver, use a
3631 <a href="#Function_literals">function literal</a> or
3632 <a href="#Method_values">method value</a>.
3636 It is legal to derive a function value from a method of an interface type.
3637 The resulting function takes an explicit receiver of that interface type.
3640 <h3 id="Method_values">Method values</h3>
3643 If the expression <code>x</code> has static type <code>T</code> and
3644 <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3645 <code>x.M</code> is called a <i>method value</i>.
3646 The method value <code>x.M</code> is a function value that is callable
3647 with the same arguments as a method call of <code>x.M</code>.
3648 The expression <code>x</code> is evaluated and saved during the evaluation of the
3649 method value; the saved copy is then used as the receiver in any calls,
3650 which may be executed later.
3654 type S struct { *T }
3656 func (t T) M() { print(t) }
3660 f := t.M // receiver *t is evaluated and stored in f
3661 g := s.M // receiver *(s.T) is evaluated and stored in g
3662 *t = 42 // does not affect stored receivers in f and g
3666 The type <code>T</code> may be an interface or non-interface type.
3670 As in the discussion of <a href="#Method_expressions">method expressions</a> above,
3671 consider a struct type <code>T</code> with two methods,
3672 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3673 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3680 func (tv T) Mv(a int) int { return 0 } // value receiver
3681 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3697 yields a function value of type
3705 These two invocations are equivalent:
3714 Similarly, the expression
3722 yields a function value of type
3726 func(float32) float32
3730 As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
3731 using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
3735 As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
3736 using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
3740 f := t.Mv; f(7) // like t.Mv(7)
3741 f := pt.Mp; f(7) // like pt.Mp(7)
3742 f := pt.Mv; f(7) // like (*pt).Mv(7)
3743 f := t.Mp; f(7) // like (&t).Mp(7)
3744 f := makeT().Mp // invalid: result of makeT() is not addressable
3748 Although the examples above use non-interface types, it is also legal to create a method value
3749 from a value of interface type.
3753 var i interface { M(int) } = myVal
3754 f := i.M; f(7) // like i.M(7)
3758 <h3 id="Index_expressions">Index expressions</h3>
3761 A primary expression of the form
3769 denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
3770 The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
3771 The following rules apply:
3775 If <code>a</code> is neither a map nor a type parameter:
3778 <li>the index <code>x</code> must be an untyped constant or its
3779 <a href="#Core_types">core type</a> must be an <a href="#Numeric_types">integer</a></li>
3780 <li>a constant index must be non-negative and
3781 <a href="#Representability">representable</a> by a value of type <code>int</code></li>
3782 <li>a constant index that is untyped is given type <code>int</code></li>
3783 <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
3784 otherwise it is <i>out of range</i></li>
3788 For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
3791 <li>a <a href="#Constants">constant</a> index must be in range</li>
3792 <li>if <code>x</code> is out of range at run time,
3793 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3794 <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
3795 <code>a[x]</code> is the element type of <code>A</code></li>
3799 For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
3802 <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
3806 For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
3809 <li>if <code>x</code> is out of range at run time,
3810 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3811 <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
3812 <code>a[x]</code> is the element type of <code>S</code></li>
3816 For <code>a</code> of <a href="#String_types">string type</a>:
3819 <li>a <a href="#Constants">constant</a> index must be in range
3820 if the string <code>a</code> is also constant</li>
3821 <li>if <code>x</code> is out of range at run time,
3822 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3823 <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
3824 <code>a[x]</code> is <code>byte</code></li>
3825 <li><code>a[x]</code> may not be assigned to</li>
3829 For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
3832 <li><code>x</code>'s type must be
3833 <a href="#Assignability">assignable</a>
3834 to the key type of <code>M</code></li>
3835 <li>if the map contains an entry with key <code>x</code>,
3836 <code>a[x]</code> is the map element with key <code>x</code>
3837 and the type of <code>a[x]</code> is the element type of <code>M</code></li>
3838 <li>if the map is <code>nil</code> or does not contain such an entry,
3839 <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
3840 for the element type of <code>M</code></li>
3844 For <code>a</code> of <a href="#Type_parameter_declarations">type parameter type</a> <code>P</code>:
3847 <li>The index expression <code>a[x]</code> must be valid for values
3848 of all types in <code>P</code>'s type set.</li>
3849 <li>The element types of all types in <code>P</code>'s type set must be identical.
3850 In this context, the element type of a string type is <code>byte</code>.</li>
3851 <li>If there is a map type in the type set of <code>P</code>,
3852 all types in that type set must be map types, and the respective key types
3853 must be all identical.</li>
3854 <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
3855 or the map element with key <code>x</code> of the type argument
3856 that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
3857 the type of the (identical) element types.</li>
3858 <li><code>a[x]</code> may not be assigned to if <code>P</code>'s type set
3859 includes string types.
3863 Otherwise <code>a[x]</code> is illegal.
3867 An index expression on a map <code>a</code> of type <code>map[K]V</code>
3868 used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
3878 yields an additional untyped boolean value. The value of <code>ok</code> is
3879 <code>true</code> if the key <code>x</code> is present in the map, and
3880 <code>false</code> otherwise.
3884 Assigning to an element of a <code>nil</code> map causes a
3885 <a href="#Run_time_panics">run-time panic</a>.
3889 <h3 id="Slice_expressions">Slice expressions</h3>
3892 Slice expressions construct a substring or slice from a string, array, pointer
3893 to array, or slice. There are two variants: a simple form that specifies a low
3894 and high bound, and a full form that also specifies a bound on the capacity.
3897 <h4>Simple slice expressions</h4>
3900 The primary expression
3908 constructs a substring or slice. The <a href="#Core_types">core type</a> of
3909 <code>a</code> must be a string, array, pointer to array, slice, or a
3910 <a href="#Core_types"><code>bytestring</code></a>.
3911 The <i>indices</i> <code>low</code> and
3912 <code>high</code> select which elements of operand <code>a</code> appear
3913 in the result. The result has indices starting at 0 and length equal to
3914 <code>high</code> - <code>low</code>.
3915 After slicing the array <code>a</code>
3919 a := [5]int{1, 2, 3, 4, 5}
3924 the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
3934 For convenience, any of the indices may be omitted. A missing <code>low</code>
3935 index defaults to zero; a missing <code>high</code> index defaults to the length of the
3940 a[2:] // same as a[2 : len(a)]
3941 a[:3] // same as a[0 : 3]
3942 a[:] // same as a[0 : len(a)]
3946 If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
3947 <code>(*a)[low : high]</code>.
3951 For arrays or strings, the indices are <i>in range</i> if
3952 <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
3953 otherwise they are <i>out of range</i>.
3954 For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
3955 A <a href="#Constants">constant</a> index must be non-negative and
3956 <a href="#Representability">representable</a> by a value of type
3957 <code>int</code>; for arrays or constant strings, constant indices must also be in range.
3958 If both indices are constant, they must satisfy <code>low <= high</code>.
3959 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3963 Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
3964 the result of the slice operation is a non-constant value of the same type as the operand.
3965 For untyped string operands the result is a non-constant value of type <code>string</code>.
3966 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
3967 and the result of the slice operation is a slice with the same element type as the array.
3971 If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
3972 is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
3973 array with the operand.
3978 s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
3979 s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
3980 s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
3983 s3 := s[:0] // s3 == nil
3987 <h4>Full slice expressions</h4>
3990 The primary expression
3998 constructs a slice of the same type, and with the same length and elements as the simple slice
3999 expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
4000 by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
4001 The <a href="#Core_types">core type</a> of <code>a</code> must be an array, pointer to array,
4002 or slice (but not a string).
4003 After slicing the array <code>a</code>
4007 a := [5]int{1, 2, 3, 4, 5}
4012 the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
4021 As for simple slice expressions, if <code>a</code> is a pointer to an array,
4022 <code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
4023 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
4027 The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
4028 otherwise they are <i>out of range</i>.
4029 A <a href="#Constants">constant</a> index must be non-negative and
4030 <a href="#Representability">representable</a> by a value of type
4031 <code>int</code>; for arrays, constant indices must also be in range.
4032 If multiple indices are constant, the constants that are present must be in range relative to each
4034 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4037 <h3 id="Type_assertions">Type assertions</h3>
4040 For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
4041 but not a <a href="#Type_parameter_declarations">type parameter</a>, and a type <code>T</code>,
4042 the primary expression
4050 asserts that <code>x</code> is not <code>nil</code>
4051 and that the value stored in <code>x</code> is of type <code>T</code>.
4052 The notation <code>x.(T)</code> is called a <i>type assertion</i>.
4055 More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
4056 that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
4057 to the type <code>T</code>.
4058 In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
4059 otherwise the type assertion is invalid since it is not possible for <code>x</code>
4060 to store a value of type <code>T</code>.
4061 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
4062 of <code>x</code> <a href="#Implementing_an_interface">implements</a> the interface <code>T</code>.
4065 If the type assertion holds, the value of the expression is the value
4066 stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
4067 a <a href="#Run_time_panics">run-time panic</a> occurs.
4068 In other words, even though the dynamic type of <code>x</code>
4069 is known only at run time, the type of <code>x.(T)</code> is
4070 known to be <code>T</code> in a correct program.
4074 var x interface{} = 7 // x has dynamic type int and value 7
4075 i := x.(int) // i has type int and value 7
4077 type I interface { m() }
4080 s := y.(string) // illegal: string does not implement I (missing method m)
4081 r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
4087 A type assertion used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
4094 var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
4098 yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
4099 if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
4100 the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
4101 No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
4105 <h3 id="Calls">Calls</h3>
4108 Given an expression <code>f</code> with a <a href="#Core_types">core type</a>
4109 <code>F</code> of <a href="#Function_types">function type</a>,
4117 calls <code>f</code> with arguments <code>a1, a2, … an</code>.
4118 Except for one special case, arguments must be single-valued expressions
4119 <a href="#Assignability">assignable</a> to the parameter types of
4120 <code>F</code> and are evaluated before the function is called.
4121 The type of the expression is the result type
4123 A method invocation is similar but the method itself
4124 is specified as a selector upon a value of the receiver type for
4129 math.Atan2(x, y) // function call
4131 pt.Scale(3.5) // method call with receiver pt
4135 If <code>f</code> denotes a generic function, it must be
4136 <a href="#Instantiations">instantiated</a> before it can be called
4137 or used as a function value.
4141 In a function call, the function value and arguments are evaluated in
4142 <a href="#Order_of_evaluation">the usual order</a>.
4143 After they are evaluated, the parameters of the call are passed by value to the function
4144 and the called function begins execution.
4145 The return parameters of the function are passed by value
4146 back to the caller when the function returns.
4150 Calling a <code>nil</code> function value
4151 causes a <a href="#Run_time_panics">run-time panic</a>.
4155 As a special case, if the return values of a function or method
4156 <code>g</code> are equal in number and individually
4157 assignable to the parameters of another function or method
4158 <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
4159 will invoke <code>f</code> after binding the return values of
4160 <code>g</code> to the parameters of <code>f</code> in order. The call
4161 of <code>f</code> must contain no parameters other than the call of <code>g</code>,
4162 and <code>g</code> must have at least one return value.
4163 If <code>f</code> has a final <code>...</code> parameter, it is
4164 assigned the return values of <code>g</code> that remain after
4165 assignment of regular parameters.
4169 func Split(s string, pos int) (string, string) {
4170 return s[0:pos], s[pos:]
4173 func Join(s, t string) string {
4177 if Join(Split(value, len(value)/2)) != value {
4178 log.Panic("test fails")
4183 A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
4184 of (the type of) <code>x</code> contains <code>m</code> and the
4185 argument list can be assigned to the parameter list of <code>m</code>.
4186 If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
4187 set contains <code>m</code>, <code>x.m()</code> is shorthand
4188 for <code>(&x).m()</code>:
4197 There is no distinct method type and there are no method literals.
4200 <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
4203 If <code>f</code> is <a href="#Function_types">variadic</a> with a final
4204 parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
4205 the type of <code>p</code> is equivalent to type <code>[]T</code>.
4206 If <code>f</code> is invoked with no actual arguments for <code>p</code>,
4207 the value passed to <code>p</code> is <code>nil</code>.
4208 Otherwise, the value passed is a new slice
4209 of type <code>[]T</code> with a new underlying array whose successive elements
4210 are the actual arguments, which all must be <a href="#Assignability">assignable</a>
4211 to <code>T</code>. The length and capacity of the slice is therefore
4212 the number of arguments bound to <code>p</code> and may differ for each
4217 Given the function and calls
4220 func Greeting(prefix string, who ...string)
4222 Greeting("hello:", "Joe", "Anna", "Eileen")
4226 within <code>Greeting</code>, <code>who</code> will have the value
4227 <code>nil</code> in the first call, and
4228 <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
4232 If the final argument is assignable to a slice type <code>[]T</code> and
4233 is followed by <code>...</code>, it is passed unchanged as the value
4234 for a <code>...T</code> parameter. In this case no new slice is created.
4238 Given the slice <code>s</code> and call
4242 s := []string{"James", "Jasmine"}
4243 Greeting("goodbye:", s...)
4247 within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
4248 with the same underlying array.
4251 <h3 id="Instantiations">Instantiations</h3>
4254 A generic function or type is <i>instantiated</i> by substituting <i>type arguments</i>
4255 for the type parameters.
4256 Instantiation proceeds in two steps:
4261 Each type argument is substituted for its corresponding type parameter in the generic
4263 This substitution happens across the entire function or type declaration,
4264 including the type parameter list itself and any types in that list.
4268 After substitution, each type argument must <a href="#Satisfying_a_type_constraint">satisfy</a>
4269 the <a href="#Type_parameter_declarations">constraint</a> (instantiated, if necessary)
4270 of the corresponding type parameter. Otherwise instantiation fails.
4275 Instantiating a type results in a new non-generic <a href="#Types">named type</a>;
4276 instantiating a function produces a new non-generic function.
4280 type parameter list type arguments after substitution
4282 [P any] int int satisfies any
4283 [S ~[]E, E any] []int, int []int satisfies ~[]int, int satisfies any
4284 [P io.Writer] string illegal: string doesn't satisfy io.Writer
4285 [P comparable] any any satisfies (but does not implement) comparable
4289 For a generic function, type arguments may be provided explicitly, or they
4290 may be partially or completely <a href="#Type_inference">inferred</a>.
4291 A generic function that is <i>not</i> <a href="#Calls">called</a> requires a
4292 type argument list for instantiation; if the list is partial, all
4293 remaining type arguments must be inferrable.
4294 A generic function that is called may provide a (possibly partial) type
4295 argument list, or may omit it entirely if the omitted type arguments are
4296 inferrable from the ordinary (non-type) function arguments.
4300 func min[T ~int|~float64](x, y T) T { … }
4302 f := min // illegal: min must be instantiated with type arguments when used without being called
4303 minInt := min[int] // minInt has type func(x, y int) int
4304 a := minInt(2, 3) // a has value 2 of type int
4305 b := min[float64](2.0, 3) // b has value 2.0 of type float64
4306 c := min(b, -1) // c has value -1.0 of type float64
4310 A partial type argument list cannot be empty; at least the first argument must be present.
4311 The list is a prefix of the full list of type arguments, leaving the remaining arguments
4312 to be inferred. Loosely speaking, type arguments may be omitted from "right to left".
4316 func apply[S ~[]E, E any](s S, f func(E) E) S { … }
4318 f0 := apply[] // illegal: type argument list cannot be empty
4319 f1 := apply[[]int] // type argument for S explicitly provided, type argument for E inferred
4320 f2 := apply[[]string, string] // both type arguments explicitly provided
4323 r := apply(bytes, func(byte) byte { … }) // both type arguments inferred from the function arguments
4327 For a generic type, all type arguments must always be provided explicitly.
4330 <h3 id="Type_inference">Type inference</h3>
4333 Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
4334 Each step attempts to use known information to infer additional type arguments.
4335 Type inference stops as soon as all type arguments are known.
4336 After type inference is complete, it is still necessary to substitute all type arguments
4337 for type parameters and verify that each type argument
4338 <a href="#Implementing_an_interface">implements</a> the relevant constraint;
4339 it is possible for an inferred type argument to fail to implement a constraint, in which
4340 case instantiation fails.
4344 Type inference is based on
4349 a <a href="#Type_parameter_declarations">type parameter list</a>
4352 a substitution map <i>M</i> initialized with the known type arguments, if any
4355 a (possibly empty) list of ordinary function arguments (in case of a function call only)
4360 and then proceeds with the following steps:
4365 apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
4366 to all <i>typed</i> ordinary function arguments
4369 apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
4372 apply function argument type inference to all <i>untyped</i> ordinary function arguments
4373 using the default type for each of the untyped function arguments
4376 apply constraint type inference
4381 If there are no ordinary or untyped function arguments, the respective steps are skipped.
4382 Constraint type inference is skipped if the previous step didn't infer any new type arguments,
4383 but it is run at least once if there are missing type arguments.
4387 The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
4388 The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
4389 If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
4392 <h4 id="Type_unification">Type unification</h4>
4395 Type inference is based on <i>type unification</i>. A single unification step
4396 applies to a <a href="#Type_inference">substitution map</a> and two types, either
4397 or both of which may be or contain type parameters. The substitution map tracks
4398 the known (explicitly provided or already inferred) type arguments: the map
4399 contains an entry <code>P</code> → <code>A</code> for each type
4400 parameter <code>P</code> and corresponding known type argument <code>A</code>.
4401 During unification, known type arguments take the place of their corresponding type
4402 parameters when comparing types. Unification is the process of finding substitution
4403 map entries that make the two types equivalent.
4407 For unification, two types that don't contain any type parameters from the current type
4408 parameter list are <i>equivalent</i>
4409 if they are identical, or if they are channel types that are identical ignoring channel
4410 direction, or if their underlying types are equivalent.
4414 Unification works by comparing the structure of pairs of types: their structure
4415 disregarding type parameters must be identical, and types other than type parameters
4417 A type parameter in one type may match any complete subtype in the other type;
4418 each successful match causes an entry to be added to the substitution map.
4419 If the structure differs, or types other than type parameters are not equivalent,
4424 TODO(gri) Somewhere we need to describe the process of adding an entry to the
4425 substitution map: if the entry is already present, the type argument
4426 values are themselves unified.
4430 For example, if <code>T1</code> and <code>T2</code> are type parameters,
4431 <code>[]map[int]bool</code> can be unified with any of the following:
4435 []map[int]bool // types are identical
4436 T1 // adds T1 → []map[int]bool to substitution map
4437 []T1 // adds T1 → map[int]bool to substitution map
4438 []map[T1]T2 // adds T1 → int and T2 → bool to substitution map
4442 On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
4446 int // int is not a slice
4447 struct{} // a struct is not a slice
4448 []struct{} // a struct is not a map
4449 []map[T1]string // map element types don't match
4453 As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
4454 <code>D</code> and a type literal <code>L</code> are never equivalent,
4455 unification compares the underlying type of <code>D</code> with <code>L</code> instead.
4456 For example, given the defined type
4460 type Vector []float64
4464 and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
4465 <code>[]E</code> and adds an entry <code>E</code> → <code>float64</code> to
4466 the substitution map.
4469 <h4 id="Function_argument_type_inference">Function argument type inference</h4>
4471 <!-- In this section and the section on constraint type inference we start with examples
4472 rather than have the examples follow the rules as is customary elsewhere in spec.
4473 Hopefully this helps building an intuition and makes the rules easier to follow. -->
4476 Function argument type inference infers type arguments from function arguments:
4477 if a function parameter is declared with a type <code>T</code> that uses
4479 <a href="#Type_unification">unifying</a> the type of the corresponding
4480 function argument with <code>T</code> may infer type arguments for the type
4481 parameters used by <code>T</code>.
4485 For instance, given the generic function
4489 func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
4497 var vector []float64
4498 scaledVector := scale(vector, 42)
4502 the type argument for <code>Number</code> can be inferred from the function argument
4503 <code>vector</code> by unifying the type of <code>vector</code> with the corresponding
4504 parameter type: <code>[]float64</code> and <code>[]Number</code>
4505 match in structure and <code>float64</code> matches with <code>Number</code>.
4506 This adds the entry <code>Number</code> → <code>float64</code> to the
4507 <a href="#Type_unification">substitution map</a>.
4508 Untyped arguments, such as the second function argument <code>42</code> here, are ignored
4509 in the first round of function argument type inference and only considered if there are
4510 unresolved type parameters left.
4514 Inference happens in two separate phases; each phase operates on a specific list of
4515 (parameter, argument) pairs:
4520 The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
4521 type uses type parameters and where the function argument is <i>typed</i>.
4524 The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
4525 type parameter. In this list, the respective function arguments are untyped.
4530 Any other (parameter, argument) pair is ignored.
4534 By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
4535 (or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
4536 of untyped values are always predeclared non-composite types, they can never match against
4537 a composite type, so it is sufficient to only consider parameter types that are single type
4542 Each list is processed in a separate phase:
4547 In the first phase, the parameter and argument types of each pair in <i>Lt</i>
4548 are unified. If unification succeeds for a pair, it may yield new entries that
4549 are added to the substitution map <i>M</i>. If unification fails, type inference
4553 The second phase considers the entries of list <i>Lu</i>. Type parameters for
4554 which the type argument has already been determined are ignored in this phase.
4555 For each remaining pair, the parameter type (which is a single type parameter) and
4556 the <a href="#Constants">default type</a> of the corresponding untyped argument is
4557 unified. If unification fails, type inference fails.
4562 While unification is successful, processing of each list continues until all list elements
4563 are considered, even if all type arguments are inferred before the last list element has
4572 func min[T ~int|~float64](x, y T) T
4575 min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
4576 min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
4577 min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
4581 In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
4582 yields the substitution map entry <code>T</code> → <code>float64</code>. Because
4583 processing continues until all untyped arguments are considered, an error is reported. This
4584 ensures that type inference does not depend on the order of the untyped arguments.
4587 <h4 id="Constraint_type_inference">Constraint type inference</h4>
4590 Constraint type inference infers type arguments by considering type constraints.
4591 If a type parameter <code>P</code> has a constraint with a
4592 <a href="#Core_types">core type</a> <code>C</code>,
4593 <a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
4594 may infer additional type arguments, either the type argument for <code>P</code>,
4595 or if that is already known, possibly the type arguments for type parameters
4596 used in <code>C</code>.
4600 For instance, consider the type parameter list with type parameters <code>List</code> and
4605 [List ~[]Elem, Elem any]
4609 Constraint type inference can deduce the type of <code>Elem</code> from the type argument
4610 for <code>List</code> because <code>Elem</code> is a type parameter in the core type
4611 <code>[]Elem</code> of <code>List</code>.
4612 If the type argument is <code>Bytes</code>:
4620 unifying the underlying type of <code>Bytes</code> with the core type means
4621 unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
4622 the <a href="#Type_unification">substitution map</a> entry
4623 <code>Elem</code> → <code>byte</code>.
4624 Thus, in this example, constraint type inference can infer the second type argument from the
4629 Using the core type of a constraint may lose some information: In the (unlikely) case that
4630 the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
4631 <code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
4632 <code>N</code> itself. In this case, constraint type inference may succeed but instantiation
4633 will fail because the inferred type is not in the type set of the constraint.
4634 Thus, constraint type inference uses the <i>adjusted core type</i> of
4635 a constraint: if the type set contains a single type, use that type; otherwise use the
4636 constraint's core type.
4640 Generally, constraint type inference proceeds in two phases: Starting with a given
4641 substitution map <i>M</i>
4646 For all type parameters with an adjusted core type, unify the type parameter with that
4647 type. If any unification fails, constraint type inference fails.
4651 At this point, some entries in <i>M</i> may map type parameters to other
4652 type parameters or to types containing type parameters. For each entry
4653 <code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
4654 contains type parameters <code>Q</code> for which there exist entries
4655 <code>Q</code> → <code>B</code> in <i>M</i>, substitute those
4656 <code>Q</code> with the respective <code>B</code> in <code>A</code>.
4657 Stop when no further substitution is possible.
4662 The result of constraint type inference is the final substitution map <i>M</i> from type
4663 parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
4664 appears in any of the <code>A</code>.
4668 For instance, given the type parameter list
4672 [A any, B []C, C *A]
4676 and the single provided type argument <code>int</code> for type parameter <code>A</code>,
4677 the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
4681 In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
4682 with the core type of their respective constraints. This adds the entries
4683 <code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
4687 At this point there are two entries in <i>M</i> where the right-hand side
4688 is or contains type parameters for which there exists other entries in <i>M</i>:
4689 <code>[]C</code> and <code>*A</code>.
4690 In the second phase, these type parameters are replaced with their respective
4691 types. It doesn't matter in which order this happens. Starting with the state
4692 of <i>M</i> after the first phase:
4696 <code>A</code> → <code>int</code>,
4697 <code>B</code> → <code>[]C</code>,
4698 <code>C</code> → <code>*A</code>
4702 Replace <code>A</code> on the right-hand side of → with <code>int</code>:
4706 <code>A</code> → <code>int</code>,
4707 <code>B</code> → <code>[]C</code>,
4708 <code>C</code> → <code>*int</code>
4712 Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
4716 <code>A</code> → <code>int</code>,
4717 <code>B</code> → <code>[]*int</code>,
4718 <code>C</code> → <code>*int</code>
4722 At this point no further substitution is possible and the map is full.
4723 Therefore, <code>M</code> represents the final map of type parameters
4724 to type arguments for the given type parameter list.
4727 <h3 id="Operators">Operators</h3>
4730 Operators combine operands into expressions.
4734 Expression = UnaryExpr | Expression binary_op Expression .
4735 UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
4737 binary_op = "||" | "&&" | rel_op | add_op | mul_op .
4738 rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
4739 add_op = "+" | "-" | "|" | "^" .
4740 mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
4742 unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
4746 Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
4747 For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
4748 unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
4749 For operations involving constants only, see the section on
4750 <a href="#Constant_expressions">constant expressions</a>.
4754 Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
4755 and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
4756 to the type of the other operand.
4760 The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
4761 or be an untyped constant <a href="#Representability">representable</a> by a
4762 value of type <code>uint</code>.
4763 If the left operand of a non-constant shift expression is an untyped constant,
4764 it is first implicitly converted to the type it would assume if the shift expression were
4765 replaced by its left operand alone.
4772 // The results of the following examples are given for 64-bit ints.
4773 var i = 1<<s // 1 has type int
4774 var j int32 = 1<<s // 1 has type int32; j == 0
4775 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
4776 var m int = 1.0<<s // 1.0 has type int; m == 1<<33
4777 var n = 1.0<<s == j // 1.0 has type int32; n == true
4778 var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
4779 var p = 1<<s == 1<<33 // 1 has type int; p == true
4780 var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
4781 var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
4782 var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
4783 var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift
4784 var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift
4785 var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
4786 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
4787 var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
4789 // The results of the following examples are given for 32-bit ints,
4790 // which means the shifts will overflow.
4791 var mm int = 1.0<<s // 1.0 has type int; mm == 0
4792 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
4793 var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
4794 var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
4795 var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
4798 <h4 id="Operator_precedence">Operator precedence</h4>
4800 Unary operators have the highest precedence.
4801 As the <code>++</code> and <code>--</code> operators form
4802 statements, not expressions, they fall
4803 outside the operator hierarchy.
4804 As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
4806 There are five precedence levels for binary operators.
4807 Multiplication operators bind strongest, followed by addition
4808 operators, comparison operators, <code>&&</code> (logical AND),
4809 and finally <code>||</code> (logical OR):
4812 <pre class="grammar">
4814 5 * / % << >> & &^
4816 3 == != < <= > >=
4822 Binary operators of the same precedence associate from left to right.
4823 For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
4832 x == y+1 && <-chanInt > 0
4836 <h3 id="Arithmetic_operators">Arithmetic operators</h3>
4838 Arithmetic operators apply to numeric values and yield a result of the same
4839 type as the first operand. The four standard arithmetic operators (<code>+</code>,
4840 <code>-</code>, <code>*</code>, <code>/</code>) apply to
4841 <a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
4842 <a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>.
4843 The bitwise logical and shift operators apply to integers only.
4846 <pre class="grammar">
4847 + sum integers, floats, complex values, strings
4848 - difference integers, floats, complex values
4849 * product integers, floats, complex values
4850 / quotient integers, floats, complex values
4851 % remainder integers
4853 & bitwise AND integers
4854 | bitwise OR integers
4855 ^ bitwise XOR integers
4856 &^ bit clear (AND NOT) integers
4858 << left shift integer << integer >= 0
4859 >> right shift integer >> integer >= 0
4863 If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>,
4864 the operator must apply to each type in that type set.
4865 The operands are represented as values of the type argument that the type parameter
4866 is <a href="#Instantiations">instantiated</a> with, and the operation is computed
4867 with the precision of that type argument. For example, given the function:
4871 func dotProduct[F ~float32|~float64](v1, v2 []F) F {
4873 for i, x := range v1 {
4882 the product <code>x * y</code> and the addition <code>s += x * y</code>
4883 are computed with <code>float32</code> or <code>float64</code> precision,
4884 respectively, depending on the type argument for <code>F</code>.
4887 <h4 id="Integer_operators">Integer operators</h4>
4890 For two integer values <code>x</code> and <code>y</code>, the integer quotient
4891 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
4896 x = q*y + r and |r| < |y|
4900 with <code>x / y</code> truncated towards zero
4901 (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
4913 The one exception to this rule is that if the dividend <code>x</code> is
4914 the most negative value for the int type of <code>x</code>, the quotient
4915 <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
4916 due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
4924 int64 -9223372036854775808
4928 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
4929 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4930 If the dividend is non-negative and the divisor is a constant power of 2,
4931 the division may be replaced by a right shift, and computing the remainder may
4932 be replaced by a bitwise AND operation:
4936 x x / 4 x % 4 x >> 2 x & 3
4942 The shift operators shift the left operand by the shift count specified by the
4943 right operand, which must be non-negative. If the shift count is negative at run time,
4944 a <a href="#Run_time_panics">run-time panic</a> occurs.
4945 The shift operators implement arithmetic shifts if the left operand is a signed
4946 integer and logical shifts if it is an unsigned integer.
4947 There is no upper limit on the shift count. Shifts behave
4948 as if the left operand is shifted <code>n</code> times by 1 for a shift
4949 count of <code>n</code>.
4950 As a result, <code>x << 1</code> is the same as <code>x*2</code>
4951 and <code>x >> 1</code> is the same as
4952 <code>x/2</code> but truncated towards negative infinity.
4956 For integer operands, the unary operators
4957 <code>+</code>, <code>-</code>, and <code>^</code> are defined as
4961 <pre class="grammar">
4963 -x negation is 0 - x
4964 ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
4965 and m = -1 for signed x
4969 <h4 id="Integer_overflow">Integer overflow</h4>
4972 For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>,
4973 <code>-</code>, <code>*</code>, and <code><<</code> are
4974 computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
4975 the unsigned integer's type.
4976 Loosely speaking, these unsigned integer operations
4977 discard high bits upon overflow, and programs may rely on "wrap around".
4981 For signed integers, the operations <code>+</code>,
4982 <code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
4983 overflow and the resulting value exists and is deterministically defined
4984 by the signed integer representation, the operation, and its operands.
4985 Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
4986 A compiler may not optimize code under the assumption that overflow does
4987 not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
4990 <h4 id="Floating_point_operators">Floating-point operators</h4>
4993 For floating-point and complex numbers,
4994 <code>+x</code> is the same as <code>x</code>,
4995 while <code>-x</code> is the negation of <code>x</code>.
4996 The result of a floating-point or complex division by zero is not specified beyond the
4997 IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
4998 occurs is implementation-specific.
5002 An implementation may combine multiple floating-point operations into a single
5003 fused operation, possibly across statements, and produce a result that differs
5004 from the value obtained by executing and rounding the instructions individually.
5005 An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to
5006 the precision of the target type, preventing fusion that would discard that rounding.
5010 For instance, some architectures provide a "fused multiply and add" (FMA) instruction
5011 that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
5012 These examples show when a Go implementation can use that instruction:
5016 // FMA allowed for computing r, because x*y is not explicitly rounded:
5020 *p = x*y; r = *p + z
5021 r = x*y + float64(z)
5023 // FMA disallowed for computing r, because it would omit rounding of x*y:
5024 r = float64(x*y) + z
5025 r = z; r += float64(x*y)
5026 t = float64(x*y); r = t + z
5029 <h4 id="String_concatenation">String concatenation</h4>
5032 Strings can be concatenated using the <code>+</code> operator
5033 or the <code>+=</code> assignment operator:
5037 s := "hi" + string(c)
5038 s += " and good bye"
5042 String addition creates a new string by concatenating the operands.
5045 <h3 id="Comparison_operators">Comparison operators</h3>
5048 Comparison operators compare two operands and yield an untyped boolean value.
5051 <pre class="grammar">
5057 >= greater or equal
5061 In any comparison, the first operand
5062 must be <a href="#Assignability">assignable</a>
5063 to the type of the second operand, or vice versa.
5066 The equality operators <code>==</code> and <code>!=</code> apply
5067 to operands of <i>comparable</i> types.
5068 The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
5069 apply to operands of <i>ordered</i> types.
5070 These terms and the result of the comparisons are defined as follows:
5075 Boolean types are comparable.
5076 Two boolean values are equal if they are either both
5077 <code>true</code> or both <code>false</code>.
5081 Integer types are comparable and ordered.
5082 Two integer values are compared in the usual way.
5086 Floating-point types are comparable and ordered.
5087 Two floating-point values are compared as defined by the IEEE-754 standard.
5091 Complex types are comparable.
5092 Two complex values <code>u</code> and <code>v</code> are
5093 equal if both <code>real(u) == real(v)</code> and
5094 <code>imag(u) == imag(v)</code>.
5098 String types are comparable and ordered.
5099 Two string values are compared lexically byte-wise.
5103 Pointer types are comparable.
5104 Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
5105 Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
5109 Channel types are comparable.
5110 Two channel values are equal if they were created by the same call to
5111 <a href="#Making_slices_maps_and_channels"><code>make</code></a>
5112 or if both have value <code>nil</code>.
5116 Interface types that are not type parameters are comparable.
5117 Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
5118 and equal dynamic values or if both have value <code>nil</code>.
5122 A value <code>x</code> of non-interface type <code>X</code> and
5123 a value <code>t</code> of interface type <code>T</code> can be compared
5124 if type <code>X</code> is comparable and
5125 <code>X</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
5126 They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
5127 and <code>t</code>'s dynamic value is equal to <code>x</code>.
5131 Struct types are comparable if all their field types are comparable.
5132 Two struct values are equal if their corresponding
5133 non-<a href="#Blank_identifier">blank</a> field values are equal.
5134 The fields are compared in source order, and comparison stops as
5135 soon as two field values differ (or all fields have been compared).
5139 Array types are comparable if their array element types are comparable.
5140 Two array values are equal if their corresponding element values are equal.
5141 The elements are compared in ascending index order, and comparison stops
5142 as soon as two element values differ (or all elements have been compared).
5146 Type parameters are comparable if they are strictly comparable (see below).
5151 A comparison of two interface values with identical dynamic types
5152 causes a <a href="#Run_time_panics">run-time panic</a> if that type
5153 is not comparable. This behavior applies not only to direct interface
5154 value comparisons but also when comparing arrays of interface values
5155 or structs with interface-valued fields.
5159 Slice, map, and function types are not comparable.
5160 However, as a special case, a slice, map, or function value may
5161 be compared to the predeclared identifier <code>nil</code>.
5162 Comparison of pointer, channel, and interface values to <code>nil</code>
5163 is also allowed and follows from the general rules above.
5167 const c = 3 < 4 // c is the untyped boolean constant true
5172 // The result of a comparison is an untyped boolean.
5173 // The usual assignment rules apply.
5174 b3 = x == y // b3 has type bool
5175 b4 bool = x == y // b4 has type bool
5176 b5 MyBool = x == y // b5 has type MyBool
5181 A type is <i>strictly comparable</i> if it is comparable and not an interface
5182 type nor composed of interface types.
5188 Boolean, numeric, string, pointer, and channel types are strictly comparable.
5192 Struct types are strictly comparable if all their field types are strictly comparable.
5196 Array types are strictly comparable if their array element types are strictly comparable.
5200 Type parameters are strictly comparable if all types in their type set are strictly comparable.
5204 <h3 id="Logical_operators">Logical operators</h3>
5207 Logical operators apply to <a href="#Boolean_types">boolean</a> values
5208 and yield a result of the same type as the operands.
5209 The right operand is evaluated conditionally.
5212 <pre class="grammar">
5213 && conditional AND p && q is "if p then q else false"
5214 || conditional OR p || q is "if p then true else q"
5219 <h3 id="Address_operators">Address operators</h3>
5222 For an operand <code>x</code> of type <code>T</code>, the address operation
5223 <code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
5224 The operand must be <i>addressable</i>,
5225 that is, either a variable, pointer indirection, or slice indexing
5226 operation; or a field selector of an addressable struct operand;
5227 or an array indexing operation of an addressable array.
5228 As an exception to the addressability requirement, <code>x</code> may also be a
5229 (possibly parenthesized)
5230 <a href="#Composite_literals">composite literal</a>.
5231 If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
5232 then the evaluation of <code>&x</code> does too.
5236 For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
5237 indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
5238 to by <code>x</code>.
5239 If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
5240 will cause a <a href="#Run_time_panics">run-time panic</a>.
5251 *x // causes a run-time panic
5252 &*x // causes a run-time panic
5256 <h3 id="Receive_operator">Receive operator</h3>
5259 For an operand <code>ch</code> whose <a href="#Core_types">core type</a> is a
5260 <a href="#Channel_types">channel</a>,
5261 the value of the receive operation <code><-ch</code> is the value received
5262 from the channel <code>ch</code>. The channel direction must permit receive operations,
5263 and the type of the receive operation is the element type of the channel.
5264 The expression blocks until a value is available.
5265 Receiving from a <code>nil</code> channel blocks forever.
5266 A receive operation on a <a href="#Close">closed</a> channel can always proceed
5267 immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
5268 after any previously sent values have been received.
5275 <-strobe // wait until clock pulse and discard received value
5279 A receive expression used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
5286 var x, ok T = <-ch
5290 yields an additional untyped boolean result reporting whether the
5291 communication succeeded. The value of <code>ok</code> is <code>true</code>
5292 if the value received was delivered by a successful send operation to the
5293 channel, or <code>false</code> if it is a zero value generated because the
5294 channel is closed and empty.
5298 <h3 id="Conversions">Conversions</h3>
5301 A conversion changes the <a href="#Types">type</a> of an expression
5302 to the type specified by the conversion.
5303 A conversion may appear literally in the source, or it may be <i>implied</i>
5304 by the context in which an expression appears.
5308 An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
5309 where <code>T</code> is a type and <code>x</code> is an expression
5310 that can be converted to type <code>T</code>.
5314 Conversion = Type "(" Expression [ "," ] ")" .
5318 If the type starts with the operator <code>*</code> or <code><-</code>,
5319 or if the type starts with the keyword <code>func</code>
5320 and has no result list, it must be parenthesized when
5321 necessary to avoid ambiguity:
5325 *Point(p) // same as *(Point(p))
5326 (*Point)(p) // p is converted to *Point
5327 <-chan int(c) // same as <-(chan int(c))
5328 (<-chan int)(c) // c is converted to <-chan int
5329 func()(x) // function signature func() x
5330 (func())(x) // x is converted to func()
5331 (func() int)(x) // x is converted to func() int
5332 func() int(x) // x is converted to func() int (unambiguous)
5336 A <a href="#Constants">constant</a> value <code>x</code> can be converted to
5337 type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
5338 by a value of <code>T</code>.
5339 As a special case, an integer constant <code>x</code> can be explicitly converted to a
5340 <a href="#String_types">string type</a> using the
5341 <a href="#Conversions_to_and_from_a_string_type">same rule</a>
5342 as for non-constant <code>x</code>.
5346 Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a>
5347 yields a typed constant.
5351 uint(iota) // iota value of type uint
5352 float32(2.718281828) // 2.718281828 of type float32
5353 complex128(1) // 1.0 + 0.0i of type complex128
5354 float32(0.49999999) // 0.5 of type float32
5355 float64(-1e-1000) // 0.0 of type float64
5356 string('x') // "x" of type string
5357 string(0x266c) // "♬" of type string
5358 myString("foo" + "bar") // "foobar" of type myString
5359 string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
5360 (*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
5361 int(1.2) // illegal: 1.2 cannot be represented as an int
5362 string(65.0) // illegal: 65.0 is not an integer constant
5366 Converting a constant to a type parameter yields a <i>non-constant</i> value of that type,
5367 with the value represented as a value of the type argument that the type parameter
5368 is <a href="#Instantiations">instantiated</a> with.
5369 For example, given the function:
5373 func f[P ~float32|~float64]() {
5379 the conversion <code>P(1.1)</code> results in a non-constant value of type <code>P</code>
5380 and the value <code>1.1</code> is represented as a <code>float32</code> or a <code>float64</code>
5381 depending on the type argument for <code>f</code>.
5382 Accordingly, if <code>f</code> is instantiated with a <code>float32</code> type,
5383 the numeric value of the expression <code>P(1.1) + 1.2</code> will be computed
5384 with the same precision as the corresponding non-constant <code>float32</code>
5389 A non-constant value <code>x</code> can be converted to type <code>T</code>
5390 in any of these cases:
5395 <code>x</code> is <a href="#Assignability">assignable</a>
5399 ignoring struct tags (see below),
5400 <code>x</code>'s type and <code>T</code> are not
5401 <a href="#Type_parameter_declarations">type parameters</a> but have
5402 <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
5405 ignoring struct tags (see below),
5406 <code>x</code>'s type and <code>T</code> are pointer types
5407 that are not <a href="#Types">named types</a>,
5408 and their pointer base types are not type parameters but
5409 have identical underlying types.
5412 <code>x</code>'s type and <code>T</code> are both integer or floating
5416 <code>x</code>'s type and <code>T</code> are both complex types.
5419 <code>x</code> is an integer or a slice of bytes or runes
5420 and <code>T</code> is a string type.
5423 <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
5426 <code>x</code> is a slice, <code>T</code> is an array or a pointer to an array,
5427 and the slice and array types have <a href="#Type_identity">identical</a> element types.
5432 Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type
5433 parameters, <code>x</code>
5434 can also be converted to type <code>T</code> if one of the following conditions applies:
5439 Both <code>V</code> and <code>T</code> are type parameters and a value of each
5440 type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s
5444 Only <code>V</code> is a type parameter and a value of each
5445 type in <code>V</code>'s type set can be converted to <code>T</code>.
5448 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
5449 type in <code>T</code>'s type set.
5454 <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
5455 for identity for the purpose of conversion:
5459 type Person struct {
5468 Name string `json:"name"`
5470 Street string `json:"street"`
5471 City string `json:"city"`
5475 var person = (*Person)(data) // ignoring tags, the underlying types are identical
5479 Specific rules apply to (non-constant) conversions between numeric types or
5480 to and from a string type.
5481 These conversions may change the representation of <code>x</code>
5482 and incur a run-time cost.
5483 All other conversions only change the type but not the representation
5488 There is no linguistic mechanism to convert between pointers and integers.
5489 The package <a href="#Package_unsafe"><code>unsafe</code></a>
5490 implements this functionality under restricted circumstances.
5493 <h4>Conversions between numeric types</h4>
5496 For the conversion of non-constant numeric values, the following rules apply:
5501 When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is
5502 sign extended to implicit infinite precision; otherwise it is zero extended.
5503 It is then truncated to fit in the result type's size.
5504 For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
5505 The conversion always yields a valid value; there is no indication of overflow.
5508 When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded
5509 (truncation towards zero).
5512 When converting an integer or floating-point number to a floating-point type,
5513 or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded
5514 to the precision specified by the destination type.
5515 For instance, the value of a variable <code>x</code> of type <code>float32</code>
5516 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
5517 but float32(x) represents the result of rounding <code>x</code>'s value to
5518 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
5519 of precision, but <code>float32(x + 0.1)</code> does not.
5524 In all non-constant conversions involving floating-point or complex values,
5525 if the result type cannot represent the value the conversion
5526 succeeds but the result value is implementation-dependent.
5529 <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
5533 Converting a signed or unsigned integer value to a string type yields a
5534 string containing the UTF-8 representation of the integer. Values outside
5535 the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
5539 string(-1) // "\ufffd" == "\xef\xbf\xbd"
5540 string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
5542 type myString string
5543 myString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
5548 Converting a slice of bytes to a string type yields
5549 a string whose successive bytes are the elements of the slice.
5552 string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5553 string([]byte{}) // ""
5554 string([]byte(nil)) // ""
5557 string(bytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5560 string([]myByte{'w', 'o', 'r', 'l', 'd', '!'}) // "world!"
5561 myString([]myByte{'\xf0', '\x9f', '\x8c', '\x8d'}) // "🌍"
5566 Converting a slice of runes to a string type yields
5567 a string that is the concatenation of the individual rune values
5568 converted to strings.
5571 string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5572 string([]rune{}) // ""
5573 string([]rune(nil)) // ""
5576 string(runes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5579 string([]myRune{0x266b, 0x266c}) // "\u266b\u266c" == "♫♬"
5580 myString([]myRune{0x1f30e}) // "\U0001f30e" == "🌎"
5585 Converting a value of a string type to a slice of bytes type
5586 yields a slice whose successive elements are the bytes of the string.
5589 []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5590 []byte("") // []byte{}
5592 bytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5594 []myByte("world!") // []myByte{'w', 'o', 'r', 'l', 'd', '!'}
5595 []myByte(myString("🌏")) // []myByte{'\xf0', '\x9f', '\x8c', '\x8f'}
5600 Converting a value of a string type to a slice of runes type
5601 yields a slice containing the individual Unicode code points of the string.
5604 []rune(myString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
5605 []rune("") // []rune{}
5607 runes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
5609 []myRune("♫♬") // []myRune{0x266b, 0x266c}
5610 []myRune(myString("🌐")) // []myRune{0x1f310}
5615 <h4 id="Conversions_from_slice_to_array_or_array_pointer">Conversions from slice to array or array pointer</h4>
5618 Converting a slice to an array yields an array containing the elements of the underlying array of the slice.
5619 Similarly, converting a slice to an array pointer yields a pointer to the underlying array of the slice.
5620 In both cases, if the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
5621 a <a href="#Run_time_panics">run-time panic</a> occurs.
5625 s := make([]byte, 2, 4)
5628 a1 := [1]byte(s[1:]) // a1[0] == s[1]
5629 a2 := [2]byte(s) // a2[0] == s[0]
5630 a4 := [4]byte(s) // panics: len([4]byte) > len(s)
5632 s0 := (*[0]byte)(s) // s0 != nil
5633 s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
5634 s2 := (*[2]byte)(s) // &s2[0] == &s[0]
5635 s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
5638 t0 := [0]string(t) // ok for nil slice t
5639 t1 := (*[0]string)(t) // t1 == nil
5640 t2 := (*[1]string)(t) // panics: len([1]string) > len(t)
5642 u := make([]byte, 0)
5643 u0 := (*[0]byte)(u) // u0 != nil
5646 <h3 id="Constant_expressions">Constant expressions</h3>
5649 Constant expressions may contain only <a href="#Constants">constant</a>
5650 operands and are evaluated at compile time.
5654 Untyped boolean, numeric, and string constants may be used as operands
5655 wherever it is legal to use an operand of boolean, numeric, or string type,
5660 A constant <a href="#Comparison_operators">comparison</a> always yields
5661 an untyped boolean constant. If the left operand of a constant
5662 <a href="#Operators">shift expression</a> is an untyped constant, the
5663 result is an integer constant; otherwise it is a constant of the same
5664 type as the left operand, which must be of
5665 <a href="#Numeric_types">integer type</a>.
5669 Any other operation on untyped constants results in an untyped constant of the
5670 same kind; that is, a boolean, integer, floating-point, complex, or string
5672 If the untyped operands of a binary operation (other than a shift) are of
5673 different kinds, the result is of the operand's kind that appears later in this
5674 list: integer, rune, floating-point, complex.
5675 For example, an untyped integer constant divided by an
5676 untyped complex constant yields an untyped complex constant.
5680 const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
5681 const b = 15 / 4 // b == 3 (untyped integer constant)
5682 const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
5683 const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
5684 const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
5685 const d = 1 << 3.0 // d == 8 (untyped integer constant)
5686 const e = 1.0 << 3 // e == 8 (untyped integer constant)
5687 const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
5688 const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
5689 const h = "foo" > "bar" // h == true (untyped boolean constant)
5690 const j = true // j == true (untyped boolean constant)
5691 const k = 'w' + 1 // k == 'x' (untyped rune constant)
5692 const l = "hi" // l == "hi" (untyped string constant)
5693 const m = string(k) // m == "x" (type string)
5694 const Σ = 1 - 0.707i // (untyped complex constant)
5695 const Δ = Σ + 2.0e-4 // (untyped complex constant)
5696 const Φ = iota*1i - 1/1i // (untyped complex constant)
5700 Applying the built-in function <code>complex</code> to untyped
5701 integer, rune, or floating-point constants yields
5702 an untyped complex constant.
5706 const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
5707 const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
5711 Constant expressions are always evaluated exactly; intermediate values and the
5712 constants themselves may require precision significantly larger than supported
5713 by any predeclared type in the language. The following are legal declarations:
5717 const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
5718 const Four int8 = Huge >> 98 // Four == 4 (type int8)
5722 The divisor of a constant division or remainder operation must not be zero:
5726 3.14 / 0.0 // illegal: division by zero
5730 The values of <i>typed</i> constants must always be accurately
5731 <a href="#Representability">representable</a> by values
5732 of the constant type. The following constant expressions are illegal:
5736 uint(-1) // -1 cannot be represented as a uint
5737 int(3.14) // 3.14 cannot be represented as an int
5738 int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
5739 Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
5740 Four * 100 // product 400 cannot be represented as an int8 (type of Four)
5744 The mask used by the unary bitwise complement operator <code>^</code> matches
5745 the rule for non-constants: the mask is all 1s for unsigned constants
5746 and -1 for signed and untyped constants.
5750 ^1 // untyped integer constant, equal to -2
5751 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
5752 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
5753 int8(^1) // same as int8(-2)
5754 ^int8(1) // same as -1 ^ int8(1) = -2
5758 Implementation restriction: A compiler may use rounding while
5759 computing untyped floating-point or complex constant expressions; see
5760 the implementation restriction in the section
5761 on <a href="#Constants">constants</a>. This rounding may cause a
5762 floating-point constant expression to be invalid in an integer
5763 context, even if it would be integral when calculated using infinite
5764 precision, and vice versa.
5768 <h3 id="Order_of_evaluation">Order of evaluation</h3>
5771 At package level, <a href="#Package_initialization">initialization dependencies</a>
5772 determine the evaluation order of individual initialization expressions in
5773 <a href="#Variable_declarations">variable declarations</a>.
5774 Otherwise, when evaluating the <a href="#Operands">operands</a> of an
5775 expression, assignment, or
5776 <a href="#Return_statements">return statement</a>,
5777 all function calls, method calls, and
5778 communication operations are evaluated in lexical left-to-right
5783 For example, in the (function-local) assignment
5786 y[f()], ok = g(h(), i()+x[j()], <-c), k()
5789 the function calls and communication happen in the order
5790 <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
5791 <code><-c</code>, <code>g()</code>, and <code>k()</code>.
5792 However, the order of those events compared to the evaluation
5793 and indexing of <code>x</code> and the evaluation
5794 of <code>y</code> is not specified.
5799 f := func() int { a++; return a }
5800 x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
5801 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
5802 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
5806 At package level, initialization dependencies override the left-to-right rule
5807 for individual initialization expressions, but not for operands within each
5812 var a, b, c = f() + v(), g(), sqr(u()) + v()
5814 func f() int { return c }
5815 func g() int { return a }
5816 func sqr(x int) int { return x*x }
5818 // functions u and v are independent of all other variables and functions
5822 The function calls happen in the order
5823 <code>u()</code>, <code>sqr()</code>, <code>v()</code>,
5824 <code>f()</code>, <code>v()</code>, and <code>g()</code>.
5828 Floating-point operations within a single expression are evaluated according to
5829 the associativity of the operators. Explicit parentheses affect the evaluation
5830 by overriding the default associativity.
5831 In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
5832 is performed before adding <code>x</code>.
5835 <h2 id="Statements">Statements</h2>
5838 Statements control execution.
5843 Declaration | LabeledStmt | SimpleStmt |
5844 GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
5845 FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
5848 SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
5851 <h3 id="Terminating_statements">Terminating statements</h3>
5854 A <i>terminating statement</i> interrupts the regular flow of control in
5855 a <a href="#Blocks">block</a>. The following statements are terminating:
5860 A <a href="#Return_statements">"return"</a> or
5861 <a href="#Goto_statements">"goto"</a> statement.
5862 <!-- ul below only for regular layout -->
5867 A call to the built-in function
5868 <a href="#Handling_panics"><code>panic</code></a>.
5869 <!-- ul below only for regular layout -->
5874 A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
5875 <!-- ul below only for regular layout -->
5880 An <a href="#If_statements">"if" statement</a> in which:
5882 <li>the "else" branch is present, and</li>
5883 <li>both branches are terminating statements.</li>
5888 A <a href="#For_statements">"for" statement</a> in which:
5890 <li>there are no "break" statements referring to the "for" statement, and</li>
5891 <li>the loop condition is absent, and</li>
5892 <li>the "for" statement does not use a range clause.</li>
5897 A <a href="#Switch_statements">"switch" statement</a> in which:
5899 <li>there are no "break" statements referring to the "switch" statement,</li>
5900 <li>there is a default case, and</li>
5901 <li>the statement lists in each case, including the default, end in a terminating
5902 statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
5908 A <a href="#Select_statements">"select" statement</a> in which:
5910 <li>there are no "break" statements referring to the "select" statement, and</li>
5911 <li>the statement lists in each case, including the default if present,
5912 end in a terminating statement.</li>
5917 A <a href="#Labeled_statements">labeled statement</a> labeling
5918 a terminating statement.
5923 All other statements are not terminating.
5927 A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
5928 is not empty and its final non-empty statement is terminating.
5932 <h3 id="Empty_statements">Empty statements</h3>
5935 The empty statement does nothing.
5943 <h3 id="Labeled_statements">Labeled statements</h3>
5946 A labeled statement may be the target of a <code>goto</code>,
5947 <code>break</code> or <code>continue</code> statement.
5951 LabeledStmt = Label ":" Statement .
5952 Label = identifier .
5956 Error: log.Panic("error encountered")
5960 <h3 id="Expression_statements">Expression statements</h3>
5963 With the exception of specific built-in functions,
5964 function and method <a href="#Calls">calls</a> and
5965 <a href="#Receive_operator">receive operations</a>
5966 can appear in statement context. Such statements may be parenthesized.
5970 ExpressionStmt = Expression .
5974 The following built-in functions are not permitted in statement context:
5978 append cap complex imag len make new real
5979 unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice
5987 len("foo") // illegal if len is the built-in function
5991 <h3 id="Send_statements">Send statements</h3>
5994 A send statement sends a value on a channel.
5995 The channel expression's <a href="#Core_types">core type</a>
5996 must be a <a href="#Channel_types">channel</a>,
5997 the channel direction must permit send operations,
5998 and the type of the value to be sent must be <a href="#Assignability">assignable</a>
5999 to the channel's element type.
6003 SendStmt = Channel "<-" Expression .
6004 Channel = Expression .
6008 Both the channel and the value expression are evaluated before communication
6009 begins. Communication blocks until the send can proceed.
6010 A send on an unbuffered channel can proceed if a receiver is ready.
6011 A send on a buffered channel can proceed if there is room in the buffer.
6012 A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
6013 A send on a <code>nil</code> channel blocks forever.
6017 ch <- 3 // send value 3 to channel ch
6021 <h3 id="IncDec_statements">IncDec statements</h3>
6024 The "++" and "--" statements increment or decrement their operands
6025 by the untyped <a href="#Constants">constant</a> <code>1</code>.
6026 As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
6027 or a map index expression.
6031 IncDecStmt = Expression ( "++" | "--" ) .
6035 The following <a href="#Assignment_statements">assignment statements</a> are semantically
6039 <pre class="grammar">
6040 IncDec statement Assignment
6046 <h3 id="Assignment_statements">Assignment statements</h3>
6049 An <i>assignment</i> replaces the current value stored in a <a href="#Variables">variable</a>
6050 with a new value specified by an <a href="#Expressions">expression</a>.
6051 An assignment statement may assign a single value to a single variable, or multiple values to a
6052 matching number of variables.
6056 Assignment = ExpressionList assign_op ExpressionList .
6058 assign_op = [ add_op | mul_op ] "=" .
6062 Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
6063 a map index expression, or (for <code>=</code> assignments only) the
6064 <a href="#Blank_identifier">blank identifier</a>.
6065 Operands may be parenthesized.
6072 (k) = <-ch // same as: k = <-ch
6076 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
6077 <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
6078 is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
6079 <code>(y)</code> but evaluates <code>x</code>
6080 only once. The <i>op</i><code>=</code> construct is a single token.
6081 In assignment operations, both the left- and right-hand expression lists
6082 must contain exactly one single-valued expression, and the left-hand
6083 expression must not be the blank identifier.
6088 i &^= 1<<n
6092 A tuple assignment assigns the individual elements of a multi-valued
6093 operation to a list of variables. There are two forms. In the
6094 first, the right hand operand is a single multi-valued expression
6095 such as a function call, a <a href="#Channel_types">channel</a> or
6096 <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
6097 The number of operands on the left
6098 hand side must match the number of values. For instance, if
6099 <code>f</code> is a function returning two values,
6107 assigns the first value to <code>x</code> and the second to <code>y</code>.
6108 In the second form, the number of operands on the left must equal the number
6109 of expressions on the right, each of which must be single-valued, and the
6110 <i>n</i>th expression on the right is assigned to the <i>n</i>th
6111 operand on the left:
6115 one, two, three = '一', '二', '三'
6119 The <a href="#Blank_identifier">blank identifier</a> provides a way to
6120 ignore right-hand side values in an assignment:
6124 _ = x // evaluate x but ignore it
6125 x, _ = f() // evaluate f() but ignore second result value
6129 The assignment proceeds in two phases.
6130 First, the operands of <a href="#Index_expressions">index expressions</a>
6131 and <a href="#Address_operators">pointer indirections</a>
6132 (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
6133 on the left and the expressions on the right are all
6134 <a href="#Order_of_evaluation">evaluated in the usual order</a>.
6135 Second, the assignments are carried out in left-to-right order.
6139 a, b = b, a // exchange a and b
6143 i, x[i] = 1, 2 // set i = 1, x[0] = 2
6146 x[i], i = 2, 1 // set x[0] = 2, i = 1
6148 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
6150 x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
6152 type Point struct { x, y int }
6154 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
6158 for i, x[i] = range x { // set i, x[2] = 0, x[0]
6161 // after this loop, i == 0 and x == []int{3, 5, 3}
6165 In assignments, each value must be <a href="#Assignability">assignable</a>
6166 to the type of the operand to which it is assigned, with the following special cases:
6171 Any typed value may be assigned to the blank identifier.
6175 If an untyped constant
6176 is assigned to a variable of interface type or the blank identifier,
6177 the constant is first implicitly <a href="#Conversions">converted</a> to its
6178 <a href="#Constants">default type</a>.
6182 If an untyped boolean value is assigned to a variable of interface type or
6183 the blank identifier, it is first implicitly converted to type <code>bool</code>.
6187 <h3 id="If_statements">If statements</h3>
6190 "If" statements specify the conditional execution of two branches
6191 according to the value of a boolean expression. If the expression
6192 evaluates to true, the "if" branch is executed, otherwise, if
6193 present, the "else" branch is executed.
6197 IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
6207 The expression may be preceded by a simple statement, which
6208 executes before the expression is evaluated.
6212 if x := f(); x < y {
6214 } else if x > z {
6222 <h3 id="Switch_statements">Switch statements</h3>
6225 "Switch" statements provide multi-way execution.
6226 An expression or type is compared to the "cases"
6227 inside the "switch" to determine which branch
6232 SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
6236 There are two forms: expression switches and type switches.
6237 In an expression switch, the cases contain expressions that are compared
6238 against the value of the switch expression.
6239 In a type switch, the cases contain types that are compared against the
6240 type of a specially annotated switch expression.
6241 The switch expression is evaluated exactly once in a switch statement.
6244 <h4 id="Expression_switches">Expression switches</h4>
6247 In an expression switch,
6248 the switch expression is evaluated and
6249 the case expressions, which need not be constants,
6250 are evaluated left-to-right and top-to-bottom; the first one that equals the
6252 triggers execution of the statements of the associated case;
6253 the other cases are skipped.
6254 If no case matches and there is a "default" case,
6255 its statements are executed.
6256 There can be at most one default case and it may appear anywhere in the
6258 A missing switch expression is equivalent to the boolean value
6263 ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
6264 ExprCaseClause = ExprSwitchCase ":" StatementList .
6265 ExprSwitchCase = "case" ExpressionList | "default" .
6269 If the switch expression evaluates to an untyped constant, it is first implicitly
6270 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
6271 The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
6272 The switch expression type must be <a href="#Comparison_operators">comparable</a>.
6276 If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
6277 to the type of the switch expression.
6278 For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
6279 of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
6283 In other words, the switch expression is treated as if it were used to declare and
6284 initialize a temporary variable <code>t</code> without explicit type; it is that
6285 value of <code>t</code> against which each case expression <code>x</code> is tested
6290 In a case or default clause, the last non-empty statement
6291 may be a (possibly <a href="#Labeled_statements">labeled</a>)
6292 <a href="#Fallthrough_statements">"fallthrough" statement</a> to
6293 indicate that control should flow from the end of this clause to
6294 the first statement of the next clause.
6295 Otherwise control flows to the end of the "switch" statement.
6296 A "fallthrough" statement may appear as the last statement of all
6297 but the last clause of an expression switch.
6301 The switch expression may be preceded by a simple statement, which
6302 executes before the expression is evaluated.
6308 case 0, 1, 2, 3: s1()
6309 case 4, 5, 6, 7: s2()
6312 switch x := f(); { // missing switch expression means "true"
6313 case x < 0: return -x
6325 Implementation restriction: A compiler may disallow multiple case
6326 expressions evaluating to the same constant.
6327 For instance, the current compilers disallow duplicate integer,
6328 floating point, or string constants in case expressions.
6331 <h4 id="Type_switches">Type switches</h4>
6334 A type switch compares types rather than values. It is otherwise similar
6335 to an expression switch. It is marked by a special switch expression that
6336 has the form of a <a href="#Type_assertions">type assertion</a>
6337 using the keyword <code>type</code> rather than an actual type:
6347 Cases then match actual types <code>T</code> against the dynamic type of the
6348 expression <code>x</code>. As with type assertions, <code>x</code> must be of
6349 <a href="#Interface_types">interface type</a>, but not a
6350 <a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type
6351 <code>T</code> listed in a case must implement the type of <code>x</code>.
6352 The types listed in the cases of a type switch must all be
6353 <a href="#Type_identity">different</a>.
6357 TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
6358 TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
6359 TypeCaseClause = TypeSwitchCase ":" StatementList .
6360 TypeSwitchCase = "case" TypeList | "default" .
6364 The TypeSwitchGuard may include a
6365 <a href="#Short_variable_declarations">short variable declaration</a>.
6366 When that form is used, the variable is declared at the end of the
6367 TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
6368 In clauses with a case listing exactly one type, the variable
6369 has that type; otherwise, the variable has the type of the expression
6370 in the TypeSwitchGuard.
6374 Instead of a type, a case may use the predeclared identifier
6375 <a href="#Predeclared_identifiers"><code>nil</code></a>;
6376 that case is selected when the expression in the TypeSwitchGuard
6377 is a <code>nil</code> interface value.
6378 There may be at most one <code>nil</code> case.
6382 Given an expression <code>x</code> of type <code>interface{}</code>,
6383 the following type switch:
6387 switch i := x.(type) {
6389 printString("x is nil") // type of i is type of x (interface{})
6391 printInt(i) // type of i is int
6393 printFloat64(i) // type of i is float64
6394 case func(int) float64:
6395 printFunction(i) // type of i is func(int) float64
6397 printString("type is bool or string") // type of i is type of x (interface{})
6399 printString("don't know the type") // type of i is type of x (interface{})
6408 v := x // x is evaluated exactly once
6410 i := v // type of i is type of x (interface{})
6411 printString("x is nil")
6412 } else if i, isInt := v.(int); isInt {
6413 printInt(i) // type of i is int
6414 } else if i, isFloat64 := v.(float64); isFloat64 {
6415 printFloat64(i) // type of i is float64
6416 } else if i, isFunc := v.(func(int) float64); isFunc {
6417 printFunction(i) // type of i is func(int) float64
6419 _, isBool := v.(bool)
6420 _, isString := v.(string)
6421 if isBool || isString {
6422 i := v // type of i is type of x (interface{})
6423 printString("type is bool or string")
6425 i := v // type of i is type of x (interface{})
6426 printString("don't know the type")
6432 A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a>
6433 may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns
6434 out to duplicate another entry in the switch, the first matching case is chosen.
6438 func f[P any](x any) int {
6453 var v1 = f[string]("foo") // v1 == 0
6454 var v2 = f[byte]([]byte{}) // v2 == 2
6458 The type switch guard may be preceded by a simple statement, which
6459 executes before the guard is evaluated.
6463 The "fallthrough" statement is not permitted in a type switch.
6466 <h3 id="For_statements">For statements</h3>
6469 A "for" statement specifies repeated execution of a block. There are three forms:
6470 The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
6474 ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
6475 Condition = Expression .
6478 <h4 id="For_condition">For statements with single condition</h4>
6481 In its simplest form, a "for" statement specifies the repeated execution of
6482 a block as long as a boolean condition evaluates to true.
6483 The condition is evaluated before each iteration.
6484 If the condition is absent, it is equivalent to the boolean value
6494 <h4 id="For_clause">For statements with <code>for</code> clause</h4>
6497 A "for" statement with a ForClause is also controlled by its condition, but
6498 additionally it may specify an <i>init</i>
6499 and a <i>post</i> statement, such as an assignment,
6500 an increment or decrement statement. The init statement may be a
6501 <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
6502 Variables declared by the init statement are re-used in each iteration.
6506 ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
6507 InitStmt = SimpleStmt .
6508 PostStmt = SimpleStmt .
6512 for i := 0; i < 10; i++ {
6518 If non-empty, the init statement is executed once before evaluating the
6519 condition for the first iteration;
6520 the post statement is executed after each execution of the block (and
6521 only if the block was executed).
6522 Any element of the ForClause may be empty but the
6523 <a href="#Semicolons">semicolons</a> are
6524 required unless there is only a condition.
6525 If the condition is absent, it is equivalent to the boolean value
6530 for cond { S() } is the same as for ; cond ; { S() }
6531 for { S() } is the same as for true { S() }
6534 <h4 id="For_range">For statements with <code>range</code> clause</h4>
6537 A "for" statement with a "range" clause
6538 iterates through all entries of an array, slice, string or map,
6539 or values received on a channel. For each entry it assigns <i>iteration values</i>
6540 to corresponding <i>iteration variables</i> if present and then executes the block.
6544 RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
6548 The expression on the right in the "range" clause is called the <i>range expression</i>,
6549 its <a href="#Core_types">core type</a> must be
6550 an array, pointer to an array, slice, string, map, or channel permitting
6551 <a href="#Receive_operator">receive operations</a>.
6552 As with an assignment, if present the operands on the left must be
6553 <a href="#Address_operators">addressable</a> or map index expressions; they
6554 denote the iteration variables. If the range expression is a channel, at most
6555 one iteration variable is permitted, otherwise there may be up to two.
6556 If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
6557 the range clause is equivalent to the same clause without that identifier.
6561 The range expression <code>x</code> is evaluated once before beginning the loop,
6562 with one exception: if at most one iteration variable is present and
6563 <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
6564 the range expression is not evaluated.
6568 Function calls on the left are evaluated once per iteration.
6569 For each iteration, iteration values are produced as follows
6570 if the respective iteration variables are present:
6573 <pre class="grammar">
6574 Range expression 1st value 2nd value
6576 array or slice a [n]E, *[n]E, or []E index i int a[i] E
6577 string s string type index i int see below rune
6578 map m map[K]V key k K m[k] V
6579 channel c chan E, <-chan E element e E
6584 For an array, pointer to array, or slice value <code>a</code>, the index iteration
6585 values are produced in increasing order, starting at element index 0.
6586 If at most one iteration variable is present, the range loop produces
6587 iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
6588 or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
6592 For a string value, the "range" clause iterates over the Unicode code points
6593 in the string starting at byte index 0. On successive iterations, the index value will be the
6594 index of the first byte of successive UTF-8-encoded code points in the string,
6595 and the second value, of type <code>rune</code>, will be the value of
6596 the corresponding code point. If the iteration encounters an invalid
6597 UTF-8 sequence, the second value will be <code>0xFFFD</code>,
6598 the Unicode replacement character, and the next iteration will advance
6599 a single byte in the string.
6603 The iteration order over maps is not specified
6604 and is not guaranteed to be the same from one iteration to the next.
6605 If a map entry that has not yet been reached is removed during iteration,
6606 the corresponding iteration value will not be produced. If a map entry is
6607 created during iteration, that entry may be produced during the iteration or
6608 may be skipped. The choice may vary for each entry created and from one
6609 iteration to the next.
6610 If the map is <code>nil</code>, the number of iterations is 0.
6614 For channels, the iteration values produced are the successive values sent on
6615 the channel until the channel is <a href="#Close">closed</a>. If the channel
6616 is <code>nil</code>, the range expression blocks forever.
6621 The iteration values are assigned to the respective
6622 iteration variables as in an <a href="#Assignment_statements">assignment statement</a>.
6626 The iteration variables may be declared by the "range" clause using a form of
6627 <a href="#Short_variable_declarations">short variable declaration</a>
6629 In this case their types are set to the types of the respective iteration values
6630 and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
6631 statement; they are re-used in each iteration.
6632 If the iteration variables are declared outside the "for" statement,
6633 after execution their values will be those of the last iteration.
6637 var testdata *struct {
6640 for i, _ := range testdata.a {
6641 // testdata.a is never evaluated; len(testdata.a) is constant
6642 // i ranges from 0 to 6
6647 for i, s := range a {
6649 // type of s is string
6655 var val interface{} // element type of m is assignable to val
6656 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
6657 for key, val = range m {
6660 // key == last map key encountered in iteration
6663 var ch chan Work = producer()
6673 <h3 id="Go_statements">Go statements</h3>
6676 A "go" statement starts the execution of a function call
6677 as an independent concurrent thread of control, or <i>goroutine</i>,
6678 within the same address space.
6682 GoStmt = "go" Expression .
6686 The expression must be a function or method call; it cannot be parenthesized.
6687 Calls of built-in functions are restricted as for
6688 <a href="#Expression_statements">expression statements</a>.
6692 The function value and parameters are
6693 <a href="#Calls">evaluated as usual</a>
6694 in the calling goroutine, but
6695 unlike with a regular call, program execution does not wait
6696 for the invoked function to complete.
6697 Instead, the function begins executing independently
6699 When the function terminates, its goroutine also terminates.
6700 If the function has any return values, they are discarded when the
6706 go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
6710 <h3 id="Select_statements">Select statements</h3>
6713 A "select" statement chooses which of a set of possible
6714 <a href="#Send_statements">send</a> or
6715 <a href="#Receive_operator">receive</a>
6716 operations will proceed.
6717 It looks similar to a
6718 <a href="#Switch_statements">"switch"</a> statement but with the
6719 cases all referring to communication operations.
6723 SelectStmt = "select" "{" { CommClause } "}" .
6724 CommClause = CommCase ":" StatementList .
6725 CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
6726 RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
6727 RecvExpr = Expression .
6731 A case with a RecvStmt may assign the result of a RecvExpr to one or
6732 two variables, which may be declared using a
6733 <a href="#Short_variable_declarations">short variable declaration</a>.
6734 The RecvExpr must be a (possibly parenthesized) receive operation.
6735 There can be at most one default case and it may appear anywhere
6736 in the list of cases.
6740 Execution of a "select" statement proceeds in several steps:
6745 For all the cases in the statement, the channel operands of receive operations
6746 and the channel and right-hand-side expressions of send statements are
6747 evaluated exactly once, in source order, upon entering the "select" statement.
6748 The result is a set of channels to receive from or send to,
6749 and the corresponding values to send.
6750 Any side effects in that evaluation will occur irrespective of which (if any)
6751 communication operation is selected to proceed.
6752 Expressions on the left-hand side of a RecvStmt with a short variable declaration
6753 or assignment are not yet evaluated.
6757 If one or more of the communications can proceed,
6758 a single one that can proceed is chosen via a uniform pseudo-random selection.
6759 Otherwise, if there is a default case, that case is chosen.
6760 If there is no default case, the "select" statement blocks until
6761 at least one of the communications can proceed.
6765 Unless the selected case is the default case, the respective communication
6766 operation is executed.
6770 If the selected case is a RecvStmt with a short variable declaration or
6771 an assignment, the left-hand side expressions are evaluated and the
6772 received value (or values) are assigned.
6776 The statement list of the selected case is executed.
6781 Since communication on <code>nil</code> channels can never proceed,
6782 a select with only <code>nil</code> channels and no default case blocks forever.
6787 var c, c1, c2, c3, c4 chan int
6791 print("received ", i1, " from c1\n")
6793 print("sent ", i2, " to c2\n")
6794 case i3, ok := (<-c3): // same as: i3, ok := <-c3
6796 print("received ", i3, " from c3\n")
6798 print("c3 is closed\n")
6800 case a[f()] = <-c4:
6802 // case t := <-c4
6805 print("no communication\n")
6808 for { // send random sequence of bits to c
6810 case c <- 0: // note: no statement, no fallthrough, no folding of cases
6815 select {} // block forever
6819 <h3 id="Return_statements">Return statements</h3>
6822 A "return" statement in a function <code>F</code> terminates the execution
6823 of <code>F</code>, and optionally provides one or more result values.
6824 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6825 are executed before <code>F</code> returns to its caller.
6829 ReturnStmt = "return" [ ExpressionList ] .
6833 In a function without a result type, a "return" statement must not
6834 specify any result values.
6843 There are three ways to return values from a function with a result
6848 <li>The return value or values may be explicitly listed
6849 in the "return" statement. Each expression must be single-valued
6850 and <a href="#Assignability">assignable</a>
6851 to the corresponding element of the function's result type.
6853 func simpleF() int {
6857 func complexF1() (re float64, im float64) {
6862 <li>The expression list in the "return" statement may be a single
6863 call to a multi-valued function. The effect is as if each value
6864 returned from that function were assigned to a temporary
6865 variable with the type of the respective value, followed by a
6866 "return" statement listing these variables, at which point the
6867 rules of the previous case apply.
6869 func complexF2() (re float64, im float64) {
6874 <li>The expression list may be empty if the function's result
6875 type specifies names for its <a href="#Function_types">result parameters</a>.
6876 The result parameters act as ordinary local variables
6877 and the function may assign values to them as necessary.
6878 The "return" statement returns the values of these variables.
6880 func complexF3() (re float64, im float64) {
6886 func (devnull) Write(p []byte) (n int, _ error) {
6895 Regardless of how they are declared, all the result values are initialized to
6896 the <a href="#The_zero_value">zero values</a> for their type upon entry to the
6897 function. A "return" statement that specifies results sets the result parameters before
6898 any deferred functions are executed.
6902 Implementation restriction: A compiler may disallow an empty expression list
6903 in a "return" statement if a different entity (constant, type, or variable)
6904 with the same name as a result parameter is in
6905 <a href="#Declarations_and_scope">scope</a> at the place of the return.
6909 func f(n int) (res int, err error) {
6910 if _, err := f(n-1); err != nil {
6911 return // invalid return statement: err is shadowed
6917 <h3 id="Break_statements">Break statements</h3>
6920 A "break" statement terminates execution of the innermost
6921 <a href="#For_statements">"for"</a>,
6922 <a href="#Switch_statements">"switch"</a>, or
6923 <a href="#Select_statements">"select"</a> statement
6924 within the same function.
6928 BreakStmt = "break" [ Label ] .
6932 If there is a label, it must be that of an enclosing
6933 "for", "switch", or "select" statement,
6934 and that is the one whose execution terminates.
6939 for i = 0; i < n; i++ {
6940 for j = 0; j < m; j++ {
6953 <h3 id="Continue_statements">Continue statements</h3>
6956 A "continue" statement begins the next iteration of the
6957 innermost enclosing <a href="#For_statements">"for" loop</a>
6958 by advancing control to the end of the loop block.
6959 The "for" loop must be within the same function.
6963 ContinueStmt = "continue" [ Label ] .
6967 If there is a label, it must be that of an enclosing
6968 "for" statement, and that is the one whose execution
6974 for y, row := range rows {
6975 for x, data := range row {
6976 if data == endOfRow {
6979 row[x] = data + bias(x, y)
6984 <h3 id="Goto_statements">Goto statements</h3>
6987 A "goto" statement transfers control to the statement with the corresponding label
6988 within the same function.
6992 GotoStmt = "goto" Label .
7000 Executing the "goto" statement must not cause any variables to come into
7001 <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
7002 For instance, this example:
7012 is erroneous because the jump to label <code>L</code> skips
7013 the creation of <code>v</code>.
7017 A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
7018 For instance, this example:
7035 is erroneous because the label <code>L1</code> is inside
7036 the "for" statement's block but the <code>goto</code> is not.
7039 <h3 id="Fallthrough_statements">Fallthrough statements</h3>
7042 A "fallthrough" statement transfers control to the first statement of the
7043 next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
7044 It may be used only as the final non-empty statement in such a clause.
7048 FallthroughStmt = "fallthrough" .
7052 <h3 id="Defer_statements">Defer statements</h3>
7055 A "defer" statement invokes a function whose execution is deferred
7056 to the moment the surrounding function returns, either because the
7057 surrounding function executed a <a href="#Return_statements">return statement</a>,
7058 reached the end of its <a href="#Function_declarations">function body</a>,
7059 or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
7063 DeferStmt = "defer" Expression .
7067 The expression must be a function or method call; it cannot be parenthesized.
7068 Calls of built-in functions are restricted as for
7069 <a href="#Expression_statements">expression statements</a>.
7073 Each time a "defer" statement
7074 executes, the function value and parameters to the call are
7075 <a href="#Calls">evaluated as usual</a>
7076 and saved anew but the actual function is not invoked.
7077 Instead, deferred functions are invoked immediately before
7078 the surrounding function returns, in the reverse order
7079 they were deferred. That is, if the surrounding function
7080 returns through an explicit <a href="#Return_statements">return statement</a>,
7081 deferred functions are executed <i>after</i> any result parameters are set
7082 by that return statement but <i>before</i> the function returns to its caller.
7083 If a deferred function value evaluates
7084 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
7085 when the function is invoked, not when the "defer" statement is executed.
7089 For instance, if the deferred function is
7090 a <a href="#Function_literals">function literal</a> and the surrounding
7091 function has <a href="#Function_types">named result parameters</a> that
7092 are in scope within the literal, the deferred function may access and modify
7093 the result parameters before they are returned.
7094 If the deferred function has any return values, they are discarded when
7095 the function completes.
7096 (See also the section on <a href="#Handling_panics">handling panics</a>.)
7101 defer unlock(l) // unlocking happens before surrounding function returns
7103 // prints 3 2 1 0 before surrounding function returns
7104 for i := 0; i <= 3; i++ {
7109 func f() (result int) {
7111 // result is accessed after it was set to 6 by the return statement
7118 <h2 id="Built-in_functions">Built-in functions</h2>
7121 Built-in functions are
7122 <a href="#Predeclared_identifiers">predeclared</a>.
7123 They are called like any other function but some of them
7124 accept a type instead of an expression as the first argument.
7128 The built-in functions do not have standard Go types,
7129 so they can only appear in <a href="#Calls">call expressions</a>;
7130 they cannot be used as function values.
7133 <h3 id="Close">Close</h3>
7136 For an argument <code>ch</code> with a <a href="#Core_types">core type</a>
7137 that is a <a href="#Channel_types">channel</a>, the built-in function <code>close</code>
7138 records that no more values will be sent on the channel.
7139 It is an error if <code>ch</code> is a receive-only channel.
7140 Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
7141 Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
7142 After calling <code>close</code>, and after any previously
7143 sent values have been received, receive operations will return
7144 the zero value for the channel's type without blocking.
7145 The multi-valued <a href="#Receive_operator">receive operation</a>
7146 returns a received value along with an indication of whether the channel is closed.
7149 <h3 id="Length_and_capacity">Length and capacity</h3>
7152 The built-in functions <code>len</code> and <code>cap</code> take arguments
7153 of various types and return a result of type <code>int</code>.
7154 The implementation guarantees that the result always fits into an <code>int</code>.
7157 <pre class="grammar">
7158 Call Argument type Result
7160 len(s) string type string length in bytes
7161 [n]T, *[n]T array length (== n)
7163 map[K]T map length (number of defined keys)
7164 chan T number of elements queued in channel buffer
7165 type parameter see below
7167 cap(s) [n]T, *[n]T array length (== n)
7169 chan T channel buffer capacity
7170 type parameter see below
7174 If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>,
7175 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
7176 each type in <code>P</code>'s type set.
7177 The result is the length (or capacity, respectively) of the argument whose type
7178 corresponds to the type argument with which <code>P</code> was
7179 <a href="#Instantiations">instantiated</a>.
7183 The capacity of a slice is the number of elements for which there is
7184 space allocated in the underlying array.
7185 At any time the following relationship holds:
7189 0 <= len(s) <= cap(s)
7193 The length of a <code>nil</code> slice, map or channel is 0.
7194 The capacity of a <code>nil</code> slice or channel is 0.
7198 The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
7199 <code>s</code> is a string constant. The expressions <code>len(s)</code> and
7200 <code>cap(s)</code> are constants if the type of <code>s</code> is an array
7201 or pointer to an array and the expression <code>s</code> does not contain
7202 <a href="#Receive_operator">channel receives</a> or (non-constant)
7203 <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
7204 Otherwise, invocations of <code>len</code> and <code>cap</code> are not
7205 constant and <code>s</code> is evaluated.
7210 c1 = imag(2i) // imag(2i) = 2.0 is a constant
7211 c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
7212 c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
7213 c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
7214 c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
7219 <h3 id="Allocation">Allocation</h3>
7222 The built-in function <code>new</code> takes a type <code>T</code>,
7223 allocates storage for a <a href="#Variables">variable</a> of that type
7224 at run time, and returns a value of type <code>*T</code>
7225 <a href="#Pointer_types">pointing</a> to it.
7226 The variable is initialized as described in the section on
7227 <a href="#The_zero_value">initial values</a>.
7230 <pre class="grammar">
7239 type S struct { a int; b float64 }
7244 allocates storage for a variable of type <code>S</code>,
7245 initializes it (<code>a=0</code>, <code>b=0.0</code>),
7246 and returns a value of type <code>*S</code> containing the address
7250 <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
7253 The built-in function <code>make</code> takes a type <code>T</code>,
7254 optionally followed by a type-specific list of expressions.
7255 The <a href="#Core_types">core type</a> of <code>T</code> must
7256 be a slice, map or channel.
7257 It returns a value of type <code>T</code> (not <code>*T</code>).
7258 The memory is initialized as described in the section on
7259 <a href="#The_zero_value">initial values</a>.
7262 <pre class="grammar">
7263 Call Core type Result
7265 make(T, n) slice slice of type T with length n and capacity n
7266 make(T, n, m) slice slice of type T with length n and capacity m
7268 make(T) map map of type T
7269 make(T, n) map map of type T with initial space for approximately n elements
7271 make(T) channel unbuffered channel of type T
7272 make(T, n) channel buffered channel of type T, buffer size n
7277 Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>,
7278 have a <a href="#Interface_types">type set</a> containing only integer types,
7279 or be an untyped <a href="#Constants">constant</a>.
7280 A constant size argument must be non-negative and <a href="#Representability">representable</a>
7281 by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
7282 If both <code>n</code> and <code>m</code> are provided and are constant, then
7283 <code>n</code> must be no larger than <code>m</code>.
7284 For slices and channels, if <code>n</code> is negative or larger than <code>m</code> at run time,
7285 a <a href="#Run_time_panics">run-time panic</a> occurs.
7289 s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
7290 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
7291 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
7292 s := make([]int, 10, 0) // illegal: len(s) > cap(s)
7293 c := make(chan int, 10) // channel with a buffer size of 10
7294 m := make(map[string]int, 100) // map with initial space for approximately 100 elements
7298 Calling <code>make</code> with a map type and size hint <code>n</code> will
7299 create a map with initial space to hold <code>n</code> map elements.
7300 The precise behavior is implementation-dependent.
7304 <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
7307 The built-in functions <code>append</code> and <code>copy</code> assist in
7308 common slice operations.
7309 For both functions, the result is independent of whether the memory referenced
7310 by the arguments overlaps.
7314 The <a href="#Function_types">variadic</a> function <code>append</code>
7315 appends zero or more values <code>x</code> to a slice <code>s</code>
7316 and returns the resulting slice of the same type as <code>s</code>.
7317 The <a href="#Core_types">core type</a> of <code>s</code> must be a slice
7318 of type <code>[]E</code>.
7319 The values <code>x</code> are passed to a parameter of type <code>...E</code>
7320 and the respective <a href="#Passing_arguments_to_..._parameters">parameter
7321 passing rules</a> apply.
7322 As a special case, if the core type of <code>s</code> is <code>[]byte</code>,
7323 <code>append</code> also accepts a second argument with core type
7324 <a href="#Core_types"><code>bytestring</code></a> followed by <code>...</code>.
7325 This form appends the bytes of the byte slice or string.
7328 <pre class="grammar">
7329 append(s S, x ...E) S // core type of S is []E
7333 If the capacity of <code>s</code> is not large enough to fit the additional
7334 values, <code>append</code> <a href="#Allocation">allocates</a> a new, sufficiently large underlying
7335 array that fits both the existing slice elements and the additional values.
7336 Otherwise, <code>append</code> re-uses the underlying array.
7341 s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
7342 s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
7343 s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
7344 s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
7347 t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"}
7350 b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
7354 The function <code>copy</code> copies slice elements from
7355 a source <code>src</code> to a destination <code>dst</code> and returns the
7356 number of elements copied.
7357 The <a href="#Core_types">core types</a> of both arguments must be slices
7358 with <a href="#Type_identity">identical</a> element type.
7359 The number of elements copied is the minimum of
7360 <code>len(src)</code> and <code>len(dst)</code>.
7361 As a special case, if the destination's core type is <code>[]byte</code>,
7362 <code>copy</code> also accepts a source argument with core type
7363 </a> <a href="#Core_types"><code>bytestring</code></a>.
7364 This form copies the bytes from the byte slice or string into the byte slice.
7367 <pre class="grammar">
7368 copy(dst, src []T) int
7369 copy(dst []byte, src string) int
7377 var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
7378 var s = make([]int, 6)
7379 var b = make([]byte, 5)
7380 n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
7381 n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
7382 n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
7386 <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
7389 The built-in function <code>delete</code> removes the element with key
7390 <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
7391 value <code>k</code> must be <a href="#Assignability">assignable</a>
7392 to the key type of <code>m</code>.
7395 <pre class="grammar">
7396 delete(m, k) // remove element m[k] from map m
7400 If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>,
7401 all types in that type set must be maps, and they must all have identical key types.
7405 If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
7406 does not exist, <code>delete</code> is a no-op.
7410 <h3 id="Complex_numbers">Manipulating complex numbers</h3>
7413 Three functions assemble and disassemble complex numbers.
7414 The built-in function <code>complex</code> constructs a complex
7415 value from a floating-point real and imaginary part, while
7416 <code>real</code> and <code>imag</code>
7417 extract the real and imaginary parts of a complex value.
7420 <pre class="grammar">
7421 complex(realPart, imaginaryPart floatT) complexT
7422 real(complexT) floatT
7423 imag(complexT) floatT
7427 The type of the arguments and return value correspond.
7428 For <code>complex</code>, the two arguments must be of the same
7429 <a href="#Numeric_types">floating-point type</a> and the return type is the
7430 <a href="#Numeric_types">complex type</a>
7431 with the corresponding floating-point constituents:
7432 <code>complex64</code> for <code>float32</code> arguments, and
7433 <code>complex128</code> for <code>float64</code> arguments.
7434 If one of the arguments evaluates to an untyped constant, it is first implicitly
7435 <a href="#Conversions">converted</a> to the type of the other argument.
7436 If both arguments evaluate to untyped constants, they must be non-complex
7437 numbers or their imaginary parts must be zero, and the return value of
7438 the function is an untyped complex constant.
7442 For <code>real</code> and <code>imag</code>, the argument must be
7443 of complex type, and the return type is the corresponding floating-point
7444 type: <code>float32</code> for a <code>complex64</code> argument, and
7445 <code>float64</code> for a <code>complex128</code> argument.
7446 If the argument evaluates to an untyped constant, it must be a number,
7447 and the return value of the function is an untyped floating-point constant.
7451 The <code>real</code> and <code>imag</code> functions together form the inverse of
7452 <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
7453 <code>z == Z(complex(real(z), imag(z)))</code>.
7457 If the operands of these functions are all constants, the return
7458 value is a constant.
7462 var a = complex(2, -2) // complex128
7463 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
7464 x := float32(math.Cos(math.Pi/2)) // float32
7465 var c64 = complex(5, -x) // complex64
7466 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
7467 _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
7468 var rl = real(c64) // float32
7469 var im = imag(a) // float64
7470 const c = imag(b) // untyped constant -1.4
7471 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
7475 Arguments of type parameter type are not permitted.
7478 <h3 id="Handling_panics">Handling panics</h3>
7480 <p> Two built-in functions, <code>panic</code> and <code>recover</code>,
7481 assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
7482 and program-defined error conditions.
7485 <pre class="grammar">
7486 func panic(interface{})
7487 func recover() interface{}
7491 While executing a function <code>F</code>,
7492 an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
7493 terminates the execution of <code>F</code>.
7494 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
7495 are then executed as usual.
7496 Next, any deferred functions run by <code>F</code>'s caller are run,
7497 and so on up to any deferred by the top-level function in the executing goroutine.
7498 At that point, the program is terminated and the error
7499 condition is reported, including the value of the argument to <code>panic</code>.
7500 This termination sequence is called <i>panicking</i>.
7505 panic("unreachable")
7506 panic(Error("cannot parse"))
7510 The <code>recover</code> function allows a program to manage behavior
7511 of a panicking goroutine.
7512 Suppose a function <code>G</code> defers a function <code>D</code> that calls
7513 <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
7515 When the running of deferred functions reaches <code>D</code>,
7516 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>.
7517 If <code>D</code> returns normally, without starting a new
7518 <code>panic</code>, the panicking sequence stops. In that case,
7519 the state of functions called between <code>G</code> and the call to <code>panic</code>
7520 is discarded, and normal execution resumes.
7521 Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
7522 execution terminates by returning to its caller.
7526 The return value of <code>recover</code> is <code>nil</code> if any of the following conditions holds:
7530 <code>panic</code>'s argument was <code>nil</code>;
7533 the goroutine is not panicking;
7536 <code>recover</code> was not called directly by a deferred function.
7541 The <code>protect</code> function in the example below invokes
7542 the function argument <code>g</code> and protects callers from
7543 run-time panics raised by <code>g</code>.
7547 func protect(g func()) {
7549 log.Println("done") // Println executes normally even if there is a panic
7550 if x := recover(); x != nil {
7551 log.Printf("run time panic: %v", x)
7554 log.Println("start")
7560 <h3 id="Bootstrapping">Bootstrapping</h3>
7563 Current implementations provide several built-in functions useful during
7564 bootstrapping. These functions are documented for completeness but are not
7565 guaranteed to stay in the language. They do not return a result.
7568 <pre class="grammar">
7571 print prints all arguments; formatting of arguments is implementation-specific
7572 println like print but prints spaces between arguments and a newline at the end
7576 Implementation restriction: <code>print</code> and <code>println</code> need not
7577 accept arbitrary argument types, but printing of boolean, numeric, and string
7578 <a href="#Types">types</a> must be supported.
7581 <h2 id="Packages">Packages</h2>
7584 Go programs are constructed by linking together <i>packages</i>.
7585 A package in turn is constructed from one or more source files
7586 that together declare constants, types, variables and functions
7587 belonging to the package and which are accessible in all files
7588 of the same package. Those elements may be
7589 <a href="#Exported_identifiers">exported</a> and used in another package.
7592 <h3 id="Source_file_organization">Source file organization</h3>
7595 Each source file consists of a package clause defining the package
7596 to which it belongs, followed by a possibly empty set of import
7597 declarations that declare packages whose contents it wishes to use,
7598 followed by a possibly empty set of declarations of functions,
7599 types, variables, and constants.
7603 SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
7606 <h3 id="Package_clause">Package clause</h3>
7609 A package clause begins each source file and defines the package
7610 to which the file belongs.
7614 PackageClause = "package" PackageName .
7615 PackageName = identifier .
7619 The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
7627 A set of files sharing the same PackageName form the implementation of a package.
7628 An implementation may require that all source files for a package inhabit the same directory.
7631 <h3 id="Import_declarations">Import declarations</h3>
7634 An import declaration states that the source file containing the declaration
7635 depends on functionality of the <i>imported</i> package
7636 (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
7637 and enables access to <a href="#Exported_identifiers">exported</a> identifiers
7639 The import names an identifier (PackageName) to be used for access and an ImportPath
7640 that specifies the package to be imported.
7644 ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
7645 ImportSpec = [ "." | PackageName ] ImportPath .
7646 ImportPath = string_lit .
7650 The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
7651 to access exported identifiers of the package within the importing source file.
7652 It is declared in the <a href="#Blocks">file block</a>.
7653 If the PackageName is omitted, it defaults to the identifier specified in the
7654 <a href="#Package_clause">package clause</a> of the imported package.
7655 If an explicit period (<code>.</code>) appears instead of a name, all the
7656 package's exported identifiers declared in that package's
7657 <a href="#Blocks">package block</a> will be declared in the importing source
7658 file's file block and must be accessed without a qualifier.
7662 The interpretation of the ImportPath is implementation-dependent but
7663 it is typically a substring of the full file name of the compiled
7664 package and may be relative to a repository of installed packages.
7668 Implementation restriction: A compiler may restrict ImportPaths to
7669 non-empty strings using only characters belonging to
7670 <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
7671 L, M, N, P, and S general categories (the Graphic characters without
7672 spaces) and may also exclude the characters
7673 <code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
7674 and the Unicode replacement character U+FFFD.
7678 Consider a compiled a package containing the package clause
7679 <code>package math</code>, which exports function <code>Sin</code>, and
7680 installed the compiled package in the file identified by
7681 <code>"lib/math"</code>.
7682 This table illustrates how <code>Sin</code> is accessed in files
7683 that import the package after the
7684 various types of import declaration.
7687 <pre class="grammar">
7688 Import declaration Local name of Sin
7690 import "lib/math" math.Sin
7691 import m "lib/math" m.Sin
7692 import . "lib/math" Sin
7696 An import declaration declares a dependency relation between
7697 the importing and imported package.
7698 It is illegal for a package to import itself, directly or indirectly,
7699 or to directly import a package without
7700 referring to any of its exported identifiers. To import a package solely for
7701 its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
7702 identifier as explicit package name:
7710 <h3 id="An_example_package">An example package</h3>
7713 Here is a complete Go package that implements a concurrent prime sieve.
7721 // Send the sequence 2, 3, 4, … to channel 'ch'.
7722 func generate(ch chan<- int) {
7724 ch <- i // Send 'i' to channel 'ch'.
7728 // Copy the values from channel 'src' to channel 'dst',
7729 // removing those divisible by 'prime'.
7730 func filter(src <-chan int, dst chan<- int, prime int) {
7731 for i := range src { // Loop over values received from 'src'.
7733 dst <- i // Send 'i' to channel 'dst'.
7738 // The prime sieve: Daisy-chain filter processes together.
7740 ch := make(chan int) // Create a new channel.
7741 go generate(ch) // Start generate() as a subprocess.
7744 fmt.Print(prime, "\n")
7745 ch1 := make(chan int)
7746 go filter(ch, ch1, prime)
7756 <h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
7758 <h3 id="The_zero_value">The zero value</h3>
7760 When storage is allocated for a <a href="#Variables">variable</a>,
7761 either through a declaration or a call of <code>new</code>, or when
7762 a new value is created, either through a composite literal or a call
7763 of <code>make</code>,
7764 and no explicit initialization is provided, the variable or value is
7765 given a default value. Each element of such a variable or value is
7766 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
7767 <code>0</code> for numeric types, <code>""</code>
7768 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
7769 This initialization is done recursively, so for instance each element of an
7770 array of structs will have its fields zeroed if no value is specified.
7773 These two simple declarations are equivalent:
7786 type T struct { i int; f float64; next *T }
7791 the following holds:
7801 The same would also be true after
7808 <h3 id="Package_initialization">Package initialization</h3>
7811 Within a package, package-level variable initialization proceeds stepwise,
7812 with each step selecting the variable earliest in <i>declaration order</i>
7813 which has no dependencies on uninitialized variables.
7817 More precisely, a package-level variable is considered <i>ready for
7818 initialization</i> if it is not yet initialized and either has
7819 no <a href="#Variable_declarations">initialization expression</a> or
7820 its initialization expression has no <i>dependencies</i> on uninitialized variables.
7821 Initialization proceeds by repeatedly initializing the next package-level
7822 variable that is earliest in declaration order and ready for initialization,
7823 until there are no variables ready for initialization.
7827 If any variables are still uninitialized when this
7828 process ends, those variables are part of one or more initialization cycles,
7829 and the program is not valid.
7833 Multiple variables on the left-hand side of a variable declaration initialized
7834 by single (multi-valued) expression on the right-hand side are initialized
7835 together: If any of the variables on the left-hand side is initialized, all
7836 those variables are initialized in the same step.
7841 var a, b = f() // a and b are initialized together, before x is initialized
7845 For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
7846 variables are treated like any other variables in declarations.
7850 The declaration order of variables declared in multiple files is determined
7851 by the order in which the files are presented to the compiler: Variables
7852 declared in the first file are declared before any of the variables declared
7853 in the second file, and so on.
7857 Dependency analysis does not rely on the actual values of the
7858 variables, only on lexical <i>references</i> to them in the source,
7859 analyzed transitively. For instance, if a variable <code>x</code>'s
7860 initialization expression refers to a function whose body refers to
7861 variable <code>y</code> then <code>x</code> depends on <code>y</code>.
7867 A reference to a variable or function is an identifier denoting that
7868 variable or function.
7872 A reference to a method <code>m</code> is a
7873 <a href="#Method_values">method value</a> or
7874 <a href="#Method_expressions">method expression</a> of the form
7875 <code>t.m</code>, where the (static) type of <code>t</code> is
7876 not an interface type, and the method <code>m</code> is in the
7877 <a href="#Method_sets">method set</a> of <code>t</code>.
7878 It is immaterial whether the resulting function value
7879 <code>t.m</code> is invoked.
7883 A variable, function, or method <code>x</code> depends on a variable
7884 <code>y</code> if <code>x</code>'s initialization expression or body
7885 (for functions and methods) contains a reference to <code>y</code>
7886 or to a function or method that depends on <code>y</code>.
7891 For example, given the declarations
7899 d = 3 // == 5 after initialization has finished
7909 the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
7910 Note that the order of subexpressions in initialization expressions is irrelevant:
7911 <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
7912 order in this example.
7916 Dependency analysis is performed per package; only references referring
7917 to variables, functions, and (non-interface) methods declared in the current
7918 package are considered. If other, hidden, data dependencies exists between
7919 variables, the initialization order between those variables is unspecified.
7923 For instance, given the declarations
7927 var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
7928 var _ = sideEffect() // unrelated to x, a, or b
7932 type I interface { ab() []int }
7934 func (T) ab() []int { return []int{a, b} }
7938 the variable <code>a</code> will be initialized after <code>b</code> but
7939 whether <code>x</code> is initialized before <code>b</code>, between
7940 <code>b</code> and <code>a</code>, or after <code>a</code>, and
7941 thus also the moment at which <code>sideEffect()</code> is called (before
7942 or after <code>x</code> is initialized) is not specified.
7946 Variables may also be initialized using functions named <code>init</code>
7947 declared in the package block, with no arguments and no result parameters.
7955 Multiple such functions may be defined per package, even within a single
7956 source file. In the package block, the <code>init</code> identifier can
7957 be used only to declare <code>init</code> functions, yet the identifier
7958 itself is not <a href="#Declarations_and_scope">declared</a>. Thus
7959 <code>init</code> functions cannot be referred to from anywhere
7964 A package with no imports is initialized by assigning initial values
7965 to all its package-level variables followed by calling all <code>init</code>
7966 functions in the order they appear in the source, possibly in multiple files,
7967 as presented to the compiler.
7968 If a package has imports, the imported packages are initialized
7969 before initializing the package itself. If multiple packages import
7970 a package, the imported package will be initialized only once.
7971 The importing of packages, by construction, guarantees that there
7972 can be no cyclic initialization dependencies.
7976 Package initialization—variable initialization and the invocation of
7977 <code>init</code> functions—happens in a single goroutine,
7978 sequentially, one package at a time.
7979 An <code>init</code> function may launch other goroutines, which can run
7980 concurrently with the initialization code. However, initialization
7982 the <code>init</code> functions: it will not invoke the next one
7983 until the previous one has returned.
7987 To ensure reproducible initialization behavior, build systems are encouraged
7988 to present multiple files belonging to the same package in lexical file name
7989 order to a compiler.
7993 <h3 id="Program_execution">Program execution</h3>
7995 A complete program is created by linking a single, unimported package
7996 called the <i>main package</i> with all the packages it imports, transitively.
7997 The main package must
7998 have package name <code>main</code> and
7999 declare a function <code>main</code> that takes no
8000 arguments and returns no value.
8008 Program execution begins by initializing the main package and then
8009 invoking the function <code>main</code>.
8010 When that function invocation returns, the program exits.
8011 It does not wait for other (non-<code>main</code>) goroutines to complete.
8014 <h2 id="Errors">Errors</h2>
8017 The predeclared type <code>error</code> is defined as
8021 type error interface {
8027 It is the conventional interface for representing an error condition,
8028 with the nil value representing no error.
8029 For instance, a function to read data from a file might be defined:
8033 func Read(f *File, b []byte) (n int, err error)
8036 <h2 id="Run_time_panics">Run-time panics</h2>
8039 Execution errors such as attempting to index an array out
8040 of bounds trigger a <i>run-time panic</i> equivalent to a call of
8041 the built-in function <a href="#Handling_panics"><code>panic</code></a>
8042 with a value of the implementation-defined interface type <code>runtime.Error</code>.
8043 That type satisfies the predeclared interface type
8044 <a href="#Errors"><code>error</code></a>.
8045 The exact error values that
8046 represent distinct run-time error conditions are unspecified.
8052 type Error interface {
8054 // and perhaps other methods
8058 <h2 id="System_considerations">System considerations</h2>
8060 <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
8063 The built-in package <code>unsafe</code>, known to the compiler
8064 and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
8065 provides facilities for low-level programming including operations
8066 that violate the type system. A package using <code>unsafe</code>
8067 must be vetted manually for type safety and may not be portable.
8068 The package provides the following interface:
8071 <pre class="grammar">
8074 type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
8075 type Pointer *ArbitraryType
8077 func Alignof(variable ArbitraryType) uintptr
8078 func Offsetof(selector ArbitraryType) uintptr
8079 func Sizeof(variable ArbitraryType) uintptr
8081 type IntegerType int // shorthand for an integer type; it is not a real type
8082 func Add(ptr Pointer, len IntegerType) Pointer
8083 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
8084 func SliceData(slice []ArbitraryType) *ArbitraryType
8085 func String(ptr *byte, len IntegerType) string
8086 func StringData(str string) *byte
8090 These conversions also apply to type parameters with suitable core types.
8091 Determine if we can simply use core type instead of underlying type here,
8092 of if the general conversion rules take care of this.
8096 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
8097 value may not be <a href="#Address_operators">dereferenced</a>.
8098 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be
8099 <a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
8100 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
8105 bits = *(*uint64)(unsafe.Pointer(&f))
8107 type ptr unsafe.Pointer
8108 bits = *(*uint64)(ptr(&f))
8114 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
8115 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
8116 as if <code>v</code> was declared via <code>var v = x</code>.
8119 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
8120 <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
8121 or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
8122 If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
8123 without pointer indirections through fields of the struct.
8124 For a struct <code>s</code> with field <code>f</code>:
8128 uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
8132 Computer architectures may require memory addresses to be <i>aligned</i>;
8133 that is, for addresses of a variable to be a multiple of a factor,
8134 the variable's type's <i>alignment</i>. The function <code>Alignof</code>
8135 takes an expression denoting a variable of any type and returns the
8136 alignment of the (type of the) variable in bytes. For a variable
8141 uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
8145 A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
8146 is a <a href="#Type_parameter_declarations">type parameter</a>, or if it is an
8147 array or struct type containing elements
8148 or fields of variable size. Otherwise the size is <i>constant</i>.
8149 Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
8150 are compile-time <a href="#Constant_expressions">constant expressions</a> of
8151 type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
8152 the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
8157 The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
8158 and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
8159 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8160 A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
8161 if it is an untyped constant it is given type <code>int</code>.
8162 The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
8166 The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
8167 and whose length and capacity are <code>len</code>.
8168 <code>Slice(ptr, len)</code> is equivalent to
8172 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
8176 except that, as a special case, if <code>ptr</code>
8177 is <code>nil</code> and <code>len</code> is zero,
8178 <code>Slice</code> returns <code>nil</code>.
8182 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8183 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
8184 if it is an untyped constant it is given type <code>int</code>.
8185 At run time, if <code>len</code> is negative,
8186 or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
8187 a <a href="#Run_time_panics">run-time panic</a> occurs.
8191 The function <code>SliceData</code> returns a pointer to the underlying array of the <code>slice</code> argument.
8192 If the slice's capacity <code>cap(slice)</code> is not zero, that pointer is <code>&slice[:1][0]</code>.
8193 If <code>slice</code> is <code>nil</code>, the result is <code>nil</code>.
8194 Otherwise it is a non-<code>nil</code> pointer to an unspecified memory address.
8198 The function <code>String</code> returns a <code>string</code> value whose underlying bytes start at
8199 <code>ptr</code> and whose length is <code>len</code>.
8200 The same requirements apply to the <code>ptr</code> and <code>len</code> argument as in the function
8201 <code>Slice</code>. If <code>len</code> is zero, the result is the empty string <code>""</code>.
8202 Since Go strings are immutable, the bytes passed to <code>String</code> must not be modified afterwards.
8206 The function <code>StringData</code> returns a pointer to the underlying bytes of the <code>str</code> argument.
8207 For an empty string the return value is unspecified, and may be <code>nil</code>.
8208 Since Go strings are immutable, the bytes returned by <code>StringData</code> must not be modified.
8211 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
8214 For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
8217 <pre class="grammar">
8222 uint32, int32, float32 4
8223 uint64, int64, float64, complex64 8
8228 The following minimal alignment properties are guaranteed:
8231 <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
8234 <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
8235 all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
8238 <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
8239 the alignment of a variable of the array's element type.
8244 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.