2 "Title": "The Go Programming Language Specification",
3 "Subtitle": "Version of March 10, 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 grammar 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 Extended Backus-Naur Form (EBNF):
35 Production = production_name "=" [ Expression ] "." .
36 Expression = Alternative { "|" Alternative } .
37 Alternative = Term { Term } .
38 Term = production_name | token [ "…" token ] | Group | Option | Repetition .
39 Group = "(" Expression ")" .
40 Option = "[" Expression "]" .
41 Repetition = "{" Expression "}" .
45 Productions are expressions constructed from terms and the following
46 operators, in increasing precedence:
51 [] option (0 or 1 times)
52 {} repetition (0 to n times)
56 Lower-case production names are used to identify lexical tokens.
57 Non-terminals are in CamelCase. Lexical tokens are enclosed in
58 double quotes <code>""</code> or back quotes <code>``</code>.
62 The form <code>a … b</code> represents the set of characters from
63 <code>a</code> through <code>b</code> as alternatives. The horizontal
64 ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various
65 enumerations or code snippets that are not further specified. The character <code>…</code>
66 (as opposed to the three characters <code>...</code>) is not a token of the Go
70 <h2 id="Source_code_representation">Source code representation</h2>
73 Source code is Unicode text encoded in
74 <a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
75 canonicalized, so a single accented code point is distinct from the
76 same character constructed from combining an accent and a letter;
77 those are treated as two code points. For simplicity, this document
78 will use the unqualified term <i>character</i> to refer to a Unicode code point
82 Each code point is distinct; for instance, upper and lower case letters
83 are different characters.
86 Implementation restriction: For compatibility with other tools, a
87 compiler may disallow the NUL character (U+0000) in the source text.
90 Implementation restriction: For compatibility with other tools, a
91 compiler may ignore a UTF-8-encoded byte order mark
92 (U+FEFF) if it is the first Unicode code point in the source text.
93 A byte order mark may be disallowed anywhere else in the source.
96 <h3 id="Characters">Characters</h3>
99 The following terms are used to denote specific Unicode character classes:
102 newline = /* the Unicode code point U+000A */ .
103 unicode_char = /* an arbitrary Unicode code point except newline */ .
104 unicode_letter = /* a Unicode code point classified as "Letter" */ .
105 unicode_digit = /* a Unicode code point classified as "Number, decimal digit" */ .
109 In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
110 Section 4.5 "General Category" defines a set of character categories.
111 Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
112 as Unicode letters, and those in the Number category Nd as Unicode digits.
115 <h3 id="Letters_and_digits">Letters and digits</h3>
118 The underscore character <code>_</code> (U+005F) is considered a letter.
121 letter = unicode_letter | "_" .
122 decimal_digit = "0" … "9" .
123 binary_digit = "0" | "1" .
124 octal_digit = "0" … "7" .
125 hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
128 <h2 id="Lexical_elements">Lexical elements</h2>
130 <h3 id="Comments">Comments</h3>
133 Comments serve as program documentation. There are two forms:
138 <i>Line comments</i> start with the character sequence <code>//</code>
139 and stop at the end of the line.
142 <i>General comments</i> start with the character sequence <code>/*</code>
143 and stop with the first subsequent character sequence <code>*/</code>.
148 A comment cannot start inside a <a href="#Rune_literals">rune</a> or
149 <a href="#String_literals">string literal</a>, or inside a comment.
150 A general comment containing no newlines acts like a space.
151 Any other comment acts like a newline.
154 <h3 id="Tokens">Tokens</h3>
157 Tokens form the vocabulary of the Go language.
158 There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators
159 and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from
160 spaces (U+0020), horizontal tabs (U+0009),
161 carriage returns (U+000D), and newlines (U+000A),
162 is ignored except as it separates tokens
163 that would otherwise combine into a single token. Also, a newline or end of file
164 may trigger the insertion of a <a href="#Semicolons">semicolon</a>.
165 While breaking the input into tokens,
166 the next token is the longest sequence of characters that form a
170 <h3 id="Semicolons">Semicolons</h3>
173 The formal grammar uses semicolons <code>";"</code> as terminators in
174 a number of productions. Go programs may omit most of these semicolons
175 using the following two rules:
180 When the input is broken into tokens, a semicolon is automatically inserted
181 into the token stream immediately after a line's final token if that token is
184 <a href="#Identifiers">identifier</a>
188 <a href="#Integer_literals">integer</a>,
189 <a href="#Floating-point_literals">floating-point</a>,
190 <a href="#Imaginary_literals">imaginary</a>,
191 <a href="#Rune_literals">rune</a>, or
192 <a href="#String_literals">string</a> literal
195 <li>one of the <a href="#Keywords">keywords</a>
197 <code>continue</code>,
198 <code>fallthrough</code>, or
202 <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a>
213 To allow complex statements to occupy a single line, a semicolon
214 may be omitted before a closing <code>")"</code> or <code>"}"</code>.
219 To reflect idiomatic use, code examples in this document elide semicolons
224 <h3 id="Identifiers">Identifiers</h3>
227 Identifiers name program entities such as variables and types.
228 An identifier is a sequence of one or more letters and digits.
229 The first character in an identifier must be a letter.
232 identifier = letter { letter | unicode_digit } .
237 ThisVariableIsExported
242 Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>.
246 <h3 id="Keywords">Keywords</h3>
249 The following keywords are reserved and may not be used as identifiers.
251 <pre class="grammar">
252 break default func interface select
253 case defer go map struct
254 chan else goto package switch
255 const fallthrough if range type
256 continue for import return var
259 <h3 id="Operators_and_punctuation">Operators and punctuation</h3>
262 The following character sequences represent <a href="#Operators">operators</a>
263 (including <a href="#Assignments">assignment operators</a>) and punctuation:
265 <pre class="grammar">
266 + & += &= && == != ( )
267 - | -= |= || < <= [ ]
268 * ^ *= ^= <- > >= { }
269 / << /= <<= ++ = := , ;
270 % >> %= >>= -- ! ... . :
274 <h3 id="Integer_literals">Integer literals</h3>
277 An integer literal is a sequence of digits representing an
278 <a href="#Constants">integer constant</a>.
279 An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code>
280 for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal,
281 and <code>0x</code> or <code>0X</code> for hexadecimal.
282 A single <code>0</code> is considered a decimal zero.
283 In hexadecimal literals, letters <code>a</code> through <code>f</code>
284 and <code>A</code> through <code>F</code> represent values 10 through 15.
288 For readability, an underscore character <code>_</code> may appear after
289 a base prefix or between successive digits; such underscores do not change
293 int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
294 decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
295 binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
296 octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
297 hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
299 decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
300 binary_digits = binary_digit { [ "_" ] binary_digit } .
301 octal_digits = octal_digit { [ "_" ] octal_digit } .
302 hex_digits = hex_digit { [ "_" ] hex_digit } .
311 0O600 // second character is capital letter 'O'
315 170141183460469231731687303715884105727
316 170_141183_460469_231731_687303_715884_105727
318 _42 // an identifier, not an integer literal
319 42_ // invalid: _ must separate successive digits
320 4__2 // invalid: only one _ at a time
321 0_xBadFace // invalid: _ must separate successive digits
325 <h3 id="Floating-point_literals">Floating-point literals</h3>
328 A floating-point literal is a decimal or hexadecimal representation of a
329 <a href="#Constants">floating-point constant</a>.
333 A decimal floating-point literal consists of an integer part (decimal digits),
334 a decimal point, a fractional part (decimal digits), and an exponent part
335 (<code>e</code> or <code>E</code> followed by an optional sign and decimal digits).
336 One of the integer part or the fractional part may be elided; one of the decimal point
337 or the exponent part may be elided.
338 An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.
342 A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code>
343 prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
344 and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits).
345 One of the integer part or the fractional part may be elided; the radix point may be elided as well,
346 but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
347 An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.
351 For readability, an underscore character <code>_</code> may appear after
352 a base prefix or between successive digits; such underscores do not change
357 float_lit = decimal_float_lit | hex_float_lit .
359 decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
360 decimal_digits decimal_exponent |
361 "." decimal_digits [ decimal_exponent ] .
362 decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
364 hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
365 hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
368 hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
386 0x1.Fp+0 // == 1.9375
388 0X_1FFFP-16 // == 0.1249847412109375
389 0x15e-2 // == 0x15e - 2 (integer subtraction)
391 0x.p1 // invalid: mantissa has no digits
392 1p-2 // invalid: p exponent requires hexadecimal mantissa
393 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent
394 1_.5 // invalid: _ must separate successive digits
395 1._5 // invalid: _ must separate successive digits
396 1.5_e1 // invalid: _ must separate successive digits
397 1.5e_1 // invalid: _ must separate successive digits
398 1.5e1_ // invalid: _ must separate successive digits
402 <h3 id="Imaginary_literals">Imaginary literals</h3>
405 An imaginary literal represents the imaginary part of a
406 <a href="#Constants">complex constant</a>.
407 It consists of an <a href="#Integer_literals">integer</a> or
408 <a href="#Floating-point_literals">floating-point</a> literal
409 followed by the lower-case letter <code>i</code>.
410 The value of an imaginary literal is the value of the respective
411 integer or floating-point literal multiplied by the imaginary unit <i>i</i>.
415 imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
419 For backward compatibility, an imaginary literal's integer part consisting
420 entirely of decimal digits (and possibly underscores) is considered a decimal
421 integer, even if it starts with a leading <code>0</code>.
426 0123i // == 123i for backward-compatibility
427 0o123i // == 0o123 * 1i == 83i
428 0xabci // == 0xabc * 1i == 2748i
436 0x1p-2i // == 0x1p-2 * 1i == 0.25i
440 <h3 id="Rune_literals">Rune literals</h3>
443 A rune literal represents a <a href="#Constants">rune constant</a>,
444 an integer value identifying a Unicode code point.
445 A rune literal is expressed as one or more characters enclosed in single quotes,
446 as in <code>'x'</code> or <code>'\n'</code>.
447 Within the quotes, any character may appear except newline and unescaped single
448 quote. A single quoted character represents the Unicode value
449 of the character itself,
450 while multi-character sequences beginning with a backslash encode
451 values in various formats.
455 The simplest form represents the single character within the quotes;
456 since Go source text is Unicode characters encoded in UTF-8, multiple
457 UTF-8-encoded bytes may represent a single integer value. For
458 instance, the literal <code>'a'</code> holds a single byte representing
459 a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
460 <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
461 a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
465 Several backslash escapes allow arbitrary values to be encoded as
466 ASCII text. There are four ways to represent the integer value
467 as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
468 digits; <code>\u</code> followed by exactly four hexadecimal digits;
469 <code>\U</code> followed by exactly eight hexadecimal digits, and a
470 plain backslash <code>\</code> followed by exactly three octal digits.
471 In each case the value of the literal is the value represented by
472 the digits in the corresponding base.
476 Although these representations all result in an integer, they have
477 different valid ranges. Octal escapes must represent a value between
478 0 and 255 inclusive. Hexadecimal escapes satisfy this condition
479 by construction. The escapes <code>\u</code> and <code>\U</code>
480 represent Unicode code points so within them some values are illegal,
481 in particular those above <code>0x10FFFF</code> and surrogate halves.
485 After a backslash, certain single-character escapes represent special values:
488 <pre class="grammar">
489 \a U+0007 alert or bell
492 \n U+000A line feed or newline
493 \r U+000D carriage return
494 \t U+0009 horizontal tab
495 \v U+000B vertical tab
497 \' U+0027 single quote (valid escape only within rune literals)
498 \" U+0022 double quote (valid escape only within string literals)
502 All other sequences starting with a backslash are illegal inside rune literals.
505 rune_lit = "'" ( unicode_value | byte_value ) "'" .
506 unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
507 byte_value = octal_byte_value | hex_byte_value .
508 octal_byte_value = `\` octal_digit octal_digit octal_digit .
509 hex_byte_value = `\` "x" hex_digit hex_digit .
510 little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
511 big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
512 hex_digit hex_digit hex_digit hex_digit .
513 escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
528 '\'' // rune literal containing single quote character
529 'aa' // illegal: too many characters
530 '\xa' // illegal: too few hexadecimal digits
531 '\0' // illegal: too few octal digits
532 '\uDFFF' // illegal: surrogate half
533 '\U00110000' // illegal: invalid Unicode code point
537 <h3 id="String_literals">String literals</h3>
540 A string literal represents a <a href="#Constants">string constant</a>
541 obtained from concatenating a sequence of characters. There are two forms:
542 raw string literals and interpreted string literals.
546 Raw string literals are character sequences between back quotes, as in
547 <code>`foo`</code>. Within the quotes, any character may appear except
548 back quote. The value of a raw string literal is the
549 string composed of the uninterpreted (implicitly UTF-8-encoded) characters
551 in particular, backslashes have no special meaning and the string may
553 Carriage return characters ('\r') inside raw string literals
554 are discarded from the raw string value.
558 Interpreted string literals are character sequences between double
559 quotes, as in <code>"bar"</code>.
560 Within the quotes, any character may appear except newline and unescaped double quote.
561 The text between the quotes forms the
562 value of the literal, with backslash escapes interpreted as they
563 are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and
564 <code>\"</code> is legal), with the same restrictions.
565 The three-digit octal (<code>\</code><i>nnn</i>)
566 and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual
567 <i>bytes</i> of the resulting string; all other escapes represent
568 the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.
569 Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent
570 a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>,
571 <code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent
572 the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character
577 string_lit = raw_string_lit | interpreted_string_lit .
578 raw_string_lit = "`" { unicode_char | newline } "`" .
579 interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
583 `abc` // same as "abc"
585 \n` // same as "\\n\n\\n"
592 "\uD800" // illegal: surrogate half
593 "\U00110000" // illegal: invalid Unicode code point
597 These examples all represent the same string:
601 "日本語" // UTF-8 input text
602 `日本語` // UTF-8 input text as a raw literal
603 "\u65e5\u672c\u8a9e" // the explicit Unicode code points
604 "\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points
605 "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes
609 If the source code represents a character as two code points, such as
610 a combining form involving an accent and a letter, the result will be
611 an error if placed in a rune literal (it is not a single code
612 point), and will appear as two code points if placed in a string
617 <h2 id="Constants">Constants</h2>
619 <p>There are <i>boolean constants</i>,
620 <i>rune constants</i>,
621 <i>integer constants</i>,
622 <i>floating-point constants</i>, <i>complex constants</i>,
623 and <i>string constants</i>. Rune, integer, floating-point,
624 and complex constants are
625 collectively called <i>numeric constants</i>.
629 A constant value is represented by a
630 <a href="#Rune_literals">rune</a>,
631 <a href="#Integer_literals">integer</a>,
632 <a href="#Floating-point_literals">floating-point</a>,
633 <a href="#Imaginary_literals">imaginary</a>,
635 <a href="#String_literals">string</a> literal,
636 an identifier denoting a constant,
637 a <a href="#Constant_expressions">constant expression</a>,
638 a <a href="#Conversions">conversion</a> with a result that is a constant, or
639 the result value of some built-in functions such as
640 <code>unsafe.Sizeof</code> applied to <a href="#Package_unsafe">certain values</a>,
641 <code>cap</code> or <code>len</code> applied to
642 <a href="#Length_and_capacity">some expressions</a>,
643 <code>real</code> and <code>imag</code> applied to a complex constant
644 and <code>complex</code> applied to numeric constants.
645 The boolean truth values are represented by the predeclared constants
646 <code>true</code> and <code>false</code>. The predeclared identifier
647 <a href="#Iota">iota</a> denotes an integer constant.
651 In general, complex constants are a form of
652 <a href="#Constant_expressions">constant expression</a>
653 and are discussed in that section.
657 Numeric constants represent exact values of arbitrary precision and do not overflow.
658 Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
659 and not-a-number values.
663 Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
664 Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
665 and certain <a href="#Constant_expressions">constant expressions</a>
666 containing only untyped constant operands are untyped.
670 A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
671 or <a href="#Conversions">conversion</a>, or implicitly when used in a
672 <a href="#Variable_declarations">variable declaration</a> or an
673 <a href="#Assignments">assignment</a> or as an
674 operand in an <a href="#Expressions">expression</a>.
675 It is an error if the constant value
676 cannot be <a href="#Representability">represented</a> as a value of the respective type.
677 If the type is a type parameter, the constant is converted into a non-constant
678 value of the type parameter.
682 An untyped constant has a <i>default type</i> which is the type to which the
683 constant is implicitly converted in contexts where a typed value is required,
684 for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
685 such as <code>i := 0</code> where there is no explicit type.
686 The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
687 <code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
688 respectively, depending on whether it is a boolean, rune, integer, floating-point,
689 complex, or string constant.
693 Implementation restriction: Although numeric constants have arbitrary
694 precision in the language, a compiler may implement them using an
695 internal representation with limited precision. That said, every
700 <li>Represent integer constants with at least 256 bits.</li>
702 <li>Represent floating-point constants, including the parts of
703 a complex constant, with a mantissa of at least 256 bits
704 and a signed binary exponent of at least 16 bits.</li>
706 <li>Give an error if unable to represent an integer constant
709 <li>Give an error if unable to represent a floating-point or
710 complex constant due to overflow.</li>
712 <li>Round to the nearest representable constant if unable to
713 represent a floating-point or complex constant due to limits
718 These requirements apply both to literal constants and to the result
719 of evaluating <a href="#Constant_expressions">constant
724 <h2 id="Variables">Variables</h2>
727 A variable is a storage location for holding a <i>value</i>.
728 The set of permissible values is determined by the
729 variable's <i><a href="#Types">type</a></i>.
733 A <a href="#Variable_declarations">variable declaration</a>
734 or, for function parameters and results, the signature
735 of a <a href="#Function_declarations">function declaration</a>
736 or <a href="#Function_literals">function literal</a> reserves
737 storage for a named variable.
739 Calling the built-in function <a href="#Allocation"><code>new</code></a>
740 or taking the address of a <a href="#Composite_literals">composite literal</a>
741 allocates storage for a variable at run time.
742 Such an anonymous variable is referred to via a (possibly implicit)
743 <a href="#Address_operators">pointer indirection</a>.
747 <i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
748 and <a href="#Struct_types">struct</a> types have elements and fields that may
749 be <a href="#Address_operators">addressed</a> individually. Each such element
750 acts like a variable.
754 The <i>static type</i> (or just <i>type</i>) of a variable is the
755 type given in its declaration, the type provided in the
756 <code>new</code> call or composite literal, or the type of
757 an element of a structured variable.
758 Variables of interface type also have a distinct <i>dynamic type</i>,
759 which is the (non-interface) type of the value assigned to the variable at run time
760 (unless the value is the predeclared identifier <code>nil</code>,
762 The dynamic type may vary during execution but values stored in interface
763 variables are always <a href="#Assignability">assignable</a>
764 to the static type of the variable.
768 var x interface{} // x is nil and has static type interface{}
769 var v *T // v has value nil, static type *T
770 x = 42 // x has value 42 and dynamic type int
771 x = v // x has value (*T)(nil) and dynamic type *T
775 A variable's value is retrieved by referring to the variable in an
776 <a href="#Expressions">expression</a>; it is the most recent value
777 <a href="#Assignments">assigned</a> to the variable.
778 If a variable has not yet been assigned a value, its value is the
779 <a href="#The_zero_value">zero value</a> for its type.
783 <h2 id="Types">Types</h2>
786 A type determines a set of values together with operations and methods specific
787 to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be
788 followed by <a href="#Instantiations">type arguments</a> if the type is generic.
789 A type may also be specified using a <i>type literal</i>, which composes a type
794 Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
795 TypeName = identifier | QualifiedIdent .
796 TypeArgs = "[" TypeList [ "," ] "]" .
797 TypeList = Type { "," Type } .
798 TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
799 SliceType | MapType | ChannelType .
803 The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
804 Others are introduced with <a href="#Type_declarations">type declarations</a>
805 or <a href="#Type_parameter_declarations">type parameter lists</a>.
806 <i>Composite types</i>—array, struct, pointer, function,
807 interface, slice, map, and channel types—may be constructed using
812 Predeclared types, defined types, and type parameters are called <i>named types</i>.
813 An alias denotes a named type if the type given in the alias declaration is a named type.
816 <h3 id="Boolean_types">Boolean types</h3>
819 A <i>boolean type</i> represents the set of Boolean truth values
820 denoted by the predeclared constants <code>true</code>
821 and <code>false</code>. The predeclared boolean type is <code>bool</code>;
822 it is a <a href="#Type_definitions">defined type</a>.
825 <h3 id="Numeric_types">Numeric types</h3>
828 An <i>integer</i>, <i>floating-point</i>, or <i>complex</i> type
829 represents the set of integer, floating-point, or complex values, respectively.
830 They are collectively called <i>numeric types</i>.
831 The predeclared architecture-independent numeric types are:
834 <pre class="grammar">
835 uint8 the set of all unsigned 8-bit integers (0 to 255)
836 uint16 the set of all unsigned 16-bit integers (0 to 65535)
837 uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
838 uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
840 int8 the set of all signed 8-bit integers (-128 to 127)
841 int16 the set of all signed 16-bit integers (-32768 to 32767)
842 int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
843 int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
845 float32 the set of all IEEE-754 32-bit floating-point numbers
846 float64 the set of all IEEE-754 64-bit floating-point numbers
848 complex64 the set of all complex numbers with float32 real and imaginary parts
849 complex128 the set of all complex numbers with float64 real and imaginary parts
856 The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
857 <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
861 There is also a set of predeclared integer types with implementation-specific sizes:
864 <pre class="grammar">
865 uint either 32 or 64 bits
866 int same size as uint
867 uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
871 To avoid portability issues all numeric types are <a href="#Type_definitions">defined
872 types</a> and thus distinct except
873 <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
874 <code>rune</code>, which is an alias for <code>int32</code>.
876 are required when different numeric types are mixed in an expression
877 or assignment. For instance, <code>int32</code> and <code>int</code>
878 are not the same type even though they may have the same size on a
879 particular architecture.
882 <h3 id="String_types">String types</h3>
885 A <i>string type</i> represents the set of string values.
886 A string value is a (possibly empty) sequence of bytes.
887 The number of bytes is called the length of the string and is never negative.
888 Strings are immutable: once created,
889 it is impossible to change the contents of a string.
890 The predeclared string type is <code>string</code>;
891 it is a <a href="#Type_definitions">defined type</a>.
895 The length of a string <code>s</code> can be discovered using
896 the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
897 The length is a compile-time constant if the string is a constant.
898 A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
899 0 through <code>len(s)-1</code>.
900 It is illegal to take the address of such an element; if
901 <code>s[i]</code> is the <code>i</code>'th byte of a
902 string, <code>&s[i]</code> is invalid.
906 <h3 id="Array_types">Array types</h3>
909 An array is a numbered sequence of elements of a single
910 type, called the element type.
911 The number of elements is called the length of the array and is never negative.
915 ArrayType = "[" ArrayLength "]" ElementType .
916 ArrayLength = Expression .
921 The length is part of the array's type; it must evaluate to a
922 non-negative <a href="#Constants">constant</a>
923 <a href="#Representability">representable</a> by a value
924 of type <code>int</code>.
925 The length of array <code>a</code> can be discovered
926 using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
927 The elements can be addressed by integer <a href="#Index_expressions">indices</a>
928 0 through <code>len(a)-1</code>.
929 Array types are always one-dimensional but may be composed to form
930 multi-dimensional types.
935 [2*N] struct { x, y int32 }
938 [2][2][2]float64 // same as [2]([2]([2]float64))
941 <h3 id="Slice_types">Slice types</h3>
944 A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
945 provides access to a numbered sequence of elements from that array.
946 A slice type denotes the set of all slices of arrays of its element type.
947 The number of elements is called the length of the slice and is never negative.
948 The value of an uninitialized slice is <code>nil</code>.
952 SliceType = "[" "]" ElementType .
956 The length of a slice <code>s</code> can be discovered by the built-in function
957 <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
958 execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
959 0 through <code>len(s)-1</code>. The slice index of a
960 given element may be less than the index of the same element in the
964 A slice, once initialized, is always associated with an underlying
965 array that holds its elements. A slice therefore shares storage
966 with its array and with other slices of the same array; by contrast,
967 distinct arrays always represent distinct storage.
970 The array underlying a slice may extend past the end of the slice.
971 The <i>capacity</i> is a measure of that extent: it is the sum of
972 the length of the slice and the length of the array beyond the slice;
973 a slice of length up to that capacity can be created by
974 <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
975 The capacity of a slice <code>a</code> can be discovered using the
976 built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
980 A new, initialized slice value for a given element type <code>T</code> may be
981 made using the built-in function
982 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
983 which takes a slice type
984 and parameters specifying the length and optionally the capacity.
985 A slice created with <code>make</code> always allocates a new, hidden array
986 to which the returned slice value refers. That is, executing
990 make([]T, length, capacity)
994 produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
995 it, so these two expressions are equivalent:
1004 Like arrays, slices are always one-dimensional but may be composed to construct
1005 higher-dimensional objects.
1006 With arrays of arrays, the inner arrays are, by construction, always the same length;
1007 however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
1008 Moreover, the inner slices must be initialized individually.
1011 <h3 id="Struct_types">Struct types</h3>
1014 A struct is a sequence of named elements, called fields, each of which has a
1015 name and a type. Field names may be specified explicitly (IdentifierList) or
1016 implicitly (EmbeddedField).
1017 Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
1018 be <a href="#Uniqueness_of_identifiers">unique</a>.
1022 StructType = "struct" "{" { FieldDecl ";" } "}" .
1023 FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
1024 EmbeddedField = [ "*" ] TypeName .
1032 // A struct with 6 fields.
1036 _ float32 // padding
1043 A field declared with a type but no explicit field name is called an <i>embedded field</i>.
1044 An embedded field must be specified as
1045 a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
1046 and <code>T</code> itself may not be
1047 a pointer type. The unqualified type name acts as the field name.
1051 // A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
1053 T1 // field name is T1
1054 *T2 // field name is T2
1055 P.T3 // field name is T3
1056 *P.T4 // field name is T4
1057 x, y int // field names are x and y
1062 The following declaration is illegal because field names must be unique
1068 T // conflicts with embedded field *T and *P.T
1069 *T // conflicts with embedded field T and *P.T
1070 *P.T // conflicts with embedded field T and *T
1075 A field or <a href="#Method_declarations">method</a> <code>f</code> of an
1076 embedded field in a struct <code>x</code> is called <i>promoted</i> if
1077 <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
1078 that field or method <code>f</code>.
1082 Promoted fields act like ordinary fields
1083 of a struct except that they cannot be used as field names in
1084 <a href="#Composite_literals">composite literals</a> of the struct.
1088 Given a struct type <code>S</code> and a <a href="#Type_definitions">defined type</a>
1089 <code>T</code>, promoted methods are included in the method set of the struct as follows:
1093 If <code>S</code> contains an embedded field <code>T</code>,
1094 the <a href="#Method_sets">method sets</a> of <code>S</code>
1095 and <code>*S</code> both include promoted methods with receiver
1096 <code>T</code>. The method set of <code>*S</code> also
1097 includes promoted methods with receiver <code>*T</code>.
1101 If <code>S</code> contains an embedded field <code>*T</code>,
1102 the method sets of <code>S</code> and <code>*S</code> both
1103 include promoted methods with receiver <code>T</code> or
1109 A field declaration may be followed by an optional string literal <i>tag</i>,
1110 which becomes an attribute for all the fields in the corresponding
1111 field declaration. An empty tag string is equivalent to an absent tag.
1112 The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
1113 and take part in <a href="#Type_identity">type identity</a> for structs
1114 but are otherwise ignored.
1119 x, y float64 "" // an empty tag string is like an absent tag
1120 name string "any string is permitted as a tag"
1121 _ [4]byte "ceci n'est pas un champ de structure"
1124 // A struct corresponding to a TimeStamp protocol buffer.
1125 // The tag strings define the protocol buffer field numbers;
1126 // they follow the convention outlined by the reflect package.
1128 microsec uint64 `protobuf:"1"`
1129 serverIP6 uint64 `protobuf:"2"`
1133 <h3 id="Pointer_types">Pointer types</h3>
1136 A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
1137 type, called the <i>base type</i> of the pointer.
1138 The value of an uninitialized pointer is <code>nil</code>.
1142 PointerType = "*" BaseType .
1151 <h3 id="Function_types">Function types</h3>
1154 A function type denotes the set of all functions with the same parameter
1155 and result types. The value of an uninitialized variable of function type
1156 is <code>nil</code>.
1160 FunctionType = "func" Signature .
1161 Signature = Parameters [ Result ] .
1162 Result = Parameters | Type .
1163 Parameters = "(" [ ParameterList [ "," ] ] ")" .
1164 ParameterList = ParameterDecl { "," ParameterDecl } .
1165 ParameterDecl = [ IdentifierList ] [ "..." ] Type .
1169 Within a list of parameters or results, the names (IdentifierList)
1170 must either all be present or all be absent. If present, each name
1171 stands for one item (parameter or result) of the specified type and
1172 all non-<a href="#Blank_identifier">blank</a> names in the signature
1173 must be <a href="#Uniqueness_of_identifiers">unique</a>.
1174 If absent, each type stands for one item of that type.
1175 Parameter and result
1176 lists are always parenthesized except that if there is exactly
1177 one unnamed result it may be written as an unparenthesized type.
1181 The final incoming parameter in a function signature may have
1182 a type prefixed with <code>...</code>.
1183 A function with such a parameter is called <i>variadic</i> and
1184 may be invoked with zero or more arguments for that parameter.
1190 func(a, _ int, z float32) bool
1191 func(a, b int, z float32) (bool)
1192 func(prefix string, values ...int)
1193 func(a, b int, z float64, opt ...interface{}) (success bool)
1194 func(int, int, float64) (float64, *[]int)
1195 func(n int) func(p *T)
1198 <h3 id="Interface_types">Interface types</h3>
1201 An interface type defines a <i>type set</i>.
1202 A variable of interface type can store a value of any type that is in the type
1203 set of the interface. Such a type is said to
1204 <a href="#Implementing_an_interface">implement the interface</a>.
1205 The value of an uninitialized variable of interface type is <code>nil</code>.
1209 InterfaceType = "interface" "{" { InterfaceElem ";" } "}" .
1210 InterfaceElem = MethodElem | TypeElem .
1211 MethodElem = MethodName Signature .
1212 MethodName = identifier .
1213 TypeElem = TypeTerm { "|" TypeTerm } .
1214 TypeTerm = Type | UnderlyingType .
1215 UnderlyingType = "~" Type .
1219 An interface type is specified by a list of <i>interface elements</i>.
1220 An interface element is either a <i>method</i> or a <i>type element</i>,
1221 where a type element is a union of one or more <i>type terms</i>.
1222 A type term is either a single type or a single underlying type.
1225 <h4 id="Basic_interfaces">Basic interfaces</h4>
1228 In its most basic form an interface specifies a (possibly empty) list of methods.
1229 The type set defined by such an interface is the set of types which implement all of
1230 those methods, and the corresponding <a href="#Method_sets">method set</a> consists
1231 exactly of the methods specified by the interface.
1232 Interfaces whose type sets can be defined entirely by a list of methods are called
1233 <i>basic interfaces.</i>
1237 // A simple File interface.
1239 Read([]byte) (int, error)
1240 Write([]byte) (int, error)
1246 The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
1247 and not <a href="#Blank_identifier">blank</a>.
1253 String() string // illegal: String not unique
1254 _(x int) // illegal: method must have non-blank name
1259 More than one type may implement an interface.
1260 For instance, if two types <code>S1</code> and <code>S2</code>
1265 func (p T) Read(p []byte) (n int, err error)
1266 func (p T) Write(p []byte) (n int, err error)
1267 func (p T) Close() error
1271 (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
1272 then the <code>File</code> interface is implemented by both <code>S1</code> and
1273 <code>S2</code>, regardless of what other methods
1274 <code>S1</code> and <code>S2</code> may have or share.
1278 Every type that is a member of the type set of an interface implements that interface.
1279 Any given type may implement several distinct interfaces.
1280 For instance, all types implement the <i>empty interface</i> which stands for the set of all types:
1288 For convenience, the predeclared type <code>any</code> is an alias for the empty interface.
1292 Similarly, consider this interface specification,
1293 which appears within a <a href="#Type_declarations">type declaration</a>
1294 to define an interface called <code>Locker</code>:
1298 type Locker interface {
1305 If <code>S1</code> and <code>S2</code> also implement
1309 func (p T) Lock() { … }
1310 func (p T) Unlock() { … }
1314 they implement the <code>Locker</code> interface as well
1315 as the <code>File</code> interface.
1318 <h4 id="Embedded_interfaces">Embedded interfaces</h4>
1321 In a slightly more general form
1322 an interface <code>T</code> may use a (possibly qualified) interface type
1323 name <code>E</code> as an interface element. This is called
1324 <i>embedding</i> interface <code>E</code> in <code>T</code>.
1325 The type set of <code>T</code> is the <i>intersection</i> of the type sets
1326 defined by <code>T</code>'s explicitly declared methods and the type sets
1327 of <code>T</code>’s embedded interfaces.
1328 In other words, the type set of <code>T</code> is the set of all types that implement all the
1329 explicitly declared methods of <code>T</code> and also all the methods of
1334 type Reader interface {
1335 Read(p []byte) (n int, err error)
1339 type Writer interface {
1340 Write(p []byte) (n int, err error)
1344 // ReadWriter's methods are Read, Write, and Close.
1345 type ReadWriter interface {
1346 Reader // includes methods of Reader in ReadWriter's method set
1347 Writer // includes methods of Writer in ReadWriter's method set
1352 When embedding interfaces, methods with the
1353 <a href="#Uniqueness_of_identifiers">same</a> names must
1354 have <a href="#Type_identity">identical</a> signatures.
1358 type ReadCloser interface {
1359 Reader // includes methods of Reader in ReadCloser's method set
1360 Close() // illegal: signatures of Reader.Close and Close are different
1364 <h4 id="General_interfaces">General interfaces</h4>
1367 In their most general form, an interface element may also be an arbitrary type term
1368 <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
1369 or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
1370 Together with method specifications, these elements enable the precise
1371 definition of an interface's type set as follows:
1375 <li>The type set of the empty interface is the set of all non-interface types.
1378 <li>The type set of a non-empty interface is the intersection of the type sets
1379 of its interface elements.
1382 <li>The type set of a method specification is the set of types
1383 whose method sets include that method.
1386 <li>The type set of a non-interface type term is the set consisting
1390 <li>The type set of a term of the form <code>~T</code>
1391 is the set of types whose underlying type is <code>T</code>.
1394 <li>The type set of a <i>union</i> of terms
1395 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
1396 is the union of the type sets of the terms.
1401 By construction, an interface's type set never contains an interface type.
1405 // An interface representing only the type int.
1410 // An interface representing all types with underlying type int.
1415 // An interface representing all types with underlying type int that implement the String method.
1421 // An interface representing an empty type set: there is no type that is both an int and a string.
1429 In a term of the form <code>~T</code>, the underlying type of <code>T</code>
1430 must be itself, and <code>T</code> cannot be an interface.
1437 ~[]byte // the underlying type of []byte is itself
1438 ~MyInt // illegal: the underlying type of MyInt is not MyInt
1439 ~error // illegal: error is an interface
1444 Union elements denote unions of type sets:
1448 // The Float interface represents all floating-point types
1449 // (including any named types whose underlying types are
1450 // either float32 or float64).
1451 type Float interface {
1457 In a union, a term cannot be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all
1458 non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
1459 Given a type parameter <code>P</code>:
1464 P // illegal: P is a type parameter
1465 int | P // illegal: P is a type parameter
1466 ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
1467 float32 | Float // overlapping type sets but Float is an interface
1472 Implementation restriction:
1473 A union (with more than one term) cannot contain the
1474 <a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code>
1475 or interfaces that specify methods, or embed <code>comparable</code> or interfaces
1476 that specify methods.
1480 Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type
1481 constraints, or as elements of other interfaces used as constraints.
1482 They cannot be the types of values or variables, or components of other,
1483 non-interface types.
1487 var x Float // illegal: Float is not a basic interface
1489 var x interface{} = Float(nil) // illegal
1491 type Floatish struct {
1497 An interface type <code>T</code> may not embed any type element
1498 that is, contains, or embeds <code>T</code>, recursively.
1502 // illegal: Bad cannot embed itself
1503 type Bad interface {
1507 // illegal: Bad1 cannot embed itself using Bad2
1508 type Bad1 interface {
1511 type Bad2 interface {
1515 // illegal: Bad3 cannot embed a union containing Bad3
1516 type Bad3 interface {
1517 ~int | ~string | Bad3
1521 <h4 id="Implementing_an_interface">Implementing an interface</h4>
1524 A type <code>T</code> implements an interface <code>I</code> if
1529 <code>T</code> is not an interface and is an element of the type set of <code>I</code>; or
1532 <code>T</code> is an interface and the type set of <code>T</code> is a subset of the
1533 type set of <code>I</code>.
1538 A value of type <code>T</code> implements an interface if <code>T</code>
1539 implements the interface.
1542 <h3 id="Map_types">Map types</h3>
1545 A map is an unordered group of elements of one type, called the
1546 element type, indexed by a set of unique <i>keys</i> of another type,
1547 called the key type.
1548 The value of an uninitialized map is <code>nil</code>.
1552 MapType = "map" "[" KeyType "]" ElementType .
1557 The <a href="#Comparison_operators">comparison operators</a>
1558 <code>==</code> and <code>!=</code> must be fully defined
1559 for operands of the key type; thus the key type must not be a function, map, or
1561 If the key type is an interface type, these
1562 comparison operators must be defined for the dynamic key values;
1563 failure will cause a <a href="#Run_time_panics">run-time panic</a>.
1568 map[*T]struct{ x, y float64 }
1569 map[string]interface{}
1573 The number of map elements is called its length.
1574 For a map <code>m</code>, it can be discovered using the
1575 built-in function <a href="#Length_and_capacity"><code>len</code></a>
1576 and may change during execution. Elements may be added during execution
1577 using <a href="#Assignments">assignments</a> and retrieved with
1578 <a href="#Index_expressions">index expressions</a>; they may be removed with the
1579 <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
1582 A new, empty map value is made using the built-in
1583 function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1584 which takes the map type and an optional capacity hint as arguments:
1588 make(map[string]int)
1589 make(map[string]int, 100)
1593 The initial capacity does not bound its size:
1594 maps grow to accommodate the number of items
1595 stored in them, with the exception of <code>nil</code> maps.
1596 A <code>nil</code> map is equivalent to an empty map except that no elements
1599 <h3 id="Channel_types">Channel types</h3>
1602 A channel provides a mechanism for
1603 <a href="#Go_statements">concurrently executing functions</a>
1605 <a href="#Send_statements">sending</a> and
1606 <a href="#Receive_operator">receiving</a>
1607 values of a specified element type.
1608 The value of an uninitialized channel is <code>nil</code>.
1612 ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
1616 The optional <code><-</code> operator specifies the channel <i>direction</i>,
1617 <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
1618 otherwise it is <i>bidirectional</i>.
1619 A channel may be constrained only to send or only to receive by
1620 <a href="#Assignments">assignment</a> or
1621 explicit <a href="#Conversions">conversion</a>.
1625 chan T // can be used to send and receive values of type T
1626 chan<- float64 // can only be used to send float64s
1627 <-chan int // can only be used to receive ints
1631 The <code><-</code> operator associates with the leftmost <code>chan</code>
1636 chan<- chan int // same as chan<- (chan int)
1637 chan<- <-chan int // same as chan<- (<-chan int)
1638 <-chan <-chan int // same as <-chan (<-chan int)
1639 chan (<-chan int)
1643 A new, initialized channel
1644 value can be made using the built-in function
1645 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1646 which takes the channel type and an optional <i>capacity</i> as arguments:
1654 The capacity, in number of elements, sets the size of the buffer in the channel.
1655 If the capacity is zero or absent, the channel is unbuffered and communication
1656 succeeds only when both a sender and receiver are ready. Otherwise, the channel
1657 is buffered and communication succeeds without blocking if the buffer
1658 is not full (sends) or not empty (receives).
1659 A <code>nil</code> channel is never ready for communication.
1663 A channel may be closed with the built-in function
1664 <a href="#Close"><code>close</code></a>.
1665 The multi-valued assignment form of the
1666 <a href="#Receive_operator">receive operator</a>
1667 reports whether a received value was sent before
1668 the channel was closed.
1672 A single channel may be used in
1673 <a href="#Send_statements">send statements</a>,
1674 <a href="#Receive_operator">receive operations</a>,
1675 and calls to the built-in functions
1676 <a href="#Length_and_capacity"><code>cap</code></a> and
1677 <a href="#Length_and_capacity"><code>len</code></a>
1678 by any number of goroutines without further synchronization.
1679 Channels act as first-in-first-out queues.
1680 For example, if one goroutine sends values on a channel
1681 and a second goroutine receives them, the values are
1682 received in the order sent.
1685 <h2 id="Properties_of_types_and_values">Properties of types and values</h2>
1687 <h3 id="Underlying_types">Underlying types</h3>
1690 Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
1691 is one of the predeclared boolean, numeric, or string types, or a type literal,
1692 the corresponding underlying type is <code>T</code> itself.
1693 Otherwise, <code>T</code>'s underlying type is the underlying type of the
1694 type to which <code>T</code> refers in its declaration.
1695 For a type parameter that is the underlying type of its
1696 <a href="#Type_constraints">type constraint</a>, which is always an interface.
1712 func f[P any](x P) { … }
1716 The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
1717 and <code>B2</code> is <code>string</code>.
1718 The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
1719 The underlying type of <code>P</code> is <code>interface{}</code>.
1722 <h3 id="Core_types">Core types</h3>
1725 Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the
1726 <a href="#Underlying_types">underlying type</a> of <code>T</code>.
1730 An interface <code>T</code> has a core type if one of the following
1731 conditions is satisfied:
1736 There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a>
1737 of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or
1740 the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a>
1741 with identical element type <code>E</code>, and all directional channels have the same
1747 No other interfaces have a core type.
1751 The core type of an interface is, depending on the condition that is satisfied, either:
1756 the type <code>U</code>; or
1759 the type <code>chan E</code> if <code>T</code> contains only bidirectional
1760 channels, or the type <code>chan<- E</code> or <code><-chan E</code>
1761 depending on the direction of the directional channels present.
1766 By definition, a core type is never a <a href="#Type_definitions">defined type</a>,
1767 <a href="#Type_parameter_declarations">type parameter</a>, or
1768 <a href="#Interface_types">interface type</a>.
1772 Examples of interfaces with core types:
1776 type Celsius float32
1779 interface{ int } // int
1780 interface{ Celsius|Kelvin } // float32
1781 interface{ ~chan int } // chan int
1782 interface{ ~chan int|~chan<- int } // chan<- int
1783 interface{ ~[]*data; String() string } // []*data
1787 Examples of interfaces without core types:
1791 interface{} // no single underlying type
1792 interface{ Celsius|float64 } // no single underlying type
1793 interface{ chan int | chan<- string } // channels have different element types
1794 interface{ <-chan int | chan<- int } // directional channels have different directions
1797 <h3 id="Type_identity">Type identity</h3>
1800 Two types are either <i>identical</i> or <i>different</i>.
1804 A <a href="#Types">named type</a> is always different from any other type.
1805 Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
1806 structurally equivalent; that is, they have the same literal structure and corresponding
1807 components have identical types. In detail:
1811 <li>Two array types are identical if they have identical element types and
1812 the same array length.</li>
1814 <li>Two slice types are identical if they have identical element types.</li>
1816 <li>Two struct types are identical if they have the same sequence of fields,
1817 and if corresponding fields have the same names, and identical types,
1819 <a href="#Exported_identifiers">Non-exported</a> field names from different
1820 packages are always different.</li>
1822 <li>Two pointer types are identical if they have identical base types.</li>
1824 <li>Two function types are identical if they have the same number of parameters
1825 and result values, corresponding parameter and result types are
1826 identical, and either both functions are variadic or neither is.
1827 Parameter and result names are not required to match.</li>
1829 <li>Two interface types are identical if they define the same type set.
1832 <li>Two map types are identical if they have identical key and element types.</li>
1834 <li>Two channel types are identical if they have identical element types and
1835 the same direction.</li>
1837 <li>Two <a href="#Instantiations">instantiated</a> types are identical if
1838 their defined types and all type arguments are identical.
1843 Given the declarations
1850 A2 = struct{ a, b int }
1852 A4 = func(A3, float64) *A0
1853 A5 = func(x int, _ float64) *[]string
1857 B2 struct{ a, b int }
1858 B3 struct{ a, c int }
1859 B4 func(int, float64) *B0
1860 B5 func(x int, y float64) *A1
1863 D0[P1, P2 any] struct{ x P1; y P2 }
1864 E0 = D0[int, string]
1869 these types are identical:
1873 A0, A1, and []string
1874 A2 and struct{ a, b int }
1876 A4, func(int, float64) *[]string, and A5
1879 D0[int, string] and E0
1881 struct{ a, b *T5 } and struct{ a, b *T5 }
1882 func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
1886 <code>B0</code> and <code>B1</code> are different because they are new types
1887 created by distinct <a href="#Type_definitions">type definitions</a>;
1888 <code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
1889 are different because <code>B0</code> is different from <code>[]string</code>;
1890 and <code>P1</code> and <code>P2</code> are different because they are different
1892 <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are
1893 different because the former is an <a href="#Instantiations">instantiated</a>
1894 defined type while the latter is a type literal
1895 (but they are still <a href="#Assignability">assignable</a>).
1898 <h3 id="Assignability">Assignability</h3>
1901 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>
1902 ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
1907 <code>V</code> and <code>T</code> are identical.
1910 <code>V</code> and <code>T</code> have identical
1911 <a href="#Underlying_types">underlying types</a> and at least one of <code>V</code>
1912 or <code>T</code> is not a <a href="#Types">named type</a>.
1915 <code>V</code> and <code>T</code> are channel types with
1916 identical element types, <code>V</code> is a bidirectional channel,
1917 and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
1920 <code>T</code> is an interface type, but not a type parameter, and
1921 <code>x</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
1924 <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
1925 is a pointer, function, slice, map, channel, or interface type,
1926 but not a type parameter.
1929 <code>x</code> is an untyped <a href="#Constants">constant</a>
1930 <a href="#Representability">representable</a>
1931 by a value of type <code>T</code>.
1936 Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code>
1937 is assignable to a variable of type <code>T</code> if one of the following conditions applies:
1942 <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
1943 a type parameter, and <code>x</code> is assignable to each type in
1944 <code>T</code>'s type set.
1947 <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
1948 a type parameter, and <code>x</code> is assignable to each type in
1949 <code>T</code>'s type set.
1952 <code>V</code> is a type parameter and <code>T</code> is not a named type,
1953 and values of each type in <code>V</code>'s type set are assignable
1958 <h3 id="Representability">Representability</h3>
1961 A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
1962 by a value of type <code>T</code>,
1963 where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>,
1964 if one of the following conditions applies:
1969 <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
1973 <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
1974 precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
1975 negative zero further simplified to an unsigned zero. Note that constant values never result
1976 in an IEEE negative zero, NaN, or infinity.
1980 <code>T</code> is a complex type, and <code>x</code>'s
1981 <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
1982 are representable by values of <code>T</code>'s component type (<code>float32</code> or
1983 <code>float64</code>).
1988 If <code>T</code> is a type parameter,
1989 <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
1990 by a value of each type in <code>T</code>'s type set.
1994 x T x is representable by a value of T because
1996 'a' byte 97 is in the set of byte values
1997 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
1998 "foo" string "foo" is in the set of string values
1999 1024 int16 1024 is in the set of 16-bit integers
2000 42.0 byte 42 is in the set of unsigned 8-bit integers
2001 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
2002 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
2003 -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
2004 0i int 0 is an integer value
2005 (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
2009 x T x is not representable by a value of T because
2011 0 bool 0 is not in the set of boolean values
2012 'a' string 'a' is a rune, it is not in the set of string values
2013 1024 byte 1024 is not in the set of unsigned 8-bit integers
2014 -1 uint16 -1 is not in the set of unsigned 16-bit integers
2015 1.1 int 1.1 is not an integer value
2016 42i float32 (0 + 42i) is not in the set of float32 values
2017 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
2020 <h3 id="Method_sets">Method sets</h3>
2023 The <i>method set</i> of a type determines the methods that can be
2024 <a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type.
2025 Every type has a (possibly empty) method set associated with it:
2029 <li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all
2030 <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
2034 The method set of a pointer to a defined type <code>T</code>
2035 (where <code>T</code> is neither a pointer nor an interface)
2036 is the set of all methods declared with receiver <code>*T</code> or <code>T</code>.
2039 <li>The method set of an <a href="#Interface_types">interface type</a> is the intersection
2040 of the method sets of each type in the interface's <a href="#Interface_types">type set</a>
2041 (the resulting method set is usually just the set of declared methods in the interface).
2046 Further rules apply to structs (and pointer to structs) containing embedded fields,
2047 as described in the section on <a href="#Struct_types">struct types</a>.
2048 Any other type has an empty method set.
2052 In a method set, each method must have a
2053 <a href="#Uniqueness_of_identifiers">unique</a>
2054 non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
2057 <h2 id="Blocks">Blocks</h2>
2060 A <i>block</i> is a possibly empty sequence of declarations and statements
2061 within matching brace brackets.
2065 Block = "{" StatementList "}" .
2066 StatementList = { Statement ";" } .
2070 In addition to explicit blocks in the source code, there are implicit blocks:
2074 <li>The <i>universe block</i> encompasses all Go source text.</li>
2076 <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
2077 Go source text for that package.</li>
2079 <li>Each file has a <i>file block</i> containing all Go source text
2082 <li>Each <a href="#If_statements">"if"</a>,
2083 <a href="#For_statements">"for"</a>, and
2084 <a href="#Switch_statements">"switch"</a>
2085 statement is considered to be in its own implicit block.</li>
2087 <li>Each clause in a <a href="#Switch_statements">"switch"</a>
2088 or <a href="#Select_statements">"select"</a> statement
2089 acts as an implicit block.</li>
2093 Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
2097 <h2 id="Declarations_and_scope">Declarations and scope</h2>
2100 A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
2101 <a href="#Constant_declarations">constant</a>,
2102 <a href="#Type_declarations">type</a>,
2103 <a href="#Type_parameter_declarations">type parameter</a>,
2104 <a href="#Variable_declarations">variable</a>,
2105 <a href="#Function_declarations">function</a>,
2106 <a href="#Labeled_statements">label</a>, or
2107 <a href="#Import_declarations">package</a>.
2108 Every identifier in a program must be declared.
2109 No identifier may be declared twice in the same block, and
2110 no identifier may be declared in both the file and package block.
2114 The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
2115 in a declaration, but it does not introduce a binding and thus is not declared.
2116 In the package block, the identifier <code>init</code> may only be used for
2117 <a href="#Package_initialization"><code>init</code> function</a> declarations,
2118 and like the blank identifier it does not introduce a new binding.
2122 Declaration = ConstDecl | TypeDecl | VarDecl .
2123 TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
2127 The <i>scope</i> of a declared identifier is the extent of source text in which
2128 the identifier denotes the specified constant, type, variable, function, label, or package.
2132 Go is lexically scoped using <a href="#Blocks">blocks</a>:
2136 <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
2138 <li>The scope of an identifier denoting a constant, type, variable,
2139 or function (but not method) declared at top level (outside any
2140 function) is the package block.</li>
2142 <li>The scope of the package name of an imported package is the file block
2143 of the file containing the import declaration.</li>
2145 <li>The scope of an identifier denoting a method receiver, function parameter,
2146 or result variable is the function body.</li>
2148 <li>The scope of an identifier denoting a type parameter of a function
2149 or declared by a method receiver is the function body and all parameter lists of the
2153 <li>The scope of an identifier denoting a type parameter of a type
2154 begins after the name of the type and ends at the end
2155 of the TypeSpec.</li>
2157 <li>The scope of a constant or variable identifier declared
2158 inside a function begins at the end of the ConstSpec or VarSpec
2159 (ShortVarDecl for short variable declarations)
2160 and ends at the end of the innermost containing block.</li>
2162 <li>The scope of a type identifier declared inside a function
2163 begins at the identifier in the TypeSpec
2164 and ends at the end of the innermost containing block.</li>
2168 An identifier declared in a block may be redeclared in an inner block.
2169 While the identifier of the inner declaration is in scope, it denotes
2170 the entity declared by the inner declaration.
2174 The <a href="#Package_clause">package clause</a> is not a declaration; the package name
2175 does not appear in any scope. Its purpose is to identify the files belonging
2176 to the same <a href="#Packages">package</a> and to specify the default package name for import
2181 <h3 id="Label_scopes">Label scopes</h3>
2184 Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
2185 used in the <a href="#Break_statements">"break"</a>,
2186 <a href="#Continue_statements">"continue"</a>, and
2187 <a href="#Goto_statements">"goto"</a> statements.
2188 It is illegal to define a label that is never used.
2189 In contrast to other identifiers, labels are not block scoped and do
2190 not conflict with identifiers that are not labels. The scope of a label
2191 is the body of the function in which it is declared and excludes
2192 the body of any nested function.
2196 <h3 id="Blank_identifier">Blank identifier</h3>
2199 The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
2200 It serves as an anonymous placeholder instead of a regular (non-blank)
2201 identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
2202 as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>.
2206 <h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
2209 The following identifiers are implicitly declared in the
2210 <a href="#Blocks">universe block</a>:
2212 <pre class="grammar">
2214 any bool byte comparable
2215 complex64 complex128 error float32 float64
2216 int int8 int16 int32 int64 rune string
2217 uint uint8 uint16 uint32 uint64 uintptr
2226 append cap close complex copy delete imag len
2227 make new panic print println real recover
2230 <h3 id="Exported_identifiers">Exported identifiers</h3>
2233 An identifier may be <i>exported</i> to permit access to it from another package.
2234 An identifier is exported if both:
2237 <li>the first character of the identifier's name is a Unicode upper case
2238 letter (Unicode class "Lu"); and</li>
2239 <li>the identifier is declared in the <a href="#Blocks">package block</a>
2240 or it is a <a href="#Struct_types">field name</a> or
2241 <a href="#MethodName">method name</a>.</li>
2244 All other identifiers are not exported.
2247 <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
2250 Given a set of identifiers, an identifier is called <i>unique</i> if it is
2251 <i>different</i> from every other in the set.
2252 Two identifiers are different if they are spelled differently, or if they
2253 appear in different <a href="#Packages">packages</a> and are not
2254 <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
2257 <h3 id="Constant_declarations">Constant declarations</h3>
2260 A constant declaration binds a list of identifiers (the names of
2261 the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
2262 The number of identifiers must be equal
2263 to the number of expressions, and the <i>n</i>th identifier on
2264 the left is bound to the value of the <i>n</i>th expression on the
2269 ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
2270 ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
2272 IdentifierList = identifier { "," identifier } .
2273 ExpressionList = Expression { "," Expression } .
2277 If the type is present, all constants take the type specified, and
2278 the expressions must be <a href="#Assignability">assignable</a> to that type,
2279 which must not be a type parameter.
2280 If the type is omitted, the constants take the
2281 individual types of the corresponding expressions.
2282 If the expression values are untyped <a href="#Constants">constants</a>,
2283 the declared constants remain untyped and the constant identifiers
2284 denote the constant values. For instance, if the expression is a
2285 floating-point literal, the constant identifier denotes a floating-point
2286 constant, even if the literal's fractional part is zero.
2290 const Pi float64 = 3.14159265358979323846
2291 const zero = 0.0 // untyped floating-point constant
2294 eof = -1 // untyped integer constant
2296 const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
2297 const u, v float32 = 0, 3 // u = 0.0, v = 3.0
2301 Within a parenthesized <code>const</code> declaration list the
2302 expression list may be omitted from any but the first ConstSpec.
2303 Such an empty list is equivalent to the textual substitution of the
2304 first preceding non-empty expression list and its type if any.
2305 Omitting the list of expressions is therefore equivalent to
2306 repeating the previous list. The number of identifiers must be equal
2307 to the number of expressions in the previous list.
2308 Together with the <a href="#Iota"><code>iota</code> constant generator</a>
2309 this mechanism permits light-weight declaration of sequential values:
2321 numberOfDays // this constant is not exported
2326 <h3 id="Iota">Iota</h3>
2329 Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
2330 <code>iota</code> represents successive untyped integer <a href="#Constants">
2331 constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
2332 in that constant declaration, starting at zero.
2333 It can be used to construct a set of related constants:
2338 c0 = iota // c0 == 0
2339 c1 = iota // c1 == 1
2340 c2 = iota // c2 == 2
2344 a = 1 << iota // a == 1 (iota == 0)
2345 b = 1 << iota // b == 2 (iota == 1)
2346 c = 3 // c == 3 (iota == 2, unused)
2347 d = 1 << iota // d == 8 (iota == 3)
2351 u = iota * 42 // u == 0 (untyped integer constant)
2352 v float64 = iota * 42 // v == 42.0 (float64 constant)
2353 w = iota * 42 // w == 84 (untyped integer constant)
2356 const x = iota // x == 0
2357 const y = iota // y == 0
2361 By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
2366 bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
2367 bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
2368 _, _ // (iota == 2, unused)
2369 bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
2374 This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
2375 of the last non-empty expression list.
2379 <h3 id="Type_declarations">Type declarations</h3>
2382 A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
2383 Type declarations come in two forms: alias declarations and type definitions.
2387 TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
2388 TypeSpec = AliasDecl | TypeDef .
2391 <h4 id="Alias_declarations">Alias declarations</h4>
2394 An alias declaration binds an identifier to the given type.
2398 AliasDecl = identifier "=" Type .
2402 Within the <a href="#Declarations_and_scope">scope</a> of
2403 the identifier, it serves as an <i>alias</i> for the type.
2408 nodeList = []*Node // nodeList and []*Node are identical types
2409 Polar = polar // Polar and polar denote identical types
2414 <h4 id="Type_definitions">Type definitions</h4>
2417 A type definition creates a new, distinct type with the same
2418 <a href="#Types">underlying type</a> and operations as the given type
2419 and binds an identifier, the <i>type name</i>, to it.
2423 TypeDef = identifier [ TypeParameters ] Type .
2427 The new type is called a <i>defined type</i>.
2428 It is <a href="#Type_identity">different</a> from any other type,
2429 including the type it is created from.
2434 Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
2435 polar Point // polar and Point denote different types
2438 type TreeNode struct {
2439 left, right *TreeNode
2443 type Block interface {
2445 Encrypt(src, dst []byte)
2446 Decrypt(src, dst []byte)
2451 A defined type may have <a href="#Method_declarations">methods</a> associated with it.
2452 It does not inherit any methods bound to the given type,
2453 but the <a href="#Method_sets">method set</a>
2454 of an interface type or of elements of a composite type remains unchanged:
2458 // A Mutex is a data type with two methods, Lock and Unlock.
2459 type Mutex struct { /* Mutex fields */ }
2460 func (m *Mutex) Lock() { /* Lock implementation */ }
2461 func (m *Mutex) Unlock() { /* Unlock implementation */ }
2463 // NewMutex has the same composition as Mutex but its method set is empty.
2466 // The method set of PtrMutex's underlying type *Mutex remains unchanged,
2467 // but the method set of PtrMutex is empty.
2468 type PtrMutex *Mutex
2470 // The method set of *PrintableMutex contains the methods
2471 // Lock and Unlock bound to its embedded field Mutex.
2472 type PrintableMutex struct {
2476 // MyBlock is an interface type that has the same method set as Block.
2481 Type definitions may be used to define different boolean, numeric,
2482 or string types and associate methods with them:
2489 EST TimeZone = -(5 + iota)
2495 func (tz TimeZone) String() string {
2496 return fmt.Sprintf("GMT%+dh", tz)
2501 If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>,
2502 the type name denotes a <i>generic type</i>.
2503 Generic types must be <a href="#Instantiations">instantiated</a> when they
2508 type List[T any] struct {
2515 In a type definition the given type cannot be a type parameter.
2519 type T[P any] P // illegal: P is a type parameter
2522 type L T // illegal: T is a type parameter declared by the enclosing function
2527 A generic type may also have <a href="#Method_declarations">methods</a> associated with it.
2528 In this case, the method receivers must declare the same number of type parameters as
2529 present in the generic type definition.
2533 // The method Len returns the number of elements in the linked list l.
2534 func (l *List[T]) Len() int { … }
2537 <h3 id="Type_parameter_declarations">Type parameter declarations</h3>
2540 A type parameter list declares the <i>type parameters</i> of a generic function or type declaration.
2541 The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a>
2542 except that the type parameter names must all be present and the list is enclosed
2543 in square brackets rather than parentheses.
2547 TypeParameters = "[" TypeParamList [ "," ] "]" .
2548 TypeParamList = TypeParamDecl { "," TypeParamDecl } .
2549 TypeParamDecl = IdentifierList TypeConstraint .
2553 All non-blank names in the list must be unique.
2554 Each name declares a type parameter, which is a new and different <a href="#Types">named type</a>
2555 that acts as a place holder for an (as of yet) unknown type in the declaration.
2556 The type parameter is replaced with a <i>type argument</i> upon
2557 <a href="#Instantiations">instantiation</a> of the generic function or type.
2562 [S interface{ ~[]byte|string }]
2569 Just as each ordinary function parameter has a parameter type, each type parameter
2570 has a corresponding (meta-)type which is called its
2571 <a href="#Type_constraints"><i>type constraint</i></a>.
2575 A parsing ambiguity arises when the type parameter list for a generic type
2576 declares a single type parameter <code>P</code> with a constraint <code>C</code>
2577 such that the text <code>P C</code> forms a valid expression:
2588 In these rare cases, the type parameter list is indistinguishable from an
2589 expression and the type declaration is parsed as an array type declaration.
2590 To resolve the ambiguity, embed the constraint in an
2591 <a href="#Interface_types">interface</a> or use a trailing comma:
2595 type T[P interface{*C}] …
2600 Type parameters may also be declared by the receiver specification
2601 of a <a href="#Method_declarations">method declaration</a> associated
2602 with a generic type.
2606 This section needs to explain if and what kind of cycles are permitted
2607 using type parameters in a type parameter list.
2610 <h4 id="Type_constraints">Type constraints</h4>
2613 A type constraint is an <a href="#Interface_types">interface</a> that defines the
2614 set of permissible type arguments for the respective type parameter and controls the
2615 operations supported by values of that type parameter.
2619 TypeConstraint = TypeElem .
2623 If the constraint is an interface literal of the form <code>interface{E}</code> where
2624 <code>E</code> is an embedded type element (not a method), in a type parameter list
2625 the enclosing <code>interface{ … }</code> may be omitted for convenience:
2629 [T []P] // = [T interface{[]P}]
2630 [T ~int] // = [T interface{~int}]
2631 [T int|string] // = [T interface{int|string}]
2632 type Constraint ~int // illegal: ~int is not inside a type parameter list
2636 We should be able to simplify the rules for comparable or delegate some of them
2637 elsewhere since we have a section that clearly defines how interfaces implement
2638 other interfaces based on their type sets. But this should get us going for now.
2642 The <a href="#Predeclared_identifiers">predeclared</a>
2643 <a href="#Interface_types">interface type</a> <code>comparable</code>
2644 denotes the set of all non-interface types that are
2645 <a href="#Comparison_operators">comparable</a>. Specifically,
2646 a type <code>T</code> implements <code>comparable</code> if:
2651 <code>T</code> is not an interface type and <code>T</code> supports the operations
2652 <code>==</code> and <code>!=</code>; or
2655 <code>T</code> is an interface type and each type in <code>T</code>'s
2656 <a href="#Interface_types">type set</a> implements <code>comparable</code>.
2661 Even though interfaces that are not type parameters can be
2662 <a href="#Comparison_operators">compared</a>
2663 (possibly causing a run-time panic) they do not implement
2664 <code>comparable</code>.
2668 int // implements comparable
2669 []byte // does not implement comparable (slices cannot be compared)
2670 interface{} // does not implement comparable (see above)
2671 interface{ ~int | ~string } // type parameter only: implements comparable
2672 interface{ comparable } // type parameter only: implements comparable
2673 interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (not all types in the type set are comparable)
2677 The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
2678 <code>comparable</code> may only be used as type constraints. They cannot be the types of
2679 values or variables, or components of other, non-interface types.
2682 <h3 id="Variable_declarations">Variable declarations</h3>
2685 A variable declaration creates one or more <a href="#Variables">variables</a>,
2686 binds corresponding identifiers to them, and gives each a type and an initial value.
2690 VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
2691 VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
2698 var x, y float32 = -1, -2
2701 u, v, s = 2.0, 3.0, "bar"
2703 var re, im = complexSqrt(-1)
2704 var _, found = entries[name] // map lookup; only interested in "found"
2708 If a list of expressions is given, the variables are initialized
2709 with the expressions following the rules for <a href="#Assignments">assignments</a>.
2710 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
2714 If a type is present, each variable is given that type.
2715 Otherwise, each variable is given the type of the corresponding
2716 initialization value in the assignment.
2717 If that value is an untyped constant, it is first implicitly
2718 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
2719 if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
2720 The predeclared value <code>nil</code> cannot be used to initialize a variable
2721 with no explicit type.
2725 var d = math.Sin(0.5) // d is float64
2726 var i = 42 // i is int
2727 var t, ok = x.(T) // t is T, ok is bool
2728 var n = nil // illegal
2732 Implementation restriction: A compiler may make it illegal to declare a variable
2733 inside a <a href="#Function_declarations">function body</a> if the variable is
2737 <h3 id="Short_variable_declarations">Short variable declarations</h3>
2740 A <i>short variable declaration</i> uses the syntax:
2744 ShortVarDecl = IdentifierList ":=" ExpressionList .
2748 It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
2749 with initializer expressions but no types:
2752 <pre class="grammar">
2753 "var" IdentifierList = ExpressionList .
2758 f := func() int { return 7 }
2759 ch := make(chan int)
2760 r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
2761 _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
2765 Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
2766 variables provided they were originally declared earlier in the same block
2767 (or the parameter lists if the block is the function body) with the same type,
2768 and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
2769 As a consequence, redeclaration can only appear in a multi-variable short declaration.
2770 Redeclaration does not introduce a new variable; it just assigns a new value to the original.
2774 field1, offset := nextField(str, 0)
2775 field2, offset := nextField(str, offset) // redeclares offset
2776 a, a := 1, 2 // illegal: double declaration of a or no new variable if a was declared elsewhere
2780 Short variable declarations may appear only inside functions.
2781 In some contexts such as the initializers for
2782 <a href="#If_statements">"if"</a>,
2783 <a href="#For_statements">"for"</a>, or
2784 <a href="#Switch_statements">"switch"</a> statements,
2785 they can be used to declare local temporary variables.
2788 <h3 id="Function_declarations">Function declarations</h3>
2791 Given the importance of functions, this section has always
2792 been woefully underdeveloped. Would be nice to expand this
2797 A function declaration binds an identifier, the <i>function name</i>,
2802 FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
2803 FunctionName = identifier .
2804 FunctionBody = Block .
2808 If the function's <a href="#Function_types">signature</a> declares
2809 result parameters, the function body's statement list must end in
2810 a <a href="#Terminating_statements">terminating statement</a>.
2814 func IndexRune(s string, r rune) int {
2815 for i, c := range s {
2820 // invalid: missing return statement
2825 If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>,
2826 the function name denotes a <i>generic function</i>.
2827 A generic function must be <a href="#Instantiations">instantiated</a> before it can be
2828 called or used as a value.
2832 func min[T ~int|~float64](x, y T) T {
2841 A function declaration without type parameters may omit the body.
2842 Such a declaration provides the signature for a function implemented outside Go,
2843 such as an assembly routine.
2847 func flushICache(begin, end uintptr) // implemented externally
2850 <h3 id="Method_declarations">Method declarations</h3>
2853 A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
2854 A method declaration binds an identifier, the <i>method name</i>, to a method,
2855 and associates the method with the receiver's <i>base type</i>.
2859 MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
2860 Receiver = Parameters .
2864 The receiver is specified via an extra parameter section preceding the method
2865 name. That parameter section must declare a single non-variadic parameter, the receiver.
2866 Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
2867 pointer to a defined type <code>T</code>, possibly followed by a list of type parameter
2868 names <code>[P1, P2, …]</code> enclosed in square brackets.
2869 <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be
2870 a pointer or interface type and it must be defined in the same package as the method.
2871 The method is said to be <i>bound</i> to its receiver base type and the method name
2872 is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
2877 A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
2878 <a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
2879 If the receiver's value is not referenced inside the body of the method,
2880 its identifier may be omitted in the declaration. The same applies in
2881 general to parameters of functions and methods.
2885 For a base type, the non-blank names of methods bound to it must be unique.
2886 If the base type is a <a href="#Struct_types">struct type</a>,
2887 the non-blank method and field names must be distinct.
2891 Given defined type <code>Point</code> the declarations
2895 func (p *Point) Length() float64 {
2896 return math.Sqrt(p.x * p.x + p.y * p.y)
2899 func (p *Point) Scale(factor float64) {
2906 bind the methods <code>Length</code> and <code>Scale</code>,
2907 with receiver type <code>*Point</code>,
2908 to the base type <code>Point</code>.
2912 If the receiver base type is a <a href="#Type_declarations">generic type</a>, the
2913 receiver specification must declare corresponding type parameters for the method
2914 to use. This makes the receiver type parameters available to the method.
2915 Syntactically, this type parameter declaration looks like an
2916 <a href="#Instantiations">instantiation</a> of the receiver base type: the type
2917 arguments must be identifiers denoting the type parameters being declared, one
2918 for each type parameter of the receiver base type.
2919 The type parameter names do not need to match their corresponding parameter names in the
2920 receiver base type definition, and all non-blank parameter names must be unique in the
2921 receiver parameter section and the method signature.
2922 The receiver type parameter constraints are implied by the receiver base type definition:
2923 corresponding type parameters have corresponding constraints.
2927 type Pair[A, B any] struct {
2932 func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B
2933 func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair
2936 <h2 id="Expressions">Expressions</h2>
2939 An expression specifies the computation of a value by applying
2940 operators and functions to operands.
2943 <h3 id="Operands">Operands</h3>
2946 Operands denote the elementary values in an expression. An operand may be a
2947 literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
2948 non-<a href="#Blank_identifier">blank</a> identifier denoting a
2949 <a href="#Constant_declarations">constant</a>,
2950 <a href="#Variable_declarations">variable</a>, or
2951 <a href="#Function_declarations">function</a>,
2952 or a parenthesized expression.
2956 Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
2957 Literal = BasicLit | CompositeLit | FunctionLit .
2958 BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
2959 OperandName = identifier | QualifiedIdent .
2963 An operand name denoting a <a href="#Function_declarations">generic function</a>
2964 may be followed by a list of <a href="#Instantiations">type arguments</a>; the
2965 resulting operand is an <a href="#Instantiations">instantiated</a> function.
2969 The <a href="#Blank_identifier">blank identifier</a> may appear as an
2970 operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
2974 Implementation restriction: A compiler need not report an error if an operand's
2975 type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty
2976 <a href="#Interface_types">type set</a>. Functions with such type parameters
2977 cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead
2978 to an error at the instantiation site.
2981 <h3 id="Qualified_identifiers">Qualified identifiers</h3>
2984 A <i>qualified identifier</i> is an identifier qualified with a package name prefix.
2985 Both the package name and the identifier must not be
2986 <a href="#Blank_identifier">blank</a>.
2990 QualifiedIdent = PackageName "." identifier .
2994 A qualified identifier accesses an identifier in a different package, which
2995 must be <a href="#Import_declarations">imported</a>.
2996 The identifier must be <a href="#Exported_identifiers">exported</a> and
2997 declared in the <a href="#Blocks">package block</a> of that package.
3001 math.Sin // denotes the Sin function in package math
3004 <h3 id="Composite_literals">Composite literals</h3>
3007 Composite literals construct new composite values each time they are evaluated.
3008 They consist of the type of the literal followed by a brace-bound list of elements.
3009 Each element may optionally be preceded by a corresponding key.
3013 CompositeLit = LiteralType LiteralValue .
3014 LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
3015 SliceType | MapType | TypeName .
3016 LiteralValue = "{" [ ElementList [ "," ] ] "}" .
3017 ElementList = KeyedElement { "," KeyedElement } .
3018 KeyedElement = [ Key ":" ] Element .
3019 Key = FieldName | Expression | LiteralValue .
3020 FieldName = identifier .
3021 Element = Expression | LiteralValue .
3025 The LiteralType's <a href="#Core_types">core type</a> <code>T</code>
3026 must be a struct, array, slice, or map type
3027 (the grammar enforces this constraint except when the type is given
3029 The types of the elements and keys must be <a href="#Assignability">assignable</a>
3030 to the respective field, element, and key types of type <code>T</code>;
3031 there is no additional conversion.
3032 The key is interpreted as a field name for struct literals,
3033 an index for array and slice literals, and a key for map literals.
3034 For map literals, all elements must have a key. It is an error
3035 to specify multiple elements with the same field name or
3036 constant key value. For non-constant map keys, see the section on
3037 <a href="#Order_of_evaluation">evaluation order</a>.
3041 For struct literals the following rules apply:
3044 <li>A key must be a field name declared in the struct type.
3046 <li>An element list that does not contain any keys must
3047 list an element for each struct field in the
3048 order in which the fields are declared.
3050 <li>If any element has a key, every element must have a key.
3052 <li>An element list that contains keys does not need to
3053 have an element for each struct field. Omitted fields
3054 get the zero value for that field.
3056 <li>A literal may omit the element list; such a literal evaluates
3057 to the zero value for its type.
3059 <li>It is an error to specify an element for a non-exported
3060 field of a struct belonging to a different package.
3065 Given the declarations
3068 type Point3D struct { x, y, z float64 }
3069 type Line struct { p, q Point3D }
3077 origin := Point3D{} // zero value for Point3D
3078 line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
3082 For array and slice literals the following rules apply:
3085 <li>Each element has an associated integer index marking
3086 its position in the array.
3088 <li>An element with a key uses the key as its index. The
3089 key must be a non-negative constant
3090 <a href="#Representability">representable</a> by
3091 a value of type <code>int</code>; and if it is typed
3092 it must be of <a href="#Numeric_types">integer type</a>.
3094 <li>An element without a key uses the previous element's index plus one.
3095 If the first element has no key, its index is zero.
3100 <a href="#Address_operators">Taking the address</a> of a composite literal
3101 generates a pointer to a unique <a href="#Variables">variable</a> initialized
3102 with the literal's value.
3106 var pointer *Point3D = &Point3D{y: 1000}
3110 Note that the <a href="#The_zero_value">zero value</a> for a slice or map
3111 type is not the same as an initialized but empty value of the same type.
3112 Consequently, taking the address of an empty slice or map composite literal
3113 does not have the same effect as allocating a new slice or map value with
3114 <a href="#Allocation">new</a>.
3118 p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
3119 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
3123 The length of an array literal is the length specified in the literal type.
3124 If fewer elements than the length are provided in the literal, the missing
3125 elements are set to the zero value for the array element type.
3126 It is an error to provide elements with index values outside the index range
3127 of the array. The notation <code>...</code> specifies an array length equal
3128 to the maximum element index plus one.
3132 buffer := [10]string{} // len(buffer) == 10
3133 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
3134 days := [...]string{"Sat", "Sun"} // len(days) == 2
3138 A slice literal describes the entire underlying array literal.
3139 Thus the length and capacity of a slice literal are the maximum
3140 element index plus one. A slice literal has the form
3148 and is shorthand for a slice operation applied to an array:
3152 tmp := [n]T{x1, x2, … xn}
3157 Within a composite literal of array, slice, or map type <code>T</code>,
3158 elements or map keys that are themselves composite literals may elide the respective
3159 literal type if it is identical to the element or key type of <code>T</code>.
3160 Similarly, elements or keys that are addresses of composite literals may elide
3161 the <code>&T</code> when the element or key type is <code>*T</code>.
3165 [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
3166 [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
3167 [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
3168 map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
3169 map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
3172 [2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
3173 [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
3177 A parsing ambiguity arises when a composite literal using the
3178 TypeName form of the LiteralType appears as an operand between the
3179 <a href="#Keywords">keyword</a> and the opening brace of the block
3180 of an "if", "for", or "switch" statement, and the composite literal
3181 is not enclosed in parentheses, square brackets, or curly braces.
3182 In this rare case, the opening brace of the literal is erroneously parsed
3183 as the one introducing the block of statements. To resolve the ambiguity,
3184 the composite literal must appear within parentheses.
3188 if x == (T{a,b,c}[i]) { … }
3189 if (x == T{a,b,c}[i]) { … }
3193 Examples of valid array, slice, and map literals:
3197 // list of prime numbers
3198 primes := []int{2, 3, 5, 7, 9, 2147483647}
3200 // vowels[ch] is true if ch is a vowel
3201 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
3203 // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
3204 filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
3206 // frequencies in Hz for equal-tempered scale (A4 = 440Hz)
3207 noteFrequency := map[string]float32{
3208 "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
3209 "G0": 24.50, "A0": 27.50, "B0": 30.87,
3214 <h3 id="Function_literals">Function literals</h3>
3217 A function literal represents an anonymous <a href="#Function_declarations">function</a>.
3218 Function literals cannot declare type parameters.
3222 FunctionLit = "func" Signature FunctionBody .
3226 func(a, b int, z float64) bool { return a*b < int(z) }
3230 A function literal can be assigned to a variable or invoked directly.
3234 f := func(x, y int) int { return x + y }
3235 func(ch chan int) { ch <- ACK }(replyChan)
3239 Function literals are <i>closures</i>: they may refer to variables
3240 defined in a surrounding function. Those variables are then shared between
3241 the surrounding function and the function literal, and they survive as long
3242 as they are accessible.
3246 <h3 id="Primary_expressions">Primary expressions</h3>
3249 Primary expressions are the operands for unary and binary expressions.
3257 PrimaryExpr Selector |
3260 PrimaryExpr TypeAssertion |
3261 PrimaryExpr Arguments .
3263 Selector = "." identifier .
3264 Index = "[" Expression "]" .
3265 Slice = "[" [ Expression ] ":" [ Expression ] "]" |
3266 "[" [ Expression ] ":" Expression ":" Expression "]" .
3267 TypeAssertion = "." "(" Type ")" .
3268 Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
3285 <h3 id="Selectors">Selectors</h3>
3288 For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
3289 that is not a <a href="#Package_clause">package name</a>, the
3290 <i>selector expression</i>
3298 denotes the field or method <code>f</code> of the value <code>x</code>
3299 (or sometimes <code>*x</code>; see below).
3300 The identifier <code>f</code> is called the (field or method) <i>selector</i>;
3301 it must not be the <a href="#Blank_identifier">blank identifier</a>.
3302 The type of the selector expression is the type of <code>f</code>.
3303 If <code>x</code> is a package name, see the section on
3304 <a href="#Qualified_identifiers">qualified identifiers</a>.
3308 A selector <code>f</code> may denote a field or method <code>f</code> of
3309 a type <code>T</code>, or it may refer
3310 to a field or method <code>f</code> of a nested
3311 <a href="#Struct_types">embedded field</a> of <code>T</code>.
3312 The number of embedded fields traversed
3313 to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
3314 The depth of a field or method <code>f</code>
3315 declared in <code>T</code> is zero.
3316 The depth of a field or method <code>f</code> declared in
3317 an embedded field <code>A</code> in <code>T</code> is the
3318 depth of <code>f</code> in <code>A</code> plus one.
3322 The following rules apply to selectors:
3327 For a value <code>x</code> of type <code>T</code> or <code>*T</code>
3328 where <code>T</code> is not a pointer or interface type,
3329 <code>x.f</code> denotes the field or method at the shallowest depth
3330 in <code>T</code> where there is such an <code>f</code>.
3331 If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
3332 with shallowest depth, the selector expression is illegal.
3336 For a value <code>x</code> of type <code>I</code> where <code>I</code>
3337 is an interface type, <code>x.f</code> denotes the actual method with name
3338 <code>f</code> of the dynamic value of <code>x</code>.
3339 If there is no method with name <code>f</code> in the
3340 <a href="#Method_sets">method set</a> of <code>I</code>, the selector
3341 expression is illegal.
3345 As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
3346 pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
3347 (but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
3351 In all other cases, <code>x.f</code> is illegal.
3355 If <code>x</code> is of pointer type and has the value
3356 <code>nil</code> and <code>x.f</code> denotes a struct field,
3357 assigning to or evaluating <code>x.f</code>
3358 causes a <a href="#Run_time_panics">run-time panic</a>.
3362 If <code>x</code> is of interface type and has the value
3363 <code>nil</code>, <a href="#Calls">calling</a> or
3364 <a href="#Method_values">evaluating</a> the method <code>x.f</code>
3365 causes a <a href="#Run_time_panics">run-time panic</a>.
3370 For example, given the declarations:
3396 var t T2 // with t.T0 != nil
3397 var p *T2 // with p != nil and (*p).T0 != nil
3414 q.x // (*(*q).T0).x (*q).x is a valid field selector
3416 p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
3417 p.M1() // ((*p).T1).M1() M1 expects T1 receiver
3418 p.M2() // p.M2() M2 expects *T2 receiver
3419 t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
3423 but the following is invalid:
3427 q.M0() // (*q).M0 is valid but not a field selector
3431 <h3 id="Method_expressions">Method expressions</h3>
3434 If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3435 <code>T.M</code> is a function that is callable as a regular function
3436 with the same arguments as <code>M</code> prefixed by an additional
3437 argument that is the receiver of the method.
3441 MethodExpr = ReceiverType "." MethodName .
3442 ReceiverType = Type .
3446 Consider a struct type <code>T</code> with two methods,
3447 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3448 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3455 func (tv T) Mv(a int) int { return 0 } // value receiver
3456 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3470 yields a function equivalent to <code>Mv</code> but
3471 with an explicit receiver as its first argument; it has signature
3475 func(tv T, a int) int
3479 That function may be called normally with an explicit receiver, so
3480 these five invocations are equivalent:
3487 f1 := T.Mv; f1(t, 7)
3488 f2 := (T).Mv; f2(t, 7)
3492 Similarly, the expression
3500 yields a function value representing <code>Mp</code> with signature
3504 func(tp *T, f float32) float32
3508 For a method with a value receiver, one can derive a function
3509 with an explicit pointer receiver, so
3517 yields a function value representing <code>Mv</code> with signature
3521 func(tv *T, a int) int
3525 Such a function indirects through the receiver to create a value
3526 to pass as the receiver to the underlying method;
3527 the method does not overwrite the value whose address is passed in
3532 The final case, a value-receiver function for a pointer-receiver method,
3533 is illegal because pointer-receiver methods are not in the method set
3538 Function values derived from methods are called with function call syntax;
3539 the receiver is provided as the first argument to the call.
3540 That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
3541 as <code>f(t, 7)</code> not <code>t.f(7)</code>.
3542 To construct a function that binds the receiver, use a
3543 <a href="#Function_literals">function literal</a> or
3544 <a href="#Method_values">method value</a>.
3548 It is legal to derive a function value from a method of an interface type.
3549 The resulting function takes an explicit receiver of that interface type.
3552 <h3 id="Method_values">Method values</h3>
3555 If the expression <code>x</code> has static type <code>T</code> and
3556 <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3557 <code>x.M</code> is called a <i>method value</i>.
3558 The method value <code>x.M</code> is a function value that is callable
3559 with the same arguments as a method call of <code>x.M</code>.
3560 The expression <code>x</code> is evaluated and saved during the evaluation of the
3561 method value; the saved copy is then used as the receiver in any calls,
3562 which may be executed later.
3566 type S struct { *T }
3568 func (t T) M() { print(t) }
3572 f := t.M // receiver *t is evaluated and stored in f
3573 g := s.M // receiver *(s.T) is evaluated and stored in g
3574 *t = 42 // does not affect stored receivers in f and g
3578 The type <code>T</code> may be an interface or non-interface type.
3582 As in the discussion of <a href="#Method_expressions">method expressions</a> above,
3583 consider a struct type <code>T</code> with two methods,
3584 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3585 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3592 func (tv T) Mv(a int) int { return 0 } // value receiver
3593 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3609 yields a function value of type
3617 These two invocations are equivalent:
3626 Similarly, the expression
3634 yields a function value of type
3638 func(float32) float32
3642 As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
3643 using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
3647 As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
3648 using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
3652 f := t.Mv; f(7) // like t.Mv(7)
3653 f := pt.Mp; f(7) // like pt.Mp(7)
3654 f := pt.Mv; f(7) // like (*pt).Mv(7)
3655 f := t.Mp; f(7) // like (&t).Mp(7)
3656 f := makeT().Mp // invalid: result of makeT() is not addressable
3660 Although the examples above use non-interface types, it is also legal to create a method value
3661 from a value of interface type.
3665 var i interface { M(int) } = myVal
3666 f := i.M; f(7) // like i.M(7)
3670 <h3 id="Index_expressions">Index expressions</h3>
3673 A primary expression of the form
3681 denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
3682 The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
3683 The following rules apply:
3687 If <code>a</code> is neither a map nor a type parameter:
3690 <li>the index <code>x</code> must be an untyped constant or its
3691 <a href="#Core_types">core type</a> must be an <a href="#Numeric_types">integer</a></li>
3692 <li>a constant index must be non-negative and
3693 <a href="#Representability">representable</a> by a value of type <code>int</code></li>
3694 <li>a constant index that is untyped is given type <code>int</code></li>
3695 <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
3696 otherwise it is <i>out of range</i></li>
3700 For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
3703 <li>a <a href="#Constants">constant</a> index must be in range</li>
3704 <li>if <code>x</code> is out of range at run time,
3705 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3706 <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
3707 <code>a[x]</code> is the element type of <code>A</code></li>
3711 For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
3714 <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
3718 For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
3721 <li>if <code>x</code> is out of range at run time,
3722 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3723 <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
3724 <code>a[x]</code> is the element type of <code>S</code></li>
3728 For <code>a</code> of <a href="#String_types">string type</a>:
3731 <li>a <a href="#Constants">constant</a> index must be in range
3732 if the string <code>a</code> is also constant</li>
3733 <li>if <code>x</code> is out of range at run time,
3734 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3735 <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
3736 <code>a[x]</code> is <code>byte</code></li>
3737 <li><code>a[x]</code> may not be assigned to</li>
3741 For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
3744 <li><code>x</code>'s type must be
3745 <a href="#Assignability">assignable</a>
3746 to the key type of <code>M</code></li>
3747 <li>if the map contains an entry with key <code>x</code>,
3748 <code>a[x]</code> is the map element with key <code>x</code>
3749 and the type of <code>a[x]</code> is the element type of <code>M</code></li>
3750 <li>if the map is <code>nil</code> or does not contain such an entry,
3751 <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
3752 for the element type of <code>M</code></li>
3756 For <code>a</code> of <a href="#Type_parameter_declarations">type parameter type</a> <code>P</code>:
3759 <li>The index expression <code>a[x]</code> must be valid for values
3760 of all types in <code>P</code>'s type set.</li>
3761 <li>The element types of all types in <code>P</code>'s type set must be identical.
3762 In this context, the element type of a string type is <code>byte</code>.</li>
3763 <li>If there is a map type in the type set of <code>P</code>,
3764 all types in that type set must be map types, and the respective key types
3765 must be all identical.</li>
3766 <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
3767 or the map element with key <code>x</code> of the type argument
3768 that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
3769 the type of the (identical) element types.</li>
3770 <li><code>a[x]</code> may not be assigned to if <code>P</code>'s type set
3771 includes string types.
3775 Otherwise <code>a[x]</code> is illegal.
3779 An index expression on a map <code>a</code> of type <code>map[K]V</code>
3780 used in an <a href="#Assignments">assignment</a> or initialization of the special form
3790 yields an additional untyped boolean value. The value of <code>ok</code> is
3791 <code>true</code> if the key <code>x</code> is present in the map, and
3792 <code>false</code> otherwise.
3796 Assigning to an element of a <code>nil</code> map causes a
3797 <a href="#Run_time_panics">run-time panic</a>.
3801 <h3 id="Slice_expressions">Slice expressions</h3>
3804 Slice expressions construct a substring or slice from a string, array, pointer
3805 to array, or slice. There are two variants: a simple form that specifies a low
3806 and high bound, and a full form that also specifies a bound on the capacity.
3809 <h4>Simple slice expressions</h4>
3812 The primary expression
3820 constructs a substring or slice. The <a href="#Core_types">core type</a> of
3821 <code>a</code> must be a string, array, pointer to array, or slice.
3822 The <i>indices</i> <code>low</code> and
3823 <code>high</code> select which elements of operand <code>a</code> appear
3824 in the result. The result has indices starting at 0 and length equal to
3825 <code>high</code> - <code>low</code>.
3826 After slicing the array <code>a</code>
3830 a := [5]int{1, 2, 3, 4, 5}
3835 the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
3845 For convenience, any of the indices may be omitted. A missing <code>low</code>
3846 index defaults to zero; a missing <code>high</code> index defaults to the length of the
3851 a[2:] // same as a[2 : len(a)]
3852 a[:3] // same as a[0 : 3]
3853 a[:] // same as a[0 : len(a)]
3857 If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
3858 <code>(*a)[low : high]</code>.
3862 For arrays or strings, the indices are <i>in range</i> if
3863 <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
3864 otherwise they are <i>out of range</i>.
3865 For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
3866 A <a href="#Constants">constant</a> index must be non-negative and
3867 <a href="#Representability">representable</a> by a value of type
3868 <code>int</code>; for arrays or constant strings, constant indices must also be in range.
3869 If both indices are constant, they must satisfy <code>low <= high</code>.
3870 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3874 Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
3875 the result of the slice operation is a non-constant value of the same type as the operand.
3876 For untyped string operands the result is a non-constant value of type <code>string</code>.
3877 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
3878 and the result of the slice operation is a slice with the same element type as the array.
3882 If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
3883 is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
3884 array with the operand.
3889 s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
3890 s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
3891 s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
3895 <h4>Full slice expressions</h4>
3898 The primary expression
3906 constructs a slice of the same type, and with the same length and elements as the simple slice
3907 expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
3908 by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
3909 The <a href="#Core_types">core type</a> of <code>a</code> must be an array, pointer to array,
3910 or slice (but not a string).
3911 After slicing the array <code>a</code>
3915 a := [5]int{1, 2, 3, 4, 5}
3920 the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
3929 As for simple slice expressions, if <code>a</code> is a pointer to an array,
3930 <code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
3931 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
3935 The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
3936 otherwise they are <i>out of range</i>.
3937 A <a href="#Constants">constant</a> index must be non-negative and
3938 <a href="#Representability">representable</a> by a value of type
3939 <code>int</code>; for arrays, constant indices must also be in range.
3940 If multiple indices are constant, the constants that are present must be in range relative to each
3942 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3945 <h3 id="Type_assertions">Type assertions</h3>
3948 For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
3949 but not a <a href="#Type_parameter_declarations">type parameter</a>, and a type <code>T</code>,
3950 the primary expression
3958 asserts that <code>x</code> is not <code>nil</code>
3959 and that the value stored in <code>x</code> is of type <code>T</code>.
3960 The notation <code>x.(T)</code> is called a <i>type assertion</i>.
3963 More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
3964 that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
3965 to the type <code>T</code>.
3966 In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
3967 otherwise the type assertion is invalid since it is not possible for <code>x</code>
3968 to store a value of type <code>T</code>.
3969 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
3970 of <code>x</code> <a href="#Implementing_an_interface">implements</a> the interface <code>T</code>.
3973 If the type assertion holds, the value of the expression is the value
3974 stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
3975 a <a href="#Run_time_panics">run-time panic</a> occurs.
3976 In other words, even though the dynamic type of <code>x</code>
3977 is known only at run time, the type of <code>x.(T)</code> is
3978 known to be <code>T</code> in a correct program.
3982 var x interface{} = 7 // x has dynamic type int and value 7
3983 i := x.(int) // i has type int and value 7
3985 type I interface { m() }
3988 s := y.(string) // illegal: string does not implement I (missing method m)
3989 r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
3995 A type assertion used in an <a href="#Assignments">assignment</a> or initialization of the special form
4002 var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
4006 yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
4007 if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
4008 the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
4009 No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
4013 <h3 id="Calls">Calls</h3>
4016 Given an expression <code>f</code> with a <a href="#Core_types">core type</a>
4017 <code>F</code> of <a href="#Function_types">function type</a>,
4025 calls <code>f</code> with arguments <code>a1, a2, … an</code>.
4026 Except for one special case, arguments must be single-valued expressions
4027 <a href="#Assignability">assignable</a> to the parameter types of
4028 <code>F</code> and are evaluated before the function is called.
4029 The type of the expression is the result type
4031 A method invocation is similar but the method itself
4032 is specified as a selector upon a value of the receiver type for
4037 math.Atan2(x, y) // function call
4039 pt.Scale(3.5) // method call with receiver pt
4043 If <code>f</code> denotes a generic function, it must be
4044 <a href="#Instantiations">instantiated</a> before it can be called
4045 or used as a function value.
4049 In a function call, the function value and arguments are evaluated in
4050 <a href="#Order_of_evaluation">the usual order</a>.
4051 After they are evaluated, the parameters of the call are passed by value to the function
4052 and the called function begins execution.
4053 The return parameters of the function are passed by value
4054 back to the caller when the function returns.
4058 Calling a <code>nil</code> function value
4059 causes a <a href="#Run_time_panics">run-time panic</a>.
4063 As a special case, if the return values of a function or method
4064 <code>g</code> are equal in number and individually
4065 assignable to the parameters of another function or method
4066 <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
4067 will invoke <code>f</code> after binding the return values of
4068 <code>g</code> to the parameters of <code>f</code> in order. The call
4069 of <code>f</code> must contain no parameters other than the call of <code>g</code>,
4070 and <code>g</code> must have at least one return value.
4071 If <code>f</code> has a final <code>...</code> parameter, it is
4072 assigned the return values of <code>g</code> that remain after
4073 assignment of regular parameters.
4077 func Split(s string, pos int) (string, string) {
4078 return s[0:pos], s[pos:]
4081 func Join(s, t string) string {
4085 if Join(Split(value, len(value)/2)) != value {
4086 log.Panic("test fails")
4091 A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
4092 of (the type of) <code>x</code> contains <code>m</code> and the
4093 argument list can be assigned to the parameter list of <code>m</code>.
4094 If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
4095 set contains <code>m</code>, <code>x.m()</code> is shorthand
4096 for <code>(&x).m()</code>:
4105 There is no distinct method type and there are no method literals.
4108 <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
4111 If <code>f</code> is <a href="#Function_types">variadic</a> with a final
4112 parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
4113 the type of <code>p</code> is equivalent to type <code>[]T</code>.
4114 If <code>f</code> is invoked with no actual arguments for <code>p</code>,
4115 the value passed to <code>p</code> is <code>nil</code>.
4116 Otherwise, the value passed is a new slice
4117 of type <code>[]T</code> with a new underlying array whose successive elements
4118 are the actual arguments, which all must be <a href="#Assignability">assignable</a>
4119 to <code>T</code>. The length and capacity of the slice is therefore
4120 the number of arguments bound to <code>p</code> and may differ for each
4125 Given the function and calls
4128 func Greeting(prefix string, who ...string)
4130 Greeting("hello:", "Joe", "Anna", "Eileen")
4134 within <code>Greeting</code>, <code>who</code> will have the value
4135 <code>nil</code> in the first call, and
4136 <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
4140 If the final argument is assignable to a slice type <code>[]T</code> and
4141 is followed by <code>...</code>, it is passed unchanged as the value
4142 for a <code>...T</code> parameter. In this case no new slice is created.
4146 Given the slice <code>s</code> and call
4150 s := []string{"James", "Jasmine"}
4151 Greeting("goodbye:", s...)
4155 within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
4156 with the same underlying array.
4159 <h3 id="Instantiations">Instantiations</h3>
4162 A generic function or type is <i>instantiated</i> by substituting <i>type arguments</i>
4163 for the type parameters.
4164 Instantiation proceeds in two steps:
4169 Each type argument is substituted for its corresponding type parameter in the generic
4171 This substitution happens across the entire function or type declaration,
4172 including the type parameter list itself and any types in that list.
4176 After substitution, each type argument must <a href="#Interface_types">implement</a>
4177 the <a href="#Type_parameter_declarations">constraint</a> (instantiated, if necessary)
4178 of the corresponding type parameter. Otherwise instantiation fails.
4183 Instantiating a type results in a new non-generic <a href="#Types">named type</a>;
4184 instantiating a function produces a new non-generic function.
4188 type parameter list type arguments after substitution
4190 [P any] int int implements any
4191 [S ~[]E, E any] []int, int []int implements ~[]int, int implements any
4192 [P io.Writer] string illegal: string doesn't implement io.Writer
4196 For a generic function, type arguments may be provided explicitly, or they
4197 may be partially or completely <a href="#Type_inference">inferred</a>.
4198 A generic function that is is <i>not</i> <a href="#Calls">called</a> requires a
4199 type argument list for instantiation; if the list is partial, all
4200 remaining type arguments must be inferrable.
4201 A generic function that is called may provide a (possibly partial) type
4202 argument list, or may omit it entirely if the omitted type arguments are
4203 inferrable from the ordinary (non-type) function arguments.
4207 func min[T ~int|~float64](x, y T) T { … }
4209 f := min // illegal: min must be instantiated with type arguments when used without being called
4210 minInt := min[int] // minInt has type func(x, y int) int
4211 a := minInt(2, 3) // a has value 2 of type int
4212 b := min[float64](2.0, 3) // b has value 2.0 of type float64
4213 c := min(b, -1) // c has value -1.0 of type float64
4217 A partial type argument list cannot be empty; at least the first argument must be present.
4218 The list is a prefix of the full list of type arguments, leaving the remaining arguments
4219 to be inferred. Loosely speaking, type arguments may be omitted from "right to left".
4223 func apply[S ~[]E, E any](s S, f(E) E) S { … }
4225 f0 := apply[] // illegal: type argument list cannot be empty
4226 f1 := apply[[]int] // type argument for S explicitly provided, type argument for E inferred
4227 f2 := apply[[]string, string] // both type arguments explicitly provided
4230 r := apply(bytes, func(byte) byte { … }) // both type arguments inferred from the function arguments
4234 For a generic type, all type arguments must always be provided explicitly.
4237 <h3 id="Type_inference">Type inference</h3>
4240 Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
4241 Each step attempts to use known information to infer additional type arguments.
4242 Type inference stops as soon as all type arguments are known.
4243 After type inference is complete, it is still necessary to substitute all type arguments
4244 for type parameters and verify that each type argument
4245 <a href="#Implementing_an_interface">implements</a> the relevant constraint;
4246 it is possible for an inferred type argument to fail to implement a constraint, in which
4247 case instantiation fails.
4251 Type inference is based on
4256 a <a href="#Type_parameter_declarations">type parameter list</a>
4259 a substitution map <i>M</i> initialized with the known type arguments, if any
4262 a (possibly empty) list of ordinary function arguments (in case of a function call only)
4267 and then proceeds with the following steps:
4272 apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
4273 to all <i>typed</i> ordinary function arguments
4276 apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
4279 apply function argument type inference to all <i>untyped</i> ordinary function arguments
4280 using the default type for each of the untyped function arguments
4283 apply constraint type inference
4288 If there are no ordinary or untyped function arguments, the respective steps are skipped.
4289 Constraint type inference is skipped if the previous step didn't infer any new type arguments,
4290 but it is run at least once if there are missing type arguments.
4294 The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
4295 The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
4296 If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
4299 <h4 id="Type_unification">Type unification</h4>
4302 Type inference is based on <i>type unification</i>. A single unification step
4303 applies to a <a href="#Type_inference">substitution map</a> and two types, either
4304 or both of which may be or contain type parameters. The substitution map tracks
4305 the known (explicitly provided or already inferred) type arguments: the map
4306 contains an entry <code>P</code> → <code>A</code> for each type
4307 parameter <code>P</code> and corresponding known type argument <code>A</code>.
4308 During unification, known type arguments take the place of their corresponding type
4309 parameters when comparing types. Unification is the process of finding substitution
4310 map entries that make the two types equivalent.
4314 For unification, two types that don't contain any type parameters from the current type
4315 parameter list are <i>equivalent</i>
4316 if they are identical, or if they are channel types that are identical ignoring channel
4317 direction, or if their underlying types are equivalent.
4321 Unification works by comparing the structure of pairs of types: their structure
4322 disregarding type parameters must be identical, and types other than type parameters
4324 A type parameter in one type may match any complete subtype in the other type;
4325 each successful match causes an entry to be added to the substitution map.
4326 If the structure differs, or types other than type parameters are not equivalent,
4331 TODO(gri) Somewhere we need to describe the process of adding an entry to the
4332 substitution map: if the entry is already present, the type argument
4333 values are themselves unified.
4337 For example, if <code>T1</code> and <code>T2</code> are type parameters,
4338 <code>[]map[int]bool</code> can be unified with any of the following:
4342 []map[int]bool // types are identical
4343 T1 // adds T1 → []map[int]bool to substitution map
4344 []T1 // adds T1 → map[int]bool to substitution map
4345 []map[T1]T2 // adds T1 → int and T2 → bool to substitution map
4349 On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
4353 int // int is not a slice
4354 struct{} // a struct is not a slice
4355 []struct{} // a struct is not a map
4356 []map[T1]string // map element types don't match
4360 As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
4361 <code>D</code> and a type literal <code>L</code> are never equivalent,
4362 unification compares the underlying type of <code>D</code> with <code>L</code> instead.
4363 For example, given the defined type
4367 type Vector []float64
4371 and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
4372 <code>[]E</code> and adds an entry <code>E</code> → <code>float64</code> to
4373 the substitution map.
4376 <h4 id="Function_argument_type_inference">Function argument type inference</h4>
4378 <!-- In this section and the section on constraint type inference we start with examples
4379 rather than have the examples follow the rules as is customary elsewhere in spec.
4380 Hopefully this helps building an intuition and makes the rules easier to follow. -->
4383 Function argument type inference infers type arguments from function arguments:
4384 if a function parameter is declared with a type <code>T</code> that uses
4386 <a href="#Type_unification">unifying</a> the type of the corresponding
4387 function argument with <code>T</code> may infer type arguments for the type
4388 parameters used by <code>T</code>.
4392 For instance, given the generic function
4396 func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
4404 var vector []float64
4405 scaledVector := scale(vector, 42)
4409 the type argument for <code>Number</code> can be inferred from the function argument
4410 <code>vector</code> by unifying the type of <code>vector</code> with the corresponding
4411 parameter type: <code>[]float64</code> and <code>[]Number</code>
4412 match in structure and <code>float64</code> matches with <code>Number</code>.
4413 This adds the entry <code>Number</code> → <code>float64</code> to the
4414 <a href="#Type_unification">substitution map</a>.
4415 Untyped arguments, such as the second function argument <code>42</code> here, are ignored
4416 in the first round of function argument type inference and only considered if there are
4417 unresolved type parameters left.
4421 Inference happens in two separate phases; each phase operates on a specific list of
4422 (parameter, argument) pairs:
4427 The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
4428 type uses type parameters and where the function argument is <i>typed</i>.
4431 The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
4432 type parameter. In this list, the respective function arguments are untyped.
4437 Any other (parameter, argument) pair is ignored.
4441 By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
4442 (or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
4443 of untyped values are always predeclared non-composite types, they can never match against
4444 a composite type, so it is sufficient to only consider parameter types that are single type
4449 Each list is processed in a separate phase:
4454 In the first phase, the parameter and argument types of each pair in <i>Lt</i>
4455 are unified. If unification succeeds for a pair, it may yield new entries that
4456 are added to the substitution map <i>M</i>. If unification fails, type inference
4460 The second phase considers the entries of list <i>Lu</i>. Type parameters for
4461 which the type argument has already been determined are ignored in this phase.
4462 For each remaining pair, the parameter type (which is a single type parameter) and
4463 the <a href="#Constants">default type</a> of the corresponding untyped argument is
4464 unified. If unification fails, type inference fails.
4469 While unification is successful, processing of each list continues until all list elements
4470 are considered, even if all type arguments are inferred before the last list element has
4479 func min[T ~int|~float64](x, y T) T
4482 min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
4483 min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
4484 min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
4488 In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
4489 yields the substitution map entry <code>T</code> → <code>float64</code>. Because
4490 processing continues until all untyped arguments are considered, an error is reported. This
4491 ensures that type inference does not depend on the order of the untyped arguments.
4494 <h4 id="Constraint_type_inference">Constraint type inference</h4>
4497 Constraint type inference infers type arguments by considering type constraints.
4498 If a type parameter <code>P</code> has a constraint with a
4499 <a href="#Core_types">core type</a> <code>C</code>,
4500 <a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
4501 may infer additional type arguments, either the type argument for <code>P</code>,
4502 or if that is already known, possibly the type arguments for type parameters
4503 used in <code>C</code>.
4507 For instance, consider the type parameter list with type parameters <code>List</code> and
4512 [List ~[]Elem, Elem any]
4516 Constraint type inference can deduce the type of <code>Elem</code> from the type argument
4517 for <code>List</code> because <code>Elem</code> is a type parameter in the core type
4518 <code>[]Elem</code> of <code>List</code>.
4519 If the type argument is <code>Bytes</code>:
4527 unifying the underlying type of <code>Bytes</code> with the core type means
4528 unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
4529 the <a href="#Type_unification">substitution map</a> entry
4530 <code>Elem</code> → <code>byte</code>.
4531 Thus, in this example, constraint type inference can infer the second type argument from the
4536 Using the core type of a constraint may lose some information: In the (unlikely) case that
4537 the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
4538 <code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
4539 <code>N</code> itself. In this case, constraint type inference may succeed but instantiation
4540 will fail because the inferred type is not in the type set of the constraint.
4541 Thus, constraint type inference uses the <i>adjusted core type</i> of
4542 a constraint: if the type set contains a single type, use that type; otherwise use the
4543 constraint's core type.
4547 Generally, constraint type inference proceeds in two phases: Starting with a given
4548 substitution map <i>M</i>
4553 For all type parameters with an adjusted core type, unify the type parameter with that
4554 type. If any unification fails, constraint type inference fails.
4558 At this point, some entries in <i>M</i> may map type parameters to other
4559 type parameters or to types containing type parameters. For each entry
4560 <code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
4561 contains type parameters <code>Q</code> for which there exist entries
4562 <code>Q</code> → <code>B</code> in <i>M</i>, substitute those
4563 <code>Q</code> with the respective <code>B</code> in <code>A</code>.
4564 Stop when no further substitution is possible.
4569 The result of constraint type inference is the final substitution map <i>M</i> from type
4570 parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
4571 appears in any of the <code>A</code>.
4575 For instance, given the type parameter list
4579 [A any, B []C, C *A]
4583 and the single provided type argument <code>int</code> for type parameter <code>A</code>,
4584 the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
4588 In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
4589 with the core type of their respective constraints. This adds the entries
4590 <code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
4594 At this point there are two entries in <i>M</i> where the right-hand side
4595 is or contains type parameters for which there exists other entries in <i>M</i>:
4596 <code>[]C</code> and <code>*A</code>.
4597 In the second phase, these type parameters are replaced with their respective
4598 types. It doesn't matter in which order this happens. Starting with the state
4599 of <i>M</i> after the first phase:
4603 <code>A</code> → <code>int</code>,
4604 <code>B</code> → <code>[]C</code>,
4605 <code>C</code> → <code>*A</code>
4609 Replace <code>A</code> on the right-hand side of → with <code>int</code>:
4613 <code>A</code> → <code>int</code>,
4614 <code>B</code> → <code>[]C</code>,
4615 <code>C</code> → <code>*int</code>
4619 Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
4623 <code>A</code> → <code>int</code>,
4624 <code>B</code> → <code>[]*int</code>,
4625 <code>C</code> → <code>*int</code>
4629 At this point no further substitution is possible and the map is full.
4630 Therefore, <code>M</code> represents the final map of type parameters
4631 to type arguments for the given type parameter list.
4634 <h3 id="Operators">Operators</h3>
4637 Operators combine operands into expressions.
4641 Expression = UnaryExpr | Expression binary_op Expression .
4642 UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
4644 binary_op = "||" | "&&" | rel_op | add_op | mul_op .
4645 rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
4646 add_op = "+" | "-" | "|" | "^" .
4647 mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
4649 unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
4653 Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
4654 For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
4655 unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
4656 For operations involving constants only, see the section on
4657 <a href="#Constant_expressions">constant expressions</a>.
4661 Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
4662 and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
4663 to the type of the other operand.
4667 The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
4668 or be an untyped constant <a href="#Representability">representable</a> by a
4669 value of type <code>uint</code>.
4670 If the left operand of a non-constant shift expression is an untyped constant,
4671 it is first implicitly converted to the type it would assume if the shift expression were
4672 replaced by its left operand alone.
4679 // The results of the following examples are given for 64-bit ints.
4680 var i = 1<<s // 1 has type int
4681 var j int32 = 1<<s // 1 has type int32; j == 0
4682 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
4683 var m int = 1.0<<s // 1.0 has type int; m == 1<<33
4684 var n = 1.0<<s == j // 1.0 has type int32; n == true
4685 var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
4686 var p = 1<<s == 1<<33 // 1 has type int; p == true
4687 var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
4688 var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
4689 var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
4690 var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift
4691 var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift
4692 var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
4693 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
4694 var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
4696 // The results of the following examples are given for 32-bit ints,
4697 // which means the shifts will overflow.
4698 var mm int = 1.0<<s // 1.0 has type int; mm == 0
4699 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
4700 var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
4701 var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
4702 var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
4705 <h4 id="Operator_precedence">Operator precedence</h4>
4707 Unary operators have the highest precedence.
4708 As the <code>++</code> and <code>--</code> operators form
4709 statements, not expressions, they fall
4710 outside the operator hierarchy.
4711 As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
4713 There are five precedence levels for binary operators.
4714 Multiplication operators bind strongest, followed by addition
4715 operators, comparison operators, <code>&&</code> (logical AND),
4716 and finally <code>||</code> (logical OR):
4719 <pre class="grammar">
4721 5 * / % << >> & &^
4723 3 == != < <= > >=
4729 Binary operators of the same precedence associate from left to right.
4730 For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
4739 x == y+1 && <-chanInt > 0
4743 <h3 id="Arithmetic_operators">Arithmetic operators</h3>
4745 Arithmetic operators apply to numeric values and yield a result of the same
4746 type as the first operand. The four standard arithmetic operators (<code>+</code>,
4747 <code>-</code>, <code>*</code>, <code>/</code>) apply to
4748 <a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
4749 <a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>.
4750 The bitwise logical and shift operators apply to integers only.
4753 <pre class="grammar">
4754 + sum integers, floats, complex values, strings
4755 - difference integers, floats, complex values
4756 * product integers, floats, complex values
4757 / quotient integers, floats, complex values
4758 % remainder integers
4760 & bitwise AND integers
4761 | bitwise OR integers
4762 ^ bitwise XOR integers
4763 &^ bit clear (AND NOT) integers
4765 << left shift integer << integer >= 0
4766 >> right shift integer >> integer >= 0
4770 If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>,
4771 the operator must apply to each type in that type set.
4772 The operands are represented as values of the type argument that the type parameter
4773 is <a href="#Instantiations">instantiated</a> with, and the operation is computed
4774 with the precision of that type argument. For example, given the function:
4778 func dotProduct[F ~float32|~float64](v1, v2 []F) F {
4780 for i, x := range v1 {
4789 the product <code>x * y</code> and the addition <code>s += x * y</code>
4790 are computed with <code>float32</code> or <code>float64</code> precision,
4791 respectively, depending on the type argument for <code>F</code>.
4794 <h4 id="Integer_operators">Integer operators</h4>
4797 For two integer values <code>x</code> and <code>y</code>, the integer quotient
4798 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
4803 x = q*y + r and |r| < |y|
4807 with <code>x / y</code> truncated towards zero
4808 (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
4820 The one exception to this rule is that if the dividend <code>x</code> is
4821 the most negative value for the int type of <code>x</code>, the quotient
4822 <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
4823 due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
4831 int64 -9223372036854775808
4835 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
4836 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4837 If the dividend is non-negative and the divisor is a constant power of 2,
4838 the division may be replaced by a right shift, and computing the remainder may
4839 be replaced by a bitwise AND operation:
4843 x x / 4 x % 4 x >> 2 x & 3
4849 The shift operators shift the left operand by the shift count specified by the
4850 right operand, which must be non-negative. If the shift count is negative at run time,
4851 a <a href="#Run_time_panics">run-time panic</a> occurs.
4852 The shift operators implement arithmetic shifts if the left operand is a signed
4853 integer and logical shifts if it is an unsigned integer.
4854 There is no upper limit on the shift count. Shifts behave
4855 as if the left operand is shifted <code>n</code> times by 1 for a shift
4856 count of <code>n</code>.
4857 As a result, <code>x << 1</code> is the same as <code>x*2</code>
4858 and <code>x >> 1</code> is the same as
4859 <code>x/2</code> but truncated towards negative infinity.
4863 For integer operands, the unary operators
4864 <code>+</code>, <code>-</code>, and <code>^</code> are defined as
4868 <pre class="grammar">
4870 -x negation is 0 - x
4871 ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
4872 and m = -1 for signed x
4876 <h4 id="Integer_overflow">Integer overflow</h4>
4879 For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>,
4880 <code>-</code>, <code>*</code>, and <code><<</code> are
4881 computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
4882 the unsigned integer's type.
4883 Loosely speaking, these unsigned integer operations
4884 discard high bits upon overflow, and programs may rely on "wrap around".
4888 For signed integers, the operations <code>+</code>,
4889 <code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
4890 overflow and the resulting value exists and is deterministically defined
4891 by the signed integer representation, the operation, and its operands.
4892 Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
4893 A compiler may not optimize code under the assumption that overflow does
4894 not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
4897 <h4 id="Floating_point_operators">Floating-point operators</h4>
4900 For floating-point and complex numbers,
4901 <code>+x</code> is the same as <code>x</code>,
4902 while <code>-x</code> is the negation of <code>x</code>.
4903 The result of a floating-point or complex division by zero is not specified beyond the
4904 IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
4905 occurs is implementation-specific.
4909 An implementation may combine multiple floating-point operations into a single
4910 fused operation, possibly across statements, and produce a result that differs
4911 from the value obtained by executing and rounding the instructions individually.
4912 An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to
4913 the precision of the target type, preventing fusion that would discard that rounding.
4917 For instance, some architectures provide a "fused multiply and add" (FMA) instruction
4918 that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
4919 These examples show when a Go implementation can use that instruction:
4923 // FMA allowed for computing r, because x*y is not explicitly rounded:
4927 *p = x*y; r = *p + z
4928 r = x*y + float64(z)
4930 // FMA disallowed for computing r, because it would omit rounding of x*y:
4931 r = float64(x*y) + z
4932 r = z; r += float64(x*y)
4933 t = float64(x*y); r = t + z
4936 <h4 id="String_concatenation">String concatenation</h4>
4939 Strings can be concatenated using the <code>+</code> operator
4940 or the <code>+=</code> assignment operator:
4944 s := "hi" + string(c)
4945 s += " and good bye"
4949 String addition creates a new string by concatenating the operands.
4952 <h3 id="Comparison_operators">Comparison operators</h3>
4955 Comparison operators compare two operands and yield an untyped boolean value.
4958 <pre class="grammar">
4964 >= greater or equal
4968 In any comparison, the first operand
4969 must be <a href="#Assignability">assignable</a>
4970 to the type of the second operand, or vice versa.
4973 The equality operators <code>==</code> and <code>!=</code> apply
4974 to operands that are <i>comparable</i>.
4975 The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
4976 apply to operands that are <i>ordered</i>.
4977 These terms and the result of the comparisons are defined as follows:
4982 Boolean values are comparable.
4983 Two boolean values are equal if they are either both
4984 <code>true</code> or both <code>false</code>.
4988 Integer values are comparable and ordered, in the usual way.
4992 Floating-point values are comparable and ordered,
4993 as defined by the IEEE-754 standard.
4997 Complex values are comparable.
4998 Two complex values <code>u</code> and <code>v</code> are
4999 equal if both <code>real(u) == real(v)</code> and
5000 <code>imag(u) == imag(v)</code>.
5004 String values are comparable and ordered, lexically byte-wise.
5008 Pointer values are comparable.
5009 Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
5010 Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
5014 Channel values are comparable.
5015 Two channel values are equal if they were created by the same call to
5016 <a href="#Making_slices_maps_and_channels"><code>make</code></a>
5017 or if both have value <code>nil</code>.
5021 Interface values are comparable.
5022 Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
5023 and equal dynamic values or if both have value <code>nil</code>.
5027 A value <code>x</code> of non-interface type <code>X</code> and
5028 a value <code>t</code> of interface type <code>T</code> are comparable when values
5029 of type <code>X</code> are comparable and
5030 <code>X</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
5031 They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
5032 and <code>t</code>'s dynamic value is equal to <code>x</code>.
5036 Struct values are comparable if all their fields are comparable.
5037 Two struct values are equal if their corresponding
5038 non-<a href="#Blank_identifier">blank</a> fields are equal.
5042 Array values are comparable if values of the array element type are comparable.
5043 Two array values are equal if their corresponding elements are equal.
5048 A comparison of two interface values with identical dynamic types
5049 causes a <a href="#Run_time_panics">run-time panic</a> if values
5050 of that type are not comparable. This behavior applies not only to direct interface
5051 value comparisons but also when comparing arrays of interface values
5052 or structs with interface-valued fields.
5056 Slice, map, and function values are not comparable.
5057 However, as a special case, a slice, map, or function value may
5058 be compared to the predeclared identifier <code>nil</code>.
5059 Comparison of pointer, channel, and interface values to <code>nil</code>
5060 is also allowed and follows from the general rules above.
5064 const c = 3 < 4 // c is the untyped boolean constant true
5069 // The result of a comparison is an untyped boolean.
5070 // The usual assignment rules apply.
5071 b3 = x == y // b3 has type bool
5072 b4 bool = x == y // b4 has type bool
5073 b5 MyBool = x == y // b5 has type MyBool
5077 <h3 id="Logical_operators">Logical operators</h3>
5080 Logical operators apply to <a href="#Boolean_types">boolean</a> values
5081 and yield a result of the same type as the operands.
5082 The right operand is evaluated conditionally.
5085 <pre class="grammar">
5086 && conditional AND p && q is "if p then q else false"
5087 || conditional OR p || q is "if p then true else q"
5092 <h3 id="Address_operators">Address operators</h3>
5095 For an operand <code>x</code> of type <code>T</code>, the address operation
5096 <code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
5097 The operand must be <i>addressable</i>,
5098 that is, either a variable, pointer indirection, or slice indexing
5099 operation; or a field selector of an addressable struct operand;
5100 or an array indexing operation of an addressable array.
5101 As an exception to the addressability requirement, <code>x</code> may also be a
5102 (possibly parenthesized)
5103 <a href="#Composite_literals">composite literal</a>.
5104 If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
5105 then the evaluation of <code>&x</code> does too.
5109 For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
5110 indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
5111 to by <code>x</code>.
5112 If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
5113 will cause a <a href="#Run_time_panics">run-time panic</a>.
5124 *x // causes a run-time panic
5125 &*x // causes a run-time panic
5129 <h3 id="Receive_operator">Receive operator</h3>
5132 For an operand <code>ch</code> whose <a href="#Core_types">core type</a> is a
5133 <a href="#Channel_types">channel</a>,
5134 the value of the receive operation <code><-ch</code> is the value received
5135 from the channel <code>ch</code>. The channel direction must permit receive operations,
5136 and the type of the receive operation is the element type of the channel.
5137 The expression blocks until a value is available.
5138 Receiving from a <code>nil</code> channel blocks forever.
5139 A receive operation on a <a href="#Close">closed</a> channel can always proceed
5140 immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
5141 after any previously sent values have been received.
5148 <-strobe // wait until clock pulse and discard received value
5152 A receive expression used in an <a href="#Assignments">assignment</a> or initialization of the special form
5159 var x, ok T = <-ch
5163 yields an additional untyped boolean result reporting whether the
5164 communication succeeded. The value of <code>ok</code> is <code>true</code>
5165 if the value received was delivered by a successful send operation to the
5166 channel, or <code>false</code> if it is a zero value generated because the
5167 channel is closed and empty.
5171 <h3 id="Conversions">Conversions</h3>
5174 A conversion changes the <a href="#Types">type</a> of an expression
5175 to the type specified by the conversion.
5176 A conversion may appear literally in the source, or it may be <i>implied</i>
5177 by the context in which an expression appears.
5181 An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
5182 where <code>T</code> is a type and <code>x</code> is an expression
5183 that can be converted to type <code>T</code>.
5187 Conversion = Type "(" Expression [ "," ] ")" .
5191 If the type starts with the operator <code>*</code> or <code><-</code>,
5192 or if the type starts with the keyword <code>func</code>
5193 and has no result list, it must be parenthesized when
5194 necessary to avoid ambiguity:
5198 *Point(p) // same as *(Point(p))
5199 (*Point)(p) // p is converted to *Point
5200 <-chan int(c) // same as <-(chan int(c))
5201 (<-chan int)(c) // c is converted to <-chan int
5202 func()(x) // function signature func() x
5203 (func())(x) // x is converted to func()
5204 (func() int)(x) // x is converted to func() int
5205 func() int(x) // x is converted to func() int (unambiguous)
5209 A <a href="#Constants">constant</a> value <code>x</code> can be converted to
5210 type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
5211 by a value of <code>T</code>.
5212 As a special case, an integer constant <code>x</code> can be explicitly converted to a
5213 <a href="#String_types">string type</a> using the
5214 <a href="#Conversions_to_and_from_a_string_type">same rule</a>
5215 as for non-constant <code>x</code>.
5219 Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a>
5220 yields a typed constant.
5224 uint(iota) // iota value of type uint
5225 float32(2.718281828) // 2.718281828 of type float32
5226 complex128(1) // 1.0 + 0.0i of type complex128
5227 float32(0.49999999) // 0.5 of type float32
5228 float64(-1e-1000) // 0.0 of type float64
5229 string('x') // "x" of type string
5230 string(0x266c) // "♬" of type string
5231 MyString("foo" + "bar") // "foobar" of type MyString
5232 string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
5233 (*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
5234 int(1.2) // illegal: 1.2 cannot be represented as an int
5235 string(65.0) // illegal: 65.0 is not an integer constant
5239 Converting a constant to a type parameter yields a <i>non-constant</i> value of that type,
5240 with the value represented as a value of the type argument that the type parameter
5241 is <a href="#Instantiations">instantiated</a> with.
5242 For example, given the function:
5246 func f[P ~float32|~float64]() {
5252 the conversion <code>P(1.1)</code> results in a non-constant value of type <code>P</code>
5253 and the value <code>1.1</code> is represented as a <code>float32</code> or a <code>float64</code>
5254 depending on the type argument for <code>f</code>.
5255 Accordingly, if <code>f</code> is instantiated with a <code>float32</code> type,
5256 the numeric value of the expression <code>P(1.1) + 1.2</code> will be computed
5257 with the same precision as the corresponding non-constant <code>float32</code>
5262 A non-constant value <code>x</code> can be converted to type <code>T</code>
5263 in any of these cases:
5268 <code>x</code> is <a href="#Assignability">assignable</a>
5272 ignoring struct tags (see below),
5273 <code>x</code>'s type and <code>T</code> are not
5274 <a href="#Type_parameter_declarations">type parameters</a> but have
5275 <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
5278 ignoring struct tags (see below),
5279 <code>x</code>'s type and <code>T</code> are pointer types
5280 that are not <a href="#Types">named types</a>,
5281 and their pointer base types are not type parameters but
5282 have identical underlying types.
5285 <code>x</code>'s type and <code>T</code> are both integer or floating
5289 <code>x</code>'s type and <code>T</code> are both complex types.
5292 <code>x</code> is an integer or a slice of bytes or runes
5293 and <code>T</code> is a string type.
5296 <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
5299 <code>x</code> is a slice, <code>T</code> is a pointer to an array,
5300 and the slice and array types have <a href="#Type_identity">identical</a> element types.
5305 Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type
5306 parameters, <code>x</code>
5307 can also be converted to type <code>T</code> if one of the following conditions applies:
5312 Both <code>V</code> and <code>T</code> are type parameters and a value of each
5313 type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s
5317 Only <code>V</code> is a type parameter and a value of each
5318 type in <code>V</code>'s type set can be converted to <code>T</code>.
5321 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
5322 type in <code>T</code>'s type set.
5327 <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
5328 for identity for the purpose of conversion:
5332 type Person struct {
5341 Name string `json:"name"`
5343 Street string `json:"street"`
5344 City string `json:"city"`
5348 var person = (*Person)(data) // ignoring tags, the underlying types are identical
5352 Specific rules apply to (non-constant) conversions between numeric types or
5353 to and from a string type.
5354 These conversions may change the representation of <code>x</code>
5355 and incur a run-time cost.
5356 All other conversions only change the type but not the representation
5361 There is no linguistic mechanism to convert between pointers and integers.
5362 The package <a href="#Package_unsafe"><code>unsafe</code></a>
5363 implements this functionality under restricted circumstances.
5366 <h4>Conversions between numeric types</h4>
5369 For the conversion of non-constant numeric values, the following rules apply:
5374 When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is
5375 sign extended to implicit infinite precision; otherwise it is zero extended.
5376 It is then truncated to fit in the result type's size.
5377 For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
5378 The conversion always yields a valid value; there is no indication of overflow.
5381 When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded
5382 (truncation towards zero).
5385 When converting an integer or floating-point number to a floating-point type,
5386 or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded
5387 to the precision specified by the destination type.
5388 For instance, the value of a variable <code>x</code> of type <code>float32</code>
5389 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
5390 but float32(x) represents the result of rounding <code>x</code>'s value to
5391 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
5392 of precision, but <code>float32(x + 0.1)</code> does not.
5397 In all non-constant conversions involving floating-point or complex values,
5398 if the result type cannot represent the value the conversion
5399 succeeds but the result value is implementation-dependent.
5402 <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
5406 Converting a signed or unsigned integer value to a string type yields a
5407 string containing the UTF-8 representation of the integer. Values outside
5408 the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
5412 string(-1) // "\ufffd" == "\xef\xbf\xbd"
5413 string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
5414 type MyString string
5415 MyString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
5420 Converting a slice of bytes to a string type yields
5421 a string whose successive bytes are the elements of the slice.
5424 string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5425 string([]byte{}) // ""
5426 string([]byte(nil)) // ""
5429 string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5434 Converting a slice of runes to a string type yields
5435 a string that is the concatenation of the individual rune values
5436 converted to strings.
5439 string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5440 string([]rune{}) // ""
5441 string([]rune(nil)) // ""
5444 string(MyRunes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5449 Converting a value of a string type to a slice of bytes type
5450 yields a slice whose successive elements are the bytes of the string.
5453 []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5454 []byte("") // []byte{}
5456 MyBytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5461 Converting a value of a string type to a slice of runes type
5462 yields a slice containing the individual Unicode code points of the string.
5465 []rune(MyString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
5466 []rune("") // []rune{}
5468 MyRunes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
5473 <h4 id="Conversions_from_slice_to_array_pointer">Conversions from slice to array pointer</h4>
5476 Converting a slice to an array pointer yields a pointer to the underlying array of the slice.
5477 If the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
5478 a <a href="#Run_time_panics">run-time panic</a> occurs.
5482 s := make([]byte, 2, 4)
5483 s0 := (*[0]byte)(s) // s0 != nil
5484 s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
5485 s2 := (*[2]byte)(s) // &s2[0] == &s[0]
5486 s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
5489 t0 := (*[0]string)(t) // t0 == nil
5490 t1 := (*[1]string)(t) // panics: len([1]string) > len(t)
5492 u := make([]byte, 0)
5493 u0 := (*[0]byte)(u) // u0 != nil
5496 <h3 id="Constant_expressions">Constant expressions</h3>
5499 Constant expressions may contain only <a href="#Constants">constant</a>
5500 operands and are evaluated at compile time.
5504 Untyped boolean, numeric, and string constants may be used as operands
5505 wherever it is legal to use an operand of boolean, numeric, or string type,
5510 A constant <a href="#Comparison_operators">comparison</a> always yields
5511 an untyped boolean constant. If the left operand of a constant
5512 <a href="#Operators">shift expression</a> is an untyped constant, the
5513 result is an integer constant; otherwise it is a constant of the same
5514 type as the left operand, which must be of
5515 <a href="#Numeric_types">integer type</a>.
5519 Any other operation on untyped constants results in an untyped constant of the
5520 same kind; that is, a boolean, integer, floating-point, complex, or string
5522 If the untyped operands of a binary operation (other than a shift) are of
5523 different kinds, the result is of the operand's kind that appears later in this
5524 list: integer, rune, floating-point, complex.
5525 For example, an untyped integer constant divided by an
5526 untyped complex constant yields an untyped complex constant.
5530 const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
5531 const b = 15 / 4 // b == 3 (untyped integer constant)
5532 const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
5533 const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
5534 const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
5535 const d = 1 << 3.0 // d == 8 (untyped integer constant)
5536 const e = 1.0 << 3 // e == 8 (untyped integer constant)
5537 const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
5538 const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
5539 const h = "foo" > "bar" // h == true (untyped boolean constant)
5540 const j = true // j == true (untyped boolean constant)
5541 const k = 'w' + 1 // k == 'x' (untyped rune constant)
5542 const l = "hi" // l == "hi" (untyped string constant)
5543 const m = string(k) // m == "x" (type string)
5544 const Σ = 1 - 0.707i // (untyped complex constant)
5545 const Δ = Σ + 2.0e-4 // (untyped complex constant)
5546 const Φ = iota*1i - 1/1i // (untyped complex constant)
5550 Applying the built-in function <code>complex</code> to untyped
5551 integer, rune, or floating-point constants yields
5552 an untyped complex constant.
5556 const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
5557 const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
5561 Constant expressions are always evaluated exactly; intermediate values and the
5562 constants themselves may require precision significantly larger than supported
5563 by any predeclared type in the language. The following are legal declarations:
5567 const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
5568 const Four int8 = Huge >> 98 // Four == 4 (type int8)
5572 The divisor of a constant division or remainder operation must not be zero:
5576 3.14 / 0.0 // illegal: division by zero
5580 The values of <i>typed</i> constants must always be accurately
5581 <a href="#Representability">representable</a> by values
5582 of the constant type. The following constant expressions are illegal:
5586 uint(-1) // -1 cannot be represented as a uint
5587 int(3.14) // 3.14 cannot be represented as an int
5588 int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
5589 Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
5590 Four * 100 // product 400 cannot be represented as an int8 (type of Four)
5594 The mask used by the unary bitwise complement operator <code>^</code> matches
5595 the rule for non-constants: the mask is all 1s for unsigned constants
5596 and -1 for signed and untyped constants.
5600 ^1 // untyped integer constant, equal to -2
5601 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
5602 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
5603 int8(^1) // same as int8(-2)
5604 ^int8(1) // same as -1 ^ int8(1) = -2
5608 Implementation restriction: A compiler may use rounding while
5609 computing untyped floating-point or complex constant expressions; see
5610 the implementation restriction in the section
5611 on <a href="#Constants">constants</a>. This rounding may cause a
5612 floating-point constant expression to be invalid in an integer
5613 context, even if it would be integral when calculated using infinite
5614 precision, and vice versa.
5618 <h3 id="Order_of_evaluation">Order of evaluation</h3>
5621 At package level, <a href="#Package_initialization">initialization dependencies</a>
5622 determine the evaluation order of individual initialization expressions in
5623 <a href="#Variable_declarations">variable declarations</a>.
5624 Otherwise, when evaluating the <a href="#Operands">operands</a> of an
5625 expression, assignment, or
5626 <a href="#Return_statements">return statement</a>,
5627 all function calls, method calls, and
5628 communication operations are evaluated in lexical left-to-right
5633 For example, in the (function-local) assignment
5636 y[f()], ok = g(h(), i()+x[j()], <-c), k()
5639 the function calls and communication happen in the order
5640 <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
5641 <code><-c</code>, <code>g()</code>, and <code>k()</code>.
5642 However, the order of those events compared to the evaluation
5643 and indexing of <code>x</code> and the evaluation
5644 of <code>y</code> is not specified.
5649 f := func() int { a++; return a }
5650 x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
5651 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
5652 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
5656 At package level, initialization dependencies override the left-to-right rule
5657 for individual initialization expressions, but not for operands within each
5662 var a, b, c = f() + v(), g(), sqr(u()) + v()
5664 func f() int { return c }
5665 func g() int { return a }
5666 func sqr(x int) int { return x*x }
5668 // functions u and v are independent of all other variables and functions
5672 The function calls happen in the order
5673 <code>u()</code>, <code>sqr()</code>, <code>v()</code>,
5674 <code>f()</code>, <code>v()</code>, and <code>g()</code>.
5678 Floating-point operations within a single expression are evaluated according to
5679 the associativity of the operators. Explicit parentheses affect the evaluation
5680 by overriding the default associativity.
5681 In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
5682 is performed before adding <code>x</code>.
5685 <h2 id="Statements">Statements</h2>
5688 Statements control execution.
5693 Declaration | LabeledStmt | SimpleStmt |
5694 GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
5695 FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
5698 SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
5701 <h3 id="Terminating_statements">Terminating statements</h3>
5704 A <i>terminating statement</i> interrupts the regular flow of control in
5705 a <a href="#Blocks">block</a>. The following statements are terminating:
5710 A <a href="#Return_statements">"return"</a> or
5711 <a href="#Goto_statements">"goto"</a> statement.
5712 <!-- ul below only for regular layout -->
5717 A call to the built-in function
5718 <a href="#Handling_panics"><code>panic</code></a>.
5719 <!-- ul below only for regular layout -->
5724 A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
5725 <!-- ul below only for regular layout -->
5730 An <a href="#If_statements">"if" statement</a> in which:
5732 <li>the "else" branch is present, and</li>
5733 <li>both branches are terminating statements.</li>
5738 A <a href="#For_statements">"for" statement</a> in which:
5740 <li>there are no "break" statements referring to the "for" statement, and</li>
5741 <li>the loop condition is absent, and</li>
5742 <li>the "for" statement does not use a range clause.</li>
5747 A <a href="#Switch_statements">"switch" statement</a> in which:
5749 <li>there are no "break" statements referring to the "switch" statement,</li>
5750 <li>there is a default case, and</li>
5751 <li>the statement lists in each case, including the default, end in a terminating
5752 statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
5758 A <a href="#Select_statements">"select" statement</a> in which:
5760 <li>there are no "break" statements referring to the "select" statement, and</li>
5761 <li>the statement lists in each case, including the default if present,
5762 end in a terminating statement.</li>
5767 A <a href="#Labeled_statements">labeled statement</a> labeling
5768 a terminating statement.
5773 All other statements are not terminating.
5777 A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
5778 is not empty and its final non-empty statement is terminating.
5782 <h3 id="Empty_statements">Empty statements</h3>
5785 The empty statement does nothing.
5793 <h3 id="Labeled_statements">Labeled statements</h3>
5796 A labeled statement may be the target of a <code>goto</code>,
5797 <code>break</code> or <code>continue</code> statement.
5801 LabeledStmt = Label ":" Statement .
5802 Label = identifier .
5806 Error: log.Panic("error encountered")
5810 <h3 id="Expression_statements">Expression statements</h3>
5813 With the exception of specific built-in functions,
5814 function and method <a href="#Calls">calls</a> and
5815 <a href="#Receive_operator">receive operations</a>
5816 can appear in statement context. Such statements may be parenthesized.
5820 ExpressionStmt = Expression .
5824 The following built-in functions are not permitted in statement context:
5828 append cap complex imag len make new real
5829 unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice
5837 len("foo") // illegal if len is the built-in function
5841 <h3 id="Send_statements">Send statements</h3>
5844 A send statement sends a value on a channel.
5845 The channel expression's <a href="#Core_types">core type</a>
5846 must be a <a href="#Channel_types">channel</a>,
5847 the channel direction must permit send operations,
5848 and the type of the value to be sent must be <a href="#Assignability">assignable</a>
5849 to the channel's element type.
5853 SendStmt = Channel "<-" Expression .
5854 Channel = Expression .
5858 Both the channel and the value expression are evaluated before communication
5859 begins. Communication blocks until the send can proceed.
5860 A send on an unbuffered channel can proceed if a receiver is ready.
5861 A send on a buffered channel can proceed if there is room in the buffer.
5862 A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
5863 A send on a <code>nil</code> channel blocks forever.
5867 ch <- 3 // send value 3 to channel ch
5871 <h3 id="IncDec_statements">IncDec statements</h3>
5874 The "++" and "--" statements increment or decrement their operands
5875 by the untyped <a href="#Constants">constant</a> <code>1</code>.
5876 As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
5877 or a map index expression.
5881 IncDecStmt = Expression ( "++" | "--" ) .
5885 The following <a href="#Assignments">assignment statements</a> are semantically
5889 <pre class="grammar">
5890 IncDec statement Assignment
5896 <h3 id="Assignments">Assignments</h3>
5899 Assignment = ExpressionList assign_op ExpressionList .
5901 assign_op = [ add_op | mul_op ] "=" .
5905 Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
5906 a map index expression, or (for <code>=</code> assignments only) the
5907 <a href="#Blank_identifier">blank identifier</a>.
5908 Operands may be parenthesized.
5915 (k) = <-ch // same as: k = <-ch
5919 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
5920 <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
5921 is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
5922 <code>(y)</code> but evaluates <code>x</code>
5923 only once. The <i>op</i><code>=</code> construct is a single token.
5924 In assignment operations, both the left- and right-hand expression lists
5925 must contain exactly one single-valued expression, and the left-hand
5926 expression must not be the blank identifier.
5931 i &^= 1<<n
5935 A tuple assignment assigns the individual elements of a multi-valued
5936 operation to a list of variables. There are two forms. In the
5937 first, the right hand operand is a single multi-valued expression
5938 such as a function call, a <a href="#Channel_types">channel</a> or
5939 <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
5940 The number of operands on the left
5941 hand side must match the number of values. For instance, if
5942 <code>f</code> is a function returning two values,
5950 assigns the first value to <code>x</code> and the second to <code>y</code>.
5951 In the second form, the number of operands on the left must equal the number
5952 of expressions on the right, each of which must be single-valued, and the
5953 <i>n</i>th expression on the right is assigned to the <i>n</i>th
5954 operand on the left:
5958 one, two, three = '一', '二', '三'
5962 The <a href="#Blank_identifier">blank identifier</a> provides a way to
5963 ignore right-hand side values in an assignment:
5967 _ = x // evaluate x but ignore it
5968 x, _ = f() // evaluate f() but ignore second result value
5972 The assignment proceeds in two phases.
5973 First, the operands of <a href="#Index_expressions">index expressions</a>
5974 and <a href="#Address_operators">pointer indirections</a>
5975 (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
5976 on the left and the expressions on the right are all
5977 <a href="#Order_of_evaluation">evaluated in the usual order</a>.
5978 Second, the assignments are carried out in left-to-right order.
5982 a, b = b, a // exchange a and b
5986 i, x[i] = 1, 2 // set i = 1, x[0] = 2
5989 x[i], i = 2, 1 // set x[0] = 2, i = 1
5991 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
5993 x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
5995 type Point struct { x, y int }
5997 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
6001 for i, x[i] = range x { // set i, x[2] = 0, x[0]
6004 // after this loop, i == 0 and x == []int{3, 5, 3}
6008 In assignments, each value must be <a href="#Assignability">assignable</a>
6009 to the type of the operand to which it is assigned, with the following special cases:
6014 Any typed value may be assigned to the blank identifier.
6018 If an untyped constant
6019 is assigned to a variable of interface type or the blank identifier,
6020 the constant is first implicitly <a href="#Conversions">converted</a> to its
6021 <a href="#Constants">default type</a>.
6025 If an untyped boolean value is assigned to a variable of interface type or
6026 the blank identifier, it is first implicitly converted to type <code>bool</code>.
6030 <h3 id="If_statements">If statements</h3>
6033 "If" statements specify the conditional execution of two branches
6034 according to the value of a boolean expression. If the expression
6035 evaluates to true, the "if" branch is executed, otherwise, if
6036 present, the "else" branch is executed.
6040 IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
6050 The expression may be preceded by a simple statement, which
6051 executes before the expression is evaluated.
6055 if x := f(); x < y {
6057 } else if x > z {
6065 <h3 id="Switch_statements">Switch statements</h3>
6068 "Switch" statements provide multi-way execution.
6069 An expression or type is compared to the "cases"
6070 inside the "switch" to determine which branch
6075 SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
6079 There are two forms: expression switches and type switches.
6080 In an expression switch, the cases contain expressions that are compared
6081 against the value of the switch expression.
6082 In a type switch, the cases contain types that are compared against the
6083 type of a specially annotated switch expression.
6084 The switch expression is evaluated exactly once in a switch statement.
6087 <h4 id="Expression_switches">Expression switches</h4>
6090 In an expression switch,
6091 the switch expression is evaluated and
6092 the case expressions, which need not be constants,
6093 are evaluated left-to-right and top-to-bottom; the first one that equals the
6095 triggers execution of the statements of the associated case;
6096 the other cases are skipped.
6097 If no case matches and there is a "default" case,
6098 its statements are executed.
6099 There can be at most one default case and it may appear anywhere in the
6101 A missing switch expression is equivalent to the boolean value
6106 ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
6107 ExprCaseClause = ExprSwitchCase ":" StatementList .
6108 ExprSwitchCase = "case" ExpressionList | "default" .
6112 If the switch expression evaluates to an untyped constant, it is first implicitly
6113 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
6114 The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
6115 The switch expression type must be <a href="#Comparison_operators">comparable</a>.
6119 If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
6120 to the type of the switch expression.
6121 For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
6122 of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
6126 In other words, the switch expression is treated as if it were used to declare and
6127 initialize a temporary variable <code>t</code> without explicit type; it is that
6128 value of <code>t</code> against which each case expression <code>x</code> is tested
6133 In a case or default clause, the last non-empty statement
6134 may be a (possibly <a href="#Labeled_statements">labeled</a>)
6135 <a href="#Fallthrough_statements">"fallthrough" statement</a> to
6136 indicate that control should flow from the end of this clause to
6137 the first statement of the next clause.
6138 Otherwise control flows to the end of the "switch" statement.
6139 A "fallthrough" statement may appear as the last statement of all
6140 but the last clause of an expression switch.
6144 The switch expression may be preceded by a simple statement, which
6145 executes before the expression is evaluated.
6151 case 0, 1, 2, 3: s1()
6152 case 4, 5, 6, 7: s2()
6155 switch x := f(); { // missing switch expression means "true"
6156 case x < 0: return -x
6168 Implementation restriction: A compiler may disallow multiple case
6169 expressions evaluating to the same constant.
6170 For instance, the current compilers disallow duplicate integer,
6171 floating point, or string constants in case expressions.
6174 <h4 id="Type_switches">Type switches</h4>
6177 A type switch compares types rather than values. It is otherwise similar
6178 to an expression switch. It is marked by a special switch expression that
6179 has the form of a <a href="#Type_assertions">type assertion</a>
6180 using the keyword <code>type</code> rather than an actual type:
6190 Cases then match actual types <code>T</code> against the dynamic type of the
6191 expression <code>x</code>. As with type assertions, <code>x</code> must be of
6192 <a href="#Interface_types">interface type</a>, but not a
6193 <a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type
6194 <code>T</code> listed in a case must implement the type of <code>x</code>.
6195 The types listed in the cases of a type switch must all be
6196 <a href="#Type_identity">different</a>.
6200 TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
6201 TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
6202 TypeCaseClause = TypeSwitchCase ":" StatementList .
6203 TypeSwitchCase = "case" TypeList | "default" .
6207 The TypeSwitchGuard may include a
6208 <a href="#Short_variable_declarations">short variable declaration</a>.
6209 When that form is used, the variable is declared at the end of the
6210 TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
6211 In clauses with a case listing exactly one type, the variable
6212 has that type; otherwise, the variable has the type of the expression
6213 in the TypeSwitchGuard.
6217 Instead of a type, a case may use the predeclared identifier
6218 <a href="#Predeclared_identifiers"><code>nil</code></a>;
6219 that case is selected when the expression in the TypeSwitchGuard
6220 is a <code>nil</code> interface value.
6221 There may be at most one <code>nil</code> case.
6225 Given an expression <code>x</code> of type <code>interface{}</code>,
6226 the following type switch:
6230 switch i := x.(type) {
6232 printString("x is nil") // type of i is type of x (interface{})
6234 printInt(i) // type of i is int
6236 printFloat64(i) // type of i is float64
6237 case func(int) float64:
6238 printFunction(i) // type of i is func(int) float64
6240 printString("type is bool or string") // type of i is type of x (interface{})
6242 printString("don't know the type") // type of i is type of x (interface{})
6251 v := x // x is evaluated exactly once
6253 i := v // type of i is type of x (interface{})
6254 printString("x is nil")
6255 } else if i, isInt := v.(int); isInt {
6256 printInt(i) // type of i is int
6257 } else if i, isFloat64 := v.(float64); isFloat64 {
6258 printFloat64(i) // type of i is float64
6259 } else if i, isFunc := v.(func(int) float64); isFunc {
6260 printFunction(i) // type of i is func(int) float64
6262 _, isBool := v.(bool)
6263 _, isString := v.(string)
6264 if isBool || isString {
6265 i := v // type of i is type of x (interface{})
6266 printString("type is bool or string")
6268 i := v // type of i is type of x (interface{})
6269 printString("don't know the type")
6275 A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a>
6276 may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns
6277 out to duplicate another entry in the switch, the first matching case is chosen.
6281 func f[P any](x any) int {
6296 var v1 = f[string]("foo") // v1 == 0
6297 var v2 = f[byte]([]byte{}) // v2 == 2
6301 The type switch guard may be preceded by a simple statement, which
6302 executes before the guard is evaluated.
6306 The "fallthrough" statement is not permitted in a type switch.
6309 <h3 id="For_statements">For statements</h3>
6312 A "for" statement specifies repeated execution of a block. There are three forms:
6313 The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
6317 ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
6318 Condition = Expression .
6321 <h4 id="For_condition">For statements with single condition</h4>
6324 In its simplest form, a "for" statement specifies the repeated execution of
6325 a block as long as a boolean condition evaluates to true.
6326 The condition is evaluated before each iteration.
6327 If the condition is absent, it is equivalent to the boolean value
6337 <h4 id="For_clause">For statements with <code>for</code> clause</h4>
6340 A "for" statement with a ForClause is also controlled by its condition, but
6341 additionally it may specify an <i>init</i>
6342 and a <i>post</i> statement, such as an assignment,
6343 an increment or decrement statement. The init statement may be a
6344 <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
6345 Variables declared by the init statement are re-used in each iteration.
6349 ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
6350 InitStmt = SimpleStmt .
6351 PostStmt = SimpleStmt .
6355 for i := 0; i < 10; i++ {
6361 If non-empty, the init statement is executed once before evaluating the
6362 condition for the first iteration;
6363 the post statement is executed after each execution of the block (and
6364 only if the block was executed).
6365 Any element of the ForClause may be empty but the
6366 <a href="#Semicolons">semicolons</a> are
6367 required unless there is only a condition.
6368 If the condition is absent, it is equivalent to the boolean value
6373 for cond { S() } is the same as for ; cond ; { S() }
6374 for { S() } is the same as for true { S() }
6377 <h4 id="For_range">For statements with <code>range</code> clause</h4>
6380 A "for" statement with a "range" clause
6381 iterates through all entries of an array, slice, string or map,
6382 or values received on a channel. For each entry it assigns <i>iteration values</i>
6383 to corresponding <i>iteration variables</i> if present and then executes the block.
6387 RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
6391 The expression on the right in the "range" clause is called the <i>range expression</i>,
6392 its <a href="#Core_types">core type</a> must be
6393 an array, pointer to an array, slice, string, map, or channel permitting
6394 <a href="#Receive_operator">receive operations</a>.
6395 As with an assignment, if present the operands on the left must be
6396 <a href="#Address_operators">addressable</a> or map index expressions; they
6397 denote the iteration variables. If the range expression is a channel, at most
6398 one iteration variable is permitted, otherwise there may be up to two.
6399 If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
6400 the range clause is equivalent to the same clause without that identifier.
6404 The range expression <code>x</code> is evaluated once before beginning the loop,
6405 with one exception: if at most one iteration variable is present and
6406 <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
6407 the range expression is not evaluated.
6411 Function calls on the left are evaluated once per iteration.
6412 For each iteration, iteration values are produced as follows
6413 if the respective iteration variables are present:
6416 <pre class="grammar">
6417 Range expression 1st value 2nd value
6419 array or slice a [n]E, *[n]E, or []E index i int a[i] E
6420 string s string type index i int see below rune
6421 map m map[K]V key k K m[k] V
6422 channel c chan E, <-chan E element e E
6427 For an array, pointer to array, or slice value <code>a</code>, the index iteration
6428 values are produced in increasing order, starting at element index 0.
6429 If at most one iteration variable is present, the range loop produces
6430 iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
6431 or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
6435 For a string value, the "range" clause iterates over the Unicode code points
6436 in the string starting at byte index 0. On successive iterations, the index value will be the
6437 index of the first byte of successive UTF-8-encoded code points in the string,
6438 and the second value, of type <code>rune</code>, will be the value of
6439 the corresponding code point. If the iteration encounters an invalid
6440 UTF-8 sequence, the second value will be <code>0xFFFD</code>,
6441 the Unicode replacement character, and the next iteration will advance
6442 a single byte in the string.
6446 The iteration order over maps is not specified
6447 and is not guaranteed to be the same from one iteration to the next.
6448 If a map entry that has not yet been reached is removed during iteration,
6449 the corresponding iteration value will not be produced. If a map entry is
6450 created during iteration, that entry may be produced during the iteration or
6451 may be skipped. The choice may vary for each entry created and from one
6452 iteration to the next.
6453 If the map is <code>nil</code>, the number of iterations is 0.
6457 For channels, the iteration values produced are the successive values sent on
6458 the channel until the channel is <a href="#Close">closed</a>. If the channel
6459 is <code>nil</code>, the range expression blocks forever.
6464 The iteration values are assigned to the respective
6465 iteration variables as in an <a href="#Assignments">assignment statement</a>.
6469 The iteration variables may be declared by the "range" clause using a form of
6470 <a href="#Short_variable_declarations">short variable declaration</a>
6472 In this case their types are set to the types of the respective iteration values
6473 and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
6474 statement; they are re-used in each iteration.
6475 If the iteration variables are declared outside the "for" statement,
6476 after execution their values will be those of the last iteration.
6480 var testdata *struct {
6483 for i, _ := range testdata.a {
6484 // testdata.a is never evaluated; len(testdata.a) is constant
6485 // i ranges from 0 to 6
6490 for i, s := range a {
6492 // type of s is string
6498 var val interface{} // element type of m is assignable to val
6499 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
6500 for key, val = range m {
6503 // key == last map key encountered in iteration
6506 var ch chan Work = producer()
6516 <h3 id="Go_statements">Go statements</h3>
6519 A "go" statement starts the execution of a function call
6520 as an independent concurrent thread of control, or <i>goroutine</i>,
6521 within the same address space.
6525 GoStmt = "go" Expression .
6529 The expression must be a function or method call; it cannot be parenthesized.
6530 Calls of built-in functions are restricted as for
6531 <a href="#Expression_statements">expression statements</a>.
6535 The function value and parameters are
6536 <a href="#Calls">evaluated as usual</a>
6537 in the calling goroutine, but
6538 unlike with a regular call, program execution does not wait
6539 for the invoked function to complete.
6540 Instead, the function begins executing independently
6542 When the function terminates, its goroutine also terminates.
6543 If the function has any return values, they are discarded when the
6549 go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
6553 <h3 id="Select_statements">Select statements</h3>
6556 A "select" statement chooses which of a set of possible
6557 <a href="#Send_statements">send</a> or
6558 <a href="#Receive_operator">receive</a>
6559 operations will proceed.
6560 It looks similar to a
6561 <a href="#Switch_statements">"switch"</a> statement but with the
6562 cases all referring to communication operations.
6566 SelectStmt = "select" "{" { CommClause } "}" .
6567 CommClause = CommCase ":" StatementList .
6568 CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
6569 RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
6570 RecvExpr = Expression .
6574 A case with a RecvStmt may assign the result of a RecvExpr to one or
6575 two variables, which may be declared using a
6576 <a href="#Short_variable_declarations">short variable declaration</a>.
6577 The RecvExpr must be a (possibly parenthesized) receive operation.
6578 There can be at most one default case and it may appear anywhere
6579 in the list of cases.
6583 Execution of a "select" statement proceeds in several steps:
6588 For all the cases in the statement, the channel operands of receive operations
6589 and the channel and right-hand-side expressions of send statements are
6590 evaluated exactly once, in source order, upon entering the "select" statement.
6591 The result is a set of channels to receive from or send to,
6592 and the corresponding values to send.
6593 Any side effects in that evaluation will occur irrespective of which (if any)
6594 communication operation is selected to proceed.
6595 Expressions on the left-hand side of a RecvStmt with a short variable declaration
6596 or assignment are not yet evaluated.
6600 If one or more of the communications can proceed,
6601 a single one that can proceed is chosen via a uniform pseudo-random selection.
6602 Otherwise, if there is a default case, that case is chosen.
6603 If there is no default case, the "select" statement blocks until
6604 at least one of the communications can proceed.
6608 Unless the selected case is the default case, the respective communication
6609 operation is executed.
6613 If the selected case is a RecvStmt with a short variable declaration or
6614 an assignment, the left-hand side expressions are evaluated and the
6615 received value (or values) are assigned.
6619 The statement list of the selected case is executed.
6624 Since communication on <code>nil</code> channels can never proceed,
6625 a select with only <code>nil</code> channels and no default case blocks forever.
6630 var c, c1, c2, c3, c4 chan int
6634 print("received ", i1, " from c1\n")
6636 print("sent ", i2, " to c2\n")
6637 case i3, ok := (<-c3): // same as: i3, ok := <-c3
6639 print("received ", i3, " from c3\n")
6641 print("c3 is closed\n")
6643 case a[f()] = <-c4:
6645 // case t := <-c4
6648 print("no communication\n")
6651 for { // send random sequence of bits to c
6653 case c <- 0: // note: no statement, no fallthrough, no folding of cases
6658 select {} // block forever
6662 <h3 id="Return_statements">Return statements</h3>
6665 A "return" statement in a function <code>F</code> terminates the execution
6666 of <code>F</code>, and optionally provides one or more result values.
6667 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6668 are executed before <code>F</code> returns to its caller.
6672 ReturnStmt = "return" [ ExpressionList ] .
6676 In a function without a result type, a "return" statement must not
6677 specify any result values.
6686 There are three ways to return values from a function with a result
6691 <li>The return value or values may be explicitly listed
6692 in the "return" statement. Each expression must be single-valued
6693 and <a href="#Assignability">assignable</a>
6694 to the corresponding element of the function's result type.
6696 func simpleF() int {
6700 func complexF1() (re float64, im float64) {
6705 <li>The expression list in the "return" statement may be a single
6706 call to a multi-valued function. The effect is as if each value
6707 returned from that function were assigned to a temporary
6708 variable with the type of the respective value, followed by a
6709 "return" statement listing these variables, at which point the
6710 rules of the previous case apply.
6712 func complexF2() (re float64, im float64) {
6717 <li>The expression list may be empty if the function's result
6718 type specifies names for its <a href="#Function_types">result parameters</a>.
6719 The result parameters act as ordinary local variables
6720 and the function may assign values to them as necessary.
6721 The "return" statement returns the values of these variables.
6723 func complexF3() (re float64, im float64) {
6729 func (devnull) Write(p []byte) (n int, _ error) {
6738 Regardless of how they are declared, all the result values are initialized to
6739 the <a href="#The_zero_value">zero values</a> for their type upon entry to the
6740 function. A "return" statement that specifies results sets the result parameters before
6741 any deferred functions are executed.
6745 Implementation restriction: A compiler may disallow an empty expression list
6746 in a "return" statement if a different entity (constant, type, or variable)
6747 with the same name as a result parameter is in
6748 <a href="#Declarations_and_scope">scope</a> at the place of the return.
6752 func f(n int) (res int, err error) {
6753 if _, err := f(n-1); err != nil {
6754 return // invalid return statement: err is shadowed
6760 <h3 id="Break_statements">Break statements</h3>
6763 A "break" statement terminates execution of the innermost
6764 <a href="#For_statements">"for"</a>,
6765 <a href="#Switch_statements">"switch"</a>, or
6766 <a href="#Select_statements">"select"</a> statement
6767 within the same function.
6771 BreakStmt = "break" [ Label ] .
6775 If there is a label, it must be that of an enclosing
6776 "for", "switch", or "select" statement,
6777 and that is the one whose execution terminates.
6782 for i = 0; i < n; i++ {
6783 for j = 0; j < m; j++ {
6796 <h3 id="Continue_statements">Continue statements</h3>
6799 A "continue" statement begins the next iteration of the
6800 innermost <a href="#For_statements">"for" loop</a> at its post statement.
6801 The "for" loop must be within the same function.
6805 ContinueStmt = "continue" [ Label ] .
6809 If there is a label, it must be that of an enclosing
6810 "for" statement, and that is the one whose execution
6816 for y, row := range rows {
6817 for x, data := range row {
6818 if data == endOfRow {
6821 row[x] = data + bias(x, y)
6826 <h3 id="Goto_statements">Goto statements</h3>
6829 A "goto" statement transfers control to the statement with the corresponding label
6830 within the same function.
6834 GotoStmt = "goto" Label .
6842 Executing the "goto" statement must not cause any variables to come into
6843 <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
6844 For instance, this example:
6854 is erroneous because the jump to label <code>L</code> skips
6855 the creation of <code>v</code>.
6859 A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
6860 For instance, this example:
6877 is erroneous because the label <code>L1</code> is inside
6878 the "for" statement's block but the <code>goto</code> is not.
6881 <h3 id="Fallthrough_statements">Fallthrough statements</h3>
6884 A "fallthrough" statement transfers control to the first statement of the
6885 next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
6886 It may be used only as the final non-empty statement in such a clause.
6890 FallthroughStmt = "fallthrough" .
6894 <h3 id="Defer_statements">Defer statements</h3>
6897 A "defer" statement invokes a function whose execution is deferred
6898 to the moment the surrounding function returns, either because the
6899 surrounding function executed a <a href="#Return_statements">return statement</a>,
6900 reached the end of its <a href="#Function_declarations">function body</a>,
6901 or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
6905 DeferStmt = "defer" Expression .
6909 The expression must be a function or method call; it cannot be parenthesized.
6910 Calls of built-in functions are restricted as for
6911 <a href="#Expression_statements">expression statements</a>.
6915 Each time a "defer" statement
6916 executes, the function value and parameters to the call are
6917 <a href="#Calls">evaluated as usual</a>
6918 and saved anew but the actual function is not invoked.
6919 Instead, deferred functions are invoked immediately before
6920 the surrounding function returns, in the reverse order
6921 they were deferred. That is, if the surrounding function
6922 returns through an explicit <a href="#Return_statements">return statement</a>,
6923 deferred functions are executed <i>after</i> any result parameters are set
6924 by that return statement but <i>before</i> the function returns to its caller.
6925 If a deferred function value evaluates
6926 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
6927 when the function is invoked, not when the "defer" statement is executed.
6931 For instance, if the deferred function is
6932 a <a href="#Function_literals">function literal</a> and the surrounding
6933 function has <a href="#Function_types">named result parameters</a> that
6934 are in scope within the literal, the deferred function may access and modify
6935 the result parameters before they are returned.
6936 If the deferred function has any return values, they are discarded when
6937 the function completes.
6938 (See also the section on <a href="#Handling_panics">handling panics</a>.)
6943 defer unlock(l) // unlocking happens before surrounding function returns
6945 // prints 3 2 1 0 before surrounding function returns
6946 for i := 0; i <= 3; i++ {
6951 func f() (result int) {
6953 // result is accessed after it was set to 6 by the return statement
6960 <h2 id="Built-in_functions">Built-in functions</h2>
6963 Built-in functions are
6964 <a href="#Predeclared_identifiers">predeclared</a>.
6965 They are called like any other function but some of them
6966 accept a type instead of an expression as the first argument.
6970 The built-in functions do not have standard Go types,
6971 so they can only appear in <a href="#Calls">call expressions</a>;
6972 they cannot be used as function values.
6975 <h3 id="Close">Close</h3>
6978 For an argument <code>ch</code> with a <a href="#Core_types">core type</a>
6979 that is a <a href="#Channel_types">channel</a>, the built-in function <code>close</code>
6980 records that no more values will be sent on the channel.
6981 It is an error if <code>ch</code> is a receive-only channel.
6982 Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
6983 Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
6984 After calling <code>close</code>, and after any previously
6985 sent values have been received, receive operations will return
6986 the zero value for the channel's type without blocking.
6987 The multi-valued <a href="#Receive_operator">receive operation</a>
6988 returns a received value along with an indication of whether the channel is closed.
6991 <h3 id="Length_and_capacity">Length and capacity</h3>
6994 The built-in functions <code>len</code> and <code>cap</code> take arguments
6995 of various types and return a result of type <code>int</code>.
6996 The implementation guarantees that the result always fits into an <code>int</code>.
6999 <pre class="grammar">
7000 Call Argument type Result
7002 len(s) string type string length in bytes
7003 [n]T, *[n]T array length (== n)
7005 map[K]T map length (number of defined keys)
7006 chan T number of elements queued in channel buffer
7007 type parameter see below
7009 cap(s) [n]T, *[n]T array length (== n)
7011 chan T channel buffer capacity
7012 type parameter see below
7016 If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>,
7017 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
7018 each type in <code>P</code>'s type set.
7019 The result is the length (or capacity, respectively) of the argument whose type
7020 corresponds to the type argument with which <code>P</code> was
7021 <a href="#Instantiations">instantiated</a>.
7025 The capacity of a slice is the number of elements for which there is
7026 space allocated in the underlying array.
7027 At any time the following relationship holds:
7031 0 <= len(s) <= cap(s)
7035 The length of a <code>nil</code> slice, map or channel is 0.
7036 The capacity of a <code>nil</code> slice or channel is 0.
7040 The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
7041 <code>s</code> is a string constant. The expressions <code>len(s)</code> and
7042 <code>cap(s)</code> are constants if the type of <code>s</code> is an array
7043 or pointer to an array and the expression <code>s</code> does not contain
7044 <a href="#Receive_operator">channel receives</a> or (non-constant)
7045 <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
7046 Otherwise, invocations of <code>len</code> and <code>cap</code> are not
7047 constant and <code>s</code> is evaluated.
7052 c1 = imag(2i) // imag(2i) = 2.0 is a constant
7053 c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
7054 c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
7055 c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
7056 c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
7061 <h3 id="Allocation">Allocation</h3>
7064 The built-in function <code>new</code> takes a type <code>T</code>,
7065 allocates storage for a <a href="#Variables">variable</a> of that type
7066 at run time, and returns a value of type <code>*T</code>
7067 <a href="#Pointer_types">pointing</a> to it.
7068 The variable is initialized as described in the section on
7069 <a href="#The_zero_value">initial values</a>.
7072 <pre class="grammar">
7081 type S struct { a int; b float64 }
7086 allocates storage for a variable of type <code>S</code>,
7087 initializes it (<code>a=0</code>, <code>b=0.0</code>),
7088 and returns a value of type <code>*S</code> containing the address
7092 <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
7095 The built-in function <code>make</code> takes a type <code>T</code>,
7096 optionally followed by a type-specific list of expressions.
7097 The <a href="#Core_types">core type</a> of <code>T</code> must
7098 be a slice, map or channel.
7099 It returns a value of type <code>T</code> (not <code>*T</code>).
7100 The memory is initialized as described in the section on
7101 <a href="#The_zero_value">initial values</a>.
7104 <pre class="grammar">
7105 Call Core type Result
7107 make(T, n) slice slice of type T with length n and capacity n
7108 make(T, n, m) slice slice of type T with length n and capacity m
7110 make(T) map map of type T
7111 make(T, n) map map of type T with initial space for approximately n elements
7113 make(T) channel unbuffered channel of type T
7114 make(T, n) channel buffered channel of type T, buffer size n
7119 Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>
7120 or an untyped <a href="#Constants">constant</a>.
7121 A constant size argument must be non-negative and <a href="#Representability">representable</a>
7122 by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
7123 If both <code>n</code> and <code>m</code> are provided and are constant, then
7124 <code>n</code> must be no larger than <code>m</code>.
7125 If <code>n</code> is negative or larger than <code>m</code> at run time,
7126 a <a href="#Run_time_panics">run-time panic</a> occurs.
7130 s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
7131 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
7132 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
7133 s := make([]int, 10, 0) // illegal: len(s) > cap(s)
7134 c := make(chan int, 10) // channel with a buffer size of 10
7135 m := make(map[string]int, 100) // map with initial space for approximately 100 elements
7139 Calling <code>make</code> with a map type and size hint <code>n</code> will
7140 create a map with initial space to hold <code>n</code> map elements.
7141 The precise behavior is implementation-dependent.
7145 <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
7148 The built-in functions <code>append</code> and <code>copy</code> assist in
7149 common slice operations.
7150 For both functions, the result is independent of whether the memory referenced
7151 by the arguments overlaps.
7155 The <a href="#Function_types">variadic</a> function <code>append</code>
7156 appends zero or more values <code>x</code> to a slice <code>s</code>
7157 and returns the resulting slice.
7158 The <a href="#Core_types">core type</a> of <code>s</code> must be a slice
7159 of the form <code>[]E</code>.
7160 The values <code>x</code> are passed to a parameter of type <code>...E</code>
7161 and the respective <a href="#Passing_arguments_to_..._parameters">parameter
7162 passing rules</a> apply.
7163 As a special case, if the core type of <code>s</code> is <code>[]byte</code>,
7164 <code>append</code> also accepts a second argument with core type <code>string</code>
7165 followed by <code>...</code>. This form appends the bytes of the string.
7168 <pre class="grammar">
7169 append(s S, x ...E) S // E is the element type of the core type of S
7173 If the capacity of <code>s</code> is not large enough to fit the additional
7174 values, <code>append</code> allocates a new, sufficiently large underlying
7175 array that fits both the existing slice elements and the additional values.
7176 Otherwise, <code>append</code> re-uses the underlying array.
7181 s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
7182 s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
7183 s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
7184 s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
7187 t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"}
7190 b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
7194 The function <code>copy</code> copies slice elements from
7195 a source <code>src</code> to a destination <code>dst</code> and returns the
7196 number of elements copied.
7197 The <a href="#Core_types">core types</a> of both arguments must be slices
7198 with <a href="#Type_identity">identical</a> element type.
7199 The number of elements copied is the minimum of
7200 <code>len(src)</code> and <code>len(dst)</code>.
7201 As a special case, if the destination's core type is <code>[]byte</code>,
7202 <code>copy</code> also accepts a source argument with core type <code>string</code>.
7203 This form copies the bytes from the string into the byte slice.
7206 <pre class="grammar">
7207 copy(dst, src []T) int
7208 copy(dst []byte, src string) int
7216 var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
7217 var s = make([]int, 6)
7218 var b = make([]byte, 5)
7219 n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
7220 n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
7221 n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
7225 <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
7228 The built-in function <code>delete</code> removes the element with key
7229 <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
7230 value <code>k</code> must be <a href="#Assignability">assignable</a>
7231 to the key type of <code>m</code>.
7234 <pre class="grammar">
7235 delete(m, k) // remove element m[k] from map m
7239 If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>,
7240 all types in that type set must be maps, and they must all have identical key types.
7244 If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
7245 does not exist, <code>delete</code> is a no-op.
7249 <h3 id="Complex_numbers">Manipulating complex numbers</h3>
7252 Three functions assemble and disassemble complex numbers.
7253 The built-in function <code>complex</code> constructs a complex
7254 value from a floating-point real and imaginary part, while
7255 <code>real</code> and <code>imag</code>
7256 extract the real and imaginary parts of a complex value.
7259 <pre class="grammar">
7260 complex(realPart, imaginaryPart floatT) complexT
7261 real(complexT) floatT
7262 imag(complexT) floatT
7266 The type of the arguments and return value correspond.
7267 For <code>complex</code>, the two arguments must be of the same
7268 <a href="#Numeric_types">floating-point type</a> and the return type is the
7269 <a href="#Numeric_types">complex type</a>
7270 with the corresponding floating-point constituents:
7271 <code>complex64</code> for <code>float32</code> arguments, and
7272 <code>complex128</code> for <code>float64</code> arguments.
7273 If one of the arguments evaluates to an untyped constant, it is first implicitly
7274 <a href="#Conversions">converted</a> to the type of the other argument.
7275 If both arguments evaluate to untyped constants, they must be non-complex
7276 numbers or their imaginary parts must be zero, and the return value of
7277 the function is an untyped complex constant.
7281 For <code>real</code> and <code>imag</code>, the argument must be
7282 of complex type, and the return type is the corresponding floating-point
7283 type: <code>float32</code> for a <code>complex64</code> argument, and
7284 <code>float64</code> for a <code>complex128</code> argument.
7285 If the argument evaluates to an untyped constant, it must be a number,
7286 and the return value of the function is an untyped floating-point constant.
7290 The <code>real</code> and <code>imag</code> functions together form the inverse of
7291 <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
7292 <code>z == Z(complex(real(z), imag(z)))</code>.
7296 If the operands of these functions are all constants, the return
7297 value is a constant.
7301 var a = complex(2, -2) // complex128
7302 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
7303 x := float32(math.Cos(math.Pi/2)) // float32
7304 var c64 = complex(5, -x) // complex64
7305 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
7306 _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
7307 var rl = real(c64) // float32
7308 var im = imag(a) // float64
7309 const c = imag(b) // untyped constant -1.4
7310 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
7314 Arguments of type parameter type are not permitted.
7317 <h3 id="Handling_panics">Handling panics</h3>
7319 <p> Two built-in functions, <code>panic</code> and <code>recover</code>,
7320 assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
7321 and program-defined error conditions.
7324 <pre class="grammar">
7325 func panic(interface{})
7326 func recover() interface{}
7330 While executing a function <code>F</code>,
7331 an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
7332 terminates the execution of <code>F</code>.
7333 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
7334 are then executed as usual.
7335 Next, any deferred functions run by <code>F</code>'s caller are run,
7336 and so on up to any deferred by the top-level function in the executing goroutine.
7337 At that point, the program is terminated and the error
7338 condition is reported, including the value of the argument to <code>panic</code>.
7339 This termination sequence is called <i>panicking</i>.
7344 panic("unreachable")
7345 panic(Error("cannot parse"))
7349 The <code>recover</code> function allows a program to manage behavior
7350 of a panicking goroutine.
7351 Suppose a function <code>G</code> defers a function <code>D</code> that calls
7352 <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
7354 When the running of deferred functions reaches <code>D</code>,
7355 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>.
7356 If <code>D</code> returns normally, without starting a new
7357 <code>panic</code>, the panicking sequence stops. In that case,
7358 the state of functions called between <code>G</code> and the call to <code>panic</code>
7359 is discarded, and normal execution resumes.
7360 Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
7361 execution terminates by returning to its caller.
7365 The return value of <code>recover</code> is <code>nil</code> if any of the following conditions holds:
7369 <code>panic</code>'s argument was <code>nil</code>;
7372 the goroutine is not panicking;
7375 <code>recover</code> was not called directly by a deferred function.
7380 The <code>protect</code> function in the example below invokes
7381 the function argument <code>g</code> and protects callers from
7382 run-time panics raised by <code>g</code>.
7386 func protect(g func()) {
7388 log.Println("done") // Println executes normally even if there is a panic
7389 if x := recover(); x != nil {
7390 log.Printf("run time panic: %v", x)
7393 log.Println("start")
7399 <h3 id="Bootstrapping">Bootstrapping</h3>
7402 Current implementations provide several built-in functions useful during
7403 bootstrapping. These functions are documented for completeness but are not
7404 guaranteed to stay in the language. They do not return a result.
7407 <pre class="grammar">
7410 print prints all arguments; formatting of arguments is implementation-specific
7411 println like print but prints spaces between arguments and a newline at the end
7415 Implementation restriction: <code>print</code> and <code>println</code> need not
7416 accept arbitrary argument types, but printing of boolean, numeric, and string
7417 <a href="#Types">types</a> must be supported.
7420 <h2 id="Packages">Packages</h2>
7423 Go programs are constructed by linking together <i>packages</i>.
7424 A package in turn is constructed from one or more source files
7425 that together declare constants, types, variables and functions
7426 belonging to the package and which are accessible in all files
7427 of the same package. Those elements may be
7428 <a href="#Exported_identifiers">exported</a> and used in another package.
7431 <h3 id="Source_file_organization">Source file organization</h3>
7434 Each source file consists of a package clause defining the package
7435 to which it belongs, followed by a possibly empty set of import
7436 declarations that declare packages whose contents it wishes to use,
7437 followed by a possibly empty set of declarations of functions,
7438 types, variables, and constants.
7442 SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
7445 <h3 id="Package_clause">Package clause</h3>
7448 A package clause begins each source file and defines the package
7449 to which the file belongs.
7453 PackageClause = "package" PackageName .
7454 PackageName = identifier .
7458 The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
7466 A set of files sharing the same PackageName form the implementation of a package.
7467 An implementation may require that all source files for a package inhabit the same directory.
7470 <h3 id="Import_declarations">Import declarations</h3>
7473 An import declaration states that the source file containing the declaration
7474 depends on functionality of the <i>imported</i> package
7475 (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
7476 and enables access to <a href="#Exported_identifiers">exported</a> identifiers
7478 The import names an identifier (PackageName) to be used for access and an ImportPath
7479 that specifies the package to be imported.
7483 ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
7484 ImportSpec = [ "." | PackageName ] ImportPath .
7485 ImportPath = string_lit .
7489 The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
7490 to access exported identifiers of the package within the importing source file.
7491 It is declared in the <a href="#Blocks">file block</a>.
7492 If the PackageName is omitted, it defaults to the identifier specified in the
7493 <a href="#Package_clause">package clause</a> of the imported package.
7494 If an explicit period (<code>.</code>) appears instead of a name, all the
7495 package's exported identifiers declared in that package's
7496 <a href="#Blocks">package block</a> will be declared in the importing source
7497 file's file block and must be accessed without a qualifier.
7501 The interpretation of the ImportPath is implementation-dependent but
7502 it is typically a substring of the full file name of the compiled
7503 package and may be relative to a repository of installed packages.
7507 Implementation restriction: A compiler may restrict ImportPaths to
7508 non-empty strings using only characters belonging to
7509 <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
7510 L, M, N, P, and S general categories (the Graphic characters without
7511 spaces) and may also exclude the characters
7512 <code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
7513 and the Unicode replacement character U+FFFD.
7517 Assume we have compiled a package containing the package clause
7518 <code>package math</code>, which exports function <code>Sin</code>, and
7519 installed the compiled package in the file identified by
7520 <code>"lib/math"</code>.
7521 This table illustrates how <code>Sin</code> is accessed in files
7522 that import the package after the
7523 various types of import declaration.
7526 <pre class="grammar">
7527 Import declaration Local name of Sin
7529 import "lib/math" math.Sin
7530 import m "lib/math" m.Sin
7531 import . "lib/math" Sin
7535 An import declaration declares a dependency relation between
7536 the importing and imported package.
7537 It is illegal for a package to import itself, directly or indirectly,
7538 or to directly import a package without
7539 referring to any of its exported identifiers. To import a package solely for
7540 its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
7541 identifier as explicit package name:
7549 <h3 id="An_example_package">An example package</h3>
7552 Here is a complete Go package that implements a concurrent prime sieve.
7560 // Send the sequence 2, 3, 4, … to channel 'ch'.
7561 func generate(ch chan<- int) {
7563 ch <- i // Send 'i' to channel 'ch'.
7567 // Copy the values from channel 'src' to channel 'dst',
7568 // removing those divisible by 'prime'.
7569 func filter(src <-chan int, dst chan<- int, prime int) {
7570 for i := range src { // Loop over values received from 'src'.
7572 dst <- i // Send 'i' to channel 'dst'.
7577 // The prime sieve: Daisy-chain filter processes together.
7579 ch := make(chan int) // Create a new channel.
7580 go generate(ch) // Start generate() as a subprocess.
7583 fmt.Print(prime, "\n")
7584 ch1 := make(chan int)
7585 go filter(ch, ch1, prime)
7595 <h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
7597 <h3 id="The_zero_value">The zero value</h3>
7599 When storage is allocated for a <a href="#Variables">variable</a>,
7600 either through a declaration or a call of <code>new</code>, or when
7601 a new value is created, either through a composite literal or a call
7602 of <code>make</code>,
7603 and no explicit initialization is provided, the variable or value is
7604 given a default value. Each element of such a variable or value is
7605 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
7606 <code>0</code> for numeric types, <code>""</code>
7607 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
7608 This initialization is done recursively, so for instance each element of an
7609 array of structs will have its fields zeroed if no value is specified.
7612 These two simple declarations are equivalent:
7625 type T struct { i int; f float64; next *T }
7630 the following holds:
7640 The same would also be true after
7647 <h3 id="Package_initialization">Package initialization</h3>
7650 Within a package, package-level variable initialization proceeds stepwise,
7651 with each step selecting the variable earliest in <i>declaration order</i>
7652 which has no dependencies on uninitialized variables.
7656 More precisely, a package-level variable is considered <i>ready for
7657 initialization</i> if it is not yet initialized and either has
7658 no <a href="#Variable_declarations">initialization expression</a> or
7659 its initialization expression has no <i>dependencies</i> on uninitialized variables.
7660 Initialization proceeds by repeatedly initializing the next package-level
7661 variable that is earliest in declaration order and ready for initialization,
7662 until there are no variables ready for initialization.
7666 If any variables are still uninitialized when this
7667 process ends, those variables are part of one or more initialization cycles,
7668 and the program is not valid.
7672 Multiple variables on the left-hand side of a variable declaration initialized
7673 by single (multi-valued) expression on the right-hand side are initialized
7674 together: If any of the variables on the left-hand side is initialized, all
7675 those variables are initialized in the same step.
7680 var a, b = f() // a and b are initialized together, before x is initialized
7684 For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
7685 variables are treated like any other variables in declarations.
7689 The declaration order of variables declared in multiple files is determined
7690 by the order in which the files are presented to the compiler: Variables
7691 declared in the first file are declared before any of the variables declared
7692 in the second file, and so on.
7696 Dependency analysis does not rely on the actual values of the
7697 variables, only on lexical <i>references</i> to them in the source,
7698 analyzed transitively. For instance, if a variable <code>x</code>'s
7699 initialization expression refers to a function whose body refers to
7700 variable <code>y</code> then <code>x</code> depends on <code>y</code>.
7706 A reference to a variable or function is an identifier denoting that
7707 variable or function.
7711 A reference to a method <code>m</code> is a
7712 <a href="#Method_values">method value</a> or
7713 <a href="#Method_expressions">method expression</a> of the form
7714 <code>t.m</code>, where the (static) type of <code>t</code> is
7715 not an interface type, and the method <code>m</code> is in the
7716 <a href="#Method_sets">method set</a> of <code>t</code>.
7717 It is immaterial whether the resulting function value
7718 <code>t.m</code> is invoked.
7722 A variable, function, or method <code>x</code> depends on a variable
7723 <code>y</code> if <code>x</code>'s initialization expression or body
7724 (for functions and methods) contains a reference to <code>y</code>
7725 or to a function or method that depends on <code>y</code>.
7730 For example, given the declarations
7738 d = 3 // == 5 after initialization has finished
7748 the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
7749 Note that the order of subexpressions in initialization expressions is irrelevant:
7750 <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
7751 order in this example.
7755 Dependency analysis is performed per package; only references referring
7756 to variables, functions, and (non-interface) methods declared in the current
7757 package are considered. If other, hidden, data dependencies exists between
7758 variables, the initialization order between those variables is unspecified.
7762 For instance, given the declarations
7766 var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
7767 var _ = sideEffect() // unrelated to x, a, or b
7771 type I interface { ab() []int }
7773 func (T) ab() []int { return []int{a, b} }
7777 the variable <code>a</code> will be initialized after <code>b</code> but
7778 whether <code>x</code> is initialized before <code>b</code>, between
7779 <code>b</code> and <code>a</code>, or after <code>a</code>, and
7780 thus also the moment at which <code>sideEffect()</code> is called (before
7781 or after <code>x</code> is initialized) is not specified.
7785 Variables may also be initialized using functions named <code>init</code>
7786 declared in the package block, with no arguments and no result parameters.
7794 Multiple such functions may be defined per package, even within a single
7795 source file. In the package block, the <code>init</code> identifier can
7796 be used only to declare <code>init</code> functions, yet the identifier
7797 itself is not <a href="#Declarations_and_scope">declared</a>. Thus
7798 <code>init</code> functions cannot be referred to from anywhere
7803 A package with no imports is initialized by assigning initial values
7804 to all its package-level variables followed by calling all <code>init</code>
7805 functions in the order they appear in the source, possibly in multiple files,
7806 as presented to the compiler.
7807 If a package has imports, the imported packages are initialized
7808 before initializing the package itself. If multiple packages import
7809 a package, the imported package will be initialized only once.
7810 The importing of packages, by construction, guarantees that there
7811 can be no cyclic initialization dependencies.
7815 Package initialization—variable initialization and the invocation of
7816 <code>init</code> functions—happens in a single goroutine,
7817 sequentially, one package at a time.
7818 An <code>init</code> function may launch other goroutines, which can run
7819 concurrently with the initialization code. However, initialization
7821 the <code>init</code> functions: it will not invoke the next one
7822 until the previous one has returned.
7826 To ensure reproducible initialization behavior, build systems are encouraged
7827 to present multiple files belonging to the same package in lexical file name
7828 order to a compiler.
7832 <h3 id="Program_execution">Program execution</h3>
7834 A complete program is created by linking a single, unimported package
7835 called the <i>main package</i> with all the packages it imports, transitively.
7836 The main package must
7837 have package name <code>main</code> and
7838 declare a function <code>main</code> that takes no
7839 arguments and returns no value.
7847 Program execution begins by initializing the main package and then
7848 invoking the function <code>main</code>.
7849 When that function invocation returns, the program exits.
7850 It does not wait for other (non-<code>main</code>) goroutines to complete.
7853 <h2 id="Errors">Errors</h2>
7856 The predeclared type <code>error</code> is defined as
7860 type error interface {
7866 It is the conventional interface for representing an error condition,
7867 with the nil value representing no error.
7868 For instance, a function to read data from a file might be defined:
7872 func Read(f *File, b []byte) (n int, err error)
7875 <h2 id="Run_time_panics">Run-time panics</h2>
7878 Execution errors such as attempting to index an array out
7879 of bounds trigger a <i>run-time panic</i> equivalent to a call of
7880 the built-in function <a href="#Handling_panics"><code>panic</code></a>
7881 with a value of the implementation-defined interface type <code>runtime.Error</code>.
7882 That type satisfies the predeclared interface type
7883 <a href="#Errors"><code>error</code></a>.
7884 The exact error values that
7885 represent distinct run-time error conditions are unspecified.
7891 type Error interface {
7893 // and perhaps other methods
7897 <h2 id="System_considerations">System considerations</h2>
7899 <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
7902 The built-in package <code>unsafe</code>, known to the compiler
7903 and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
7904 provides facilities for low-level programming including operations
7905 that violate the type system. A package using <code>unsafe</code>
7906 must be vetted manually for type safety and may not be portable.
7907 The package provides the following interface:
7910 <pre class="grammar">
7913 type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
7914 type Pointer *ArbitraryType
7916 func Alignof(variable ArbitraryType) uintptr
7917 func Offsetof(selector ArbitraryType) uintptr
7918 func Sizeof(variable ArbitraryType) uintptr
7920 type IntegerType int // shorthand for an integer type; it is not a real type
7921 func Add(ptr Pointer, len IntegerType) Pointer
7922 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
7926 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
7927 value may not be <a href="#Address_operators">dereferenced</a>.
7928 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be converted to
7929 a type of underlying type <code>Pointer</code> and vice versa.
7930 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
7935 bits = *(*uint64)(unsafe.Pointer(&f))
7937 type ptr unsafe.Pointer
7938 bits = *(*uint64)(ptr(&f))
7944 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
7945 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
7946 as if <code>v</code> was declared via <code>var v = x</code>.
7949 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
7950 <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
7951 or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
7952 If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
7953 without pointer indirections through fields of the struct.
7954 For a struct <code>s</code> with field <code>f</code>:
7958 uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
7962 Computer architectures may require memory addresses to be <i>aligned</i>;
7963 that is, for addresses of a variable to be a multiple of a factor,
7964 the variable's type's <i>alignment</i>. The function <code>Alignof</code>
7965 takes an expression denoting a variable of any type and returns the
7966 alignment of the (type of the) variable in bytes. For a variable
7971 uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
7975 A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
7976 is a type parameter, or if it is an array or struct type containing elements
7977 or fields of variable size. Otherwise the size is <i>constant</i>.
7978 Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
7979 are compile-time <a href="#Constant_expressions">constant expressions</a> of
7980 type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
7981 the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
7986 The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
7987 and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
7988 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
7989 A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
7990 if it is an untyped constant it is given type <code>int</code>.
7991 The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
7995 The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
7996 and whose length and capacity are <code>len</code>.
7997 <code>Slice(ptr, len)</code> is equivalent to
8001 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
8005 except that, as a special case, if <code>ptr</code>
8006 is <code>nil</code> and <code>len</code> is zero,
8007 <code>Slice</code> returns <code>nil</code>.
8011 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8012 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
8013 if it is an untyped constant it is given type <code>int</code>.
8014 At run time, if <code>len</code> is negative,
8015 or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
8016 a <a href="#Run_time_panics">run-time panic</a> occurs.
8019 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
8022 For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
8025 <pre class="grammar">
8030 uint32, int32, float32 4
8031 uint64, int64, float64, complex64 8
8036 The following minimal alignment properties are guaranteed:
8039 <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
8042 <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
8043 all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
8046 <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
8047 the alignment of a variable of the array's element type.
8052 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.