2 "Title": "The Go Programming Language Specification",
3 "Subtitle": "Version of June 7, 2023",
7 <h2 id="Introduction">Introduction</h2>
10 This is the reference manual for the Go programming language.
11 The pre-Go1.18 version, without generics, can be found
12 <a href="/doc/go1.17_spec.html">here</a>.
13 For more information and other documents, see <a href="/">golang.org</a>.
17 Go is a general-purpose language designed with systems programming
18 in mind. It is strongly typed and garbage-collected and has explicit
19 support for concurrent programming. Programs are constructed from
20 <i>packages</i>, whose properties allow efficient management of
25 The syntax is compact and simple to parse, allowing for easy analysis
26 by automatic tools such as integrated development environments.
29 <h2 id="Notation">Notation</h2>
31 The syntax is specified using a
32 <a href="https://en.wikipedia.org/wiki/Wirth_syntax_notation">variant</a>
33 of Extended Backus-Naur Form (EBNF):
37 Syntax = { Production } .
38 Production = production_name "=" [ Expression ] "." .
39 Expression = Term { "|" Term } .
40 Term = Factor { Factor } .
41 Factor = production_name | token [ "…" token ] | Group | Option | Repetition .
42 Group = "(" Expression ")" .
43 Option = "[" Expression "]" .
44 Repetition = "{" Expression "}" .
48 Productions are expressions constructed from terms and the following
49 operators, in increasing precedence:
54 [] option (0 or 1 times)
55 {} repetition (0 to n times)
59 Lowercase production names are used to identify lexical (terminal) tokens.
60 Non-terminals are in CamelCase. Lexical tokens are enclosed in
61 double quotes <code>""</code> or back quotes <code>``</code>.
65 The form <code>a … b</code> represents the set of characters from
66 <code>a</code> through <code>b</code> as alternatives. The horizontal
67 ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various
68 enumerations or code snippets that are not further specified. The character <code>…</code>
69 (as opposed to the three characters <code>...</code>) is not a token of the Go
73 <h2 id="Source_code_representation">Source code representation</h2>
76 Source code is Unicode text encoded in
77 <a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
78 canonicalized, so a single accented code point is distinct from the
79 same character constructed from combining an accent and a letter;
80 those are treated as two code points. For simplicity, this document
81 will use the unqualified term <i>character</i> to refer to a Unicode code point
85 Each code point is distinct; for instance, uppercase and lowercase letters
86 are different characters.
89 Implementation restriction: For compatibility with other tools, a
90 compiler may disallow the NUL character (U+0000) in the source text.
93 Implementation restriction: For compatibility with other tools, a
94 compiler may ignore a UTF-8-encoded byte order mark
95 (U+FEFF) if it is the first Unicode code point in the source text.
96 A byte order mark may be disallowed anywhere else in the source.
99 <h3 id="Characters">Characters</h3>
102 The following terms are used to denote specific Unicode character categories:
105 newline = /* the Unicode code point U+000A */ .
106 unicode_char = /* an arbitrary Unicode code point except newline */ .
107 unicode_letter = /* a Unicode code point categorized as "Letter" */ .
108 unicode_digit = /* a Unicode code point categorized as "Number, decimal digit" */ .
112 In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
113 Section 4.5 "General Category" defines a set of character categories.
114 Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
115 as Unicode letters, and those in the Number category Nd as Unicode digits.
118 <h3 id="Letters_and_digits">Letters and digits</h3>
121 The underscore character <code>_</code> (U+005F) is considered a lowercase letter.
124 letter = unicode_letter | "_" .
125 decimal_digit = "0" … "9" .
126 binary_digit = "0" | "1" .
127 octal_digit = "0" … "7" .
128 hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
131 <h2 id="Lexical_elements">Lexical elements</h2>
133 <h3 id="Comments">Comments</h3>
136 Comments serve as program documentation. There are two forms:
141 <i>Line comments</i> start with the character sequence <code>//</code>
142 and stop at the end of the line.
145 <i>General comments</i> start with the character sequence <code>/*</code>
146 and stop with the first subsequent character sequence <code>*/</code>.
151 A comment cannot start inside a <a href="#Rune_literals">rune</a> or
152 <a href="#String_literals">string literal</a>, or inside a comment.
153 A general comment containing no newlines acts like a space.
154 Any other comment acts like a newline.
157 <h3 id="Tokens">Tokens</h3>
160 Tokens form the vocabulary of the Go language.
161 There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators
162 and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from
163 spaces (U+0020), horizontal tabs (U+0009),
164 carriage returns (U+000D), and newlines (U+000A),
165 is ignored except as it separates tokens
166 that would otherwise combine into a single token. Also, a newline or end of file
167 may trigger the insertion of a <a href="#Semicolons">semicolon</a>.
168 While breaking the input into tokens,
169 the next token is the longest sequence of characters that form a
173 <h3 id="Semicolons">Semicolons</h3>
176 The formal syntax uses semicolons <code>";"</code> as terminators in
177 a number of productions. Go programs may omit most of these semicolons
178 using the following two rules:
183 When the input is broken into tokens, a semicolon is automatically inserted
184 into the token stream immediately after a line's final token if that token is
187 <a href="#Identifiers">identifier</a>
191 <a href="#Integer_literals">integer</a>,
192 <a href="#Floating-point_literals">floating-point</a>,
193 <a href="#Imaginary_literals">imaginary</a>,
194 <a href="#Rune_literals">rune</a>, or
195 <a href="#String_literals">string</a> literal
198 <li>one of the <a href="#Keywords">keywords</a>
200 <code>continue</code>,
201 <code>fallthrough</code>, or
205 <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a>
216 To allow complex statements to occupy a single line, a semicolon
217 may be omitted before a closing <code>")"</code> or <code>"}"</code>.
222 To reflect idiomatic use, code examples in this document elide semicolons
227 <h3 id="Identifiers">Identifiers</h3>
230 Identifiers name program entities such as variables and types.
231 An identifier is a sequence of one or more letters and digits.
232 The first character in an identifier must be a letter.
235 identifier = letter { letter | unicode_digit } .
240 ThisVariableIsExported
245 Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>.
249 <h3 id="Keywords">Keywords</h3>
252 The following keywords are reserved and may not be used as identifiers.
254 <pre class="grammar">
255 break default func interface select
256 case defer go map struct
257 chan else goto package switch
258 const fallthrough if range type
259 continue for import return var
262 <h3 id="Operators_and_punctuation">Operators and punctuation</h3>
265 The following character sequences represent <a href="#Operators">operators</a>
266 (including <a href="#Assignment_statements">assignment operators</a>) and punctuation:
268 <pre class="grammar">
269 + & += &= && == != ( )
270 - | -= |= || < <= [ ]
271 * ^ *= ^= <- > >= { }
272 / << /= <<= ++ = := , ;
273 % >> %= >>= -- ! ... . :
277 <h3 id="Integer_literals">Integer literals</h3>
280 An integer literal is a sequence of digits representing an
281 <a href="#Constants">integer constant</a>.
282 An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code>
283 for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal,
284 and <code>0x</code> or <code>0X</code> for hexadecimal.
285 A single <code>0</code> is considered a decimal zero.
286 In hexadecimal literals, letters <code>a</code> through <code>f</code>
287 and <code>A</code> through <code>F</code> represent values 10 through 15.
291 For readability, an underscore character <code>_</code> may appear after
292 a base prefix or between successive digits; such underscores do not change
296 int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
297 decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
298 binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
299 octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
300 hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
302 decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
303 binary_digits = binary_digit { [ "_" ] binary_digit } .
304 octal_digits = octal_digit { [ "_" ] octal_digit } .
305 hex_digits = hex_digit { [ "_" ] hex_digit } .
314 0O600 // second character is capital letter 'O'
318 170141183460469231731687303715884105727
319 170_141183_460469_231731_687303_715884_105727
321 _42 // an identifier, not an integer literal
322 42_ // invalid: _ must separate successive digits
323 4__2 // invalid: only one _ at a time
324 0_xBadFace // invalid: _ must separate successive digits
328 <h3 id="Floating-point_literals">Floating-point literals</h3>
331 A floating-point literal is a decimal or hexadecimal representation of a
332 <a href="#Constants">floating-point constant</a>.
336 A decimal floating-point literal consists of an integer part (decimal digits),
337 a decimal point, a fractional part (decimal digits), and an exponent part
338 (<code>e</code> or <code>E</code> followed by an optional sign and decimal digits).
339 One of the integer part or the fractional part may be elided; one of the decimal point
340 or the exponent part may be elided.
341 An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.
345 A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code>
346 prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
347 and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits).
348 One of the integer part or the fractional part may be elided; the radix point may be elided as well,
349 but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
350 An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.
354 For readability, an underscore character <code>_</code> may appear after
355 a base prefix or between successive digits; such underscores do not change
360 float_lit = decimal_float_lit | hex_float_lit .
362 decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
363 decimal_digits decimal_exponent |
364 "." decimal_digits [ decimal_exponent ] .
365 decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
367 hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
368 hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
371 hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
389 0x1.Fp+0 // == 1.9375
391 0X_1FFFP-16 // == 0.1249847412109375
392 0x15e-2 // == 0x15e - 2 (integer subtraction)
394 0x.p1 // invalid: mantissa has no digits
395 1p-2 // invalid: p exponent requires hexadecimal mantissa
396 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent
397 1_.5 // invalid: _ must separate successive digits
398 1._5 // invalid: _ must separate successive digits
399 1.5_e1 // invalid: _ must separate successive digits
400 1.5e_1 // invalid: _ must separate successive digits
401 1.5e1_ // invalid: _ must separate successive digits
405 <h3 id="Imaginary_literals">Imaginary literals</h3>
408 An imaginary literal represents the imaginary part of a
409 <a href="#Constants">complex constant</a>.
410 It consists of an <a href="#Integer_literals">integer</a> or
411 <a href="#Floating-point_literals">floating-point</a> literal
412 followed by the lowercase letter <code>i</code>.
413 The value of an imaginary literal is the value of the respective
414 integer or floating-point literal multiplied by the imaginary unit <i>i</i>.
418 imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
422 For backward compatibility, an imaginary literal's integer part consisting
423 entirely of decimal digits (and possibly underscores) is considered a decimal
424 integer, even if it starts with a leading <code>0</code>.
429 0123i // == 123i for backward-compatibility
430 0o123i // == 0o123 * 1i == 83i
431 0xabci // == 0xabc * 1i == 2748i
439 0x1p-2i // == 0x1p-2 * 1i == 0.25i
443 <h3 id="Rune_literals">Rune literals</h3>
446 A rune literal represents a <a href="#Constants">rune constant</a>,
447 an integer value identifying a Unicode code point.
448 A rune literal is expressed as one or more characters enclosed in single quotes,
449 as in <code>'x'</code> or <code>'\n'</code>.
450 Within the quotes, any character may appear except newline and unescaped single
451 quote. A single quoted character represents the Unicode value
452 of the character itself,
453 while multi-character sequences beginning with a backslash encode
454 values in various formats.
458 The simplest form represents the single character within the quotes;
459 since Go source text is Unicode characters encoded in UTF-8, multiple
460 UTF-8-encoded bytes may represent a single integer value. For
461 instance, the literal <code>'a'</code> holds a single byte representing
462 a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
463 <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
464 a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
468 Several backslash escapes allow arbitrary values to be encoded as
469 ASCII text. There are four ways to represent the integer value
470 as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
471 digits; <code>\u</code> followed by exactly four hexadecimal digits;
472 <code>\U</code> followed by exactly eight hexadecimal digits, and a
473 plain backslash <code>\</code> followed by exactly three octal digits.
474 In each case the value of the literal is the value represented by
475 the digits in the corresponding base.
479 Although these representations all result in an integer, they have
480 different valid ranges. Octal escapes must represent a value between
481 0 and 255 inclusive. Hexadecimal escapes satisfy this condition
482 by construction. The escapes <code>\u</code> and <code>\U</code>
483 represent Unicode code points so within them some values are illegal,
484 in particular those above <code>0x10FFFF</code> and surrogate halves.
488 After a backslash, certain single-character escapes represent special values:
491 <pre class="grammar">
492 \a U+0007 alert or bell
495 \n U+000A line feed or newline
496 \r U+000D carriage return
497 \t U+0009 horizontal tab
498 \v U+000B vertical tab
500 \' U+0027 single quote (valid escape only within rune literals)
501 \" U+0022 double quote (valid escape only within string literals)
505 An unrecognized character following a backslash in a rune literal is illegal.
509 rune_lit = "'" ( unicode_value | byte_value ) "'" .
510 unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
511 byte_value = octal_byte_value | hex_byte_value .
512 octal_byte_value = `\` octal_digit octal_digit octal_digit .
513 hex_byte_value = `\` "x" hex_digit hex_digit .
514 little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
515 big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
516 hex_digit hex_digit hex_digit hex_digit .
517 escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
532 '\'' // rune literal containing single quote character
533 'aa' // illegal: too many characters
534 '\k' // illegal: k is not recognized after a backslash
535 '\xa' // illegal: too few hexadecimal digits
536 '\0' // illegal: too few octal digits
537 '\400' // illegal: octal value over 255
538 '\uDFFF' // illegal: surrogate half
539 '\U00110000' // illegal: invalid Unicode code point
543 <h3 id="String_literals">String literals</h3>
546 A string literal represents a <a href="#Constants">string constant</a>
547 obtained from concatenating a sequence of characters. There are two forms:
548 raw string literals and interpreted string literals.
552 Raw string literals are character sequences between back quotes, as in
553 <code>`foo`</code>. Within the quotes, any character may appear except
554 back quote. The value of a raw string literal is the
555 string composed of the uninterpreted (implicitly UTF-8-encoded) characters
557 in particular, backslashes have no special meaning and the string may
559 Carriage return characters ('\r') inside raw string literals
560 are discarded from the raw string value.
564 Interpreted string literals are character sequences between double
565 quotes, as in <code>"bar"</code>.
566 Within the quotes, any character may appear except newline and unescaped double quote.
567 The text between the quotes forms the
568 value of the literal, with backslash escapes interpreted as they
569 are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and
570 <code>\"</code> is legal), with the same restrictions.
571 The three-digit octal (<code>\</code><i>nnn</i>)
572 and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual
573 <i>bytes</i> of the resulting string; all other escapes represent
574 the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.
575 Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent
576 a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>,
577 <code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent
578 the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character
583 string_lit = raw_string_lit | interpreted_string_lit .
584 raw_string_lit = "`" { unicode_char | newline } "`" .
585 interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
589 `abc` // same as "abc"
591 \n` // same as "\\n\n\\n"
598 "\uD800" // illegal: surrogate half
599 "\U00110000" // illegal: invalid Unicode code point
603 These examples all represent the same string:
607 "日本語" // UTF-8 input text
608 `日本語` // UTF-8 input text as a raw literal
609 "\u65e5\u672c\u8a9e" // the explicit Unicode code points
610 "\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points
611 "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes
615 If the source code represents a character as two code points, such as
616 a combining form involving an accent and a letter, the result will be
617 an error if placed in a rune literal (it is not a single code
618 point), and will appear as two code points if placed in a string
623 <h2 id="Constants">Constants</h2>
625 <p>There are <i>boolean constants</i>,
626 <i>rune constants</i>,
627 <i>integer constants</i>,
628 <i>floating-point constants</i>, <i>complex constants</i>,
629 and <i>string constants</i>. Rune, integer, floating-point,
630 and complex constants are
631 collectively called <i>numeric constants</i>.
635 A constant value is represented by a
636 <a href="#Rune_literals">rune</a>,
637 <a href="#Integer_literals">integer</a>,
638 <a href="#Floating-point_literals">floating-point</a>,
639 <a href="#Imaginary_literals">imaginary</a>,
641 <a href="#String_literals">string</a> literal,
642 an identifier denoting a constant,
643 a <a href="#Constant_expressions">constant expression</a>,
644 a <a href="#Conversions">conversion</a> with a result that is a constant, or
645 the result value of some built-in functions such as
646 <code>min</code> or <code>max</code> applied to constant arguments,
647 <code>unsafe.Sizeof</code> applied to <a href="#Package_unsafe">certain values</a>,
648 <code>cap</code> or <code>len</code> applied to
649 <a href="#Length_and_capacity">some expressions</a>,
650 <code>real</code> and <code>imag</code> applied to a complex constant
651 and <code>complex</code> applied to numeric constants.
652 The boolean truth values are represented by the predeclared constants
653 <code>true</code> and <code>false</code>. The predeclared identifier
654 <a href="#Iota">iota</a> denotes an integer constant.
658 In general, complex constants are a form of
659 <a href="#Constant_expressions">constant expression</a>
660 and are discussed in that section.
664 Numeric constants represent exact values of arbitrary precision and do not overflow.
665 Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
666 and not-a-number values.
670 Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
671 Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
672 and certain <a href="#Constant_expressions">constant expressions</a>
673 containing only untyped constant operands are untyped.
677 A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
678 or <a href="#Conversions">conversion</a>, or implicitly when used in a
679 <a href="#Variable_declarations">variable declaration</a> or an
680 <a href="#Assignment_statements">assignment statement</a> or as an
681 operand in an <a href="#Expressions">expression</a>.
682 It is an error if the constant value
683 cannot be <a href="#Representability">represented</a> as a value of the respective type.
684 If the type is a type parameter, the constant is converted into a non-constant
685 value of the type parameter.
689 An untyped constant has a <i>default type</i> which is the type to which the
690 constant is implicitly converted in contexts where a typed value is required,
691 for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
692 such as <code>i := 0</code> where there is no explicit type.
693 The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
694 <code>int</code>, <code>float64</code>, <code>complex128</code>, or <code>string</code>
695 respectively, depending on whether it is a boolean, rune, integer, floating-point,
696 complex, or string constant.
700 Implementation restriction: Although numeric constants have arbitrary
701 precision in the language, a compiler may implement them using an
702 internal representation with limited precision. That said, every
707 <li>Represent integer constants with at least 256 bits.</li>
709 <li>Represent floating-point constants, including the parts of
710 a complex constant, with a mantissa of at least 256 bits
711 and a signed binary exponent of at least 16 bits.</li>
713 <li>Give an error if unable to represent an integer constant
716 <li>Give an error if unable to represent a floating-point or
717 complex constant due to overflow.</li>
719 <li>Round to the nearest representable constant if unable to
720 represent a floating-point or complex constant due to limits
725 These requirements apply both to literal constants and to the result
726 of evaluating <a href="#Constant_expressions">constant
731 <h2 id="Variables">Variables</h2>
734 A variable is a storage location for holding a <i>value</i>.
735 The set of permissible values is determined by the
736 variable's <i><a href="#Types">type</a></i>.
740 A <a href="#Variable_declarations">variable declaration</a>
741 or, for function parameters and results, the signature
742 of a <a href="#Function_declarations">function declaration</a>
743 or <a href="#Function_literals">function literal</a> reserves
744 storage for a named variable.
746 Calling the built-in function <a href="#Allocation"><code>new</code></a>
747 or taking the address of a <a href="#Composite_literals">composite literal</a>
748 allocates storage for a variable at run time.
749 Such an anonymous variable is referred to via a (possibly implicit)
750 <a href="#Address_operators">pointer indirection</a>.
754 <i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
755 and <a href="#Struct_types">struct</a> types have elements and fields that may
756 be <a href="#Address_operators">addressed</a> individually. Each such element
757 acts like a variable.
761 The <i>static type</i> (or just <i>type</i>) of a variable is the
762 type given in its declaration, the type provided in the
763 <code>new</code> call or composite literal, or the type of
764 an element of a structured variable.
765 Variables of interface type also have a distinct <i>dynamic type</i>,
766 which is the (non-interface) type of the value assigned to the variable at run time
767 (unless the value is the predeclared identifier <code>nil</code>,
769 The dynamic type may vary during execution but values stored in interface
770 variables are always <a href="#Assignability">assignable</a>
771 to the static type of the variable.
775 var x interface{} // x is nil and has static type interface{}
776 var v *T // v has value nil, static type *T
777 x = 42 // x has value 42 and dynamic type int
778 x = v // x has value (*T)(nil) and dynamic type *T
782 A variable's value is retrieved by referring to the variable in an
783 <a href="#Expressions">expression</a>; it is the most recent value
784 <a href="#Assignment_statements">assigned</a> to the variable.
785 If a variable has not yet been assigned a value, its value is the
786 <a href="#The_zero_value">zero value</a> for its type.
790 <h2 id="Types">Types</h2>
793 A type determines a set of values together with operations and methods specific
794 to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be
795 followed by <a href="#Instantiations">type arguments</a> if the type is generic.
796 A type may also be specified using a <i>type literal</i>, which composes a type
801 Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
802 TypeName = identifier | QualifiedIdent .
803 TypeArgs = "[" TypeList [ "," ] "]" .
804 TypeList = Type { "," Type } .
805 TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
806 SliceType | MapType | ChannelType .
810 The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
811 Others are introduced with <a href="#Type_declarations">type declarations</a>
812 or <a href="#Type_parameter_declarations">type parameter lists</a>.
813 <i>Composite types</i>—array, struct, pointer, function,
814 interface, slice, map, and channel types—may be constructed using
819 Predeclared types, defined types, and type parameters are called <i>named types</i>.
820 An alias denotes a named type if the type given in the alias declaration is a named type.
823 <h3 id="Boolean_types">Boolean types</h3>
826 A <i>boolean type</i> represents the set of Boolean truth values
827 denoted by the predeclared constants <code>true</code>
828 and <code>false</code>. The predeclared boolean type is <code>bool</code>;
829 it is a <a href="#Type_definitions">defined type</a>.
832 <h3 id="Numeric_types">Numeric types</h3>
835 An <i>integer</i>, <i>floating-point</i>, or <i>complex</i> type
836 represents the set of integer, floating-point, or complex values, respectively.
837 They are collectively called <i>numeric types</i>.
838 The predeclared architecture-independent numeric types are:
841 <pre class="grammar">
842 uint8 the set of all unsigned 8-bit integers (0 to 255)
843 uint16 the set of all unsigned 16-bit integers (0 to 65535)
844 uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
845 uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
847 int8 the set of all signed 8-bit integers (-128 to 127)
848 int16 the set of all signed 16-bit integers (-32768 to 32767)
849 int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
850 int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
852 float32 the set of all IEEE-754 32-bit floating-point numbers
853 float64 the set of all IEEE-754 64-bit floating-point numbers
855 complex64 the set of all complex numbers with float32 real and imaginary parts
856 complex128 the set of all complex numbers with float64 real and imaginary parts
863 The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
864 <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
868 There is also a set of predeclared integer types with implementation-specific sizes:
871 <pre class="grammar">
872 uint either 32 or 64 bits
873 int same size as uint
874 uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
878 To avoid portability issues all numeric types are <a href="#Type_definitions">defined
879 types</a> and thus distinct except
880 <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
881 <code>rune</code>, which is an alias for <code>int32</code>.
883 are required when different numeric types are mixed in an expression
884 or assignment. For instance, <code>int32</code> and <code>int</code>
885 are not the same type even though they may have the same size on a
886 particular architecture.
889 <h3 id="String_types">String types</h3>
892 A <i>string type</i> represents the set of string values.
893 A string value is a (possibly empty) sequence of bytes.
894 The number of bytes is called the length of the string and is never negative.
895 Strings are immutable: once created,
896 it is impossible to change the contents of a string.
897 The predeclared string type is <code>string</code>;
898 it is a <a href="#Type_definitions">defined type</a>.
902 The length of a string <code>s</code> can be discovered using
903 the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
904 The length is a compile-time constant if the string is a constant.
905 A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
906 0 through <code>len(s)-1</code>.
907 It is illegal to take the address of such an element; if
908 <code>s[i]</code> is the <code>i</code>'th byte of a
909 string, <code>&s[i]</code> is invalid.
913 <h3 id="Array_types">Array types</h3>
916 An array is a numbered sequence of elements of a single
917 type, called the element type.
918 The number of elements is called the length of the array and is never negative.
922 ArrayType = "[" ArrayLength "]" ElementType .
923 ArrayLength = Expression .
928 The length is part of the array's type; it must evaluate to a
929 non-negative <a href="#Constants">constant</a>
930 <a href="#Representability">representable</a> by a value
931 of type <code>int</code>.
932 The length of array <code>a</code> can be discovered
933 using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
934 The elements can be addressed by integer <a href="#Index_expressions">indices</a>
935 0 through <code>len(a)-1</code>.
936 Array types are always one-dimensional but may be composed to form
937 multi-dimensional types.
942 [2*N] struct { x, y int32 }
945 [2][2][2]float64 // same as [2]([2]([2]float64))
949 An array type <code>T</code> may not have an element of type <code>T</code>,
950 or of a type containing <code>T</code> as a component, directly or indirectly,
951 if those containing types are only array or struct types.
955 // invalid array types
957 T1 [10]T1 // element type of T1 is T1
958 T2 [10]struct{ f T2 } // T2 contains T2 as component of a struct
959 T3 [10]T4 // T3 contains T3 as component of a struct in T4
960 T4 struct{ f T3 } // T4 contains T4 as component of array T3 in a struct
965 T5 [10]*T5 // T5 contains T5 as component of a pointer
966 T6 [10]func() T6 // T6 contains T6 as component of a function type
967 T7 [10]struct{ f []T7 } // T7 contains T7 as component of a slice in a struct
971 <h3 id="Slice_types">Slice types</h3>
974 A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
975 provides access to a numbered sequence of elements from that array.
976 A slice type denotes the set of all slices of arrays of its element type.
977 The number of elements is called the length of the slice and is never negative.
978 The value of an uninitialized slice is <code>nil</code>.
982 SliceType = "[" "]" ElementType .
986 The length of a slice <code>s</code> can be discovered by the built-in function
987 <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
988 execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
989 0 through <code>len(s)-1</code>. The slice index of a
990 given element may be less than the index of the same element in the
994 A slice, once initialized, is always associated with an underlying
995 array that holds its elements. A slice therefore shares storage
996 with its array and with other slices of the same array; by contrast,
997 distinct arrays always represent distinct storage.
1000 The array underlying a slice may extend past the end of the slice.
1001 The <i>capacity</i> is a measure of that extent: it is the sum of
1002 the length of the slice and the length of the array beyond the slice;
1003 a slice of length up to that capacity can be created by
1004 <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
1005 The capacity of a slice <code>a</code> can be discovered using the
1006 built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
1010 A new, initialized slice value for a given element type <code>T</code> may be
1011 made using the built-in function
1012 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1013 which takes a slice type
1014 and parameters specifying the length and optionally the capacity.
1015 A slice created with <code>make</code> always allocates a new, hidden array
1016 to which the returned slice value refers. That is, executing
1020 make([]T, length, capacity)
1024 produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
1025 it, so these two expressions are equivalent:
1029 make([]int, 50, 100)
1034 Like arrays, slices are always one-dimensional but may be composed to construct
1035 higher-dimensional objects.
1036 With arrays of arrays, the inner arrays are, by construction, always the same length;
1037 however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
1038 Moreover, the inner slices must be initialized individually.
1041 <h3 id="Struct_types">Struct types</h3>
1044 A struct is a sequence of named elements, called fields, each of which has a
1045 name and a type. Field names may be specified explicitly (IdentifierList) or
1046 implicitly (EmbeddedField).
1047 Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
1048 be <a href="#Uniqueness_of_identifiers">unique</a>.
1052 StructType = "struct" "{" { FieldDecl ";" } "}" .
1053 FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
1054 EmbeddedField = [ "*" ] TypeName [ TypeArgs ] .
1062 // A struct with 6 fields.
1066 _ float32 // padding
1073 A field declared with a type but no explicit field name is called an <i>embedded field</i>.
1074 An embedded field must be specified as
1075 a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
1076 and <code>T</code> itself may not be
1077 a pointer type. The unqualified type name acts as the field name.
1081 // A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
1083 T1 // field name is T1
1084 *T2 // field name is T2
1085 P.T3 // field name is T3
1086 *P.T4 // field name is T4
1087 x, y int // field names are x and y
1092 The following declaration is illegal because field names must be unique
1098 T // conflicts with embedded field *T and *P.T
1099 *T // conflicts with embedded field T and *P.T
1100 *P.T // conflicts with embedded field T and *T
1105 A field or <a href="#Method_declarations">method</a> <code>f</code> of an
1106 embedded field in a struct <code>x</code> is called <i>promoted</i> if
1107 <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
1108 that field or method <code>f</code>.
1112 Promoted fields act like ordinary fields
1113 of a struct except that they cannot be used as field names in
1114 <a href="#Composite_literals">composite literals</a> of the struct.
1118 Given a struct type <code>S</code> and a <a href="#Types">named type</a>
1119 <code>T</code>, promoted methods are included in the method set of the struct as follows:
1123 If <code>S</code> contains an embedded field <code>T</code>,
1124 the <a href="#Method_sets">method sets</a> of <code>S</code>
1125 and <code>*S</code> both include promoted methods with receiver
1126 <code>T</code>. The method set of <code>*S</code> also
1127 includes promoted methods with receiver <code>*T</code>.
1131 If <code>S</code> contains an embedded field <code>*T</code>,
1132 the method sets of <code>S</code> and <code>*S</code> both
1133 include promoted methods with receiver <code>T</code> or
1139 A field declaration may be followed by an optional string literal <i>tag</i>,
1140 which becomes an attribute for all the fields in the corresponding
1141 field declaration. An empty tag string is equivalent to an absent tag.
1142 The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
1143 and take part in <a href="#Type_identity">type identity</a> for structs
1144 but are otherwise ignored.
1149 x, y float64 "" // an empty tag string is like an absent tag
1150 name string "any string is permitted as a tag"
1151 _ [4]byte "ceci n'est pas un champ de structure"
1154 // A struct corresponding to a TimeStamp protocol buffer.
1155 // The tag strings define the protocol buffer field numbers;
1156 // they follow the convention outlined by the reflect package.
1158 microsec uint64 `protobuf:"1"`
1159 serverIP6 uint64 `protobuf:"2"`
1164 A struct type <code>T</code> may not contain a field of type <code>T</code>,
1165 or of a type containing <code>T</code> as a component, directly or indirectly,
1166 if those containing types are only array or struct types.
1170 // invalid struct types
1172 T1 struct{ T1 } // T1 contains a field of T1
1173 T2 struct{ f [10]T2 } // T2 contains T2 as component of an array
1174 T3 struct{ T4 } // T3 contains T3 as component of an array in struct T4
1175 T4 struct{ f [10]T3 } // T4 contains T4 as component of struct T3 in an array
1178 // valid struct types
1180 T5 struct{ f *T5 } // T5 contains T5 as component of a pointer
1181 T6 struct{ f func() T6 } // T6 contains T6 as component of a function type
1182 T7 struct{ f [10][]T7 } // T7 contains T7 as component of a slice in an array
1186 <h3 id="Pointer_types">Pointer types</h3>
1189 A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
1190 type, called the <i>base type</i> of the pointer.
1191 The value of an uninitialized pointer is <code>nil</code>.
1195 PointerType = "*" BaseType .
1204 <h3 id="Function_types">Function types</h3>
1207 A function type denotes the set of all functions with the same parameter
1208 and result types. The value of an uninitialized variable of function type
1209 is <code>nil</code>.
1213 FunctionType = "func" Signature .
1214 Signature = Parameters [ Result ] .
1215 Result = Parameters | Type .
1216 Parameters = "(" [ ParameterList [ "," ] ] ")" .
1217 ParameterList = ParameterDecl { "," ParameterDecl } .
1218 ParameterDecl = [ IdentifierList ] [ "..." ] Type .
1222 Within a list of parameters or results, the names (IdentifierList)
1223 must either all be present or all be absent. If present, each name
1224 stands for one item (parameter or result) of the specified type and
1225 all non-<a href="#Blank_identifier">blank</a> names in the signature
1226 must be <a href="#Uniqueness_of_identifiers">unique</a>.
1227 If absent, each type stands for one item of that type.
1228 Parameter and result
1229 lists are always parenthesized except that if there is exactly
1230 one unnamed result it may be written as an unparenthesized type.
1234 The final incoming parameter in a function signature may have
1235 a type prefixed with <code>...</code>.
1236 A function with such a parameter is called <i>variadic</i> and
1237 may be invoked with zero or more arguments for that parameter.
1243 func(a, _ int, z float32) bool
1244 func(a, b int, z float32) (bool)
1245 func(prefix string, values ...int)
1246 func(a, b int, z float64, opt ...interface{}) (success bool)
1247 func(int, int, float64) (float64, *[]int)
1248 func(n int) func(p *T)
1251 <h3 id="Interface_types">Interface types</h3>
1254 An interface type defines a <i>type set</i>.
1255 A variable of interface type can store a value of any type that is in the type
1256 set of the interface. Such a type is said to
1257 <a href="#Implementing_an_interface">implement the interface</a>.
1258 The value of an uninitialized variable of interface type is <code>nil</code>.
1262 InterfaceType = "interface" "{" { InterfaceElem ";" } "}" .
1263 InterfaceElem = MethodElem | TypeElem .
1264 MethodElem = MethodName Signature .
1265 MethodName = identifier .
1266 TypeElem = TypeTerm { "|" TypeTerm } .
1267 TypeTerm = Type | UnderlyingType .
1268 UnderlyingType = "~" Type .
1272 An interface type is specified by a list of <i>interface elements</i>.
1273 An interface element is either a <i>method</i> or a <i>type element</i>,
1274 where a type element is a union of one or more <i>type terms</i>.
1275 A type term is either a single type or a single underlying type.
1278 <h4 id="Basic_interfaces">Basic interfaces</h4>
1281 In its most basic form an interface specifies a (possibly empty) list of methods.
1282 The type set defined by such an interface is the set of types which implement all of
1283 those methods, and the corresponding <a href="#Method_sets">method set</a> consists
1284 exactly of the methods specified by the interface.
1285 Interfaces whose type sets can be defined entirely by a list of methods are called
1286 <i>basic interfaces.</i>
1290 // A simple File interface.
1292 Read([]byte) (int, error)
1293 Write([]byte) (int, error)
1299 The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
1300 and not <a href="#Blank_identifier">blank</a>.
1306 String() string // illegal: String not unique
1307 _(x int) // illegal: method must have non-blank name
1312 More than one type may implement an interface.
1313 For instance, if two types <code>S1</code> and <code>S2</code>
1318 func (p T) Read(p []byte) (n int, err error)
1319 func (p T) Write(p []byte) (n int, err error)
1320 func (p T) Close() error
1324 (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
1325 then the <code>File</code> interface is implemented by both <code>S1</code> and
1326 <code>S2</code>, regardless of what other methods
1327 <code>S1</code> and <code>S2</code> may have or share.
1331 Every type that is a member of the type set of an interface implements that interface.
1332 Any given type may implement several distinct interfaces.
1333 For instance, all types implement the <i>empty interface</i> which stands for the set
1334 of all (non-interface) types:
1342 For convenience, the predeclared type <code>any</code> is an alias for the empty interface.
1346 Similarly, consider this interface specification,
1347 which appears within a <a href="#Type_declarations">type declaration</a>
1348 to define an interface called <code>Locker</code>:
1352 type Locker interface {
1359 If <code>S1</code> and <code>S2</code> also implement
1363 func (p T) Lock() { … }
1364 func (p T) Unlock() { … }
1368 they implement the <code>Locker</code> interface as well
1369 as the <code>File</code> interface.
1372 <h4 id="Embedded_interfaces">Embedded interfaces</h4>
1375 In a slightly more general form
1376 an interface <code>T</code> may use a (possibly qualified) interface type
1377 name <code>E</code> as an interface element. This is called
1378 <i>embedding</i> interface <code>E</code> in <code>T</code>.
1379 The type set of <code>T</code> is the <i>intersection</i> of the type sets
1380 defined by <code>T</code>'s explicitly declared methods and the type sets
1381 of <code>T</code>’s embedded interfaces.
1382 In other words, the type set of <code>T</code> is the set of all types that implement all the
1383 explicitly declared methods of <code>T</code> and also all the methods of
1388 type Reader interface {
1389 Read(p []byte) (n int, err error)
1393 type Writer interface {
1394 Write(p []byte) (n int, err error)
1398 // ReadWriter's methods are Read, Write, and Close.
1399 type ReadWriter interface {
1400 Reader // includes methods of Reader in ReadWriter's method set
1401 Writer // includes methods of Writer in ReadWriter's method set
1406 When embedding interfaces, methods with the
1407 <a href="#Uniqueness_of_identifiers">same</a> names must
1408 have <a href="#Type_identity">identical</a> signatures.
1412 type ReadCloser interface {
1413 Reader // includes methods of Reader in ReadCloser's method set
1414 Close() // illegal: signatures of Reader.Close and Close are different
1418 <h4 id="General_interfaces">General interfaces</h4>
1421 In their most general form, an interface element may also be an arbitrary type term
1422 <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
1423 or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
1424 Together with method specifications, these elements enable the precise
1425 definition of an interface's type set as follows:
1429 <li>The type set of the empty interface is the set of all non-interface types.
1432 <li>The type set of a non-empty interface is the intersection of the type sets
1433 of its interface elements.
1436 <li>The type set of a method specification is the set of all non-interface types
1437 whose method sets include that method.
1440 <li>The type set of a non-interface type term is the set consisting
1444 <li>The type set of a term of the form <code>~T</code>
1445 is the set of all types whose underlying type is <code>T</code>.
1448 <li>The type set of a <i>union</i> of terms
1449 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
1450 is the union of the type sets of the terms.
1455 The quantification "the set of all non-interface types" refers not just to all (non-interface)
1456 types declared in the program at hand, but all possible types in all possible programs, and
1458 Similarly, given the set of all non-interface types that implement a particular method, the
1459 intersection of the method sets of those types will contain exactly that method, even if all
1460 types in the program at hand always pair that method with another method.
1464 By construction, an interface's type set never contains an interface type.
1468 // An interface representing only the type int.
1473 // An interface representing all types with underlying type int.
1478 // An interface representing all types with underlying type int that implement the String method.
1484 // An interface representing an empty type set: there is no type that is both an int and a string.
1492 In a term of the form <code>~T</code>, the underlying type of <code>T</code>
1493 must be itself, and <code>T</code> cannot be an interface.
1500 ~[]byte // the underlying type of []byte is itself
1501 ~MyInt // illegal: the underlying type of MyInt is not MyInt
1502 ~error // illegal: error is an interface
1507 Union elements denote unions of type sets:
1511 // The Float interface represents all floating-point types
1512 // (including any named types whose underlying types are
1513 // either float32 or float64).
1514 type Float interface {
1520 The type <code>T</code> in a term of the form <code>T</code> or <code>~T</code> cannot
1521 be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all
1522 non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
1523 Given a type parameter <code>P</code>:
1528 P // illegal: P is a type parameter
1529 int | ~P // illegal: P is a type parameter
1530 ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
1531 float32 | Float // overlapping type sets but Float is an interface
1536 Implementation restriction:
1537 A union (with more than one term) cannot contain the
1538 <a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code>
1539 or interfaces that specify methods, or embed <code>comparable</code> or interfaces
1540 that specify methods.
1544 Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type
1545 constraints, or as elements of other interfaces used as constraints.
1546 They cannot be the types of values or variables, or components of other,
1547 non-interface types.
1551 var x Float // illegal: Float is not a basic interface
1553 var x interface{} = Float(nil) // illegal
1555 type Floatish struct {
1561 An interface type <code>T</code> may not embed a type element
1562 that is, contains, or embeds <code>T</code>, directly or indirectly.
1566 // illegal: Bad may not embed itself
1567 type Bad interface {
1571 // illegal: Bad1 may not embed itself using Bad2
1572 type Bad1 interface {
1575 type Bad2 interface {
1579 // illegal: Bad3 may not embed a union containing Bad3
1580 type Bad3 interface {
1581 ~int | ~string | Bad3
1584 // illegal: Bad4 may not embed an array containing Bad4 as element type
1585 type Bad4 interface {
1590 <h4 id="Implementing_an_interface">Implementing an interface</h4>
1593 A type <code>T</code> implements an interface <code>I</code> if
1598 <code>T</code> is not an interface and is an element of the type set of <code>I</code>; or
1601 <code>T</code> is an interface and the type set of <code>T</code> is a subset of the
1602 type set of <code>I</code>.
1607 A value of type <code>T</code> implements an interface if <code>T</code>
1608 implements the interface.
1611 <h3 id="Map_types">Map types</h3>
1614 A map is an unordered group of elements of one type, called the
1615 element type, indexed by a set of unique <i>keys</i> of another type,
1616 called the key type.
1617 The value of an uninitialized map is <code>nil</code>.
1621 MapType = "map" "[" KeyType "]" ElementType .
1626 The <a href="#Comparison_operators">comparison operators</a>
1627 <code>==</code> and <code>!=</code> must be fully defined
1628 for operands of the key type; thus the key type must not be a function, map, or
1630 If the key type is an interface type, these
1631 comparison operators must be defined for the dynamic key values;
1632 failure will cause a <a href="#Run_time_panics">run-time panic</a>.
1637 map[*T]struct{ x, y float64 }
1638 map[string]interface{}
1642 The number of map elements is called its length.
1643 For a map <code>m</code>, it can be discovered using the
1644 built-in function <a href="#Length_and_capacity"><code>len</code></a>
1645 and may change during execution. Elements may be added during execution
1646 using <a href="#Assignment_statements">assignments</a> and retrieved with
1647 <a href="#Index_expressions">index expressions</a>; they may be removed with the
1648 <a href="#Deletion_of_map_elements"><code>delete</code></a> and
1649 <a href="#Clear"><code>clear</code></a> built-in function.
1653 A new, empty map value is made using the built-in
1654 function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1655 which takes the map type and an optional capacity hint as arguments:
1659 make(map[string]int)
1660 make(map[string]int, 100)
1664 The initial capacity does not bound its size:
1665 maps grow to accommodate the number of items
1666 stored in them, with the exception of <code>nil</code> maps.
1667 A <code>nil</code> map is equivalent to an empty map except that no elements
1670 <h3 id="Channel_types">Channel types</h3>
1673 A channel provides a mechanism for
1674 <a href="#Go_statements">concurrently executing functions</a>
1676 <a href="#Send_statements">sending</a> and
1677 <a href="#Receive_operator">receiving</a>
1678 values of a specified element type.
1679 The value of an uninitialized channel is <code>nil</code>.
1683 ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
1687 The optional <code><-</code> operator specifies the channel <i>direction</i>,
1688 <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
1689 otherwise it is <i>bidirectional</i>.
1690 A channel may be constrained only to send or only to receive by
1691 <a href="#Assignment_statements">assignment</a> or
1692 explicit <a href="#Conversions">conversion</a>.
1696 chan T // can be used to send and receive values of type T
1697 chan<- float64 // can only be used to send float64s
1698 <-chan int // can only be used to receive ints
1702 The <code><-</code> operator associates with the leftmost <code>chan</code>
1707 chan<- chan int // same as chan<- (chan int)
1708 chan<- <-chan int // same as chan<- (<-chan int)
1709 <-chan <-chan int // same as <-chan (<-chan int)
1710 chan (<-chan int)
1714 A new, initialized channel
1715 value can be made using the built-in function
1716 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1717 which takes the channel type and an optional <i>capacity</i> as arguments:
1725 The capacity, in number of elements, sets the size of the buffer in the channel.
1726 If the capacity is zero or absent, the channel is unbuffered and communication
1727 succeeds only when both a sender and receiver are ready. Otherwise, the channel
1728 is buffered and communication succeeds without blocking if the buffer
1729 is not full (sends) or not empty (receives).
1730 A <code>nil</code> channel is never ready for communication.
1734 A channel may be closed with the built-in function
1735 <a href="#Close"><code>close</code></a>.
1736 The multi-valued assignment form of the
1737 <a href="#Receive_operator">receive operator</a>
1738 reports whether a received value was sent before
1739 the channel was closed.
1743 A single channel may be used in
1744 <a href="#Send_statements">send statements</a>,
1745 <a href="#Receive_operator">receive operations</a>,
1746 and calls to the built-in functions
1747 <a href="#Length_and_capacity"><code>cap</code></a> and
1748 <a href="#Length_and_capacity"><code>len</code></a>
1749 by any number of goroutines without further synchronization.
1750 Channels act as first-in-first-out queues.
1751 For example, if one goroutine sends values on a channel
1752 and a second goroutine receives them, the values are
1753 received in the order sent.
1756 <h2 id="Properties_of_types_and_values">Properties of types and values</h2>
1758 <h3 id="Underlying_types">Underlying types</h3>
1761 Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
1762 is one of the predeclared boolean, numeric, or string types, or a type literal,
1763 the corresponding underlying type is <code>T</code> itself.
1764 Otherwise, <code>T</code>'s underlying type is the underlying type of the
1765 type to which <code>T</code> refers in its declaration.
1766 For a type parameter that is the underlying type of its
1767 <a href="#Type_constraints">type constraint</a>, which is always an interface.
1783 func f[P any](x P) { … }
1787 The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
1788 and <code>B2</code> is <code>string</code>.
1789 The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
1790 The underlying type of <code>P</code> is <code>interface{}</code>.
1793 <h3 id="Core_types">Core types</h3>
1796 Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the
1797 <a href="#Underlying_types">underlying type</a> of <code>T</code>.
1801 An interface <code>T</code> has a core type if one of the following
1802 conditions is satisfied:
1807 There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a>
1808 of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or
1811 the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a>
1812 with identical element type <code>E</code>, and all directional channels have the same
1818 No other interfaces have a core type.
1822 The core type of an interface is, depending on the condition that is satisfied, either:
1827 the type <code>U</code>; or
1830 the type <code>chan E</code> if <code>T</code> contains only bidirectional
1831 channels, or the type <code>chan<- E</code> or <code><-chan E</code>
1832 depending on the direction of the directional channels present.
1837 By definition, a core type is never a <a href="#Type_definitions">defined type</a>,
1838 <a href="#Type_parameter_declarations">type parameter</a>, or
1839 <a href="#Interface_types">interface type</a>.
1843 Examples of interfaces with core types:
1847 type Celsius float32
1850 interface{ int } // int
1851 interface{ Celsius|Kelvin } // float32
1852 interface{ ~chan int } // chan int
1853 interface{ ~chan int|~chan<- int } // chan<- int
1854 interface{ ~[]*data; String() string } // []*data
1858 Examples of interfaces without core types:
1862 interface{} // no single underlying type
1863 interface{ Celsius|float64 } // no single underlying type
1864 interface{ chan int | chan<- string } // channels have different element types
1865 interface{ <-chan int | chan<- int } // directional channels have different directions
1869 Some operations (<a href="#Slice_expressions">slice expressions</a>,
1870 <a href="#Appending_and_copying_slices"><code>append</code> and <code>copy</code></a>)
1871 rely on a slightly more loose form of core types which accept byte slices and strings.
1872 Specifically, if there are exactly two types, <code>[]byte</code> and <code>string</code>,
1873 which are the underlying types of all types in the type set of interface <code>T</code>,
1874 the core type of <code>T</code> is called <code>bytestring</code>.
1878 Examples of interfaces with <code>bytestring</code> core types:
1882 interface{ int } // int (same as ordinary core type)
1883 interface{ []byte | string } // bytestring
1884 interface{ ~[]byte | myString } // bytestring
1888 Note that <code>bytestring</code> is not a real type; it cannot be used to declare
1889 variables or compose other types. It exists solely to describe the behavior of some
1890 operations that read from a sequence of bytes, which may be a byte slice or a string.
1893 <h3 id="Type_identity">Type identity</h3>
1896 Two types are either <i>identical</i> or <i>different</i>.
1900 A <a href="#Types">named type</a> is always different from any other type.
1901 Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
1902 structurally equivalent; that is, they have the same literal structure and corresponding
1903 components have identical types. In detail:
1907 <li>Two array types are identical if they have identical element types and
1908 the same array length.</li>
1910 <li>Two slice types are identical if they have identical element types.</li>
1912 <li>Two struct types are identical if they have the same sequence of fields,
1913 and if corresponding fields have the same names, and identical types,
1915 <a href="#Exported_identifiers">Non-exported</a> field names from different
1916 packages are always different.</li>
1918 <li>Two pointer types are identical if they have identical base types.</li>
1920 <li>Two function types are identical if they have the same number of parameters
1921 and result values, corresponding parameter and result types are
1922 identical, and either both functions are variadic or neither is.
1923 Parameter and result names are not required to match.</li>
1925 <li>Two interface types are identical if they define the same type set.
1928 <li>Two map types are identical if they have identical key and element types.</li>
1930 <li>Two channel types are identical if they have identical element types and
1931 the same direction.</li>
1933 <li>Two <a href="#Instantiations">instantiated</a> types are identical if
1934 their defined types and all type arguments are identical.
1939 Given the declarations
1946 A2 = struct{ a, b int }
1948 A4 = func(A3, float64) *A0
1949 A5 = func(x int, _ float64) *[]string
1953 B2 struct{ a, b int }
1954 B3 struct{ a, c int }
1955 B4 func(int, float64) *B0
1956 B5 func(x int, y float64) *A1
1959 D0[P1, P2 any] struct{ x P1; y P2 }
1960 E0 = D0[int, string]
1965 these types are identical:
1969 A0, A1, and []string
1970 A2 and struct{ a, b int }
1972 A4, func(int, float64) *[]string, and A5
1975 D0[int, string] and E0
1977 struct{ a, b *B5 } and struct{ a, b *B5 }
1978 func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
1982 <code>B0</code> and <code>B1</code> are different because they are new types
1983 created by distinct <a href="#Type_definitions">type definitions</a>;
1984 <code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
1985 are different because <code>B0</code> is different from <code>[]string</code>;
1986 and <code>P1</code> and <code>P2</code> are different because they are different
1988 <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are
1989 different because the former is an <a href="#Instantiations">instantiated</a>
1990 defined type while the latter is a type literal
1991 (but they are still <a href="#Assignability">assignable</a>).
1994 <h3 id="Assignability">Assignability</h3>
1997 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>
1998 ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
2003 <code>V</code> and <code>T</code> are identical.
2006 <code>V</code> and <code>T</code> have identical
2007 <a href="#Underlying_types">underlying types</a>
2008 but are not type parameters and at least one of <code>V</code>
2009 or <code>T</code> is not a <a href="#Types">named type</a>.
2012 <code>V</code> and <code>T</code> are channel types with
2013 identical element types, <code>V</code> is a bidirectional channel,
2014 and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
2017 <code>T</code> is an interface type, but not a type parameter, and
2018 <code>x</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
2021 <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
2022 is a pointer, function, slice, map, channel, or interface type,
2023 but not a type parameter.
2026 <code>x</code> is an untyped <a href="#Constants">constant</a>
2027 <a href="#Representability">representable</a>
2028 by a value of type <code>T</code>.
2033 Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code>
2034 is assignable to a variable of type <code>T</code> if one of the following conditions applies:
2039 <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
2040 a type parameter, and <code>x</code> is assignable to each type in
2041 <code>T</code>'s type set.
2044 <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
2045 a type parameter, and <code>x</code> is assignable to each type in
2046 <code>T</code>'s type set.
2049 <code>V</code> is a type parameter and <code>T</code> is not a named type,
2050 and values of each type in <code>V</code>'s type set are assignable
2055 <h3 id="Representability">Representability</h3>
2058 A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
2059 by a value of type <code>T</code>,
2060 where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>,
2061 if one of the following conditions applies:
2066 <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
2070 <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
2071 precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
2072 negative zero further simplified to an unsigned zero. Note that constant values never result
2073 in an IEEE negative zero, NaN, or infinity.
2077 <code>T</code> is a complex type, and <code>x</code>'s
2078 <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
2079 are representable by values of <code>T</code>'s component type (<code>float32</code> or
2080 <code>float64</code>).
2085 If <code>T</code> is a type parameter,
2086 <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
2087 by a value of each type in <code>T</code>'s type set.
2091 x T x is representable by a value of T because
2093 'a' byte 97 is in the set of byte values
2094 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
2095 "foo" string "foo" is in the set of string values
2096 1024 int16 1024 is in the set of 16-bit integers
2097 42.0 byte 42 is in the set of unsigned 8-bit integers
2098 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
2099 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
2100 -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
2101 0i int 0 is an integer value
2102 (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
2106 x T x is not representable by a value of T because
2108 0 bool 0 is not in the set of boolean values
2109 'a' string 'a' is a rune, it is not in the set of string values
2110 1024 byte 1024 is not in the set of unsigned 8-bit integers
2111 -1 uint16 -1 is not in the set of unsigned 16-bit integers
2112 1.1 int 1.1 is not an integer value
2113 42i float32 (0 + 42i) is not in the set of float32 values
2114 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
2117 <h3 id="Method_sets">Method sets</h3>
2120 The <i>method set</i> of a type determines the methods that can be
2121 <a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type.
2122 Every type has a (possibly empty) method set associated with it:
2126 <li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all
2127 <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
2131 The method set of a pointer to a defined type <code>T</code>
2132 (where <code>T</code> is neither a pointer nor an interface)
2133 is the set of all methods declared with receiver <code>*T</code> or <code>T</code>.
2136 <li>The method set of an <a href="#Interface_types">interface type</a> is the intersection
2137 of the method sets of each type in the interface's <a href="#Interface_types">type set</a>
2138 (the resulting method set is usually just the set of declared methods in the interface).
2143 Further rules apply to structs (and pointer to structs) containing embedded fields,
2144 as described in the section on <a href="#Struct_types">struct types</a>.
2145 Any other type has an empty method set.
2149 In a method set, each method must have a
2150 <a href="#Uniqueness_of_identifiers">unique</a>
2151 non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
2154 <h2 id="Blocks">Blocks</h2>
2157 A <i>block</i> is a possibly empty sequence of declarations and statements
2158 within matching brace brackets.
2162 Block = "{" StatementList "}" .
2163 StatementList = { Statement ";" } .
2167 In addition to explicit blocks in the source code, there are implicit blocks:
2171 <li>The <i>universe block</i> encompasses all Go source text.</li>
2173 <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
2174 Go source text for that package.</li>
2176 <li>Each file has a <i>file block</i> containing all Go source text
2179 <li>Each <a href="#If_statements">"if"</a>,
2180 <a href="#For_statements">"for"</a>, and
2181 <a href="#Switch_statements">"switch"</a>
2182 statement is considered to be in its own implicit block.</li>
2184 <li>Each clause in a <a href="#Switch_statements">"switch"</a>
2185 or <a href="#Select_statements">"select"</a> statement
2186 acts as an implicit block.</li>
2190 Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
2194 <h2 id="Declarations_and_scope">Declarations and scope</h2>
2197 A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
2198 <a href="#Constant_declarations">constant</a>,
2199 <a href="#Type_declarations">type</a>,
2200 <a href="#Type_parameter_declarations">type parameter</a>,
2201 <a href="#Variable_declarations">variable</a>,
2202 <a href="#Function_declarations">function</a>,
2203 <a href="#Labeled_statements">label</a>, or
2204 <a href="#Import_declarations">package</a>.
2205 Every identifier in a program must be declared.
2206 No identifier may be declared twice in the same block, and
2207 no identifier may be declared in both the file and package block.
2211 The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
2212 in a declaration, but it does not introduce a binding and thus is not declared.
2213 In the package block, the identifier <code>init</code> may only be used for
2214 <a href="#Package_initialization"><code>init</code> function</a> declarations,
2215 and like the blank identifier it does not introduce a new binding.
2219 Declaration = ConstDecl | TypeDecl | VarDecl .
2220 TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
2224 The <i>scope</i> of a declared identifier is the extent of source text in which
2225 the identifier denotes the specified constant, type, variable, function, label, or package.
2229 Go is lexically scoped using <a href="#Blocks">blocks</a>:
2233 <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
2235 <li>The scope of an identifier denoting a constant, type, variable,
2236 or function (but not method) declared at top level (outside any
2237 function) is the package block.</li>
2239 <li>The scope of the package name of an imported package is the file block
2240 of the file containing the import declaration.</li>
2242 <li>The scope of an identifier denoting a method receiver, function parameter,
2243 or result variable is the function body.</li>
2245 <li>The scope of an identifier denoting a type parameter of a function
2246 or declared by a method receiver begins after the name of the function
2247 and ends at the end of the function body.</li>
2249 <li>The scope of an identifier denoting a type parameter of a type
2250 begins after the name of the type and ends at the end
2251 of the TypeSpec.</li>
2253 <li>The scope of a constant or variable identifier declared
2254 inside a function begins at the end of the ConstSpec or VarSpec
2255 (ShortVarDecl for short variable declarations)
2256 and ends at the end of the innermost containing block.</li>
2258 <li>The scope of a type identifier declared inside a function
2259 begins at the identifier in the TypeSpec
2260 and ends at the end of the innermost containing block.</li>
2264 An identifier declared in a block may be redeclared in an inner block.
2265 While the identifier of the inner declaration is in scope, it denotes
2266 the entity declared by the inner declaration.
2270 The <a href="#Package_clause">package clause</a> is not a declaration; the package name
2271 does not appear in any scope. Its purpose is to identify the files belonging
2272 to the same <a href="#Packages">package</a> and to specify the default package name for import
2277 <h3 id="Label_scopes">Label scopes</h3>
2280 Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
2281 used in the <a href="#Break_statements">"break"</a>,
2282 <a href="#Continue_statements">"continue"</a>, and
2283 <a href="#Goto_statements">"goto"</a> statements.
2284 It is illegal to define a label that is never used.
2285 In contrast to other identifiers, labels are not block scoped and do
2286 not conflict with identifiers that are not labels. The scope of a label
2287 is the body of the function in which it is declared and excludes
2288 the body of any nested function.
2292 <h3 id="Blank_identifier">Blank identifier</h3>
2295 The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
2296 It serves as an anonymous placeholder instead of a regular (non-blank)
2297 identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
2298 as an <a href="#Operands">operand</a>, and in <a href="#Assignment_statements">assignment statements</a>.
2302 <h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
2305 The following identifiers are implicitly declared in the
2306 <a href="#Blocks">universe block</a>:
2308 <pre class="grammar">
2310 any bool byte comparable
2311 complex64 complex128 error float32 float64
2312 int int8 int16 int32 int64 rune string
2313 uint uint8 uint16 uint32 uint64 uintptr
2322 append cap clear close complex copy delete imag len
2323 make max min new panic print println real recover
2326 <h3 id="Exported_identifiers">Exported identifiers</h3>
2329 An identifier may be <i>exported</i> to permit access to it from another package.
2330 An identifier is exported if both:
2333 <li>the first character of the identifier's name is a Unicode uppercase
2334 letter (Unicode character category Lu); and</li>
2335 <li>the identifier is declared in the <a href="#Blocks">package block</a>
2336 or it is a <a href="#Struct_types">field name</a> or
2337 <a href="#MethodName">method name</a>.</li>
2340 All other identifiers are not exported.
2343 <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
2346 Given a set of identifiers, an identifier is called <i>unique</i> if it is
2347 <i>different</i> from every other in the set.
2348 Two identifiers are different if they are spelled differently, or if they
2349 appear in different <a href="#Packages">packages</a> and are not
2350 <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
2353 <h3 id="Constant_declarations">Constant declarations</h3>
2356 A constant declaration binds a list of identifiers (the names of
2357 the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
2358 The number of identifiers must be equal
2359 to the number of expressions, and the <i>n</i>th identifier on
2360 the left is bound to the value of the <i>n</i>th expression on the
2365 ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
2366 ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
2368 IdentifierList = identifier { "," identifier } .
2369 ExpressionList = Expression { "," Expression } .
2373 If the type is present, all constants take the type specified, and
2374 the expressions must be <a href="#Assignability">assignable</a> to that type,
2375 which must not be a type parameter.
2376 If the type is omitted, the constants take the
2377 individual types of the corresponding expressions.
2378 If the expression values are untyped <a href="#Constants">constants</a>,
2379 the declared constants remain untyped and the constant identifiers
2380 denote the constant values. For instance, if the expression is a
2381 floating-point literal, the constant identifier denotes a floating-point
2382 constant, even if the literal's fractional part is zero.
2386 const Pi float64 = 3.14159265358979323846
2387 const zero = 0.0 // untyped floating-point constant
2390 eof = -1 // untyped integer constant
2392 const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
2393 const u, v float32 = 0, 3 // u = 0.0, v = 3.0
2397 Within a parenthesized <code>const</code> declaration list the
2398 expression list may be omitted from any but the first ConstSpec.
2399 Such an empty list is equivalent to the textual substitution of the
2400 first preceding non-empty expression list and its type if any.
2401 Omitting the list of expressions is therefore equivalent to
2402 repeating the previous list. The number of identifiers must be equal
2403 to the number of expressions in the previous list.
2404 Together with the <a href="#Iota"><code>iota</code> constant generator</a>
2405 this mechanism permits light-weight declaration of sequential values:
2417 numberOfDays // this constant is not exported
2422 <h3 id="Iota">Iota</h3>
2425 Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
2426 <code>iota</code> represents successive untyped integer <a href="#Constants">
2427 constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
2428 in that constant declaration, starting at zero.
2429 It can be used to construct a set of related constants:
2434 c0 = iota // c0 == 0
2435 c1 = iota // c1 == 1
2436 c2 = iota // c2 == 2
2440 a = 1 << iota // a == 1 (iota == 0)
2441 b = 1 << iota // b == 2 (iota == 1)
2442 c = 3 // c == 3 (iota == 2, unused)
2443 d = 1 << iota // d == 8 (iota == 3)
2447 u = iota * 42 // u == 0 (untyped integer constant)
2448 v float64 = iota * 42 // v == 42.0 (float64 constant)
2449 w = iota * 42 // w == 84 (untyped integer constant)
2452 const x = iota // x == 0
2453 const y = iota // y == 0
2457 By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
2462 bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
2463 bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
2464 _, _ // (iota == 2, unused)
2465 bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
2470 This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
2471 of the last non-empty expression list.
2475 <h3 id="Type_declarations">Type declarations</h3>
2478 A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
2479 Type declarations come in two forms: alias declarations and type definitions.
2483 TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
2484 TypeSpec = AliasDecl | TypeDef .
2487 <h4 id="Alias_declarations">Alias declarations</h4>
2490 An alias declaration binds an identifier to the given type.
2494 AliasDecl = identifier "=" Type .
2498 Within the <a href="#Declarations_and_scope">scope</a> of
2499 the identifier, it serves as an <i>alias</i> for the type.
2504 nodeList = []*Node // nodeList and []*Node are identical types
2505 Polar = polar // Polar and polar denote identical types
2510 <h4 id="Type_definitions">Type definitions</h4>
2513 A type definition creates a new, distinct type with the same
2514 <a href="#Types">underlying type</a> and operations as the given type
2515 and binds an identifier, the <i>type name</i>, to it.
2519 TypeDef = identifier [ TypeParameters ] Type .
2523 The new type is called a <i>defined type</i>.
2524 It is <a href="#Type_identity">different</a> from any other type,
2525 including the type it is created from.
2530 Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
2531 polar Point // polar and Point denote different types
2534 type TreeNode struct {
2535 left, right *TreeNode
2539 type Block interface {
2541 Encrypt(src, dst []byte)
2542 Decrypt(src, dst []byte)
2547 A defined type may have <a href="#Method_declarations">methods</a> associated with it.
2548 It does not inherit any methods bound to the given type,
2549 but the <a href="#Method_sets">method set</a>
2550 of an interface type or of elements of a composite type remains unchanged:
2554 // A Mutex is a data type with two methods, Lock and Unlock.
2555 type Mutex struct { /* Mutex fields */ }
2556 func (m *Mutex) Lock() { /* Lock implementation */ }
2557 func (m *Mutex) Unlock() { /* Unlock implementation */ }
2559 // NewMutex has the same composition as Mutex but its method set is empty.
2562 // The method set of PtrMutex's underlying type *Mutex remains unchanged,
2563 // but the method set of PtrMutex is empty.
2564 type PtrMutex *Mutex
2566 // The method set of *PrintableMutex contains the methods
2567 // Lock and Unlock bound to its embedded field Mutex.
2568 type PrintableMutex struct {
2572 // MyBlock is an interface type that has the same method set as Block.
2577 Type definitions may be used to define different boolean, numeric,
2578 or string types and associate methods with them:
2585 EST TimeZone = -(5 + iota)
2591 func (tz TimeZone) String() string {
2592 return fmt.Sprintf("GMT%+dh", tz)
2597 If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>,
2598 the type name denotes a <i>generic type</i>.
2599 Generic types must be <a href="#Instantiations">instantiated</a> when they
2604 type List[T any] struct {
2611 In a type definition the given type cannot be a type parameter.
2615 type T[P any] P // illegal: P is a type parameter
2618 type L T // illegal: T is a type parameter declared by the enclosing function
2623 A generic type may also have <a href="#Method_declarations">methods</a> associated with it.
2624 In this case, the method receivers must declare the same number of type parameters as
2625 present in the generic type definition.
2629 // The method Len returns the number of elements in the linked list l.
2630 func (l *List[T]) Len() int { … }
2633 <h3 id="Type_parameter_declarations">Type parameter declarations</h3>
2636 A type parameter list declares the <i>type parameters</i> of a generic function or type declaration.
2637 The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a>
2638 except that the type parameter names must all be present and the list is enclosed
2639 in square brackets rather than parentheses.
2643 TypeParameters = "[" TypeParamList [ "," ] "]" .
2644 TypeParamList = TypeParamDecl { "," TypeParamDecl } .
2645 TypeParamDecl = IdentifierList TypeConstraint .
2649 All non-blank names in the list must be unique.
2650 Each name declares a type parameter, which is a new and different <a href="#Types">named type</a>
2651 that acts as a placeholder for an (as of yet) unknown type in the declaration.
2652 The type parameter is replaced with a <i>type argument</i> upon
2653 <a href="#Instantiations">instantiation</a> of the generic function or type.
2658 [S interface{ ~[]byte|string }]
2665 Just as each ordinary function parameter has a parameter type, each type parameter
2666 has a corresponding (meta-)type which is called its
2667 <a href="#Type_constraints"><i>type constraint</i></a>.
2671 A parsing ambiguity arises when the type parameter list for a generic type
2672 declares a single type parameter <code>P</code> with a constraint <code>C</code>
2673 such that the text <code>P C</code> forms a valid expression:
2684 In these rare cases, the type parameter list is indistinguishable from an
2685 expression and the type declaration is parsed as an array type declaration.
2686 To resolve the ambiguity, embed the constraint in an
2687 <a href="#Interface_types">interface</a> or use a trailing comma:
2691 type T[P interface{*C}] …
2696 Type parameters may also be declared by the receiver specification
2697 of a <a href="#Method_declarations">method declaration</a> associated
2698 with a generic type.
2702 Within a type parameter list of a generic type <code>T</code>, a type constraint
2703 may not (directly, or indirectly through the type parameter list of another
2704 generic type) refer to <code>T</code>.
2708 type T1[P T1[P]] … // illegal: T1 refers to itself
2709 type T2[P interface{ T2[int] }] … // illegal: T2 refers to itself
2710 type T3[P interface{ m(T3[int])}] … // illegal: T3 refers to itself
2711 type T4[P T5[P]] … // illegal: T4 refers to T5 and
2712 type T5[P T4[P]] … // T5 refers to T4
2714 type T6[P int] struct{ f *T6[P] } // ok: reference to T6 is not in type parameter list
2717 <h4 id="Type_constraints">Type constraints</h4>
2720 A <i>type constraint</i> is an <a href="#Interface_types">interface</a> that defines the
2721 set of permissible type arguments for the respective type parameter and controls the
2722 operations supported by values of that type parameter.
2726 TypeConstraint = TypeElem .
2730 If the constraint is an interface literal of the form <code>interface{E}</code> where
2731 <code>E</code> is an embedded <a href="#Interface_types">type element</a> (not a method), in a type parameter list
2732 the enclosing <code>interface{ … }</code> may be omitted for convenience:
2736 [T []P] // = [T interface{[]P}]
2737 [T ~int] // = [T interface{~int}]
2738 [T int|string] // = [T interface{int|string}]
2739 type Constraint ~int // illegal: ~int is not in a type parameter list
2743 We should be able to simplify the rules for comparable or delegate some of them
2744 elsewhere since we have a section that clearly defines how interfaces implement
2745 other interfaces based on their type sets. But this should get us going for now.
2749 The <a href="#Predeclared_identifiers">predeclared</a>
2750 <a href="#Interface_types">interface type</a> <code>comparable</code>
2751 denotes the set of all non-interface types that are
2752 <a href="#Comparison_operators">strictly comparable</a>.
2756 Even though interfaces that are not type parameters are <a href="#Comparison_operators">comparable</a>,
2757 they are not strictly comparable and therefore they do not implement <code>comparable</code>.
2758 However, they <a href="#Satisfying_a_type_constraint">satisfy</a> <code>comparable</code>.
2762 int // implements comparable (int is strictly comparable)
2763 []byte // does not implement comparable (slices cannot be compared)
2764 interface{} // does not implement comparable (see above)
2765 interface{ ~int | ~string } // type parameter only: implements comparable (int, string types are strictly comparable)
2766 interface{ comparable } // type parameter only: implements comparable (comparable implements itself)
2767 interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (slices are not comparable)
2768 interface{ ~struct{ any } } // type parameter only: does not implement comparable (field any is not strictly comparable)
2772 The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
2773 <code>comparable</code> may only be used as type constraints. They cannot be the types of
2774 values or variables, or components of other, non-interface types.
2777 <h4 id="Satisfying_a_type_constraint">Satisfying a type constraint</h4>
2780 A type argument <code>T</code><i> satisfies</i> a type constraint <code>C</code>
2781 if <code>T</code> is an element of the type set defined by <code>C</code>; i.e.,
2782 if <code>T</code> <a href="#Implementing_an_interface">implements</a> <code>C</code>.
2783 As an exception, a <a href="#Comparison_operators">strictly comparable</a>
2784 type constraint may also be satisfied by a <a href="#Comparison_operators">comparable</a>
2785 (not necessarily strictly comparable) type argument.
2790 A type T <i>satisfies</i> a constraint <code>C</code> if
2795 <code>T</code> <a href="#Implementing_an_interface">implements</a> <code>C</code>; or
2798 <code>C</code> can be written in the form <code>interface{ comparable; E }</code>,
2799 where <code>E</code> is a <a href="#Basic_interfaces">basic interface</a> and
2800 <code>T</code> is <a href="#Comparison_operators">comparable</a> and implements <code>E</code>.
2805 type argument type constraint // constraint satisfaction
2807 int interface{ ~int } // satisfied: int implements interface{ ~int }
2808 string comparable // satisfied: string implements comparable (string is strictly comparable)
2809 []byte comparable // not satisfied: slices are not comparable
2810 any interface{ comparable; int } // not satisfied: any does not implement interface{ int }
2811 any comparable // satisfied: any is comparable and implements the basic interface any
2812 struct{f any} comparable // satisfied: struct{f any} is comparable and implements the basic interface any
2813 any interface{ comparable; m() } // not satisfied: any does not implement the basic interface interface{ m() }
2814 interface{ m() } interface{ comparable; m() } // satisfied: interface{ m() } is comparable and implements the basic interface interface{ m() }
2818 Because of the exception in the constraint satisfaction rule, comparing operands of type parameter type
2819 may panic at run-time (even though comparable type parameters are always strictly comparable).
2822 <h3 id="Variable_declarations">Variable declarations</h3>
2825 A variable declaration creates one or more <a href="#Variables">variables</a>,
2826 binds corresponding identifiers to them, and gives each a type and an initial value.
2830 VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
2831 VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
2838 var x, y float32 = -1, -2
2841 u, v, s = 2.0, 3.0, "bar"
2843 var re, im = complexSqrt(-1)
2844 var _, found = entries[name] // map lookup; only interested in "found"
2848 If a list of expressions is given, the variables are initialized
2849 with the expressions following the rules for <a href="#Assignment_statements">assignment statements</a>.
2850 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
2854 If a type is present, each variable is given that type.
2855 Otherwise, each variable is given the type of the corresponding
2856 initialization value in the assignment.
2857 If that value is an untyped constant, it is first implicitly
2858 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
2859 if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
2860 The predeclared value <code>nil</code> cannot be used to initialize a variable
2861 with no explicit type.
2865 var d = math.Sin(0.5) // d is float64
2866 var i = 42 // i is int
2867 var t, ok = x.(T) // t is T, ok is bool
2868 var n = nil // illegal
2872 Implementation restriction: A compiler may make it illegal to declare a variable
2873 inside a <a href="#Function_declarations">function body</a> if the variable is
2877 <h3 id="Short_variable_declarations">Short variable declarations</h3>
2880 A <i>short variable declaration</i> uses the syntax:
2884 ShortVarDecl = IdentifierList ":=" ExpressionList .
2888 It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
2889 with initializer expressions but no types:
2892 <pre class="grammar">
2893 "var" IdentifierList "=" ExpressionList .
2898 f := func() int { return 7 }
2899 ch := make(chan int)
2900 r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
2901 _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
2905 Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
2906 variables provided they were originally declared earlier in the same block
2907 (or the parameter lists if the block is the function body) with the same type,
2908 and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
2909 As a consequence, redeclaration can only appear in a multi-variable short declaration.
2910 Redeclaration does not introduce a new variable; it just assigns a new value to the original.
2911 The non-blank variable names on the left side of <code>:=</code>
2912 must be <a href="#Uniqueness_of_identifiers">unique</a>.
2916 field1, offset := nextField(str, 0)
2917 field2, offset := nextField(str, offset) // redeclares offset
2918 x, y, x := 1, 2, 3 // illegal: x repeated on left side of :=
2922 Short variable declarations may appear only inside functions.
2923 In some contexts such as the initializers for
2924 <a href="#If_statements">"if"</a>,
2925 <a href="#For_statements">"for"</a>, or
2926 <a href="#Switch_statements">"switch"</a> statements,
2927 they can be used to declare local temporary variables.
2930 <h3 id="Function_declarations">Function declarations</h3>
2933 Given the importance of functions, this section has always
2934 been woefully underdeveloped. Would be nice to expand this
2939 A function declaration binds an identifier, the <i>function name</i>,
2944 FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
2945 FunctionName = identifier .
2946 FunctionBody = Block .
2950 If the function's <a href="#Function_types">signature</a> declares
2951 result parameters, the function body's statement list must end in
2952 a <a href="#Terminating_statements">terminating statement</a>.
2956 func IndexRune(s string, r rune) int {
2957 for i, c := range s {
2962 // invalid: missing return statement
2967 If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>,
2968 the function name denotes a <i>generic function</i>.
2969 A generic function must be <a href="#Instantiations">instantiated</a> before it can be
2970 called or used as a value.
2974 func min[T ~int|~float64](x, y T) T {
2983 A function declaration without type parameters may omit the body.
2984 Such a declaration provides the signature for a function implemented outside Go,
2985 such as an assembly routine.
2989 func flushICache(begin, end uintptr) // implemented externally
2992 <h3 id="Method_declarations">Method declarations</h3>
2995 A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
2996 A method declaration binds an identifier, the <i>method name</i>, to a method,
2997 and associates the method with the receiver's <i>base type</i>.
3001 MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
3002 Receiver = Parameters .
3006 The receiver is specified via an extra parameter section preceding the method
3007 name. That parameter section must declare a single non-variadic parameter, the receiver.
3008 Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
3009 pointer to a defined type <code>T</code>, possibly followed by a list of type parameter
3010 names <code>[P1, P2, …]</code> enclosed in square brackets.
3011 <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be
3012 a pointer or interface type and it must be defined in the same package as the method.
3013 The method is said to be <i>bound</i> to its receiver base type and the method name
3014 is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
3019 A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
3020 <a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
3021 If the receiver's value is not referenced inside the body of the method,
3022 its identifier may be omitted in the declaration. The same applies in
3023 general to parameters of functions and methods.
3027 For a base type, the non-blank names of methods bound to it must be unique.
3028 If the base type is a <a href="#Struct_types">struct type</a>,
3029 the non-blank method and field names must be distinct.
3033 Given defined type <code>Point</code> the declarations
3037 func (p *Point) Length() float64 {
3038 return math.Sqrt(p.x * p.x + p.y * p.y)
3041 func (p *Point) Scale(factor float64) {
3048 bind the methods <code>Length</code> and <code>Scale</code>,
3049 with receiver type <code>*Point</code>,
3050 to the base type <code>Point</code>.
3054 If the receiver base type is a <a href="#Type_declarations">generic type</a>, the
3055 receiver specification must declare corresponding type parameters for the method
3056 to use. This makes the receiver type parameters available to the method.
3057 Syntactically, this type parameter declaration looks like an
3058 <a href="#Instantiations">instantiation</a> of the receiver base type: the type
3059 arguments must be identifiers denoting the type parameters being declared, one
3060 for each type parameter of the receiver base type.
3061 The type parameter names do not need to match their corresponding parameter names in the
3062 receiver base type definition, and all non-blank parameter names must be unique in the
3063 receiver parameter section and the method signature.
3064 The receiver type parameter constraints are implied by the receiver base type definition:
3065 corresponding type parameters have corresponding constraints.
3069 type Pair[A, B any] struct {
3074 func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B
3075 func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair
3078 <h2 id="Expressions">Expressions</h2>
3081 An expression specifies the computation of a value by applying
3082 operators and functions to operands.
3085 <h3 id="Operands">Operands</h3>
3088 Operands denote the elementary values in an expression. An operand may be a
3089 literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
3090 non-<a href="#Blank_identifier">blank</a> identifier denoting a
3091 <a href="#Constant_declarations">constant</a>,
3092 <a href="#Variable_declarations">variable</a>, or
3093 <a href="#Function_declarations">function</a>,
3094 or a parenthesized expression.
3098 Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
3099 Literal = BasicLit | CompositeLit | FunctionLit .
3100 BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
3101 OperandName = identifier | QualifiedIdent .
3105 An operand name denoting a <a href="#Function_declarations">generic function</a>
3106 may be followed by a list of <a href="#Instantiations">type arguments</a>; the
3107 resulting operand is an <a href="#Instantiations">instantiated</a> function.
3111 The <a href="#Blank_identifier">blank identifier</a> may appear as an
3112 operand only on the left-hand side of an <a href="#Assignment_statements">assignment statement</a>.
3116 Implementation restriction: A compiler need not report an error if an operand's
3117 type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty
3118 <a href="#Interface_types">type set</a>. Functions with such type parameters
3119 cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead
3120 to an error at the instantiation site.
3123 <h3 id="Qualified_identifiers">Qualified identifiers</h3>
3126 A <i>qualified identifier</i> is an identifier qualified with a package name prefix.
3127 Both the package name and the identifier must not be
3128 <a href="#Blank_identifier">blank</a>.
3132 QualifiedIdent = PackageName "." identifier .
3136 A qualified identifier accesses an identifier in a different package, which
3137 must be <a href="#Import_declarations">imported</a>.
3138 The identifier must be <a href="#Exported_identifiers">exported</a> and
3139 declared in the <a href="#Blocks">package block</a> of that package.
3143 math.Sin // denotes the Sin function in package math
3146 <h3 id="Composite_literals">Composite literals</h3>
3149 Composite literals construct new composite values each time they are evaluated.
3150 They consist of the type of the literal followed by a brace-bound list of elements.
3151 Each element may optionally be preceded by a corresponding key.
3155 CompositeLit = LiteralType LiteralValue .
3156 LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
3157 SliceType | MapType | TypeName [ TypeArgs ] .
3158 LiteralValue = "{" [ ElementList [ "," ] ] "}" .
3159 ElementList = KeyedElement { "," KeyedElement } .
3160 KeyedElement = [ Key ":" ] Element .
3161 Key = FieldName | Expression | LiteralValue .
3162 FieldName = identifier .
3163 Element = Expression | LiteralValue .
3167 The LiteralType's <a href="#Core_types">core type</a> <code>T</code>
3168 must be a struct, array, slice, or map type
3169 (the syntax enforces this constraint except when the type is given
3171 The types of the elements and keys must be <a href="#Assignability">assignable</a>
3172 to the respective field, element, and key types of type <code>T</code>;
3173 there is no additional conversion.
3174 The key is interpreted as a field name for struct literals,
3175 an index for array and slice literals, and a key for map literals.
3176 For map literals, all elements must have a key. It is an error
3177 to specify multiple elements with the same field name or
3178 constant key value. For non-constant map keys, see the section on
3179 <a href="#Order_of_evaluation">evaluation order</a>.
3183 For struct literals the following rules apply:
3186 <li>A key must be a field name declared in the struct type.
3188 <li>An element list that does not contain any keys must
3189 list an element for each struct field in the
3190 order in which the fields are declared.
3192 <li>If any element has a key, every element must have a key.
3194 <li>An element list that contains keys does not need to
3195 have an element for each struct field. Omitted fields
3196 get the zero value for that field.
3198 <li>A literal may omit the element list; such a literal evaluates
3199 to the zero value for its type.
3201 <li>It is an error to specify an element for a non-exported
3202 field of a struct belonging to a different package.
3207 Given the declarations
3210 type Point3D struct { x, y, z float64 }
3211 type Line struct { p, q Point3D }
3219 origin := Point3D{} // zero value for Point3D
3220 line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
3224 For array and slice literals the following rules apply:
3227 <li>Each element has an associated integer index marking
3228 its position in the array.
3230 <li>An element with a key uses the key as its index. The
3231 key must be a non-negative constant
3232 <a href="#Representability">representable</a> by
3233 a value of type <code>int</code>; and if it is typed
3234 it must be of <a href="#Numeric_types">integer type</a>.
3236 <li>An element without a key uses the previous element's index plus one.
3237 If the first element has no key, its index is zero.
3242 <a href="#Address_operators">Taking the address</a> of a composite literal
3243 generates a pointer to a unique <a href="#Variables">variable</a> initialized
3244 with the literal's value.
3248 var pointer *Point3D = &Point3D{y: 1000}
3252 Note that the <a href="#The_zero_value">zero value</a> for a slice or map
3253 type is not the same as an initialized but empty value of the same type.
3254 Consequently, taking the address of an empty slice or map composite literal
3255 does not have the same effect as allocating a new slice or map value with
3256 <a href="#Allocation">new</a>.
3260 p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
3261 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
3265 The length of an array literal is the length specified in the literal type.
3266 If fewer elements than the length are provided in the literal, the missing
3267 elements are set to the zero value for the array element type.
3268 It is an error to provide elements with index values outside the index range
3269 of the array. The notation <code>...</code> specifies an array length equal
3270 to the maximum element index plus one.
3274 buffer := [10]string{} // len(buffer) == 10
3275 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
3276 days := [...]string{"Sat", "Sun"} // len(days) == 2
3280 A slice literal describes the entire underlying array literal.
3281 Thus the length and capacity of a slice literal are the maximum
3282 element index plus one. A slice literal has the form
3290 and is shorthand for a slice operation applied to an array:
3294 tmp := [n]T{x1, x2, … xn}
3299 Within a composite literal of array, slice, or map type <code>T</code>,
3300 elements or map keys that are themselves composite literals may elide the respective
3301 literal type if it is identical to the element or key type of <code>T</code>.
3302 Similarly, elements or keys that are addresses of composite literals may elide
3303 the <code>&T</code> when the element or key type is <code>*T</code>.
3307 [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
3308 [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
3309 [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
3310 map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
3311 map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
3314 [2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
3315 [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
3319 A parsing ambiguity arises when a composite literal using the
3320 TypeName form of the LiteralType appears as an operand between the
3321 <a href="#Keywords">keyword</a> and the opening brace of the block
3322 of an "if", "for", or "switch" statement, and the composite literal
3323 is not enclosed in parentheses, square brackets, or curly braces.
3324 In this rare case, the opening brace of the literal is erroneously parsed
3325 as the one introducing the block of statements. To resolve the ambiguity,
3326 the composite literal must appear within parentheses.
3330 if x == (T{a,b,c}[i]) { … }
3331 if (x == T{a,b,c}[i]) { … }
3335 Examples of valid array, slice, and map literals:
3339 // list of prime numbers
3340 primes := []int{2, 3, 5, 7, 9, 2147483647}
3342 // vowels[ch] is true if ch is a vowel
3343 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
3345 // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
3346 filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
3348 // frequencies in Hz for equal-tempered scale (A4 = 440Hz)
3349 noteFrequency := map[string]float32{
3350 "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
3351 "G0": 24.50, "A0": 27.50, "B0": 30.87,
3356 <h3 id="Function_literals">Function literals</h3>
3359 A function literal represents an anonymous <a href="#Function_declarations">function</a>.
3360 Function literals cannot declare type parameters.
3364 FunctionLit = "func" Signature FunctionBody .
3368 func(a, b int, z float64) bool { return a*b < int(z) }
3372 A function literal can be assigned to a variable or invoked directly.
3376 f := func(x, y int) int { return x + y }
3377 func(ch chan int) { ch <- ACK }(replyChan)
3381 Function literals are <i>closures</i>: they may refer to variables
3382 defined in a surrounding function. Those variables are then shared between
3383 the surrounding function and the function literal, and they survive as long
3384 as they are accessible.
3388 <h3 id="Primary_expressions">Primary expressions</h3>
3391 Primary expressions are the operands for unary and binary expressions.
3399 PrimaryExpr Selector |
3402 PrimaryExpr TypeAssertion |
3403 PrimaryExpr Arguments .
3405 Selector = "." identifier .
3406 Index = "[" Expression [ "," ] "]" .
3407 Slice = "[" [ Expression ] ":" [ Expression ] "]" |
3408 "[" [ Expression ] ":" Expression ":" Expression "]" .
3409 TypeAssertion = "." "(" Type ")" .
3410 Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
3427 <h3 id="Selectors">Selectors</h3>
3430 For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
3431 that is not a <a href="#Package_clause">package name</a>, the
3432 <i>selector expression</i>
3440 denotes the field or method <code>f</code> of the value <code>x</code>
3441 (or sometimes <code>*x</code>; see below).
3442 The identifier <code>f</code> is called the (field or method) <i>selector</i>;
3443 it must not be the <a href="#Blank_identifier">blank identifier</a>.
3444 The type of the selector expression is the type of <code>f</code>.
3445 If <code>x</code> is a package name, see the section on
3446 <a href="#Qualified_identifiers">qualified identifiers</a>.
3450 A selector <code>f</code> may denote a field or method <code>f</code> of
3451 a type <code>T</code>, or it may refer
3452 to a field or method <code>f</code> of a nested
3453 <a href="#Struct_types">embedded field</a> of <code>T</code>.
3454 The number of embedded fields traversed
3455 to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
3456 The depth of a field or method <code>f</code>
3457 declared in <code>T</code> is zero.
3458 The depth of a field or method <code>f</code> declared in
3459 an embedded field <code>A</code> in <code>T</code> is the
3460 depth of <code>f</code> in <code>A</code> plus one.
3464 The following rules apply to selectors:
3469 For a value <code>x</code> of type <code>T</code> or <code>*T</code>
3470 where <code>T</code> is not a pointer or interface type,
3471 <code>x.f</code> denotes the field or method at the shallowest depth
3472 in <code>T</code> where there is such an <code>f</code>.
3473 If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
3474 with shallowest depth, the selector expression is illegal.
3478 For a value <code>x</code> of type <code>I</code> where <code>I</code>
3479 is an interface type, <code>x.f</code> denotes the actual method with name
3480 <code>f</code> of the dynamic value of <code>x</code>.
3481 If there is no method with name <code>f</code> in the
3482 <a href="#Method_sets">method set</a> of <code>I</code>, the selector
3483 expression is illegal.
3487 As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
3488 pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
3489 (but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
3493 In all other cases, <code>x.f</code> is illegal.
3497 If <code>x</code> is of pointer type and has the value
3498 <code>nil</code> and <code>x.f</code> denotes a struct field,
3499 assigning to or evaluating <code>x.f</code>
3500 causes a <a href="#Run_time_panics">run-time panic</a>.
3504 If <code>x</code> is of interface type and has the value
3505 <code>nil</code>, <a href="#Calls">calling</a> or
3506 <a href="#Method_values">evaluating</a> the method <code>x.f</code>
3507 causes a <a href="#Run_time_panics">run-time panic</a>.
3512 For example, given the declarations:
3538 var t T2 // with t.T0 != nil
3539 var p *T2 // with p != nil and (*p).T0 != nil
3556 q.x // (*(*q).T0).x (*q).x is a valid field selector
3558 p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
3559 p.M1() // ((*p).T1).M1() M1 expects T1 receiver
3560 p.M2() // p.M2() M2 expects *T2 receiver
3561 t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
3565 but the following is invalid:
3569 q.M0() // (*q).M0 is valid but not a field selector
3573 <h3 id="Method_expressions">Method expressions</h3>
3576 If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3577 <code>T.M</code> is a function that is callable as a regular function
3578 with the same arguments as <code>M</code> prefixed by an additional
3579 argument that is the receiver of the method.
3583 MethodExpr = ReceiverType "." MethodName .
3584 ReceiverType = Type .
3588 Consider a struct type <code>T</code> with two methods,
3589 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3590 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3597 func (tv T) Mv(a int) int { return 0 } // value receiver
3598 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3612 yields a function equivalent to <code>Mv</code> but
3613 with an explicit receiver as its first argument; it has signature
3617 func(tv T, a int) int
3621 That function may be called normally with an explicit receiver, so
3622 these five invocations are equivalent:
3629 f1 := T.Mv; f1(t, 7)
3630 f2 := (T).Mv; f2(t, 7)
3634 Similarly, the expression
3642 yields a function value representing <code>Mp</code> with signature
3646 func(tp *T, f float32) float32
3650 For a method with a value receiver, one can derive a function
3651 with an explicit pointer receiver, so
3659 yields a function value representing <code>Mv</code> with signature
3663 func(tv *T, a int) int
3667 Such a function indirects through the receiver to create a value
3668 to pass as the receiver to the underlying method;
3669 the method does not overwrite the value whose address is passed in
3674 The final case, a value-receiver function for a pointer-receiver method,
3675 is illegal because pointer-receiver methods are not in the method set
3680 Function values derived from methods are called with function call syntax;
3681 the receiver is provided as the first argument to the call.
3682 That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
3683 as <code>f(t, 7)</code> not <code>t.f(7)</code>.
3684 To construct a function that binds the receiver, use a
3685 <a href="#Function_literals">function literal</a> or
3686 <a href="#Method_values">method value</a>.
3690 It is legal to derive a function value from a method of an interface type.
3691 The resulting function takes an explicit receiver of that interface type.
3694 <h3 id="Method_values">Method values</h3>
3697 If the expression <code>x</code> has static type <code>T</code> and
3698 <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3699 <code>x.M</code> is called a <i>method value</i>.
3700 The method value <code>x.M</code> is a function value that is callable
3701 with the same arguments as a method call of <code>x.M</code>.
3702 The expression <code>x</code> is evaluated and saved during the evaluation of the
3703 method value; the saved copy is then used as the receiver in any calls,
3704 which may be executed later.
3708 type S struct { *T }
3710 func (t T) M() { print(t) }
3714 f := t.M // receiver *t is evaluated and stored in f
3715 g := s.M // receiver *(s.T) is evaluated and stored in g
3716 *t = 42 // does not affect stored receivers in f and g
3720 The type <code>T</code> may be an interface or non-interface type.
3724 As in the discussion of <a href="#Method_expressions">method expressions</a> above,
3725 consider a struct type <code>T</code> with two methods,
3726 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3727 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3734 func (tv T) Mv(a int) int { return 0 } // value receiver
3735 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3751 yields a function value of type
3759 These two invocations are equivalent:
3768 Similarly, the expression
3776 yields a function value of type
3780 func(float32) float32
3784 As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
3785 using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
3789 As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
3790 using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
3794 f := t.Mv; f(7) // like t.Mv(7)
3795 f := pt.Mp; f(7) // like pt.Mp(7)
3796 f := pt.Mv; f(7) // like (*pt).Mv(7)
3797 f := t.Mp; f(7) // like (&t).Mp(7)
3798 f := makeT().Mp // invalid: result of makeT() is not addressable
3802 Although the examples above use non-interface types, it is also legal to create a method value
3803 from a value of interface type.
3807 var i interface { M(int) } = myVal
3808 f := i.M; f(7) // like i.M(7)
3812 <h3 id="Index_expressions">Index expressions</h3>
3815 A primary expression of the form
3823 denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
3824 The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
3825 The following rules apply:
3829 If <code>a</code> is neither a map nor a type parameter:
3832 <li>the index <code>x</code> must be an untyped constant or its
3833 <a href="#Core_types">core type</a> must be an <a href="#Numeric_types">integer</a></li>
3834 <li>a constant index must be non-negative and
3835 <a href="#Representability">representable</a> by a value of type <code>int</code></li>
3836 <li>a constant index that is untyped is given type <code>int</code></li>
3837 <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
3838 otherwise it is <i>out of range</i></li>
3842 For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
3845 <li>a <a href="#Constants">constant</a> index must be in range</li>
3846 <li>if <code>x</code> is out of range at run time,
3847 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3848 <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
3849 <code>a[x]</code> is the element type of <code>A</code></li>
3853 For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
3856 <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
3860 For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
3863 <li>if <code>x</code> is out of range at run time,
3864 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3865 <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
3866 <code>a[x]</code> is the element type of <code>S</code></li>
3870 For <code>a</code> of <a href="#String_types">string type</a>:
3873 <li>a <a href="#Constants">constant</a> index must be in range
3874 if the string <code>a</code> is also constant</li>
3875 <li>if <code>x</code> is out of range at run time,
3876 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3877 <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
3878 <code>a[x]</code> is <code>byte</code></li>
3879 <li><code>a[x]</code> may not be assigned to</li>
3883 For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
3886 <li><code>x</code>'s type must be
3887 <a href="#Assignability">assignable</a>
3888 to the key type of <code>M</code></li>
3889 <li>if the map contains an entry with key <code>x</code>,
3890 <code>a[x]</code> is the map element with key <code>x</code>
3891 and the type of <code>a[x]</code> is the element type of <code>M</code></li>
3892 <li>if the map is <code>nil</code> or does not contain such an entry,
3893 <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
3894 for the element type of <code>M</code></li>
3898 For <code>a</code> of <a href="#Type_parameter_declarations">type parameter type</a> <code>P</code>:
3901 <li>The index expression <code>a[x]</code> must be valid for values
3902 of all types in <code>P</code>'s type set.</li>
3903 <li>The element types of all types in <code>P</code>'s type set must be identical.
3904 In this context, the element type of a string type is <code>byte</code>.</li>
3905 <li>If there is a map type in the type set of <code>P</code>,
3906 all types in that type set must be map types, and the respective key types
3907 must be all identical.</li>
3908 <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
3909 or the map element with key <code>x</code> of the type argument
3910 that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
3911 the type of the (identical) element types.</li>
3912 <li><code>a[x]</code> may not be assigned to if <code>P</code>'s type set
3913 includes string types.
3917 Otherwise <code>a[x]</code> is illegal.
3921 An index expression on a map <code>a</code> of type <code>map[K]V</code>
3922 used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
3932 yields an additional untyped boolean value. The value of <code>ok</code> is
3933 <code>true</code> if the key <code>x</code> is present in the map, and
3934 <code>false</code> otherwise.
3938 Assigning to an element of a <code>nil</code> map causes a
3939 <a href="#Run_time_panics">run-time panic</a>.
3943 <h3 id="Slice_expressions">Slice expressions</h3>
3946 Slice expressions construct a substring or slice from a string, array, pointer
3947 to array, or slice. There are two variants: a simple form that specifies a low
3948 and high bound, and a full form that also specifies a bound on the capacity.
3951 <h4>Simple slice expressions</h4>
3954 The primary expression
3962 constructs a substring or slice. The <a href="#Core_types">core type</a> of
3963 <code>a</code> must be a string, array, pointer to array, slice, or a
3964 <a href="#Core_types"><code>bytestring</code></a>.
3965 The <i>indices</i> <code>low</code> and
3966 <code>high</code> select which elements of operand <code>a</code> appear
3967 in the result. The result has indices starting at 0 and length equal to
3968 <code>high</code> - <code>low</code>.
3969 After slicing the array <code>a</code>
3973 a := [5]int{1, 2, 3, 4, 5}
3978 the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
3988 For convenience, any of the indices may be omitted. A missing <code>low</code>
3989 index defaults to zero; a missing <code>high</code> index defaults to the length of the
3994 a[2:] // same as a[2 : len(a)]
3995 a[:3] // same as a[0 : 3]
3996 a[:] // same as a[0 : len(a)]
4000 If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
4001 <code>(*a)[low : high]</code>.
4005 For arrays or strings, the indices are <i>in range</i> if
4006 <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
4007 otherwise they are <i>out of range</i>.
4008 For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
4009 A <a href="#Constants">constant</a> index must be non-negative and
4010 <a href="#Representability">representable</a> by a value of type
4011 <code>int</code>; for arrays or constant strings, constant indices must also be in range.
4012 If both indices are constant, they must satisfy <code>low <= high</code>.
4013 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4017 Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
4018 the result of the slice operation is a non-constant value of the same type as the operand.
4019 For untyped string operands the result is a non-constant value of type <code>string</code>.
4020 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
4021 and the result of the slice operation is a slice with the same element type as the array.
4025 If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
4026 is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
4027 array with the operand.
4032 s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
4033 s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
4034 s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
4037 s3 := s[:0] // s3 == nil
4041 <h4>Full slice expressions</h4>
4044 The primary expression
4052 constructs a slice of the same type, and with the same length and elements as the simple slice
4053 expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
4054 by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
4055 The <a href="#Core_types">core type</a> of <code>a</code> must be an array, pointer to array,
4056 or slice (but not a string).
4057 After slicing the array <code>a</code>
4061 a := [5]int{1, 2, 3, 4, 5}
4066 the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
4075 As for simple slice expressions, if <code>a</code> is a pointer to an array,
4076 <code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
4077 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
4081 The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
4082 otherwise they are <i>out of range</i>.
4083 A <a href="#Constants">constant</a> index must be non-negative and
4084 <a href="#Representability">representable</a> by a value of type
4085 <code>int</code>; for arrays, constant indices must also be in range.
4086 If multiple indices are constant, the constants that are present must be in range relative to each
4088 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4091 <h3 id="Type_assertions">Type assertions</h3>
4094 For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
4095 but not a <a href="#Type_parameter_declarations">type parameter</a>, and a type <code>T</code>,
4096 the primary expression
4104 asserts that <code>x</code> is not <code>nil</code>
4105 and that the value stored in <code>x</code> is of type <code>T</code>.
4106 The notation <code>x.(T)</code> is called a <i>type assertion</i>.
4109 More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
4110 that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
4111 to the type <code>T</code>.
4112 In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
4113 otherwise the type assertion is invalid since it is not possible for <code>x</code>
4114 to store a value of type <code>T</code>.
4115 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
4116 of <code>x</code> <a href="#Implementing_an_interface">implements</a> the interface <code>T</code>.
4119 If the type assertion holds, the value of the expression is the value
4120 stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
4121 a <a href="#Run_time_panics">run-time panic</a> occurs.
4122 In other words, even though the dynamic type of <code>x</code>
4123 is known only at run time, the type of <code>x.(T)</code> is
4124 known to be <code>T</code> in a correct program.
4128 var x interface{} = 7 // x has dynamic type int and value 7
4129 i := x.(int) // i has type int and value 7
4131 type I interface { m() }
4134 s := y.(string) // illegal: string does not implement I (missing method m)
4135 r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
4141 A type assertion used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
4148 var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
4152 yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
4153 if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
4154 the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
4155 No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
4159 <h3 id="Calls">Calls</h3>
4162 Given an expression <code>f</code> with a <a href="#Core_types">core type</a>
4163 <code>F</code> of <a href="#Function_types">function type</a>,
4171 calls <code>f</code> with arguments <code>a1, a2, … an</code>.
4172 Except for one special case, arguments must be single-valued expressions
4173 <a href="#Assignability">assignable</a> to the parameter types of
4174 <code>F</code> and are evaluated before the function is called.
4175 The type of the expression is the result type
4177 A method invocation is similar but the method itself
4178 is specified as a selector upon a value of the receiver type for
4183 math.Atan2(x, y) // function call
4185 pt.Scale(3.5) // method call with receiver pt
4189 If <code>f</code> denotes a generic function, it must be
4190 <a href="#Instantiations">instantiated</a> before it can be called
4191 or used as a function value.
4195 In a function call, the function value and arguments are evaluated in
4196 <a href="#Order_of_evaluation">the usual order</a>.
4197 After they are evaluated, the parameters of the call are passed by value to the function
4198 and the called function begins execution.
4199 The return parameters of the function are passed by value
4200 back to the caller when the function returns.
4204 Calling a <code>nil</code> function value
4205 causes a <a href="#Run_time_panics">run-time panic</a>.
4209 As a special case, if the return values of a function or method
4210 <code>g</code> are equal in number and individually
4211 assignable to the parameters of another function or method
4212 <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
4213 will invoke <code>f</code> after binding the return values of
4214 <code>g</code> to the parameters of <code>f</code> in order. The call
4215 of <code>f</code> must contain no parameters other than the call of <code>g</code>,
4216 and <code>g</code> must have at least one return value.
4217 If <code>f</code> has a final <code>...</code> parameter, it is
4218 assigned the return values of <code>g</code> that remain after
4219 assignment of regular parameters.
4223 func Split(s string, pos int) (string, string) {
4224 return s[0:pos], s[pos:]
4227 func Join(s, t string) string {
4231 if Join(Split(value, len(value)/2)) != value {
4232 log.Panic("test fails")
4237 A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
4238 of (the type of) <code>x</code> contains <code>m</code> and the
4239 argument list can be assigned to the parameter list of <code>m</code>.
4240 If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
4241 set contains <code>m</code>, <code>x.m()</code> is shorthand
4242 for <code>(&x).m()</code>:
4251 There is no distinct method type and there are no method literals.
4254 <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
4257 If <code>f</code> is <a href="#Function_types">variadic</a> with a final
4258 parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
4259 the type of <code>p</code> is equivalent to type <code>[]T</code>.
4260 If <code>f</code> is invoked with no actual arguments for <code>p</code>,
4261 the value passed to <code>p</code> is <code>nil</code>.
4262 Otherwise, the value passed is a new slice
4263 of type <code>[]T</code> with a new underlying array whose successive elements
4264 are the actual arguments, which all must be <a href="#Assignability">assignable</a>
4265 to <code>T</code>. The length and capacity of the slice is therefore
4266 the number of arguments bound to <code>p</code> and may differ for each
4271 Given the function and calls
4274 func Greeting(prefix string, who ...string)
4276 Greeting("hello:", "Joe", "Anna", "Eileen")
4280 within <code>Greeting</code>, <code>who</code> will have the value
4281 <code>nil</code> in the first call, and
4282 <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
4286 If the final argument is assignable to a slice type <code>[]T</code> and
4287 is followed by <code>...</code>, it is passed unchanged as the value
4288 for a <code>...T</code> parameter. In this case no new slice is created.
4292 Given the slice <code>s</code> and call
4296 s := []string{"James", "Jasmine"}
4297 Greeting("goodbye:", s...)
4301 within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
4302 with the same underlying array.
4305 <h3 id="Instantiations">Instantiations</h3>
4308 A generic function or type is <i>instantiated</i> by substituting <i>type arguments</i>
4309 for the type parameters.
4310 Instantiation proceeds in two steps:
4315 Each type argument is substituted for its corresponding type parameter in the generic
4317 This substitution happens across the entire function or type declaration,
4318 including the type parameter list itself and any types in that list.
4322 After substitution, each type argument must <a href="#Satisfying_a_type_constraint">satisfy</a>
4323 the <a href="#Type_parameter_declarations">constraint</a> (instantiated, if necessary)
4324 of the corresponding type parameter. Otherwise instantiation fails.
4329 Instantiating a type results in a new non-generic <a href="#Types">named type</a>;
4330 instantiating a function produces a new non-generic function.
4334 type parameter list type arguments after substitution
4336 [P any] int int satisfies any
4337 [S ~[]E, E any] []int, int []int satisfies ~[]int, int satisfies any
4338 [P io.Writer] string illegal: string doesn't satisfy io.Writer
4339 [P comparable] any any satisfies (but does not implement) comparable
4343 For a generic function, type arguments may be provided explicitly, or they
4344 may be partially or completely <a href="#Type_inference">inferred</a>.
4345 A generic function that is <i>not</i> <a href="#Calls">called</a> requires a
4346 type argument list for instantiation; if the list is partial, all
4347 remaining type arguments must be inferrable.
4348 A generic function that is called may provide a (possibly partial) type
4349 argument list, or may omit it entirely if the omitted type arguments are
4350 inferrable from the ordinary (non-type) function arguments.
4354 func min[T ~int|~float64](x, y T) T { … }
4356 f := min // illegal: min must be instantiated with type arguments when used without being called
4357 minInt := min[int] // minInt has type func(x, y int) int
4358 a := minInt(2, 3) // a has value 2 of type int
4359 b := min[float64](2.0, 3) // b has value 2.0 of type float64
4360 c := min(b, -1) // c has value -1.0 of type float64
4364 A partial type argument list cannot be empty; at least the first argument must be present.
4365 The list is a prefix of the full list of type arguments, leaving the remaining arguments
4366 to be inferred. Loosely speaking, type arguments may be omitted from "right to left".
4370 func apply[S ~[]E, E any](s S, f func(E) E) S { … }
4372 f0 := apply[] // illegal: type argument list cannot be empty
4373 f1 := apply[[]int] // type argument for S explicitly provided, type argument for E inferred
4374 f2 := apply[[]string, string] // both type arguments explicitly provided
4377 r := apply(bytes, func(byte) byte { … }) // both type arguments inferred from the function arguments
4381 For a generic type, all type arguments must always be provided explicitly.
4384 <h3 id="Type_inference">Type inference</h3>
4387 <em>NOTE: This section is not yet up-to-date for Go 1.21.</em>
4391 Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
4392 Each step attempts to use known information to infer additional type arguments.
4393 Type inference stops as soon as all type arguments are known.
4394 After type inference is complete, it is still necessary to substitute all type arguments
4395 for type parameters and verify that each type argument
4396 <a href="#Implementing_an_interface">implements</a> the relevant constraint;
4397 it is possible for an inferred type argument to fail to implement a constraint, in which
4398 case instantiation fails.
4402 Type inference is based on
4407 a <a href="#Type_parameter_declarations">type parameter list</a>
4410 a substitution map <i>M</i> initialized with the known type arguments, if any
4413 a (possibly empty) list of ordinary function arguments (in case of a function call only)
4418 and then proceeds with the following steps:
4423 apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
4424 to all <i>typed</i> ordinary function arguments
4427 apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
4430 apply function argument type inference to all <i>untyped</i> ordinary function arguments
4431 using the default type for each of the untyped function arguments
4434 apply constraint type inference
4439 If there are no ordinary or untyped function arguments, the respective steps are skipped.
4440 Constraint type inference is skipped if the previous step didn't infer any new type arguments,
4441 but it is run at least once if there are missing type arguments.
4445 The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
4446 The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
4447 If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
4450 <h4 id="Type_unification">Type unification</h4>
4453 Type inference is based on <i>type unification</i>. A single unification step
4454 applies to a <a href="#Type_inference">substitution map</a> and two types, either
4455 or both of which may be or contain type parameters. The substitution map tracks
4456 the known (explicitly provided or already inferred) type arguments: the map
4457 contains an entry <code>P</code> → <code>A</code> for each type
4458 parameter <code>P</code> and corresponding known type argument <code>A</code>.
4459 During unification, known type arguments take the place of their corresponding type
4460 parameters when comparing types. Unification is the process of finding substitution
4461 map entries that make the two types equivalent.
4465 For unification, two types that don't contain any type parameters from the current type
4466 parameter list are <i>equivalent</i>
4467 if they are identical, or if they are channel types that are identical ignoring channel
4468 direction, or if their underlying types are equivalent.
4472 Unification works by comparing the structure of pairs of types: their structure
4473 disregarding type parameters must be identical, and types other than type parameters
4475 A type parameter in one type may match any complete subtype in the other type;
4476 each successful match causes an entry to be added to the substitution map.
4477 If the structure differs, or types other than type parameters are not equivalent,
4482 TODO(gri) Somewhere we need to describe the process of adding an entry to the
4483 substitution map: if the entry is already present, the type argument
4484 values are themselves unified.
4488 For example, if <code>T1</code> and <code>T2</code> are type parameters,
4489 <code>[]map[int]bool</code> can be unified with any of the following:
4493 []map[int]bool // types are identical
4494 T1 // adds T1 → []map[int]bool to substitution map
4495 []T1 // adds T1 → map[int]bool to substitution map
4496 []map[T1]T2 // adds T1 → int and T2 → bool to substitution map
4500 On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
4504 int // int is not a slice
4505 struct{} // a struct is not a slice
4506 []struct{} // a struct is not a map
4507 []map[T1]string // map element types don't match
4511 As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
4512 <code>D</code> and a type literal <code>L</code> are never equivalent,
4513 unification compares the underlying type of <code>D</code> with <code>L</code> instead.
4514 For example, given the defined type
4518 type Vector []float64
4522 and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
4523 <code>[]E</code> and adds an entry <code>E</code> → <code>float64</code> to
4524 the substitution map.
4527 <h4 id="Function_argument_type_inference">Function argument type inference</h4>
4529 <!-- In this section and the section on constraint type inference we start with examples
4530 rather than have the examples follow the rules as is customary elsewhere in spec.
4531 Hopefully this helps building an intuition and makes the rules easier to follow. -->
4534 Function argument type inference infers type arguments from function arguments:
4535 if a function parameter is declared with a type <code>T</code> that uses
4537 <a href="#Type_unification">unifying</a> the type of the corresponding
4538 function argument with <code>T</code> may infer type arguments for the type
4539 parameters used by <code>T</code>.
4543 For instance, given the generic function
4547 func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
4555 var vector []float64
4556 scaledVector := scale(vector, 42)
4560 the type argument for <code>Number</code> can be inferred from the function argument
4561 <code>vector</code> by unifying the type of <code>vector</code> with the corresponding
4562 parameter type: <code>[]float64</code> and <code>[]Number</code>
4563 match in structure and <code>float64</code> matches with <code>Number</code>.
4564 This adds the entry <code>Number</code> → <code>float64</code> to the
4565 <a href="#Type_unification">substitution map</a>.
4566 Untyped arguments, such as the second function argument <code>42</code> here, are ignored
4567 in the first round of function argument type inference and only considered if there are
4568 unresolved type parameters left.
4572 Inference happens in two separate phases; each phase operates on a specific list of
4573 (parameter, argument) pairs:
4578 The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
4579 type uses type parameters and where the function argument is <i>typed</i>.
4582 The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
4583 type parameter. In this list, the respective function arguments are untyped.
4588 Any other (parameter, argument) pair is ignored.
4592 By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
4593 (or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
4594 of untyped values are always predeclared non-composite types, they can never match against
4595 a composite type, so it is sufficient to only consider parameter types that are single type
4600 Each list is processed in a separate phase:
4605 In the first phase, the parameter and argument types of each pair in <i>Lt</i>
4606 are unified. If unification succeeds for a pair, it may yield new entries that
4607 are added to the substitution map <i>M</i>. If unification fails, type inference
4611 The second phase considers the entries of list <i>Lu</i>. Type parameters for
4612 which the type argument has already been determined are ignored in this phase.
4613 For each remaining pair, the parameter type (which is a single type parameter) and
4614 the <a href="#Constants">default type</a> of the corresponding untyped argument is
4615 unified. If unification fails, type inference fails.
4620 While unification is successful, processing of each list continues until all list elements
4621 are considered, even if all type arguments are inferred before the last list element has
4630 func min[T ~int|~float64](x, y T) T
4633 min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
4634 min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
4635 min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
4639 In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
4640 yields the substitution map entry <code>T</code> → <code>float64</code>. Because
4641 processing continues until all untyped arguments are considered, an error is reported. This
4642 ensures that type inference does not depend on the order of the untyped arguments.
4645 <h4 id="Constraint_type_inference">Constraint type inference</h4>
4648 Constraint type inference infers type arguments by considering type constraints.
4649 If a type parameter <code>P</code> has a constraint with a
4650 <a href="#Core_types">core type</a> <code>C</code>,
4651 <a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
4652 may infer additional type arguments, either the type argument for <code>P</code>,
4653 or if that is already known, possibly the type arguments for type parameters
4654 used in <code>C</code>.
4658 For instance, consider the type parameter list with type parameters <code>List</code> and
4663 [List ~[]Elem, Elem any]
4667 Constraint type inference can deduce the type of <code>Elem</code> from the type argument
4668 for <code>List</code> because <code>Elem</code> is a type parameter in the core type
4669 <code>[]Elem</code> of <code>List</code>.
4670 If the type argument is <code>Bytes</code>:
4678 unifying the underlying type of <code>Bytes</code> with the core type means
4679 unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
4680 the <a href="#Type_unification">substitution map</a> entry
4681 <code>Elem</code> → <code>byte</code>.
4682 Thus, in this example, constraint type inference can infer the second type argument from the
4687 Using the core type of a constraint may lose some information: In the (unlikely) case that
4688 the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
4689 <code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
4690 <code>N</code> itself. In this case, constraint type inference may succeed but instantiation
4691 will fail because the inferred type is not in the type set of the constraint.
4692 Thus, constraint type inference uses the <i>adjusted core type</i> of
4693 a constraint: if the type set contains a single type, use that type; otherwise use the
4694 constraint's core type.
4698 Generally, constraint type inference proceeds in two phases: Starting with a given
4699 substitution map <i>M</i>
4704 For all type parameters with an adjusted core type, unify the type parameter with that
4705 type. If any unification fails, constraint type inference fails.
4709 At this point, some entries in <i>M</i> may map type parameters to other
4710 type parameters or to types containing type parameters. For each entry
4711 <code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
4712 contains type parameters <code>Q</code> for which there exist entries
4713 <code>Q</code> → <code>B</code> in <i>M</i>, substitute those
4714 <code>Q</code> with the respective <code>B</code> in <code>A</code>.
4715 Stop when no further substitution is possible.
4720 The result of constraint type inference is the final substitution map <i>M</i> from type
4721 parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
4722 appears in any of the <code>A</code>.
4726 For instance, given the type parameter list
4730 [A any, B []C, C *A]
4734 and the single provided type argument <code>int</code> for type parameter <code>A</code>,
4735 the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
4739 In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
4740 with the core type of their respective constraints. This adds the entries
4741 <code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
4745 At this point there are two entries in <i>M</i> where the right-hand side
4746 is or contains type parameters for which there exists other entries in <i>M</i>:
4747 <code>[]C</code> and <code>*A</code>.
4748 In the second phase, these type parameters are replaced with their respective
4749 types. It doesn't matter in which order this happens. Starting with the state
4750 of <i>M</i> after the first phase:
4754 <code>A</code> → <code>int</code>,
4755 <code>B</code> → <code>[]C</code>,
4756 <code>C</code> → <code>*A</code>
4760 Replace <code>A</code> on the right-hand side of → with <code>int</code>:
4764 <code>A</code> → <code>int</code>,
4765 <code>B</code> → <code>[]C</code>,
4766 <code>C</code> → <code>*int</code>
4770 Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
4774 <code>A</code> → <code>int</code>,
4775 <code>B</code> → <code>[]*int</code>,
4776 <code>C</code> → <code>*int</code>
4780 At this point no further substitution is possible and the map is full.
4781 Therefore, <code>M</code> represents the final map of type parameters
4782 to type arguments for the given type parameter list.
4785 <h3 id="Operators">Operators</h3>
4788 Operators combine operands into expressions.
4792 Expression = UnaryExpr | Expression binary_op Expression .
4793 UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
4795 binary_op = "||" | "&&" | rel_op | add_op | mul_op .
4796 rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
4797 add_op = "+" | "-" | "|" | "^" .
4798 mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
4800 unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
4804 Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
4805 For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
4806 unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
4807 For operations involving constants only, see the section on
4808 <a href="#Constant_expressions">constant expressions</a>.
4812 Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
4813 and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
4814 to the type of the other operand.
4818 The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
4819 or be an untyped constant <a href="#Representability">representable</a> by a
4820 value of type <code>uint</code>.
4821 If the left operand of a non-constant shift expression is an untyped constant,
4822 it is first implicitly converted to the type it would assume if the shift expression were
4823 replaced by its left operand alone.
4830 // The results of the following examples are given for 64-bit ints.
4831 var i = 1<<s // 1 has type int
4832 var j int32 = 1<<s // 1 has type int32; j == 0
4833 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
4834 var m int = 1.0<<s // 1.0 has type int; m == 1<<33
4835 var n = 1.0<<s == j // 1.0 has type int32; n == true
4836 var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
4837 var p = 1<<s == 1<<33 // 1 has type int; p == true
4838 var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
4839 var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
4840 var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
4841 var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift
4842 var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift
4843 var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
4844 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
4845 var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
4847 // The results of the following examples are given for 32-bit ints,
4848 // which means the shifts will overflow.
4849 var mm int = 1.0<<s // 1.0 has type int; mm == 0
4850 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
4851 var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
4852 var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
4853 var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
4856 <h4 id="Operator_precedence">Operator precedence</h4>
4858 Unary operators have the highest precedence.
4859 As the <code>++</code> and <code>--</code> operators form
4860 statements, not expressions, they fall
4861 outside the operator hierarchy.
4862 As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
4864 There are five precedence levels for binary operators.
4865 Multiplication operators bind strongest, followed by addition
4866 operators, comparison operators, <code>&&</code> (logical AND),
4867 and finally <code>||</code> (logical OR):
4870 <pre class="grammar">
4872 5 * / % << >> & &^
4874 3 == != < <= > >=
4880 Binary operators of the same precedence associate from left to right.
4881 For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
4890 x == y+1 && <-chanInt > 0
4894 <h3 id="Arithmetic_operators">Arithmetic operators</h3>
4896 Arithmetic operators apply to numeric values and yield a result of the same
4897 type as the first operand. The four standard arithmetic operators (<code>+</code>,
4898 <code>-</code>, <code>*</code>, <code>/</code>) apply to
4899 <a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
4900 <a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>.
4901 The bitwise logical and shift operators apply to integers only.
4904 <pre class="grammar">
4905 + sum integers, floats, complex values, strings
4906 - difference integers, floats, complex values
4907 * product integers, floats, complex values
4908 / quotient integers, floats, complex values
4909 % remainder integers
4911 & bitwise AND integers
4912 | bitwise OR integers
4913 ^ bitwise XOR integers
4914 &^ bit clear (AND NOT) integers
4916 << left shift integer << integer >= 0
4917 >> right shift integer >> integer >= 0
4921 If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>,
4922 the operator must apply to each type in that type set.
4923 The operands are represented as values of the type argument that the type parameter
4924 is <a href="#Instantiations">instantiated</a> with, and the operation is computed
4925 with the precision of that type argument. For example, given the function:
4929 func dotProduct[F ~float32|~float64](v1, v2 []F) F {
4931 for i, x := range v1 {
4940 the product <code>x * y</code> and the addition <code>s += x * y</code>
4941 are computed with <code>float32</code> or <code>float64</code> precision,
4942 respectively, depending on the type argument for <code>F</code>.
4945 <h4 id="Integer_operators">Integer operators</h4>
4948 For two integer values <code>x</code> and <code>y</code>, the integer quotient
4949 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
4954 x = q*y + r and |r| < |y|
4958 with <code>x / y</code> truncated towards zero
4959 (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
4971 The one exception to this rule is that if the dividend <code>x</code> is
4972 the most negative value for the int type of <code>x</code>, the quotient
4973 <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
4974 due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
4982 int64 -9223372036854775808
4986 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
4987 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4988 If the dividend is non-negative and the divisor is a constant power of 2,
4989 the division may be replaced by a right shift, and computing the remainder may
4990 be replaced by a bitwise AND operation:
4994 x x / 4 x % 4 x >> 2 x & 3
5000 The shift operators shift the left operand by the shift count specified by the
5001 right operand, which must be non-negative. If the shift count is negative at run time,
5002 a <a href="#Run_time_panics">run-time panic</a> occurs.
5003 The shift operators implement arithmetic shifts if the left operand is a signed
5004 integer and logical shifts if it is an unsigned integer.
5005 There is no upper limit on the shift count. Shifts behave
5006 as if the left operand is shifted <code>n</code> times by 1 for a shift
5007 count of <code>n</code>.
5008 As a result, <code>x << 1</code> is the same as <code>x*2</code>
5009 and <code>x >> 1</code> is the same as
5010 <code>x/2</code> but truncated towards negative infinity.
5014 For integer operands, the unary operators
5015 <code>+</code>, <code>-</code>, and <code>^</code> are defined as
5019 <pre class="grammar">
5021 -x negation is 0 - x
5022 ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
5023 and m = -1 for signed x
5027 <h4 id="Integer_overflow">Integer overflow</h4>
5030 For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>,
5031 <code>-</code>, <code>*</code>, and <code><<</code> are
5032 computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
5033 the unsigned integer's type.
5034 Loosely speaking, these unsigned integer operations
5035 discard high bits upon overflow, and programs may rely on "wrap around".
5039 For signed integers, the operations <code>+</code>,
5040 <code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
5041 overflow and the resulting value exists and is deterministically defined
5042 by the signed integer representation, the operation, and its operands.
5043 Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
5044 A compiler may not optimize code under the assumption that overflow does
5045 not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
5048 <h4 id="Floating_point_operators">Floating-point operators</h4>
5051 For floating-point and complex numbers,
5052 <code>+x</code> is the same as <code>x</code>,
5053 while <code>-x</code> is the negation of <code>x</code>.
5054 The result of a floating-point or complex division by zero is not specified beyond the
5055 IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
5056 occurs is implementation-specific.
5060 An implementation may combine multiple floating-point operations into a single
5061 fused operation, possibly across statements, and produce a result that differs
5062 from the value obtained by executing and rounding the instructions individually.
5063 An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to
5064 the precision of the target type, preventing fusion that would discard that rounding.
5068 For instance, some architectures provide a "fused multiply and add" (FMA) instruction
5069 that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
5070 These examples show when a Go implementation can use that instruction:
5074 // FMA allowed for computing r, because x*y is not explicitly rounded:
5078 *p = x*y; r = *p + z
5079 r = x*y + float64(z)
5081 // FMA disallowed for computing r, because it would omit rounding of x*y:
5082 r = float64(x*y) + z
5083 r = z; r += float64(x*y)
5084 t = float64(x*y); r = t + z
5087 <h4 id="String_concatenation">String concatenation</h4>
5090 Strings can be concatenated using the <code>+</code> operator
5091 or the <code>+=</code> assignment operator:
5095 s := "hi" + string(c)
5096 s += " and good bye"
5100 String addition creates a new string by concatenating the operands.
5103 <h3 id="Comparison_operators">Comparison operators</h3>
5106 Comparison operators compare two operands and yield an untyped boolean value.
5109 <pre class="grammar">
5115 >= greater or equal
5119 In any comparison, the first operand
5120 must be <a href="#Assignability">assignable</a>
5121 to the type of the second operand, or vice versa.
5124 The equality operators <code>==</code> and <code>!=</code> apply
5125 to operands of <i>comparable</i> types.
5126 The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
5127 apply to operands of <i>ordered</i> types.
5128 These terms and the result of the comparisons are defined as follows:
5133 Boolean types are comparable.
5134 Two boolean values are equal if they are either both
5135 <code>true</code> or both <code>false</code>.
5139 Integer types are comparable and ordered.
5140 Two integer values are compared in the usual way.
5144 Floating-point types are comparable and ordered.
5145 Two floating-point values are compared as defined by the IEEE-754 standard.
5149 Complex types are comparable.
5150 Two complex values <code>u</code> and <code>v</code> are
5151 equal if both <code>real(u) == real(v)</code> and
5152 <code>imag(u) == imag(v)</code>.
5156 String types are comparable and ordered.
5157 Two string values are compared lexically byte-wise.
5161 Pointer types are comparable.
5162 Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
5163 Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
5167 Channel types are comparable.
5168 Two channel values are equal if they were created by the same call to
5169 <a href="#Making_slices_maps_and_channels"><code>make</code></a>
5170 or if both have value <code>nil</code>.
5174 Interface types that are not type parameters are comparable.
5175 Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
5176 and equal dynamic values or if both have value <code>nil</code>.
5180 A value <code>x</code> of non-interface type <code>X</code> and
5181 a value <code>t</code> of interface type <code>T</code> can be compared
5182 if type <code>X</code> is comparable and
5183 <code>X</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
5184 They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
5185 and <code>t</code>'s dynamic value is equal to <code>x</code>.
5189 Struct types are comparable if all their field types are comparable.
5190 Two struct values are equal if their corresponding
5191 non-<a href="#Blank_identifier">blank</a> field values are equal.
5192 The fields are compared in source order, and comparison stops as
5193 soon as two field values differ (or all fields have been compared).
5197 Array types are comparable if their array element types are comparable.
5198 Two array values are equal if their corresponding element values are equal.
5199 The elements are compared in ascending index order, and comparison stops
5200 as soon as two element values differ (or all elements have been compared).
5204 Type parameters are comparable if they are strictly comparable (see below).
5209 A comparison of two interface values with identical dynamic types
5210 causes a <a href="#Run_time_panics">run-time panic</a> if that type
5211 is not comparable. This behavior applies not only to direct interface
5212 value comparisons but also when comparing arrays of interface values
5213 or structs with interface-valued fields.
5217 Slice, map, and function types are not comparable.
5218 However, as a special case, a slice, map, or function value may
5219 be compared to the predeclared identifier <code>nil</code>.
5220 Comparison of pointer, channel, and interface values to <code>nil</code>
5221 is also allowed and follows from the general rules above.
5225 const c = 3 < 4 // c is the untyped boolean constant true
5230 // The result of a comparison is an untyped boolean.
5231 // The usual assignment rules apply.
5232 b3 = x == y // b3 has type bool
5233 b4 bool = x == y // b4 has type bool
5234 b5 MyBool = x == y // b5 has type MyBool
5239 A type is <i>strictly comparable</i> if it is comparable and not an interface
5240 type nor composed of interface types.
5246 Boolean, numeric, string, pointer, and channel types are strictly comparable.
5250 Struct types are strictly comparable if all their field types are strictly comparable.
5254 Array types are strictly comparable if their array element types are strictly comparable.
5258 Type parameters are strictly comparable if all types in their type set are strictly comparable.
5262 <h3 id="Logical_operators">Logical operators</h3>
5265 Logical operators apply to <a href="#Boolean_types">boolean</a> values
5266 and yield a result of the same type as the operands.
5267 The right operand is evaluated conditionally.
5270 <pre class="grammar">
5271 && conditional AND p && q is "if p then q else false"
5272 || conditional OR p || q is "if p then true else q"
5277 <h3 id="Address_operators">Address operators</h3>
5280 For an operand <code>x</code> of type <code>T</code>, the address operation
5281 <code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
5282 The operand must be <i>addressable</i>,
5283 that is, either a variable, pointer indirection, or slice indexing
5284 operation; or a field selector of an addressable struct operand;
5285 or an array indexing operation of an addressable array.
5286 As an exception to the addressability requirement, <code>x</code> may also be a
5287 (possibly parenthesized)
5288 <a href="#Composite_literals">composite literal</a>.
5289 If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
5290 then the evaluation of <code>&x</code> does too.
5294 For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
5295 indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
5296 to by <code>x</code>.
5297 If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
5298 will cause a <a href="#Run_time_panics">run-time panic</a>.
5309 *x // causes a run-time panic
5310 &*x // causes a run-time panic
5314 <h3 id="Receive_operator">Receive operator</h3>
5317 For an operand <code>ch</code> whose <a href="#Core_types">core type</a> is a
5318 <a href="#Channel_types">channel</a>,
5319 the value of the receive operation <code><-ch</code> is the value received
5320 from the channel <code>ch</code>. The channel direction must permit receive operations,
5321 and the type of the receive operation is the element type of the channel.
5322 The expression blocks until a value is available.
5323 Receiving from a <code>nil</code> channel blocks forever.
5324 A receive operation on a <a href="#Close">closed</a> channel can always proceed
5325 immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
5326 after any previously sent values have been received.
5333 <-strobe // wait until clock pulse and discard received value
5337 A receive expression used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
5344 var x, ok T = <-ch
5348 yields an additional untyped boolean result reporting whether the
5349 communication succeeded. The value of <code>ok</code> is <code>true</code>
5350 if the value received was delivered by a successful send operation to the
5351 channel, or <code>false</code> if it is a zero value generated because the
5352 channel is closed and empty.
5356 <h3 id="Conversions">Conversions</h3>
5359 A conversion changes the <a href="#Types">type</a> of an expression
5360 to the type specified by the conversion.
5361 A conversion may appear literally in the source, or it may be <i>implied</i>
5362 by the context in which an expression appears.
5366 An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
5367 where <code>T</code> is a type and <code>x</code> is an expression
5368 that can be converted to type <code>T</code>.
5372 Conversion = Type "(" Expression [ "," ] ")" .
5376 If the type starts with the operator <code>*</code> or <code><-</code>,
5377 or if the type starts with the keyword <code>func</code>
5378 and has no result list, it must be parenthesized when
5379 necessary to avoid ambiguity:
5383 *Point(p) // same as *(Point(p))
5384 (*Point)(p) // p is converted to *Point
5385 <-chan int(c) // same as <-(chan int(c))
5386 (<-chan int)(c) // c is converted to <-chan int
5387 func()(x) // function signature func() x
5388 (func())(x) // x is converted to func()
5389 (func() int)(x) // x is converted to func() int
5390 func() int(x) // x is converted to func() int (unambiguous)
5394 A <a href="#Constants">constant</a> value <code>x</code> can be converted to
5395 type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
5396 by a value of <code>T</code>.
5397 As a special case, an integer constant <code>x</code> can be explicitly converted to a
5398 <a href="#String_types">string type</a> using the
5399 <a href="#Conversions_to_and_from_a_string_type">same rule</a>
5400 as for non-constant <code>x</code>.
5404 Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a>
5405 yields a typed constant.
5409 uint(iota) // iota value of type uint
5410 float32(2.718281828) // 2.718281828 of type float32
5411 complex128(1) // 1.0 + 0.0i of type complex128
5412 float32(0.49999999) // 0.5 of type float32
5413 float64(-1e-1000) // 0.0 of type float64
5414 string('x') // "x" of type string
5415 string(0x266c) // "♬" of type string
5416 myString("foo" + "bar") // "foobar" of type myString
5417 string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
5418 (*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
5419 int(1.2) // illegal: 1.2 cannot be represented as an int
5420 string(65.0) // illegal: 65.0 is not an integer constant
5424 Converting a constant to a type parameter yields a <i>non-constant</i> value of that type,
5425 with the value represented as a value of the type argument that the type parameter
5426 is <a href="#Instantiations">instantiated</a> with.
5427 For example, given the function:
5431 func f[P ~float32|~float64]() {
5437 the conversion <code>P(1.1)</code> results in a non-constant value of type <code>P</code>
5438 and the value <code>1.1</code> is represented as a <code>float32</code> or a <code>float64</code>
5439 depending on the type argument for <code>f</code>.
5440 Accordingly, if <code>f</code> is instantiated with a <code>float32</code> type,
5441 the numeric value of the expression <code>P(1.1) + 1.2</code> will be computed
5442 with the same precision as the corresponding non-constant <code>float32</code>
5447 A non-constant value <code>x</code> can be converted to type <code>T</code>
5448 in any of these cases:
5453 <code>x</code> is <a href="#Assignability">assignable</a>
5457 ignoring struct tags (see below),
5458 <code>x</code>'s type and <code>T</code> are not
5459 <a href="#Type_parameter_declarations">type parameters</a> but have
5460 <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
5463 ignoring struct tags (see below),
5464 <code>x</code>'s type and <code>T</code> are pointer types
5465 that are not <a href="#Types">named types</a>,
5466 and their pointer base types are not type parameters but
5467 have identical underlying types.
5470 <code>x</code>'s type and <code>T</code> are both integer or floating
5474 <code>x</code>'s type and <code>T</code> are both complex types.
5477 <code>x</code> is an integer or a slice of bytes or runes
5478 and <code>T</code> is a string type.
5481 <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
5484 <code>x</code> is a slice, <code>T</code> is an array or a pointer to an array,
5485 and the slice and array types have <a href="#Type_identity">identical</a> element types.
5490 Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type
5491 parameters, <code>x</code>
5492 can also be converted to type <code>T</code> if one of the following conditions applies:
5497 Both <code>V</code> and <code>T</code> are type parameters and a value of each
5498 type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s
5502 Only <code>V</code> is a type parameter and a value of each
5503 type in <code>V</code>'s type set can be converted to <code>T</code>.
5506 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
5507 type in <code>T</code>'s type set.
5512 <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
5513 for identity for the purpose of conversion:
5517 type Person struct {
5526 Name string `json:"name"`
5528 Street string `json:"street"`
5529 City string `json:"city"`
5533 var person = (*Person)(data) // ignoring tags, the underlying types are identical
5537 Specific rules apply to (non-constant) conversions between numeric types or
5538 to and from a string type.
5539 These conversions may change the representation of <code>x</code>
5540 and incur a run-time cost.
5541 All other conversions only change the type but not the representation
5546 There is no linguistic mechanism to convert between pointers and integers.
5547 The package <a href="#Package_unsafe"><code>unsafe</code></a>
5548 implements this functionality under restricted circumstances.
5551 <h4>Conversions between numeric types</h4>
5554 For the conversion of non-constant numeric values, the following rules apply:
5559 When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is
5560 sign extended to implicit infinite precision; otherwise it is zero extended.
5561 It is then truncated to fit in the result type's size.
5562 For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
5563 The conversion always yields a valid value; there is no indication of overflow.
5566 When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded
5567 (truncation towards zero).
5570 When converting an integer or floating-point number to a floating-point type,
5571 or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded
5572 to the precision specified by the destination type.
5573 For instance, the value of a variable <code>x</code> of type <code>float32</code>
5574 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
5575 but float32(x) represents the result of rounding <code>x</code>'s value to
5576 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
5577 of precision, but <code>float32(x + 0.1)</code> does not.
5582 In all non-constant conversions involving floating-point or complex values,
5583 if the result type cannot represent the value the conversion
5584 succeeds but the result value is implementation-dependent.
5587 <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
5591 Converting a signed or unsigned integer value to a string type yields a
5592 string containing the UTF-8 representation of the integer. Values outside
5593 the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
5597 string(-1) // "\ufffd" == "\xef\xbf\xbd"
5598 string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
5600 type myString string
5601 myString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
5606 Converting a slice of bytes to a string type yields
5607 a string whose successive bytes are the elements of the slice.
5610 string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5611 string([]byte{}) // ""
5612 string([]byte(nil)) // ""
5615 string(bytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5618 string([]myByte{'w', 'o', 'r', 'l', 'd', '!'}) // "world!"
5619 myString([]myByte{'\xf0', '\x9f', '\x8c', '\x8d'}) // "🌍"
5624 Converting a slice of runes to a string type yields
5625 a string that is the concatenation of the individual rune values
5626 converted to strings.
5629 string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5630 string([]rune{}) // ""
5631 string([]rune(nil)) // ""
5634 string(runes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5637 string([]myRune{0x266b, 0x266c}) // "\u266b\u266c" == "♫♬"
5638 myString([]myRune{0x1f30e}) // "\U0001f30e" == "🌎"
5643 Converting a value of a string type to a slice of bytes type
5644 yields a slice whose successive elements are the bytes of the string.
5647 []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5648 []byte("") // []byte{}
5650 bytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5652 []myByte("world!") // []myByte{'w', 'o', 'r', 'l', 'd', '!'}
5653 []myByte(myString("🌏")) // []myByte{'\xf0', '\x9f', '\x8c', '\x8f'}
5658 Converting a value of a string type to a slice of runes type
5659 yields a slice containing the individual Unicode code points of the string.
5662 []rune(myString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
5663 []rune("") // []rune{}
5665 runes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
5667 []myRune("♫♬") // []myRune{0x266b, 0x266c}
5668 []myRune(myString("🌐")) // []myRune{0x1f310}
5673 <h4 id="Conversions_from_slice_to_array_or_array_pointer">Conversions from slice to array or array pointer</h4>
5676 Converting a slice to an array yields an array containing the elements of the underlying array of the slice.
5677 Similarly, converting a slice to an array pointer yields a pointer to the underlying array of the slice.
5678 In both cases, if the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
5679 a <a href="#Run_time_panics">run-time panic</a> occurs.
5683 s := make([]byte, 2, 4)
5686 a1 := [1]byte(s[1:]) // a1[0] == s[1]
5687 a2 := [2]byte(s) // a2[0] == s[0]
5688 a4 := [4]byte(s) // panics: len([4]byte) > len(s)
5690 s0 := (*[0]byte)(s) // s0 != nil
5691 s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
5692 s2 := (*[2]byte)(s) // &s2[0] == &s[0]
5693 s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
5696 t0 := [0]string(t) // ok for nil slice t
5697 t1 := (*[0]string)(t) // t1 == nil
5698 t2 := (*[1]string)(t) // panics: len([1]string) > len(t)
5700 u := make([]byte, 0)
5701 u0 := (*[0]byte)(u) // u0 != nil
5704 <h3 id="Constant_expressions">Constant expressions</h3>
5707 Constant expressions may contain only <a href="#Constants">constant</a>
5708 operands and are evaluated at compile time.
5712 Untyped boolean, numeric, and string constants may be used as operands
5713 wherever it is legal to use an operand of boolean, numeric, or string type,
5718 A constant <a href="#Comparison_operators">comparison</a> always yields
5719 an untyped boolean constant. If the left operand of a constant
5720 <a href="#Operators">shift expression</a> is an untyped constant, the
5721 result is an integer constant; otherwise it is a constant of the same
5722 type as the left operand, which must be of
5723 <a href="#Numeric_types">integer type</a>.
5727 Any other operation on untyped constants results in an untyped constant of the
5728 same kind; that is, a boolean, integer, floating-point, complex, or string
5730 If the untyped operands of a binary operation (other than a shift) are of
5731 different kinds, the result is of the operand's kind that appears later in this
5732 list: integer, rune, floating-point, complex.
5733 For example, an untyped integer constant divided by an
5734 untyped complex constant yields an untyped complex constant.
5738 const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
5739 const b = 15 / 4 // b == 3 (untyped integer constant)
5740 const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
5741 const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
5742 const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
5743 const d = 1 << 3.0 // d == 8 (untyped integer constant)
5744 const e = 1.0 << 3 // e == 8 (untyped integer constant)
5745 const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
5746 const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
5747 const h = "foo" > "bar" // h == true (untyped boolean constant)
5748 const j = true // j == true (untyped boolean constant)
5749 const k = 'w' + 1 // k == 'x' (untyped rune constant)
5750 const l = "hi" // l == "hi" (untyped string constant)
5751 const m = string(k) // m == "x" (type string)
5752 const Σ = 1 - 0.707i // (untyped complex constant)
5753 const Δ = Σ + 2.0e-4 // (untyped complex constant)
5754 const Φ = iota*1i - 1/1i // (untyped complex constant)
5758 Applying the built-in function <code>complex</code> to untyped
5759 integer, rune, or floating-point constants yields
5760 an untyped complex constant.
5764 const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
5765 const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
5769 Constant expressions are always evaluated exactly; intermediate values and the
5770 constants themselves may require precision significantly larger than supported
5771 by any predeclared type in the language. The following are legal declarations:
5775 const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
5776 const Four int8 = Huge >> 98 // Four == 4 (type int8)
5780 The divisor of a constant division or remainder operation must not be zero:
5784 3.14 / 0.0 // illegal: division by zero
5788 The values of <i>typed</i> constants must always be accurately
5789 <a href="#Representability">representable</a> by values
5790 of the constant type. The following constant expressions are illegal:
5794 uint(-1) // -1 cannot be represented as a uint
5795 int(3.14) // 3.14 cannot be represented as an int
5796 int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
5797 Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
5798 Four * 100 // product 400 cannot be represented as an int8 (type of Four)
5802 The mask used by the unary bitwise complement operator <code>^</code> matches
5803 the rule for non-constants: the mask is all 1s for unsigned constants
5804 and -1 for signed and untyped constants.
5808 ^1 // untyped integer constant, equal to -2
5809 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
5810 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
5811 int8(^1) // same as int8(-2)
5812 ^int8(1) // same as -1 ^ int8(1) = -2
5816 Implementation restriction: A compiler may use rounding while
5817 computing untyped floating-point or complex constant expressions; see
5818 the implementation restriction in the section
5819 on <a href="#Constants">constants</a>. This rounding may cause a
5820 floating-point constant expression to be invalid in an integer
5821 context, even if it would be integral when calculated using infinite
5822 precision, and vice versa.
5826 <h3 id="Order_of_evaluation">Order of evaluation</h3>
5829 At package level, <a href="#Package_initialization">initialization dependencies</a>
5830 determine the evaluation order of individual initialization expressions in
5831 <a href="#Variable_declarations">variable declarations</a>.
5832 Otherwise, when evaluating the <a href="#Operands">operands</a> of an
5833 expression, assignment, or
5834 <a href="#Return_statements">return statement</a>,
5835 all function calls, method calls, and
5836 communication operations are evaluated in lexical left-to-right
5841 For example, in the (function-local) assignment
5844 y[f()], ok = g(h(), i()+x[j()], <-c), k()
5847 the function calls and communication happen in the order
5848 <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
5849 <code><-c</code>, <code>g()</code>, and <code>k()</code>.
5850 However, the order of those events compared to the evaluation
5851 and indexing of <code>x</code> and the evaluation
5852 of <code>y</code> is not specified.
5857 f := func() int { a++; return a }
5858 x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
5859 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
5860 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
5864 At package level, initialization dependencies override the left-to-right rule
5865 for individual initialization expressions, but not for operands within each
5870 var a, b, c = f() + v(), g(), sqr(u()) + v()
5872 func f() int { return c }
5873 func g() int { return a }
5874 func sqr(x int) int { return x*x }
5876 // functions u and v are independent of all other variables and functions
5880 The function calls happen in the order
5881 <code>u()</code>, <code>sqr()</code>, <code>v()</code>,
5882 <code>f()</code>, <code>v()</code>, and <code>g()</code>.
5886 Floating-point operations within a single expression are evaluated according to
5887 the associativity of the operators. Explicit parentheses affect the evaluation
5888 by overriding the default associativity.
5889 In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
5890 is performed before adding <code>x</code>.
5893 <h2 id="Statements">Statements</h2>
5896 Statements control execution.
5901 Declaration | LabeledStmt | SimpleStmt |
5902 GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
5903 FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
5906 SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
5909 <h3 id="Terminating_statements">Terminating statements</h3>
5912 A <i>terminating statement</i> interrupts the regular flow of control in
5913 a <a href="#Blocks">block</a>. The following statements are terminating:
5918 A <a href="#Return_statements">"return"</a> or
5919 <a href="#Goto_statements">"goto"</a> statement.
5920 <!-- ul below only for regular layout -->
5925 A call to the built-in function
5926 <a href="#Handling_panics"><code>panic</code></a>.
5927 <!-- ul below only for regular layout -->
5932 A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
5933 <!-- ul below only for regular layout -->
5938 An <a href="#If_statements">"if" statement</a> in which:
5940 <li>the "else" branch is present, and</li>
5941 <li>both branches are terminating statements.</li>
5946 A <a href="#For_statements">"for" statement</a> in which:
5948 <li>there are no "break" statements referring to the "for" statement, and</li>
5949 <li>the loop condition is absent, and</li>
5950 <li>the "for" statement does not use a range clause.</li>
5955 A <a href="#Switch_statements">"switch" statement</a> in which:
5957 <li>there are no "break" statements referring to the "switch" statement,</li>
5958 <li>there is a default case, and</li>
5959 <li>the statement lists in each case, including the default, end in a terminating
5960 statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
5966 A <a href="#Select_statements">"select" statement</a> in which:
5968 <li>there are no "break" statements referring to the "select" statement, and</li>
5969 <li>the statement lists in each case, including the default if present,
5970 end in a terminating statement.</li>
5975 A <a href="#Labeled_statements">labeled statement</a> labeling
5976 a terminating statement.
5981 All other statements are not terminating.
5985 A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
5986 is not empty and its final non-empty statement is terminating.
5990 <h3 id="Empty_statements">Empty statements</h3>
5993 The empty statement does nothing.
6001 <h3 id="Labeled_statements">Labeled statements</h3>
6004 A labeled statement may be the target of a <code>goto</code>,
6005 <code>break</code> or <code>continue</code> statement.
6009 LabeledStmt = Label ":" Statement .
6010 Label = identifier .
6014 Error: log.Panic("error encountered")
6018 <h3 id="Expression_statements">Expression statements</h3>
6021 With the exception of specific built-in functions,
6022 function and method <a href="#Calls">calls</a> and
6023 <a href="#Receive_operator">receive operations</a>
6024 can appear in statement context. Such statements may be parenthesized.
6028 ExpressionStmt = Expression .
6032 The following built-in functions are not permitted in statement context:
6036 append cap complex imag len make new real
6037 unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice unsafe.SliceData unsafe.String unsafe.StringData
6045 len("foo") // illegal if len is the built-in function
6049 <h3 id="Send_statements">Send statements</h3>
6052 A send statement sends a value on a channel.
6053 The channel expression's <a href="#Core_types">core type</a>
6054 must be a <a href="#Channel_types">channel</a>,
6055 the channel direction must permit send operations,
6056 and the type of the value to be sent must be <a href="#Assignability">assignable</a>
6057 to the channel's element type.
6061 SendStmt = Channel "<-" Expression .
6062 Channel = Expression .
6066 Both the channel and the value expression are evaluated before communication
6067 begins. Communication blocks until the send can proceed.
6068 A send on an unbuffered channel can proceed if a receiver is ready.
6069 A send on a buffered channel can proceed if there is room in the buffer.
6070 A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
6071 A send on a <code>nil</code> channel blocks forever.
6075 ch <- 3 // send value 3 to channel ch
6079 <h3 id="IncDec_statements">IncDec statements</h3>
6082 The "++" and "--" statements increment or decrement their operands
6083 by the untyped <a href="#Constants">constant</a> <code>1</code>.
6084 As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
6085 or a map index expression.
6089 IncDecStmt = Expression ( "++" | "--" ) .
6093 The following <a href="#Assignment_statements">assignment statements</a> are semantically
6097 <pre class="grammar">
6098 IncDec statement Assignment
6104 <h3 id="Assignment_statements">Assignment statements</h3>
6107 An <i>assignment</i> replaces the current value stored in a <a href="#Variables">variable</a>
6108 with a new value specified by an <a href="#Expressions">expression</a>.
6109 An assignment statement may assign a single value to a single variable, or multiple values to a
6110 matching number of variables.
6114 Assignment = ExpressionList assign_op ExpressionList .
6116 assign_op = [ add_op | mul_op ] "=" .
6120 Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
6121 a map index expression, or (for <code>=</code> assignments only) the
6122 <a href="#Blank_identifier">blank identifier</a>.
6123 Operands may be parenthesized.
6130 (k) = <-ch // same as: k = <-ch
6134 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
6135 <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
6136 is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
6137 <code>(y)</code> but evaluates <code>x</code>
6138 only once. The <i>op</i><code>=</code> construct is a single token.
6139 In assignment operations, both the left- and right-hand expression lists
6140 must contain exactly one single-valued expression, and the left-hand
6141 expression must not be the blank identifier.
6146 i &^= 1<<n
6150 A tuple assignment assigns the individual elements of a multi-valued
6151 operation to a list of variables. There are two forms. In the
6152 first, the right hand operand is a single multi-valued expression
6153 such as a function call, a <a href="#Channel_types">channel</a> or
6154 <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
6155 The number of operands on the left
6156 hand side must match the number of values. For instance, if
6157 <code>f</code> is a function returning two values,
6165 assigns the first value to <code>x</code> and the second to <code>y</code>.
6166 In the second form, the number of operands on the left must equal the number
6167 of expressions on the right, each of which must be single-valued, and the
6168 <i>n</i>th expression on the right is assigned to the <i>n</i>th
6169 operand on the left:
6173 one, two, three = '一', '二', '三'
6177 The <a href="#Blank_identifier">blank identifier</a> provides a way to
6178 ignore right-hand side values in an assignment:
6182 _ = x // evaluate x but ignore it
6183 x, _ = f() // evaluate f() but ignore second result value
6187 The assignment proceeds in two phases.
6188 First, the operands of <a href="#Index_expressions">index expressions</a>
6189 and <a href="#Address_operators">pointer indirections</a>
6190 (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
6191 on the left and the expressions on the right are all
6192 <a href="#Order_of_evaluation">evaluated in the usual order</a>.
6193 Second, the assignments are carried out in left-to-right order.
6197 a, b = b, a // exchange a and b
6201 i, x[i] = 1, 2 // set i = 1, x[0] = 2
6204 x[i], i = 2, 1 // set x[0] = 2, i = 1
6206 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
6208 x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
6210 type Point struct { x, y int }
6212 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
6216 for i, x[i] = range x { // set i, x[2] = 0, x[0]
6219 // after this loop, i == 0 and x is []int{3, 5, 3}
6223 In assignments, each value must be <a href="#Assignability">assignable</a>
6224 to the type of the operand to which it is assigned, with the following special cases:
6229 Any typed value may be assigned to the blank identifier.
6233 If an untyped constant
6234 is assigned to a variable of interface type or the blank identifier,
6235 the constant is first implicitly <a href="#Conversions">converted</a> to its
6236 <a href="#Constants">default type</a>.
6240 If an untyped boolean value is assigned to a variable of interface type or
6241 the blank identifier, it is first implicitly converted to type <code>bool</code>.
6245 <h3 id="If_statements">If statements</h3>
6248 "If" statements specify the conditional execution of two branches
6249 according to the value of a boolean expression. If the expression
6250 evaluates to true, the "if" branch is executed, otherwise, if
6251 present, the "else" branch is executed.
6255 IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
6265 The expression may be preceded by a simple statement, which
6266 executes before the expression is evaluated.
6270 if x := f(); x < y {
6272 } else if x > z {
6280 <h3 id="Switch_statements">Switch statements</h3>
6283 "Switch" statements provide multi-way execution.
6284 An expression or type is compared to the "cases"
6285 inside the "switch" to determine which branch
6290 SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
6294 There are two forms: expression switches and type switches.
6295 In an expression switch, the cases contain expressions that are compared
6296 against the value of the switch expression.
6297 In a type switch, the cases contain types that are compared against the
6298 type of a specially annotated switch expression.
6299 The switch expression is evaluated exactly once in a switch statement.
6302 <h4 id="Expression_switches">Expression switches</h4>
6305 In an expression switch,
6306 the switch expression is evaluated and
6307 the case expressions, which need not be constants,
6308 are evaluated left-to-right and top-to-bottom; the first one that equals the
6310 triggers execution of the statements of the associated case;
6311 the other cases are skipped.
6312 If no case matches and there is a "default" case,
6313 its statements are executed.
6314 There can be at most one default case and it may appear anywhere in the
6316 A missing switch expression is equivalent to the boolean value
6321 ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
6322 ExprCaseClause = ExprSwitchCase ":" StatementList .
6323 ExprSwitchCase = "case" ExpressionList | "default" .
6327 If the switch expression evaluates to an untyped constant, it is first implicitly
6328 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
6329 The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
6330 The switch expression type must be <a href="#Comparison_operators">comparable</a>.
6334 If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
6335 to the type of the switch expression.
6336 For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
6337 of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
6341 In other words, the switch expression is treated as if it were used to declare and
6342 initialize a temporary variable <code>t</code> without explicit type; it is that
6343 value of <code>t</code> against which each case expression <code>x</code> is tested
6348 In a case or default clause, the last non-empty statement
6349 may be a (possibly <a href="#Labeled_statements">labeled</a>)
6350 <a href="#Fallthrough_statements">"fallthrough" statement</a> to
6351 indicate that control should flow from the end of this clause to
6352 the first statement of the next clause.
6353 Otherwise control flows to the end of the "switch" statement.
6354 A "fallthrough" statement may appear as the last statement of all
6355 but the last clause of an expression switch.
6359 The switch expression may be preceded by a simple statement, which
6360 executes before the expression is evaluated.
6366 case 0, 1, 2, 3: s1()
6367 case 4, 5, 6, 7: s2()
6370 switch x := f(); { // missing switch expression means "true"
6371 case x < 0: return -x
6383 Implementation restriction: A compiler may disallow multiple case
6384 expressions evaluating to the same constant.
6385 For instance, the current compilers disallow duplicate integer,
6386 floating point, or string constants in case expressions.
6389 <h4 id="Type_switches">Type switches</h4>
6392 A type switch compares types rather than values. It is otherwise similar
6393 to an expression switch. It is marked by a special switch expression that
6394 has the form of a <a href="#Type_assertions">type assertion</a>
6395 using the keyword <code>type</code> rather than an actual type:
6405 Cases then match actual types <code>T</code> against the dynamic type of the
6406 expression <code>x</code>. As with type assertions, <code>x</code> must be of
6407 <a href="#Interface_types">interface type</a>, but not a
6408 <a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type
6409 <code>T</code> listed in a case must implement the type of <code>x</code>.
6410 The types listed in the cases of a type switch must all be
6411 <a href="#Type_identity">different</a>.
6415 TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
6416 TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
6417 TypeCaseClause = TypeSwitchCase ":" StatementList .
6418 TypeSwitchCase = "case" TypeList | "default" .
6422 The TypeSwitchGuard may include a
6423 <a href="#Short_variable_declarations">short variable declaration</a>.
6424 When that form is used, the variable is declared at the end of the
6425 TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
6426 In clauses with a case listing exactly one type, the variable
6427 has that type; otherwise, the variable has the type of the expression
6428 in the TypeSwitchGuard.
6432 Instead of a type, a case may use the predeclared identifier
6433 <a href="#Predeclared_identifiers"><code>nil</code></a>;
6434 that case is selected when the expression in the TypeSwitchGuard
6435 is a <code>nil</code> interface value.
6436 There may be at most one <code>nil</code> case.
6440 Given an expression <code>x</code> of type <code>interface{}</code>,
6441 the following type switch:
6445 switch i := x.(type) {
6447 printString("x is nil") // type of i is type of x (interface{})
6449 printInt(i) // type of i is int
6451 printFloat64(i) // type of i is float64
6452 case func(int) float64:
6453 printFunction(i) // type of i is func(int) float64
6455 printString("type is bool or string") // type of i is type of x (interface{})
6457 printString("don't know the type") // type of i is type of x (interface{})
6466 v := x // x is evaluated exactly once
6468 i := v // type of i is type of x (interface{})
6469 printString("x is nil")
6470 } else if i, isInt := v.(int); isInt {
6471 printInt(i) // type of i is int
6472 } else if i, isFloat64 := v.(float64); isFloat64 {
6473 printFloat64(i) // type of i is float64
6474 } else if i, isFunc := v.(func(int) float64); isFunc {
6475 printFunction(i) // type of i is func(int) float64
6477 _, isBool := v.(bool)
6478 _, isString := v.(string)
6479 if isBool || isString {
6480 i := v // type of i is type of x (interface{})
6481 printString("type is bool or string")
6483 i := v // type of i is type of x (interface{})
6484 printString("don't know the type")
6490 A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a>
6491 may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns
6492 out to duplicate another entry in the switch, the first matching case is chosen.
6496 func f[P any](x any) int {
6511 var v1 = f[string]("foo") // v1 == 0
6512 var v2 = f[byte]([]byte{}) // v2 == 2
6516 The type switch guard may be preceded by a simple statement, which
6517 executes before the guard is evaluated.
6521 The "fallthrough" statement is not permitted in a type switch.
6524 <h3 id="For_statements">For statements</h3>
6527 A "for" statement specifies repeated execution of a block. There are three forms:
6528 The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
6532 ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
6533 Condition = Expression .
6536 <h4 id="For_condition">For statements with single condition</h4>
6539 In its simplest form, a "for" statement specifies the repeated execution of
6540 a block as long as a boolean condition evaluates to true.
6541 The condition is evaluated before each iteration.
6542 If the condition is absent, it is equivalent to the boolean value
6552 <h4 id="For_clause">For statements with <code>for</code> clause</h4>
6555 A "for" statement with a ForClause is also controlled by its condition, but
6556 additionally it may specify an <i>init</i>
6557 and a <i>post</i> statement, such as an assignment,
6558 an increment or decrement statement. The init statement may be a
6559 <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
6560 Variables declared by the init statement are re-used in each iteration.
6564 ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
6565 InitStmt = SimpleStmt .
6566 PostStmt = SimpleStmt .
6570 for i := 0; i < 10; i++ {
6576 If non-empty, the init statement is executed once before evaluating the
6577 condition for the first iteration;
6578 the post statement is executed after each execution of the block (and
6579 only if the block was executed).
6580 Any element of the ForClause may be empty but the
6581 <a href="#Semicolons">semicolons</a> are
6582 required unless there is only a condition.
6583 If the condition is absent, it is equivalent to the boolean value
6588 for cond { S() } is the same as for ; cond ; { S() }
6589 for { S() } is the same as for true { S() }
6592 <h4 id="For_range">For statements with <code>range</code> clause</h4>
6595 A "for" statement with a "range" clause
6596 iterates through all entries of an array, slice, string or map,
6597 or values received on a channel. For each entry it assigns <i>iteration values</i>
6598 to corresponding <i>iteration variables</i> if present and then executes the block.
6602 RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
6606 The expression on the right in the "range" clause is called the <i>range expression</i>,
6607 its <a href="#Core_types">core type</a> must be
6608 an array, pointer to an array, slice, string, map, or channel permitting
6609 <a href="#Receive_operator">receive operations</a>.
6610 As with an assignment, if present the operands on the left must be
6611 <a href="#Address_operators">addressable</a> or map index expressions; they
6612 denote the iteration variables. If the range expression is a channel, at most
6613 one iteration variable is permitted, otherwise there may be up to two.
6614 If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
6615 the range clause is equivalent to the same clause without that identifier.
6619 The range expression <code>x</code> is evaluated once before beginning the loop,
6620 with one exception: if at most one iteration variable is present and
6621 <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
6622 the range expression is not evaluated.
6626 Function calls on the left are evaluated once per iteration.
6627 For each iteration, iteration values are produced as follows
6628 if the respective iteration variables are present:
6631 <pre class="grammar">
6632 Range expression 1st value 2nd value
6634 array or slice a [n]E, *[n]E, or []E index i int a[i] E
6635 string s string type index i int see below rune
6636 map m map[K]V key k K m[k] V
6637 channel c chan E, <-chan E element e E
6642 For an array, pointer to array, or slice value <code>a</code>, the index iteration
6643 values are produced in increasing order, starting at element index 0.
6644 If at most one iteration variable is present, the range loop produces
6645 iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
6646 or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
6650 For a string value, the "range" clause iterates over the Unicode code points
6651 in the string starting at byte index 0. On successive iterations, the index value will be the
6652 index of the first byte of successive UTF-8-encoded code points in the string,
6653 and the second value, of type <code>rune</code>, will be the value of
6654 the corresponding code point. If the iteration encounters an invalid
6655 UTF-8 sequence, the second value will be <code>0xFFFD</code>,
6656 the Unicode replacement character, and the next iteration will advance
6657 a single byte in the string.
6661 The iteration order over maps is not specified
6662 and is not guaranteed to be the same from one iteration to the next.
6663 If a map entry that has not yet been reached is removed during iteration,
6664 the corresponding iteration value will not be produced. If a map entry is
6665 created during iteration, that entry may be produced during the iteration or
6666 may be skipped. The choice may vary for each entry created and from one
6667 iteration to the next.
6668 If the map is <code>nil</code>, the number of iterations is 0.
6672 For channels, the iteration values produced are the successive values sent on
6673 the channel until the channel is <a href="#Close">closed</a>. If the channel
6674 is <code>nil</code>, the range expression blocks forever.
6679 The iteration values are assigned to the respective
6680 iteration variables as in an <a href="#Assignment_statements">assignment statement</a>.
6684 The iteration variables may be declared by the "range" clause using a form of
6685 <a href="#Short_variable_declarations">short variable declaration</a>
6687 In this case their types are set to the types of the respective iteration values
6688 and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
6689 statement; they are re-used in each iteration.
6690 If the iteration variables are declared outside the "for" statement,
6691 after execution their values will be those of the last iteration.
6695 var testdata *struct {
6698 for i, _ := range testdata.a {
6699 // testdata.a is never evaluated; len(testdata.a) is constant
6700 // i ranges from 0 to 6
6705 for i, s := range a {
6707 // type of s is string
6713 var val interface{} // element type of m is assignable to val
6714 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
6715 for key, val = range m {
6718 // key == last map key encountered in iteration
6721 var ch chan Work = producer()
6731 <h3 id="Go_statements">Go statements</h3>
6734 A "go" statement starts the execution of a function call
6735 as an independent concurrent thread of control, or <i>goroutine</i>,
6736 within the same address space.
6740 GoStmt = "go" Expression .
6744 The expression must be a function or method call; it cannot be parenthesized.
6745 Calls of built-in functions are restricted as for
6746 <a href="#Expression_statements">expression statements</a>.
6750 The function value and parameters are
6751 <a href="#Calls">evaluated as usual</a>
6752 in the calling goroutine, but
6753 unlike with a regular call, program execution does not wait
6754 for the invoked function to complete.
6755 Instead, the function begins executing independently
6757 When the function terminates, its goroutine also terminates.
6758 If the function has any return values, they are discarded when the
6764 go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
6768 <h3 id="Select_statements">Select statements</h3>
6771 A "select" statement chooses which of a set of possible
6772 <a href="#Send_statements">send</a> or
6773 <a href="#Receive_operator">receive</a>
6774 operations will proceed.
6775 It looks similar to a
6776 <a href="#Switch_statements">"switch"</a> statement but with the
6777 cases all referring to communication operations.
6781 SelectStmt = "select" "{" { CommClause } "}" .
6782 CommClause = CommCase ":" StatementList .
6783 CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
6784 RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
6785 RecvExpr = Expression .
6789 A case with a RecvStmt may assign the result of a RecvExpr to one or
6790 two variables, which may be declared using a
6791 <a href="#Short_variable_declarations">short variable declaration</a>.
6792 The RecvExpr must be a (possibly parenthesized) receive operation.
6793 There can be at most one default case and it may appear anywhere
6794 in the list of cases.
6798 Execution of a "select" statement proceeds in several steps:
6803 For all the cases in the statement, the channel operands of receive operations
6804 and the channel and right-hand-side expressions of send statements are
6805 evaluated exactly once, in source order, upon entering the "select" statement.
6806 The result is a set of channels to receive from or send to,
6807 and the corresponding values to send.
6808 Any side effects in that evaluation will occur irrespective of which (if any)
6809 communication operation is selected to proceed.
6810 Expressions on the left-hand side of a RecvStmt with a short variable declaration
6811 or assignment are not yet evaluated.
6815 If one or more of the communications can proceed,
6816 a single one that can proceed is chosen via a uniform pseudo-random selection.
6817 Otherwise, if there is a default case, that case is chosen.
6818 If there is no default case, the "select" statement blocks until
6819 at least one of the communications can proceed.
6823 Unless the selected case is the default case, the respective communication
6824 operation is executed.
6828 If the selected case is a RecvStmt with a short variable declaration or
6829 an assignment, the left-hand side expressions are evaluated and the
6830 received value (or values) are assigned.
6834 The statement list of the selected case is executed.
6839 Since communication on <code>nil</code> channels can never proceed,
6840 a select with only <code>nil</code> channels and no default case blocks forever.
6845 var c, c1, c2, c3, c4 chan int
6849 print("received ", i1, " from c1\n")
6851 print("sent ", i2, " to c2\n")
6852 case i3, ok := (<-c3): // same as: i3, ok := <-c3
6854 print("received ", i3, " from c3\n")
6856 print("c3 is closed\n")
6858 case a[f()] = <-c4:
6860 // case t := <-c4
6863 print("no communication\n")
6866 for { // send random sequence of bits to c
6868 case c <- 0: // note: no statement, no fallthrough, no folding of cases
6873 select {} // block forever
6877 <h3 id="Return_statements">Return statements</h3>
6880 A "return" statement in a function <code>F</code> terminates the execution
6881 of <code>F</code>, and optionally provides one or more result values.
6882 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6883 are executed before <code>F</code> returns to its caller.
6887 ReturnStmt = "return" [ ExpressionList ] .
6891 In a function without a result type, a "return" statement must not
6892 specify any result values.
6901 There are three ways to return values from a function with a result
6906 <li>The return value or values may be explicitly listed
6907 in the "return" statement. Each expression must be single-valued
6908 and <a href="#Assignability">assignable</a>
6909 to the corresponding element of the function's result type.
6911 func simpleF() int {
6915 func complexF1() (re float64, im float64) {
6920 <li>The expression list in the "return" statement may be a single
6921 call to a multi-valued function. The effect is as if each value
6922 returned from that function were assigned to a temporary
6923 variable with the type of the respective value, followed by a
6924 "return" statement listing these variables, at which point the
6925 rules of the previous case apply.
6927 func complexF2() (re float64, im float64) {
6932 <li>The expression list may be empty if the function's result
6933 type specifies names for its <a href="#Function_types">result parameters</a>.
6934 The result parameters act as ordinary local variables
6935 and the function may assign values to them as necessary.
6936 The "return" statement returns the values of these variables.
6938 func complexF3() (re float64, im float64) {
6944 func (devnull) Write(p []byte) (n int, _ error) {
6953 Regardless of how they are declared, all the result values are initialized to
6954 the <a href="#The_zero_value">zero values</a> for their type upon entry to the
6955 function. A "return" statement that specifies results sets the result parameters before
6956 any deferred functions are executed.
6960 Implementation restriction: A compiler may disallow an empty expression list
6961 in a "return" statement if a different entity (constant, type, or variable)
6962 with the same name as a result parameter is in
6963 <a href="#Declarations_and_scope">scope</a> at the place of the return.
6967 func f(n int) (res int, err error) {
6968 if _, err := f(n-1); err != nil {
6969 return // invalid return statement: err is shadowed
6975 <h3 id="Break_statements">Break statements</h3>
6978 A "break" statement terminates execution of the innermost
6979 <a href="#For_statements">"for"</a>,
6980 <a href="#Switch_statements">"switch"</a>, or
6981 <a href="#Select_statements">"select"</a> statement
6982 within the same function.
6986 BreakStmt = "break" [ Label ] .
6990 If there is a label, it must be that of an enclosing
6991 "for", "switch", or "select" statement,
6992 and that is the one whose execution terminates.
6997 for i = 0; i < n; i++ {
6998 for j = 0; j < m; j++ {
7011 <h3 id="Continue_statements">Continue statements</h3>
7014 A "continue" statement begins the next iteration of the
7015 innermost enclosing <a href="#For_statements">"for" loop</a>
7016 by advancing control to the end of the loop block.
7017 The "for" loop must be within the same function.
7021 ContinueStmt = "continue" [ Label ] .
7025 If there is a label, it must be that of an enclosing
7026 "for" statement, and that is the one whose execution
7032 for y, row := range rows {
7033 for x, data := range row {
7034 if data == endOfRow {
7037 row[x] = data + bias(x, y)
7042 <h3 id="Goto_statements">Goto statements</h3>
7045 A "goto" statement transfers control to the statement with the corresponding label
7046 within the same function.
7050 GotoStmt = "goto" Label .
7058 Executing the "goto" statement must not cause any variables to come into
7059 <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
7060 For instance, this example:
7070 is erroneous because the jump to label <code>L</code> skips
7071 the creation of <code>v</code>.
7075 A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
7076 For instance, this example:
7093 is erroneous because the label <code>L1</code> is inside
7094 the "for" statement's block but the <code>goto</code> is not.
7097 <h3 id="Fallthrough_statements">Fallthrough statements</h3>
7100 A "fallthrough" statement transfers control to the first statement of the
7101 next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
7102 It may be used only as the final non-empty statement in such a clause.
7106 FallthroughStmt = "fallthrough" .
7110 <h3 id="Defer_statements">Defer statements</h3>
7113 A "defer" statement invokes a function whose execution is deferred
7114 to the moment the surrounding function returns, either because the
7115 surrounding function executed a <a href="#Return_statements">return statement</a>,
7116 reached the end of its <a href="#Function_declarations">function body</a>,
7117 or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
7121 DeferStmt = "defer" Expression .
7125 The expression must be a function or method call; it cannot be parenthesized.
7126 Calls of built-in functions are restricted as for
7127 <a href="#Expression_statements">expression statements</a>.
7131 Each time a "defer" statement
7132 executes, the function value and parameters to the call are
7133 <a href="#Calls">evaluated as usual</a>
7134 and saved anew but the actual function is not invoked.
7135 Instead, deferred functions are invoked immediately before
7136 the surrounding function returns, in the reverse order
7137 they were deferred. That is, if the surrounding function
7138 returns through an explicit <a href="#Return_statements">return statement</a>,
7139 deferred functions are executed <i>after</i> any result parameters are set
7140 by that return statement but <i>before</i> the function returns to its caller.
7141 If a deferred function value evaluates
7142 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
7143 when the function is invoked, not when the "defer" statement is executed.
7147 For instance, if the deferred function is
7148 a <a href="#Function_literals">function literal</a> and the surrounding
7149 function has <a href="#Function_types">named result parameters</a> that
7150 are in scope within the literal, the deferred function may access and modify
7151 the result parameters before they are returned.
7152 If the deferred function has any return values, they are discarded when
7153 the function completes.
7154 (See also the section on <a href="#Handling_panics">handling panics</a>.)
7159 defer unlock(l) // unlocking happens before surrounding function returns
7161 // prints 3 2 1 0 before surrounding function returns
7162 for i := 0; i <= 3; i++ {
7167 func f() (result int) {
7169 // result is accessed after it was set to 6 by the return statement
7176 <h2 id="Built-in_functions">Built-in functions</h2>
7179 Built-in functions are
7180 <a href="#Predeclared_identifiers">predeclared</a>.
7181 They are called like any other function but some of them
7182 accept a type instead of an expression as the first argument.
7186 The built-in functions do not have standard Go types,
7187 so they can only appear in <a href="#Calls">call expressions</a>;
7188 they cannot be used as function values.
7192 <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
7195 The built-in functions <code>append</code> and <code>copy</code> assist in
7196 common slice operations.
7197 For both functions, the result is independent of whether the memory referenced
7198 by the arguments overlaps.
7202 The <a href="#Function_types">variadic</a> function <code>append</code>
7203 appends zero or more values <code>x</code> to a slice <code>s</code>
7204 and returns the resulting slice of the same type as <code>s</code>.
7205 The <a href="#Core_types">core type</a> of <code>s</code> must be a slice
7206 of type <code>[]E</code>.
7207 The values <code>x</code> are passed to a parameter of type <code>...E</code>
7208 and the respective <a href="#Passing_arguments_to_..._parameters">parameter
7209 passing rules</a> apply.
7210 As a special case, if the core type of <code>s</code> is <code>[]byte</code>,
7211 <code>append</code> also accepts a second argument with core type
7212 <a href="#Core_types"><code>bytestring</code></a> followed by <code>...</code>.
7213 This form appends the bytes of the byte slice or string.
7216 <pre class="grammar">
7217 append(s S, x ...E) S // core type of S is []E
7221 If the capacity of <code>s</code> is not large enough to fit the additional
7222 values, <code>append</code> <a href="#Allocation">allocates</a> a new, sufficiently large underlying
7223 array that fits both the existing slice elements and the additional values.
7224 Otherwise, <code>append</code> re-uses the underlying array.
7229 s1 := append(s0, 2) // append a single element s1 is []int{0, 0, 2}
7230 s2 := append(s1, 3, 5, 7) // append multiple elements s2 is []int{0, 0, 2, 3, 5, 7}
7231 s3 := append(s2, s0...) // append a slice s3 is []int{0, 0, 2, 3, 5, 7, 0, 0}
7232 s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 is []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
7235 t = append(t, 42, 3.1415, "foo") // t is []interface{}{42, 3.1415, "foo"}
7238 b = append(b, "bar"...) // append string contents b is []byte{'b', 'a', 'r' }
7242 The function <code>copy</code> copies slice elements from
7243 a source <code>src</code> to a destination <code>dst</code> and returns the
7244 number of elements copied.
7245 The <a href="#Core_types">core types</a> of both arguments must be slices
7246 with <a href="#Type_identity">identical</a> element type.
7247 The number of elements copied is the minimum of
7248 <code>len(src)</code> and <code>len(dst)</code>.
7249 As a special case, if the destination's core type is <code>[]byte</code>,
7250 <code>copy</code> also accepts a source argument with core type
7251 </a> <a href="#Core_types"><code>bytestring</code></a>.
7252 This form copies the bytes from the byte slice or string into the byte slice.
7255 <pre class="grammar">
7256 copy(dst, src []T) int
7257 copy(dst []byte, src string) int
7265 var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
7266 var s = make([]int, 6)
7267 var b = make([]byte, 5)
7268 n1 := copy(s, a[0:]) // n1 == 6, s is []int{0, 1, 2, 3, 4, 5}
7269 n2 := copy(s, s[2:]) // n2 == 4, s is []int{2, 3, 4, 5, 4, 5}
7270 n3 := copy(b, "Hello, World!") // n3 == 5, b is []byte("Hello")
7274 <h3 id="Clear">Clear</h3>
7277 The built-in function <code>clear</code> takes an argument of <a href="#Map_types">map</a>,
7278 <a href="#Slice_types">slice</a>, or <a href="#Type_parameter_declarations">type parameter</a> type,
7279 and deletes or zeroes out all elements.
7282 <pre class="grammar">
7283 Call Argument type Result
7285 clear(m) map[K]T deletes all entries, resulting in an
7286 empty map (len(m) == 0)
7288 clear(s) []T sets all elements up to the length of
7289 <code>s</code> to the zero value of T
7291 clear(t) type parameter see below
7295 If the argument type is a <a href="#Type_parameter_declarations">type parameter</a>,
7296 all types in its type set must be maps or slices, and <code>clear</code>
7297 performs the operation corresponding to the actual type argument.
7301 If the map or slice is <code>nil</code>, <code>clear</code> is a no-op.
7305 <h3 id="Close">Close</h3>
7308 For an argument <code>ch</code> with a <a href="#Core_types">core type</a>
7309 that is a <a href="#Channel_types">channel</a>, the built-in function <code>close</code>
7310 records that no more values will be sent on the channel.
7311 It is an error if <code>ch</code> is a receive-only channel.
7312 Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
7313 Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
7314 After calling <code>close</code>, and after any previously
7315 sent values have been received, receive operations will return
7316 the zero value for the channel's type without blocking.
7317 The multi-valued <a href="#Receive_operator">receive operation</a>
7318 returns a received value along with an indication of whether the channel is closed.
7322 <h3 id="Complex_numbers">Manipulating complex numbers</h3>
7325 Three functions assemble and disassemble complex numbers.
7326 The built-in function <code>complex</code> constructs a complex
7327 value from a floating-point real and imaginary part, while
7328 <code>real</code> and <code>imag</code>
7329 extract the real and imaginary parts of a complex value.
7332 <pre class="grammar">
7333 complex(realPart, imaginaryPart floatT) complexT
7334 real(complexT) floatT
7335 imag(complexT) floatT
7339 The type of the arguments and return value correspond.
7340 For <code>complex</code>, the two arguments must be of the same
7341 <a href="#Numeric_types">floating-point type</a> and the return type is the
7342 <a href="#Numeric_types">complex type</a>
7343 with the corresponding floating-point constituents:
7344 <code>complex64</code> for <code>float32</code> arguments, and
7345 <code>complex128</code> for <code>float64</code> arguments.
7346 If one of the arguments evaluates to an untyped constant, it is first implicitly
7347 <a href="#Conversions">converted</a> to the type of the other argument.
7348 If both arguments evaluate to untyped constants, they must be non-complex
7349 numbers or their imaginary parts must be zero, and the return value of
7350 the function is an untyped complex constant.
7354 For <code>real</code> and <code>imag</code>, the argument must be
7355 of complex type, and the return type is the corresponding floating-point
7356 type: <code>float32</code> for a <code>complex64</code> argument, and
7357 <code>float64</code> for a <code>complex128</code> argument.
7358 If the argument evaluates to an untyped constant, it must be a number,
7359 and the return value of the function is an untyped floating-point constant.
7363 The <code>real</code> and <code>imag</code> functions together form the inverse of
7364 <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
7365 <code>z == Z(complex(real(z), imag(z)))</code>.
7369 If the operands of these functions are all constants, the return
7370 value is a constant.
7374 var a = complex(2, -2) // complex128
7375 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
7376 x := float32(math.Cos(math.Pi/2)) // float32
7377 var c64 = complex(5, -x) // complex64
7378 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
7379 _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
7380 var rl = real(c64) // float32
7381 var im = imag(a) // float64
7382 const c = imag(b) // untyped constant -1.4
7383 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
7387 Arguments of type parameter type are not permitted.
7391 <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
7394 The built-in function <code>delete</code> removes the element with key
7395 <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
7396 value <code>k</code> must be <a href="#Assignability">assignable</a>
7397 to the key type of <code>m</code>.
7400 <pre class="grammar">
7401 delete(m, k) // remove element m[k] from map m
7405 If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>,
7406 all types in that type set must be maps, and they must all have identical key types.
7410 If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
7411 does not exist, <code>delete</code> is a no-op.
7415 <h3 id="Length_and_capacity">Length and capacity</h3>
7418 The built-in functions <code>len</code> and <code>cap</code> take arguments
7419 of various types and return a result of type <code>int</code>.
7420 The implementation guarantees that the result always fits into an <code>int</code>.
7423 <pre class="grammar">
7424 Call Argument type Result
7426 len(s) string type string length in bytes
7427 [n]T, *[n]T array length (== n)
7429 map[K]T map length (number of defined keys)
7430 chan T number of elements queued in channel buffer
7431 type parameter see below
7433 cap(s) [n]T, *[n]T array length (== n)
7435 chan T channel buffer capacity
7436 type parameter see below
7440 If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>,
7441 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
7442 each type in <code>P</code>'s type set.
7443 The result is the length (or capacity, respectively) of the argument whose type
7444 corresponds to the type argument with which <code>P</code> was
7445 <a href="#Instantiations">instantiated</a>.
7449 The capacity of a slice is the number of elements for which there is
7450 space allocated in the underlying array.
7451 At any time the following relationship holds:
7455 0 <= len(s) <= cap(s)
7459 The length of a <code>nil</code> slice, map or channel is 0.
7460 The capacity of a <code>nil</code> slice or channel is 0.
7464 The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
7465 <code>s</code> is a string constant. The expressions <code>len(s)</code> and
7466 <code>cap(s)</code> are constants if the type of <code>s</code> is an array
7467 or pointer to an array and the expression <code>s</code> does not contain
7468 <a href="#Receive_operator">channel receives</a> or (non-constant)
7469 <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
7470 Otherwise, invocations of <code>len</code> and <code>cap</code> are not
7471 constant and <code>s</code> is evaluated.
7476 c1 = imag(2i) // imag(2i) = 2.0 is a constant
7477 c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
7478 c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
7479 c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
7480 c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
7486 <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
7489 The built-in function <code>make</code> takes a type <code>T</code>,
7490 optionally followed by a type-specific list of expressions.
7491 The <a href="#Core_types">core type</a> of <code>T</code> must
7492 be a slice, map or channel.
7493 It returns a value of type <code>T</code> (not <code>*T</code>).
7494 The memory is initialized as described in the section on
7495 <a href="#The_zero_value">initial values</a>.
7498 <pre class="grammar">
7499 Call Core type Result
7501 make(T, n) slice slice of type T with length n and capacity n
7502 make(T, n, m) slice slice of type T with length n and capacity m
7504 make(T) map map of type T
7505 make(T, n) map map of type T with initial space for approximately n elements
7507 make(T) channel unbuffered channel of type T
7508 make(T, n) channel buffered channel of type T, buffer size n
7512 Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>,
7513 have a <a href="#Interface_types">type set</a> containing only integer types,
7514 or be an untyped <a href="#Constants">constant</a>.
7515 A constant size argument must be non-negative and <a href="#Representability">representable</a>
7516 by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
7517 If both <code>n</code> and <code>m</code> are provided and are constant, then
7518 <code>n</code> must be no larger than <code>m</code>.
7519 For slices and channels, if <code>n</code> is negative or larger than <code>m</code> at run time,
7520 a <a href="#Run_time_panics">run-time panic</a> occurs.
7524 s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
7525 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
7526 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
7527 s := make([]int, 10, 0) // illegal: len(s) > cap(s)
7528 c := make(chan int, 10) // channel with a buffer size of 10
7529 m := make(map[string]int, 100) // map with initial space for approximately 100 elements
7533 Calling <code>make</code> with a map type and size hint <code>n</code> will
7534 create a map with initial space to hold <code>n</code> map elements.
7535 The precise behavior is implementation-dependent.
7539 <h3 id="Min_and_max">Min and max</h3>
7542 The built-in functions <code>min</code> and <code>max</code> compute the
7543 smallest—or largest, respectively—value of a fixed number of
7544 arguments of <a href="#Comparison_operators">ordered types</a>.
7545 There must be at least one argument.
7549 The same type rules as for <a href="#Operators">operators</a> apply:
7550 for <a href="#Comparison_operators">ordered</a> arguments <code>x</code> and
7551 <code>y</code>, <code>min(x, y)</code> is valid if <code>x + y</code> is valid,
7552 and the type of <code>min(x, y)</code> is the type of <code>x + y</code>
7553 (and similarly for <code>max</code>).
7554 If all arguments are constant, the result is constant.
7559 m := min(x) // m == x
7560 m := min(x, y) // m is the smaller of x and y
7561 m := max(x, y, 10) // m is the larger of x and y but at least 10
7562 c := max(1, 2.0, 10) // c == 10.0 (floating-point kind)
7563 f := max(0, float32(x)) // type of f is float32
7565 _ = min(s...) // invalid: slice arguments are not permitted
7566 t := max("", "foo", "bar") // t == "foo" (string kind)
7570 For numeric arguments, assuming all NaNs are equal, <code>min</code> and <code>max</code> are
7571 commutative and associative:
7575 min(x, y) == min(y, x)
7576 min(x, y, z) == min(min(x, y), z) == min(x, min(y, z))
7580 For floating-point arguments negative zero, NaN, and infinity the following rules apply:
7584 x y min(x, y) max(x, y)
7586 -0.0 0.0 -0.0 0.0 // negative zero is smaller than (non-negative) zero
7587 -Inf y -Inf y // negative infinity is smaller than any other number
7588 +Inf y y +Inf // positive infinity is larger than any other number
7589 NaN y NaN NaN // if any argument is a NaN, the result is a NaN
7593 For string arguments the result for <code>min</code> is the first argument
7594 with the smallest (or for <code>max</code>, largest) value,
7595 compared lexically byte-wise:
7599 min(x, y) == if x <= y then x else y
7600 min(x, y, z) == min(min(x, y), z)
7603 <h3 id="Allocation">Allocation</h3>
7606 The built-in function <code>new</code> takes a type <code>T</code>,
7607 allocates storage for a <a href="#Variables">variable</a> of that type
7608 at run time, and returns a value of type <code>*T</code>
7609 <a href="#Pointer_types">pointing</a> to it.
7610 The variable is initialized as described in the section on
7611 <a href="#The_zero_value">initial values</a>.
7614 <pre class="grammar">
7623 type S struct { a int; b float64 }
7628 allocates storage for a variable of type <code>S</code>,
7629 initializes it (<code>a=0</code>, <code>b=0.0</code>),
7630 and returns a value of type <code>*S</code> containing the address
7635 <h3 id="Handling_panics">Handling panics</h3>
7637 <p> Two built-in functions, <code>panic</code> and <code>recover</code>,
7638 assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
7639 and program-defined error conditions.
7642 <pre class="grammar">
7643 func panic(interface{})
7644 func recover() interface{}
7648 While executing a function <code>F</code>,
7649 an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
7650 terminates the execution of <code>F</code>.
7651 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
7652 are then executed as usual.
7653 Next, any deferred functions run by <code>F</code>'s caller are run,
7654 and so on up to any deferred by the top-level function in the executing goroutine.
7655 At that point, the program is terminated and the error
7656 condition is reported, including the value of the argument to <code>panic</code>.
7657 This termination sequence is called <i>panicking</i>.
7662 panic("unreachable")
7663 panic(Error("cannot parse"))
7667 The <code>recover</code> function allows a program to manage behavior
7668 of a panicking goroutine.
7669 Suppose a function <code>G</code> defers a function <code>D</code> that calls
7670 <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
7672 When the running of deferred functions reaches <code>D</code>,
7673 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>.
7674 If <code>D</code> returns normally, without starting a new
7675 <code>panic</code>, the panicking sequence stops. In that case,
7676 the state of functions called between <code>G</code> and the call to <code>panic</code>
7677 is discarded, and normal execution resumes.
7678 Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
7679 execution terminates by returning to its caller.
7683 The return value of <code>recover</code> is <code>nil</code> when the
7684 goroutine is not panicking or <code>recover</code> was not called directly by a deferred function.
7685 Conversely, if a goroutine is panicking and <code>recover</code> was called directly by a deferred function,
7686 the return value of <code>recover</code> is guaranteed not to be <code>nil</code>.
7687 To ensure this, calling <code>panic</code> with a <code>nil</code> interface value (or an untyped <code>nil</code>)
7688 causes a <a href="#Run_time_panics">run-time panic</a>.
7692 The <code>protect</code> function in the example below invokes
7693 the function argument <code>g</code> and protects callers from
7694 run-time panics raised by <code>g</code>.
7698 func protect(g func()) {
7700 log.Println("done") // Println executes normally even if there is a panic
7701 if x := recover(); x != nil {
7702 log.Printf("run time panic: %v", x)
7705 log.Println("start")
7711 <h3 id="Bootstrapping">Bootstrapping</h3>
7714 Current implementations provide several built-in functions useful during
7715 bootstrapping. These functions are documented for completeness but are not
7716 guaranteed to stay in the language. They do not return a result.
7719 <pre class="grammar">
7722 print prints all arguments; formatting of arguments is implementation-specific
7723 println like print but prints spaces between arguments and a newline at the end
7727 Implementation restriction: <code>print</code> and <code>println</code> need not
7728 accept arbitrary argument types, but printing of boolean, numeric, and string
7729 <a href="#Types">types</a> must be supported.
7733 <h2 id="Packages">Packages</h2>
7736 Go programs are constructed by linking together <i>packages</i>.
7737 A package in turn is constructed from one or more source files
7738 that together declare constants, types, variables and functions
7739 belonging to the package and which are accessible in all files
7740 of the same package. Those elements may be
7741 <a href="#Exported_identifiers">exported</a> and used in another package.
7744 <h3 id="Source_file_organization">Source file organization</h3>
7747 Each source file consists of a package clause defining the package
7748 to which it belongs, followed by a possibly empty set of import
7749 declarations that declare packages whose contents it wishes to use,
7750 followed by a possibly empty set of declarations of functions,
7751 types, variables, and constants.
7755 SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
7758 <h3 id="Package_clause">Package clause</h3>
7761 A package clause begins each source file and defines the package
7762 to which the file belongs.
7766 PackageClause = "package" PackageName .
7767 PackageName = identifier .
7771 The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
7779 A set of files sharing the same PackageName form the implementation of a package.
7780 An implementation may require that all source files for a package inhabit the same directory.
7783 <h3 id="Import_declarations">Import declarations</h3>
7786 An import declaration states that the source file containing the declaration
7787 depends on functionality of the <i>imported</i> package
7788 (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
7789 and enables access to <a href="#Exported_identifiers">exported</a> identifiers
7791 The import names an identifier (PackageName) to be used for access and an ImportPath
7792 that specifies the package to be imported.
7796 ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
7797 ImportSpec = [ "." | PackageName ] ImportPath .
7798 ImportPath = string_lit .
7802 The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
7803 to access exported identifiers of the package within the importing source file.
7804 It is declared in the <a href="#Blocks">file block</a>.
7805 If the PackageName is omitted, it defaults to the identifier specified in the
7806 <a href="#Package_clause">package clause</a> of the imported package.
7807 If an explicit period (<code>.</code>) appears instead of a name, all the
7808 package's exported identifiers declared in that package's
7809 <a href="#Blocks">package block</a> will be declared in the importing source
7810 file's file block and must be accessed without a qualifier.
7814 The interpretation of the ImportPath is implementation-dependent but
7815 it is typically a substring of the full file name of the compiled
7816 package and may be relative to a repository of installed packages.
7820 Implementation restriction: A compiler may restrict ImportPaths to
7821 non-empty strings using only characters belonging to
7822 <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
7823 L, M, N, P, and S general categories (the Graphic characters without
7824 spaces) and may also exclude the characters
7825 <code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
7826 and the Unicode replacement character U+FFFD.
7830 Consider a compiled a package containing the package clause
7831 <code>package math</code>, which exports function <code>Sin</code>, and
7832 installed the compiled package in the file identified by
7833 <code>"lib/math"</code>.
7834 This table illustrates how <code>Sin</code> is accessed in files
7835 that import the package after the
7836 various types of import declaration.
7839 <pre class="grammar">
7840 Import declaration Local name of Sin
7842 import "lib/math" math.Sin
7843 import m "lib/math" m.Sin
7844 import . "lib/math" Sin
7848 An import declaration declares a dependency relation between
7849 the importing and imported package.
7850 It is illegal for a package to import itself, directly or indirectly,
7851 or to directly import a package without
7852 referring to any of its exported identifiers. To import a package solely for
7853 its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
7854 identifier as explicit package name:
7862 <h3 id="An_example_package">An example package</h3>
7865 Here is a complete Go package that implements a concurrent prime sieve.
7873 // Send the sequence 2, 3, 4, … to channel 'ch'.
7874 func generate(ch chan<- int) {
7876 ch <- i // Send 'i' to channel 'ch'.
7880 // Copy the values from channel 'src' to channel 'dst',
7881 // removing those divisible by 'prime'.
7882 func filter(src <-chan int, dst chan<- int, prime int) {
7883 for i := range src { // Loop over values received from 'src'.
7885 dst <- i // Send 'i' to channel 'dst'.
7890 // The prime sieve: Daisy-chain filter processes together.
7892 ch := make(chan int) // Create a new channel.
7893 go generate(ch) // Start generate() as a subprocess.
7896 fmt.Print(prime, "\n")
7897 ch1 := make(chan int)
7898 go filter(ch, ch1, prime)
7908 <h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
7910 <h3 id="The_zero_value">The zero value</h3>
7912 When storage is allocated for a <a href="#Variables">variable</a>,
7913 either through a declaration or a call of <code>new</code>, or when
7914 a new value is created, either through a composite literal or a call
7915 of <code>make</code>,
7916 and no explicit initialization is provided, the variable or value is
7917 given a default value. Each element of such a variable or value is
7918 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
7919 <code>0</code> for numeric types, <code>""</code>
7920 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
7921 This initialization is done recursively, so for instance each element of an
7922 array of structs will have its fields zeroed if no value is specified.
7925 These two simple declarations are equivalent:
7938 type T struct { i int; f float64; next *T }
7943 the following holds:
7953 The same would also be true after
7960 <h3 id="Package_initialization">Package initialization</h3>
7963 Within a package, package-level variable initialization proceeds stepwise,
7964 with each step selecting the variable earliest in <i>declaration order</i>
7965 which has no dependencies on uninitialized variables.
7969 More precisely, a package-level variable is considered <i>ready for
7970 initialization</i> if it is not yet initialized and either has
7971 no <a href="#Variable_declarations">initialization expression</a> or
7972 its initialization expression has no <i>dependencies</i> on uninitialized variables.
7973 Initialization proceeds by repeatedly initializing the next package-level
7974 variable that is earliest in declaration order and ready for initialization,
7975 until there are no variables ready for initialization.
7979 If any variables are still uninitialized when this
7980 process ends, those variables are part of one or more initialization cycles,
7981 and the program is not valid.
7985 Multiple variables on the left-hand side of a variable declaration initialized
7986 by single (multi-valued) expression on the right-hand side are initialized
7987 together: If any of the variables on the left-hand side is initialized, all
7988 those variables are initialized in the same step.
7993 var a, b = f() // a and b are initialized together, before x is initialized
7997 For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
7998 variables are treated like any other variables in declarations.
8002 The declaration order of variables declared in multiple files is determined
8003 by the order in which the files are presented to the compiler: Variables
8004 declared in the first file are declared before any of the variables declared
8005 in the second file, and so on.
8009 Dependency analysis does not rely on the actual values of the
8010 variables, only on lexical <i>references</i> to them in the source,
8011 analyzed transitively. For instance, if a variable <code>x</code>'s
8012 initialization expression refers to a function whose body refers to
8013 variable <code>y</code> then <code>x</code> depends on <code>y</code>.
8019 A reference to a variable or function is an identifier denoting that
8020 variable or function.
8024 A reference to a method <code>m</code> is a
8025 <a href="#Method_values">method value</a> or
8026 <a href="#Method_expressions">method expression</a> of the form
8027 <code>t.m</code>, where the (static) type of <code>t</code> is
8028 not an interface type, and the method <code>m</code> is in the
8029 <a href="#Method_sets">method set</a> of <code>t</code>.
8030 It is immaterial whether the resulting function value
8031 <code>t.m</code> is invoked.
8035 A variable, function, or method <code>x</code> depends on a variable
8036 <code>y</code> if <code>x</code>'s initialization expression or body
8037 (for functions and methods) contains a reference to <code>y</code>
8038 or to a function or method that depends on <code>y</code>.
8043 For example, given the declarations
8051 d = 3 // == 5 after initialization has finished
8061 the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
8062 Note that the order of subexpressions in initialization expressions is irrelevant:
8063 <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
8064 order in this example.
8068 Dependency analysis is performed per package; only references referring
8069 to variables, functions, and (non-interface) methods declared in the current
8070 package are considered. If other, hidden, data dependencies exists between
8071 variables, the initialization order between those variables is unspecified.
8075 For instance, given the declarations
8079 var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
8080 var _ = sideEffect() // unrelated to x, a, or b
8084 type I interface { ab() []int }
8086 func (T) ab() []int { return []int{a, b} }
8090 the variable <code>a</code> will be initialized after <code>b</code> but
8091 whether <code>x</code> is initialized before <code>b</code>, between
8092 <code>b</code> and <code>a</code>, or after <code>a</code>, and
8093 thus also the moment at which <code>sideEffect()</code> is called (before
8094 or after <code>x</code> is initialized) is not specified.
8098 Variables may also be initialized using functions named <code>init</code>
8099 declared in the package block, with no arguments and no result parameters.
8107 Multiple such functions may be defined per package, even within a single
8108 source file. In the package block, the <code>init</code> identifier can
8109 be used only to declare <code>init</code> functions, yet the identifier
8110 itself is not <a href="#Declarations_and_scope">declared</a>. Thus
8111 <code>init</code> functions cannot be referred to from anywhere
8116 A package with no imports is initialized by assigning initial values
8117 to all its package-level variables followed by calling all <code>init</code>
8118 functions in the order they appear in the source, possibly in multiple files,
8119 as presented to the compiler.
8120 If a package has imports, the imported packages are initialized
8121 before initializing the package itself. If multiple packages import
8122 a package, the imported package will be initialized only once.
8123 The importing of packages, by construction, guarantees that there
8124 can be no cyclic initialization dependencies.
8128 Package initialization—variable initialization and the invocation of
8129 <code>init</code> functions—happens in a single goroutine,
8130 sequentially, one package at a time.
8131 An <code>init</code> function may launch other goroutines, which can run
8132 concurrently with the initialization code. However, initialization
8134 the <code>init</code> functions: it will not invoke the next one
8135 until the previous one has returned.
8139 To ensure reproducible initialization behavior, build systems are encouraged
8140 to present multiple files belonging to the same package in lexical file name
8141 order to a compiler.
8145 <h3 id="Program_execution">Program execution</h3>
8147 A complete program is created by linking a single, unimported package
8148 called the <i>main package</i> with all the packages it imports, transitively.
8149 The main package must
8150 have package name <code>main</code> and
8151 declare a function <code>main</code> that takes no
8152 arguments and returns no value.
8160 Program execution begins by initializing the main package and then
8161 invoking the function <code>main</code>.
8162 When that function invocation returns, the program exits.
8163 It does not wait for other (non-<code>main</code>) goroutines to complete.
8166 <h2 id="Errors">Errors</h2>
8169 The predeclared type <code>error</code> is defined as
8173 type error interface {
8179 It is the conventional interface for representing an error condition,
8180 with the nil value representing no error.
8181 For instance, a function to read data from a file might be defined:
8185 func Read(f *File, b []byte) (n int, err error)
8188 <h2 id="Run_time_panics">Run-time panics</h2>
8191 Execution errors such as attempting to index an array out
8192 of bounds trigger a <i>run-time panic</i> equivalent to a call of
8193 the built-in function <a href="#Handling_panics"><code>panic</code></a>
8194 with a value of the implementation-defined interface type <code>runtime.Error</code>.
8195 That type satisfies the predeclared interface type
8196 <a href="#Errors"><code>error</code></a>.
8197 The exact error values that
8198 represent distinct run-time error conditions are unspecified.
8204 type Error interface {
8206 // and perhaps other methods
8210 <h2 id="System_considerations">System considerations</h2>
8212 <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
8215 The built-in package <code>unsafe</code>, known to the compiler
8216 and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
8217 provides facilities for low-level programming including operations
8218 that violate the type system. A package using <code>unsafe</code>
8219 must be vetted manually for type safety and may not be portable.
8220 The package provides the following interface:
8223 <pre class="grammar">
8226 type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
8227 type Pointer *ArbitraryType
8229 func Alignof(variable ArbitraryType) uintptr
8230 func Offsetof(selector ArbitraryType) uintptr
8231 func Sizeof(variable ArbitraryType) uintptr
8233 type IntegerType int // shorthand for an integer type; it is not a real type
8234 func Add(ptr Pointer, len IntegerType) Pointer
8235 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
8236 func SliceData(slice []ArbitraryType) *ArbitraryType
8237 func String(ptr *byte, len IntegerType) string
8238 func StringData(str string) *byte
8242 These conversions also apply to type parameters with suitable core types.
8243 Determine if we can simply use core type instead of underlying type here,
8244 of if the general conversion rules take care of this.
8248 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
8249 value may not be <a href="#Address_operators">dereferenced</a>.
8250 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be
8251 <a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
8252 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
8257 bits = *(*uint64)(unsafe.Pointer(&f))
8259 type ptr unsafe.Pointer
8260 bits = *(*uint64)(ptr(&f))
8266 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
8267 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
8268 as if <code>v</code> was declared via <code>var v = x</code>.
8271 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
8272 <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
8273 or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
8274 If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
8275 without pointer indirections through fields of the struct.
8276 For a struct <code>s</code> with field <code>f</code>:
8280 uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
8284 Computer architectures may require memory addresses to be <i>aligned</i>;
8285 that is, for addresses of a variable to be a multiple of a factor,
8286 the variable's type's <i>alignment</i>. The function <code>Alignof</code>
8287 takes an expression denoting a variable of any type and returns the
8288 alignment of the (type of the) variable in bytes. For a variable
8293 uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
8297 A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
8298 is a <a href="#Type_parameter_declarations">type parameter</a>, or if it is an
8299 array or struct type containing elements
8300 or fields of variable size. Otherwise the size is <i>constant</i>.
8301 Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
8302 are compile-time <a href="#Constant_expressions">constant expressions</a> of
8303 type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
8304 the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
8309 The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
8310 and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
8311 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8312 A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
8313 if it is an untyped constant it is given type <code>int</code>.
8314 The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
8318 The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
8319 and whose length and capacity are <code>len</code>.
8320 <code>Slice(ptr, len)</code> is equivalent to
8324 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
8328 except that, as a special case, if <code>ptr</code>
8329 is <code>nil</code> and <code>len</code> is zero,
8330 <code>Slice</code> returns <code>nil</code>.
8334 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8335 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
8336 if it is an untyped constant it is given type <code>int</code>.
8337 At run time, if <code>len</code> is negative,
8338 or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
8339 a <a href="#Run_time_panics">run-time panic</a> occurs.
8343 The function <code>SliceData</code> returns a pointer to the underlying array of the <code>slice</code> argument.
8344 If the slice's capacity <code>cap(slice)</code> is not zero, that pointer is <code>&slice[:1][0]</code>.
8345 If <code>slice</code> is <code>nil</code>, the result is <code>nil</code>.
8346 Otherwise it is a non-<code>nil</code> pointer to an unspecified memory address.
8350 The function <code>String</code> returns a <code>string</code> value whose underlying bytes start at
8351 <code>ptr</code> and whose length is <code>len</code>.
8352 The same requirements apply to the <code>ptr</code> and <code>len</code> argument as in the function
8353 <code>Slice</code>. If <code>len</code> is zero, the result is the empty string <code>""</code>.
8354 Since Go strings are immutable, the bytes passed to <code>String</code> must not be modified afterwards.
8358 The function <code>StringData</code> returns a pointer to the underlying bytes of the <code>str</code> argument.
8359 For an empty string the return value is unspecified, and may be <code>nil</code>.
8360 Since Go strings are immutable, the bytes returned by <code>StringData</code> must not be modified.
8363 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
8366 For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
8369 <pre class="grammar">
8374 uint32, int32, float32 4
8375 uint64, int64, float64, complex64 8
8380 The following minimal alignment properties are guaranteed:
8383 <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
8386 <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
8387 all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
8390 <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
8391 the alignment of a variable of the array's element type.
8396 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.