2 "Title": "The Go Programming Language Specification",
3 "Subtitle": "Version of May 25, 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 Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
4388 Each step attempts to use known information to infer additional type arguments.
4389 Type inference stops as soon as all type arguments are known.
4390 After type inference is complete, it is still necessary to substitute all type arguments
4391 for type parameters and verify that each type argument
4392 <a href="#Implementing_an_interface">implements</a> the relevant constraint;
4393 it is possible for an inferred type argument to fail to implement a constraint, in which
4394 case instantiation fails.
4398 Type inference is based on
4403 a <a href="#Type_parameter_declarations">type parameter list</a>
4406 a substitution map <i>M</i> initialized with the known type arguments, if any
4409 a (possibly empty) list of ordinary function arguments (in case of a function call only)
4414 and then proceeds with the following steps:
4419 apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
4420 to all <i>typed</i> ordinary function arguments
4423 apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
4426 apply function argument type inference to all <i>untyped</i> ordinary function arguments
4427 using the default type for each of the untyped function arguments
4430 apply constraint type inference
4435 If there are no ordinary or untyped function arguments, the respective steps are skipped.
4436 Constraint type inference is skipped if the previous step didn't infer any new type arguments,
4437 but it is run at least once if there are missing type arguments.
4441 The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
4442 The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
4443 If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
4446 <h4 id="Type_unification">Type unification</h4>
4449 Type inference is based on <i>type unification</i>. A single unification step
4450 applies to a <a href="#Type_inference">substitution map</a> and two types, either
4451 or both of which may be or contain type parameters. The substitution map tracks
4452 the known (explicitly provided or already inferred) type arguments: the map
4453 contains an entry <code>P</code> → <code>A</code> for each type
4454 parameter <code>P</code> and corresponding known type argument <code>A</code>.
4455 During unification, known type arguments take the place of their corresponding type
4456 parameters when comparing types. Unification is the process of finding substitution
4457 map entries that make the two types equivalent.
4461 For unification, two types that don't contain any type parameters from the current type
4462 parameter list are <i>equivalent</i>
4463 if they are identical, or if they are channel types that are identical ignoring channel
4464 direction, or if their underlying types are equivalent.
4468 Unification works by comparing the structure of pairs of types: their structure
4469 disregarding type parameters must be identical, and types other than type parameters
4471 A type parameter in one type may match any complete subtype in the other type;
4472 each successful match causes an entry to be added to the substitution map.
4473 If the structure differs, or types other than type parameters are not equivalent,
4478 TODO(gri) Somewhere we need to describe the process of adding an entry to the
4479 substitution map: if the entry is already present, the type argument
4480 values are themselves unified.
4484 For example, if <code>T1</code> and <code>T2</code> are type parameters,
4485 <code>[]map[int]bool</code> can be unified with any of the following:
4489 []map[int]bool // types are identical
4490 T1 // adds T1 → []map[int]bool to substitution map
4491 []T1 // adds T1 → map[int]bool to substitution map
4492 []map[T1]T2 // adds T1 → int and T2 → bool to substitution map
4496 On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
4500 int // int is not a slice
4501 struct{} // a struct is not a slice
4502 []struct{} // a struct is not a map
4503 []map[T1]string // map element types don't match
4507 As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
4508 <code>D</code> and a type literal <code>L</code> are never equivalent,
4509 unification compares the underlying type of <code>D</code> with <code>L</code> instead.
4510 For example, given the defined type
4514 type Vector []float64
4518 and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
4519 <code>[]E</code> and adds an entry <code>E</code> → <code>float64</code> to
4520 the substitution map.
4523 <h4 id="Function_argument_type_inference">Function argument type inference</h4>
4525 <!-- In this section and the section on constraint type inference we start with examples
4526 rather than have the examples follow the rules as is customary elsewhere in spec.
4527 Hopefully this helps building an intuition and makes the rules easier to follow. -->
4530 Function argument type inference infers type arguments from function arguments:
4531 if a function parameter is declared with a type <code>T</code> that uses
4533 <a href="#Type_unification">unifying</a> the type of the corresponding
4534 function argument with <code>T</code> may infer type arguments for the type
4535 parameters used by <code>T</code>.
4539 For instance, given the generic function
4543 func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
4551 var vector []float64
4552 scaledVector := scale(vector, 42)
4556 the type argument for <code>Number</code> can be inferred from the function argument
4557 <code>vector</code> by unifying the type of <code>vector</code> with the corresponding
4558 parameter type: <code>[]float64</code> and <code>[]Number</code>
4559 match in structure and <code>float64</code> matches with <code>Number</code>.
4560 This adds the entry <code>Number</code> → <code>float64</code> to the
4561 <a href="#Type_unification">substitution map</a>.
4562 Untyped arguments, such as the second function argument <code>42</code> here, are ignored
4563 in the first round of function argument type inference and only considered if there are
4564 unresolved type parameters left.
4568 Inference happens in two separate phases; each phase operates on a specific list of
4569 (parameter, argument) pairs:
4574 The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
4575 type uses type parameters and where the function argument is <i>typed</i>.
4578 The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
4579 type parameter. In this list, the respective function arguments are untyped.
4584 Any other (parameter, argument) pair is ignored.
4588 By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
4589 (or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
4590 of untyped values are always predeclared non-composite types, they can never match against
4591 a composite type, so it is sufficient to only consider parameter types that are single type
4596 Each list is processed in a separate phase:
4601 In the first phase, the parameter and argument types of each pair in <i>Lt</i>
4602 are unified. If unification succeeds for a pair, it may yield new entries that
4603 are added to the substitution map <i>M</i>. If unification fails, type inference
4607 The second phase considers the entries of list <i>Lu</i>. Type parameters for
4608 which the type argument has already been determined are ignored in this phase.
4609 For each remaining pair, the parameter type (which is a single type parameter) and
4610 the <a href="#Constants">default type</a> of the corresponding untyped argument is
4611 unified. If unification fails, type inference fails.
4616 While unification is successful, processing of each list continues until all list elements
4617 are considered, even if all type arguments are inferred before the last list element has
4626 func min[T ~int|~float64](x, y T) T
4629 min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
4630 min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
4631 min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
4635 In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
4636 yields the substitution map entry <code>T</code> → <code>float64</code>. Because
4637 processing continues until all untyped arguments are considered, an error is reported. This
4638 ensures that type inference does not depend on the order of the untyped arguments.
4641 <h4 id="Constraint_type_inference">Constraint type inference</h4>
4644 Constraint type inference infers type arguments by considering type constraints.
4645 If a type parameter <code>P</code> has a constraint with a
4646 <a href="#Core_types">core type</a> <code>C</code>,
4647 <a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
4648 may infer additional type arguments, either the type argument for <code>P</code>,
4649 or if that is already known, possibly the type arguments for type parameters
4650 used in <code>C</code>.
4654 For instance, consider the type parameter list with type parameters <code>List</code> and
4659 [List ~[]Elem, Elem any]
4663 Constraint type inference can deduce the type of <code>Elem</code> from the type argument
4664 for <code>List</code> because <code>Elem</code> is a type parameter in the core type
4665 <code>[]Elem</code> of <code>List</code>.
4666 If the type argument is <code>Bytes</code>:
4674 unifying the underlying type of <code>Bytes</code> with the core type means
4675 unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
4676 the <a href="#Type_unification">substitution map</a> entry
4677 <code>Elem</code> → <code>byte</code>.
4678 Thus, in this example, constraint type inference can infer the second type argument from the
4683 Using the core type of a constraint may lose some information: In the (unlikely) case that
4684 the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
4685 <code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
4686 <code>N</code> itself. In this case, constraint type inference may succeed but instantiation
4687 will fail because the inferred type is not in the type set of the constraint.
4688 Thus, constraint type inference uses the <i>adjusted core type</i> of
4689 a constraint: if the type set contains a single type, use that type; otherwise use the
4690 constraint's core type.
4694 Generally, constraint type inference proceeds in two phases: Starting with a given
4695 substitution map <i>M</i>
4700 For all type parameters with an adjusted core type, unify the type parameter with that
4701 type. If any unification fails, constraint type inference fails.
4705 At this point, some entries in <i>M</i> may map type parameters to other
4706 type parameters or to types containing type parameters. For each entry
4707 <code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
4708 contains type parameters <code>Q</code> for which there exist entries
4709 <code>Q</code> → <code>B</code> in <i>M</i>, substitute those
4710 <code>Q</code> with the respective <code>B</code> in <code>A</code>.
4711 Stop when no further substitution is possible.
4716 The result of constraint type inference is the final substitution map <i>M</i> from type
4717 parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
4718 appears in any of the <code>A</code>.
4722 For instance, given the type parameter list
4726 [A any, B []C, C *A]
4730 and the single provided type argument <code>int</code> for type parameter <code>A</code>,
4731 the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
4735 In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
4736 with the core type of their respective constraints. This adds the entries
4737 <code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
4741 At this point there are two entries in <i>M</i> where the right-hand side
4742 is or contains type parameters for which there exists other entries in <i>M</i>:
4743 <code>[]C</code> and <code>*A</code>.
4744 In the second phase, these type parameters are replaced with their respective
4745 types. It doesn't matter in which order this happens. Starting with the state
4746 of <i>M</i> after the first phase:
4750 <code>A</code> → <code>int</code>,
4751 <code>B</code> → <code>[]C</code>,
4752 <code>C</code> → <code>*A</code>
4756 Replace <code>A</code> on the right-hand side of → with <code>int</code>:
4760 <code>A</code> → <code>int</code>,
4761 <code>B</code> → <code>[]C</code>,
4762 <code>C</code> → <code>*int</code>
4766 Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
4770 <code>A</code> → <code>int</code>,
4771 <code>B</code> → <code>[]*int</code>,
4772 <code>C</code> → <code>*int</code>
4776 At this point no further substitution is possible and the map is full.
4777 Therefore, <code>M</code> represents the final map of type parameters
4778 to type arguments for the given type parameter list.
4781 <h3 id="Operators">Operators</h3>
4784 Operators combine operands into expressions.
4788 Expression = UnaryExpr | Expression binary_op Expression .
4789 UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
4791 binary_op = "||" | "&&" | rel_op | add_op | mul_op .
4792 rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
4793 add_op = "+" | "-" | "|" | "^" .
4794 mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
4796 unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
4800 Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
4801 For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
4802 unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
4803 For operations involving constants only, see the section on
4804 <a href="#Constant_expressions">constant expressions</a>.
4808 Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
4809 and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
4810 to the type of the other operand.
4814 The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
4815 or be an untyped constant <a href="#Representability">representable</a> by a
4816 value of type <code>uint</code>.
4817 If the left operand of a non-constant shift expression is an untyped constant,
4818 it is first implicitly converted to the type it would assume if the shift expression were
4819 replaced by its left operand alone.
4826 // The results of the following examples are given for 64-bit ints.
4827 var i = 1<<s // 1 has type int
4828 var j int32 = 1<<s // 1 has type int32; j == 0
4829 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
4830 var m int = 1.0<<s // 1.0 has type int; m == 1<<33
4831 var n = 1.0<<s == j // 1.0 has type int32; n == true
4832 var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
4833 var p = 1<<s == 1<<33 // 1 has type int; p == true
4834 var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
4835 var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
4836 var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
4837 var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift
4838 var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift
4839 var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
4840 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
4841 var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
4843 // The results of the following examples are given for 32-bit ints,
4844 // which means the shifts will overflow.
4845 var mm int = 1.0<<s // 1.0 has type int; mm == 0
4846 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
4847 var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
4848 var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
4849 var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
4852 <h4 id="Operator_precedence">Operator precedence</h4>
4854 Unary operators have the highest precedence.
4855 As the <code>++</code> and <code>--</code> operators form
4856 statements, not expressions, they fall
4857 outside the operator hierarchy.
4858 As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
4860 There are five precedence levels for binary operators.
4861 Multiplication operators bind strongest, followed by addition
4862 operators, comparison operators, <code>&&</code> (logical AND),
4863 and finally <code>||</code> (logical OR):
4866 <pre class="grammar">
4868 5 * / % << >> & &^
4870 3 == != < <= > >=
4876 Binary operators of the same precedence associate from left to right.
4877 For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
4886 x == y+1 && <-chanInt > 0
4890 <h3 id="Arithmetic_operators">Arithmetic operators</h3>
4892 Arithmetic operators apply to numeric values and yield a result of the same
4893 type as the first operand. The four standard arithmetic operators (<code>+</code>,
4894 <code>-</code>, <code>*</code>, <code>/</code>) apply to
4895 <a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
4896 <a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>.
4897 The bitwise logical and shift operators apply to integers only.
4900 <pre class="grammar">
4901 + sum integers, floats, complex values, strings
4902 - difference integers, floats, complex values
4903 * product integers, floats, complex values
4904 / quotient integers, floats, complex values
4905 % remainder integers
4907 & bitwise AND integers
4908 | bitwise OR integers
4909 ^ bitwise XOR integers
4910 &^ bit clear (AND NOT) integers
4912 << left shift integer << integer >= 0
4913 >> right shift integer >> integer >= 0
4917 If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>,
4918 the operator must apply to each type in that type set.
4919 The operands are represented as values of the type argument that the type parameter
4920 is <a href="#Instantiations">instantiated</a> with, and the operation is computed
4921 with the precision of that type argument. For example, given the function:
4925 func dotProduct[F ~float32|~float64](v1, v2 []F) F {
4927 for i, x := range v1 {
4936 the product <code>x * y</code> and the addition <code>s += x * y</code>
4937 are computed with <code>float32</code> or <code>float64</code> precision,
4938 respectively, depending on the type argument for <code>F</code>.
4941 <h4 id="Integer_operators">Integer operators</h4>
4944 For two integer values <code>x</code> and <code>y</code>, the integer quotient
4945 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
4950 x = q*y + r and |r| < |y|
4954 with <code>x / y</code> truncated towards zero
4955 (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
4967 The one exception to this rule is that if the dividend <code>x</code> is
4968 the most negative value for the int type of <code>x</code>, the quotient
4969 <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
4970 due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
4978 int64 -9223372036854775808
4982 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
4983 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4984 If the dividend is non-negative and the divisor is a constant power of 2,
4985 the division may be replaced by a right shift, and computing the remainder may
4986 be replaced by a bitwise AND operation:
4990 x x / 4 x % 4 x >> 2 x & 3
4996 The shift operators shift the left operand by the shift count specified by the
4997 right operand, which must be non-negative. If the shift count is negative at run time,
4998 a <a href="#Run_time_panics">run-time panic</a> occurs.
4999 The shift operators implement arithmetic shifts if the left operand is a signed
5000 integer and logical shifts if it is an unsigned integer.
5001 There is no upper limit on the shift count. Shifts behave
5002 as if the left operand is shifted <code>n</code> times by 1 for a shift
5003 count of <code>n</code>.
5004 As a result, <code>x << 1</code> is the same as <code>x*2</code>
5005 and <code>x >> 1</code> is the same as
5006 <code>x/2</code> but truncated towards negative infinity.
5010 For integer operands, the unary operators
5011 <code>+</code>, <code>-</code>, and <code>^</code> are defined as
5015 <pre class="grammar">
5017 -x negation is 0 - x
5018 ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
5019 and m = -1 for signed x
5023 <h4 id="Integer_overflow">Integer overflow</h4>
5026 For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>,
5027 <code>-</code>, <code>*</code>, and <code><<</code> are
5028 computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
5029 the unsigned integer's type.
5030 Loosely speaking, these unsigned integer operations
5031 discard high bits upon overflow, and programs may rely on "wrap around".
5035 For signed integers, the operations <code>+</code>,
5036 <code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
5037 overflow and the resulting value exists and is deterministically defined
5038 by the signed integer representation, the operation, and its operands.
5039 Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
5040 A compiler may not optimize code under the assumption that overflow does
5041 not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
5044 <h4 id="Floating_point_operators">Floating-point operators</h4>
5047 For floating-point and complex numbers,
5048 <code>+x</code> is the same as <code>x</code>,
5049 while <code>-x</code> is the negation of <code>x</code>.
5050 The result of a floating-point or complex division by zero is not specified beyond the
5051 IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
5052 occurs is implementation-specific.
5056 An implementation may combine multiple floating-point operations into a single
5057 fused operation, possibly across statements, and produce a result that differs
5058 from the value obtained by executing and rounding the instructions individually.
5059 An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to
5060 the precision of the target type, preventing fusion that would discard that rounding.
5064 For instance, some architectures provide a "fused multiply and add" (FMA) instruction
5065 that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
5066 These examples show when a Go implementation can use that instruction:
5070 // FMA allowed for computing r, because x*y is not explicitly rounded:
5074 *p = x*y; r = *p + z
5075 r = x*y + float64(z)
5077 // FMA disallowed for computing r, because it would omit rounding of x*y:
5078 r = float64(x*y) + z
5079 r = z; r += float64(x*y)
5080 t = float64(x*y); r = t + z
5083 <h4 id="String_concatenation">String concatenation</h4>
5086 Strings can be concatenated using the <code>+</code> operator
5087 or the <code>+=</code> assignment operator:
5091 s := "hi" + string(c)
5092 s += " and good bye"
5096 String addition creates a new string by concatenating the operands.
5099 <h3 id="Comparison_operators">Comparison operators</h3>
5102 Comparison operators compare two operands and yield an untyped boolean value.
5105 <pre class="grammar">
5111 >= greater or equal
5115 In any comparison, the first operand
5116 must be <a href="#Assignability">assignable</a>
5117 to the type of the second operand, or vice versa.
5120 The equality operators <code>==</code> and <code>!=</code> apply
5121 to operands of <i>comparable</i> types.
5122 The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
5123 apply to operands of <i>ordered</i> types.
5124 These terms and the result of the comparisons are defined as follows:
5129 Boolean types are comparable.
5130 Two boolean values are equal if they are either both
5131 <code>true</code> or both <code>false</code>.
5135 Integer types are comparable and ordered.
5136 Two integer values are compared in the usual way.
5140 Floating-point types are comparable and ordered.
5141 Two floating-point values are compared as defined by the IEEE-754 standard.
5145 Complex types are comparable.
5146 Two complex values <code>u</code> and <code>v</code> are
5147 equal if both <code>real(u) == real(v)</code> and
5148 <code>imag(u) == imag(v)</code>.
5152 String types are comparable and ordered.
5153 Two string values are compared lexically byte-wise.
5157 Pointer types are comparable.
5158 Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
5159 Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
5163 Channel types are comparable.
5164 Two channel values are equal if they were created by the same call to
5165 <a href="#Making_slices_maps_and_channels"><code>make</code></a>
5166 or if both have value <code>nil</code>.
5170 Interface types that are not type parameters are comparable.
5171 Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
5172 and equal dynamic values or if both have value <code>nil</code>.
5176 A value <code>x</code> of non-interface type <code>X</code> and
5177 a value <code>t</code> of interface type <code>T</code> can be compared
5178 if type <code>X</code> is comparable and
5179 <code>X</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
5180 They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
5181 and <code>t</code>'s dynamic value is equal to <code>x</code>.
5185 Struct types are comparable if all their field types are comparable.
5186 Two struct values are equal if their corresponding
5187 non-<a href="#Blank_identifier">blank</a> field values are equal.
5188 The fields are compared in source order, and comparison stops as
5189 soon as two field values differ (or all fields have been compared).
5193 Array types are comparable if their array element types are comparable.
5194 Two array values are equal if their corresponding element values are equal.
5195 The elements are compared in ascending index order, and comparison stops
5196 as soon as two element values differ (or all elements have been compared).
5200 Type parameters are comparable if they are strictly comparable (see below).
5205 A comparison of two interface values with identical dynamic types
5206 causes a <a href="#Run_time_panics">run-time panic</a> if that type
5207 is not comparable. This behavior applies not only to direct interface
5208 value comparisons but also when comparing arrays of interface values
5209 or structs with interface-valued fields.
5213 Slice, map, and function types are not comparable.
5214 However, as a special case, a slice, map, or function value may
5215 be compared to the predeclared identifier <code>nil</code>.
5216 Comparison of pointer, channel, and interface values to <code>nil</code>
5217 is also allowed and follows from the general rules above.
5221 const c = 3 < 4 // c is the untyped boolean constant true
5226 // The result of a comparison is an untyped boolean.
5227 // The usual assignment rules apply.
5228 b3 = x == y // b3 has type bool
5229 b4 bool = x == y // b4 has type bool
5230 b5 MyBool = x == y // b5 has type MyBool
5235 A type is <i>strictly comparable</i> if it is comparable and not an interface
5236 type nor composed of interface types.
5242 Boolean, numeric, string, pointer, and channel types are strictly comparable.
5246 Struct types are strictly comparable if all their field types are strictly comparable.
5250 Array types are strictly comparable if their array element types are strictly comparable.
5254 Type parameters are strictly comparable if all types in their type set are strictly comparable.
5258 <h3 id="Logical_operators">Logical operators</h3>
5261 Logical operators apply to <a href="#Boolean_types">boolean</a> values
5262 and yield a result of the same type as the operands.
5263 The right operand is evaluated conditionally.
5266 <pre class="grammar">
5267 && conditional AND p && q is "if p then q else false"
5268 || conditional OR p || q is "if p then true else q"
5273 <h3 id="Address_operators">Address operators</h3>
5276 For an operand <code>x</code> of type <code>T</code>, the address operation
5277 <code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
5278 The operand must be <i>addressable</i>,
5279 that is, either a variable, pointer indirection, or slice indexing
5280 operation; or a field selector of an addressable struct operand;
5281 or an array indexing operation of an addressable array.
5282 As an exception to the addressability requirement, <code>x</code> may also be a
5283 (possibly parenthesized)
5284 <a href="#Composite_literals">composite literal</a>.
5285 If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
5286 then the evaluation of <code>&x</code> does too.
5290 For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
5291 indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
5292 to by <code>x</code>.
5293 If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
5294 will cause a <a href="#Run_time_panics">run-time panic</a>.
5305 *x // causes a run-time panic
5306 &*x // causes a run-time panic
5310 <h3 id="Receive_operator">Receive operator</h3>
5313 For an operand <code>ch</code> whose <a href="#Core_types">core type</a> is a
5314 <a href="#Channel_types">channel</a>,
5315 the value of the receive operation <code><-ch</code> is the value received
5316 from the channel <code>ch</code>. The channel direction must permit receive operations,
5317 and the type of the receive operation is the element type of the channel.
5318 The expression blocks until a value is available.
5319 Receiving from a <code>nil</code> channel blocks forever.
5320 A receive operation on a <a href="#Close">closed</a> channel can always proceed
5321 immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
5322 after any previously sent values have been received.
5329 <-strobe // wait until clock pulse and discard received value
5333 A receive expression used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
5340 var x, ok T = <-ch
5344 yields an additional untyped boolean result reporting whether the
5345 communication succeeded. The value of <code>ok</code> is <code>true</code>
5346 if the value received was delivered by a successful send operation to the
5347 channel, or <code>false</code> if it is a zero value generated because the
5348 channel is closed and empty.
5352 <h3 id="Conversions">Conversions</h3>
5355 A conversion changes the <a href="#Types">type</a> of an expression
5356 to the type specified by the conversion.
5357 A conversion may appear literally in the source, or it may be <i>implied</i>
5358 by the context in which an expression appears.
5362 An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
5363 where <code>T</code> is a type and <code>x</code> is an expression
5364 that can be converted to type <code>T</code>.
5368 Conversion = Type "(" Expression [ "," ] ")" .
5372 If the type starts with the operator <code>*</code> or <code><-</code>,
5373 or if the type starts with the keyword <code>func</code>
5374 and has no result list, it must be parenthesized when
5375 necessary to avoid ambiguity:
5379 *Point(p) // same as *(Point(p))
5380 (*Point)(p) // p is converted to *Point
5381 <-chan int(c) // same as <-(chan int(c))
5382 (<-chan int)(c) // c is converted to <-chan int
5383 func()(x) // function signature func() x
5384 (func())(x) // x is converted to func()
5385 (func() int)(x) // x is converted to func() int
5386 func() int(x) // x is converted to func() int (unambiguous)
5390 A <a href="#Constants">constant</a> value <code>x</code> can be converted to
5391 type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
5392 by a value of <code>T</code>.
5393 As a special case, an integer constant <code>x</code> can be explicitly converted to a
5394 <a href="#String_types">string type</a> using the
5395 <a href="#Conversions_to_and_from_a_string_type">same rule</a>
5396 as for non-constant <code>x</code>.
5400 Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a>
5401 yields a typed constant.
5405 uint(iota) // iota value of type uint
5406 float32(2.718281828) // 2.718281828 of type float32
5407 complex128(1) // 1.0 + 0.0i of type complex128
5408 float32(0.49999999) // 0.5 of type float32
5409 float64(-1e-1000) // 0.0 of type float64
5410 string('x') // "x" of type string
5411 string(0x266c) // "♬" of type string
5412 myString("foo" + "bar") // "foobar" of type myString
5413 string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
5414 (*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
5415 int(1.2) // illegal: 1.2 cannot be represented as an int
5416 string(65.0) // illegal: 65.0 is not an integer constant
5420 Converting a constant to a type parameter yields a <i>non-constant</i> value of that type,
5421 with the value represented as a value of the type argument that the type parameter
5422 is <a href="#Instantiations">instantiated</a> with.
5423 For example, given the function:
5427 func f[P ~float32|~float64]() {
5433 the conversion <code>P(1.1)</code> results in a non-constant value of type <code>P</code>
5434 and the value <code>1.1</code> is represented as a <code>float32</code> or a <code>float64</code>
5435 depending on the type argument for <code>f</code>.
5436 Accordingly, if <code>f</code> is instantiated with a <code>float32</code> type,
5437 the numeric value of the expression <code>P(1.1) + 1.2</code> will be computed
5438 with the same precision as the corresponding non-constant <code>float32</code>
5443 A non-constant value <code>x</code> can be converted to type <code>T</code>
5444 in any of these cases:
5449 <code>x</code> is <a href="#Assignability">assignable</a>
5453 ignoring struct tags (see below),
5454 <code>x</code>'s type and <code>T</code> are not
5455 <a href="#Type_parameter_declarations">type parameters</a> but have
5456 <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
5459 ignoring struct tags (see below),
5460 <code>x</code>'s type and <code>T</code> are pointer types
5461 that are not <a href="#Types">named types</a>,
5462 and their pointer base types are not type parameters but
5463 have identical underlying types.
5466 <code>x</code>'s type and <code>T</code> are both integer or floating
5470 <code>x</code>'s type and <code>T</code> are both complex types.
5473 <code>x</code> is an integer or a slice of bytes or runes
5474 and <code>T</code> is a string type.
5477 <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
5480 <code>x</code> is a slice, <code>T</code> is an array or a pointer to an array,
5481 and the slice and array types have <a href="#Type_identity">identical</a> element types.
5486 Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type
5487 parameters, <code>x</code>
5488 can also be converted to type <code>T</code> if one of the following conditions applies:
5493 Both <code>V</code> and <code>T</code> are type parameters and a value of each
5494 type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s
5498 Only <code>V</code> is a type parameter and a value of each
5499 type in <code>V</code>'s type set can be converted to <code>T</code>.
5502 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
5503 type in <code>T</code>'s type set.
5508 <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
5509 for identity for the purpose of conversion:
5513 type Person struct {
5522 Name string `json:"name"`
5524 Street string `json:"street"`
5525 City string `json:"city"`
5529 var person = (*Person)(data) // ignoring tags, the underlying types are identical
5533 Specific rules apply to (non-constant) conversions between numeric types or
5534 to and from a string type.
5535 These conversions may change the representation of <code>x</code>
5536 and incur a run-time cost.
5537 All other conversions only change the type but not the representation
5542 There is no linguistic mechanism to convert between pointers and integers.
5543 The package <a href="#Package_unsafe"><code>unsafe</code></a>
5544 implements this functionality under restricted circumstances.
5547 <h4>Conversions between numeric types</h4>
5550 For the conversion of non-constant numeric values, the following rules apply:
5555 When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is
5556 sign extended to implicit infinite precision; otherwise it is zero extended.
5557 It is then truncated to fit in the result type's size.
5558 For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
5559 The conversion always yields a valid value; there is no indication of overflow.
5562 When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded
5563 (truncation towards zero).
5566 When converting an integer or floating-point number to a floating-point type,
5567 or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded
5568 to the precision specified by the destination type.
5569 For instance, the value of a variable <code>x</code> of type <code>float32</code>
5570 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
5571 but float32(x) represents the result of rounding <code>x</code>'s value to
5572 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
5573 of precision, but <code>float32(x + 0.1)</code> does not.
5578 In all non-constant conversions involving floating-point or complex values,
5579 if the result type cannot represent the value the conversion
5580 succeeds but the result value is implementation-dependent.
5583 <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
5587 Converting a signed or unsigned integer value to a string type yields a
5588 string containing the UTF-8 representation of the integer. Values outside
5589 the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
5593 string(-1) // "\ufffd" == "\xef\xbf\xbd"
5594 string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
5596 type myString string
5597 myString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
5602 Converting a slice of bytes to a string type yields
5603 a string whose successive bytes are the elements of the slice.
5606 string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5607 string([]byte{}) // ""
5608 string([]byte(nil)) // ""
5611 string(bytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5614 string([]myByte{'w', 'o', 'r', 'l', 'd', '!'}) // "world!"
5615 myString([]myByte{'\xf0', '\x9f', '\x8c', '\x8d'}) // "🌍"
5620 Converting a slice of runes to a string type yields
5621 a string that is the concatenation of the individual rune values
5622 converted to strings.
5625 string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5626 string([]rune{}) // ""
5627 string([]rune(nil)) // ""
5630 string(runes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5633 string([]myRune{0x266b, 0x266c}) // "\u266b\u266c" == "♫♬"
5634 myString([]myRune{0x1f30e}) // "\U0001f30e" == "🌎"
5639 Converting a value of a string type to a slice of bytes type
5640 yields a slice whose successive elements are the bytes of the string.
5643 []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5644 []byte("") // []byte{}
5646 bytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5648 []myByte("world!") // []myByte{'w', 'o', 'r', 'l', 'd', '!'}
5649 []myByte(myString("🌏")) // []myByte{'\xf0', '\x9f', '\x8c', '\x8f'}
5654 Converting a value of a string type to a slice of runes type
5655 yields a slice containing the individual Unicode code points of the string.
5658 []rune(myString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
5659 []rune("") // []rune{}
5661 runes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
5663 []myRune("♫♬") // []myRune{0x266b, 0x266c}
5664 []myRune(myString("🌐")) // []myRune{0x1f310}
5669 <h4 id="Conversions_from_slice_to_array_or_array_pointer">Conversions from slice to array or array pointer</h4>
5672 Converting a slice to an array yields an array containing the elements of the underlying array of the slice.
5673 Similarly, converting a slice to an array pointer yields a pointer to the underlying array of the slice.
5674 In both cases, if the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
5675 a <a href="#Run_time_panics">run-time panic</a> occurs.
5679 s := make([]byte, 2, 4)
5682 a1 := [1]byte(s[1:]) // a1[0] == s[1]
5683 a2 := [2]byte(s) // a2[0] == s[0]
5684 a4 := [4]byte(s) // panics: len([4]byte) > len(s)
5686 s0 := (*[0]byte)(s) // s0 != nil
5687 s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
5688 s2 := (*[2]byte)(s) // &s2[0] == &s[0]
5689 s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
5692 t0 := [0]string(t) // ok for nil slice t
5693 t1 := (*[0]string)(t) // t1 == nil
5694 t2 := (*[1]string)(t) // panics: len([1]string) > len(t)
5696 u := make([]byte, 0)
5697 u0 := (*[0]byte)(u) // u0 != nil
5700 <h3 id="Constant_expressions">Constant expressions</h3>
5703 Constant expressions may contain only <a href="#Constants">constant</a>
5704 operands and are evaluated at compile time.
5708 Untyped boolean, numeric, and string constants may be used as operands
5709 wherever it is legal to use an operand of boolean, numeric, or string type,
5714 A constant <a href="#Comparison_operators">comparison</a> always yields
5715 an untyped boolean constant. If the left operand of a constant
5716 <a href="#Operators">shift expression</a> is an untyped constant, the
5717 result is an integer constant; otherwise it is a constant of the same
5718 type as the left operand, which must be of
5719 <a href="#Numeric_types">integer type</a>.
5723 Any other operation on untyped constants results in an untyped constant of the
5724 same kind; that is, a boolean, integer, floating-point, complex, or string
5726 If the untyped operands of a binary operation (other than a shift) are of
5727 different kinds, the result is of the operand's kind that appears later in this
5728 list: integer, rune, floating-point, complex.
5729 For example, an untyped integer constant divided by an
5730 untyped complex constant yields an untyped complex constant.
5734 const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
5735 const b = 15 / 4 // b == 3 (untyped integer constant)
5736 const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
5737 const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
5738 const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
5739 const d = 1 << 3.0 // d == 8 (untyped integer constant)
5740 const e = 1.0 << 3 // e == 8 (untyped integer constant)
5741 const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
5742 const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
5743 const h = "foo" > "bar" // h == true (untyped boolean constant)
5744 const j = true // j == true (untyped boolean constant)
5745 const k = 'w' + 1 // k == 'x' (untyped rune constant)
5746 const l = "hi" // l == "hi" (untyped string constant)
5747 const m = string(k) // m == "x" (type string)
5748 const Σ = 1 - 0.707i // (untyped complex constant)
5749 const Δ = Σ + 2.0e-4 // (untyped complex constant)
5750 const Φ = iota*1i - 1/1i // (untyped complex constant)
5754 Applying the built-in function <code>complex</code> to untyped
5755 integer, rune, or floating-point constants yields
5756 an untyped complex constant.
5760 const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
5761 const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
5765 Constant expressions are always evaluated exactly; intermediate values and the
5766 constants themselves may require precision significantly larger than supported
5767 by any predeclared type in the language. The following are legal declarations:
5771 const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
5772 const Four int8 = Huge >> 98 // Four == 4 (type int8)
5776 The divisor of a constant division or remainder operation must not be zero:
5780 3.14 / 0.0 // illegal: division by zero
5784 The values of <i>typed</i> constants must always be accurately
5785 <a href="#Representability">representable</a> by values
5786 of the constant type. The following constant expressions are illegal:
5790 uint(-1) // -1 cannot be represented as a uint
5791 int(3.14) // 3.14 cannot be represented as an int
5792 int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
5793 Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
5794 Four * 100 // product 400 cannot be represented as an int8 (type of Four)
5798 The mask used by the unary bitwise complement operator <code>^</code> matches
5799 the rule for non-constants: the mask is all 1s for unsigned constants
5800 and -1 for signed and untyped constants.
5804 ^1 // untyped integer constant, equal to -2
5805 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
5806 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
5807 int8(^1) // same as int8(-2)
5808 ^int8(1) // same as -1 ^ int8(1) = -2
5812 Implementation restriction: A compiler may use rounding while
5813 computing untyped floating-point or complex constant expressions; see
5814 the implementation restriction in the section
5815 on <a href="#Constants">constants</a>. This rounding may cause a
5816 floating-point constant expression to be invalid in an integer
5817 context, even if it would be integral when calculated using infinite
5818 precision, and vice versa.
5822 <h3 id="Order_of_evaluation">Order of evaluation</h3>
5825 At package level, <a href="#Package_initialization">initialization dependencies</a>
5826 determine the evaluation order of individual initialization expressions in
5827 <a href="#Variable_declarations">variable declarations</a>.
5828 Otherwise, when evaluating the <a href="#Operands">operands</a> of an
5829 expression, assignment, or
5830 <a href="#Return_statements">return statement</a>,
5831 all function calls, method calls, and
5832 communication operations are evaluated in lexical left-to-right
5837 For example, in the (function-local) assignment
5840 y[f()], ok = g(h(), i()+x[j()], <-c), k()
5843 the function calls and communication happen in the order
5844 <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
5845 <code><-c</code>, <code>g()</code>, and <code>k()</code>.
5846 However, the order of those events compared to the evaluation
5847 and indexing of <code>x</code> and the evaluation
5848 of <code>y</code> is not specified.
5853 f := func() int { a++; return a }
5854 x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
5855 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
5856 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
5860 At package level, initialization dependencies override the left-to-right rule
5861 for individual initialization expressions, but not for operands within each
5866 var a, b, c = f() + v(), g(), sqr(u()) + v()
5868 func f() int { return c }
5869 func g() int { return a }
5870 func sqr(x int) int { return x*x }
5872 // functions u and v are independent of all other variables and functions
5876 The function calls happen in the order
5877 <code>u()</code>, <code>sqr()</code>, <code>v()</code>,
5878 <code>f()</code>, <code>v()</code>, and <code>g()</code>.
5882 Floating-point operations within a single expression are evaluated according to
5883 the associativity of the operators. Explicit parentheses affect the evaluation
5884 by overriding the default associativity.
5885 In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
5886 is performed before adding <code>x</code>.
5889 <h2 id="Statements">Statements</h2>
5892 Statements control execution.
5897 Declaration | LabeledStmt | SimpleStmt |
5898 GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
5899 FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
5902 SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
5905 <h3 id="Terminating_statements">Terminating statements</h3>
5908 A <i>terminating statement</i> interrupts the regular flow of control in
5909 a <a href="#Blocks">block</a>. The following statements are terminating:
5914 A <a href="#Return_statements">"return"</a> or
5915 <a href="#Goto_statements">"goto"</a> statement.
5916 <!-- ul below only for regular layout -->
5921 A call to the built-in function
5922 <a href="#Handling_panics"><code>panic</code></a>.
5923 <!-- ul below only for regular layout -->
5928 A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
5929 <!-- ul below only for regular layout -->
5934 An <a href="#If_statements">"if" statement</a> in which:
5936 <li>the "else" branch is present, and</li>
5937 <li>both branches are terminating statements.</li>
5942 A <a href="#For_statements">"for" statement</a> in which:
5944 <li>there are no "break" statements referring to the "for" statement, and</li>
5945 <li>the loop condition is absent, and</li>
5946 <li>the "for" statement does not use a range clause.</li>
5951 A <a href="#Switch_statements">"switch" statement</a> in which:
5953 <li>there are no "break" statements referring to the "switch" statement,</li>
5954 <li>there is a default case, and</li>
5955 <li>the statement lists in each case, including the default, end in a terminating
5956 statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
5962 A <a href="#Select_statements">"select" statement</a> in which:
5964 <li>there are no "break" statements referring to the "select" statement, and</li>
5965 <li>the statement lists in each case, including the default if present,
5966 end in a terminating statement.</li>
5971 A <a href="#Labeled_statements">labeled statement</a> labeling
5972 a terminating statement.
5977 All other statements are not terminating.
5981 A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
5982 is not empty and its final non-empty statement is terminating.
5986 <h3 id="Empty_statements">Empty statements</h3>
5989 The empty statement does nothing.
5997 <h3 id="Labeled_statements">Labeled statements</h3>
6000 A labeled statement may be the target of a <code>goto</code>,
6001 <code>break</code> or <code>continue</code> statement.
6005 LabeledStmt = Label ":" Statement .
6006 Label = identifier .
6010 Error: log.Panic("error encountered")
6014 <h3 id="Expression_statements">Expression statements</h3>
6017 With the exception of specific built-in functions,
6018 function and method <a href="#Calls">calls</a> and
6019 <a href="#Receive_operator">receive operations</a>
6020 can appear in statement context. Such statements may be parenthesized.
6024 ExpressionStmt = Expression .
6028 The following built-in functions are not permitted in statement context:
6032 append cap complex imag len make new real
6033 unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice unsafe.SliceData unsafe.String unsafe.StringData
6041 len("foo") // illegal if len is the built-in function
6045 <h3 id="Send_statements">Send statements</h3>
6048 A send statement sends a value on a channel.
6049 The channel expression's <a href="#Core_types">core type</a>
6050 must be a <a href="#Channel_types">channel</a>,
6051 the channel direction must permit send operations,
6052 and the type of the value to be sent must be <a href="#Assignability">assignable</a>
6053 to the channel's element type.
6057 SendStmt = Channel "<-" Expression .
6058 Channel = Expression .
6062 Both the channel and the value expression are evaluated before communication
6063 begins. Communication blocks until the send can proceed.
6064 A send on an unbuffered channel can proceed if a receiver is ready.
6065 A send on a buffered channel can proceed if there is room in the buffer.
6066 A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
6067 A send on a <code>nil</code> channel blocks forever.
6071 ch <- 3 // send value 3 to channel ch
6075 <h3 id="IncDec_statements">IncDec statements</h3>
6078 The "++" and "--" statements increment or decrement their operands
6079 by the untyped <a href="#Constants">constant</a> <code>1</code>.
6080 As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
6081 or a map index expression.
6085 IncDecStmt = Expression ( "++" | "--" ) .
6089 The following <a href="#Assignment_statements">assignment statements</a> are semantically
6093 <pre class="grammar">
6094 IncDec statement Assignment
6100 <h3 id="Assignment_statements">Assignment statements</h3>
6103 An <i>assignment</i> replaces the current value stored in a <a href="#Variables">variable</a>
6104 with a new value specified by an <a href="#Expressions">expression</a>.
6105 An assignment statement may assign a single value to a single variable, or multiple values to a
6106 matching number of variables.
6110 Assignment = ExpressionList assign_op ExpressionList .
6112 assign_op = [ add_op | mul_op ] "=" .
6116 Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
6117 a map index expression, or (for <code>=</code> assignments only) the
6118 <a href="#Blank_identifier">blank identifier</a>.
6119 Operands may be parenthesized.
6126 (k) = <-ch // same as: k = <-ch
6130 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
6131 <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
6132 is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
6133 <code>(y)</code> but evaluates <code>x</code>
6134 only once. The <i>op</i><code>=</code> construct is a single token.
6135 In assignment operations, both the left- and right-hand expression lists
6136 must contain exactly one single-valued expression, and the left-hand
6137 expression must not be the blank identifier.
6142 i &^= 1<<n
6146 A tuple assignment assigns the individual elements of a multi-valued
6147 operation to a list of variables. There are two forms. In the
6148 first, the right hand operand is a single multi-valued expression
6149 such as a function call, a <a href="#Channel_types">channel</a> or
6150 <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
6151 The number of operands on the left
6152 hand side must match the number of values. For instance, if
6153 <code>f</code> is a function returning two values,
6161 assigns the first value to <code>x</code> and the second to <code>y</code>.
6162 In the second form, the number of operands on the left must equal the number
6163 of expressions on the right, each of which must be single-valued, and the
6164 <i>n</i>th expression on the right is assigned to the <i>n</i>th
6165 operand on the left:
6169 one, two, three = '一', '二', '三'
6173 The <a href="#Blank_identifier">blank identifier</a> provides a way to
6174 ignore right-hand side values in an assignment:
6178 _ = x // evaluate x but ignore it
6179 x, _ = f() // evaluate f() but ignore second result value
6183 The assignment proceeds in two phases.
6184 First, the operands of <a href="#Index_expressions">index expressions</a>
6185 and <a href="#Address_operators">pointer indirections</a>
6186 (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
6187 on the left and the expressions on the right are all
6188 <a href="#Order_of_evaluation">evaluated in the usual order</a>.
6189 Second, the assignments are carried out in left-to-right order.
6193 a, b = b, a // exchange a and b
6197 i, x[i] = 1, 2 // set i = 1, x[0] = 2
6200 x[i], i = 2, 1 // set x[0] = 2, i = 1
6202 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
6204 x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
6206 type Point struct { x, y int }
6208 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
6212 for i, x[i] = range x { // set i, x[2] = 0, x[0]
6215 // after this loop, i == 0 and x is []int{3, 5, 3}
6219 In assignments, each value must be <a href="#Assignability">assignable</a>
6220 to the type of the operand to which it is assigned, with the following special cases:
6225 Any typed value may be assigned to the blank identifier.
6229 If an untyped constant
6230 is assigned to a variable of interface type or the blank identifier,
6231 the constant is first implicitly <a href="#Conversions">converted</a> to its
6232 <a href="#Constants">default type</a>.
6236 If an untyped boolean value is assigned to a variable of interface type or
6237 the blank identifier, it is first implicitly converted to type <code>bool</code>.
6241 <h3 id="If_statements">If statements</h3>
6244 "If" statements specify the conditional execution of two branches
6245 according to the value of a boolean expression. If the expression
6246 evaluates to true, the "if" branch is executed, otherwise, if
6247 present, the "else" branch is executed.
6251 IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
6261 The expression may be preceded by a simple statement, which
6262 executes before the expression is evaluated.
6266 if x := f(); x < y {
6268 } else if x > z {
6276 <h3 id="Switch_statements">Switch statements</h3>
6279 "Switch" statements provide multi-way execution.
6280 An expression or type is compared to the "cases"
6281 inside the "switch" to determine which branch
6286 SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
6290 There are two forms: expression switches and type switches.
6291 In an expression switch, the cases contain expressions that are compared
6292 against the value of the switch expression.
6293 In a type switch, the cases contain types that are compared against the
6294 type of a specially annotated switch expression.
6295 The switch expression is evaluated exactly once in a switch statement.
6298 <h4 id="Expression_switches">Expression switches</h4>
6301 In an expression switch,
6302 the switch expression is evaluated and
6303 the case expressions, which need not be constants,
6304 are evaluated left-to-right and top-to-bottom; the first one that equals the
6306 triggers execution of the statements of the associated case;
6307 the other cases are skipped.
6308 If no case matches and there is a "default" case,
6309 its statements are executed.
6310 There can be at most one default case and it may appear anywhere in the
6312 A missing switch expression is equivalent to the boolean value
6317 ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
6318 ExprCaseClause = ExprSwitchCase ":" StatementList .
6319 ExprSwitchCase = "case" ExpressionList | "default" .
6323 If the switch expression evaluates to an untyped constant, it is first implicitly
6324 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
6325 The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
6326 The switch expression type must be <a href="#Comparison_operators">comparable</a>.
6330 If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
6331 to the type of the switch expression.
6332 For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
6333 of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
6337 In other words, the switch expression is treated as if it were used to declare and
6338 initialize a temporary variable <code>t</code> without explicit type; it is that
6339 value of <code>t</code> against which each case expression <code>x</code> is tested
6344 In a case or default clause, the last non-empty statement
6345 may be a (possibly <a href="#Labeled_statements">labeled</a>)
6346 <a href="#Fallthrough_statements">"fallthrough" statement</a> to
6347 indicate that control should flow from the end of this clause to
6348 the first statement of the next clause.
6349 Otherwise control flows to the end of the "switch" statement.
6350 A "fallthrough" statement may appear as the last statement of all
6351 but the last clause of an expression switch.
6355 The switch expression may be preceded by a simple statement, which
6356 executes before the expression is evaluated.
6362 case 0, 1, 2, 3: s1()
6363 case 4, 5, 6, 7: s2()
6366 switch x := f(); { // missing switch expression means "true"
6367 case x < 0: return -x
6379 Implementation restriction: A compiler may disallow multiple case
6380 expressions evaluating to the same constant.
6381 For instance, the current compilers disallow duplicate integer,
6382 floating point, or string constants in case expressions.
6385 <h4 id="Type_switches">Type switches</h4>
6388 A type switch compares types rather than values. It is otherwise similar
6389 to an expression switch. It is marked by a special switch expression that
6390 has the form of a <a href="#Type_assertions">type assertion</a>
6391 using the keyword <code>type</code> rather than an actual type:
6401 Cases then match actual types <code>T</code> against the dynamic type of the
6402 expression <code>x</code>. As with type assertions, <code>x</code> must be of
6403 <a href="#Interface_types">interface type</a>, but not a
6404 <a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type
6405 <code>T</code> listed in a case must implement the type of <code>x</code>.
6406 The types listed in the cases of a type switch must all be
6407 <a href="#Type_identity">different</a>.
6411 TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
6412 TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
6413 TypeCaseClause = TypeSwitchCase ":" StatementList .
6414 TypeSwitchCase = "case" TypeList | "default" .
6418 The TypeSwitchGuard may include a
6419 <a href="#Short_variable_declarations">short variable declaration</a>.
6420 When that form is used, the variable is declared at the end of the
6421 TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
6422 In clauses with a case listing exactly one type, the variable
6423 has that type; otherwise, the variable has the type of the expression
6424 in the TypeSwitchGuard.
6428 Instead of a type, a case may use the predeclared identifier
6429 <a href="#Predeclared_identifiers"><code>nil</code></a>;
6430 that case is selected when the expression in the TypeSwitchGuard
6431 is a <code>nil</code> interface value.
6432 There may be at most one <code>nil</code> case.
6436 Given an expression <code>x</code> of type <code>interface{}</code>,
6437 the following type switch:
6441 switch i := x.(type) {
6443 printString("x is nil") // type of i is type of x (interface{})
6445 printInt(i) // type of i is int
6447 printFloat64(i) // type of i is float64
6448 case func(int) float64:
6449 printFunction(i) // type of i is func(int) float64
6451 printString("type is bool or string") // type of i is type of x (interface{})
6453 printString("don't know the type") // type of i is type of x (interface{})
6462 v := x // x is evaluated exactly once
6464 i := v // type of i is type of x (interface{})
6465 printString("x is nil")
6466 } else if i, isInt := v.(int); isInt {
6467 printInt(i) // type of i is int
6468 } else if i, isFloat64 := v.(float64); isFloat64 {
6469 printFloat64(i) // type of i is float64
6470 } else if i, isFunc := v.(func(int) float64); isFunc {
6471 printFunction(i) // type of i is func(int) float64
6473 _, isBool := v.(bool)
6474 _, isString := v.(string)
6475 if isBool || isString {
6476 i := v // type of i is type of x (interface{})
6477 printString("type is bool or string")
6479 i := v // type of i is type of x (interface{})
6480 printString("don't know the type")
6486 A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a>
6487 may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns
6488 out to duplicate another entry in the switch, the first matching case is chosen.
6492 func f[P any](x any) int {
6507 var v1 = f[string]("foo") // v1 == 0
6508 var v2 = f[byte]([]byte{}) // v2 == 2
6512 The type switch guard may be preceded by a simple statement, which
6513 executes before the guard is evaluated.
6517 The "fallthrough" statement is not permitted in a type switch.
6520 <h3 id="For_statements">For statements</h3>
6523 A "for" statement specifies repeated execution of a block. There are three forms:
6524 The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
6528 ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
6529 Condition = Expression .
6532 <h4 id="For_condition">For statements with single condition</h4>
6535 In its simplest form, a "for" statement specifies the repeated execution of
6536 a block as long as a boolean condition evaluates to true.
6537 The condition is evaluated before each iteration.
6538 If the condition is absent, it is equivalent to the boolean value
6548 <h4 id="For_clause">For statements with <code>for</code> clause</h4>
6551 A "for" statement with a ForClause is also controlled by its condition, but
6552 additionally it may specify an <i>init</i>
6553 and a <i>post</i> statement, such as an assignment,
6554 an increment or decrement statement. The init statement may be a
6555 <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
6556 Variables declared by the init statement are re-used in each iteration.
6560 ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
6561 InitStmt = SimpleStmt .
6562 PostStmt = SimpleStmt .
6566 for i := 0; i < 10; i++ {
6572 If non-empty, the init statement is executed once before evaluating the
6573 condition for the first iteration;
6574 the post statement is executed after each execution of the block (and
6575 only if the block was executed).
6576 Any element of the ForClause may be empty but the
6577 <a href="#Semicolons">semicolons</a> are
6578 required unless there is only a condition.
6579 If the condition is absent, it is equivalent to the boolean value
6584 for cond { S() } is the same as for ; cond ; { S() }
6585 for { S() } is the same as for true { S() }
6588 <h4 id="For_range">For statements with <code>range</code> clause</h4>
6591 A "for" statement with a "range" clause
6592 iterates through all entries of an array, slice, string or map,
6593 or values received on a channel. For each entry it assigns <i>iteration values</i>
6594 to corresponding <i>iteration variables</i> if present and then executes the block.
6598 RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
6602 The expression on the right in the "range" clause is called the <i>range expression</i>,
6603 its <a href="#Core_types">core type</a> must be
6604 an array, pointer to an array, slice, string, map, or channel permitting
6605 <a href="#Receive_operator">receive operations</a>.
6606 As with an assignment, if present the operands on the left must be
6607 <a href="#Address_operators">addressable</a> or map index expressions; they
6608 denote the iteration variables. If the range expression is a channel, at most
6609 one iteration variable is permitted, otherwise there may be up to two.
6610 If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
6611 the range clause is equivalent to the same clause without that identifier.
6615 The range expression <code>x</code> is evaluated once before beginning the loop,
6616 with one exception: if at most one iteration variable is present and
6617 <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
6618 the range expression is not evaluated.
6622 Function calls on the left are evaluated once per iteration.
6623 For each iteration, iteration values are produced as follows
6624 if the respective iteration variables are present:
6627 <pre class="grammar">
6628 Range expression 1st value 2nd value
6630 array or slice a [n]E, *[n]E, or []E index i int a[i] E
6631 string s string type index i int see below rune
6632 map m map[K]V key k K m[k] V
6633 channel c chan E, <-chan E element e E
6638 For an array, pointer to array, or slice value <code>a</code>, the index iteration
6639 values are produced in increasing order, starting at element index 0.
6640 If at most one iteration variable is present, the range loop produces
6641 iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
6642 or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
6646 For a string value, the "range" clause iterates over the Unicode code points
6647 in the string starting at byte index 0. On successive iterations, the index value will be the
6648 index of the first byte of successive UTF-8-encoded code points in the string,
6649 and the second value, of type <code>rune</code>, will be the value of
6650 the corresponding code point. If the iteration encounters an invalid
6651 UTF-8 sequence, the second value will be <code>0xFFFD</code>,
6652 the Unicode replacement character, and the next iteration will advance
6653 a single byte in the string.
6657 The iteration order over maps is not specified
6658 and is not guaranteed to be the same from one iteration to the next.
6659 If a map entry that has not yet been reached is removed during iteration,
6660 the corresponding iteration value will not be produced. If a map entry is
6661 created during iteration, that entry may be produced during the iteration or
6662 may be skipped. The choice may vary for each entry created and from one
6663 iteration to the next.
6664 If the map is <code>nil</code>, the number of iterations is 0.
6668 For channels, the iteration values produced are the successive values sent on
6669 the channel until the channel is <a href="#Close">closed</a>. If the channel
6670 is <code>nil</code>, the range expression blocks forever.
6675 The iteration values are assigned to the respective
6676 iteration variables as in an <a href="#Assignment_statements">assignment statement</a>.
6680 The iteration variables may be declared by the "range" clause using a form of
6681 <a href="#Short_variable_declarations">short variable declaration</a>
6683 In this case their types are set to the types of the respective iteration values
6684 and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
6685 statement; they are re-used in each iteration.
6686 If the iteration variables are declared outside the "for" statement,
6687 after execution their values will be those of the last iteration.
6691 var testdata *struct {
6694 for i, _ := range testdata.a {
6695 // testdata.a is never evaluated; len(testdata.a) is constant
6696 // i ranges from 0 to 6
6701 for i, s := range a {
6703 // type of s is string
6709 var val interface{} // element type of m is assignable to val
6710 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
6711 for key, val = range m {
6714 // key == last map key encountered in iteration
6717 var ch chan Work = producer()
6727 <h3 id="Go_statements">Go statements</h3>
6730 A "go" statement starts the execution of a function call
6731 as an independent concurrent thread of control, or <i>goroutine</i>,
6732 within the same address space.
6736 GoStmt = "go" Expression .
6740 The expression must be a function or method call; it cannot be parenthesized.
6741 Calls of built-in functions are restricted as for
6742 <a href="#Expression_statements">expression statements</a>.
6746 The function value and parameters are
6747 <a href="#Calls">evaluated as usual</a>
6748 in the calling goroutine, but
6749 unlike with a regular call, program execution does not wait
6750 for the invoked function to complete.
6751 Instead, the function begins executing independently
6753 When the function terminates, its goroutine also terminates.
6754 If the function has any return values, they are discarded when the
6760 go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
6764 <h3 id="Select_statements">Select statements</h3>
6767 A "select" statement chooses which of a set of possible
6768 <a href="#Send_statements">send</a> or
6769 <a href="#Receive_operator">receive</a>
6770 operations will proceed.
6771 It looks similar to a
6772 <a href="#Switch_statements">"switch"</a> statement but with the
6773 cases all referring to communication operations.
6777 SelectStmt = "select" "{" { CommClause } "}" .
6778 CommClause = CommCase ":" StatementList .
6779 CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
6780 RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
6781 RecvExpr = Expression .
6785 A case with a RecvStmt may assign the result of a RecvExpr to one or
6786 two variables, which may be declared using a
6787 <a href="#Short_variable_declarations">short variable declaration</a>.
6788 The RecvExpr must be a (possibly parenthesized) receive operation.
6789 There can be at most one default case and it may appear anywhere
6790 in the list of cases.
6794 Execution of a "select" statement proceeds in several steps:
6799 For all the cases in the statement, the channel operands of receive operations
6800 and the channel and right-hand-side expressions of send statements are
6801 evaluated exactly once, in source order, upon entering the "select" statement.
6802 The result is a set of channels to receive from or send to,
6803 and the corresponding values to send.
6804 Any side effects in that evaluation will occur irrespective of which (if any)
6805 communication operation is selected to proceed.
6806 Expressions on the left-hand side of a RecvStmt with a short variable declaration
6807 or assignment are not yet evaluated.
6811 If one or more of the communications can proceed,
6812 a single one that can proceed is chosen via a uniform pseudo-random selection.
6813 Otherwise, if there is a default case, that case is chosen.
6814 If there is no default case, the "select" statement blocks until
6815 at least one of the communications can proceed.
6819 Unless the selected case is the default case, the respective communication
6820 operation is executed.
6824 If the selected case is a RecvStmt with a short variable declaration or
6825 an assignment, the left-hand side expressions are evaluated and the
6826 received value (or values) are assigned.
6830 The statement list of the selected case is executed.
6835 Since communication on <code>nil</code> channels can never proceed,
6836 a select with only <code>nil</code> channels and no default case blocks forever.
6841 var c, c1, c2, c3, c4 chan int
6845 print("received ", i1, " from c1\n")
6847 print("sent ", i2, " to c2\n")
6848 case i3, ok := (<-c3): // same as: i3, ok := <-c3
6850 print("received ", i3, " from c3\n")
6852 print("c3 is closed\n")
6854 case a[f()] = <-c4:
6856 // case t := <-c4
6859 print("no communication\n")
6862 for { // send random sequence of bits to c
6864 case c <- 0: // note: no statement, no fallthrough, no folding of cases
6869 select {} // block forever
6873 <h3 id="Return_statements">Return statements</h3>
6876 A "return" statement in a function <code>F</code> terminates the execution
6877 of <code>F</code>, and optionally provides one or more result values.
6878 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6879 are executed before <code>F</code> returns to its caller.
6883 ReturnStmt = "return" [ ExpressionList ] .
6887 In a function without a result type, a "return" statement must not
6888 specify any result values.
6897 There are three ways to return values from a function with a result
6902 <li>The return value or values may be explicitly listed
6903 in the "return" statement. Each expression must be single-valued
6904 and <a href="#Assignability">assignable</a>
6905 to the corresponding element of the function's result type.
6907 func simpleF() int {
6911 func complexF1() (re float64, im float64) {
6916 <li>The expression list in the "return" statement may be a single
6917 call to a multi-valued function. The effect is as if each value
6918 returned from that function were assigned to a temporary
6919 variable with the type of the respective value, followed by a
6920 "return" statement listing these variables, at which point the
6921 rules of the previous case apply.
6923 func complexF2() (re float64, im float64) {
6928 <li>The expression list may be empty if the function's result
6929 type specifies names for its <a href="#Function_types">result parameters</a>.
6930 The result parameters act as ordinary local variables
6931 and the function may assign values to them as necessary.
6932 The "return" statement returns the values of these variables.
6934 func complexF3() (re float64, im float64) {
6940 func (devnull) Write(p []byte) (n int, _ error) {
6949 Regardless of how they are declared, all the result values are initialized to
6950 the <a href="#The_zero_value">zero values</a> for their type upon entry to the
6951 function. A "return" statement that specifies results sets the result parameters before
6952 any deferred functions are executed.
6956 Implementation restriction: A compiler may disallow an empty expression list
6957 in a "return" statement if a different entity (constant, type, or variable)
6958 with the same name as a result parameter is in
6959 <a href="#Declarations_and_scope">scope</a> at the place of the return.
6963 func f(n int) (res int, err error) {
6964 if _, err := f(n-1); err != nil {
6965 return // invalid return statement: err is shadowed
6971 <h3 id="Break_statements">Break statements</h3>
6974 A "break" statement terminates execution of the innermost
6975 <a href="#For_statements">"for"</a>,
6976 <a href="#Switch_statements">"switch"</a>, or
6977 <a href="#Select_statements">"select"</a> statement
6978 within the same function.
6982 BreakStmt = "break" [ Label ] .
6986 If there is a label, it must be that of an enclosing
6987 "for", "switch", or "select" statement,
6988 and that is the one whose execution terminates.
6993 for i = 0; i < n; i++ {
6994 for j = 0; j < m; j++ {
7007 <h3 id="Continue_statements">Continue statements</h3>
7010 A "continue" statement begins the next iteration of the
7011 innermost enclosing <a href="#For_statements">"for" loop</a>
7012 by advancing control to the end of the loop block.
7013 The "for" loop must be within the same function.
7017 ContinueStmt = "continue" [ Label ] .
7021 If there is a label, it must be that of an enclosing
7022 "for" statement, and that is the one whose execution
7028 for y, row := range rows {
7029 for x, data := range row {
7030 if data == endOfRow {
7033 row[x] = data + bias(x, y)
7038 <h3 id="Goto_statements">Goto statements</h3>
7041 A "goto" statement transfers control to the statement with the corresponding label
7042 within the same function.
7046 GotoStmt = "goto" Label .
7054 Executing the "goto" statement must not cause any variables to come into
7055 <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
7056 For instance, this example:
7066 is erroneous because the jump to label <code>L</code> skips
7067 the creation of <code>v</code>.
7071 A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
7072 For instance, this example:
7089 is erroneous because the label <code>L1</code> is inside
7090 the "for" statement's block but the <code>goto</code> is not.
7093 <h3 id="Fallthrough_statements">Fallthrough statements</h3>
7096 A "fallthrough" statement transfers control to the first statement of the
7097 next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
7098 It may be used only as the final non-empty statement in such a clause.
7102 FallthroughStmt = "fallthrough" .
7106 <h3 id="Defer_statements">Defer statements</h3>
7109 A "defer" statement invokes a function whose execution is deferred
7110 to the moment the surrounding function returns, either because the
7111 surrounding function executed a <a href="#Return_statements">return statement</a>,
7112 reached the end of its <a href="#Function_declarations">function body</a>,
7113 or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
7117 DeferStmt = "defer" Expression .
7121 The expression must be a function or method call; it cannot be parenthesized.
7122 Calls of built-in functions are restricted as for
7123 <a href="#Expression_statements">expression statements</a>.
7127 Each time a "defer" statement
7128 executes, the function value and parameters to the call are
7129 <a href="#Calls">evaluated as usual</a>
7130 and saved anew but the actual function is not invoked.
7131 Instead, deferred functions are invoked immediately before
7132 the surrounding function returns, in the reverse order
7133 they were deferred. That is, if the surrounding function
7134 returns through an explicit <a href="#Return_statements">return statement</a>,
7135 deferred functions are executed <i>after</i> any result parameters are set
7136 by that return statement but <i>before</i> the function returns to its caller.
7137 If a deferred function value evaluates
7138 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
7139 when the function is invoked, not when the "defer" statement is executed.
7143 For instance, if the deferred function is
7144 a <a href="#Function_literals">function literal</a> and the surrounding
7145 function has <a href="#Function_types">named result parameters</a> that
7146 are in scope within the literal, the deferred function may access and modify
7147 the result parameters before they are returned.
7148 If the deferred function has any return values, they are discarded when
7149 the function completes.
7150 (See also the section on <a href="#Handling_panics">handling panics</a>.)
7155 defer unlock(l) // unlocking happens before surrounding function returns
7157 // prints 3 2 1 0 before surrounding function returns
7158 for i := 0; i <= 3; i++ {
7163 func f() (result int) {
7165 // result is accessed after it was set to 6 by the return statement
7172 <h2 id="Built-in_functions">Built-in functions</h2>
7175 Built-in functions are
7176 <a href="#Predeclared_identifiers">predeclared</a>.
7177 They are called like any other function but some of them
7178 accept a type instead of an expression as the first argument.
7182 The built-in functions do not have standard Go types,
7183 so they can only appear in <a href="#Calls">call expressions</a>;
7184 they cannot be used as function values.
7188 <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
7191 The built-in functions <code>append</code> and <code>copy</code> assist in
7192 common slice operations.
7193 For both functions, the result is independent of whether the memory referenced
7194 by the arguments overlaps.
7198 The <a href="#Function_types">variadic</a> function <code>append</code>
7199 appends zero or more values <code>x</code> to a slice <code>s</code>
7200 and returns the resulting slice of the same type as <code>s</code>.
7201 The <a href="#Core_types">core type</a> of <code>s</code> must be a slice
7202 of type <code>[]E</code>.
7203 The values <code>x</code> are passed to a parameter of type <code>...E</code>
7204 and the respective <a href="#Passing_arguments_to_..._parameters">parameter
7205 passing rules</a> apply.
7206 As a special case, if the core type of <code>s</code> is <code>[]byte</code>,
7207 <code>append</code> also accepts a second argument with core type
7208 <a href="#Core_types"><code>bytestring</code></a> followed by <code>...</code>.
7209 This form appends the bytes of the byte slice or string.
7212 <pre class="grammar">
7213 append(s S, x ...E) S // core type of S is []E
7217 If the capacity of <code>s</code> is not large enough to fit the additional
7218 values, <code>append</code> <a href="#Allocation">allocates</a> a new, sufficiently large underlying
7219 array that fits both the existing slice elements and the additional values.
7220 Otherwise, <code>append</code> re-uses the underlying array.
7225 s1 := append(s0, 2) // append a single element s1 is []int{0, 0, 2}
7226 s2 := append(s1, 3, 5, 7) // append multiple elements s2 is []int{0, 0, 2, 3, 5, 7}
7227 s3 := append(s2, s0...) // append a slice s3 is []int{0, 0, 2, 3, 5, 7, 0, 0}
7228 s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 is []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
7231 t = append(t, 42, 3.1415, "foo") // t is []interface{}{42, 3.1415, "foo"}
7234 b = append(b, "bar"...) // append string contents b is []byte{'b', 'a', 'r' }
7238 The function <code>copy</code> copies slice elements from
7239 a source <code>src</code> to a destination <code>dst</code> and returns the
7240 number of elements copied.
7241 The <a href="#Core_types">core types</a> of both arguments must be slices
7242 with <a href="#Type_identity">identical</a> element type.
7243 The number of elements copied is the minimum of
7244 <code>len(src)</code> and <code>len(dst)</code>.
7245 As a special case, if the destination's core type is <code>[]byte</code>,
7246 <code>copy</code> also accepts a source argument with core type
7247 </a> <a href="#Core_types"><code>bytestring</code></a>.
7248 This form copies the bytes from the byte slice or string into the byte slice.
7251 <pre class="grammar">
7252 copy(dst, src []T) int
7253 copy(dst []byte, src string) int
7261 var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
7262 var s = make([]int, 6)
7263 var b = make([]byte, 5)
7264 n1 := copy(s, a[0:]) // n1 == 6, s is []int{0, 1, 2, 3, 4, 5}
7265 n2 := copy(s, s[2:]) // n2 == 4, s is []int{2, 3, 4, 5, 4, 5}
7266 n3 := copy(b, "Hello, World!") // n3 == 5, b is []byte("Hello")
7270 <h3 id="Clear">Clear</h3>
7273 The built-in function <code>clear</code> takes an argument of <a href="#Map_types">map</a>,
7274 <a href="#Slice_types">slice</a>, or <a href="#Type_parameter_declarations">type parameter</a> type,
7275 and deletes or zeroes out all elements.
7278 <pre class="grammar">
7279 Call Argument type Result
7281 clear(m) map[K]T deletes all entries, resulting in an
7282 empty map (len(m) == 0)
7284 clear(s) []T sets all elements up to the length of
7285 <code>s</code> to the zero value of T
7287 clear(t) type parameter see below
7291 If the argument type is a <a href="#Type_parameter_declarations">type parameter</a>,
7292 all types in its type set must be maps or slices, and <code>clear</code>
7293 performs the operation corresponding to the actual type argument.
7297 If the map or slice is <code>nil</code>, <code>clear</code> is a no-op.
7301 <h3 id="Close">Close</h3>
7304 For an argument <code>ch</code> with a <a href="#Core_types">core type</a>
7305 that is a <a href="#Channel_types">channel</a>, the built-in function <code>close</code>
7306 records that no more values will be sent on the channel.
7307 It is an error if <code>ch</code> is a receive-only channel.
7308 Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
7309 Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
7310 After calling <code>close</code>, and after any previously
7311 sent values have been received, receive operations will return
7312 the zero value for the channel's type without blocking.
7313 The multi-valued <a href="#Receive_operator">receive operation</a>
7314 returns a received value along with an indication of whether the channel is closed.
7318 <h3 id="Complex_numbers">Manipulating complex numbers</h3>
7321 Three functions assemble and disassemble complex numbers.
7322 The built-in function <code>complex</code> constructs a complex
7323 value from a floating-point real and imaginary part, while
7324 <code>real</code> and <code>imag</code>
7325 extract the real and imaginary parts of a complex value.
7328 <pre class="grammar">
7329 complex(realPart, imaginaryPart floatT) complexT
7330 real(complexT) floatT
7331 imag(complexT) floatT
7335 The type of the arguments and return value correspond.
7336 For <code>complex</code>, the two arguments must be of the same
7337 <a href="#Numeric_types">floating-point type</a> and the return type is the
7338 <a href="#Numeric_types">complex type</a>
7339 with the corresponding floating-point constituents:
7340 <code>complex64</code> for <code>float32</code> arguments, and
7341 <code>complex128</code> for <code>float64</code> arguments.
7342 If one of the arguments evaluates to an untyped constant, it is first implicitly
7343 <a href="#Conversions">converted</a> to the type of the other argument.
7344 If both arguments evaluate to untyped constants, they must be non-complex
7345 numbers or their imaginary parts must be zero, and the return value of
7346 the function is an untyped complex constant.
7350 For <code>real</code> and <code>imag</code>, the argument must be
7351 of complex type, and the return type is the corresponding floating-point
7352 type: <code>float32</code> for a <code>complex64</code> argument, and
7353 <code>float64</code> for a <code>complex128</code> argument.
7354 If the argument evaluates to an untyped constant, it must be a number,
7355 and the return value of the function is an untyped floating-point constant.
7359 The <code>real</code> and <code>imag</code> functions together form the inverse of
7360 <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
7361 <code>z == Z(complex(real(z), imag(z)))</code>.
7365 If the operands of these functions are all constants, the return
7366 value is a constant.
7370 var a = complex(2, -2) // complex128
7371 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
7372 x := float32(math.Cos(math.Pi/2)) // float32
7373 var c64 = complex(5, -x) // complex64
7374 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
7375 _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
7376 var rl = real(c64) // float32
7377 var im = imag(a) // float64
7378 const c = imag(b) // untyped constant -1.4
7379 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
7383 Arguments of type parameter type are not permitted.
7387 <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
7390 The built-in function <code>delete</code> removes the element with key
7391 <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
7392 value <code>k</code> must be <a href="#Assignability">assignable</a>
7393 to the key type of <code>m</code>.
7396 <pre class="grammar">
7397 delete(m, k) // remove element m[k] from map m
7401 If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>,
7402 all types in that type set must be maps, and they must all have identical key types.
7406 If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
7407 does not exist, <code>delete</code> is a no-op.
7411 <h3 id="Length_and_capacity">Length and capacity</h3>
7414 The built-in functions <code>len</code> and <code>cap</code> take arguments
7415 of various types and return a result of type <code>int</code>.
7416 The implementation guarantees that the result always fits into an <code>int</code>.
7419 <pre class="grammar">
7420 Call Argument type Result
7422 len(s) string type string length in bytes
7423 [n]T, *[n]T array length (== n)
7425 map[K]T map length (number of defined keys)
7426 chan T number of elements queued in channel buffer
7427 type parameter see below
7429 cap(s) [n]T, *[n]T array length (== n)
7431 chan T channel buffer capacity
7432 type parameter see below
7436 If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>,
7437 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
7438 each type in <code>P</code>'s type set.
7439 The result is the length (or capacity, respectively) of the argument whose type
7440 corresponds to the type argument with which <code>P</code> was
7441 <a href="#Instantiations">instantiated</a>.
7445 The capacity of a slice is the number of elements for which there is
7446 space allocated in the underlying array.
7447 At any time the following relationship holds:
7451 0 <= len(s) <= cap(s)
7455 The length of a <code>nil</code> slice, map or channel is 0.
7456 The capacity of a <code>nil</code> slice or channel is 0.
7460 The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
7461 <code>s</code> is a string constant. The expressions <code>len(s)</code> and
7462 <code>cap(s)</code> are constants if the type of <code>s</code> is an array
7463 or pointer to an array and the expression <code>s</code> does not contain
7464 <a href="#Receive_operator">channel receives</a> or (non-constant)
7465 <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
7466 Otherwise, invocations of <code>len</code> and <code>cap</code> are not
7467 constant and <code>s</code> is evaluated.
7472 c1 = imag(2i) // imag(2i) = 2.0 is a constant
7473 c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
7474 c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
7475 c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
7476 c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
7482 <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
7485 The built-in function <code>make</code> takes a type <code>T</code>,
7486 optionally followed by a type-specific list of expressions.
7487 The <a href="#Core_types">core type</a> of <code>T</code> must
7488 be a slice, map or channel.
7489 It returns a value of type <code>T</code> (not <code>*T</code>).
7490 The memory is initialized as described in the section on
7491 <a href="#The_zero_value">initial values</a>.
7494 <pre class="grammar">
7495 Call Core type Result
7497 make(T, n) slice slice of type T with length n and capacity n
7498 make(T, n, m) slice slice of type T with length n and capacity m
7500 make(T) map map of type T
7501 make(T, n) map map of type T with initial space for approximately n elements
7503 make(T) channel unbuffered channel of type T
7504 make(T, n) channel buffered channel of type T, buffer size n
7508 Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>,
7509 have a <a href="#Interface_types">type set</a> containing only integer types,
7510 or be an untyped <a href="#Constants">constant</a>.
7511 A constant size argument must be non-negative and <a href="#Representability">representable</a>
7512 by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
7513 If both <code>n</code> and <code>m</code> are provided and are constant, then
7514 <code>n</code> must be no larger than <code>m</code>.
7515 For slices and channels, if <code>n</code> is negative or larger than <code>m</code> at run time,
7516 a <a href="#Run_time_panics">run-time panic</a> occurs.
7520 s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
7521 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
7522 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
7523 s := make([]int, 10, 0) // illegal: len(s) > cap(s)
7524 c := make(chan int, 10) // channel with a buffer size of 10
7525 m := make(map[string]int, 100) // map with initial space for approximately 100 elements
7529 Calling <code>make</code> with a map type and size hint <code>n</code> will
7530 create a map with initial space to hold <code>n</code> map elements.
7531 The precise behavior is implementation-dependent.
7535 <h3 id="Min_and_max">Min and max</h3>
7538 The built-in functions <code>min</code> and <code>max</code> compute the
7539 smallest—or largest, respectively—value of a fixed number of
7540 arguments of <a href="#Comparison_operators">ordered types</a>.
7541 There must be at least one argument.
7545 The same type rules as for <a href="#Operators">operators</a> apply:
7546 for <a href="#Comparison_operators">ordered</a> arguments <code>x</code> and
7547 <code>y</code>, <code>min(x, y)</code> is valid if <code>x + y</code> is valid,
7548 and the type of <code>min(x, y)</code> is the type of <code>x + y</code>
7549 (and similarly for <code>max</code>).
7550 If all arguments are constant, the result is constant.
7555 m := min(x) // m == x
7556 m := min(x, y) // m is the smaller of x and y
7557 m := max(x, y, 10) // m is the larger of x and y but at least 10
7558 c := max(1, 2.0, 10) // c == 10.0 (floating-point kind)
7559 f := max(0, float32(x)) // type of f is float32
7561 _ = min(s...) // invalid: slice arguments are not permitted
7562 t := max("", "foo", "bar") // t == "foo" (string kind)
7566 For numeric arguments, <code>min</code> and <code>max</code> are
7567 commutative and associative:
7571 min(x, y) == min(y, x)
7572 min(x, y, z) == min(min(x, y), z) == min(x, min(y, z))
7576 For floating-point arguments negative zero, NaN, and infinity the following rules apply:
7580 x y min(x, y) max(x, y)
7582 -0.0 0.0 -0.0 0.0 // negative zero is smaller than (non-negative) zero
7583 -Inf y -Inf y // negative infinity is smaller than any other number
7584 +Inf y y +Inf // positive infinity is larger than any other number
7585 NaN y NaN NaN // if any argument is a NaN, the result is a NaN
7589 For string arguments the result for <code>min</code> is the first argument
7590 with the smallest (or for <code>max</code>, largest) value,
7591 compared lexically byte-wise:
7595 min(x, y) == if x <= y then x else y
7596 min(x, y, z) == min(min(x, y), z)
7599 <h3 id="Allocation">Allocation</h3>
7602 The built-in function <code>new</code> takes a type <code>T</code>,
7603 allocates storage for a <a href="#Variables">variable</a> of that type
7604 at run time, and returns a value of type <code>*T</code>
7605 <a href="#Pointer_types">pointing</a> to it.
7606 The variable is initialized as described in the section on
7607 <a href="#The_zero_value">initial values</a>.
7610 <pre class="grammar">
7619 type S struct { a int; b float64 }
7624 allocates storage for a variable of type <code>S</code>,
7625 initializes it (<code>a=0</code>, <code>b=0.0</code>),
7626 and returns a value of type <code>*S</code> containing the address
7631 <h3 id="Handling_panics">Handling panics</h3>
7633 <p> Two built-in functions, <code>panic</code> and <code>recover</code>,
7634 assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
7635 and program-defined error conditions.
7638 <pre class="grammar">
7639 func panic(interface{})
7640 func recover() interface{}
7644 While executing a function <code>F</code>,
7645 an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
7646 terminates the execution of <code>F</code>.
7647 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
7648 are then executed as usual.
7649 Next, any deferred functions run by <code>F</code>'s caller are run,
7650 and so on up to any deferred by the top-level function in the executing goroutine.
7651 At that point, the program is terminated and the error
7652 condition is reported, including the value of the argument to <code>panic</code>.
7653 This termination sequence is called <i>panicking</i>.
7658 panic("unreachable")
7659 panic(Error("cannot parse"))
7663 The <code>recover</code> function allows a program to manage behavior
7664 of a panicking goroutine.
7665 Suppose a function <code>G</code> defers a function <code>D</code> that calls
7666 <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
7668 When the running of deferred functions reaches <code>D</code>,
7669 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>.
7670 If <code>D</code> returns normally, without starting a new
7671 <code>panic</code>, the panicking sequence stops. In that case,
7672 the state of functions called between <code>G</code> and the call to <code>panic</code>
7673 is discarded, and normal execution resumes.
7674 Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
7675 execution terminates by returning to its caller.
7679 The return value of <code>recover</code> is <code>nil</code> when the
7680 goroutine is not panicking or <code>recover</code> was not called directly by a deferred function.
7681 Conversely, if a goroutine is panicking and <code>recover</code> was called directly by a deferred function,
7682 the return value of <code>recover</code> is guaranteed not to be <code>nil</code>.
7683 To ensure this, calling <code>panic</code> with a <code>nil</code> interface value (or an untyped <code>nil</code>)
7684 causes a <a href="#Run_time_panics">run-time panic</a>.
7688 The <code>protect</code> function in the example below invokes
7689 the function argument <code>g</code> and protects callers from
7690 run-time panics raised by <code>g</code>.
7694 func protect(g func()) {
7696 log.Println("done") // Println executes normally even if there is a panic
7697 if x := recover(); x != nil {
7698 log.Printf("run time panic: %v", x)
7701 log.Println("start")
7707 <h3 id="Bootstrapping">Bootstrapping</h3>
7710 Current implementations provide several built-in functions useful during
7711 bootstrapping. These functions are documented for completeness but are not
7712 guaranteed to stay in the language. They do not return a result.
7715 <pre class="grammar">
7718 print prints all arguments; formatting of arguments is implementation-specific
7719 println like print but prints spaces between arguments and a newline at the end
7723 Implementation restriction: <code>print</code> and <code>println</code> need not
7724 accept arbitrary argument types, but printing of boolean, numeric, and string
7725 <a href="#Types">types</a> must be supported.
7729 <h2 id="Packages">Packages</h2>
7732 Go programs are constructed by linking together <i>packages</i>.
7733 A package in turn is constructed from one or more source files
7734 that together declare constants, types, variables and functions
7735 belonging to the package and which are accessible in all files
7736 of the same package. Those elements may be
7737 <a href="#Exported_identifiers">exported</a> and used in another package.
7740 <h3 id="Source_file_organization">Source file organization</h3>
7743 Each source file consists of a package clause defining the package
7744 to which it belongs, followed by a possibly empty set of import
7745 declarations that declare packages whose contents it wishes to use,
7746 followed by a possibly empty set of declarations of functions,
7747 types, variables, and constants.
7751 SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
7754 <h3 id="Package_clause">Package clause</h3>
7757 A package clause begins each source file and defines the package
7758 to which the file belongs.
7762 PackageClause = "package" PackageName .
7763 PackageName = identifier .
7767 The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
7775 A set of files sharing the same PackageName form the implementation of a package.
7776 An implementation may require that all source files for a package inhabit the same directory.
7779 <h3 id="Import_declarations">Import declarations</h3>
7782 An import declaration states that the source file containing the declaration
7783 depends on functionality of the <i>imported</i> package
7784 (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
7785 and enables access to <a href="#Exported_identifiers">exported</a> identifiers
7787 The import names an identifier (PackageName) to be used for access and an ImportPath
7788 that specifies the package to be imported.
7792 ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
7793 ImportSpec = [ "." | PackageName ] ImportPath .
7794 ImportPath = string_lit .
7798 The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
7799 to access exported identifiers of the package within the importing source file.
7800 It is declared in the <a href="#Blocks">file block</a>.
7801 If the PackageName is omitted, it defaults to the identifier specified in the
7802 <a href="#Package_clause">package clause</a> of the imported package.
7803 If an explicit period (<code>.</code>) appears instead of a name, all the
7804 package's exported identifiers declared in that package's
7805 <a href="#Blocks">package block</a> will be declared in the importing source
7806 file's file block and must be accessed without a qualifier.
7810 The interpretation of the ImportPath is implementation-dependent but
7811 it is typically a substring of the full file name of the compiled
7812 package and may be relative to a repository of installed packages.
7816 Implementation restriction: A compiler may restrict ImportPaths to
7817 non-empty strings using only characters belonging to
7818 <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
7819 L, M, N, P, and S general categories (the Graphic characters without
7820 spaces) and may also exclude the characters
7821 <code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
7822 and the Unicode replacement character U+FFFD.
7826 Consider a compiled a package containing the package clause
7827 <code>package math</code>, which exports function <code>Sin</code>, and
7828 installed the compiled package in the file identified by
7829 <code>"lib/math"</code>.
7830 This table illustrates how <code>Sin</code> is accessed in files
7831 that import the package after the
7832 various types of import declaration.
7835 <pre class="grammar">
7836 Import declaration Local name of Sin
7838 import "lib/math" math.Sin
7839 import m "lib/math" m.Sin
7840 import . "lib/math" Sin
7844 An import declaration declares a dependency relation between
7845 the importing and imported package.
7846 It is illegal for a package to import itself, directly or indirectly,
7847 or to directly import a package without
7848 referring to any of its exported identifiers. To import a package solely for
7849 its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
7850 identifier as explicit package name:
7858 <h3 id="An_example_package">An example package</h3>
7861 Here is a complete Go package that implements a concurrent prime sieve.
7869 // Send the sequence 2, 3, 4, … to channel 'ch'.
7870 func generate(ch chan<- int) {
7872 ch <- i // Send 'i' to channel 'ch'.
7876 // Copy the values from channel 'src' to channel 'dst',
7877 // removing those divisible by 'prime'.
7878 func filter(src <-chan int, dst chan<- int, prime int) {
7879 for i := range src { // Loop over values received from 'src'.
7881 dst <- i // Send 'i' to channel 'dst'.
7886 // The prime sieve: Daisy-chain filter processes together.
7888 ch := make(chan int) // Create a new channel.
7889 go generate(ch) // Start generate() as a subprocess.
7892 fmt.Print(prime, "\n")
7893 ch1 := make(chan int)
7894 go filter(ch, ch1, prime)
7904 <h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
7906 <h3 id="The_zero_value">The zero value</h3>
7908 When storage is allocated for a <a href="#Variables">variable</a>,
7909 either through a declaration or a call of <code>new</code>, or when
7910 a new value is created, either through a composite literal or a call
7911 of <code>make</code>,
7912 and no explicit initialization is provided, the variable or value is
7913 given a default value. Each element of such a variable or value is
7914 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
7915 <code>0</code> for numeric types, <code>""</code>
7916 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
7917 This initialization is done recursively, so for instance each element of an
7918 array of structs will have its fields zeroed if no value is specified.
7921 These two simple declarations are equivalent:
7934 type T struct { i int; f float64; next *T }
7939 the following holds:
7949 The same would also be true after
7956 <h3 id="Package_initialization">Package initialization</h3>
7959 Within a package, package-level variable initialization proceeds stepwise,
7960 with each step selecting the variable earliest in <i>declaration order</i>
7961 which has no dependencies on uninitialized variables.
7965 More precisely, a package-level variable is considered <i>ready for
7966 initialization</i> if it is not yet initialized and either has
7967 no <a href="#Variable_declarations">initialization expression</a> or
7968 its initialization expression has no <i>dependencies</i> on uninitialized variables.
7969 Initialization proceeds by repeatedly initializing the next package-level
7970 variable that is earliest in declaration order and ready for initialization,
7971 until there are no variables ready for initialization.
7975 If any variables are still uninitialized when this
7976 process ends, those variables are part of one or more initialization cycles,
7977 and the program is not valid.
7981 Multiple variables on the left-hand side of a variable declaration initialized
7982 by single (multi-valued) expression on the right-hand side are initialized
7983 together: If any of the variables on the left-hand side is initialized, all
7984 those variables are initialized in the same step.
7989 var a, b = f() // a and b are initialized together, before x is initialized
7993 For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
7994 variables are treated like any other variables in declarations.
7998 The declaration order of variables declared in multiple files is determined
7999 by the order in which the files are presented to the compiler: Variables
8000 declared in the first file are declared before any of the variables declared
8001 in the second file, and so on.
8005 Dependency analysis does not rely on the actual values of the
8006 variables, only on lexical <i>references</i> to them in the source,
8007 analyzed transitively. For instance, if a variable <code>x</code>'s
8008 initialization expression refers to a function whose body refers to
8009 variable <code>y</code> then <code>x</code> depends on <code>y</code>.
8015 A reference to a variable or function is an identifier denoting that
8016 variable or function.
8020 A reference to a method <code>m</code> is a
8021 <a href="#Method_values">method value</a> or
8022 <a href="#Method_expressions">method expression</a> of the form
8023 <code>t.m</code>, where the (static) type of <code>t</code> is
8024 not an interface type, and the method <code>m</code> is in the
8025 <a href="#Method_sets">method set</a> of <code>t</code>.
8026 It is immaterial whether the resulting function value
8027 <code>t.m</code> is invoked.
8031 A variable, function, or method <code>x</code> depends on a variable
8032 <code>y</code> if <code>x</code>'s initialization expression or body
8033 (for functions and methods) contains a reference to <code>y</code>
8034 or to a function or method that depends on <code>y</code>.
8039 For example, given the declarations
8047 d = 3 // == 5 after initialization has finished
8057 the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
8058 Note that the order of subexpressions in initialization expressions is irrelevant:
8059 <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
8060 order in this example.
8064 Dependency analysis is performed per package; only references referring
8065 to variables, functions, and (non-interface) methods declared in the current
8066 package are considered. If other, hidden, data dependencies exists between
8067 variables, the initialization order between those variables is unspecified.
8071 For instance, given the declarations
8075 var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
8076 var _ = sideEffect() // unrelated to x, a, or b
8080 type I interface { ab() []int }
8082 func (T) ab() []int { return []int{a, b} }
8086 the variable <code>a</code> will be initialized after <code>b</code> but
8087 whether <code>x</code> is initialized before <code>b</code>, between
8088 <code>b</code> and <code>a</code>, or after <code>a</code>, and
8089 thus also the moment at which <code>sideEffect()</code> is called (before
8090 or after <code>x</code> is initialized) is not specified.
8094 Variables may also be initialized using functions named <code>init</code>
8095 declared in the package block, with no arguments and no result parameters.
8103 Multiple such functions may be defined per package, even within a single
8104 source file. In the package block, the <code>init</code> identifier can
8105 be used only to declare <code>init</code> functions, yet the identifier
8106 itself is not <a href="#Declarations_and_scope">declared</a>. Thus
8107 <code>init</code> functions cannot be referred to from anywhere
8112 A package with no imports is initialized by assigning initial values
8113 to all its package-level variables followed by calling all <code>init</code>
8114 functions in the order they appear in the source, possibly in multiple files,
8115 as presented to the compiler.
8116 If a package has imports, the imported packages are initialized
8117 before initializing the package itself. If multiple packages import
8118 a package, the imported package will be initialized only once.
8119 The importing of packages, by construction, guarantees that there
8120 can be no cyclic initialization dependencies.
8124 Package initialization—variable initialization and the invocation of
8125 <code>init</code> functions—happens in a single goroutine,
8126 sequentially, one package at a time.
8127 An <code>init</code> function may launch other goroutines, which can run
8128 concurrently with the initialization code. However, initialization
8130 the <code>init</code> functions: it will not invoke the next one
8131 until the previous one has returned.
8135 To ensure reproducible initialization behavior, build systems are encouraged
8136 to present multiple files belonging to the same package in lexical file name
8137 order to a compiler.
8141 <h3 id="Program_execution">Program execution</h3>
8143 A complete program is created by linking a single, unimported package
8144 called the <i>main package</i> with all the packages it imports, transitively.
8145 The main package must
8146 have package name <code>main</code> and
8147 declare a function <code>main</code> that takes no
8148 arguments and returns no value.
8156 Program execution begins by initializing the main package and then
8157 invoking the function <code>main</code>.
8158 When that function invocation returns, the program exits.
8159 It does not wait for other (non-<code>main</code>) goroutines to complete.
8162 <h2 id="Errors">Errors</h2>
8165 The predeclared type <code>error</code> is defined as
8169 type error interface {
8175 It is the conventional interface for representing an error condition,
8176 with the nil value representing no error.
8177 For instance, a function to read data from a file might be defined:
8181 func Read(f *File, b []byte) (n int, err error)
8184 <h2 id="Run_time_panics">Run-time panics</h2>
8187 Execution errors such as attempting to index an array out
8188 of bounds trigger a <i>run-time panic</i> equivalent to a call of
8189 the built-in function <a href="#Handling_panics"><code>panic</code></a>
8190 with a value of the implementation-defined interface type <code>runtime.Error</code>.
8191 That type satisfies the predeclared interface type
8192 <a href="#Errors"><code>error</code></a>.
8193 The exact error values that
8194 represent distinct run-time error conditions are unspecified.
8200 type Error interface {
8202 // and perhaps other methods
8206 <h2 id="System_considerations">System considerations</h2>
8208 <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
8211 The built-in package <code>unsafe</code>, known to the compiler
8212 and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
8213 provides facilities for low-level programming including operations
8214 that violate the type system. A package using <code>unsafe</code>
8215 must be vetted manually for type safety and may not be portable.
8216 The package provides the following interface:
8219 <pre class="grammar">
8222 type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
8223 type Pointer *ArbitraryType
8225 func Alignof(variable ArbitraryType) uintptr
8226 func Offsetof(selector ArbitraryType) uintptr
8227 func Sizeof(variable ArbitraryType) uintptr
8229 type IntegerType int // shorthand for an integer type; it is not a real type
8230 func Add(ptr Pointer, len IntegerType) Pointer
8231 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
8232 func SliceData(slice []ArbitraryType) *ArbitraryType
8233 func String(ptr *byte, len IntegerType) string
8234 func StringData(str string) *byte
8238 These conversions also apply to type parameters with suitable core types.
8239 Determine if we can simply use core type instead of underlying type here,
8240 of if the general conversion rules take care of this.
8244 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
8245 value may not be <a href="#Address_operators">dereferenced</a>.
8246 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be
8247 <a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
8248 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
8253 bits = *(*uint64)(unsafe.Pointer(&f))
8255 type ptr unsafe.Pointer
8256 bits = *(*uint64)(ptr(&f))
8262 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
8263 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
8264 as if <code>v</code> was declared via <code>var v = x</code>.
8267 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
8268 <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
8269 or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
8270 If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
8271 without pointer indirections through fields of the struct.
8272 For a struct <code>s</code> with field <code>f</code>:
8276 uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
8280 Computer architectures may require memory addresses to be <i>aligned</i>;
8281 that is, for addresses of a variable to be a multiple of a factor,
8282 the variable's type's <i>alignment</i>. The function <code>Alignof</code>
8283 takes an expression denoting a variable of any type and returns the
8284 alignment of the (type of the) variable in bytes. For a variable
8289 uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
8293 A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
8294 is a <a href="#Type_parameter_declarations">type parameter</a>, or if it is an
8295 array or struct type containing elements
8296 or fields of variable size. Otherwise the size is <i>constant</i>.
8297 Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
8298 are compile-time <a href="#Constant_expressions">constant expressions</a> of
8299 type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
8300 the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
8305 The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
8306 and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
8307 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8308 A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
8309 if it is an untyped constant it is given type <code>int</code>.
8310 The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
8314 The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
8315 and whose length and capacity are <code>len</code>.
8316 <code>Slice(ptr, len)</code> is equivalent to
8320 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
8324 except that, as a special case, if <code>ptr</code>
8325 is <code>nil</code> and <code>len</code> is zero,
8326 <code>Slice</code> returns <code>nil</code>.
8330 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8331 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
8332 if it is an untyped constant it is given type <code>int</code>.
8333 At run time, if <code>len</code> is negative,
8334 or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
8335 a <a href="#Run_time_panics">run-time panic</a> occurs.
8339 The function <code>SliceData</code> returns a pointer to the underlying array of the <code>slice</code> argument.
8340 If the slice's capacity <code>cap(slice)</code> is not zero, that pointer is <code>&slice[:1][0]</code>.
8341 If <code>slice</code> is <code>nil</code>, the result is <code>nil</code>.
8342 Otherwise it is a non-<code>nil</code> pointer to an unspecified memory address.
8346 The function <code>String</code> returns a <code>string</code> value whose underlying bytes start at
8347 <code>ptr</code> and whose length is <code>len</code>.
8348 The same requirements apply to the <code>ptr</code> and <code>len</code> argument as in the function
8349 <code>Slice</code>. If <code>len</code> is zero, the result is the empty string <code>""</code>.
8350 Since Go strings are immutable, the bytes passed to <code>String</code> must not be modified afterwards.
8354 The function <code>StringData</code> returns a pointer to the underlying bytes of the <code>str</code> argument.
8355 For an empty string the return value is unspecified, and may be <code>nil</code>.
8356 Since Go strings are immutable, the bytes returned by <code>StringData</code> must not be modified.
8359 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
8362 For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
8365 <pre class="grammar">
8370 uint32, int32, float32 4
8371 uint64, int64, float64, complex64 8
8376 The following minimal alignment properties are guaranteed:
8379 <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
8382 <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
8383 all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
8386 <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
8387 the alignment of a variable of the array's element type.
8392 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.