2 "Title": "The Go Programming Language Specification",
3 "Subtitle": "Version of September 21, 2022",
7 <h2 id="Introduction">Introduction</h2>
10 This is the reference manual for the Go programming language.
11 The pre-Go1.18 version, without generics, can be found
12 <a href="/doc/go1.17_spec.html">here</a>.
13 For more information and other documents, see <a href="/">golang.org</a>.
17 Go is a general-purpose language designed with systems programming
18 in mind. It is strongly typed and garbage-collected and has explicit
19 support for concurrent programming. Programs are constructed from
20 <i>packages</i>, whose properties allow efficient management of
25 The syntax is compact and simple to parse, allowing for easy analysis
26 by automatic tools such as integrated development environments.
29 <h2 id="Notation">Notation</h2>
31 The syntax is specified using a
32 <a href="https://en.wikipedia.org/wiki/Wirth_syntax_notation">variant</a>
33 of Extended Backus-Naur Form (EBNF):
37 Syntax = { Production } .
38 Production = production_name "=" [ Expression ] "." .
39 Expression = Term { "|" Term } .
40 Term = Factor { Factor } .
41 Factor = production_name | token [ "…" token ] | Group | Option | Repetition .
42 Group = "(" Expression ")" .
43 Option = "[" Expression "]" .
44 Repetition = "{" Expression "}" .
48 Productions are expressions constructed from terms and the following
49 operators, in increasing precedence:
54 [] option (0 or 1 times)
55 {} repetition (0 to n times)
59 Lowercase production names are used to identify lexical (terminal) tokens.
60 Non-terminals are in CamelCase. Lexical tokens are enclosed in
61 double quotes <code>""</code> or back quotes <code>``</code>.
65 The form <code>a … b</code> represents the set of characters from
66 <code>a</code> through <code>b</code> as alternatives. The horizontal
67 ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various
68 enumerations or code snippets that are not further specified. The character <code>…</code>
69 (as opposed to the three characters <code>...</code>) is not a token of the Go
73 <h2 id="Source_code_representation">Source code representation</h2>
76 Source code is Unicode text encoded in
77 <a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
78 canonicalized, so a single accented code point is distinct from the
79 same character constructed from combining an accent and a letter;
80 those are treated as two code points. For simplicity, this document
81 will use the unqualified term <i>character</i> to refer to a Unicode code point
85 Each code point is distinct; for instance, uppercase and lowercase letters
86 are different characters.
89 Implementation restriction: For compatibility with other tools, a
90 compiler may disallow the NUL character (U+0000) in the source text.
93 Implementation restriction: For compatibility with other tools, a
94 compiler may ignore a UTF-8-encoded byte order mark
95 (U+FEFF) if it is the first Unicode code point in the source text.
96 A byte order mark may be disallowed anywhere else in the source.
99 <h3 id="Characters">Characters</h3>
102 The following terms are used to denote specific Unicode character categories:
105 newline = /* the Unicode code point U+000A */ .
106 unicode_char = /* an arbitrary Unicode code point except newline */ .
107 unicode_letter = /* a Unicode code point categorized as "Letter" */ .
108 unicode_digit = /* a Unicode code point categorized as "Number, decimal digit" */ .
112 In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
113 Section 4.5 "General Category" defines a set of character categories.
114 Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
115 as Unicode letters, and those in the Number category Nd as Unicode digits.
118 <h3 id="Letters_and_digits">Letters and digits</h3>
121 The underscore character <code>_</code> (U+005F) is considered a lowercase letter.
124 letter = unicode_letter | "_" .
125 decimal_digit = "0" … "9" .
126 binary_digit = "0" | "1" .
127 octal_digit = "0" … "7" .
128 hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
131 <h2 id="Lexical_elements">Lexical elements</h2>
133 <h3 id="Comments">Comments</h3>
136 Comments serve as program documentation. There are two forms:
141 <i>Line comments</i> start with the character sequence <code>//</code>
142 and stop at the end of the line.
145 <i>General comments</i> start with the character sequence <code>/*</code>
146 and stop with the first subsequent character sequence <code>*/</code>.
151 A comment cannot start inside a <a href="#Rune_literals">rune</a> or
152 <a href="#String_literals">string literal</a>, or inside a comment.
153 A general comment containing no newlines acts like a space.
154 Any other comment acts like a newline.
157 <h3 id="Tokens">Tokens</h3>
160 Tokens form the vocabulary of the Go language.
161 There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators
162 and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from
163 spaces (U+0020), horizontal tabs (U+0009),
164 carriage returns (U+000D), and newlines (U+000A),
165 is ignored except as it separates tokens
166 that would otherwise combine into a single token. Also, a newline or end of file
167 may trigger the insertion of a <a href="#Semicolons">semicolon</a>.
168 While breaking the input into tokens,
169 the next token is the longest sequence of characters that form a
173 <h3 id="Semicolons">Semicolons</h3>
176 The formal syntax uses semicolons <code>";"</code> as terminators in
177 a number of productions. Go programs may omit most of these semicolons
178 using the following two rules:
183 When the input is broken into tokens, a semicolon is automatically inserted
184 into the token stream immediately after a line's final token if that token is
187 <a href="#Identifiers">identifier</a>
191 <a href="#Integer_literals">integer</a>,
192 <a href="#Floating-point_literals">floating-point</a>,
193 <a href="#Imaginary_literals">imaginary</a>,
194 <a href="#Rune_literals">rune</a>, or
195 <a href="#String_literals">string</a> literal
198 <li>one of the <a href="#Keywords">keywords</a>
200 <code>continue</code>,
201 <code>fallthrough</code>, or
205 <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a>
216 To allow complex statements to occupy a single line, a semicolon
217 may be omitted before a closing <code>")"</code> or <code>"}"</code>.
222 To reflect idiomatic use, code examples in this document elide semicolons
227 <h3 id="Identifiers">Identifiers</h3>
230 Identifiers name program entities such as variables and types.
231 An identifier is a sequence of one or more letters and digits.
232 The first character in an identifier must be a letter.
235 identifier = letter { letter | unicode_digit } .
240 ThisVariableIsExported
245 Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>.
249 <h3 id="Keywords">Keywords</h3>
252 The following keywords are reserved and may not be used as identifiers.
254 <pre class="grammar">
255 break default func interface select
256 case defer go map struct
257 chan else goto package switch
258 const fallthrough if range type
259 continue for import return var
262 <h3 id="Operators_and_punctuation">Operators and punctuation</h3>
265 The following character sequences represent <a href="#Operators">operators</a>
266 (including <a href="#Assignment_statements">assignment operators</a>) and punctuation:
268 <pre class="grammar">
269 + & += &= && == != ( )
270 - | -= |= || < <= [ ]
271 * ^ *= ^= <- > >= { }
272 / << /= <<= ++ = := , ;
273 % >> %= >>= -- ! ... . :
277 <h3 id="Integer_literals">Integer literals</h3>
280 An integer literal is a sequence of digits representing an
281 <a href="#Constants">integer constant</a>.
282 An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code>
283 for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal,
284 and <code>0x</code> or <code>0X</code> for hexadecimal.
285 A single <code>0</code> is considered a decimal zero.
286 In hexadecimal literals, letters <code>a</code> through <code>f</code>
287 and <code>A</code> through <code>F</code> represent values 10 through 15.
291 For readability, an underscore character <code>_</code> may appear after
292 a base prefix or between successive digits; such underscores do not change
296 int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
297 decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
298 binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
299 octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
300 hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
302 decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
303 binary_digits = binary_digit { [ "_" ] binary_digit } .
304 octal_digits = octal_digit { [ "_" ] octal_digit } .
305 hex_digits = hex_digit { [ "_" ] hex_digit } .
314 0O600 // second character is capital letter 'O'
318 170141183460469231731687303715884105727
319 170_141183_460469_231731_687303_715884_105727
321 _42 // an identifier, not an integer literal
322 42_ // invalid: _ must separate successive digits
323 4__2 // invalid: only one _ at a time
324 0_xBadFace // invalid: _ must separate successive digits
328 <h3 id="Floating-point_literals">Floating-point literals</h3>
331 A floating-point literal is a decimal or hexadecimal representation of a
332 <a href="#Constants">floating-point constant</a>.
336 A decimal floating-point literal consists of an integer part (decimal digits),
337 a decimal point, a fractional part (decimal digits), and an exponent part
338 (<code>e</code> or <code>E</code> followed by an optional sign and decimal digits).
339 One of the integer part or the fractional part may be elided; one of the decimal point
340 or the exponent part may be elided.
341 An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.
345 A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code>
346 prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
347 and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits).
348 One of the integer part or the fractional part may be elided; the radix point may be elided as well,
349 but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
350 An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.
354 For readability, an underscore character <code>_</code> may appear after
355 a base prefix or between successive digits; such underscores do not change
360 float_lit = decimal_float_lit | hex_float_lit .
362 decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
363 decimal_digits decimal_exponent |
364 "." decimal_digits [ decimal_exponent ] .
365 decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
367 hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
368 hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
371 hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
389 0x1.Fp+0 // == 1.9375
391 0X_1FFFP-16 // == 0.1249847412109375
392 0x15e-2 // == 0x15e - 2 (integer subtraction)
394 0x.p1 // invalid: mantissa has no digits
395 1p-2 // invalid: p exponent requires hexadecimal mantissa
396 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent
397 1_.5 // invalid: _ must separate successive digits
398 1._5 // invalid: _ must separate successive digits
399 1.5_e1 // invalid: _ must separate successive digits
400 1.5e_1 // invalid: _ must separate successive digits
401 1.5e1_ // invalid: _ must separate successive digits
405 <h3 id="Imaginary_literals">Imaginary literals</h3>
408 An imaginary literal represents the imaginary part of a
409 <a href="#Constants">complex constant</a>.
410 It consists of an <a href="#Integer_literals">integer</a> or
411 <a href="#Floating-point_literals">floating-point</a> literal
412 followed by the lowercase letter <code>i</code>.
413 The value of an imaginary literal is the value of the respective
414 integer or floating-point literal multiplied by the imaginary unit <i>i</i>.
418 imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
422 For backward compatibility, an imaginary literal's integer part consisting
423 entirely of decimal digits (and possibly underscores) is considered a decimal
424 integer, even if it starts with a leading <code>0</code>.
429 0123i // == 123i for backward-compatibility
430 0o123i // == 0o123 * 1i == 83i
431 0xabci // == 0xabc * 1i == 2748i
439 0x1p-2i // == 0x1p-2 * 1i == 0.25i
443 <h3 id="Rune_literals">Rune literals</h3>
446 A rune literal represents a <a href="#Constants">rune constant</a>,
447 an integer value identifying a Unicode code point.
448 A rune literal is expressed as one or more characters enclosed in single quotes,
449 as in <code>'x'</code> or <code>'\n'</code>.
450 Within the quotes, any character may appear except newline and unescaped single
451 quote. A single quoted character represents the Unicode value
452 of the character itself,
453 while multi-character sequences beginning with a backslash encode
454 values in various formats.
458 The simplest form represents the single character within the quotes;
459 since Go source text is Unicode characters encoded in UTF-8, multiple
460 UTF-8-encoded bytes may represent a single integer value. For
461 instance, the literal <code>'a'</code> holds a single byte representing
462 a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
463 <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
464 a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
468 Several backslash escapes allow arbitrary values to be encoded as
469 ASCII text. There are four ways to represent the integer value
470 as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
471 digits; <code>\u</code> followed by exactly four hexadecimal digits;
472 <code>\U</code> followed by exactly eight hexadecimal digits, and a
473 plain backslash <code>\</code> followed by exactly three octal digits.
474 In each case the value of the literal is the value represented by
475 the digits in the corresponding base.
479 Although these representations all result in an integer, they have
480 different valid ranges. Octal escapes must represent a value between
481 0 and 255 inclusive. Hexadecimal escapes satisfy this condition
482 by construction. The escapes <code>\u</code> and <code>\U</code>
483 represent Unicode code points so within them some values are illegal,
484 in particular those above <code>0x10FFFF</code> and surrogate halves.
488 After a backslash, certain single-character escapes represent special values:
491 <pre class="grammar">
492 \a U+0007 alert or bell
495 \n U+000A line feed or newline
496 \r U+000D carriage return
497 \t U+0009 horizontal tab
498 \v U+000B vertical tab
500 \' U+0027 single quote (valid escape only within rune literals)
501 \" U+0022 double quote (valid escape only within string literals)
505 An unrecognized character following a backslash in a rune literal is illegal.
509 rune_lit = "'" ( unicode_value | byte_value ) "'" .
510 unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
511 byte_value = octal_byte_value | hex_byte_value .
512 octal_byte_value = `\` octal_digit octal_digit octal_digit .
513 hex_byte_value = `\` "x" hex_digit hex_digit .
514 little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
515 big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
516 hex_digit hex_digit hex_digit hex_digit .
517 escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
532 '\'' // rune literal containing single quote character
533 'aa' // illegal: too many characters
534 '\k' // illegal: k is not recognized after a backslash
535 '\xa' // illegal: too few hexadecimal digits
536 '\0' // illegal: too few octal digits
537 '\400' // illegal: octal value over 255
538 '\uDFFF' // illegal: surrogate half
539 '\U00110000' // illegal: invalid Unicode code point
543 <h3 id="String_literals">String literals</h3>
546 A string literal represents a <a href="#Constants">string constant</a>
547 obtained from concatenating a sequence of characters. There are two forms:
548 raw string literals and interpreted string literals.
552 Raw string literals are character sequences between back quotes, as in
553 <code>`foo`</code>. Within the quotes, any character may appear except
554 back quote. The value of a raw string literal is the
555 string composed of the uninterpreted (implicitly UTF-8-encoded) characters
557 in particular, backslashes have no special meaning and the string may
559 Carriage return characters ('\r') inside raw string literals
560 are discarded from the raw string value.
564 Interpreted string literals are character sequences between double
565 quotes, as in <code>"bar"</code>.
566 Within the quotes, any character may appear except newline and unescaped double quote.
567 The text between the quotes forms the
568 value of the literal, with backslash escapes interpreted as they
569 are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and
570 <code>\"</code> is legal), with the same restrictions.
571 The three-digit octal (<code>\</code><i>nnn</i>)
572 and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual
573 <i>bytes</i> of the resulting string; all other escapes represent
574 the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.
575 Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent
576 a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>,
577 <code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent
578 the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character
583 string_lit = raw_string_lit | interpreted_string_lit .
584 raw_string_lit = "`" { unicode_char | newline } "`" .
585 interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
589 `abc` // same as "abc"
591 \n` // same as "\\n\n\\n"
598 "\uD800" // illegal: surrogate half
599 "\U00110000" // illegal: invalid Unicode code point
603 These examples all represent the same string:
607 "日本語" // UTF-8 input text
608 `日本語` // UTF-8 input text as a raw literal
609 "\u65e5\u672c\u8a9e" // the explicit Unicode code points
610 "\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points
611 "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes
615 If the source code represents a character as two code points, such as
616 a combining form involving an accent and a letter, the result will be
617 an error if placed in a rune literal (it is not a single code
618 point), and will appear as two code points if placed in a string
623 <h2 id="Constants">Constants</h2>
625 <p>There are <i>boolean constants</i>,
626 <i>rune constants</i>,
627 <i>integer constants</i>,
628 <i>floating-point constants</i>, <i>complex constants</i>,
629 and <i>string constants</i>. Rune, integer, floating-point,
630 and complex constants are
631 collectively called <i>numeric constants</i>.
635 A constant value is represented by a
636 <a href="#Rune_literals">rune</a>,
637 <a href="#Integer_literals">integer</a>,
638 <a href="#Floating-point_literals">floating-point</a>,
639 <a href="#Imaginary_literals">imaginary</a>,
641 <a href="#String_literals">string</a> literal,
642 an identifier denoting a constant,
643 a <a href="#Constant_expressions">constant expression</a>,
644 a <a href="#Conversions">conversion</a> with a result that is a constant, or
645 the result value of some built-in functions such as
646 <code>unsafe.Sizeof</code> applied to <a href="#Package_unsafe">certain values</a>,
647 <code>cap</code> or <code>len</code> applied to
648 <a href="#Length_and_capacity">some expressions</a>,
649 <code>real</code> and <code>imag</code> applied to a complex constant
650 and <code>complex</code> applied to numeric constants.
651 The boolean truth values are represented by the predeclared constants
652 <code>true</code> and <code>false</code>. The predeclared identifier
653 <a href="#Iota">iota</a> denotes an integer constant.
657 In general, complex constants are a form of
658 <a href="#Constant_expressions">constant expression</a>
659 and are discussed in that section.
663 Numeric constants represent exact values of arbitrary precision and do not overflow.
664 Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
665 and not-a-number values.
669 Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
670 Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
671 and certain <a href="#Constant_expressions">constant expressions</a>
672 containing only untyped constant operands are untyped.
676 A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
677 or <a href="#Conversions">conversion</a>, or implicitly when used in a
678 <a href="#Variable_declarations">variable declaration</a> or an
679 <a href="#Assignment_statements">assignment statement</a> or as an
680 operand in an <a href="#Expressions">expression</a>.
681 It is an error if the constant value
682 cannot be <a href="#Representability">represented</a> as a value of the respective type.
683 If the type is a type parameter, the constant is converted into a non-constant
684 value of the type parameter.
688 An untyped constant has a <i>default type</i> which is the type to which the
689 constant is implicitly converted in contexts where a typed value is required,
690 for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
691 such as <code>i := 0</code> where there is no explicit type.
692 The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
693 <code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
694 respectively, depending on whether it is a boolean, rune, integer, floating-point,
695 complex, or string constant.
699 Implementation restriction: Although numeric constants have arbitrary
700 precision in the language, a compiler may implement them using an
701 internal representation with limited precision. That said, every
706 <li>Represent integer constants with at least 256 bits.</li>
708 <li>Represent floating-point constants, including the parts of
709 a complex constant, with a mantissa of at least 256 bits
710 and a signed binary exponent of at least 16 bits.</li>
712 <li>Give an error if unable to represent an integer constant
715 <li>Give an error if unable to represent a floating-point or
716 complex constant due to overflow.</li>
718 <li>Round to the nearest representable constant if unable to
719 represent a floating-point or complex constant due to limits
724 These requirements apply both to literal constants and to the result
725 of evaluating <a href="#Constant_expressions">constant
730 <h2 id="Variables">Variables</h2>
733 A variable is a storage location for holding a <i>value</i>.
734 The set of permissible values is determined by the
735 variable's <i><a href="#Types">type</a></i>.
739 A <a href="#Variable_declarations">variable declaration</a>
740 or, for function parameters and results, the signature
741 of a <a href="#Function_declarations">function declaration</a>
742 or <a href="#Function_literals">function literal</a> reserves
743 storage for a named variable.
745 Calling the built-in function <a href="#Allocation"><code>new</code></a>
746 or taking the address of a <a href="#Composite_literals">composite literal</a>
747 allocates storage for a variable at run time.
748 Such an anonymous variable is referred to via a (possibly implicit)
749 <a href="#Address_operators">pointer indirection</a>.
753 <i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
754 and <a href="#Struct_types">struct</a> types have elements and fields that may
755 be <a href="#Address_operators">addressed</a> individually. Each such element
756 acts like a variable.
760 The <i>static type</i> (or just <i>type</i>) of a variable is the
761 type given in its declaration, the type provided in the
762 <code>new</code> call or composite literal, or the type of
763 an element of a structured variable.
764 Variables of interface type also have a distinct <i>dynamic type</i>,
765 which is the (non-interface) type of the value assigned to the variable at run time
766 (unless the value is the predeclared identifier <code>nil</code>,
768 The dynamic type may vary during execution but values stored in interface
769 variables are always <a href="#Assignability">assignable</a>
770 to the static type of the variable.
774 var x interface{} // x is nil and has static type interface{}
775 var v *T // v has value nil, static type *T
776 x = 42 // x has value 42 and dynamic type int
777 x = v // x has value (*T)(nil) and dynamic type *T
781 A variable's value is retrieved by referring to the variable in an
782 <a href="#Expressions">expression</a>; it is the most recent value
783 <a href="#Assignment_statements">assigned</a> to the variable.
784 If a variable has not yet been assigned a value, its value is the
785 <a href="#The_zero_value">zero value</a> for its type.
789 <h2 id="Types">Types</h2>
792 A type determines a set of values together with operations and methods specific
793 to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be
794 followed by <a href="#Instantiations">type arguments</a> if the type is generic.
795 A type may also be specified using a <i>type literal</i>, which composes a type
800 Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
801 TypeName = identifier | QualifiedIdent .
802 TypeArgs = "[" TypeList [ "," ] "]" .
803 TypeList = Type { "," Type } .
804 TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
805 SliceType | MapType | ChannelType .
809 The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
810 Others are introduced with <a href="#Type_declarations">type declarations</a>
811 or <a href="#Type_parameter_declarations">type parameter lists</a>.
812 <i>Composite types</i>—array, struct, pointer, function,
813 interface, slice, map, and channel types—may be constructed using
818 Predeclared types, defined types, and type parameters are called <i>named types</i>.
819 An alias denotes a named type if the type given in the alias declaration is a named type.
822 <h3 id="Boolean_types">Boolean types</h3>
825 A <i>boolean type</i> represents the set of Boolean truth values
826 denoted by the predeclared constants <code>true</code>
827 and <code>false</code>. The predeclared boolean type is <code>bool</code>;
828 it is a <a href="#Type_definitions">defined type</a>.
831 <h3 id="Numeric_types">Numeric types</h3>
834 An <i>integer</i>, <i>floating-point</i>, or <i>complex</i> type
835 represents the set of integer, floating-point, or complex values, respectively.
836 They are collectively called <i>numeric types</i>.
837 The predeclared architecture-independent numeric types are:
840 <pre class="grammar">
841 uint8 the set of all unsigned 8-bit integers (0 to 255)
842 uint16 the set of all unsigned 16-bit integers (0 to 65535)
843 uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
844 uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
846 int8 the set of all signed 8-bit integers (-128 to 127)
847 int16 the set of all signed 16-bit integers (-32768 to 32767)
848 int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
849 int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
851 float32 the set of all IEEE-754 32-bit floating-point numbers
852 float64 the set of all IEEE-754 64-bit floating-point numbers
854 complex64 the set of all complex numbers with float32 real and imaginary parts
855 complex128 the set of all complex numbers with float64 real and imaginary parts
862 The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
863 <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
867 There is also a set of predeclared integer types with implementation-specific sizes:
870 <pre class="grammar">
871 uint either 32 or 64 bits
872 int same size as uint
873 uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
877 To avoid portability issues all numeric types are <a href="#Type_definitions">defined
878 types</a> and thus distinct except
879 <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
880 <code>rune</code>, which is an alias for <code>int32</code>.
882 are required when different numeric types are mixed in an expression
883 or assignment. For instance, <code>int32</code> and <code>int</code>
884 are not the same type even though they may have the same size on a
885 particular architecture.
888 <h3 id="String_types">String types</h3>
891 A <i>string type</i> represents the set of string values.
892 A string value is a (possibly empty) sequence of bytes.
893 The number of bytes is called the length of the string and is never negative.
894 Strings are immutable: once created,
895 it is impossible to change the contents of a string.
896 The predeclared string type is <code>string</code>;
897 it is a <a href="#Type_definitions">defined type</a>.
901 The length of a string <code>s</code> can be discovered using
902 the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
903 The length is a compile-time constant if the string is a constant.
904 A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
905 0 through <code>len(s)-1</code>.
906 It is illegal to take the address of such an element; if
907 <code>s[i]</code> is the <code>i</code>'th byte of a
908 string, <code>&s[i]</code> is invalid.
912 <h3 id="Array_types">Array types</h3>
915 An array is a numbered sequence of elements of a single
916 type, called the element type.
917 The number of elements is called the length of the array and is never negative.
921 ArrayType = "[" ArrayLength "]" ElementType .
922 ArrayLength = Expression .
927 The length is part of the array's type; it must evaluate to a
928 non-negative <a href="#Constants">constant</a>
929 <a href="#Representability">representable</a> by a value
930 of type <code>int</code>.
931 The length of array <code>a</code> can be discovered
932 using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
933 The elements can be addressed by integer <a href="#Index_expressions">indices</a>
934 0 through <code>len(a)-1</code>.
935 Array types are always one-dimensional but may be composed to form
936 multi-dimensional types.
941 [2*N] struct { x, y int32 }
944 [2][2][2]float64 // same as [2]([2]([2]float64))
947 <h3 id="Slice_types">Slice types</h3>
950 A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
951 provides access to a numbered sequence of elements from that array.
952 A slice type denotes the set of all slices of arrays of its element type.
953 The number of elements is called the length of the slice and is never negative.
954 The value of an uninitialized slice is <code>nil</code>.
958 SliceType = "[" "]" ElementType .
962 The length of a slice <code>s</code> can be discovered by the built-in function
963 <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
964 execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
965 0 through <code>len(s)-1</code>. The slice index of a
966 given element may be less than the index of the same element in the
970 A slice, once initialized, is always associated with an underlying
971 array that holds its elements. A slice therefore shares storage
972 with its array and with other slices of the same array; by contrast,
973 distinct arrays always represent distinct storage.
976 The array underlying a slice may extend past the end of the slice.
977 The <i>capacity</i> is a measure of that extent: it is the sum of
978 the length of the slice and the length of the array beyond the slice;
979 a slice of length up to that capacity can be created by
980 <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
981 The capacity of a slice <code>a</code> can be discovered using the
982 built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
986 A new, initialized slice value for a given element type <code>T</code> may be
987 made using the built-in function
988 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
989 which takes a slice type
990 and parameters specifying the length and optionally the capacity.
991 A slice created with <code>make</code> always allocates a new, hidden array
992 to which the returned slice value refers. That is, executing
996 make([]T, length, capacity)
1000 produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
1001 it, so these two expressions are equivalent:
1005 make([]int, 50, 100)
1010 Like arrays, slices are always one-dimensional but may be composed to construct
1011 higher-dimensional objects.
1012 With arrays of arrays, the inner arrays are, by construction, always the same length;
1013 however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
1014 Moreover, the inner slices must be initialized individually.
1017 <h3 id="Struct_types">Struct types</h3>
1020 A struct is a sequence of named elements, called fields, each of which has a
1021 name and a type. Field names may be specified explicitly (IdentifierList) or
1022 implicitly (EmbeddedField).
1023 Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
1024 be <a href="#Uniqueness_of_identifiers">unique</a>.
1028 StructType = "struct" "{" { FieldDecl ";" } "}" .
1029 FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
1030 EmbeddedField = [ "*" ] TypeName [ TypeArgs ] .
1038 // A struct with 6 fields.
1042 _ float32 // padding
1049 A field declared with a type but no explicit field name is called an <i>embedded field</i>.
1050 An embedded field must be specified as
1051 a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
1052 and <code>T</code> itself may not be
1053 a pointer type. The unqualified type name acts as the field name.
1057 // A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
1059 T1 // field name is T1
1060 *T2 // field name is T2
1061 P.T3 // field name is T3
1062 *P.T4 // field name is T4
1063 x, y int // field names are x and y
1068 The following declaration is illegal because field names must be unique
1074 T // conflicts with embedded field *T and *P.T
1075 *T // conflicts with embedded field T and *P.T
1076 *P.T // conflicts with embedded field T and *T
1081 A field or <a href="#Method_declarations">method</a> <code>f</code> of an
1082 embedded field in a struct <code>x</code> is called <i>promoted</i> if
1083 <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
1084 that field or method <code>f</code>.
1088 Promoted fields act like ordinary fields
1089 of a struct except that they cannot be used as field names in
1090 <a href="#Composite_literals">composite literals</a> of the struct.
1094 Given a struct type <code>S</code> and a <a href="#Types">named type</a>
1095 <code>T</code>, promoted methods are included in the method set of the struct as follows:
1099 If <code>S</code> contains an embedded field <code>T</code>,
1100 the <a href="#Method_sets">method sets</a> of <code>S</code>
1101 and <code>*S</code> both include promoted methods with receiver
1102 <code>T</code>. The method set of <code>*S</code> also
1103 includes promoted methods with receiver <code>*T</code>.
1107 If <code>S</code> contains an embedded field <code>*T</code>,
1108 the method sets of <code>S</code> and <code>*S</code> both
1109 include promoted methods with receiver <code>T</code> or
1115 A field declaration may be followed by an optional string literal <i>tag</i>,
1116 which becomes an attribute for all the fields in the corresponding
1117 field declaration. An empty tag string is equivalent to an absent tag.
1118 The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
1119 and take part in <a href="#Type_identity">type identity</a> for structs
1120 but are otherwise ignored.
1125 x, y float64 "" // an empty tag string is like an absent tag
1126 name string "any string is permitted as a tag"
1127 _ [4]byte "ceci n'est pas un champ de structure"
1130 // A struct corresponding to a TimeStamp protocol buffer.
1131 // The tag strings define the protocol buffer field numbers;
1132 // they follow the convention outlined by the reflect package.
1134 microsec uint64 `protobuf:"1"`
1135 serverIP6 uint64 `protobuf:"2"`
1139 <h3 id="Pointer_types">Pointer types</h3>
1142 A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
1143 type, called the <i>base type</i> of the pointer.
1144 The value of an uninitialized pointer is <code>nil</code>.
1148 PointerType = "*" BaseType .
1157 <h3 id="Function_types">Function types</h3>
1160 A function type denotes the set of all functions with the same parameter
1161 and result types. The value of an uninitialized variable of function type
1162 is <code>nil</code>.
1166 FunctionType = "func" Signature .
1167 Signature = Parameters [ Result ] .
1168 Result = Parameters | Type .
1169 Parameters = "(" [ ParameterList [ "," ] ] ")" .
1170 ParameterList = ParameterDecl { "," ParameterDecl } .
1171 ParameterDecl = [ IdentifierList ] [ "..." ] Type .
1175 Within a list of parameters or results, the names (IdentifierList)
1176 must either all be present or all be absent. If present, each name
1177 stands for one item (parameter or result) of the specified type and
1178 all non-<a href="#Blank_identifier">blank</a> names in the signature
1179 must be <a href="#Uniqueness_of_identifiers">unique</a>.
1180 If absent, each type stands for one item of that type.
1181 Parameter and result
1182 lists are always parenthesized except that if there is exactly
1183 one unnamed result it may be written as an unparenthesized type.
1187 The final incoming parameter in a function signature may have
1188 a type prefixed with <code>...</code>.
1189 A function with such a parameter is called <i>variadic</i> and
1190 may be invoked with zero or more arguments for that parameter.
1196 func(a, _ int, z float32) bool
1197 func(a, b int, z float32) (bool)
1198 func(prefix string, values ...int)
1199 func(a, b int, z float64, opt ...interface{}) (success bool)
1200 func(int, int, float64) (float64, *[]int)
1201 func(n int) func(p *T)
1204 <h3 id="Interface_types">Interface types</h3>
1207 An interface type defines a <i>type set</i>.
1208 A variable of interface type can store a value of any type that is in the type
1209 set of the interface. Such a type is said to
1210 <a href="#Implementing_an_interface">implement the interface</a>.
1211 The value of an uninitialized variable of interface type is <code>nil</code>.
1215 InterfaceType = "interface" "{" { InterfaceElem ";" } "}" .
1216 InterfaceElem = MethodElem | TypeElem .
1217 MethodElem = MethodName Signature .
1218 MethodName = identifier .
1219 TypeElem = TypeTerm { "|" TypeTerm } .
1220 TypeTerm = Type | UnderlyingType .
1221 UnderlyingType = "~" Type .
1225 An interface type is specified by a list of <i>interface elements</i>.
1226 An interface element is either a <i>method</i> or a <i>type element</i>,
1227 where a type element is a union of one or more <i>type terms</i>.
1228 A type term is either a single type or a single underlying type.
1231 <h4 id="Basic_interfaces">Basic interfaces</h4>
1234 In its most basic form an interface specifies a (possibly empty) list of methods.
1235 The type set defined by such an interface is the set of types which implement all of
1236 those methods, and the corresponding <a href="#Method_sets">method set</a> consists
1237 exactly of the methods specified by the interface.
1238 Interfaces whose type sets can be defined entirely by a list of methods are called
1239 <i>basic interfaces.</i>
1243 // A simple File interface.
1245 Read([]byte) (int, error)
1246 Write([]byte) (int, error)
1252 The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
1253 and not <a href="#Blank_identifier">blank</a>.
1259 String() string // illegal: String not unique
1260 _(x int) // illegal: method must have non-blank name
1265 More than one type may implement an interface.
1266 For instance, if two types <code>S1</code> and <code>S2</code>
1271 func (p T) Read(p []byte) (n int, err error)
1272 func (p T) Write(p []byte) (n int, err error)
1273 func (p T) Close() error
1277 (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
1278 then the <code>File</code> interface is implemented by both <code>S1</code> and
1279 <code>S2</code>, regardless of what other methods
1280 <code>S1</code> and <code>S2</code> may have or share.
1284 Every type that is a member of the type set of an interface implements that interface.
1285 Any given type may implement several distinct interfaces.
1286 For instance, all types implement the <i>empty interface</i> which stands for the set
1287 of all (non-interface) types:
1295 For convenience, the predeclared type <code>any</code> is an alias for the empty interface.
1299 Similarly, consider this interface specification,
1300 which appears within a <a href="#Type_declarations">type declaration</a>
1301 to define an interface called <code>Locker</code>:
1305 type Locker interface {
1312 If <code>S1</code> and <code>S2</code> also implement
1316 func (p T) Lock() { … }
1317 func (p T) Unlock() { … }
1321 they implement the <code>Locker</code> interface as well
1322 as the <code>File</code> interface.
1325 <h4 id="Embedded_interfaces">Embedded interfaces</h4>
1328 In a slightly more general form
1329 an interface <code>T</code> may use a (possibly qualified) interface type
1330 name <code>E</code> as an interface element. This is called
1331 <i>embedding</i> interface <code>E</code> in <code>T</code>.
1332 The type set of <code>T</code> is the <i>intersection</i> of the type sets
1333 defined by <code>T</code>'s explicitly declared methods and the type sets
1334 of <code>T</code>’s embedded interfaces.
1335 In other words, the type set of <code>T</code> is the set of all types that implement all the
1336 explicitly declared methods of <code>T</code> and also all the methods of
1341 type Reader interface {
1342 Read(p []byte) (n int, err error)
1346 type Writer interface {
1347 Write(p []byte) (n int, err error)
1351 // ReadWriter's methods are Read, Write, and Close.
1352 type ReadWriter interface {
1353 Reader // includes methods of Reader in ReadWriter's method set
1354 Writer // includes methods of Writer in ReadWriter's method set
1359 When embedding interfaces, methods with the
1360 <a href="#Uniqueness_of_identifiers">same</a> names must
1361 have <a href="#Type_identity">identical</a> signatures.
1365 type ReadCloser interface {
1366 Reader // includes methods of Reader in ReadCloser's method set
1367 Close() // illegal: signatures of Reader.Close and Close are different
1371 <h4 id="General_interfaces">General interfaces</h4>
1374 In their most general form, an interface element may also be an arbitrary type term
1375 <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
1376 or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
1377 Together with method specifications, these elements enable the precise
1378 definition of an interface's type set as follows:
1382 <li>The type set of the empty interface is the set of all non-interface types.
1385 <li>The type set of a non-empty interface is the intersection of the type sets
1386 of its interface elements.
1389 <li>The type set of a method specification is the set of all non-interface types
1390 whose method sets include that method.
1393 <li>The type set of a non-interface type term is the set consisting
1397 <li>The type set of a term of the form <code>~T</code>
1398 is the set of all types whose underlying type is <code>T</code>.
1401 <li>The type set of a <i>union</i> of terms
1402 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
1403 is the union of the type sets of the terms.
1408 The quantification "the set of all non-interface types" refers not just to all (non-interface)
1409 types declared in the program at hand, but all possible types in all possible programs, and
1411 Similarly, given the set of all non-interface types that implement a particular method, the
1412 intersection of the method sets of those types will contain exactly that method, even if all
1413 types in the program at hand always pair that method with another method.
1417 By construction, an interface's type set never contains an interface type.
1421 // An interface representing only the type int.
1426 // An interface representing all types with underlying type int.
1431 // An interface representing all types with underlying type int that implement the String method.
1437 // An interface representing an empty type set: there is no type that is both an int and a string.
1445 In a term of the form <code>~T</code>, the underlying type of <code>T</code>
1446 must be itself, and <code>T</code> cannot be an interface.
1453 ~[]byte // the underlying type of []byte is itself
1454 ~MyInt // illegal: the underlying type of MyInt is not MyInt
1455 ~error // illegal: error is an interface
1460 Union elements denote unions of type sets:
1464 // The Float interface represents all floating-point types
1465 // (including any named types whose underlying types are
1466 // either float32 or float64).
1467 type Float interface {
1473 The type <code>T</code> in a term of the form <code>T</code> or <code>~T</code> cannot
1474 be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all
1475 non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
1476 Given a type parameter <code>P</code>:
1481 P // illegal: P is a type parameter
1482 int | ~P // illegal: P is a type parameter
1483 ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
1484 float32 | Float // overlapping type sets but Float is an interface
1489 Implementation restriction:
1490 A union (with more than one term) cannot contain the
1491 <a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code>
1492 or interfaces that specify methods, or embed <code>comparable</code> or interfaces
1493 that specify methods.
1497 Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type
1498 constraints, or as elements of other interfaces used as constraints.
1499 They cannot be the types of values or variables, or components of other,
1500 non-interface types.
1504 var x Float // illegal: Float is not a basic interface
1506 var x interface{} = Float(nil) // illegal
1508 type Floatish struct {
1514 An interface type <code>T</code> may not embed any type element
1515 that is, contains, or embeds <code>T</code>, recursively.
1519 // illegal: Bad cannot embed itself
1520 type Bad interface {
1524 // illegal: Bad1 cannot embed itself using Bad2
1525 type Bad1 interface {
1528 type Bad2 interface {
1532 // illegal: Bad3 cannot embed a union containing Bad3
1533 type Bad3 interface {
1534 ~int | ~string | Bad3
1538 <h4 id="Implementing_an_interface">Implementing an interface</h4>
1541 A type <code>T</code> implements an interface <code>I</code> if
1546 <code>T</code> is not an interface and is an element of the type set of <code>I</code>; or
1549 <code>T</code> is an interface and the type set of <code>T</code> is a subset of the
1550 type set of <code>I</code>.
1555 A value of type <code>T</code> implements an interface if <code>T</code>
1556 implements the interface.
1559 <h3 id="Map_types">Map types</h3>
1562 A map is an unordered group of elements of one type, called the
1563 element type, indexed by a set of unique <i>keys</i> of another type,
1564 called the key type.
1565 The value of an uninitialized map is <code>nil</code>.
1569 MapType = "map" "[" KeyType "]" ElementType .
1574 The <a href="#Comparison_operators">comparison operators</a>
1575 <code>==</code> and <code>!=</code> must be fully defined
1576 for operands of the key type; thus the key type must not be a function, map, or
1578 If the key type is an interface type, these
1579 comparison operators must be defined for the dynamic key values;
1580 failure will cause a <a href="#Run_time_panics">run-time panic</a>.
1585 map[*T]struct{ x, y float64 }
1586 map[string]interface{}
1590 The number of map elements is called its length.
1591 For a map <code>m</code>, it can be discovered using the
1592 built-in function <a href="#Length_and_capacity"><code>len</code></a>
1593 and may change during execution. Elements may be added during execution
1594 using <a href="#Assignment_statements">assignments</a> and retrieved with
1595 <a href="#Index_expressions">index expressions</a>; they may be removed with the
1596 <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
1599 A new, empty map value is made using the built-in
1600 function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1601 which takes the map type and an optional capacity hint as arguments:
1605 make(map[string]int)
1606 make(map[string]int, 100)
1610 The initial capacity does not bound its size:
1611 maps grow to accommodate the number of items
1612 stored in them, with the exception of <code>nil</code> maps.
1613 A <code>nil</code> map is equivalent to an empty map except that no elements
1616 <h3 id="Channel_types">Channel types</h3>
1619 A channel provides a mechanism for
1620 <a href="#Go_statements">concurrently executing functions</a>
1622 <a href="#Send_statements">sending</a> and
1623 <a href="#Receive_operator">receiving</a>
1624 values of a specified element type.
1625 The value of an uninitialized channel is <code>nil</code>.
1629 ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
1633 The optional <code><-</code> operator specifies the channel <i>direction</i>,
1634 <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
1635 otherwise it is <i>bidirectional</i>.
1636 A channel may be constrained only to send or only to receive by
1637 <a href="#Assignment_statements">assignment</a> or
1638 explicit <a href="#Conversions">conversion</a>.
1642 chan T // can be used to send and receive values of type T
1643 chan<- float64 // can only be used to send float64s
1644 <-chan int // can only be used to receive ints
1648 The <code><-</code> operator associates with the leftmost <code>chan</code>
1653 chan<- chan int // same as chan<- (chan int)
1654 chan<- <-chan int // same as chan<- (<-chan int)
1655 <-chan <-chan int // same as <-chan (<-chan int)
1656 chan (<-chan int)
1660 A new, initialized channel
1661 value can be made using the built-in function
1662 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1663 which takes the channel type and an optional <i>capacity</i> as arguments:
1671 The capacity, in number of elements, sets the size of the buffer in the channel.
1672 If the capacity is zero or absent, the channel is unbuffered and communication
1673 succeeds only when both a sender and receiver are ready. Otherwise, the channel
1674 is buffered and communication succeeds without blocking if the buffer
1675 is not full (sends) or not empty (receives).
1676 A <code>nil</code> channel is never ready for communication.
1680 A channel may be closed with the built-in function
1681 <a href="#Close"><code>close</code></a>.
1682 The multi-valued assignment form of the
1683 <a href="#Receive_operator">receive operator</a>
1684 reports whether a received value was sent before
1685 the channel was closed.
1689 A single channel may be used in
1690 <a href="#Send_statements">send statements</a>,
1691 <a href="#Receive_operator">receive operations</a>,
1692 and calls to the built-in functions
1693 <a href="#Length_and_capacity"><code>cap</code></a> and
1694 <a href="#Length_and_capacity"><code>len</code></a>
1695 by any number of goroutines without further synchronization.
1696 Channels act as first-in-first-out queues.
1697 For example, if one goroutine sends values on a channel
1698 and a second goroutine receives them, the values are
1699 received in the order sent.
1702 <h2 id="Properties_of_types_and_values">Properties of types and values</h2>
1704 <h3 id="Underlying_types">Underlying types</h3>
1707 Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
1708 is one of the predeclared boolean, numeric, or string types, or a type literal,
1709 the corresponding underlying type is <code>T</code> itself.
1710 Otherwise, <code>T</code>'s underlying type is the underlying type of the
1711 type to which <code>T</code> refers in its declaration.
1712 For a type parameter that is the underlying type of its
1713 <a href="#Type_constraints">type constraint</a>, which is always an interface.
1729 func f[P any](x P) { … }
1733 The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
1734 and <code>B2</code> is <code>string</code>.
1735 The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
1736 The underlying type of <code>P</code> is <code>interface{}</code>.
1739 <h3 id="Core_types">Core types</h3>
1742 Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the
1743 <a href="#Underlying_types">underlying type</a> of <code>T</code>.
1747 An interface <code>T</code> has a core type if one of the following
1748 conditions is satisfied:
1753 There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a>
1754 of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or
1757 the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a>
1758 with identical element type <code>E</code>, and all directional channels have the same
1764 No other interfaces have a core type.
1768 The core type of an interface is, depending on the condition that is satisfied, either:
1773 the type <code>U</code>; or
1776 the type <code>chan E</code> if <code>T</code> contains only bidirectional
1777 channels, or the type <code>chan<- E</code> or <code><-chan E</code>
1778 depending on the direction of the directional channels present.
1783 By definition, a core type is never a <a href="#Type_definitions">defined type</a>,
1784 <a href="#Type_parameter_declarations">type parameter</a>, or
1785 <a href="#Interface_types">interface type</a>.
1789 Examples of interfaces with core types:
1793 type Celsius float32
1796 interface{ int } // int
1797 interface{ Celsius|Kelvin } // float32
1798 interface{ ~chan int } // chan int
1799 interface{ ~chan int|~chan<- int } // chan<- int
1800 interface{ ~[]*data; String() string } // []*data
1804 Examples of interfaces without core types:
1808 interface{} // no single underlying type
1809 interface{ Celsius|float64 } // no single underlying type
1810 interface{ chan int | chan<- string } // channels have different element types
1811 interface{ <-chan int | chan<- int } // directional channels have different directions
1815 Some operations (<a href="#Slice_expressions">slice expressions</a>,
1816 <a href="#Appending_and_copying_slices"><code>append</code> and <code>copy</code></a>)
1817 rely on a slightly more loose form of core types which accept byte slices and strings.
1818 Specifically, if there are exactly two types, <code>[]byte</code> and <code>string</code>,
1819 which are the underlying types of all types in the type set of interface <code>T</code>,
1820 the core type of <code>T</code> is called <code>bytestring</code>.
1824 Examples of interfaces with <code>bytestring</code> core types:
1828 interface{ int } // int (same as ordinary core type)
1829 interface{ []byte | string } // bytestring
1830 interface{ ~[]byte | myString } // bytestring
1834 Note that <code>bytestring</code> is not a real type; it cannot be used to declare
1835 variables are compose other types. It exists solely to describe the behavior of some
1836 operations that read from a sequence of bytes, which may be a byte slice or a string.
1839 <h3 id="Type_identity">Type identity</h3>
1842 Two types are either <i>identical</i> or <i>different</i>.
1846 A <a href="#Types">named type</a> is always different from any other type.
1847 Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
1848 structurally equivalent; that is, they have the same literal structure and corresponding
1849 components have identical types. In detail:
1853 <li>Two array types are identical if they have identical element types and
1854 the same array length.</li>
1856 <li>Two slice types are identical if they have identical element types.</li>
1858 <li>Two struct types are identical if they have the same sequence of fields,
1859 and if corresponding fields have the same names, and identical types,
1861 <a href="#Exported_identifiers">Non-exported</a> field names from different
1862 packages are always different.</li>
1864 <li>Two pointer types are identical if they have identical base types.</li>
1866 <li>Two function types are identical if they have the same number of parameters
1867 and result values, corresponding parameter and result types are
1868 identical, and either both functions are variadic or neither is.
1869 Parameter and result names are not required to match.</li>
1871 <li>Two interface types are identical if they define the same type set.
1874 <li>Two map types are identical if they have identical key and element types.</li>
1876 <li>Two channel types are identical if they have identical element types and
1877 the same direction.</li>
1879 <li>Two <a href="#Instantiations">instantiated</a> types are identical if
1880 their defined types and all type arguments are identical.
1885 Given the declarations
1892 A2 = struct{ a, b int }
1894 A4 = func(A3, float64) *A0
1895 A5 = func(x int, _ float64) *[]string
1899 B2 struct{ a, b int }
1900 B3 struct{ a, c int }
1901 B4 func(int, float64) *B0
1902 B5 func(x int, y float64) *A1
1905 D0[P1, P2 any] struct{ x P1; y P2 }
1906 E0 = D0[int, string]
1911 these types are identical:
1915 A0, A1, and []string
1916 A2 and struct{ a, b int }
1918 A4, func(int, float64) *[]string, and A5
1921 D0[int, string] and E0
1923 struct{ a, b *B5 } and struct{ a, b *B5 }
1924 func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
1928 <code>B0</code> and <code>B1</code> are different because they are new types
1929 created by distinct <a href="#Type_definitions">type definitions</a>;
1930 <code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
1931 are different because <code>B0</code> is different from <code>[]string</code>;
1932 and <code>P1</code> and <code>P2</code> are different because they are different
1934 <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are
1935 different because the former is an <a href="#Instantiations">instantiated</a>
1936 defined type while the latter is a type literal
1937 (but they are still <a href="#Assignability">assignable</a>).
1940 <h3 id="Assignability">Assignability</h3>
1943 A value <code>x</code> of type <code>V</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
1944 ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
1949 <code>V</code> and <code>T</code> are identical.
1952 <code>V</code> and <code>T</code> have identical
1953 <a href="#Underlying_types">underlying types</a>
1954 but are not type parameters and at least one of <code>V</code>
1955 or <code>T</code> is not a <a href="#Types">named type</a>.
1958 <code>V</code> and <code>T</code> are channel types with
1959 identical element types, <code>V</code> is a bidirectional channel,
1960 and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
1963 <code>T</code> is an interface type, but not a type parameter, and
1964 <code>x</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
1967 <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
1968 is a pointer, function, slice, map, channel, or interface type,
1969 but not a type parameter.
1972 <code>x</code> is an untyped <a href="#Constants">constant</a>
1973 <a href="#Representability">representable</a>
1974 by a value of type <code>T</code>.
1979 Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code>
1980 is assignable to a variable of type <code>T</code> if one of the following conditions applies:
1985 <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
1986 a type parameter, and <code>x</code> is assignable to each type in
1987 <code>T</code>'s type set.
1990 <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
1991 a type parameter, and <code>x</code> is assignable to each type in
1992 <code>T</code>'s type set.
1995 <code>V</code> is a type parameter and <code>T</code> is not a named type,
1996 and values of each type in <code>V</code>'s type set are assignable
2001 <h3 id="Representability">Representability</h3>
2004 A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
2005 by a value of type <code>T</code>,
2006 where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>,
2007 if one of the following conditions applies:
2012 <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
2016 <code>T</code> is a <a href="#Numeric_types">floating-point type</a> and <code>x</code> can be rounded to <code>T</code>'s
2017 precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
2018 negative zero further simplified to an unsigned zero. Note that constant values never result
2019 in an IEEE negative zero, NaN, or infinity.
2023 <code>T</code> is a complex type, and <code>x</code>'s
2024 <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
2025 are representable by values of <code>T</code>'s component type (<code>float32</code> or
2026 <code>float64</code>).
2031 If <code>T</code> is a type parameter,
2032 <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
2033 by a value of each type in <code>T</code>'s type set.
2037 x T x is representable by a value of T because
2039 'a' byte 97 is in the set of byte values
2040 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
2041 "foo" string "foo" is in the set of string values
2042 1024 int16 1024 is in the set of 16-bit integers
2043 42.0 byte 42 is in the set of unsigned 8-bit integers
2044 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
2045 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
2046 -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
2047 0i int 0 is an integer value
2048 (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
2052 x T x is not representable by a value of T because
2054 0 bool 0 is not in the set of boolean values
2055 'a' string 'a' is a rune, it is not in the set of string values
2056 1024 byte 1024 is not in the set of unsigned 8-bit integers
2057 -1 uint16 -1 is not in the set of unsigned 16-bit integers
2058 1.1 int 1.1 is not an integer value
2059 42i float32 (0 + 42i) is not in the set of float32 values
2060 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
2063 <h3 id="Method_sets">Method sets</h3>
2066 The <i>method set</i> of a type determines the methods that can be
2067 <a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type.
2068 Every type has a (possibly empty) method set associated with it:
2072 <li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all
2073 <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
2077 The method set of a pointer to a defined type <code>T</code>
2078 (where <code>T</code> is neither a pointer nor an interface)
2079 is the set of all methods declared with receiver <code>*T</code> or <code>T</code>.
2082 <li>The method set of an <a href="#Interface_types">interface type</a> is the intersection
2083 of the method sets of each type in the interface's <a href="#Interface_types">type set</a>
2084 (the resulting method set is usually just the set of declared methods in the interface).
2089 Further rules apply to structs (and pointer to structs) containing embedded fields,
2090 as described in the section on <a href="#Struct_types">struct types</a>.
2091 Any other type has an empty method set.
2095 In a method set, each method must have a
2096 <a href="#Uniqueness_of_identifiers">unique</a>
2097 non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
2100 <h2 id="Blocks">Blocks</h2>
2103 A <i>block</i> is a possibly empty sequence of declarations and statements
2104 within matching brace brackets.
2108 Block = "{" StatementList "}" .
2109 StatementList = { Statement ";" } .
2113 In addition to explicit blocks in the source code, there are implicit blocks:
2117 <li>The <i>universe block</i> encompasses all Go source text.</li>
2119 <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
2120 Go source text for that package.</li>
2122 <li>Each file has a <i>file block</i> containing all Go source text
2125 <li>Each <a href="#If_statements">"if"</a>,
2126 <a href="#For_statements">"for"</a>, and
2127 <a href="#Switch_statements">"switch"</a>
2128 statement is considered to be in its own implicit block.</li>
2130 <li>Each clause in a <a href="#Switch_statements">"switch"</a>
2131 or <a href="#Select_statements">"select"</a> statement
2132 acts as an implicit block.</li>
2136 Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
2140 <h2 id="Declarations_and_scope">Declarations and scope</h2>
2143 A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
2144 <a href="#Constant_declarations">constant</a>,
2145 <a href="#Type_declarations">type</a>,
2146 <a href="#Type_parameter_declarations">type parameter</a>,
2147 <a href="#Variable_declarations">variable</a>,
2148 <a href="#Function_declarations">function</a>,
2149 <a href="#Labeled_statements">label</a>, or
2150 <a href="#Import_declarations">package</a>.
2151 Every identifier in a program must be declared.
2152 No identifier may be declared twice in the same block, and
2153 no identifier may be declared in both the file and package block.
2157 The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
2158 in a declaration, but it does not introduce a binding and thus is not declared.
2159 In the package block, the identifier <code>init</code> may only be used for
2160 <a href="#Package_initialization"><code>init</code> function</a> declarations,
2161 and like the blank identifier it does not introduce a new binding.
2165 Declaration = ConstDecl | TypeDecl | VarDecl .
2166 TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
2170 The <i>scope</i> of a declared identifier is the extent of source text in which
2171 the identifier denotes the specified constant, type, variable, function, label, or package.
2175 Go is lexically scoped using <a href="#Blocks">blocks</a>:
2179 <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
2181 <li>The scope of an identifier denoting a constant, type, variable,
2182 or function (but not method) declared at top level (outside any
2183 function) is the package block.</li>
2185 <li>The scope of the package name of an imported package is the file block
2186 of the file containing the import declaration.</li>
2188 <li>The scope of an identifier denoting a method receiver, function parameter,
2189 or result variable is the function body.</li>
2191 <li>The scope of an identifier denoting a type parameter of a function
2192 or declared by a method receiver begins after the name of the function
2193 and ends at the end of the function body.</li>
2195 <li>The scope of an identifier denoting a type parameter of a type
2196 begins after the name of the type and ends at the end
2197 of the TypeSpec.</li>
2199 <li>The scope of a constant or variable identifier declared
2200 inside a function begins at the end of the ConstSpec or VarSpec
2201 (ShortVarDecl for short variable declarations)
2202 and ends at the end of the innermost containing block.</li>
2204 <li>The scope of a type identifier declared inside a function
2205 begins at the identifier in the TypeSpec
2206 and ends at the end of the innermost containing block.</li>
2210 An identifier declared in a block may be redeclared in an inner block.
2211 While the identifier of the inner declaration is in scope, it denotes
2212 the entity declared by the inner declaration.
2216 The <a href="#Package_clause">package clause</a> is not a declaration; the package name
2217 does not appear in any scope. Its purpose is to identify the files belonging
2218 to the same <a href="#Packages">package</a> and to specify the default package name for import
2223 <h3 id="Label_scopes">Label scopes</h3>
2226 Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
2227 used in the <a href="#Break_statements">"break"</a>,
2228 <a href="#Continue_statements">"continue"</a>, and
2229 <a href="#Goto_statements">"goto"</a> statements.
2230 It is illegal to define a label that is never used.
2231 In contrast to other identifiers, labels are not block scoped and do
2232 not conflict with identifiers that are not labels. The scope of a label
2233 is the body of the function in which it is declared and excludes
2234 the body of any nested function.
2238 <h3 id="Blank_identifier">Blank identifier</h3>
2241 The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
2242 It serves as an anonymous placeholder instead of a regular (non-blank)
2243 identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
2244 as an <a href="#Operands">operand</a>, and in <a href="#Assignment_statements">assignment statements</a>.
2248 <h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
2251 The following identifiers are implicitly declared in the
2252 <a href="#Blocks">universe block</a>:
2254 <pre class="grammar">
2256 any bool byte comparable
2257 complex64 complex128 error float32 float64
2258 int int8 int16 int32 int64 rune string
2259 uint uint8 uint16 uint32 uint64 uintptr
2268 append cap close complex copy delete imag len
2269 make new panic print println real recover
2272 <h3 id="Exported_identifiers">Exported identifiers</h3>
2275 An identifier may be <i>exported</i> to permit access to it from another package.
2276 An identifier is exported if both:
2279 <li>the first character of the identifier's name is a Unicode uppercase
2280 letter (Unicode character category Lu); and</li>
2281 <li>the identifier is declared in the <a href="#Blocks">package block</a>
2282 or it is a <a href="#Struct_types">field name</a> or
2283 <a href="#MethodName">method name</a>.</li>
2286 All other identifiers are not exported.
2289 <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
2292 Given a set of identifiers, an identifier is called <i>unique</i> if it is
2293 <i>different</i> from every other in the set.
2294 Two identifiers are different if they are spelled differently, or if they
2295 appear in different <a href="#Packages">packages</a> and are not
2296 <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
2299 <h3 id="Constant_declarations">Constant declarations</h3>
2302 A constant declaration binds a list of identifiers (the names of
2303 the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
2304 The number of identifiers must be equal
2305 to the number of expressions, and the <i>n</i>th identifier on
2306 the left is bound to the value of the <i>n</i>th expression on the
2311 ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
2312 ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
2314 IdentifierList = identifier { "," identifier } .
2315 ExpressionList = Expression { "," Expression } .
2319 If the type is present, all constants take the type specified, and
2320 the expressions must be <a href="#Assignability">assignable</a> to that type,
2321 which must not be a type parameter.
2322 If the type is omitted, the constants take the
2323 individual types of the corresponding expressions.
2324 If the expression values are untyped <a href="#Constants">constants</a>,
2325 the declared constants remain untyped and the constant identifiers
2326 denote the constant values. For instance, if the expression is a
2327 floating-point literal, the constant identifier denotes a floating-point
2328 constant, even if the literal's fractional part is zero.
2332 const Pi float64 = 3.14159265358979323846
2333 const zero = 0.0 // untyped floating-point constant
2336 eof = -1 // untyped integer constant
2338 const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
2339 const u, v float32 = 0, 3 // u = 0.0, v = 3.0
2343 Within a parenthesized <code>const</code> declaration list the
2344 expression list may be omitted from any but the first ConstSpec.
2345 Such an empty list is equivalent to the textual substitution of the
2346 first preceding non-empty expression list and its type if any.
2347 Omitting the list of expressions is therefore equivalent to
2348 repeating the previous list. The number of identifiers must be equal
2349 to the number of expressions in the previous list.
2350 Together with the <a href="#Iota"><code>iota</code> constant generator</a>
2351 this mechanism permits light-weight declaration of sequential values:
2363 numberOfDays // this constant is not exported
2368 <h3 id="Iota">Iota</h3>
2371 Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
2372 <code>iota</code> represents successive untyped integer <a href="#Constants">
2373 constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
2374 in that constant declaration, starting at zero.
2375 It can be used to construct a set of related constants:
2380 c0 = iota // c0 == 0
2381 c1 = iota // c1 == 1
2382 c2 = iota // c2 == 2
2386 a = 1 << iota // a == 1 (iota == 0)
2387 b = 1 << iota // b == 2 (iota == 1)
2388 c = 3 // c == 3 (iota == 2, unused)
2389 d = 1 << iota // d == 8 (iota == 3)
2393 u = iota * 42 // u == 0 (untyped integer constant)
2394 v float64 = iota * 42 // v == 42.0 (float64 constant)
2395 w = iota * 42 // w == 84 (untyped integer constant)
2398 const x = iota // x == 0
2399 const y = iota // y == 0
2403 By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
2408 bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
2409 bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
2410 _, _ // (iota == 2, unused)
2411 bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
2416 This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
2417 of the last non-empty expression list.
2421 <h3 id="Type_declarations">Type declarations</h3>
2424 A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
2425 Type declarations come in two forms: alias declarations and type definitions.
2429 TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
2430 TypeSpec = AliasDecl | TypeDef .
2433 <h4 id="Alias_declarations">Alias declarations</h4>
2436 An alias declaration binds an identifier to the given type.
2440 AliasDecl = identifier "=" Type .
2444 Within the <a href="#Declarations_and_scope">scope</a> of
2445 the identifier, it serves as an <i>alias</i> for the type.
2450 nodeList = []*Node // nodeList and []*Node are identical types
2451 Polar = polar // Polar and polar denote identical types
2456 <h4 id="Type_definitions">Type definitions</h4>
2459 A type definition creates a new, distinct type with the same
2460 <a href="#Types">underlying type</a> and operations as the given type
2461 and binds an identifier, the <i>type name</i>, to it.
2465 TypeDef = identifier [ TypeParameters ] Type .
2469 The new type is called a <i>defined type</i>.
2470 It is <a href="#Type_identity">different</a> from any other type,
2471 including the type it is created from.
2476 Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
2477 polar Point // polar and Point denote different types
2480 type TreeNode struct {
2481 left, right *TreeNode
2485 type Block interface {
2487 Encrypt(src, dst []byte)
2488 Decrypt(src, dst []byte)
2493 A defined type may have <a href="#Method_declarations">methods</a> associated with it.
2494 It does not inherit any methods bound to the given type,
2495 but the <a href="#Method_sets">method set</a>
2496 of an interface type or of elements of a composite type remains unchanged:
2500 // A Mutex is a data type with two methods, Lock and Unlock.
2501 type Mutex struct { /* Mutex fields */ }
2502 func (m *Mutex) Lock() { /* Lock implementation */ }
2503 func (m *Mutex) Unlock() { /* Unlock implementation */ }
2505 // NewMutex has the same composition as Mutex but its method set is empty.
2508 // The method set of PtrMutex's underlying type *Mutex remains unchanged,
2509 // but the method set of PtrMutex is empty.
2510 type PtrMutex *Mutex
2512 // The method set of *PrintableMutex contains the methods
2513 // Lock and Unlock bound to its embedded field Mutex.
2514 type PrintableMutex struct {
2518 // MyBlock is an interface type that has the same method set as Block.
2523 Type definitions may be used to define different boolean, numeric,
2524 or string types and associate methods with them:
2531 EST TimeZone = -(5 + iota)
2537 func (tz TimeZone) String() string {
2538 return fmt.Sprintf("GMT%+dh", tz)
2543 If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>,
2544 the type name denotes a <i>generic type</i>.
2545 Generic types must be <a href="#Instantiations">instantiated</a> when they
2550 type List[T any] struct {
2557 In a type definition the given type cannot be a type parameter.
2561 type T[P any] P // illegal: P is a type parameter
2564 type L T // illegal: T is a type parameter declared by the enclosing function
2569 A generic type may also have <a href="#Method_declarations">methods</a> associated with it.
2570 In this case, the method receivers must declare the same number of type parameters as
2571 present in the generic type definition.
2575 // The method Len returns the number of elements in the linked list l.
2576 func (l *List[T]) Len() int { … }
2579 <h3 id="Type_parameter_declarations">Type parameter declarations</h3>
2582 A type parameter list declares the <i>type parameters</i> of a generic function or type declaration.
2583 The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a>
2584 except that the type parameter names must all be present and the list is enclosed
2585 in square brackets rather than parentheses.
2589 TypeParameters = "[" TypeParamList [ "," ] "]" .
2590 TypeParamList = TypeParamDecl { "," TypeParamDecl } .
2591 TypeParamDecl = IdentifierList TypeConstraint .
2595 All non-blank names in the list must be unique.
2596 Each name declares a type parameter, which is a new and different <a href="#Types">named type</a>
2597 that acts as a place holder for an (as of yet) unknown type in the declaration.
2598 The type parameter is replaced with a <i>type argument</i> upon
2599 <a href="#Instantiations">instantiation</a> of the generic function or type.
2604 [S interface{ ~[]byte|string }]
2611 Just as each ordinary function parameter has a parameter type, each type parameter
2612 has a corresponding (meta-)type which is called its
2613 <a href="#Type_constraints"><i>type constraint</i></a>.
2617 A parsing ambiguity arises when the type parameter list for a generic type
2618 declares a single type parameter <code>P</code> with a constraint <code>C</code>
2619 such that the text <code>P C</code> forms a valid expression:
2630 In these rare cases, the type parameter list is indistinguishable from an
2631 expression and the type declaration is parsed as an array type declaration.
2632 To resolve the ambiguity, embed the constraint in an
2633 <a href="#Interface_types">interface</a> or use a trailing comma:
2637 type T[P interface{*C}] …
2642 Type parameters may also be declared by the receiver specification
2643 of a <a href="#Method_declarations">method declaration</a> associated
2644 with a generic type.
2648 This section needs to explain if and what kind of cycles are permitted
2649 using type parameters in a type parameter list.
2652 <h4 id="Type_constraints">Type constraints</h4>
2655 A type constraint is an <a href="#Interface_types">interface</a> that defines the
2656 set of permissible type arguments for the respective type parameter and controls the
2657 operations supported by values of that type parameter.
2661 TypeConstraint = TypeElem .
2665 If the constraint is an interface literal of the form <code>interface{E}</code> where
2666 <code>E</code> is an embedded type element (not a method), in a type parameter list
2667 the enclosing <code>interface{ … }</code> may be omitted for convenience:
2671 [T []P] // = [T interface{[]P}]
2672 [T ~int] // = [T interface{~int}]
2673 [T int|string] // = [T interface{int|string}]
2674 type Constraint ~int // illegal: ~int is not inside a type parameter list
2678 We should be able to simplify the rules for comparable or delegate some of them
2679 elsewhere since we have a section that clearly defines how interfaces implement
2680 other interfaces based on their type sets. But this should get us going for now.
2684 The <a href="#Predeclared_identifiers">predeclared</a>
2685 <a href="#Interface_types">interface type</a> <code>comparable</code>
2686 denotes the set of all non-interface types that are
2687 <a href="#Comparison_operators">comparable</a>. Specifically,
2688 a type <code>T</code> implements <code>comparable</code> if:
2693 <code>T</code> is not an interface type and <code>T</code> supports the operations
2694 <code>==</code> and <code>!=</code>; or
2697 <code>T</code> is an interface type and each type in <code>T</code>'s
2698 <a href="#Interface_types">type set</a> implements <code>comparable</code>.
2703 Even though interfaces that are not type parameters can be
2704 <a href="#Comparison_operators">compared</a>
2705 (possibly causing a run-time panic) they do not implement
2706 <code>comparable</code>.
2710 int // implements comparable
2711 []byte // does not implement comparable (slices cannot be compared)
2712 interface{} // does not implement comparable (see above)
2713 interface{ ~int | ~string } // type parameter only: implements comparable
2714 interface{ comparable } // type parameter only: implements comparable
2715 interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (not all types in the type set are comparable)
2719 The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
2720 <code>comparable</code> may only be used as type constraints. They cannot be the types of
2721 values or variables, or components of other, non-interface types.
2724 <h3 id="Variable_declarations">Variable declarations</h3>
2727 A variable declaration creates one or more <a href="#Variables">variables</a>,
2728 binds corresponding identifiers to them, and gives each a type and an initial value.
2732 VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
2733 VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
2740 var x, y float32 = -1, -2
2743 u, v, s = 2.0, 3.0, "bar"
2745 var re, im = complexSqrt(-1)
2746 var _, found = entries[name] // map lookup; only interested in "found"
2750 If a list of expressions is given, the variables are initialized
2751 with the expressions following the rules for <a href="#Assignment_statements">assignment statements</a>.
2752 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
2756 If a type is present, each variable is given that type.
2757 Otherwise, each variable is given the type of the corresponding
2758 initialization value in the assignment.
2759 If that value is an untyped constant, it is first implicitly
2760 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
2761 if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
2762 The predeclared value <code>nil</code> cannot be used to initialize a variable
2763 with no explicit type.
2767 var d = math.Sin(0.5) // d is float64
2768 var i = 42 // i is int
2769 var t, ok = x.(T) // t is T, ok is bool
2770 var n = nil // illegal
2774 Implementation restriction: A compiler may make it illegal to declare a variable
2775 inside a <a href="#Function_declarations">function body</a> if the variable is
2779 <h3 id="Short_variable_declarations">Short variable declarations</h3>
2782 A <i>short variable declaration</i> uses the syntax:
2786 ShortVarDecl = IdentifierList ":=" ExpressionList .
2790 It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
2791 with initializer expressions but no types:
2794 <pre class="grammar">
2795 "var" IdentifierList "=" ExpressionList .
2800 f := func() int { return 7 }
2801 ch := make(chan int)
2802 r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
2803 _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
2807 Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
2808 variables provided they were originally declared earlier in the same block
2809 (or the parameter lists if the block is the function body) with the same type,
2810 and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
2811 As a consequence, redeclaration can only appear in a multi-variable short declaration.
2812 Redeclaration does not introduce a new variable; it just assigns a new value to the original.
2813 The non-blank variable names on the left side of <code>:=</code>
2814 must be <a href="#Uniqueness_of_identifiers">unique</a>.
2818 field1, offset := nextField(str, 0)
2819 field2, offset := nextField(str, offset) // redeclares offset
2820 x, y, x := 1, 2, 3 // illegal: x repeated on left side of :=
2824 Short variable declarations may appear only inside functions.
2825 In some contexts such as the initializers for
2826 <a href="#If_statements">"if"</a>,
2827 <a href="#For_statements">"for"</a>, or
2828 <a href="#Switch_statements">"switch"</a> statements,
2829 they can be used to declare local temporary variables.
2832 <h3 id="Function_declarations">Function declarations</h3>
2835 Given the importance of functions, this section has always
2836 been woefully underdeveloped. Would be nice to expand this
2841 A function declaration binds an identifier, the <i>function name</i>,
2846 FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
2847 FunctionName = identifier .
2848 FunctionBody = Block .
2852 If the function's <a href="#Function_types">signature</a> declares
2853 result parameters, the function body's statement list must end in
2854 a <a href="#Terminating_statements">terminating statement</a>.
2858 func IndexRune(s string, r rune) int {
2859 for i, c := range s {
2864 // invalid: missing return statement
2869 If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>,
2870 the function name denotes a <i>generic function</i>.
2871 A generic function must be <a href="#Instantiations">instantiated</a> before it can be
2872 called or used as a value.
2876 func min[T ~int|~float64](x, y T) T {
2885 A function declaration without type parameters may omit the body.
2886 Such a declaration provides the signature for a function implemented outside Go,
2887 such as an assembly routine.
2891 func flushICache(begin, end uintptr) // implemented externally
2894 <h3 id="Method_declarations">Method declarations</h3>
2897 A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
2898 A method declaration binds an identifier, the <i>method name</i>, to a method,
2899 and associates the method with the receiver's <i>base type</i>.
2903 MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
2904 Receiver = Parameters .
2908 The receiver is specified via an extra parameter section preceding the method
2909 name. That parameter section must declare a single non-variadic parameter, the receiver.
2910 Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
2911 pointer to a defined type <code>T</code>, possibly followed by a list of type parameter
2912 names <code>[P1, P2, …]</code> enclosed in square brackets.
2913 <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be
2914 a pointer or interface type and it must be defined in the same package as the method.
2915 The method is said to be <i>bound</i> to its receiver base type and the method name
2916 is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
2921 A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
2922 <a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
2923 If the receiver's value is not referenced inside the body of the method,
2924 its identifier may be omitted in the declaration. The same applies in
2925 general to parameters of functions and methods.
2929 For a base type, the non-blank names of methods bound to it must be unique.
2930 If the base type is a <a href="#Struct_types">struct type</a>,
2931 the non-blank method and field names must be distinct.
2935 Given defined type <code>Point</code> the declarations
2939 func (p *Point) Length() float64 {
2940 return math.Sqrt(p.x * p.x + p.y * p.y)
2943 func (p *Point) Scale(factor float64) {
2950 bind the methods <code>Length</code> and <code>Scale</code>,
2951 with receiver type <code>*Point</code>,
2952 to the base type <code>Point</code>.
2956 If the receiver base type is a <a href="#Type_declarations">generic type</a>, the
2957 receiver specification must declare corresponding type parameters for the method
2958 to use. This makes the receiver type parameters available to the method.
2959 Syntactically, this type parameter declaration looks like an
2960 <a href="#Instantiations">instantiation</a> of the receiver base type: the type
2961 arguments must be identifiers denoting the type parameters being declared, one
2962 for each type parameter of the receiver base type.
2963 The type parameter names do not need to match their corresponding parameter names in the
2964 receiver base type definition, and all non-blank parameter names must be unique in the
2965 receiver parameter section and the method signature.
2966 The receiver type parameter constraints are implied by the receiver base type definition:
2967 corresponding type parameters have corresponding constraints.
2971 type Pair[A, B any] struct {
2976 func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B
2977 func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair
2980 <h2 id="Expressions">Expressions</h2>
2983 An expression specifies the computation of a value by applying
2984 operators and functions to operands.
2987 <h3 id="Operands">Operands</h3>
2990 Operands denote the elementary values in an expression. An operand may be a
2991 literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
2992 non-<a href="#Blank_identifier">blank</a> identifier denoting a
2993 <a href="#Constant_declarations">constant</a>,
2994 <a href="#Variable_declarations">variable</a>, or
2995 <a href="#Function_declarations">function</a>,
2996 or a parenthesized expression.
3000 Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
3001 Literal = BasicLit | CompositeLit | FunctionLit .
3002 BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
3003 OperandName = identifier | QualifiedIdent .
3007 An operand name denoting a <a href="#Function_declarations">generic function</a>
3008 may be followed by a list of <a href="#Instantiations">type arguments</a>; the
3009 resulting operand is an <a href="#Instantiations">instantiated</a> function.
3013 The <a href="#Blank_identifier">blank identifier</a> may appear as an
3014 operand only on the left-hand side of an <a href="#Assignment_statements">assignment statement</a>.
3018 Implementation restriction: A compiler need not report an error if an operand's
3019 type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty
3020 <a href="#Interface_types">type set</a>. Functions with such type parameters
3021 cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead
3022 to an error at the instantiation site.
3025 <h3 id="Qualified_identifiers">Qualified identifiers</h3>
3028 A <i>qualified identifier</i> is an identifier qualified with a package name prefix.
3029 Both the package name and the identifier must not be
3030 <a href="#Blank_identifier">blank</a>.
3034 QualifiedIdent = PackageName "." identifier .
3038 A qualified identifier accesses an identifier in a different package, which
3039 must be <a href="#Import_declarations">imported</a>.
3040 The identifier must be <a href="#Exported_identifiers">exported</a> and
3041 declared in the <a href="#Blocks">package block</a> of that package.
3045 math.Sin // denotes the Sin function in package math
3048 <h3 id="Composite_literals">Composite literals</h3>
3051 Composite literals construct new composite values each time they are evaluated.
3052 They consist of the type of the literal followed by a brace-bound list of elements.
3053 Each element may optionally be preceded by a corresponding key.
3057 CompositeLit = LiteralType LiteralValue .
3058 LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
3059 SliceType | MapType | TypeName [ TypeArgs ] .
3060 LiteralValue = "{" [ ElementList [ "," ] ] "}" .
3061 ElementList = KeyedElement { "," KeyedElement } .
3062 KeyedElement = [ Key ":" ] Element .
3063 Key = FieldName | Expression | LiteralValue .
3064 FieldName = identifier .
3065 Element = Expression | LiteralValue .
3069 The LiteralType's <a href="#Core_types">core type</a> <code>T</code>
3070 must be a struct, array, slice, or map type
3071 (the syntax enforces this constraint except when the type is given
3073 The types of the elements and keys must be <a href="#Assignability">assignable</a>
3074 to the respective field, element, and key types of type <code>T</code>;
3075 there is no additional conversion.
3076 The key is interpreted as a field name for struct literals,
3077 an index for array and slice literals, and a key for map literals.
3078 For map literals, all elements must have a key. It is an error
3079 to specify multiple elements with the same field name or
3080 constant key value. For non-constant map keys, see the section on
3081 <a href="#Order_of_evaluation">evaluation order</a>.
3085 For struct literals the following rules apply:
3088 <li>A key must be a field name declared in the struct type.
3090 <li>An element list that does not contain any keys must
3091 list an element for each struct field in the
3092 order in which the fields are declared.
3094 <li>If any element has a key, every element must have a key.
3096 <li>An element list that contains keys does not need to
3097 have an element for each struct field. Omitted fields
3098 get the zero value for that field.
3100 <li>A literal may omit the element list; such a literal evaluates
3101 to the zero value for its type.
3103 <li>It is an error to specify an element for a non-exported
3104 field of a struct belonging to a different package.
3109 Given the declarations
3112 type Point3D struct { x, y, z float64 }
3113 type Line struct { p, q Point3D }
3121 origin := Point3D{} // zero value for Point3D
3122 line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
3126 For array and slice literals the following rules apply:
3129 <li>Each element has an associated integer index marking
3130 its position in the array.
3132 <li>An element with a key uses the key as its index. The
3133 key must be a non-negative constant
3134 <a href="#Representability">representable</a> by
3135 a value of type <code>int</code>; and if it is typed
3136 it must be of <a href="#Numeric_types">integer type</a>.
3138 <li>An element without a key uses the previous element's index plus one.
3139 If the first element has no key, its index is zero.
3144 <a href="#Address_operators">Taking the address</a> of a composite literal
3145 generates a pointer to a unique <a href="#Variables">variable</a> initialized
3146 with the literal's value.
3150 var pointer *Point3D = &Point3D{y: 1000}
3154 Note that the <a href="#The_zero_value">zero value</a> for a slice or map
3155 type is not the same as an initialized but empty value of the same type.
3156 Consequently, taking the address of an empty slice or map composite literal
3157 does not have the same effect as allocating a new slice or map value with
3158 <a href="#Allocation">new</a>.
3162 p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
3163 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
3167 The length of an array literal is the length specified in the literal type.
3168 If fewer elements than the length are provided in the literal, the missing
3169 elements are set to the zero value for the array element type.
3170 It is an error to provide elements with index values outside the index range
3171 of the array. The notation <code>...</code> specifies an array length equal
3172 to the maximum element index plus one.
3176 buffer := [10]string{} // len(buffer) == 10
3177 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
3178 days := [...]string{"Sat", "Sun"} // len(days) == 2
3182 A slice literal describes the entire underlying array literal.
3183 Thus the length and capacity of a slice literal are the maximum
3184 element index plus one. A slice literal has the form
3192 and is shorthand for a slice operation applied to an array:
3196 tmp := [n]T{x1, x2, … xn}
3201 Within a composite literal of array, slice, or map type <code>T</code>,
3202 elements or map keys that are themselves composite literals may elide the respective
3203 literal type if it is identical to the element or key type of <code>T</code>.
3204 Similarly, elements or keys that are addresses of composite literals may elide
3205 the <code>&T</code> when the element or key type is <code>*T</code>.
3209 [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
3210 [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
3211 [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
3212 map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
3213 map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
3216 [2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
3217 [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
3221 A parsing ambiguity arises when a composite literal using the
3222 TypeName form of the LiteralType appears as an operand between the
3223 <a href="#Keywords">keyword</a> and the opening brace of the block
3224 of an "if", "for", or "switch" statement, and the composite literal
3225 is not enclosed in parentheses, square brackets, or curly braces.
3226 In this rare case, the opening brace of the literal is erroneously parsed
3227 as the one introducing the block of statements. To resolve the ambiguity,
3228 the composite literal must appear within parentheses.
3232 if x == (T{a,b,c}[i]) { … }
3233 if (x == T{a,b,c}[i]) { … }
3237 Examples of valid array, slice, and map literals:
3241 // list of prime numbers
3242 primes := []int{2, 3, 5, 7, 9, 2147483647}
3244 // vowels[ch] is true if ch is a vowel
3245 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
3247 // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
3248 filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
3250 // frequencies in Hz for equal-tempered scale (A4 = 440Hz)
3251 noteFrequency := map[string]float32{
3252 "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
3253 "G0": 24.50, "A0": 27.50, "B0": 30.87,
3258 <h3 id="Function_literals">Function literals</h3>
3261 A function literal represents an anonymous <a href="#Function_declarations">function</a>.
3262 Function literals cannot declare type parameters.
3266 FunctionLit = "func" Signature FunctionBody .
3270 func(a, b int, z float64) bool { return a*b < int(z) }
3274 A function literal can be assigned to a variable or invoked directly.
3278 f := func(x, y int) int { return x + y }
3279 func(ch chan int) { ch <- ACK }(replyChan)
3283 Function literals are <i>closures</i>: they may refer to variables
3284 defined in a surrounding function. Those variables are then shared between
3285 the surrounding function and the function literal, and they survive as long
3286 as they are accessible.
3290 <h3 id="Primary_expressions">Primary expressions</h3>
3293 Primary expressions are the operands for unary and binary expressions.
3301 PrimaryExpr Selector |
3304 PrimaryExpr TypeAssertion |
3305 PrimaryExpr Arguments .
3307 Selector = "." identifier .
3308 Index = "[" Expression "]" .
3309 Slice = "[" [ Expression ] ":" [ Expression ] "]" |
3310 "[" [ Expression ] ":" Expression ":" Expression "]" .
3311 TypeAssertion = "." "(" Type ")" .
3312 Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
3329 <h3 id="Selectors">Selectors</h3>
3332 For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
3333 that is not a <a href="#Package_clause">package name</a>, the
3334 <i>selector expression</i>
3342 denotes the field or method <code>f</code> of the value <code>x</code>
3343 (or sometimes <code>*x</code>; see below).
3344 The identifier <code>f</code> is called the (field or method) <i>selector</i>;
3345 it must not be the <a href="#Blank_identifier">blank identifier</a>.
3346 The type of the selector expression is the type of <code>f</code>.
3347 If <code>x</code> is a package name, see the section on
3348 <a href="#Qualified_identifiers">qualified identifiers</a>.
3352 A selector <code>f</code> may denote a field or method <code>f</code> of
3353 a type <code>T</code>, or it may refer
3354 to a field or method <code>f</code> of a nested
3355 <a href="#Struct_types">embedded field</a> of <code>T</code>.
3356 The number of embedded fields traversed
3357 to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
3358 The depth of a field or method <code>f</code>
3359 declared in <code>T</code> is zero.
3360 The depth of a field or method <code>f</code> declared in
3361 an embedded field <code>A</code> in <code>T</code> is the
3362 depth of <code>f</code> in <code>A</code> plus one.
3366 The following rules apply to selectors:
3371 For a value <code>x</code> of type <code>T</code> or <code>*T</code>
3372 where <code>T</code> is not a pointer or interface type,
3373 <code>x.f</code> denotes the field or method at the shallowest depth
3374 in <code>T</code> where there is such an <code>f</code>.
3375 If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
3376 with shallowest depth, the selector expression is illegal.
3380 For a value <code>x</code> of type <code>I</code> where <code>I</code>
3381 is an interface type, <code>x.f</code> denotes the actual method with name
3382 <code>f</code> of the dynamic value of <code>x</code>.
3383 If there is no method with name <code>f</code> in the
3384 <a href="#Method_sets">method set</a> of <code>I</code>, the selector
3385 expression is illegal.
3389 As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
3390 pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
3391 (but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
3395 In all other cases, <code>x.f</code> is illegal.
3399 If <code>x</code> is of pointer type and has the value
3400 <code>nil</code> and <code>x.f</code> denotes a struct field,
3401 assigning to or evaluating <code>x.f</code>
3402 causes a <a href="#Run_time_panics">run-time panic</a>.
3406 If <code>x</code> is of interface type and has the value
3407 <code>nil</code>, <a href="#Calls">calling</a> or
3408 <a href="#Method_values">evaluating</a> the method <code>x.f</code>
3409 causes a <a href="#Run_time_panics">run-time panic</a>.
3414 For example, given the declarations:
3440 var t T2 // with t.T0 != nil
3441 var p *T2 // with p != nil and (*p).T0 != nil
3458 q.x // (*(*q).T0).x (*q).x is a valid field selector
3460 p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
3461 p.M1() // ((*p).T1).M1() M1 expects T1 receiver
3462 p.M2() // p.M2() M2 expects *T2 receiver
3463 t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
3467 but the following is invalid:
3471 q.M0() // (*q).M0 is valid but not a field selector
3475 <h3 id="Method_expressions">Method expressions</h3>
3478 If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3479 <code>T.M</code> is a function that is callable as a regular function
3480 with the same arguments as <code>M</code> prefixed by an additional
3481 argument that is the receiver of the method.
3485 MethodExpr = ReceiverType "." MethodName .
3486 ReceiverType = Type .
3490 Consider a struct type <code>T</code> with two methods,
3491 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3492 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3499 func (tv T) Mv(a int) int { return 0 } // value receiver
3500 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3514 yields a function equivalent to <code>Mv</code> but
3515 with an explicit receiver as its first argument; it has signature
3519 func(tv T, a int) int
3523 That function may be called normally with an explicit receiver, so
3524 these five invocations are equivalent:
3531 f1 := T.Mv; f1(t, 7)
3532 f2 := (T).Mv; f2(t, 7)
3536 Similarly, the expression
3544 yields a function value representing <code>Mp</code> with signature
3548 func(tp *T, f float32) float32
3552 For a method with a value receiver, one can derive a function
3553 with an explicit pointer receiver, so
3561 yields a function value representing <code>Mv</code> with signature
3565 func(tv *T, a int) int
3569 Such a function indirects through the receiver to create a value
3570 to pass as the receiver to the underlying method;
3571 the method does not overwrite the value whose address is passed in
3576 The final case, a value-receiver function for a pointer-receiver method,
3577 is illegal because pointer-receiver methods are not in the method set
3582 Function values derived from methods are called with function call syntax;
3583 the receiver is provided as the first argument to the call.
3584 That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
3585 as <code>f(t, 7)</code> not <code>t.f(7)</code>.
3586 To construct a function that binds the receiver, use a
3587 <a href="#Function_literals">function literal</a> or
3588 <a href="#Method_values">method value</a>.
3592 It is legal to derive a function value from a method of an interface type.
3593 The resulting function takes an explicit receiver of that interface type.
3596 <h3 id="Method_values">Method values</h3>
3599 If the expression <code>x</code> has static type <code>T</code> and
3600 <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3601 <code>x.M</code> is called a <i>method value</i>.
3602 The method value <code>x.M</code> is a function value that is callable
3603 with the same arguments as a method call of <code>x.M</code>.
3604 The expression <code>x</code> is evaluated and saved during the evaluation of the
3605 method value; the saved copy is then used as the receiver in any calls,
3606 which may be executed later.
3610 type S struct { *T }
3612 func (t T) M() { print(t) }
3616 f := t.M // receiver *t is evaluated and stored in f
3617 g := s.M // receiver *(s.T) is evaluated and stored in g
3618 *t = 42 // does not affect stored receivers in f and g
3622 The type <code>T</code> may be an interface or non-interface type.
3626 As in the discussion of <a href="#Method_expressions">method expressions</a> above,
3627 consider a struct type <code>T</code> with two methods,
3628 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3629 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3636 func (tv T) Mv(a int) int { return 0 } // value receiver
3637 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3653 yields a function value of type
3661 These two invocations are equivalent:
3670 Similarly, the expression
3678 yields a function value of type
3682 func(float32) float32
3686 As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
3687 using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
3691 As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
3692 using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
3696 f := t.Mv; f(7) // like t.Mv(7)
3697 f := pt.Mp; f(7) // like pt.Mp(7)
3698 f := pt.Mv; f(7) // like (*pt).Mv(7)
3699 f := t.Mp; f(7) // like (&t).Mp(7)
3700 f := makeT().Mp // invalid: result of makeT() is not addressable
3704 Although the examples above use non-interface types, it is also legal to create a method value
3705 from a value of interface type.
3709 var i interface { M(int) } = myVal
3710 f := i.M; f(7) // like i.M(7)
3714 <h3 id="Index_expressions">Index expressions</h3>
3717 A primary expression of the form
3725 denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
3726 The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
3727 The following rules apply:
3731 If <code>a</code> is neither a map nor a type parameter:
3734 <li>the index <code>x</code> must be an untyped constant or its
3735 <a href="#Core_types">core type</a> must be an <a href="#Numeric_types">integer</a></li>
3736 <li>a constant index must be non-negative and
3737 <a href="#Representability">representable</a> by a value of type <code>int</code></li>
3738 <li>a constant index that is untyped is given type <code>int</code></li>
3739 <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
3740 otherwise it is <i>out of range</i></li>
3744 For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
3747 <li>a <a href="#Constants">constant</a> index must be in range</li>
3748 <li>if <code>x</code> is out of range at run time,
3749 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3750 <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
3751 <code>a[x]</code> is the element type of <code>A</code></li>
3755 For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
3758 <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
3762 For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
3765 <li>if <code>x</code> is out of range at run time,
3766 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3767 <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
3768 <code>a[x]</code> is the element type of <code>S</code></li>
3772 For <code>a</code> of <a href="#String_types">string type</a>:
3775 <li>a <a href="#Constants">constant</a> index must be in range
3776 if the string <code>a</code> is also constant</li>
3777 <li>if <code>x</code> is out of range at run time,
3778 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3779 <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
3780 <code>a[x]</code> is <code>byte</code></li>
3781 <li><code>a[x]</code> may not be assigned to</li>
3785 For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
3788 <li><code>x</code>'s type must be
3789 <a href="#Assignability">assignable</a>
3790 to the key type of <code>M</code></li>
3791 <li>if the map contains an entry with key <code>x</code>,
3792 <code>a[x]</code> is the map element with key <code>x</code>
3793 and the type of <code>a[x]</code> is the element type of <code>M</code></li>
3794 <li>if the map is <code>nil</code> or does not contain such an entry,
3795 <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
3796 for the element type of <code>M</code></li>
3800 For <code>a</code> of <a href="#Type_parameter_declarations">type parameter type</a> <code>P</code>:
3803 <li>The index expression <code>a[x]</code> must be valid for values
3804 of all types in <code>P</code>'s type set.</li>
3805 <li>The element types of all types in <code>P</code>'s type set must be identical.
3806 In this context, the element type of a string type is <code>byte</code>.</li>
3807 <li>If there is a map type in the type set of <code>P</code>,
3808 all types in that type set must be map types, and the respective key types
3809 must be all identical.</li>
3810 <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
3811 or the map element with key <code>x</code> of the type argument
3812 that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
3813 the type of the (identical) element types.</li>
3814 <li><code>a[x]</code> may not be assigned to if <code>P</code>'s type set
3815 includes string types.
3819 Otherwise <code>a[x]</code> is illegal.
3823 An index expression on a map <code>a</code> of type <code>map[K]V</code>
3824 used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
3834 yields an additional untyped boolean value. The value of <code>ok</code> is
3835 <code>true</code> if the key <code>x</code> is present in the map, and
3836 <code>false</code> otherwise.
3840 Assigning to an element of a <code>nil</code> map causes a
3841 <a href="#Run_time_panics">run-time panic</a>.
3845 <h3 id="Slice_expressions">Slice expressions</h3>
3848 Slice expressions construct a substring or slice from a string, array, pointer
3849 to array, or slice. There are two variants: a simple form that specifies a low
3850 and high bound, and a full form that also specifies a bound on the capacity.
3853 <h4>Simple slice expressions</h4>
3856 The primary expression
3864 constructs a substring or slice. The <a href="#Core_types">core type</a> of
3865 <code>a</code> must be a string, array, pointer to array, slice, or a
3866 <a href="#Core_types"><code>bytestring</code></a>.
3867 The <i>indices</i> <code>low</code> and
3868 <code>high</code> select which elements of operand <code>a</code> appear
3869 in the result. The result has indices starting at 0 and length equal to
3870 <code>high</code> - <code>low</code>.
3871 After slicing the array <code>a</code>
3875 a := [5]int{1, 2, 3, 4, 5}
3880 the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
3890 For convenience, any of the indices may be omitted. A missing <code>low</code>
3891 index defaults to zero; a missing <code>high</code> index defaults to the length of the
3896 a[2:] // same as a[2 : len(a)]
3897 a[:3] // same as a[0 : 3]
3898 a[:] // same as a[0 : len(a)]
3902 If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
3903 <code>(*a)[low : high]</code>.
3907 For arrays or strings, the indices are <i>in range</i> if
3908 <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
3909 otherwise they are <i>out of range</i>.
3910 For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
3911 A <a href="#Constants">constant</a> index must be non-negative and
3912 <a href="#Representability">representable</a> by a value of type
3913 <code>int</code>; for arrays or constant strings, constant indices must also be in range.
3914 If both indices are constant, they must satisfy <code>low <= high</code>.
3915 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3919 Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
3920 the result of the slice operation is a non-constant value of the same type as the operand.
3921 For untyped string operands the result is a non-constant value of type <code>string</code>.
3922 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
3923 and the result of the slice operation is a slice with the same element type as the array.
3927 If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
3928 is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
3929 array with the operand.
3934 s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
3935 s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
3936 s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
3939 s3 := s[:0] // s3 == nil
3943 <h4>Full slice expressions</h4>
3946 The primary expression
3954 constructs a slice of the same type, and with the same length and elements as the simple slice
3955 expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
3956 by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
3957 The <a href="#Core_types">core type</a> of <code>a</code> must be an array, pointer to array,
3958 or slice (but not a string).
3959 After slicing the array <code>a</code>
3963 a := [5]int{1, 2, 3, 4, 5}
3968 the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
3977 As for simple slice expressions, if <code>a</code> is a pointer to an array,
3978 <code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
3979 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
3983 The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
3984 otherwise they are <i>out of range</i>.
3985 A <a href="#Constants">constant</a> index must be non-negative and
3986 <a href="#Representability">representable</a> by a value of type
3987 <code>int</code>; for arrays, constant indices must also be in range.
3988 If multiple indices are constant, the constants that are present must be in range relative to each
3990 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3993 <h3 id="Type_assertions">Type assertions</h3>
3996 For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
3997 but not a <a href="#Type_parameter_declarations">type parameter</a>, and a type <code>T</code>,
3998 the primary expression
4006 asserts that <code>x</code> is not <code>nil</code>
4007 and that the value stored in <code>x</code> is of type <code>T</code>.
4008 The notation <code>x.(T)</code> is called a <i>type assertion</i>.
4011 More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
4012 that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
4013 to the type <code>T</code>.
4014 In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
4015 otherwise the type assertion is invalid since it is not possible for <code>x</code>
4016 to store a value of type <code>T</code>.
4017 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
4018 of <code>x</code> <a href="#Implementing_an_interface">implements</a> the interface <code>T</code>.
4021 If the type assertion holds, the value of the expression is the value
4022 stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
4023 a <a href="#Run_time_panics">run-time panic</a> occurs.
4024 In other words, even though the dynamic type of <code>x</code>
4025 is known only at run time, the type of <code>x.(T)</code> is
4026 known to be <code>T</code> in a correct program.
4030 var x interface{} = 7 // x has dynamic type int and value 7
4031 i := x.(int) // i has type int and value 7
4033 type I interface { m() }
4036 s := y.(string) // illegal: string does not implement I (missing method m)
4037 r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
4043 A type assertion used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
4050 var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
4054 yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
4055 if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
4056 the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
4057 No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
4061 <h3 id="Calls">Calls</h3>
4064 Given an expression <code>f</code> with a <a href="#Core_types">core type</a>
4065 <code>F</code> of <a href="#Function_types">function type</a>,
4073 calls <code>f</code> with arguments <code>a1, a2, … an</code>.
4074 Except for one special case, arguments must be single-valued expressions
4075 <a href="#Assignability">assignable</a> to the parameter types of
4076 <code>F</code> and are evaluated before the function is called.
4077 The type of the expression is the result type
4079 A method invocation is similar but the method itself
4080 is specified as a selector upon a value of the receiver type for
4085 math.Atan2(x, y) // function call
4087 pt.Scale(3.5) // method call with receiver pt
4091 If <code>f</code> denotes a generic function, it must be
4092 <a href="#Instantiations">instantiated</a> before it can be called
4093 or used as a function value.
4097 In a function call, the function value and arguments are evaluated in
4098 <a href="#Order_of_evaluation">the usual order</a>.
4099 After they are evaluated, the parameters of the call are passed by value to the function
4100 and the called function begins execution.
4101 The return parameters of the function are passed by value
4102 back to the caller when the function returns.
4106 Calling a <code>nil</code> function value
4107 causes a <a href="#Run_time_panics">run-time panic</a>.
4111 As a special case, if the return values of a function or method
4112 <code>g</code> are equal in number and individually
4113 assignable to the parameters of another function or method
4114 <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
4115 will invoke <code>f</code> after binding the return values of
4116 <code>g</code> to the parameters of <code>f</code> in order. The call
4117 of <code>f</code> must contain no parameters other than the call of <code>g</code>,
4118 and <code>g</code> must have at least one return value.
4119 If <code>f</code> has a final <code>...</code> parameter, it is
4120 assigned the return values of <code>g</code> that remain after
4121 assignment of regular parameters.
4125 func Split(s string, pos int) (string, string) {
4126 return s[0:pos], s[pos:]
4129 func Join(s, t string) string {
4133 if Join(Split(value, len(value)/2)) != value {
4134 log.Panic("test fails")
4139 A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
4140 of (the type of) <code>x</code> contains <code>m</code> and the
4141 argument list can be assigned to the parameter list of <code>m</code>.
4142 If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
4143 set contains <code>m</code>, <code>x.m()</code> is shorthand
4144 for <code>(&x).m()</code>:
4153 There is no distinct method type and there are no method literals.
4156 <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
4159 If <code>f</code> is <a href="#Function_types">variadic</a> with a final
4160 parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
4161 the type of <code>p</code> is equivalent to type <code>[]T</code>.
4162 If <code>f</code> is invoked with no actual arguments for <code>p</code>,
4163 the value passed to <code>p</code> is <code>nil</code>.
4164 Otherwise, the value passed is a new slice
4165 of type <code>[]T</code> with a new underlying array whose successive elements
4166 are the actual arguments, which all must be <a href="#Assignability">assignable</a>
4167 to <code>T</code>. The length and capacity of the slice is therefore
4168 the number of arguments bound to <code>p</code> and may differ for each
4173 Given the function and calls
4176 func Greeting(prefix string, who ...string)
4178 Greeting("hello:", "Joe", "Anna", "Eileen")
4182 within <code>Greeting</code>, <code>who</code> will have the value
4183 <code>nil</code> in the first call, and
4184 <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
4188 If the final argument is assignable to a slice type <code>[]T</code> and
4189 is followed by <code>...</code>, it is passed unchanged as the value
4190 for a <code>...T</code> parameter. In this case no new slice is created.
4194 Given the slice <code>s</code> and call
4198 s := []string{"James", "Jasmine"}
4199 Greeting("goodbye:", s...)
4203 within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
4204 with the same underlying array.
4207 <h3 id="Instantiations">Instantiations</h3>
4210 A generic function or type is <i>instantiated</i> by substituting <i>type arguments</i>
4211 for the type parameters.
4212 Instantiation proceeds in two steps:
4217 Each type argument is substituted for its corresponding type parameter in the generic
4219 This substitution happens across the entire function or type declaration,
4220 including the type parameter list itself and any types in that list.
4224 After substitution, each type argument must <a href="#Interface_types">implement</a>
4225 the <a href="#Type_parameter_declarations">constraint</a> (instantiated, if necessary)
4226 of the corresponding type parameter. Otherwise instantiation fails.
4231 Instantiating a type results in a new non-generic <a href="#Types">named type</a>;
4232 instantiating a function produces a new non-generic function.
4236 type parameter list type arguments after substitution
4238 [P any] int int implements any
4239 [S ~[]E, E any] []int, int []int implements ~[]int, int implements any
4240 [P io.Writer] string illegal: string doesn't implement io.Writer
4244 For a generic function, type arguments may be provided explicitly, or they
4245 may be partially or completely <a href="#Type_inference">inferred</a>.
4246 A generic function that is <i>not</i> <a href="#Calls">called</a> requires a
4247 type argument list for instantiation; if the list is partial, all
4248 remaining type arguments must be inferrable.
4249 A generic function that is called may provide a (possibly partial) type
4250 argument list, or may omit it entirely if the omitted type arguments are
4251 inferrable from the ordinary (non-type) function arguments.
4255 func min[T ~int|~float64](x, y T) T { … }
4257 f := min // illegal: min must be instantiated with type arguments when used without being called
4258 minInt := min[int] // minInt has type func(x, y int) int
4259 a := minInt(2, 3) // a has value 2 of type int
4260 b := min[float64](2.0, 3) // b has value 2.0 of type float64
4261 c := min(b, -1) // c has value -1.0 of type float64
4265 A partial type argument list cannot be empty; at least the first argument must be present.
4266 The list is a prefix of the full list of type arguments, leaving the remaining arguments
4267 to be inferred. Loosely speaking, type arguments may be omitted from "right to left".
4271 func apply[S ~[]E, E any](s S, f func(E) E) S { … }
4273 f0 := apply[] // illegal: type argument list cannot be empty
4274 f1 := apply[[]int] // type argument for S explicitly provided, type argument for E inferred
4275 f2 := apply[[]string, string] // both type arguments explicitly provided
4278 r := apply(bytes, func(byte) byte { … }) // both type arguments inferred from the function arguments
4282 For a generic type, all type arguments must always be provided explicitly.
4285 <h3 id="Type_inference">Type inference</h3>
4288 Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
4289 Each step attempts to use known information to infer additional type arguments.
4290 Type inference stops as soon as all type arguments are known.
4291 After type inference is complete, it is still necessary to substitute all type arguments
4292 for type parameters and verify that each type argument
4293 <a href="#Implementing_an_interface">implements</a> the relevant constraint;
4294 it is possible for an inferred type argument to fail to implement a constraint, in which
4295 case instantiation fails.
4299 Type inference is based on
4304 a <a href="#Type_parameter_declarations">type parameter list</a>
4307 a substitution map <i>M</i> initialized with the known type arguments, if any
4310 a (possibly empty) list of ordinary function arguments (in case of a function call only)
4315 and then proceeds with the following steps:
4320 apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
4321 to all <i>typed</i> ordinary function arguments
4324 apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
4327 apply function argument type inference to all <i>untyped</i> ordinary function arguments
4328 using the default type for each of the untyped function arguments
4331 apply constraint type inference
4336 If there are no ordinary or untyped function arguments, the respective steps are skipped.
4337 Constraint type inference is skipped if the previous step didn't infer any new type arguments,
4338 but it is run at least once if there are missing type arguments.
4342 The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
4343 The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
4344 If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
4347 <h4 id="Type_unification">Type unification</h4>
4350 Type inference is based on <i>type unification</i>. A single unification step
4351 applies to a <a href="#Type_inference">substitution map</a> and two types, either
4352 or both of which may be or contain type parameters. The substitution map tracks
4353 the known (explicitly provided or already inferred) type arguments: the map
4354 contains an entry <code>P</code> → <code>A</code> for each type
4355 parameter <code>P</code> and corresponding known type argument <code>A</code>.
4356 During unification, known type arguments take the place of their corresponding type
4357 parameters when comparing types. Unification is the process of finding substitution
4358 map entries that make the two types equivalent.
4362 For unification, two types that don't contain any type parameters from the current type
4363 parameter list are <i>equivalent</i>
4364 if they are identical, or if they are channel types that are identical ignoring channel
4365 direction, or if their underlying types are equivalent.
4369 Unification works by comparing the structure of pairs of types: their structure
4370 disregarding type parameters must be identical, and types other than type parameters
4372 A type parameter in one type may match any complete subtype in the other type;
4373 each successful match causes an entry to be added to the substitution map.
4374 If the structure differs, or types other than type parameters are not equivalent,
4379 TODO(gri) Somewhere we need to describe the process of adding an entry to the
4380 substitution map: if the entry is already present, the type argument
4381 values are themselves unified.
4385 For example, if <code>T1</code> and <code>T2</code> are type parameters,
4386 <code>[]map[int]bool</code> can be unified with any of the following:
4390 []map[int]bool // types are identical
4391 T1 // adds T1 → []map[int]bool to substitution map
4392 []T1 // adds T1 → map[int]bool to substitution map
4393 []map[T1]T2 // adds T1 → int and T2 → bool to substitution map
4397 On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
4401 int // int is not a slice
4402 struct{} // a struct is not a slice
4403 []struct{} // a struct is not a map
4404 []map[T1]string // map element types don't match
4408 As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
4409 <code>D</code> and a type literal <code>L</code> are never equivalent,
4410 unification compares the underlying type of <code>D</code> with <code>L</code> instead.
4411 For example, given the defined type
4415 type Vector []float64
4419 and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
4420 <code>[]E</code> and adds an entry <code>E</code> → <code>float64</code> to
4421 the substitution map.
4424 <h4 id="Function_argument_type_inference">Function argument type inference</h4>
4426 <!-- In this section and the section on constraint type inference we start with examples
4427 rather than have the examples follow the rules as is customary elsewhere in spec.
4428 Hopefully this helps building an intuition and makes the rules easier to follow. -->
4431 Function argument type inference infers type arguments from function arguments:
4432 if a function parameter is declared with a type <code>T</code> that uses
4434 <a href="#Type_unification">unifying</a> the type of the corresponding
4435 function argument with <code>T</code> may infer type arguments for the type
4436 parameters used by <code>T</code>.
4440 For instance, given the generic function
4444 func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
4452 var vector []float64
4453 scaledVector := scale(vector, 42)
4457 the type argument for <code>Number</code> can be inferred from the function argument
4458 <code>vector</code> by unifying the type of <code>vector</code> with the corresponding
4459 parameter type: <code>[]float64</code> and <code>[]Number</code>
4460 match in structure and <code>float64</code> matches with <code>Number</code>.
4461 This adds the entry <code>Number</code> → <code>float64</code> to the
4462 <a href="#Type_unification">substitution map</a>.
4463 Untyped arguments, such as the second function argument <code>42</code> here, are ignored
4464 in the first round of function argument type inference and only considered if there are
4465 unresolved type parameters left.
4469 Inference happens in two separate phases; each phase operates on a specific list of
4470 (parameter, argument) pairs:
4475 The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
4476 type uses type parameters and where the function argument is <i>typed</i>.
4479 The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
4480 type parameter. In this list, the respective function arguments are untyped.
4485 Any other (parameter, argument) pair is ignored.
4489 By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
4490 (or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
4491 of untyped values are always predeclared non-composite types, they can never match against
4492 a composite type, so it is sufficient to only consider parameter types that are single type
4497 Each list is processed in a separate phase:
4502 In the first phase, the parameter and argument types of each pair in <i>Lt</i>
4503 are unified. If unification succeeds for a pair, it may yield new entries that
4504 are added to the substitution map <i>M</i>. If unification fails, type inference
4508 The second phase considers the entries of list <i>Lu</i>. Type parameters for
4509 which the type argument has already been determined are ignored in this phase.
4510 For each remaining pair, the parameter type (which is a single type parameter) and
4511 the <a href="#Constants">default type</a> of the corresponding untyped argument is
4512 unified. If unification fails, type inference fails.
4517 While unification is successful, processing of each list continues until all list elements
4518 are considered, even if all type arguments are inferred before the last list element has
4527 func min[T ~int|~float64](x, y T) T
4530 min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
4531 min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
4532 min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
4536 In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
4537 yields the substitution map entry <code>T</code> → <code>float64</code>. Because
4538 processing continues until all untyped arguments are considered, an error is reported. This
4539 ensures that type inference does not depend on the order of the untyped arguments.
4542 <h4 id="Constraint_type_inference">Constraint type inference</h4>
4545 Constraint type inference infers type arguments by considering type constraints.
4546 If a type parameter <code>P</code> has a constraint with a
4547 <a href="#Core_types">core type</a> <code>C</code>,
4548 <a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
4549 may infer additional type arguments, either the type argument for <code>P</code>,
4550 or if that is already known, possibly the type arguments for type parameters
4551 used in <code>C</code>.
4555 For instance, consider the type parameter list with type parameters <code>List</code> and
4560 [List ~[]Elem, Elem any]
4564 Constraint type inference can deduce the type of <code>Elem</code> from the type argument
4565 for <code>List</code> because <code>Elem</code> is a type parameter in the core type
4566 <code>[]Elem</code> of <code>List</code>.
4567 If the type argument is <code>Bytes</code>:
4575 unifying the underlying type of <code>Bytes</code> with the core type means
4576 unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
4577 the <a href="#Type_unification">substitution map</a> entry
4578 <code>Elem</code> → <code>byte</code>.
4579 Thus, in this example, constraint type inference can infer the second type argument from the
4584 Using the core type of a constraint may lose some information: In the (unlikely) case that
4585 the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
4586 <code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
4587 <code>N</code> itself. In this case, constraint type inference may succeed but instantiation
4588 will fail because the inferred type is not in the type set of the constraint.
4589 Thus, constraint type inference uses the <i>adjusted core type</i> of
4590 a constraint: if the type set contains a single type, use that type; otherwise use the
4591 constraint's core type.
4595 Generally, constraint type inference proceeds in two phases: Starting with a given
4596 substitution map <i>M</i>
4601 For all type parameters with an adjusted core type, unify the type parameter with that
4602 type. If any unification fails, constraint type inference fails.
4606 At this point, some entries in <i>M</i> may map type parameters to other
4607 type parameters or to types containing type parameters. For each entry
4608 <code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
4609 contains type parameters <code>Q</code> for which there exist entries
4610 <code>Q</code> → <code>B</code> in <i>M</i>, substitute those
4611 <code>Q</code> with the respective <code>B</code> in <code>A</code>.
4612 Stop when no further substitution is possible.
4617 The result of constraint type inference is the final substitution map <i>M</i> from type
4618 parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
4619 appears in any of the <code>A</code>.
4623 For instance, given the type parameter list
4627 [A any, B []C, C *A]
4631 and the single provided type argument <code>int</code> for type parameter <code>A</code>,
4632 the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
4636 In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
4637 with the core type of their respective constraints. This adds the entries
4638 <code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
4642 At this point there are two entries in <i>M</i> where the right-hand side
4643 is or contains type parameters for which there exists other entries in <i>M</i>:
4644 <code>[]C</code> and <code>*A</code>.
4645 In the second phase, these type parameters are replaced with their respective
4646 types. It doesn't matter in which order this happens. Starting with the state
4647 of <i>M</i> after the first phase:
4651 <code>A</code> → <code>int</code>,
4652 <code>B</code> → <code>[]C</code>,
4653 <code>C</code> → <code>*A</code>
4657 Replace <code>A</code> on the right-hand side of → with <code>int</code>:
4661 <code>A</code> → <code>int</code>,
4662 <code>B</code> → <code>[]C</code>,
4663 <code>C</code> → <code>*int</code>
4667 Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
4671 <code>A</code> → <code>int</code>,
4672 <code>B</code> → <code>[]*int</code>,
4673 <code>C</code> → <code>*int</code>
4677 At this point no further substitution is possible and the map is full.
4678 Therefore, <code>M</code> represents the final map of type parameters
4679 to type arguments for the given type parameter list.
4682 <h3 id="Operators">Operators</h3>
4685 Operators combine operands into expressions.
4689 Expression = UnaryExpr | Expression binary_op Expression .
4690 UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
4692 binary_op = "||" | "&&" | rel_op | add_op | mul_op .
4693 rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
4694 add_op = "+" | "-" | "|" | "^" .
4695 mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
4697 unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
4701 Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
4702 For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
4703 unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
4704 For operations involving constants only, see the section on
4705 <a href="#Constant_expressions">constant expressions</a>.
4709 Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
4710 and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
4711 to the type of the other operand.
4715 The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
4716 or be an untyped constant <a href="#Representability">representable</a> by a
4717 value of type <code>uint</code>.
4718 If the left operand of a non-constant shift expression is an untyped constant,
4719 it is first implicitly converted to the type it would assume if the shift expression were
4720 replaced by its left operand alone.
4727 // The results of the following examples are given for 64-bit ints.
4728 var i = 1<<s // 1 has type int
4729 var j int32 = 1<<s // 1 has type int32; j == 0
4730 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
4731 var m int = 1.0<<s // 1.0 has type int; m == 1<<33
4732 var n = 1.0<<s == j // 1.0 has type int32; n == true
4733 var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
4734 var p = 1<<s == 1<<33 // 1 has type int; p == true
4735 var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
4736 var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
4737 var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
4738 var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift
4739 var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift
4740 var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
4741 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
4742 var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
4744 // The results of the following examples are given for 32-bit ints,
4745 // which means the shifts will overflow.
4746 var mm int = 1.0<<s // 1.0 has type int; mm == 0
4747 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
4748 var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
4749 var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
4750 var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
4753 <h4 id="Operator_precedence">Operator precedence</h4>
4755 Unary operators have the highest precedence.
4756 As the <code>++</code> and <code>--</code> operators form
4757 statements, not expressions, they fall
4758 outside the operator hierarchy.
4759 As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
4761 There are five precedence levels for binary operators.
4762 Multiplication operators bind strongest, followed by addition
4763 operators, comparison operators, <code>&&</code> (logical AND),
4764 and finally <code>||</code> (logical OR):
4767 <pre class="grammar">
4769 5 * / % << >> & &^
4771 3 == != < <= > >=
4777 Binary operators of the same precedence associate from left to right.
4778 For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
4787 x == y+1 && <-chanInt > 0
4791 <h3 id="Arithmetic_operators">Arithmetic operators</h3>
4793 Arithmetic operators apply to numeric values and yield a result of the same
4794 type as the first operand. The four standard arithmetic operators (<code>+</code>,
4795 <code>-</code>, <code>*</code>, <code>/</code>) apply to
4796 <a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
4797 <a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>.
4798 The bitwise logical and shift operators apply to integers only.
4801 <pre class="grammar">
4802 + sum integers, floats, complex values, strings
4803 - difference integers, floats, complex values
4804 * product integers, floats, complex values
4805 / quotient integers, floats, complex values
4806 % remainder integers
4808 & bitwise AND integers
4809 | bitwise OR integers
4810 ^ bitwise XOR integers
4811 &^ bit clear (AND NOT) integers
4813 << left shift integer << integer >= 0
4814 >> right shift integer >> integer >= 0
4818 If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>,
4819 the operator must apply to each type in that type set.
4820 The operands are represented as values of the type argument that the type parameter
4821 is <a href="#Instantiations">instantiated</a> with, and the operation is computed
4822 with the precision of that type argument. For example, given the function:
4826 func dotProduct[F ~float32|~float64](v1, v2 []F) F {
4828 for i, x := range v1 {
4837 the product <code>x * y</code> and the addition <code>s += x * y</code>
4838 are computed with <code>float32</code> or <code>float64</code> precision,
4839 respectively, depending on the type argument for <code>F</code>.
4842 <h4 id="Integer_operators">Integer operators</h4>
4845 For two integer values <code>x</code> and <code>y</code>, the integer quotient
4846 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
4851 x = q*y + r and |r| < |y|
4855 with <code>x / y</code> truncated towards zero
4856 (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
4868 The one exception to this rule is that if the dividend <code>x</code> is
4869 the most negative value for the int type of <code>x</code>, the quotient
4870 <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
4871 due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
4879 int64 -9223372036854775808
4883 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
4884 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4885 If the dividend is non-negative and the divisor is a constant power of 2,
4886 the division may be replaced by a right shift, and computing the remainder may
4887 be replaced by a bitwise AND operation:
4891 x x / 4 x % 4 x >> 2 x & 3
4897 The shift operators shift the left operand by the shift count specified by the
4898 right operand, which must be non-negative. If the shift count is negative at run time,
4899 a <a href="#Run_time_panics">run-time panic</a> occurs.
4900 The shift operators implement arithmetic shifts if the left operand is a signed
4901 integer and logical shifts if it is an unsigned integer.
4902 There is no upper limit on the shift count. Shifts behave
4903 as if the left operand is shifted <code>n</code> times by 1 for a shift
4904 count of <code>n</code>.
4905 As a result, <code>x << 1</code> is the same as <code>x*2</code>
4906 and <code>x >> 1</code> is the same as
4907 <code>x/2</code> but truncated towards negative infinity.
4911 For integer operands, the unary operators
4912 <code>+</code>, <code>-</code>, and <code>^</code> are defined as
4916 <pre class="grammar">
4918 -x negation is 0 - x
4919 ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
4920 and m = -1 for signed x
4924 <h4 id="Integer_overflow">Integer overflow</h4>
4927 For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>,
4928 <code>-</code>, <code>*</code>, and <code><<</code> are
4929 computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
4930 the unsigned integer's type.
4931 Loosely speaking, these unsigned integer operations
4932 discard high bits upon overflow, and programs may rely on "wrap around".
4936 For signed integers, the operations <code>+</code>,
4937 <code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
4938 overflow and the resulting value exists and is deterministically defined
4939 by the signed integer representation, the operation, and its operands.
4940 Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
4941 A compiler may not optimize code under the assumption that overflow does
4942 not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
4945 <h4 id="Floating_point_operators">Floating-point operators</h4>
4948 For floating-point and complex numbers,
4949 <code>+x</code> is the same as <code>x</code>,
4950 while <code>-x</code> is the negation of <code>x</code>.
4951 The result of a floating-point or complex division by zero is not specified beyond the
4952 IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
4953 occurs is implementation-specific.
4957 An implementation may combine multiple floating-point operations into a single
4958 fused operation, possibly across statements, and produce a result that differs
4959 from the value obtained by executing and rounding the instructions individually.
4960 An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to
4961 the precision of the target type, preventing fusion that would discard that rounding.
4965 For instance, some architectures provide a "fused multiply and add" (FMA) instruction
4966 that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
4967 These examples show when a Go implementation can use that instruction:
4971 // FMA allowed for computing r, because x*y is not explicitly rounded:
4975 *p = x*y; r = *p + z
4976 r = x*y + float64(z)
4978 // FMA disallowed for computing r, because it would omit rounding of x*y:
4979 r = float64(x*y) + z
4980 r = z; r += float64(x*y)
4981 t = float64(x*y); r = t + z
4984 <h4 id="String_concatenation">String concatenation</h4>
4987 Strings can be concatenated using the <code>+</code> operator
4988 or the <code>+=</code> assignment operator:
4992 s := "hi" + string(c)
4993 s += " and good bye"
4997 String addition creates a new string by concatenating the operands.
5000 <h3 id="Comparison_operators">Comparison operators</h3>
5003 Comparison operators compare two operands and yield an untyped boolean value.
5006 <pre class="grammar">
5012 >= greater or equal
5016 In any comparison, the first operand
5017 must be <a href="#Assignability">assignable</a>
5018 to the type of the second operand, or vice versa.
5021 The equality operators <code>==</code> and <code>!=</code> apply
5022 to operands that are <i>comparable</i>.
5023 The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
5024 apply to operands that are <i>ordered</i>.
5025 These terms and the result of the comparisons are defined as follows:
5030 Boolean values are comparable.
5031 Two boolean values are equal if they are either both
5032 <code>true</code> or both <code>false</code>.
5036 Integer values are comparable and ordered, in the usual way.
5040 Floating-point values are comparable and ordered,
5041 as defined by the IEEE-754 standard.
5045 Complex values are comparable.
5046 Two complex values <code>u</code> and <code>v</code> are
5047 equal if both <code>real(u) == real(v)</code> and
5048 <code>imag(u) == imag(v)</code>.
5052 String values are comparable and ordered, lexically byte-wise.
5056 Pointer values are comparable.
5057 Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
5058 Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
5062 Channel values are comparable.
5063 Two channel values are equal if they were created by the same call to
5064 <a href="#Making_slices_maps_and_channels"><code>make</code></a>
5065 or if both have value <code>nil</code>.
5069 Interface values are comparable.
5070 Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
5071 and equal dynamic values or if both have value <code>nil</code>.
5075 A value <code>x</code> of non-interface type <code>X</code> and
5076 a value <code>t</code> of interface type <code>T</code> are comparable when values
5077 of type <code>X</code> are comparable and
5078 <code>X</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>.
5079 They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
5080 and <code>t</code>'s dynamic value is equal to <code>x</code>.
5084 Struct values are comparable if all their fields are comparable.
5085 Two struct values are equal if their corresponding
5086 non-<a href="#Blank_identifier">blank</a> fields are equal.
5090 Array values are comparable if values of the array element type are comparable.
5091 Two array values are equal if their corresponding elements are equal.
5096 A comparison of two interface values with identical dynamic types
5097 causes a <a href="#Run_time_panics">run-time panic</a> if values
5098 of that type are not comparable. This behavior applies not only to direct interface
5099 value comparisons but also when comparing arrays of interface values
5100 or structs with interface-valued fields.
5104 Slice, map, and function values are not comparable.
5105 However, as a special case, a slice, map, or function value may
5106 be compared to the predeclared identifier <code>nil</code>.
5107 Comparison of pointer, channel, and interface values to <code>nil</code>
5108 is also allowed and follows from the general rules above.
5112 const c = 3 < 4 // c is the untyped boolean constant true
5117 // The result of a comparison is an untyped boolean.
5118 // The usual assignment rules apply.
5119 b3 = x == y // b3 has type bool
5120 b4 bool = x == y // b4 has type bool
5121 b5 MyBool = x == y // b5 has type MyBool
5125 <h3 id="Logical_operators">Logical operators</h3>
5128 Logical operators apply to <a href="#Boolean_types">boolean</a> values
5129 and yield a result of the same type as the operands.
5130 The right operand is evaluated conditionally.
5133 <pre class="grammar">
5134 && conditional AND p && q is "if p then q else false"
5135 || conditional OR p || q is "if p then true else q"
5140 <h3 id="Address_operators">Address operators</h3>
5143 For an operand <code>x</code> of type <code>T</code>, the address operation
5144 <code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
5145 The operand must be <i>addressable</i>,
5146 that is, either a variable, pointer indirection, or slice indexing
5147 operation; or a field selector of an addressable struct operand;
5148 or an array indexing operation of an addressable array.
5149 As an exception to the addressability requirement, <code>x</code> may also be a
5150 (possibly parenthesized)
5151 <a href="#Composite_literals">composite literal</a>.
5152 If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
5153 then the evaluation of <code>&x</code> does too.
5157 For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
5158 indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
5159 to by <code>x</code>.
5160 If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
5161 will cause a <a href="#Run_time_panics">run-time panic</a>.
5172 *x // causes a run-time panic
5173 &*x // causes a run-time panic
5177 <h3 id="Receive_operator">Receive operator</h3>
5180 For an operand <code>ch</code> whose <a href="#Core_types">core type</a> is a
5181 <a href="#Channel_types">channel</a>,
5182 the value of the receive operation <code><-ch</code> is the value received
5183 from the channel <code>ch</code>. The channel direction must permit receive operations,
5184 and the type of the receive operation is the element type of the channel.
5185 The expression blocks until a value is available.
5186 Receiving from a <code>nil</code> channel blocks forever.
5187 A receive operation on a <a href="#Close">closed</a> channel can always proceed
5188 immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
5189 after any previously sent values have been received.
5196 <-strobe // wait until clock pulse and discard received value
5200 A receive expression used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form
5207 var x, ok T = <-ch
5211 yields an additional untyped boolean result reporting whether the
5212 communication succeeded. The value of <code>ok</code> is <code>true</code>
5213 if the value received was delivered by a successful send operation to the
5214 channel, or <code>false</code> if it is a zero value generated because the
5215 channel is closed and empty.
5219 <h3 id="Conversions">Conversions</h3>
5222 A conversion changes the <a href="#Types">type</a> of an expression
5223 to the type specified by the conversion.
5224 A conversion may appear literally in the source, or it may be <i>implied</i>
5225 by the context in which an expression appears.
5229 An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
5230 where <code>T</code> is a type and <code>x</code> is an expression
5231 that can be converted to type <code>T</code>.
5235 Conversion = Type "(" Expression [ "," ] ")" .
5239 If the type starts with the operator <code>*</code> or <code><-</code>,
5240 or if the type starts with the keyword <code>func</code>
5241 and has no result list, it must be parenthesized when
5242 necessary to avoid ambiguity:
5246 *Point(p) // same as *(Point(p))
5247 (*Point)(p) // p is converted to *Point
5248 <-chan int(c) // same as <-(chan int(c))
5249 (<-chan int)(c) // c is converted to <-chan int
5250 func()(x) // function signature func() x
5251 (func())(x) // x is converted to func()
5252 (func() int)(x) // x is converted to func() int
5253 func() int(x) // x is converted to func() int (unambiguous)
5257 A <a href="#Constants">constant</a> value <code>x</code> can be converted to
5258 type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
5259 by a value of <code>T</code>.
5260 As a special case, an integer constant <code>x</code> can be explicitly converted to a
5261 <a href="#String_types">string type</a> using the
5262 <a href="#Conversions_to_and_from_a_string_type">same rule</a>
5263 as for non-constant <code>x</code>.
5267 Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a>
5268 yields a typed constant.
5272 uint(iota) // iota value of type uint
5273 float32(2.718281828) // 2.718281828 of type float32
5274 complex128(1) // 1.0 + 0.0i of type complex128
5275 float32(0.49999999) // 0.5 of type float32
5276 float64(-1e-1000) // 0.0 of type float64
5277 string('x') // "x" of type string
5278 string(0x266c) // "♬" of type string
5279 myString("foo" + "bar") // "foobar" of type myString
5280 string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
5281 (*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
5282 int(1.2) // illegal: 1.2 cannot be represented as an int
5283 string(65.0) // illegal: 65.0 is not an integer constant
5287 Converting a constant to a type parameter yields a <i>non-constant</i> value of that type,
5288 with the value represented as a value of the type argument that the type parameter
5289 is <a href="#Instantiations">instantiated</a> with.
5290 For example, given the function:
5294 func f[P ~float32|~float64]() {
5300 the conversion <code>P(1.1)</code> results in a non-constant value of type <code>P</code>
5301 and the value <code>1.1</code> is represented as a <code>float32</code> or a <code>float64</code>
5302 depending on the type argument for <code>f</code>.
5303 Accordingly, if <code>f</code> is instantiated with a <code>float32</code> type,
5304 the numeric value of the expression <code>P(1.1) + 1.2</code> will be computed
5305 with the same precision as the corresponding non-constant <code>float32</code>
5310 A non-constant value <code>x</code> can be converted to type <code>T</code>
5311 in any of these cases:
5316 <code>x</code> is <a href="#Assignability">assignable</a>
5320 ignoring struct tags (see below),
5321 <code>x</code>'s type and <code>T</code> are not
5322 <a href="#Type_parameter_declarations">type parameters</a> but have
5323 <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
5326 ignoring struct tags (see below),
5327 <code>x</code>'s type and <code>T</code> are pointer types
5328 that are not <a href="#Types">named types</a>,
5329 and their pointer base types are not type parameters but
5330 have identical underlying types.
5333 <code>x</code>'s type and <code>T</code> are both integer or floating
5337 <code>x</code>'s type and <code>T</code> are both complex types.
5340 <code>x</code> is an integer or a slice of bytes or runes
5341 and <code>T</code> is a string type.
5344 <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
5347 <code>x</code> is a slice, <code>T</code> is a pointer to an array,
5348 and the slice and array types have <a href="#Type_identity">identical</a> element types.
5353 Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type
5354 parameters, <code>x</code>
5355 can also be converted to type <code>T</code> if one of the following conditions applies:
5360 Both <code>V</code> and <code>T</code> are type parameters and a value of each
5361 type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s
5365 Only <code>V</code> is a type parameter and a value of each
5366 type in <code>V</code>'s type set can be converted to <code>T</code>.
5369 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
5370 type in <code>T</code>'s type set.
5375 <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
5376 for identity for the purpose of conversion:
5380 type Person struct {
5389 Name string `json:"name"`
5391 Street string `json:"street"`
5392 City string `json:"city"`
5396 var person = (*Person)(data) // ignoring tags, the underlying types are identical
5400 Specific rules apply to (non-constant) conversions between numeric types or
5401 to and from a string type.
5402 These conversions may change the representation of <code>x</code>
5403 and incur a run-time cost.
5404 All other conversions only change the type but not the representation
5409 There is no linguistic mechanism to convert between pointers and integers.
5410 The package <a href="#Package_unsafe"><code>unsafe</code></a>
5411 implements this functionality under restricted circumstances.
5414 <h4>Conversions between numeric types</h4>
5417 For the conversion of non-constant numeric values, the following rules apply:
5422 When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is
5423 sign extended to implicit infinite precision; otherwise it is zero extended.
5424 It is then truncated to fit in the result type's size.
5425 For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
5426 The conversion always yields a valid value; there is no indication of overflow.
5429 When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded
5430 (truncation towards zero).
5433 When converting an integer or floating-point number to a floating-point type,
5434 or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded
5435 to the precision specified by the destination type.
5436 For instance, the value of a variable <code>x</code> of type <code>float32</code>
5437 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
5438 but float32(x) represents the result of rounding <code>x</code>'s value to
5439 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
5440 of precision, but <code>float32(x + 0.1)</code> does not.
5445 In all non-constant conversions involving floating-point or complex values,
5446 if the result type cannot represent the value the conversion
5447 succeeds but the result value is implementation-dependent.
5450 <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
5454 Converting a signed or unsigned integer value to a string type yields a
5455 string containing the UTF-8 representation of the integer. Values outside
5456 the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
5460 string(-1) // "\ufffd" == "\xef\xbf\xbd"
5461 string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
5463 type myString string
5464 myString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
5469 Converting a slice of bytes to a string type yields
5470 a string whose successive bytes are the elements of the slice.
5473 string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5474 string([]byte{}) // ""
5475 string([]byte(nil)) // ""
5478 string(bytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5481 string([]myByte{'w', 'o', 'r', 'l', 'd', '!'}) // "world!"
5482 myString([]myByte{'\xf0', '\x9f', '\x8c', '\x8d'}) // "🌍"
5487 Converting a slice of runes to a string type yields
5488 a string that is the concatenation of the individual rune values
5489 converted to strings.
5492 string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5493 string([]rune{}) // ""
5494 string([]rune(nil)) // ""
5497 string(runes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5500 string([]myRune{0x266b, 0x266c}) // "\u266b\u266c" == "♫♬"
5501 myString([]myRune{0x1f30e}) // "\U0001f30e" == "🌎"
5506 Converting a value of a string type to a slice of bytes type
5507 yields a slice whose successive elements are the bytes of the string.
5510 []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5511 []byte("") // []byte{}
5513 bytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5515 []myByte("world!") // []myByte{'w', 'o', 'r', 'l', 'd', '!'}
5516 []myByte(myString("🌏")) // []myByte{'\xf0', '\x9f', '\x8c', '\x8f'}
5521 Converting a value of a string type to a slice of runes type
5522 yields a slice containing the individual Unicode code points of the string.
5525 []rune(myString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
5526 []rune("") // []rune{}
5528 runes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
5530 []myRune("♫♬") // []myRune{0x266b, 0x266c}
5531 []myRune(myString("🌐")) // []myRune{0x1f310}
5536 <h4 id="Conversions_from_slice_to_array_or_array_pointer">Conversions from slice to array or array pointer</h4>
5539 Converting a slice to an array yields an array containing the elements of the underlying array of the slice.
5540 Similarly, converting a slice to an array pointer yields a pointer to the underlying array of the slice.
5541 In both cases, if the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
5542 a <a href="#Run_time_panics">run-time panic</a> occurs.
5546 s := make([]byte, 2, 4)
5549 a1 := [1]byte(s[1:]) // a1[0] == s[1]
5550 a2 := [2]byte(s) // a2[0] == s[0]
5551 a4 := [4]byte(s) // panics: len([4]byte) > len(s)
5553 s0 := (*[0]byte)(s) // s0 != nil
5554 s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
5555 s2 := (*[2]byte)(s) // &s2[0] == &s[0]
5556 s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
5559 t0 := [0]string(t) // ok for nil slice t
5560 t1 := (*[0]string)(t) // t1 == nil
5561 t2 := (*[1]string)(t) // panics: len([1]string) > len(t)
5563 u := make([]byte, 0)
5564 u0 := (*[0]byte)(u) // u0 != nil
5567 <h3 id="Constant_expressions">Constant expressions</h3>
5570 Constant expressions may contain only <a href="#Constants">constant</a>
5571 operands and are evaluated at compile time.
5575 Untyped boolean, numeric, and string constants may be used as operands
5576 wherever it is legal to use an operand of boolean, numeric, or string type,
5581 A constant <a href="#Comparison_operators">comparison</a> always yields
5582 an untyped boolean constant. If the left operand of a constant
5583 <a href="#Operators">shift expression</a> is an untyped constant, the
5584 result is an integer constant; otherwise it is a constant of the same
5585 type as the left operand, which must be of
5586 <a href="#Numeric_types">integer type</a>.
5590 Any other operation on untyped constants results in an untyped constant of the
5591 same kind; that is, a boolean, integer, floating-point, complex, or string
5593 If the untyped operands of a binary operation (other than a shift) are of
5594 different kinds, the result is of the operand's kind that appears later in this
5595 list: integer, rune, floating-point, complex.
5596 For example, an untyped integer constant divided by an
5597 untyped complex constant yields an untyped complex constant.
5601 const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
5602 const b = 15 / 4 // b == 3 (untyped integer constant)
5603 const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
5604 const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
5605 const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
5606 const d = 1 << 3.0 // d == 8 (untyped integer constant)
5607 const e = 1.0 << 3 // e == 8 (untyped integer constant)
5608 const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
5609 const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
5610 const h = "foo" > "bar" // h == true (untyped boolean constant)
5611 const j = true // j == true (untyped boolean constant)
5612 const k = 'w' + 1 // k == 'x' (untyped rune constant)
5613 const l = "hi" // l == "hi" (untyped string constant)
5614 const m = string(k) // m == "x" (type string)
5615 const Σ = 1 - 0.707i // (untyped complex constant)
5616 const Δ = Σ + 2.0e-4 // (untyped complex constant)
5617 const Φ = iota*1i - 1/1i // (untyped complex constant)
5621 Applying the built-in function <code>complex</code> to untyped
5622 integer, rune, or floating-point constants yields
5623 an untyped complex constant.
5627 const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
5628 const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
5632 Constant expressions are always evaluated exactly; intermediate values and the
5633 constants themselves may require precision significantly larger than supported
5634 by any predeclared type in the language. The following are legal declarations:
5638 const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
5639 const Four int8 = Huge >> 98 // Four == 4 (type int8)
5643 The divisor of a constant division or remainder operation must not be zero:
5647 3.14 / 0.0 // illegal: division by zero
5651 The values of <i>typed</i> constants must always be accurately
5652 <a href="#Representability">representable</a> by values
5653 of the constant type. The following constant expressions are illegal:
5657 uint(-1) // -1 cannot be represented as a uint
5658 int(3.14) // 3.14 cannot be represented as an int
5659 int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
5660 Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
5661 Four * 100 // product 400 cannot be represented as an int8 (type of Four)
5665 The mask used by the unary bitwise complement operator <code>^</code> matches
5666 the rule for non-constants: the mask is all 1s for unsigned constants
5667 and -1 for signed and untyped constants.
5671 ^1 // untyped integer constant, equal to -2
5672 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
5673 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
5674 int8(^1) // same as int8(-2)
5675 ^int8(1) // same as -1 ^ int8(1) = -2
5679 Implementation restriction: A compiler may use rounding while
5680 computing untyped floating-point or complex constant expressions; see
5681 the implementation restriction in the section
5682 on <a href="#Constants">constants</a>. This rounding may cause a
5683 floating-point constant expression to be invalid in an integer
5684 context, even if it would be integral when calculated using infinite
5685 precision, and vice versa.
5689 <h3 id="Order_of_evaluation">Order of evaluation</h3>
5692 At package level, <a href="#Package_initialization">initialization dependencies</a>
5693 determine the evaluation order of individual initialization expressions in
5694 <a href="#Variable_declarations">variable declarations</a>.
5695 Otherwise, when evaluating the <a href="#Operands">operands</a> of an
5696 expression, assignment, or
5697 <a href="#Return_statements">return statement</a>,
5698 all function calls, method calls, and
5699 communication operations are evaluated in lexical left-to-right
5704 For example, in the (function-local) assignment
5707 y[f()], ok = g(h(), i()+x[j()], <-c), k()
5710 the function calls and communication happen in the order
5711 <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
5712 <code><-c</code>, <code>g()</code>, and <code>k()</code>.
5713 However, the order of those events compared to the evaluation
5714 and indexing of <code>x</code> and the evaluation
5715 of <code>y</code> is not specified.
5720 f := func() int { a++; return a }
5721 x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
5722 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
5723 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
5727 At package level, initialization dependencies override the left-to-right rule
5728 for individual initialization expressions, but not for operands within each
5733 var a, b, c = f() + v(), g(), sqr(u()) + v()
5735 func f() int { return c }
5736 func g() int { return a }
5737 func sqr(x int) int { return x*x }
5739 // functions u and v are independent of all other variables and functions
5743 The function calls happen in the order
5744 <code>u()</code>, <code>sqr()</code>, <code>v()</code>,
5745 <code>f()</code>, <code>v()</code>, and <code>g()</code>.
5749 Floating-point operations within a single expression are evaluated according to
5750 the associativity of the operators. Explicit parentheses affect the evaluation
5751 by overriding the default associativity.
5752 In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
5753 is performed before adding <code>x</code>.
5756 <h2 id="Statements">Statements</h2>
5759 Statements control execution.
5764 Declaration | LabeledStmt | SimpleStmt |
5765 GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
5766 FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
5769 SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
5772 <h3 id="Terminating_statements">Terminating statements</h3>
5775 A <i>terminating statement</i> interrupts the regular flow of control in
5776 a <a href="#Blocks">block</a>. The following statements are terminating:
5781 A <a href="#Return_statements">"return"</a> or
5782 <a href="#Goto_statements">"goto"</a> statement.
5783 <!-- ul below only for regular layout -->
5788 A call to the built-in function
5789 <a href="#Handling_panics"><code>panic</code></a>.
5790 <!-- ul below only for regular layout -->
5795 A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
5796 <!-- ul below only for regular layout -->
5801 An <a href="#If_statements">"if" statement</a> in which:
5803 <li>the "else" branch is present, and</li>
5804 <li>both branches are terminating statements.</li>
5809 A <a href="#For_statements">"for" statement</a> in which:
5811 <li>there are no "break" statements referring to the "for" statement, and</li>
5812 <li>the loop condition is absent, and</li>
5813 <li>the "for" statement does not use a range clause.</li>
5818 A <a href="#Switch_statements">"switch" statement</a> in which:
5820 <li>there are no "break" statements referring to the "switch" statement,</li>
5821 <li>there is a default case, and</li>
5822 <li>the statement lists in each case, including the default, end in a terminating
5823 statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
5829 A <a href="#Select_statements">"select" statement</a> in which:
5831 <li>there are no "break" statements referring to the "select" statement, and</li>
5832 <li>the statement lists in each case, including the default if present,
5833 end in a terminating statement.</li>
5838 A <a href="#Labeled_statements">labeled statement</a> labeling
5839 a terminating statement.
5844 All other statements are not terminating.
5848 A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
5849 is not empty and its final non-empty statement is terminating.
5853 <h3 id="Empty_statements">Empty statements</h3>
5856 The empty statement does nothing.
5864 <h3 id="Labeled_statements">Labeled statements</h3>
5867 A labeled statement may be the target of a <code>goto</code>,
5868 <code>break</code> or <code>continue</code> statement.
5872 LabeledStmt = Label ":" Statement .
5873 Label = identifier .
5877 Error: log.Panic("error encountered")
5881 <h3 id="Expression_statements">Expression statements</h3>
5884 With the exception of specific built-in functions,
5885 function and method <a href="#Calls">calls</a> and
5886 <a href="#Receive_operator">receive operations</a>
5887 can appear in statement context. Such statements may be parenthesized.
5891 ExpressionStmt = Expression .
5895 The following built-in functions are not permitted in statement context:
5899 append cap complex imag len make new real
5900 unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice
5908 len("foo") // illegal if len is the built-in function
5912 <h3 id="Send_statements">Send statements</h3>
5915 A send statement sends a value on a channel.
5916 The channel expression's <a href="#Core_types">core type</a>
5917 must be a <a href="#Channel_types">channel</a>,
5918 the channel direction must permit send operations,
5919 and the type of the value to be sent must be <a href="#Assignability">assignable</a>
5920 to the channel's element type.
5924 SendStmt = Channel "<-" Expression .
5925 Channel = Expression .
5929 Both the channel and the value expression are evaluated before communication
5930 begins. Communication blocks until the send can proceed.
5931 A send on an unbuffered channel can proceed if a receiver is ready.
5932 A send on a buffered channel can proceed if there is room in the buffer.
5933 A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
5934 A send on a <code>nil</code> channel blocks forever.
5938 ch <- 3 // send value 3 to channel ch
5942 <h3 id="IncDec_statements">IncDec statements</h3>
5945 The "++" and "--" statements increment or decrement their operands
5946 by the untyped <a href="#Constants">constant</a> <code>1</code>.
5947 As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
5948 or a map index expression.
5952 IncDecStmt = Expression ( "++" | "--" ) .
5956 The following <a href="#Assignment_statements">assignment statements</a> are semantically
5960 <pre class="grammar">
5961 IncDec statement Assignment
5967 <h3 id="Assignment_statements">Assignment statements</h3>
5970 An <i>assignment</i> replaces the current value stored in a <a href="#Variables">variable</a>
5971 with a new value specified by an <a href="#Expressions">expression</a>.
5972 An assignment statement may assign a single value to a single variable, or multiple values to a
5973 matching number of variables.
5977 Assignment = ExpressionList assign_op ExpressionList .
5979 assign_op = [ add_op | mul_op ] "=" .
5983 Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
5984 a map index expression, or (for <code>=</code> assignments only) the
5985 <a href="#Blank_identifier">blank identifier</a>.
5986 Operands may be parenthesized.
5993 (k) = <-ch // same as: k = <-ch
5997 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
5998 <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
5999 is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
6000 <code>(y)</code> but evaluates <code>x</code>
6001 only once. The <i>op</i><code>=</code> construct is a single token.
6002 In assignment operations, both the left- and right-hand expression lists
6003 must contain exactly one single-valued expression, and the left-hand
6004 expression must not be the blank identifier.
6009 i &^= 1<<n
6013 A tuple assignment assigns the individual elements of a multi-valued
6014 operation to a list of variables. There are two forms. In the
6015 first, the right hand operand is a single multi-valued expression
6016 such as a function call, a <a href="#Channel_types">channel</a> or
6017 <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
6018 The number of operands on the left
6019 hand side must match the number of values. For instance, if
6020 <code>f</code> is a function returning two values,
6028 assigns the first value to <code>x</code> and the second to <code>y</code>.
6029 In the second form, the number of operands on the left must equal the number
6030 of expressions on the right, each of which must be single-valued, and the
6031 <i>n</i>th expression on the right is assigned to the <i>n</i>th
6032 operand on the left:
6036 one, two, three = '一', '二', '三'
6040 The <a href="#Blank_identifier">blank identifier</a> provides a way to
6041 ignore right-hand side values in an assignment:
6045 _ = x // evaluate x but ignore it
6046 x, _ = f() // evaluate f() but ignore second result value
6050 The assignment proceeds in two phases.
6051 First, the operands of <a href="#Index_expressions">index expressions</a>
6052 and <a href="#Address_operators">pointer indirections</a>
6053 (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
6054 on the left and the expressions on the right are all
6055 <a href="#Order_of_evaluation">evaluated in the usual order</a>.
6056 Second, the assignments are carried out in left-to-right order.
6060 a, b = b, a // exchange a and b
6064 i, x[i] = 1, 2 // set i = 1, x[0] = 2
6067 x[i], i = 2, 1 // set x[0] = 2, i = 1
6069 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
6071 x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
6073 type Point struct { x, y int }
6075 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
6079 for i, x[i] = range x { // set i, x[2] = 0, x[0]
6082 // after this loop, i == 0 and x == []int{3, 5, 3}
6086 In assignments, each value must be <a href="#Assignability">assignable</a>
6087 to the type of the operand to which it is assigned, with the following special cases:
6092 Any typed value may be assigned to the blank identifier.
6096 If an untyped constant
6097 is assigned to a variable of interface type or the blank identifier,
6098 the constant is first implicitly <a href="#Conversions">converted</a> to its
6099 <a href="#Constants">default type</a>.
6103 If an untyped boolean value is assigned to a variable of interface type or
6104 the blank identifier, it is first implicitly converted to type <code>bool</code>.
6108 <h3 id="If_statements">If statements</h3>
6111 "If" statements specify the conditional execution of two branches
6112 according to the value of a boolean expression. If the expression
6113 evaluates to true, the "if" branch is executed, otherwise, if
6114 present, the "else" branch is executed.
6118 IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
6128 The expression may be preceded by a simple statement, which
6129 executes before the expression is evaluated.
6133 if x := f(); x < y {
6135 } else if x > z {
6143 <h3 id="Switch_statements">Switch statements</h3>
6146 "Switch" statements provide multi-way execution.
6147 An expression or type is compared to the "cases"
6148 inside the "switch" to determine which branch
6153 SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
6157 There are two forms: expression switches and type switches.
6158 In an expression switch, the cases contain expressions that are compared
6159 against the value of the switch expression.
6160 In a type switch, the cases contain types that are compared against the
6161 type of a specially annotated switch expression.
6162 The switch expression is evaluated exactly once in a switch statement.
6165 <h4 id="Expression_switches">Expression switches</h4>
6168 In an expression switch,
6169 the switch expression is evaluated and
6170 the case expressions, which need not be constants,
6171 are evaluated left-to-right and top-to-bottom; the first one that equals the
6173 triggers execution of the statements of the associated case;
6174 the other cases are skipped.
6175 If no case matches and there is a "default" case,
6176 its statements are executed.
6177 There can be at most one default case and it may appear anywhere in the
6179 A missing switch expression is equivalent to the boolean value
6184 ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
6185 ExprCaseClause = ExprSwitchCase ":" StatementList .
6186 ExprSwitchCase = "case" ExpressionList | "default" .
6190 If the switch expression evaluates to an untyped constant, it is first implicitly
6191 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
6192 The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
6193 The switch expression type must be <a href="#Comparison_operators">comparable</a>.
6197 If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
6198 to the type of the switch expression.
6199 For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
6200 of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
6204 In other words, the switch expression is treated as if it were used to declare and
6205 initialize a temporary variable <code>t</code> without explicit type; it is that
6206 value of <code>t</code> against which each case expression <code>x</code> is tested
6211 In a case or default clause, the last non-empty statement
6212 may be a (possibly <a href="#Labeled_statements">labeled</a>)
6213 <a href="#Fallthrough_statements">"fallthrough" statement</a> to
6214 indicate that control should flow from the end of this clause to
6215 the first statement of the next clause.
6216 Otherwise control flows to the end of the "switch" statement.
6217 A "fallthrough" statement may appear as the last statement of all
6218 but the last clause of an expression switch.
6222 The switch expression may be preceded by a simple statement, which
6223 executes before the expression is evaluated.
6229 case 0, 1, 2, 3: s1()
6230 case 4, 5, 6, 7: s2()
6233 switch x := f(); { // missing switch expression means "true"
6234 case x < 0: return -x
6246 Implementation restriction: A compiler may disallow multiple case
6247 expressions evaluating to the same constant.
6248 For instance, the current compilers disallow duplicate integer,
6249 floating point, or string constants in case expressions.
6252 <h4 id="Type_switches">Type switches</h4>
6255 A type switch compares types rather than values. It is otherwise similar
6256 to an expression switch. It is marked by a special switch expression that
6257 has the form of a <a href="#Type_assertions">type assertion</a>
6258 using the keyword <code>type</code> rather than an actual type:
6268 Cases then match actual types <code>T</code> against the dynamic type of the
6269 expression <code>x</code>. As with type assertions, <code>x</code> must be of
6270 <a href="#Interface_types">interface type</a>, but not a
6271 <a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type
6272 <code>T</code> listed in a case must implement the type of <code>x</code>.
6273 The types listed in the cases of a type switch must all be
6274 <a href="#Type_identity">different</a>.
6278 TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
6279 TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
6280 TypeCaseClause = TypeSwitchCase ":" StatementList .
6281 TypeSwitchCase = "case" TypeList | "default" .
6285 The TypeSwitchGuard may include a
6286 <a href="#Short_variable_declarations">short variable declaration</a>.
6287 When that form is used, the variable is declared at the end of the
6288 TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
6289 In clauses with a case listing exactly one type, the variable
6290 has that type; otherwise, the variable has the type of the expression
6291 in the TypeSwitchGuard.
6295 Instead of a type, a case may use the predeclared identifier
6296 <a href="#Predeclared_identifiers"><code>nil</code></a>;
6297 that case is selected when the expression in the TypeSwitchGuard
6298 is a <code>nil</code> interface value.
6299 There may be at most one <code>nil</code> case.
6303 Given an expression <code>x</code> of type <code>interface{}</code>,
6304 the following type switch:
6308 switch i := x.(type) {
6310 printString("x is nil") // type of i is type of x (interface{})
6312 printInt(i) // type of i is int
6314 printFloat64(i) // type of i is float64
6315 case func(int) float64:
6316 printFunction(i) // type of i is func(int) float64
6318 printString("type is bool or string") // type of i is type of x (interface{})
6320 printString("don't know the type") // type of i is type of x (interface{})
6329 v := x // x is evaluated exactly once
6331 i := v // type of i is type of x (interface{})
6332 printString("x is nil")
6333 } else if i, isInt := v.(int); isInt {
6334 printInt(i) // type of i is int
6335 } else if i, isFloat64 := v.(float64); isFloat64 {
6336 printFloat64(i) // type of i is float64
6337 } else if i, isFunc := v.(func(int) float64); isFunc {
6338 printFunction(i) // type of i is func(int) float64
6340 _, isBool := v.(bool)
6341 _, isString := v.(string)
6342 if isBool || isString {
6343 i := v // type of i is type of x (interface{})
6344 printString("type is bool or string")
6346 i := v // type of i is type of x (interface{})
6347 printString("don't know the type")
6353 A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a>
6354 may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns
6355 out to duplicate another entry in the switch, the first matching case is chosen.
6359 func f[P any](x any) int {
6374 var v1 = f[string]("foo") // v1 == 0
6375 var v2 = f[byte]([]byte{}) // v2 == 2
6379 The type switch guard may be preceded by a simple statement, which
6380 executes before the guard is evaluated.
6384 The "fallthrough" statement is not permitted in a type switch.
6387 <h3 id="For_statements">For statements</h3>
6390 A "for" statement specifies repeated execution of a block. There are three forms:
6391 The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
6395 ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
6396 Condition = Expression .
6399 <h4 id="For_condition">For statements with single condition</h4>
6402 In its simplest form, a "for" statement specifies the repeated execution of
6403 a block as long as a boolean condition evaluates to true.
6404 The condition is evaluated before each iteration.
6405 If the condition is absent, it is equivalent to the boolean value
6415 <h4 id="For_clause">For statements with <code>for</code> clause</h4>
6418 A "for" statement with a ForClause is also controlled by its condition, but
6419 additionally it may specify an <i>init</i>
6420 and a <i>post</i> statement, such as an assignment,
6421 an increment or decrement statement. The init statement may be a
6422 <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
6423 Variables declared by the init statement are re-used in each iteration.
6427 ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
6428 InitStmt = SimpleStmt .
6429 PostStmt = SimpleStmt .
6433 for i := 0; i < 10; i++ {
6439 If non-empty, the init statement is executed once before evaluating the
6440 condition for the first iteration;
6441 the post statement is executed after each execution of the block (and
6442 only if the block was executed).
6443 Any element of the ForClause may be empty but the
6444 <a href="#Semicolons">semicolons</a> are
6445 required unless there is only a condition.
6446 If the condition is absent, it is equivalent to the boolean value
6451 for cond { S() } is the same as for ; cond ; { S() }
6452 for { S() } is the same as for true { S() }
6455 <h4 id="For_range">For statements with <code>range</code> clause</h4>
6458 A "for" statement with a "range" clause
6459 iterates through all entries of an array, slice, string or map,
6460 or values received on a channel. For each entry it assigns <i>iteration values</i>
6461 to corresponding <i>iteration variables</i> if present and then executes the block.
6465 RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
6469 The expression on the right in the "range" clause is called the <i>range expression</i>,
6470 its <a href="#Core_types">core type</a> must be
6471 an array, pointer to an array, slice, string, map, or channel permitting
6472 <a href="#Receive_operator">receive operations</a>.
6473 As with an assignment, if present the operands on the left must be
6474 <a href="#Address_operators">addressable</a> or map index expressions; they
6475 denote the iteration variables. If the range expression is a channel, at most
6476 one iteration variable is permitted, otherwise there may be up to two.
6477 If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
6478 the range clause is equivalent to the same clause without that identifier.
6482 The range expression <code>x</code> is evaluated once before beginning the loop,
6483 with one exception: if at most one iteration variable is present and
6484 <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
6485 the range expression is not evaluated.
6489 Function calls on the left are evaluated once per iteration.
6490 For each iteration, iteration values are produced as follows
6491 if the respective iteration variables are present:
6494 <pre class="grammar">
6495 Range expression 1st value 2nd value
6497 array or slice a [n]E, *[n]E, or []E index i int a[i] E
6498 string s string type index i int see below rune
6499 map m map[K]V key k K m[k] V
6500 channel c chan E, <-chan E element e E
6505 For an array, pointer to array, or slice value <code>a</code>, the index iteration
6506 values are produced in increasing order, starting at element index 0.
6507 If at most one iteration variable is present, the range loop produces
6508 iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
6509 or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
6513 For a string value, the "range" clause iterates over the Unicode code points
6514 in the string starting at byte index 0. On successive iterations, the index value will be the
6515 index of the first byte of successive UTF-8-encoded code points in the string,
6516 and the second value, of type <code>rune</code>, will be the value of
6517 the corresponding code point. If the iteration encounters an invalid
6518 UTF-8 sequence, the second value will be <code>0xFFFD</code>,
6519 the Unicode replacement character, and the next iteration will advance
6520 a single byte in the string.
6524 The iteration order over maps is not specified
6525 and is not guaranteed to be the same from one iteration to the next.
6526 If a map entry that has not yet been reached is removed during iteration,
6527 the corresponding iteration value will not be produced. If a map entry is
6528 created during iteration, that entry may be produced during the iteration or
6529 may be skipped. The choice may vary for each entry created and from one
6530 iteration to the next.
6531 If the map is <code>nil</code>, the number of iterations is 0.
6535 For channels, the iteration values produced are the successive values sent on
6536 the channel until the channel is <a href="#Close">closed</a>. If the channel
6537 is <code>nil</code>, the range expression blocks forever.
6542 The iteration values are assigned to the respective
6543 iteration variables as in an <a href="#Assignment_statements">assignment statement</a>.
6547 The iteration variables may be declared by the "range" clause using a form of
6548 <a href="#Short_variable_declarations">short variable declaration</a>
6550 In this case their types are set to the types of the respective iteration values
6551 and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
6552 statement; they are re-used in each iteration.
6553 If the iteration variables are declared outside the "for" statement,
6554 after execution their values will be those of the last iteration.
6558 var testdata *struct {
6561 for i, _ := range testdata.a {
6562 // testdata.a is never evaluated; len(testdata.a) is constant
6563 // i ranges from 0 to 6
6568 for i, s := range a {
6570 // type of s is string
6576 var val interface{} // element type of m is assignable to val
6577 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
6578 for key, val = range m {
6581 // key == last map key encountered in iteration
6584 var ch chan Work = producer()
6594 <h3 id="Go_statements">Go statements</h3>
6597 A "go" statement starts the execution of a function call
6598 as an independent concurrent thread of control, or <i>goroutine</i>,
6599 within the same address space.
6603 GoStmt = "go" Expression .
6607 The expression must be a function or method call; it cannot be parenthesized.
6608 Calls of built-in functions are restricted as for
6609 <a href="#Expression_statements">expression statements</a>.
6613 The function value and parameters are
6614 <a href="#Calls">evaluated as usual</a>
6615 in the calling goroutine, but
6616 unlike with a regular call, program execution does not wait
6617 for the invoked function to complete.
6618 Instead, the function begins executing independently
6620 When the function terminates, its goroutine also terminates.
6621 If the function has any return values, they are discarded when the
6627 go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
6631 <h3 id="Select_statements">Select statements</h3>
6634 A "select" statement chooses which of a set of possible
6635 <a href="#Send_statements">send</a> or
6636 <a href="#Receive_operator">receive</a>
6637 operations will proceed.
6638 It looks similar to a
6639 <a href="#Switch_statements">"switch"</a> statement but with the
6640 cases all referring to communication operations.
6644 SelectStmt = "select" "{" { CommClause } "}" .
6645 CommClause = CommCase ":" StatementList .
6646 CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
6647 RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
6648 RecvExpr = Expression .
6652 A case with a RecvStmt may assign the result of a RecvExpr to one or
6653 two variables, which may be declared using a
6654 <a href="#Short_variable_declarations">short variable declaration</a>.
6655 The RecvExpr must be a (possibly parenthesized) receive operation.
6656 There can be at most one default case and it may appear anywhere
6657 in the list of cases.
6661 Execution of a "select" statement proceeds in several steps:
6666 For all the cases in the statement, the channel operands of receive operations
6667 and the channel and right-hand-side expressions of send statements are
6668 evaluated exactly once, in source order, upon entering the "select" statement.
6669 The result is a set of channels to receive from or send to,
6670 and the corresponding values to send.
6671 Any side effects in that evaluation will occur irrespective of which (if any)
6672 communication operation is selected to proceed.
6673 Expressions on the left-hand side of a RecvStmt with a short variable declaration
6674 or assignment are not yet evaluated.
6678 If one or more of the communications can proceed,
6679 a single one that can proceed is chosen via a uniform pseudo-random selection.
6680 Otherwise, if there is a default case, that case is chosen.
6681 If there is no default case, the "select" statement blocks until
6682 at least one of the communications can proceed.
6686 Unless the selected case is the default case, the respective communication
6687 operation is executed.
6691 If the selected case is a RecvStmt with a short variable declaration or
6692 an assignment, the left-hand side expressions are evaluated and the
6693 received value (or values) are assigned.
6697 The statement list of the selected case is executed.
6702 Since communication on <code>nil</code> channels can never proceed,
6703 a select with only <code>nil</code> channels and no default case blocks forever.
6708 var c, c1, c2, c3, c4 chan int
6712 print("received ", i1, " from c1\n")
6714 print("sent ", i2, " to c2\n")
6715 case i3, ok := (<-c3): // same as: i3, ok := <-c3
6717 print("received ", i3, " from c3\n")
6719 print("c3 is closed\n")
6721 case a[f()] = <-c4:
6723 // case t := <-c4
6726 print("no communication\n")
6729 for { // send random sequence of bits to c
6731 case c <- 0: // note: no statement, no fallthrough, no folding of cases
6736 select {} // block forever
6740 <h3 id="Return_statements">Return statements</h3>
6743 A "return" statement in a function <code>F</code> terminates the execution
6744 of <code>F</code>, and optionally provides one or more result values.
6745 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6746 are executed before <code>F</code> returns to its caller.
6750 ReturnStmt = "return" [ ExpressionList ] .
6754 In a function without a result type, a "return" statement must not
6755 specify any result values.
6764 There are three ways to return values from a function with a result
6769 <li>The return value or values may be explicitly listed
6770 in the "return" statement. Each expression must be single-valued
6771 and <a href="#Assignability">assignable</a>
6772 to the corresponding element of the function's result type.
6774 func simpleF() int {
6778 func complexF1() (re float64, im float64) {
6783 <li>The expression list in the "return" statement may be a single
6784 call to a multi-valued function. The effect is as if each value
6785 returned from that function were assigned to a temporary
6786 variable with the type of the respective value, followed by a
6787 "return" statement listing these variables, at which point the
6788 rules of the previous case apply.
6790 func complexF2() (re float64, im float64) {
6795 <li>The expression list may be empty if the function's result
6796 type specifies names for its <a href="#Function_types">result parameters</a>.
6797 The result parameters act as ordinary local variables
6798 and the function may assign values to them as necessary.
6799 The "return" statement returns the values of these variables.
6801 func complexF3() (re float64, im float64) {
6807 func (devnull) Write(p []byte) (n int, _ error) {
6816 Regardless of how they are declared, all the result values are initialized to
6817 the <a href="#The_zero_value">zero values</a> for their type upon entry to the
6818 function. A "return" statement that specifies results sets the result parameters before
6819 any deferred functions are executed.
6823 Implementation restriction: A compiler may disallow an empty expression list
6824 in a "return" statement if a different entity (constant, type, or variable)
6825 with the same name as a result parameter is in
6826 <a href="#Declarations_and_scope">scope</a> at the place of the return.
6830 func f(n int) (res int, err error) {
6831 if _, err := f(n-1); err != nil {
6832 return // invalid return statement: err is shadowed
6838 <h3 id="Break_statements">Break statements</h3>
6841 A "break" statement terminates execution of the innermost
6842 <a href="#For_statements">"for"</a>,
6843 <a href="#Switch_statements">"switch"</a>, or
6844 <a href="#Select_statements">"select"</a> statement
6845 within the same function.
6849 BreakStmt = "break" [ Label ] .
6853 If there is a label, it must be that of an enclosing
6854 "for", "switch", or "select" statement,
6855 and that is the one whose execution terminates.
6860 for i = 0; i < n; i++ {
6861 for j = 0; j < m; j++ {
6874 <h3 id="Continue_statements">Continue statements</h3>
6877 A "continue" statement begins the next iteration of the
6878 innermost enclosing <a href="#For_statements">"for" loop</a>
6879 by advancing control to the end of the loop block.
6880 The "for" loop must be within the same function.
6884 ContinueStmt = "continue" [ Label ] .
6888 If there is a label, it must be that of an enclosing
6889 "for" statement, and that is the one whose execution
6895 for y, row := range rows {
6896 for x, data := range row {
6897 if data == endOfRow {
6900 row[x] = data + bias(x, y)
6905 <h3 id="Goto_statements">Goto statements</h3>
6908 A "goto" statement transfers control to the statement with the corresponding label
6909 within the same function.
6913 GotoStmt = "goto" Label .
6921 Executing the "goto" statement must not cause any variables to come into
6922 <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
6923 For instance, this example:
6933 is erroneous because the jump to label <code>L</code> skips
6934 the creation of <code>v</code>.
6938 A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
6939 For instance, this example:
6956 is erroneous because the label <code>L1</code> is inside
6957 the "for" statement's block but the <code>goto</code> is not.
6960 <h3 id="Fallthrough_statements">Fallthrough statements</h3>
6963 A "fallthrough" statement transfers control to the first statement of the
6964 next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
6965 It may be used only as the final non-empty statement in such a clause.
6969 FallthroughStmt = "fallthrough" .
6973 <h3 id="Defer_statements">Defer statements</h3>
6976 A "defer" statement invokes a function whose execution is deferred
6977 to the moment the surrounding function returns, either because the
6978 surrounding function executed a <a href="#Return_statements">return statement</a>,
6979 reached the end of its <a href="#Function_declarations">function body</a>,
6980 or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
6984 DeferStmt = "defer" Expression .
6988 The expression must be a function or method call; it cannot be parenthesized.
6989 Calls of built-in functions are restricted as for
6990 <a href="#Expression_statements">expression statements</a>.
6994 Each time a "defer" statement
6995 executes, the function value and parameters to the call are
6996 <a href="#Calls">evaluated as usual</a>
6997 and saved anew but the actual function is not invoked.
6998 Instead, deferred functions are invoked immediately before
6999 the surrounding function returns, in the reverse order
7000 they were deferred. That is, if the surrounding function
7001 returns through an explicit <a href="#Return_statements">return statement</a>,
7002 deferred functions are executed <i>after</i> any result parameters are set
7003 by that return statement but <i>before</i> the function returns to its caller.
7004 If a deferred function value evaluates
7005 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
7006 when the function is invoked, not when the "defer" statement is executed.
7010 For instance, if the deferred function is
7011 a <a href="#Function_literals">function literal</a> and the surrounding
7012 function has <a href="#Function_types">named result parameters</a> that
7013 are in scope within the literal, the deferred function may access and modify
7014 the result parameters before they are returned.
7015 If the deferred function has any return values, they are discarded when
7016 the function completes.
7017 (See also the section on <a href="#Handling_panics">handling panics</a>.)
7022 defer unlock(l) // unlocking happens before surrounding function returns
7024 // prints 3 2 1 0 before surrounding function returns
7025 for i := 0; i <= 3; i++ {
7030 func f() (result int) {
7032 // result is accessed after it was set to 6 by the return statement
7039 <h2 id="Built-in_functions">Built-in functions</h2>
7042 Built-in functions are
7043 <a href="#Predeclared_identifiers">predeclared</a>.
7044 They are called like any other function but some of them
7045 accept a type instead of an expression as the first argument.
7049 The built-in functions do not have standard Go types,
7050 so they can only appear in <a href="#Calls">call expressions</a>;
7051 they cannot be used as function values.
7054 <h3 id="Close">Close</h3>
7057 For an argument <code>ch</code> with a <a href="#Core_types">core type</a>
7058 that is a <a href="#Channel_types">channel</a>, the built-in function <code>close</code>
7059 records that no more values will be sent on the channel.
7060 It is an error if <code>ch</code> is a receive-only channel.
7061 Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
7062 Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
7063 After calling <code>close</code>, and after any previously
7064 sent values have been received, receive operations will return
7065 the zero value for the channel's type without blocking.
7066 The multi-valued <a href="#Receive_operator">receive operation</a>
7067 returns a received value along with an indication of whether the channel is closed.
7070 <h3 id="Length_and_capacity">Length and capacity</h3>
7073 The built-in functions <code>len</code> and <code>cap</code> take arguments
7074 of various types and return a result of type <code>int</code>.
7075 The implementation guarantees that the result always fits into an <code>int</code>.
7078 <pre class="grammar">
7079 Call Argument type Result
7081 len(s) string type string length in bytes
7082 [n]T, *[n]T array length (== n)
7084 map[K]T map length (number of defined keys)
7085 chan T number of elements queued in channel buffer
7086 type parameter see below
7088 cap(s) [n]T, *[n]T array length (== n)
7090 chan T channel buffer capacity
7091 type parameter see below
7095 If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>,
7096 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
7097 each type in <code>P</code>'s type set.
7098 The result is the length (or capacity, respectively) of the argument whose type
7099 corresponds to the type argument with which <code>P</code> was
7100 <a href="#Instantiations">instantiated</a>.
7104 The capacity of a slice is the number of elements for which there is
7105 space allocated in the underlying array.
7106 At any time the following relationship holds:
7110 0 <= len(s) <= cap(s)
7114 The length of a <code>nil</code> slice, map or channel is 0.
7115 The capacity of a <code>nil</code> slice or channel is 0.
7119 The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
7120 <code>s</code> is a string constant. The expressions <code>len(s)</code> and
7121 <code>cap(s)</code> are constants if the type of <code>s</code> is an array
7122 or pointer to an array and the expression <code>s</code> does not contain
7123 <a href="#Receive_operator">channel receives</a> or (non-constant)
7124 <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
7125 Otherwise, invocations of <code>len</code> and <code>cap</code> are not
7126 constant and <code>s</code> is evaluated.
7131 c1 = imag(2i) // imag(2i) = 2.0 is a constant
7132 c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
7133 c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
7134 c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
7135 c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
7140 <h3 id="Allocation">Allocation</h3>
7143 The built-in function <code>new</code> takes a type <code>T</code>,
7144 allocates storage for a <a href="#Variables">variable</a> of that type
7145 at run time, and returns a value of type <code>*T</code>
7146 <a href="#Pointer_types">pointing</a> to it.
7147 The variable is initialized as described in the section on
7148 <a href="#The_zero_value">initial values</a>.
7151 <pre class="grammar">
7160 type S struct { a int; b float64 }
7165 allocates storage for a variable of type <code>S</code>,
7166 initializes it (<code>a=0</code>, <code>b=0.0</code>),
7167 and returns a value of type <code>*S</code> containing the address
7171 <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
7174 The built-in function <code>make</code> takes a type <code>T</code>,
7175 optionally followed by a type-specific list of expressions.
7176 The <a href="#Core_types">core type</a> of <code>T</code> must
7177 be a slice, map or channel.
7178 It returns a value of type <code>T</code> (not <code>*T</code>).
7179 The memory is initialized as described in the section on
7180 <a href="#The_zero_value">initial values</a>.
7183 <pre class="grammar">
7184 Call Core type Result
7186 make(T, n) slice slice of type T with length n and capacity n
7187 make(T, n, m) slice slice of type T with length n and capacity m
7189 make(T) map map of type T
7190 make(T, n) map map of type T with initial space for approximately n elements
7192 make(T) channel unbuffered channel of type T
7193 make(T, n) channel buffered channel of type T, buffer size n
7198 Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>,
7199 have a <a href="#Interface_types">type set</a> containing only integer types,
7200 or be an untyped <a href="#Constants">constant</a>.
7201 A constant size argument must be non-negative and <a href="#Representability">representable</a>
7202 by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
7203 If both <code>n</code> and <code>m</code> are provided and are constant, then
7204 <code>n</code> must be no larger than <code>m</code>.
7205 For slices and channels, if <code>n</code> is negative or larger than <code>m</code> at run time,
7206 a <a href="#Run_time_panics">run-time panic</a> occurs.
7210 s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
7211 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
7212 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
7213 s := make([]int, 10, 0) // illegal: len(s) > cap(s)
7214 c := make(chan int, 10) // channel with a buffer size of 10
7215 m := make(map[string]int, 100) // map with initial space for approximately 100 elements
7219 Calling <code>make</code> with a map type and size hint <code>n</code> will
7220 create a map with initial space to hold <code>n</code> map elements.
7221 The precise behavior is implementation-dependent.
7225 <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
7228 The built-in functions <code>append</code> and <code>copy</code> assist in
7229 common slice operations.
7230 For both functions, the result is independent of whether the memory referenced
7231 by the arguments overlaps.
7235 The <a href="#Function_types">variadic</a> function <code>append</code>
7236 appends zero or more values <code>x</code> to a slice <code>s</code>
7237 and returns the resulting slice of the same type as <code>s</code>.
7238 The <a href="#Core_types">core type</a> of <code>s</code> must be a slice
7239 of type <code>[]E</code>.
7240 The values <code>x</code> are passed to a parameter of type <code>...E</code>
7241 and the respective <a href="#Passing_arguments_to_..._parameters">parameter
7242 passing rules</a> apply.
7243 As a special case, if the core type of <code>s</code> is <code>[]byte</code>,
7244 <code>append</code> also accepts a second argument with core type
7245 <a href="#Core_types"><code>bytestring</code></a> followed by <code>...</code>.
7246 This form appends the bytes of the byte slice or string.
7249 <pre class="grammar">
7250 append(s S, x ...E) S // core type of S is []E
7254 If the capacity of <code>s</code> is not large enough to fit the additional
7255 values, <code>append</code> allocates a new, sufficiently large underlying
7256 array that fits both the existing slice elements and the additional values.
7257 Otherwise, <code>append</code> re-uses the underlying array.
7262 s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
7263 s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
7264 s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
7265 s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
7268 t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"}
7271 b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
7275 The function <code>copy</code> copies slice elements from
7276 a source <code>src</code> to a destination <code>dst</code> and returns the
7277 number of elements copied.
7278 The <a href="#Core_types">core types</a> of both arguments must be slices
7279 with <a href="#Type_identity">identical</a> element type.
7280 The number of elements copied is the minimum of
7281 <code>len(src)</code> and <code>len(dst)</code>.
7282 As a special case, if the destination's core type is <code>[]byte</code>,
7283 <code>copy</code> also accepts a source argument with core type
7284 </a> <a href="#Core_types"><code>bytestring</code></a>.
7285 This form copies the bytes from the byte slice or string into the byte slice.
7288 <pre class="grammar">
7289 copy(dst, src []T) int
7290 copy(dst []byte, src string) int
7298 var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
7299 var s = make([]int, 6)
7300 var b = make([]byte, 5)
7301 n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
7302 n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
7303 n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
7307 <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
7310 The built-in function <code>delete</code> removes the element with key
7311 <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
7312 value <code>k</code> must be <a href="#Assignability">assignable</a>
7313 to the key type of <code>m</code>.
7316 <pre class="grammar">
7317 delete(m, k) // remove element m[k] from map m
7321 If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>,
7322 all types in that type set must be maps, and they must all have identical key types.
7326 If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
7327 does not exist, <code>delete</code> is a no-op.
7331 <h3 id="Complex_numbers">Manipulating complex numbers</h3>
7334 Three functions assemble and disassemble complex numbers.
7335 The built-in function <code>complex</code> constructs a complex
7336 value from a floating-point real and imaginary part, while
7337 <code>real</code> and <code>imag</code>
7338 extract the real and imaginary parts of a complex value.
7341 <pre class="grammar">
7342 complex(realPart, imaginaryPart floatT) complexT
7343 real(complexT) floatT
7344 imag(complexT) floatT
7348 The type of the arguments and return value correspond.
7349 For <code>complex</code>, the two arguments must be of the same
7350 <a href="#Numeric_types">floating-point type</a> and the return type is the
7351 <a href="#Numeric_types">complex type</a>
7352 with the corresponding floating-point constituents:
7353 <code>complex64</code> for <code>float32</code> arguments, and
7354 <code>complex128</code> for <code>float64</code> arguments.
7355 If one of the arguments evaluates to an untyped constant, it is first implicitly
7356 <a href="#Conversions">converted</a> to the type of the other argument.
7357 If both arguments evaluate to untyped constants, they must be non-complex
7358 numbers or their imaginary parts must be zero, and the return value of
7359 the function is an untyped complex constant.
7363 For <code>real</code> and <code>imag</code>, the argument must be
7364 of complex type, and the return type is the corresponding floating-point
7365 type: <code>float32</code> for a <code>complex64</code> argument, and
7366 <code>float64</code> for a <code>complex128</code> argument.
7367 If the argument evaluates to an untyped constant, it must be a number,
7368 and the return value of the function is an untyped floating-point constant.
7372 The <code>real</code> and <code>imag</code> functions together form the inverse of
7373 <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
7374 <code>z == Z(complex(real(z), imag(z)))</code>.
7378 If the operands of these functions are all constants, the return
7379 value is a constant.
7383 var a = complex(2, -2) // complex128
7384 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
7385 x := float32(math.Cos(math.Pi/2)) // float32
7386 var c64 = complex(5, -x) // complex64
7387 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
7388 _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
7389 var rl = real(c64) // float32
7390 var im = imag(a) // float64
7391 const c = imag(b) // untyped constant -1.4
7392 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
7396 Arguments of type parameter type are not permitted.
7399 <h3 id="Handling_panics">Handling panics</h3>
7401 <p> Two built-in functions, <code>panic</code> and <code>recover</code>,
7402 assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
7403 and program-defined error conditions.
7406 <pre class="grammar">
7407 func panic(interface{})
7408 func recover() interface{}
7412 While executing a function <code>F</code>,
7413 an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
7414 terminates the execution of <code>F</code>.
7415 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
7416 are then executed as usual.
7417 Next, any deferred functions run by <code>F</code>'s caller are run,
7418 and so on up to any deferred by the top-level function in the executing goroutine.
7419 At that point, the program is terminated and the error
7420 condition is reported, including the value of the argument to <code>panic</code>.
7421 This termination sequence is called <i>panicking</i>.
7426 panic("unreachable")
7427 panic(Error("cannot parse"))
7431 The <code>recover</code> function allows a program to manage behavior
7432 of a panicking goroutine.
7433 Suppose a function <code>G</code> defers a function <code>D</code> that calls
7434 <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
7436 When the running of deferred functions reaches <code>D</code>,
7437 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>.
7438 If <code>D</code> returns normally, without starting a new
7439 <code>panic</code>, the panicking sequence stops. In that case,
7440 the state of functions called between <code>G</code> and the call to <code>panic</code>
7441 is discarded, and normal execution resumes.
7442 Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
7443 execution terminates by returning to its caller.
7447 The return value of <code>recover</code> is <code>nil</code> if any of the following conditions holds:
7451 <code>panic</code>'s argument was <code>nil</code>;
7454 the goroutine is not panicking;
7457 <code>recover</code> was not called directly by a deferred function.
7462 The <code>protect</code> function in the example below invokes
7463 the function argument <code>g</code> and protects callers from
7464 run-time panics raised by <code>g</code>.
7468 func protect(g func()) {
7470 log.Println("done") // Println executes normally even if there is a panic
7471 if x := recover(); x != nil {
7472 log.Printf("run time panic: %v", x)
7475 log.Println("start")
7481 <h3 id="Bootstrapping">Bootstrapping</h3>
7484 Current implementations provide several built-in functions useful during
7485 bootstrapping. These functions are documented for completeness but are not
7486 guaranteed to stay in the language. They do not return a result.
7489 <pre class="grammar">
7492 print prints all arguments; formatting of arguments is implementation-specific
7493 println like print but prints spaces between arguments and a newline at the end
7497 Implementation restriction: <code>print</code> and <code>println</code> need not
7498 accept arbitrary argument types, but printing of boolean, numeric, and string
7499 <a href="#Types">types</a> must be supported.
7502 <h2 id="Packages">Packages</h2>
7505 Go programs are constructed by linking together <i>packages</i>.
7506 A package in turn is constructed from one or more source files
7507 that together declare constants, types, variables and functions
7508 belonging to the package and which are accessible in all files
7509 of the same package. Those elements may be
7510 <a href="#Exported_identifiers">exported</a> and used in another package.
7513 <h3 id="Source_file_organization">Source file organization</h3>
7516 Each source file consists of a package clause defining the package
7517 to which it belongs, followed by a possibly empty set of import
7518 declarations that declare packages whose contents it wishes to use,
7519 followed by a possibly empty set of declarations of functions,
7520 types, variables, and constants.
7524 SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
7527 <h3 id="Package_clause">Package clause</h3>
7530 A package clause begins each source file and defines the package
7531 to which the file belongs.
7535 PackageClause = "package" PackageName .
7536 PackageName = identifier .
7540 The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
7548 A set of files sharing the same PackageName form the implementation of a package.
7549 An implementation may require that all source files for a package inhabit the same directory.
7552 <h3 id="Import_declarations">Import declarations</h3>
7555 An import declaration states that the source file containing the declaration
7556 depends on functionality of the <i>imported</i> package
7557 (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
7558 and enables access to <a href="#Exported_identifiers">exported</a> identifiers
7560 The import names an identifier (PackageName) to be used for access and an ImportPath
7561 that specifies the package to be imported.
7565 ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
7566 ImportSpec = [ "." | PackageName ] ImportPath .
7567 ImportPath = string_lit .
7571 The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
7572 to access exported identifiers of the package within the importing source file.
7573 It is declared in the <a href="#Blocks">file block</a>.
7574 If the PackageName is omitted, it defaults to the identifier specified in the
7575 <a href="#Package_clause">package clause</a> of the imported package.
7576 If an explicit period (<code>.</code>) appears instead of a name, all the
7577 package's exported identifiers declared in that package's
7578 <a href="#Blocks">package block</a> will be declared in the importing source
7579 file's file block and must be accessed without a qualifier.
7583 The interpretation of the ImportPath is implementation-dependent but
7584 it is typically a substring of the full file name of the compiled
7585 package and may be relative to a repository of installed packages.
7589 Implementation restriction: A compiler may restrict ImportPaths to
7590 non-empty strings using only characters belonging to
7591 <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
7592 L, M, N, P, and S general categories (the Graphic characters without
7593 spaces) and may also exclude the characters
7594 <code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
7595 and the Unicode replacement character U+FFFD.
7599 Consider a compiled a package containing the package clause
7600 <code>package math</code>, which exports function <code>Sin</code>, and
7601 installed the compiled package in the file identified by
7602 <code>"lib/math"</code>.
7603 This table illustrates how <code>Sin</code> is accessed in files
7604 that import the package after the
7605 various types of import declaration.
7608 <pre class="grammar">
7609 Import declaration Local name of Sin
7611 import "lib/math" math.Sin
7612 import m "lib/math" m.Sin
7613 import . "lib/math" Sin
7617 An import declaration declares a dependency relation between
7618 the importing and imported package.
7619 It is illegal for a package to import itself, directly or indirectly,
7620 or to directly import a package without
7621 referring to any of its exported identifiers. To import a package solely for
7622 its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
7623 identifier as explicit package name:
7631 <h3 id="An_example_package">An example package</h3>
7634 Here is a complete Go package that implements a concurrent prime sieve.
7642 // Send the sequence 2, 3, 4, … to channel 'ch'.
7643 func generate(ch chan<- int) {
7645 ch <- i // Send 'i' to channel 'ch'.
7649 // Copy the values from channel 'src' to channel 'dst',
7650 // removing those divisible by 'prime'.
7651 func filter(src <-chan int, dst chan<- int, prime int) {
7652 for i := range src { // Loop over values received from 'src'.
7654 dst <- i // Send 'i' to channel 'dst'.
7659 // The prime sieve: Daisy-chain filter processes together.
7661 ch := make(chan int) // Create a new channel.
7662 go generate(ch) // Start generate() as a subprocess.
7665 fmt.Print(prime, "\n")
7666 ch1 := make(chan int)
7667 go filter(ch, ch1, prime)
7677 <h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
7679 <h3 id="The_zero_value">The zero value</h3>
7681 When storage is allocated for a <a href="#Variables">variable</a>,
7682 either through a declaration or a call of <code>new</code>, or when
7683 a new value is created, either through a composite literal or a call
7684 of <code>make</code>,
7685 and no explicit initialization is provided, the variable or value is
7686 given a default value. Each element of such a variable or value is
7687 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
7688 <code>0</code> for numeric types, <code>""</code>
7689 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
7690 This initialization is done recursively, so for instance each element of an
7691 array of structs will have its fields zeroed if no value is specified.
7694 These two simple declarations are equivalent:
7707 type T struct { i int; f float64; next *T }
7712 the following holds:
7722 The same would also be true after
7729 <h3 id="Package_initialization">Package initialization</h3>
7732 Within a package, package-level variable initialization proceeds stepwise,
7733 with each step selecting the variable earliest in <i>declaration order</i>
7734 which has no dependencies on uninitialized variables.
7738 More precisely, a package-level variable is considered <i>ready for
7739 initialization</i> if it is not yet initialized and either has
7740 no <a href="#Variable_declarations">initialization expression</a> or
7741 its initialization expression has no <i>dependencies</i> on uninitialized variables.
7742 Initialization proceeds by repeatedly initializing the next package-level
7743 variable that is earliest in declaration order and ready for initialization,
7744 until there are no variables ready for initialization.
7748 If any variables are still uninitialized when this
7749 process ends, those variables are part of one or more initialization cycles,
7750 and the program is not valid.
7754 Multiple variables on the left-hand side of a variable declaration initialized
7755 by single (multi-valued) expression on the right-hand side are initialized
7756 together: If any of the variables on the left-hand side is initialized, all
7757 those variables are initialized in the same step.
7762 var a, b = f() // a and b are initialized together, before x is initialized
7766 For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
7767 variables are treated like any other variables in declarations.
7771 The declaration order of variables declared in multiple files is determined
7772 by the order in which the files are presented to the compiler: Variables
7773 declared in the first file are declared before any of the variables declared
7774 in the second file, and so on.
7778 Dependency analysis does not rely on the actual values of the
7779 variables, only on lexical <i>references</i> to them in the source,
7780 analyzed transitively. For instance, if a variable <code>x</code>'s
7781 initialization expression refers to a function whose body refers to
7782 variable <code>y</code> then <code>x</code> depends on <code>y</code>.
7788 A reference to a variable or function is an identifier denoting that
7789 variable or function.
7793 A reference to a method <code>m</code> is a
7794 <a href="#Method_values">method value</a> or
7795 <a href="#Method_expressions">method expression</a> of the form
7796 <code>t.m</code>, where the (static) type of <code>t</code> is
7797 not an interface type, and the method <code>m</code> is in the
7798 <a href="#Method_sets">method set</a> of <code>t</code>.
7799 It is immaterial whether the resulting function value
7800 <code>t.m</code> is invoked.
7804 A variable, function, or method <code>x</code> depends on a variable
7805 <code>y</code> if <code>x</code>'s initialization expression or body
7806 (for functions and methods) contains a reference to <code>y</code>
7807 or to a function or method that depends on <code>y</code>.
7812 For example, given the declarations
7820 d = 3 // == 5 after initialization has finished
7830 the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
7831 Note that the order of subexpressions in initialization expressions is irrelevant:
7832 <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
7833 order in this example.
7837 Dependency analysis is performed per package; only references referring
7838 to variables, functions, and (non-interface) methods declared in the current
7839 package are considered. If other, hidden, data dependencies exists between
7840 variables, the initialization order between those variables is unspecified.
7844 For instance, given the declarations
7848 var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
7849 var _ = sideEffect() // unrelated to x, a, or b
7853 type I interface { ab() []int }
7855 func (T) ab() []int { return []int{a, b} }
7859 the variable <code>a</code> will be initialized after <code>b</code> but
7860 whether <code>x</code> is initialized before <code>b</code>, between
7861 <code>b</code> and <code>a</code>, or after <code>a</code>, and
7862 thus also the moment at which <code>sideEffect()</code> is called (before
7863 or after <code>x</code> is initialized) is not specified.
7867 Variables may also be initialized using functions named <code>init</code>
7868 declared in the package block, with no arguments and no result parameters.
7876 Multiple such functions may be defined per package, even within a single
7877 source file. In the package block, the <code>init</code> identifier can
7878 be used only to declare <code>init</code> functions, yet the identifier
7879 itself is not <a href="#Declarations_and_scope">declared</a>. Thus
7880 <code>init</code> functions cannot be referred to from anywhere
7885 A package with no imports is initialized by assigning initial values
7886 to all its package-level variables followed by calling all <code>init</code>
7887 functions in the order they appear in the source, possibly in multiple files,
7888 as presented to the compiler.
7889 If a package has imports, the imported packages are initialized
7890 before initializing the package itself. If multiple packages import
7891 a package, the imported package will be initialized only once.
7892 The importing of packages, by construction, guarantees that there
7893 can be no cyclic initialization dependencies.
7897 Package initialization—variable initialization and the invocation of
7898 <code>init</code> functions—happens in a single goroutine,
7899 sequentially, one package at a time.
7900 An <code>init</code> function may launch other goroutines, which can run
7901 concurrently with the initialization code. However, initialization
7903 the <code>init</code> functions: it will not invoke the next one
7904 until the previous one has returned.
7908 To ensure reproducible initialization behavior, build systems are encouraged
7909 to present multiple files belonging to the same package in lexical file name
7910 order to a compiler.
7914 <h3 id="Program_execution">Program execution</h3>
7916 A complete program is created by linking a single, unimported package
7917 called the <i>main package</i> with all the packages it imports, transitively.
7918 The main package must
7919 have package name <code>main</code> and
7920 declare a function <code>main</code> that takes no
7921 arguments and returns no value.
7929 Program execution begins by initializing the main package and then
7930 invoking the function <code>main</code>.
7931 When that function invocation returns, the program exits.
7932 It does not wait for other (non-<code>main</code>) goroutines to complete.
7935 <h2 id="Errors">Errors</h2>
7938 The predeclared type <code>error</code> is defined as
7942 type error interface {
7948 It is the conventional interface for representing an error condition,
7949 with the nil value representing no error.
7950 For instance, a function to read data from a file might be defined:
7954 func Read(f *File, b []byte) (n int, err error)
7957 <h2 id="Run_time_panics">Run-time panics</h2>
7960 Execution errors such as attempting to index an array out
7961 of bounds trigger a <i>run-time panic</i> equivalent to a call of
7962 the built-in function <a href="#Handling_panics"><code>panic</code></a>
7963 with a value of the implementation-defined interface type <code>runtime.Error</code>.
7964 That type satisfies the predeclared interface type
7965 <a href="#Errors"><code>error</code></a>.
7966 The exact error values that
7967 represent distinct run-time error conditions are unspecified.
7973 type Error interface {
7975 // and perhaps other methods
7979 <h2 id="System_considerations">System considerations</h2>
7981 <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
7984 The built-in package <code>unsafe</code>, known to the compiler
7985 and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
7986 provides facilities for low-level programming including operations
7987 that violate the type system. A package using <code>unsafe</code>
7988 must be vetted manually for type safety and may not be portable.
7989 The package provides the following interface:
7992 <pre class="grammar">
7995 type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
7996 type Pointer *ArbitraryType
7998 func Alignof(variable ArbitraryType) uintptr
7999 func Offsetof(selector ArbitraryType) uintptr
8000 func Sizeof(variable ArbitraryType) uintptr
8002 type IntegerType int // shorthand for an integer type; it is not a real type
8003 func Add(ptr Pointer, len IntegerType) Pointer
8004 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
8008 These conversions also apply to type parameters with suitable core types.
8009 Determine if we can simply use core type insted of underlying type here,
8010 of if the general conversion rules take care of this.
8014 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
8015 value may not be <a href="#Address_operators">dereferenced</a>.
8016 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be
8017 <a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
8018 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
8023 bits = *(*uint64)(unsafe.Pointer(&f))
8025 type ptr unsafe.Pointer
8026 bits = *(*uint64)(ptr(&f))
8032 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
8033 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
8034 as if <code>v</code> was declared via <code>var v = x</code>.
8037 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
8038 <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
8039 or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
8040 If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
8041 without pointer indirections through fields of the struct.
8042 For a struct <code>s</code> with field <code>f</code>:
8046 uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
8050 Computer architectures may require memory addresses to be <i>aligned</i>;
8051 that is, for addresses of a variable to be a multiple of a factor,
8052 the variable's type's <i>alignment</i>. The function <code>Alignof</code>
8053 takes an expression denoting a variable of any type and returns the
8054 alignment of the (type of the) variable in bytes. For a variable
8059 uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
8063 A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
8064 is a <a href="#Type_parameter_declarations">type parameter</a>, or if it is an
8065 array or struct type containing elements
8066 or fields of variable size. Otherwise the size is <i>constant</i>.
8067 Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
8068 are compile-time <a href="#Constant_expressions">constant expressions</a> of
8069 type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
8070 the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
8075 The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
8076 and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
8077 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8078 A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
8079 if it is an untyped constant it is given type <code>int</code>.
8080 The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
8084 The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
8085 and whose length and capacity are <code>len</code>.
8086 <code>Slice(ptr, len)</code> is equivalent to
8090 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
8094 except that, as a special case, if <code>ptr</code>
8095 is <code>nil</code> and <code>len</code> is zero,
8096 <code>Slice</code> returns <code>nil</code>.
8100 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
8101 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
8102 if it is an untyped constant it is given type <code>int</code>.
8103 At run time, if <code>len</code> is negative,
8104 or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
8105 a <a href="#Run_time_panics">run-time panic</a> occurs.
8108 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
8111 For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
8114 <pre class="grammar">
8119 uint32, int32, float32 4
8120 uint64, int64, float64, complex64 8
8125 The following minimal alignment properties are guaranteed:
8128 <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
8131 <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
8132 all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
8135 <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
8136 the alignment of a variable of the array's element type.
8141 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.