2 "Title": "The Go Programming Language Specification - Go 1.18 Draft (incomplete)",
3 "Subtitle": "Version of Jan 6, 2022",
7 <h2>Earlier version</h2>
10 For the pre-Go1.18 specification without generics support see
11 <a href="/doc/go1.17_spec.html">The Go Programming Language Specification</a>.
14 <h2 id="Introduction">Introduction</h2>
17 This is a reference manual for the Go programming language. For
18 more information and other documents, see <a href="/">golang.org</a>.
22 Go is a general-purpose language designed with systems programming
23 in mind. It is strongly typed and garbage-collected and has explicit
24 support for concurrent programming. Programs are constructed from
25 <i>packages</i>, whose properties allow efficient management of
30 The grammar is compact and simple to parse, allowing for easy analysis
31 by automatic tools such as integrated development environments.
34 <h2 id="Notation">Notation</h2>
36 The syntax is specified using Extended Backus-Naur Form (EBNF):
40 Production = production_name "=" [ Expression ] "." .
41 Expression = Alternative { "|" Alternative } .
42 Alternative = Term { Term } .
43 Term = production_name | token [ "…" token ] | Group | Option | Repetition .
44 Group = "(" Expression ")" .
45 Option = "[" Expression "]" .
46 Repetition = "{" Expression "}" .
50 Productions are expressions constructed from terms and the following
51 operators, in increasing precedence:
56 [] option (0 or 1 times)
57 {} repetition (0 to n times)
61 Lower-case production names are used to identify lexical tokens.
62 Non-terminals are in CamelCase. Lexical tokens are enclosed in
63 double quotes <code>""</code> or back quotes <code>``</code>.
67 The form <code>a … b</code> represents the set of characters from
68 <code>a</code> through <code>b</code> as alternatives. The horizontal
69 ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various
70 enumerations or code snippets that are not further specified. The character <code>…</code>
71 (as opposed to the three characters <code>...</code>) is not a token of the Go
75 <h2 id="Source_code_representation">Source code representation</h2>
78 Source code is Unicode text encoded in
79 <a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not
80 canonicalized, so a single accented code point is distinct from the
81 same character constructed from combining an accent and a letter;
82 those are treated as two code points. For simplicity, this document
83 will use the unqualified term <i>character</i> to refer to a Unicode code point
87 Each code point is distinct; for instance, upper and lower case letters
88 are different characters.
91 Implementation restriction: For compatibility with other tools, a
92 compiler may disallow the NUL character (U+0000) in the source text.
95 Implementation restriction: For compatibility with other tools, a
96 compiler may ignore a UTF-8-encoded byte order mark
97 (U+FEFF) if it is the first Unicode code point in the source text.
98 A byte order mark may be disallowed anywhere else in the source.
101 <h3 id="Characters">Characters</h3>
104 The following terms are used to denote specific Unicode character classes:
107 newline = /* the Unicode code point U+000A */ .
108 unicode_char = /* an arbitrary Unicode code point except newline */ .
109 unicode_letter = /* a Unicode code point classified as "Letter" */ .
110 unicode_digit = /* a Unicode code point classified as "Number, decimal digit" */ .
114 In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>,
115 Section 4.5 "General Category" defines a set of character categories.
116 Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
117 as Unicode letters, and those in the Number category Nd as Unicode digits.
120 <h3 id="Letters_and_digits">Letters and digits</h3>
123 The underscore character <code>_</code> (U+005F) is considered a letter.
126 letter = unicode_letter | "_" .
127 decimal_digit = "0" … "9" .
128 binary_digit = "0" | "1" .
129 octal_digit = "0" … "7" .
130 hex_digit = "0" … "9" | "A" … "F" | "a" … "f" .
133 <h2 id="Lexical_elements">Lexical elements</h2>
135 <h3 id="Comments">Comments</h3>
138 Comments serve as program documentation. There are two forms:
143 <i>Line comments</i> start with the character sequence <code>//</code>
144 and stop at the end of the line.
147 <i>General comments</i> start with the character sequence <code>/*</code>
148 and stop with the first subsequent character sequence <code>*/</code>.
153 A comment cannot start inside a <a href="#Rune_literals">rune</a> or
154 <a href="#String_literals">string literal</a>, or inside a comment.
155 A general comment containing no newlines acts like a space.
156 Any other comment acts like a newline.
159 <h3 id="Tokens">Tokens</h3>
162 Tokens form the vocabulary of the Go language.
163 There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators
164 and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from
165 spaces (U+0020), horizontal tabs (U+0009),
166 carriage returns (U+000D), and newlines (U+000A),
167 is ignored except as it separates tokens
168 that would otherwise combine into a single token. Also, a newline or end of file
169 may trigger the insertion of a <a href="#Semicolons">semicolon</a>.
170 While breaking the input into tokens,
171 the next token is the longest sequence of characters that form a
175 <h3 id="Semicolons">Semicolons</h3>
178 The formal grammar uses semicolons <code>";"</code> as terminators in
179 a number of productions. Go programs may omit most of these semicolons
180 using the following two rules:
185 When the input is broken into tokens, a semicolon is automatically inserted
186 into the token stream immediately after a line's final token if that token is
189 <a href="#Identifiers">identifier</a>
193 <a href="#Integer_literals">integer</a>,
194 <a href="#Floating-point_literals">floating-point</a>,
195 <a href="#Imaginary_literals">imaginary</a>,
196 <a href="#Rune_literals">rune</a>, or
197 <a href="#String_literals">string</a> literal
200 <li>one of the <a href="#Keywords">keywords</a>
202 <code>continue</code>,
203 <code>fallthrough</code>, or
207 <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a>
218 To allow complex statements to occupy a single line, a semicolon
219 may be omitted before a closing <code>")"</code> or <code>"}"</code>.
224 To reflect idiomatic use, code examples in this document elide semicolons
229 <h3 id="Identifiers">Identifiers</h3>
232 Identifiers name program entities such as variables and types.
233 An identifier is a sequence of one or more letters and digits.
234 The first character in an identifier must be a letter.
237 identifier = letter { letter | unicode_digit } .
242 ThisVariableIsExported
247 Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>.
251 <h3 id="Keywords">Keywords</h3>
254 The following keywords are reserved and may not be used as identifiers.
256 <pre class="grammar">
257 break default func interface select
258 case defer go map struct
259 chan else goto package switch
260 const fallthrough if range type
261 continue for import return var
264 <h3 id="Operators_and_punctuation">Operators and punctuation</h3>
267 The following character sequences represent <a href="#Operators">operators</a>
268 (including <a href="#Assignments">assignment operators</a>) and punctuation:
270 <pre class="grammar">
271 + & += &= && == != ( )
272 - | -= |= || < <= [ ]
273 * ^ *= ^= <- > >= { }
274 / << /= <<= ++ = := , ;
275 % >> %= >>= -- ! ... . :
279 <h3 id="Integer_literals">Integer literals</h3>
282 An integer literal is a sequence of digits representing an
283 <a href="#Constants">integer constant</a>.
284 An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code>
285 for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal,
286 and <code>0x</code> or <code>0X</code> for hexadecimal.
287 A single <code>0</code> is considered a decimal zero.
288 In hexadecimal literals, letters <code>a</code> through <code>f</code>
289 and <code>A</code> through <code>F</code> represent values 10 through 15.
293 For readability, an underscore character <code>_</code> may appear after
294 a base prefix or between successive digits; such underscores do not change
298 int_lit = decimal_lit | binary_lit | octal_lit | hex_lit .
299 decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] .
300 binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits .
301 octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits .
302 hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits .
304 decimal_digits = decimal_digit { [ "_" ] decimal_digit } .
305 binary_digits = binary_digit { [ "_" ] binary_digit } .
306 octal_digits = octal_digit { [ "_" ] octal_digit } .
307 hex_digits = hex_digit { [ "_" ] hex_digit } .
316 0O600 // second character is capital letter 'O'
320 170141183460469231731687303715884105727
321 170_141183_460469_231731_687303_715884_105727
323 _42 // an identifier, not an integer literal
324 42_ // invalid: _ must separate successive digits
325 4__2 // invalid: only one _ at a time
326 0_xBadFace // invalid: _ must separate successive digits
330 <h3 id="Floating-point_literals">Floating-point literals</h3>
333 A floating-point literal is a decimal or hexadecimal representation of a
334 <a href="#Constants">floating-point constant</a>.
338 A decimal floating-point literal consists of an integer part (decimal digits),
339 a decimal point, a fractional part (decimal digits), and an exponent part
340 (<code>e</code> or <code>E</code> followed by an optional sign and decimal digits).
341 One of the integer part or the fractional part may be elided; one of the decimal point
342 or the exponent part may be elided.
343 An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>.
347 A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code>
348 prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits),
349 and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits).
350 One of the integer part or the fractional part may be elided; the radix point may be elided as well,
351 but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.)
352 An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>.
356 For readability, an underscore character <code>_</code> may appear after
357 a base prefix or between successive digits; such underscores do not change
362 float_lit = decimal_float_lit | hex_float_lit .
364 decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] |
365 decimal_digits decimal_exponent |
366 "." decimal_digits [ decimal_exponent ] .
367 decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits .
369 hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent .
370 hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] |
373 hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits .
391 0x1.Fp+0 // == 1.9375
393 0X_1FFFP-16 // == 0.1249847412109375
394 0x15e-2 // == 0x15e - 2 (integer subtraction)
396 0x.p1 // invalid: mantissa has no digits
397 1p-2 // invalid: p exponent requires hexadecimal mantissa
398 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent
399 1_.5 // invalid: _ must separate successive digits
400 1._5 // invalid: _ must separate successive digits
401 1.5_e1 // invalid: _ must separate successive digits
402 1.5e_1 // invalid: _ must separate successive digits
403 1.5e1_ // invalid: _ must separate successive digits
407 <h3 id="Imaginary_literals">Imaginary literals</h3>
410 An imaginary literal represents the imaginary part of a
411 <a href="#Constants">complex constant</a>.
412 It consists of an <a href="#Integer_literals">integer</a> or
413 <a href="#Floating-point_literals">floating-point</a> literal
414 followed by the lower-case letter <code>i</code>.
415 The value of an imaginary literal is the value of the respective
416 integer or floating-point literal multiplied by the imaginary unit <i>i</i>.
420 imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
424 For backward compatibility, an imaginary literal's integer part consisting
425 entirely of decimal digits (and possibly underscores) is considered a decimal
426 integer, even if it starts with a leading <code>0</code>.
431 0123i // == 123i for backward-compatibility
432 0o123i // == 0o123 * 1i == 83i
433 0xabci // == 0xabc * 1i == 2748i
441 0x1p-2i // == 0x1p-2 * 1i == 0.25i
445 <h3 id="Rune_literals">Rune literals</h3>
448 A rune literal represents a <a href="#Constants">rune constant</a>,
449 an integer value identifying a Unicode code point.
450 A rune literal is expressed as one or more characters enclosed in single quotes,
451 as in <code>'x'</code> or <code>'\n'</code>.
452 Within the quotes, any character may appear except newline and unescaped single
453 quote. A single quoted character represents the Unicode value
454 of the character itself,
455 while multi-character sequences beginning with a backslash encode
456 values in various formats.
460 The simplest form represents the single character within the quotes;
461 since Go source text is Unicode characters encoded in UTF-8, multiple
462 UTF-8-encoded bytes may represent a single integer value. For
463 instance, the literal <code>'a'</code> holds a single byte representing
464 a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
465 <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
466 a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
470 Several backslash escapes allow arbitrary values to be encoded as
471 ASCII text. There are four ways to represent the integer value
472 as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
473 digits; <code>\u</code> followed by exactly four hexadecimal digits;
474 <code>\U</code> followed by exactly eight hexadecimal digits, and a
475 plain backslash <code>\</code> followed by exactly three octal digits.
476 In each case the value of the literal is the value represented by
477 the digits in the corresponding base.
481 Although these representations all result in an integer, they have
482 different valid ranges. Octal escapes must represent a value between
483 0 and 255 inclusive. Hexadecimal escapes satisfy this condition
484 by construction. The escapes <code>\u</code> and <code>\U</code>
485 represent Unicode code points so within them some values are illegal,
486 in particular those above <code>0x10FFFF</code> and surrogate halves.
490 After a backslash, certain single-character escapes represent special values:
493 <pre class="grammar">
494 \a U+0007 alert or bell
497 \n U+000A line feed or newline
498 \r U+000D carriage return
499 \t U+0009 horizontal tab
500 \v U+000B vertical tab
502 \' U+0027 single quote (valid escape only within rune literals)
503 \" U+0022 double quote (valid escape only within string literals)
507 All other sequences starting with a backslash are illegal inside rune literals.
510 rune_lit = "'" ( unicode_value | byte_value ) "'" .
511 unicode_value = unicode_char | little_u_value | big_u_value | escaped_char .
512 byte_value = octal_byte_value | hex_byte_value .
513 octal_byte_value = `\` octal_digit octal_digit octal_digit .
514 hex_byte_value = `\` "x" hex_digit hex_digit .
515 little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit .
516 big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit
517 hex_digit hex_digit hex_digit hex_digit .
518 escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .
533 '\'' // rune literal containing single quote character
534 'aa' // illegal: too many characters
535 '\xa' // illegal: too few hexadecimal digits
536 '\0' // illegal: too few octal digits
537 '\uDFFF' // illegal: surrogate half
538 '\U00110000' // illegal: invalid Unicode code point
542 <h3 id="String_literals">String literals</h3>
545 A string literal represents a <a href="#Constants">string constant</a>
546 obtained from concatenating a sequence of characters. There are two forms:
547 raw string literals and interpreted string literals.
551 Raw string literals are character sequences between back quotes, as in
552 <code>`foo`</code>. Within the quotes, any character may appear except
553 back quote. The value of a raw string literal is the
554 string composed of the uninterpreted (implicitly UTF-8-encoded) characters
556 in particular, backslashes have no special meaning and the string may
558 Carriage return characters ('\r') inside raw string literals
559 are discarded from the raw string value.
563 Interpreted string literals are character sequences between double
564 quotes, as in <code>"bar"</code>.
565 Within the quotes, any character may appear except newline and unescaped double quote.
566 The text between the quotes forms the
567 value of the literal, with backslash escapes interpreted as they
568 are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and
569 <code>\"</code> is legal), with the same restrictions.
570 The three-digit octal (<code>\</code><i>nnn</i>)
571 and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual
572 <i>bytes</i> of the resulting string; all other escapes represent
573 the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.
574 Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent
575 a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>,
576 <code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent
577 the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character
582 string_lit = raw_string_lit | interpreted_string_lit .
583 raw_string_lit = "`" { unicode_char | newline } "`" .
584 interpreted_string_lit = `"` { unicode_value | byte_value } `"` .
588 `abc` // same as "abc"
590 \n` // same as "\\n\n\\n"
597 "\uD800" // illegal: surrogate half
598 "\U00110000" // illegal: invalid Unicode code point
602 These examples all represent the same string:
606 "日本語" // UTF-8 input text
607 `日本語` // UTF-8 input text as a raw literal
608 "\u65e5\u672c\u8a9e" // the explicit Unicode code points
609 "\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points
610 "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes
614 If the source code represents a character as two code points, such as
615 a combining form involving an accent and a letter, the result will be
616 an error if placed in a rune literal (it is not a single code
617 point), and will appear as two code points if placed in a string
622 <h2 id="Constants">Constants</h2>
624 <p>There are <i>boolean constants</i>,
625 <i>rune constants</i>,
626 <i>integer constants</i>,
627 <i>floating-point constants</i>, <i>complex constants</i>,
628 and <i>string constants</i>. Rune, integer, floating-point,
629 and complex constants are
630 collectively called <i>numeric constants</i>.
634 A constant value is represented by a
635 <a href="#Rune_literals">rune</a>,
636 <a href="#Integer_literals">integer</a>,
637 <a href="#Floating-point_literals">floating-point</a>,
638 <a href="#Imaginary_literals">imaginary</a>,
640 <a href="#String_literals">string</a> literal,
641 an identifier denoting a constant,
642 a <a href="#Constant_expressions">constant expression</a>,
643 a <a href="#Conversions">conversion</a> with a result that is a constant, or
644 the result value of some built-in functions such as
645 <code>unsafe.Sizeof</code> applied to <a href="#Package_unsafe">certain values</a>,
646 <code>cap</code> or <code>len</code> applied to
647 <a href="#Length_and_capacity">some expressions</a>,
648 <code>real</code> and <code>imag</code> applied to a complex constant
649 and <code>complex</code> applied to numeric constants.
650 The boolean truth values are represented by the predeclared constants
651 <code>true</code> and <code>false</code>. The predeclared identifier
652 <a href="#Iota">iota</a> denotes an integer constant.
656 In general, complex constants are a form of
657 <a href="#Constant_expressions">constant expression</a>
658 and are discussed in that section.
662 Numeric constants represent exact values of arbitrary precision and do not overflow.
663 Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
664 and not-a-number values.
668 Constants may be <a href="#Types">typed</a> or <i>untyped</i>.
669 Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>,
670 and certain <a href="#Constant_expressions">constant expressions</a>
671 containing only untyped constant operands are untyped.
675 A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a>
676 or <a href="#Conversions">conversion</a>, or implicitly when used in a
677 <a href="#Variable_declarations">variable declaration</a> or an
678 <a href="#Assignments">assignment</a> or as an
679 operand in an <a href="#Expressions">expression</a>.
680 It is an error if the constant value
681 cannot be <a href="#Representability">represented</a> as a value of the respective type.
685 An untyped constant has a <i>default type</i> which is the type to which the
686 constant is implicitly converted in contexts where a typed value is required,
687 for instance, in a <a href="#Short_variable_declarations">short variable declaration</a>
688 such as <code>i := 0</code> where there is no explicit type.
689 The default type of an untyped constant is <code>bool</code>, <code>rune</code>,
690 <code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code>
691 respectively, depending on whether it is a boolean, rune, integer, floating-point,
692 complex, or string constant.
696 Implementation restriction: Although numeric constants have arbitrary
697 precision in the language, a compiler may implement them using an
698 internal representation with limited precision. That said, every
703 <li>Represent integer constants with at least 256 bits.</li>
705 <li>Represent floating-point constants, including the parts of
706 a complex constant, with a mantissa of at least 256 bits
707 and a signed binary exponent of at least 16 bits.</li>
709 <li>Give an error if unable to represent an integer constant
712 <li>Give an error if unable to represent a floating-point or
713 complex constant due to overflow.</li>
715 <li>Round to the nearest representable constant if unable to
716 represent a floating-point or complex constant due to limits
721 These requirements apply both to literal constants and to the result
722 of evaluating <a href="#Constant_expressions">constant
727 <h2 id="Variables">Variables</h2>
730 A variable is a storage location for holding a <i>value</i>.
731 The set of permissible values is determined by the
732 variable's <i><a href="#Types">type</a></i>.
736 A <a href="#Variable_declarations">variable declaration</a>
737 or, for function parameters and results, the signature
738 of a <a href="#Function_declarations">function declaration</a>
739 or <a href="#Function_literals">function literal</a> reserves
740 storage for a named variable.
742 Calling the built-in function <a href="#Allocation"><code>new</code></a>
743 or taking the address of a <a href="#Composite_literals">composite literal</a>
744 allocates storage for a variable at run time.
745 Such an anonymous variable is referred to via a (possibly implicit)
746 <a href="#Address_operators">pointer indirection</a>.
750 <i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>,
751 and <a href="#Struct_types">struct</a> types have elements and fields that may
752 be <a href="#Address_operators">addressed</a> individually. Each such element
753 acts like a variable.
757 The <i>static type</i> (or just <i>type</i>) of a variable is the
758 type given in its declaration, the type provided in the
759 <code>new</code> call or composite literal, or the type of
760 an element of a structured variable.
761 Variables of interface type also have a distinct <i>dynamic type</i>,
762 which is the concrete type of the value assigned to the variable at run time
763 (unless the value is the predeclared identifier <code>nil</code>,
765 The dynamic type may vary during execution but values stored in interface
766 variables are always <a href="#Assignability">assignable</a>
767 to the static type of the variable.
771 var x interface{} // x is nil and has static type interface{}
772 var v *T // v has value nil, static type *T
773 x = 42 // x has value 42 and dynamic type int
774 x = v // x has value (*T)(nil) and dynamic type *T
778 A variable's value is retrieved by referring to the variable in an
779 <a href="#Expressions">expression</a>; it is the most recent value
780 <a href="#Assignments">assigned</a> to the variable.
781 If a variable has not yet been assigned a value, its value is the
782 <a href="#The_zero_value">zero value</a> for its type.
786 <h2 id="Types">Types</h2>
789 A type determines a set of values together with operations and methods specific
790 to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be
791 followed by <a href="#Instantiations">type arguments</a> if the type is parameterized.
792 A type may also be specified using a <i>type literal</i>, which composes a type
797 Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
798 TypeName = identifier | QualifiedIdent .
799 TypeArgs = "[" TypeList [ "," ] "]" .
800 TypeList = Type { "," Type } .
801 TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
802 SliceType | MapType | ChannelType .
806 The language <a href="#Predeclared_identifiers">predeclares</a> certain type names.
807 Others are introduced with <a href="#Type_declarations">type declarations</a>
808 or <a href="#Type_parameter_lists">type parameter lists</a>.
809 <i>Composite types</i>—array, struct, pointer, function,
810 interface, slice, map, and channel types—may be constructed using
815 Predeclared types, defined types, and type parameters are called <i>named types</i>.
816 An alias denotes a named type if the type given in the alias declaration is a named type.
819 <h3 id="Underlying_types">Underlying types</h3>
822 Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code>
823 is one of the predeclared boolean, numeric, or string types, or a type literal,
824 the corresponding underlying type is <code>T</code> itself.
825 Otherwise, <code>T</code>'s underlying type is the underlying type of the
826 type to which <code>T</code> refers in its <a href="#Type_declarations">type
827 declaration</a>. Accordingly, the underlying type of a type parameter is the
828 underlying type of its <a href="#Type_constraints">type constraint</a>, which
829 is always an interface.
845 func f[P any](x P) { … }
849 The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>,
850 and <code>B2</code> is <code>string</code>.
851 The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>.
852 The underlying type of <code>P</code> is <code>interface{}</code>.
855 <h3 id="Method_sets">Method sets</h3>
858 The <i>method set</i> of a type determines the methods that can be
859 <a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type.
860 Every type has a (possibly empty) method set associated with it:
864 <li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all
865 <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>.
869 The method set of a pointer to a defined type <code>T</code>
870 (where <code>T</code> is neither a pointer nor an interface)
871 is the set of all methods declared with receiver <code>*T</code> or <code>T</code>.
874 <li>The method set of an <a href="#Interface_types">interface type</a> is the intersection
875 of the method sets of each type in the interface's <a href="#Interface_types">type set</a>
876 (the resulting method set is usually just the set of declared methods in the interface).
881 Further rules apply to structs (and pointer to structs) containing embedded fields,
882 as described in the section on <a href="#Struct_types">struct types</a>.
883 Any other type has an empty method set.
887 In a method set, each method must have a
888 <a href="#Uniqueness_of_identifiers">unique</a>
889 non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>.
892 <h3 id="Boolean_types">Boolean types</h3>
895 A <i>boolean type</i> represents the set of Boolean truth values
896 denoted by the predeclared constants <code>true</code>
897 and <code>false</code>. The predeclared boolean type is <code>bool</code>;
898 it is a <a href="#Type_definitions">defined type</a>.
901 <h3 id="Numeric_types">Numeric types</h3>
904 A <i>numeric type</i> represents sets of integer or floating-point values.
905 The predeclared architecture-independent numeric types are:
908 <pre class="grammar">
909 uint8 the set of all unsigned 8-bit integers (0 to 255)
910 uint16 the set of all unsigned 16-bit integers (0 to 65535)
911 uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
912 uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
914 int8 the set of all signed 8-bit integers (-128 to 127)
915 int16 the set of all signed 16-bit integers (-32768 to 32767)
916 int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
917 int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
919 float32 the set of all IEEE-754 32-bit floating-point numbers
920 float64 the set of all IEEE-754 64-bit floating-point numbers
922 complex64 the set of all complex numbers with float32 real and imaginary parts
923 complex128 the set of all complex numbers with float64 real and imaginary parts
930 The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
931 <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
935 There is also a set of predeclared numeric types with implementation-specific sizes:
938 <pre class="grammar">
939 uint either 32 or 64 bits
940 int same size as uint
941 uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
945 To avoid portability issues all numeric types are <a href="#Type_definitions">defined
946 types</a> and thus distinct except
947 <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
948 <code>rune</code>, which is an alias for <code>int32</code>.
950 are required when different numeric types are mixed in an expression
951 or assignment. For instance, <code>int32</code> and <code>int</code>
952 are not the same type even though they may have the same size on a
953 particular architecture.
956 <h3 id="String_types">String types</h3>
959 A <i>string type</i> represents the set of string values.
960 A string value is a (possibly empty) sequence of bytes.
961 The number of bytes is called the length of the string and is never negative.
962 Strings are immutable: once created,
963 it is impossible to change the contents of a string.
964 The predeclared string type is <code>string</code>;
965 it is a <a href="#Type_definitions">defined type</a>.
969 The length of a string <code>s</code> can be discovered using
970 the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
971 The length is a compile-time constant if the string is a constant.
972 A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
973 0 through <code>len(s)-1</code>.
974 It is illegal to take the address of such an element; if
975 <code>s[i]</code> is the <code>i</code>'th byte of a
976 string, <code>&s[i]</code> is invalid.
980 <h3 id="Array_types">Array types</h3>
983 An array is a numbered sequence of elements of a single
984 type, called the element type.
985 The number of elements is called the length of the array and is never negative.
989 ArrayType = "[" ArrayLength "]" ElementType .
990 ArrayLength = Expression .
995 The length is part of the array's type; it must evaluate to a
996 non-negative <a href="#Constants">constant</a>
997 <a href="#Representability">representable</a> by a value
998 of type <code>int</code>.
999 The length of array <code>a</code> can be discovered
1000 using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
1001 The elements can be addressed by integer <a href="#Index_expressions">indices</a>
1002 0 through <code>len(a)-1</code>.
1003 Array types are always one-dimensional but may be composed to form
1004 multi-dimensional types.
1009 [2*N] struct { x, y int32 }
1012 [2][2][2]float64 // same as [2]([2]([2]float64))
1015 <h3 id="Slice_types">Slice types</h3>
1018 A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
1019 provides access to a numbered sequence of elements from that array.
1020 A slice type denotes the set of all slices of arrays of its element type.
1021 The number of elements is called the length of the slice and is never negative.
1022 The value of an uninitialized slice is <code>nil</code>.
1026 SliceType = "[" "]" ElementType .
1030 The length of a slice <code>s</code> can be discovered by the built-in function
1031 <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
1032 execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
1033 0 through <code>len(s)-1</code>. The slice index of a
1034 given element may be less than the index of the same element in the
1038 A slice, once initialized, is always associated with an underlying
1039 array that holds its elements. A slice therefore shares storage
1040 with its array and with other slices of the same array; by contrast,
1041 distinct arrays always represent distinct storage.
1044 The array underlying a slice may extend past the end of the slice.
1045 The <i>capacity</i> is a measure of that extent: it is the sum of
1046 the length of the slice and the length of the array beyond the slice;
1047 a slice of length up to that capacity can be created by
1048 <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
1049 The capacity of a slice <code>a</code> can be discovered using the
1050 built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
1054 A new, initialized slice value for a given element type <code>T</code> is
1055 made using the built-in function
1056 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1057 which takes a slice type
1058 and parameters specifying the length and optionally the capacity.
1059 A slice created with <code>make</code> always allocates a new, hidden array
1060 to which the returned slice value refers. That is, executing
1064 make([]T, length, capacity)
1068 produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
1069 it, so these two expressions are equivalent:
1073 make([]int, 50, 100)
1078 Like arrays, slices are always one-dimensional but may be composed to construct
1079 higher-dimensional objects.
1080 With arrays of arrays, the inner arrays are, by construction, always the same length;
1081 however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
1082 Moreover, the inner slices must be initialized individually.
1085 <h3 id="Struct_types">Struct types</h3>
1088 A struct is a sequence of named elements, called fields, each of which has a
1089 name and a type. Field names may be specified explicitly (IdentifierList) or
1090 implicitly (EmbeddedField).
1091 Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
1092 be <a href="#Uniqueness_of_identifiers">unique</a>.
1096 StructType = "struct" "{" { FieldDecl ";" } "}" .
1097 FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
1098 EmbeddedField = [ "*" ] TypeName .
1106 // A struct with 6 fields.
1110 _ float32 // padding
1117 A field declared with a type but no explicit field name is called an <i>embedded field</i>.
1118 An embedded field must be specified as
1119 a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
1120 and <code>T</code> itself may not be
1121 a pointer type. The unqualified type name acts as the field name.
1125 // A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
1127 T1 // field name is T1
1128 *T2 // field name is T2
1129 P.T3 // field name is T3
1130 *P.T4 // field name is T4
1131 x, y int // field names are x and y
1136 The following declaration is illegal because field names must be unique
1142 T // conflicts with embedded field *T and *P.T
1143 *T // conflicts with embedded field T and *P.T
1144 *P.T // conflicts with embedded field T and *T
1149 A field or <a href="#Method_declarations">method</a> <code>f</code> of an
1150 embedded field in a struct <code>x</code> is called <i>promoted</i> if
1151 <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
1152 that field or method <code>f</code>.
1156 Promoted fields act like ordinary fields
1157 of a struct except that they cannot be used as field names in
1158 <a href="#Composite_literals">composite literals</a> of the struct.
1162 Given a struct type <code>S</code> and a <a href="#Type_definitions">defined type</a>
1163 <code>T</code>, promoted methods are included in the method set of the struct as follows:
1167 If <code>S</code> contains an embedded field <code>T</code>,
1168 the <a href="#Method_sets">method sets</a> of <code>S</code>
1169 and <code>*S</code> both include promoted methods with receiver
1170 <code>T</code>. The method set of <code>*S</code> also
1171 includes promoted methods with receiver <code>*T</code>.
1175 If <code>S</code> contains an embedded field <code>*T</code>,
1176 the method sets of <code>S</code> and <code>*S</code> both
1177 include promoted methods with receiver <code>T</code> or
1183 A field declaration may be followed by an optional string literal <i>tag</i>,
1184 which becomes an attribute for all the fields in the corresponding
1185 field declaration. An empty tag string is equivalent to an absent tag.
1186 The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
1187 and take part in <a href="#Type_identity">type identity</a> for structs
1188 but are otherwise ignored.
1193 x, y float64 "" // an empty tag string is like an absent tag
1194 name string "any string is permitted as a tag"
1195 _ [4]byte "ceci n'est pas un champ de structure"
1198 // A struct corresponding to a TimeStamp protocol buffer.
1199 // The tag strings define the protocol buffer field numbers;
1200 // they follow the convention outlined by the reflect package.
1202 microsec uint64 `protobuf:"1"`
1203 serverIP6 uint64 `protobuf:"2"`
1207 <h3 id="Pointer_types">Pointer types</h3>
1210 A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
1211 type, called the <i>base type</i> of the pointer.
1212 The value of an uninitialized pointer is <code>nil</code>.
1216 PointerType = "*" BaseType .
1225 <h3 id="Function_types">Function types</h3>
1228 A function type denotes the set of all functions with the same parameter
1229 and result types. The value of an uninitialized variable of function type
1230 is <code>nil</code>.
1234 FunctionType = "func" Signature .
1235 Signature = Parameters [ Result ] .
1236 Result = Parameters | Type .
1237 Parameters = "(" [ ParameterList [ "," ] ] ")" .
1238 ParameterList = ParameterDecl { "," ParameterDecl } .
1239 ParameterDecl = [ IdentifierList ] [ "..." ] Type .
1243 Within a list of parameters or results, the names (IdentifierList)
1244 must either all be present or all be absent. If present, each name
1245 stands for one item (parameter or result) of the specified type and
1246 all non-<a href="#Blank_identifier">blank</a> names in the signature
1247 must be <a href="#Uniqueness_of_identifiers">unique</a>.
1248 If absent, each type stands for one item of that type.
1249 Parameter and result
1250 lists are always parenthesized except that if there is exactly
1251 one unnamed result it may be written as an unparenthesized type.
1255 The final incoming parameter in a function signature may have
1256 a type prefixed with <code>...</code>.
1257 A function with such a parameter is called <i>variadic</i> and
1258 may be invoked with zero or more arguments for that parameter.
1264 func(a, _ int, z float32) bool
1265 func(a, b int, z float32) (bool)
1266 func(prefix string, values ...int)
1267 func(a, b int, z float64, opt ...interface{}) (success bool)
1268 func(int, int, float64) (float64, *[]int)
1269 func(n int) func(p *T)
1272 <h3 id="Interface_types">Interface types</h3>
1275 An interface type defines a <i>type set</i>.
1276 A variable of interface type can store a value of any type that is in the type
1277 set of the interface. Such a type is said to <i>implement the interface</i>.
1278 The value of an uninitialized variable of interface type is <code>nil</code>.
1282 InterfaceType = "interface" "{" { InterfaceElem ";" } "}" .
1283 InterfaceElem = MethodElem | TypeElem .
1284 MethodElem = MethodName Signature .
1285 MethodName = identifier .
1286 TypeElem = TypeTerm { "|" TypeTerm } .
1287 TypeTerm = Type | UnderlyingType .
1288 UnderlyingType = "~" Type .
1292 An interface type is specified by a list of <i>interface elements</i>.
1293 An interface element is either a <i>method</i> or a <i>type element</i>,
1294 where a type element is a union of one or more <i>type terms</i>.
1295 A type term is either a single type or a single underlying type.
1299 In its most basic form an interface specifies a (possibly empty) list of methods.
1300 The type set defined by such an interface is the set of types which implement all of
1301 those methods, and the corresponding <a href="#Method_sets">method set</a> consists
1302 exactly of the methods specified by the interface.
1306 // A simple File interface.
1308 Read([]byte) (int, error)
1309 Write([]byte) (int, error)
1315 The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
1316 and not <a href="#Blank_identifier">blank</a>.
1322 String() string // illegal: String not unique
1323 _(x int) // illegal: method must have non-blank name
1328 More than one type may implement an interface.
1329 For instance, if two types <code>S1</code> and <code>S2</code>
1334 func (p T) Read(p []byte) (n int, err error)
1335 func (p T) Write(p []byte) (n int, err error)
1336 func (p T) Close() error
1340 (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
1341 then the <code>File</code> interface is implemented by both <code>S1</code> and
1342 <code>S2</code>, regardless of what other methods
1343 <code>S1</code> and <code>S2</code> may have or share.
1347 Every type that is a member of the type set of an interface implements that interface.
1348 Any given type may implement several distinct interfaces.
1349 For instance, all types implement the <i>empty interface</i> which stands for the set of all types:
1357 For convenience, the predeclared type <code>any</code> is an alias for the empty interface.
1361 Similarly, consider this interface specification,
1362 which appears within a <a href="#Type_declarations">type declaration</a>
1363 to define an interface called <code>Locker</code>:
1367 type Locker interface {
1374 If <code>S1</code> and <code>S2</code> also implement
1378 func (p T) Lock() { … }
1379 func (p T) Unlock() { … }
1383 they implement the <code>Locker</code> interface as well
1384 as the <code>File</code> interface.
1388 In a slightly more general form
1389 an interface <code>T</code> may use a (possibly qualified) interface type
1390 name <code>E</code> as an interface element. This is called
1391 <i>embedding</i> interface <code>E</code> in <code>T</code>.
1392 The type set of <code>T</code> is the <i>intersection</i> of the type sets
1393 defined by <code>T</code>'s explicitly declared methods and the type sets
1394 of <code>T</code>’s embedded interfaces.
1395 In other words, the type set of <code>T</code> is the set of all types that implement all the
1396 explicitly declared methods of <code>T</code> and also all the methods of
1401 type Reader interface {
1402 Read(p []byte) (n int, err error)
1406 type Writer interface {
1407 Write(p []byte) (n int, err error)
1411 // ReadWriter's methods are Read, Write, and Close.
1412 type ReadWriter interface {
1413 Reader // includes methods of Reader in ReadWriter's method set
1414 Writer // includes methods of Writer in ReadWriter's method set
1419 When embedding interfaces, methods with the
1420 <a href="#Uniqueness_of_identifiers">same</a> names must
1421 have <a href="#Type_identity">identical</a> signatures.
1425 type ReadCloser interface {
1426 Reader // includes methods of Reader in ReadCloser's method set
1427 Close() // illegal: signatures of Reader.Close and Close are different
1432 Finally, in their most general form, an interface element may also be an arbitrary type term
1433 <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
1434 or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
1435 Together with method specifications, these elements enable the precise
1436 definition of an interface's type set as follows:
1440 <li>The type set of the empty interface is the set of all types.
1443 <li>The type set of a non-empty interface is the intersection of the type sets
1444 of its interface elements.
1447 <li>The type set of a method specification is the set of types
1448 whose method sets include that method.
1451 <li>The type set of a non-interface type term is the set consisting
1455 <li>The type set of a term of the form <code>~T</code>
1456 is the set of types whose underlying type is <code>T</code>.
1459 <li>The type set of a <i>union</i> of terms
1460 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
1461 is the union of the type sets of the terms.
1466 // An interface representing only the type int.
1471 // An interface representing all types with underlying type int.
1476 // An interface representing all types with underlying type int which implement the String method.
1482 // An interface representing an empty type set: there is no type that is both an int and a string.
1490 In a term of the form <code>~T</code>, the underlying type of <code>T</code>
1491 must be itself, and <code>T</code> cannot be an interface.
1498 ~[]byte // the underlying type of []byte is itself
1499 ~MyInt // illegal: the underlying type of MyInt is not MyInt
1500 ~error // illegal: error is an interface
1505 Union elements denote unions of type sets:
1509 // The Floats interface represents all floating-point types
1510 // (including any named types whose underlying types are
1511 // either float32 or float64).
1512 type Floats interface {
1518 In a union, a term cannot be a type parameter, and the type sets of all
1519 non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
1520 Given a type parameter <code>P</code>:
1525 P // illegal: the term P is a type parameter
1526 int | P // illegal: the term P is a type parameter
1527 ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
1528 float32 | Floats // overlapping type sets but Floats is an interface
1533 Implementation restriction:
1534 A union with more than one term cannot contain interface types
1535 with non-empty <a href="#Method_sets">method sets</a> or which
1536 are or embed the <a href="#Predeclared_identifiers">predeclared identifier</a>
1537 <code>comparable</code>.
1541 Interfaces that contain non-interface types, terms of the form <code>~T</code>,
1542 or unions may only be used as type constraints, or as elements of other interfaces used
1543 as constraints. They cannot be the types of values or variables, or components of other,
1544 non-interface types.
1548 var x Floats // illegal: Floats is restricted by float32 and float64
1550 var x interface{} = Floats(nil) // illegal
1552 type Floatish struct {
1557 <!-- TODO The rule below needs to be generalized to interface elements.
1558 It should be factored out and generalized to other types
1559 such as arrays and structs which are currently missing such a
1560 rule. See also #5069.
1564 An interface type <code>T</code> may not embed itself
1565 or any interface type that embeds <code>T</code>, recursively.
1569 // illegal: Bad cannot embed itself
1570 type Bad interface {
1574 // illegal: Bad1 cannot embed itself using Bad2
1575 type Bad1 interface {
1578 type Bad2 interface {
1583 <h3 id="Map_types">Map types</h3>
1586 A map is an unordered group of elements of one type, called the
1587 element type, indexed by a set of unique <i>keys</i> of another type,
1588 called the key type.
1589 The value of an uninitialized map is <code>nil</code>.
1593 MapType = "map" "[" KeyType "]" ElementType .
1598 The <a href="#Comparison_operators">comparison operators</a>
1599 <code>==</code> and <code>!=</code> must be fully defined
1600 for operands of the key type; thus the key type must not be a function, map, or
1602 If the key type is an interface type, these
1603 comparison operators must be defined for the dynamic key values;
1604 failure will cause a <a href="#Run_time_panics">run-time panic</a>.
1610 map[*T]struct{ x, y float64 }
1611 map[string]interface{}
1615 The number of map elements is called its length.
1616 For a map <code>m</code>, it can be discovered using the
1617 built-in function <a href="#Length_and_capacity"><code>len</code></a>
1618 and may change during execution. Elements may be added during execution
1619 using <a href="#Assignments">assignments</a> and retrieved with
1620 <a href="#Index_expressions">index expressions</a>; they may be removed with the
1621 <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
1624 A new, empty map value is made using the built-in
1625 function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1626 which takes the map type and an optional capacity hint as arguments:
1630 make(map[string]int)
1631 make(map[string]int, 100)
1635 The initial capacity does not bound its size:
1636 maps grow to accommodate the number of items
1637 stored in them, with the exception of <code>nil</code> maps.
1638 A <code>nil</code> map is equivalent to an empty map except that no elements
1641 <h3 id="Channel_types">Channel types</h3>
1644 A channel provides a mechanism for
1645 <a href="#Go_statements">concurrently executing functions</a>
1647 <a href="#Send_statements">sending</a> and
1648 <a href="#Receive_operator">receiving</a>
1649 values of a specified element type.
1650 The value of an uninitialized channel is <code>nil</code>.
1654 ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
1658 The optional <code><-</code> operator specifies the channel <i>direction</i>,
1659 <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
1660 otherwise it is <i>bidirectional</i>.
1661 A channel may be constrained only to send or only to receive by
1662 <a href="#Assignments">assignment</a> or
1663 explicit <a href="#Conversions">conversion</a>.
1667 chan T // can be used to send and receive values of type T
1668 chan<- float64 // can only be used to send float64s
1669 <-chan int // can only be used to receive ints
1673 The <code><-</code> operator associates with the leftmost <code>chan</code>
1678 chan<- chan int // same as chan<- (chan int)
1679 chan<- <-chan int // same as chan<- (<-chan int)
1680 <-chan <-chan int // same as <-chan (<-chan int)
1681 chan (<-chan int)
1685 A new, initialized channel
1686 value can be made using the built-in function
1687 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1688 which takes the channel type and an optional <i>capacity</i> as arguments:
1696 The capacity, in number of elements, sets the size of the buffer in the channel.
1697 If the capacity is zero or absent, the channel is unbuffered and communication
1698 succeeds only when both a sender and receiver are ready. Otherwise, the channel
1699 is buffered and communication succeeds without blocking if the buffer
1700 is not full (sends) or not empty (receives).
1701 A <code>nil</code> channel is never ready for communication.
1705 A channel may be closed with the built-in function
1706 <a href="#Close"><code>close</code></a>.
1707 The multi-valued assignment form of the
1708 <a href="#Receive_operator">receive operator</a>
1709 reports whether a received value was sent before
1710 the channel was closed.
1714 A single channel may be used in
1715 <a href="#Send_statements">send statements</a>,
1716 <a href="#Receive_operator">receive operations</a>,
1717 and calls to the built-in functions
1718 <a href="#Length_and_capacity"><code>cap</code></a> and
1719 <a href="#Length_and_capacity"><code>len</code></a>
1720 by any number of goroutines without further synchronization.
1721 Channels act as first-in-first-out queues.
1722 For example, if one goroutine sends values on a channel
1723 and a second goroutine receives them, the values are
1724 received in the order sent.
1727 <h3 id="Type_parameters">Type parameters</h3>
1730 A <i>type parameter</i> is an (unqualified) type name declared in the
1731 <a href="#Type_parameter_lists">type parameter list</a> of a
1732 <a href="#Function_declarations">function declaration</a> or
1733 <a href="#Type_definitions">type definition</a>; or in the receiver specification
1734 of a <a href="#Method_declarations">method declaration</a> that is associated
1735 with a parameterized type.
1736 A type parameter acts as a place holder for an (as of yet) unknown type in the declaration;
1737 the type parameter is replaced with a <i>type argument</i> upon
1738 <a href="#Instantiations">instantiation</a> of the parameterized function or type.
1742 The properties of a type parameter are determined by its
1743 <a href="#Type_constraints">type constraint</a>.
1746 <h2 id="Properties_of_types_and_values">Properties of types and values</h2>
1748 <h3 id="Type_identity">Type identity</h3>
1751 Two types are either <i>identical</i> or <i>different</i>.
1755 A <a href="#Types">named type</a> is always different from any other type.
1756 Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
1757 structurally equivalent; that is, they have the same literal structure and corresponding
1758 components have identical types. In detail:
1762 <li>Two array types are identical if they have identical element types and
1763 the same array length.</li>
1765 <li>Two slice types are identical if they have identical element types.</li>
1767 <li>Two struct types are identical if they have the same sequence of fields,
1768 and if corresponding fields have the same names, and identical types,
1770 <a href="#Exported_identifiers">Non-exported</a> field names from different
1771 packages are always different.</li>
1773 <li>Two pointer types are identical if they have identical base types.</li>
1775 <li>Two function types are identical if they have the same number of parameters
1776 and result values, corresponding parameter and result types are
1777 identical, and either both functions are variadic or neither is.
1778 Parameter and result names are not required to match.</li>
1780 <li>Two interface types are identical if they define the same type set.
1783 <li>Two map types are identical if they have identical key and element types.</li>
1785 <li>Two channel types are identical if they have identical element types and
1786 the same direction.</li>
1788 <li>Two <a href="#Instantiations">instantiated</a> types are identical if
1789 their defined types and all type arguments are identical.
1794 Given the declarations
1801 A2 = struct{ a, b int }
1803 A4 = func(A3, float64) *A0
1804 A5 = func(x int, _ float64) *[]string
1808 B2 struct{ a, b int }
1809 B3 struct{ a, c int }
1810 B4 func(int, float64) *B0
1811 B5 func(x int, y float64) *A1
1814 D0[P1, P2 any] struct{ x P1; y P2 }
1815 E0 = D0[int, string]
1820 these types are identical:
1824 A0, A1, and []string
1825 A2 and struct{ a, b int }
1827 A4, func(int, float64) *[]string, and A5
1830 D0[int, string] and E0
1832 struct{ a, b *T5 } and struct{ a, b *T5 }
1833 func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
1837 <code>B0</code> and <code>B1</code> are different because they are new types
1838 created by distinct <a href="#Type_definitions">type definitions</a>;
1839 <code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
1840 are different because <code>B0</code> is different from <code>[]string</code>;
1841 and <code>P1</code> and <code>P2</code> are different because they are different
1843 <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are
1844 different because the former is an <a href="#Instantiations">instantiated</a>
1845 defined type while the latter is a type literal
1846 (but they are still <a href="#Assignability">assignable</a>).
1849 <h3 id="Assignability">Assignability</h3>
1852 A value <code>x</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
1853 ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
1858 <code>x</code>'s type is identical to <code>T</code>.
1861 <code>x</code>'s type <code>V</code> and <code>T</code> have identical
1862 <a href="#Underlying_types">underlying types</a> and at least one of <code>V</code>
1863 or <code>T</code> is not a <a href="#Types">named type</a>.
1866 <code>x</code>'s type <code>V</code> and <code>T</code> are channel types with
1867 identical element types, <code>V</code> is a bidirectional channel,
1868 and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
1871 <code>T</code> is an interface type, but not a type parameter, and
1872 <code>x</code> <a href="#Interface_types">implements</a> <code>T</code>.
1875 <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
1876 is a pointer, function, slice, map, channel, or interface type,
1877 but not a type parameter.
1880 <code>x</code> is an untyped <a href="#Constants">constant</a>
1881 <a href="#Representability">representable</a>
1882 by a value of type <code>T</code>.
1887 Additionally, if <code>x's</code> type <code>V</code> or <code>T</code> are type parameters
1888 with <a href="#Structure_of_interfaces">specific types</a>, <code>x</code>
1889 is assignable to a variable of type <code>T</code> if one of the following conditions applies:
1894 <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
1895 a type parameter, and <code>x</code> is assignable to each specific type of
1899 <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
1900 a type parameter, and <code>x</code> is assignable to each specific type of
1904 <code>V</code> is a type parameter and <code>T</code> is not a named type,
1905 and values of each specific type of <code>V</code> are assignable
1910 <h3 id="Representability">Representability</h3>
1913 A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
1914 by a value of type <code>T</code>,
1915 where <code>T</code> is not a <a href="#Type_parameters">type parameter</a>,
1916 if one of the following conditions applies:
1921 <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
1925 <code>T</code> is a floating-point type and <code>x</code> can be rounded to <code>T</code>'s
1926 precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
1927 negative zero further simplified to an unsigned zero. Note that constant values never result
1928 in an IEEE negative zero, NaN, or infinity.
1932 <code>T</code> is a complex type, and <code>x</code>'s
1933 <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
1934 are representable by values of <code>T</code>'s component type (<code>float32</code> or
1935 <code>float64</code>).
1940 If <code>T</code> is a type parameter with <a href="#Structure_of_interfaces">specific types</a>,
1941 <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
1942 by a value of each specific type of <code>T</code>.
1946 x T x is representable by a value of T because
1948 'a' byte 97 is in the set of byte values
1949 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
1950 "foo" string "foo" is in the set of string values
1951 1024 int16 1024 is in the set of 16-bit integers
1952 42.0 byte 42 is in the set of unsigned 8-bit integers
1953 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
1954 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
1955 -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
1956 0i int 0 is an integer value
1957 (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
1961 x T x is not representable by a value of T because
1963 0 bool 0 is not in the set of boolean values
1964 'a' string 'a' is a rune, it is not in the set of string values
1965 1024 byte 1024 is not in the set of unsigned 8-bit integers
1966 -1 uint16 -1 is not in the set of unsigned 16-bit integers
1967 1.1 int 1.1 is not an integer value
1968 42i float32 (0 + 42i) is not in the set of float32 values
1969 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
1972 <h3 id="Structure_of_interfaces">Structure of interfaces</h3>
1975 An interface specification which contains <a href="#Interface_types">type elements</a>
1976 defines a (possibly empty) set of <i>specific types</i>.
1977 Loosely speaking, these are the types <code>T</code> that appear in the
1978 interface definition in terms of the form <code>T</code>, <code>~T</code>,
1979 or in unions of such terms.
1983 More precisely, for a given interface, the set 𝑆 of specific types is defined as follows:
1987 <li>For an interface with no type elements, 𝑆 is the empty set.
1990 <li>For an interface with type elements, 𝑆 is the intersection
1991 of the specific types of its type elements with specific types
1992 (type elements that have no specific types are ignored).
1995 <li>For a non-interface type term <code>T</code>
1996 or <code>~T</code>, 𝑆 is the set consisting of the type <code>T</code>.
1999 <li>For a <i>union</i> of terms
2000 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>,
2001 𝑆 is the union of the specific types of the terms.
2006 If 𝑆 is empty, the interface has <i>no specific types</i>.
2007 An interface may have specific types even if its <a href="#Interface_types">type set</a>
2012 Examples of interfaces with their specific types:
2016 type Celsius float32
2019 interface{} // no specific types
2020 interface{ int } // int
2021 interface{ ~string } // string
2022 interface{ int|~string } // int, string
2023 interface{ Celsius|Kelvin } // Celsius, Kelvin
2024 interface{ int; m() } // int (but type set is empty because int has no method m)
2025 interface{ int; any } // int (any has no specific types and is ignored)
2026 interface{ int; string } // no specific types (intersection is empty)
2030 An interface <code>T</code> is called <i>structural</i> if one of the following
2031 conditions is satisfied:
2036 There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a>
2037 of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or
2040 the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a>
2041 with identical element type <code>E</code>, and all directional channels have the same
2047 A structural interface has a <i>structural type</i> which is, depending on the
2048 condition that is satisfied, either:
2053 the type <code>U</code>; or
2056 the type <code>chan E</code> if <code>T</code> contains only bidirectional
2057 channels, or the type <code>chan<- E</code> or <code><-chan E</code>
2058 depending on the direction of the directional channels present.
2063 Examples of structural interfaces with their structural types:
2067 interface{ int } // int
2068 interface{ Celsius|Kelvin } // float32
2069 interface{ ~chan int } // chan int
2070 interface{ ~chan int|~chan<- int } // chan<- int
2071 interface{ ~[]*data; String() string } // []*data
2075 Examples of non-structural interfaces:
2079 interface{} // no single underlying type
2080 interface{ Celsius|float64 } // no single underlying type
2081 interface{ chan int | chan<- string } // channels have different element types
2082 interface{ <-chan int | chan<- int } // directional channels have different directions
2085 <h2 id="Blocks">Blocks</h2>
2088 A <i>block</i> is a possibly empty sequence of declarations and statements
2089 within matching brace brackets.
2093 Block = "{" StatementList "}" .
2094 StatementList = { Statement ";" } .
2098 In addition to explicit blocks in the source code, there are implicit blocks:
2102 <li>The <i>universe block</i> encompasses all Go source text.</li>
2104 <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
2105 Go source text for that package.</li>
2107 <li>Each file has a <i>file block</i> containing all Go source text
2110 <li>Each <a href="#If_statements">"if"</a>,
2111 <a href="#For_statements">"for"</a>, and
2112 <a href="#Switch_statements">"switch"</a>
2113 statement is considered to be in its own implicit block.</li>
2115 <li>Each clause in a <a href="#Switch_statements">"switch"</a>
2116 or <a href="#Select_statements">"select"</a> statement
2117 acts as an implicit block.</li>
2121 Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
2125 <h2 id="Declarations_and_scope">Declarations and scope</h2>
2128 A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
2129 <a href="#Constant_declarations">constant</a>,
2130 <a href="#Type_declarations">type</a>,
2131 <a href="#Variable_declarations">variable</a>,
2132 <a href="#Function_declarations">function</a>,
2133 <a href="#Labeled_statements">label</a>, or
2134 <a href="#Import_declarations">package</a>.
2135 Every identifier in a program must be declared.
2136 No identifier may be declared twice in the same block, and
2137 no identifier may be declared in both the file and package block.
2141 The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
2142 in a declaration, but it does not introduce a binding and thus is not declared.
2143 In the package block, the identifier <code>init</code> may only be used for
2144 <a href="#Package_initialization"><code>init</code> function</a> declarations,
2145 and like the blank identifier it does not introduce a new binding.
2149 Declaration = ConstDecl | TypeDecl | VarDecl .
2150 TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
2154 The <i>scope</i> of a declared identifier is the extent of source text in which
2155 the identifier denotes the specified constant, type, variable, function, label, or package.
2159 Go is lexically scoped using <a href="#Blocks">blocks</a>:
2163 <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
2165 <li>The scope of an identifier denoting a constant, type, variable,
2166 or function (but not method) declared at top level (outside any
2167 function) is the package block.</li>
2169 <li>The scope of the package name of an imported package is the file block
2170 of the file containing the import declaration.</li>
2172 <li>The scope of an identifier denoting a method receiver, function parameter,
2173 or result variable is the function body.</li>
2175 <li>The scope of an identifier denoting a type parameter of a type-parameterized function
2176 or declared by a method receiver is the function body and all parameter lists of the
2180 <li>The scope of an identifier denoting a type parameter of a parameterized type
2181 begins after the name of the parameterized type and ends at the end
2182 of the TypeSpec.</li>
2184 <li>The scope of a constant or variable identifier declared
2185 inside a function begins at the end of the ConstSpec or VarSpec
2186 (ShortVarDecl for short variable declarations)
2187 and ends at the end of the innermost containing block.</li>
2189 <li>The scope of a type identifier declared inside a function
2190 begins at the identifier in the TypeSpec
2191 and ends at the end of the innermost containing block.</li>
2195 An identifier declared in a block may be redeclared in an inner block.
2196 While the identifier of the inner declaration is in scope, it denotes
2197 the entity declared by the inner declaration.
2201 The <a href="#Package_clause">package clause</a> is not a declaration; the package name
2202 does not appear in any scope. Its purpose is to identify the files belonging
2203 to the same <a href="#Packages">package</a> and to specify the default package name for import
2208 <h3 id="Label_scopes">Label scopes</h3>
2211 Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
2212 used in the <a href="#Break_statements">"break"</a>,
2213 <a href="#Continue_statements">"continue"</a>, and
2214 <a href="#Goto_statements">"goto"</a> statements.
2215 It is illegal to define a label that is never used.
2216 In contrast to other identifiers, labels are not block scoped and do
2217 not conflict with identifiers that are not labels. The scope of a label
2218 is the body of the function in which it is declared and excludes
2219 the body of any nested function.
2223 <h3 id="Blank_identifier">Blank identifier</h3>
2226 The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
2227 It serves as an anonymous placeholder instead of a regular (non-blank)
2228 identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
2229 as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>.
2233 <h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
2236 The following identifiers are implicitly declared in the
2237 <a href="#Blocks">universe block</a>:
2239 <pre class="grammar">
2241 any bool byte comparable
2242 complex64 complex128 error float32 float64
2243 int int8 int16 int32 int64 rune string
2244 uint uint8 uint16 uint32 uint64 uintptr
2253 append cap close complex copy delete imag len
2254 make new panic print println real recover
2258 <h3 id="Exported_identifiers">Exported identifiers</h3>
2261 An identifier may be <i>exported</i> to permit access to it from another package.
2262 An identifier is exported if both:
2265 <li>the first character of the identifier's name is a Unicode upper case
2266 letter (Unicode class "Lu"); and</li>
2267 <li>the identifier is declared in the <a href="#Blocks">package block</a>
2268 or it is a <a href="#Struct_types">field name</a> or
2269 <a href="#MethodName">method name</a>.</li>
2272 All other identifiers are not exported.
2275 <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
2278 Given a set of identifiers, an identifier is called <i>unique</i> if it is
2279 <i>different</i> from every other in the set.
2280 Two identifiers are different if they are spelled differently, or if they
2281 appear in different <a href="#Packages">packages</a> and are not
2282 <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
2285 <h3 id="Constant_declarations">Constant declarations</h3>
2288 A constant declaration binds a list of identifiers (the names of
2289 the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
2290 The number of identifiers must be equal
2291 to the number of expressions, and the <i>n</i>th identifier on
2292 the left is bound to the value of the <i>n</i>th expression on the
2297 ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
2298 ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
2300 IdentifierList = identifier { "," identifier } .
2301 ExpressionList = Expression { "," Expression } .
2305 If the type is present, all constants take the type specified, and
2306 the expressions must be <a href="#Assignability">assignable</a> to that type.
2307 If the type is omitted, the constants take the
2308 individual types of the corresponding expressions.
2309 If the expression values are untyped <a href="#Constants">constants</a>,
2310 the declared constants remain untyped and the constant identifiers
2311 denote the constant values. For instance, if the expression is a
2312 floating-point literal, the constant identifier denotes a floating-point
2313 constant, even if the literal's fractional part is zero.
2317 const Pi float64 = 3.14159265358979323846
2318 const zero = 0.0 // untyped floating-point constant
2321 eof = -1 // untyped integer constant
2323 const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
2324 const u, v float32 = 0, 3 // u = 0.0, v = 3.0
2328 Within a parenthesized <code>const</code> declaration list the
2329 expression list may be omitted from any but the first ConstSpec.
2330 Such an empty list is equivalent to the textual substitution of the
2331 first preceding non-empty expression list and its type if any.
2332 Omitting the list of expressions is therefore equivalent to
2333 repeating the previous list. The number of identifiers must be equal
2334 to the number of expressions in the previous list.
2335 Together with the <a href="#Iota"><code>iota</code> constant generator</a>
2336 this mechanism permits light-weight declaration of sequential values:
2348 numberOfDays // this constant is not exported
2353 <h3 id="Iota">Iota</h3>
2356 Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
2357 <code>iota</code> represents successive untyped integer <a href="#Constants">
2358 constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
2359 in that constant declaration, starting at zero.
2360 It can be used to construct a set of related constants:
2365 c0 = iota // c0 == 0
2366 c1 = iota // c1 == 1
2367 c2 = iota // c2 == 2
2371 a = 1 << iota // a == 1 (iota == 0)
2372 b = 1 << iota // b == 2 (iota == 1)
2373 c = 3 // c == 3 (iota == 2, unused)
2374 d = 1 << iota // d == 8 (iota == 3)
2378 u = iota * 42 // u == 0 (untyped integer constant)
2379 v float64 = iota * 42 // v == 42.0 (float64 constant)
2380 w = iota * 42 // w == 84 (untyped integer constant)
2383 const x = iota // x == 0
2384 const y = iota // y == 0
2388 By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
2393 bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
2394 bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
2395 _, _ // (iota == 2, unused)
2396 bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
2401 This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
2402 of the last non-empty expression list.
2406 <h3 id="Type_declarations">Type declarations</h3>
2409 A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
2410 Type declarations come in two forms: alias declarations and type definitions.
2414 TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
2415 TypeSpec = AliasDecl | TypeDef .
2418 <h4 id="Alias_declarations">Alias declarations</h4>
2421 An alias declaration binds an identifier to the given type.
2425 AliasDecl = identifier "=" Type .
2429 Within the <a href="#Declarations_and_scope">scope</a> of
2430 the identifier, it serves as an <i>alias</i> for the type.
2435 nodeList = []*Node // nodeList and []*Node are identical types
2436 Polar = polar // Polar and polar denote identical types
2441 <h4 id="Type_definitions">Type definitions</h4>
2444 A type definition creates a new, distinct type with the same
2445 <a href="#Types">underlying type</a> and operations as the given type
2446 and binds an identifier, the <i>type name</i>, to it.
2450 TypeDef = identifier [ TypeParameters ] Type .
2454 The new type is called a <i>defined type</i>.
2455 It is <a href="#Type_identity">different</a> from any other type,
2456 including the type it is created from.
2461 Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
2462 polar Point // polar and Point denote different types
2465 type TreeNode struct {
2466 left, right *TreeNode
2470 type Block interface {
2472 Encrypt(src, dst []byte)
2473 Decrypt(src, dst []byte)
2478 A defined type may have <a href="#Method_declarations">methods</a> associated with it.
2479 It does not inherit any methods bound to the given type,
2480 but the <a href="#Method_sets">method set</a>
2481 of an interface type or of elements of a composite type remains unchanged:
2485 // A Mutex is a data type with two methods, Lock and Unlock.
2486 type Mutex struct { /* Mutex fields */ }
2487 func (m *Mutex) Lock() { /* Lock implementation */ }
2488 func (m *Mutex) Unlock() { /* Unlock implementation */ }
2490 // NewMutex has the same composition as Mutex but its method set is empty.
2493 // The method set of PtrMutex's underlying type *Mutex remains unchanged,
2494 // but the method set of PtrMutex is empty.
2495 type PtrMutex *Mutex
2497 // The method set of *PrintableMutex contains the methods
2498 // Lock and Unlock bound to its embedded field Mutex.
2499 type PrintableMutex struct {
2503 // MyBlock is an interface type that has the same method set as Block.
2508 Type definitions may be used to define different boolean, numeric,
2509 or string types and associate methods with them:
2516 EST TimeZone = -(5 + iota)
2522 func (tz TimeZone) String() string {
2523 return fmt.Sprintf("GMT%+dh", tz)
2528 If the type definition specifies <a href="#Type_parameter_lists">type parameters</a>,
2529 the type name denotes a <i>parameterized type</i>.
2530 Parameterized types must be <a href="#Instantiations">instantiated</a> when they
2535 type List[T any] struct {
2540 type Tree[T constraints.Ordered] struct {
2541 left, right *Tree[T]
2547 The given type cannot be a type parameter in a type definition.
2551 type T[P any] P // illegal: P is a type parameter
2554 type L T // illegal: T is a type parameter declared by the enclosing function
2559 A parameterized type may also have methods associated with it. In this case,
2560 the method receivers must declare the same number of type parameters as
2561 present in the parameterized type definition.
2565 // The method Len returns the number of elements in the linked list l.
2566 func (l *List[T]) Len() int { … }
2569 <h3 id="Type_parameter_lists">Type parameter lists</h3>
2572 A type parameter list declares the <a href="#Type_parameters">type parameters</a>
2573 in a type-parameterized function or type declaration.
2574 The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a>
2575 except that the type parameter names must all be present and the list is enclosed
2576 in square brackets rather than parentheses.
2580 TypeParameters = "[" TypeParamList [ "," ] "]" .
2581 TypeParamList = TypeParamDecl { "," TypeParamDecl } .
2582 TypeParamDecl = IdentifierList TypeConstraint .
2586 Each identifier declares a type parameter.
2587 All non-blank names in the list must be unique.
2588 Each type parameter is a new and different <a href="#Types">named type</a>.
2593 [S interface{ ~[]byte|string }]
2600 Just as each ordinary function parameter has a parameter type, each type parameter
2601 has a corresponding (meta-)type which is called its
2602 <a href="#Type_constraints"><i>type constraint</i></a>.
2606 A parsing ambiguity arises when the type parameter list for a parameterized type
2607 declares a single type parameter with a type constraint of the form <code>*C</code>
2608 or <code>(C)</code> where <code>C</code> is not a (possibly parenthesized)
2609 <a href="#Types">type literal</a>:
2618 In these rare cases, the type parameter declaration is indistinguishable from
2619 the expressions <code>P*C</code> or <code>P(C)</code> and the type declaration
2620 is parsed as an array type declaration.
2621 To resolve the ambiguity, embed the constraint in an interface or use a trailing
2626 type T[P interface{*C}] …
2630 <h4 id="Type_constraints">Type constraints</h4>
2633 A type constraint is an <a href="#Interface_types">interface</a> that defines the
2634 set of permissible type arguments for the respective type parameter and controls the
2635 operations supported by values of that type parameter.
2639 TypeConstraint = TypeElem .
2643 If the constraint is an interface literal containing exactly one embedded type element
2644 <code>interface{E}</code>, in a type parameter list the enclosing <code>interface{ … }</code>
2645 may be omitted for convenience:
2649 [T *P] // = [T interface{*P}]
2650 [T ~int] // = [T interface{~int}]
2651 [T int|string] // = [T interface{int|string}]
2652 type Constraint ~int // illegal: ~int is not inside a type parameter list
2655 <h3 id="Variable_declarations">Variable declarations</h3>
2658 A variable declaration creates one or more <a href="#Variables">variables</a>,
2659 binds corresponding identifiers to them, and gives each a type and an initial value.
2663 VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
2664 VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
2671 var x, y float32 = -1, -2
2674 u, v, s = 2.0, 3.0, "bar"
2676 var re, im = complexSqrt(-1)
2677 var _, found = entries[name] // map lookup; only interested in "found"
2681 If a list of expressions is given, the variables are initialized
2682 with the expressions following the rules for <a href="#Assignments">assignments</a>.
2683 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
2687 If a type is present, each variable is given that type.
2688 Otherwise, each variable is given the type of the corresponding
2689 initialization value in the assignment.
2690 If that value is an untyped constant, it is first implicitly
2691 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
2692 if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
2693 The predeclared value <code>nil</code> cannot be used to initialize a variable
2694 with no explicit type.
2698 var d = math.Sin(0.5) // d is float64
2699 var i = 42 // i is int
2700 var t, ok = x.(T) // t is T, ok is bool
2701 var n = nil // illegal
2705 Implementation restriction: A compiler may make it illegal to declare a variable
2706 inside a <a href="#Function_declarations">function body</a> if the variable is
2710 <h3 id="Short_variable_declarations">Short variable declarations</h3>
2713 A <i>short variable declaration</i> uses the syntax:
2717 ShortVarDecl = IdentifierList ":=" ExpressionList .
2721 It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
2722 with initializer expressions but no types:
2725 <pre class="grammar">
2726 "var" IdentifierList = ExpressionList .
2731 f := func() int { return 7 }
2732 ch := make(chan int)
2733 r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
2734 _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
2738 Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
2739 variables provided they were originally declared earlier in the same block
2740 (or the parameter lists if the block is the function body) with the same type,
2741 and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
2742 As a consequence, redeclaration can only appear in a multi-variable short declaration.
2743 Redeclaration does not introduce a new variable; it just assigns a new value to the original.
2747 field1, offset := nextField(str, 0)
2748 field2, offset := nextField(str, offset) // redeclares offset
2749 a, a := 1, 2 // illegal: double declaration of a or no new variable if a was declared elsewhere
2753 Short variable declarations may appear only inside functions.
2754 In some contexts such as the initializers for
2755 <a href="#If_statements">"if"</a>,
2756 <a href="#For_statements">"for"</a>, or
2757 <a href="#Switch_statements">"switch"</a> statements,
2758 they can be used to declare local temporary variables.
2761 <h3 id="Function_declarations">Function declarations</h3>
2764 Given the importance of functions, this section has always
2765 been woefully underdeveloped. Would be nice to expand this
2770 A function declaration binds an identifier, the <i>function name</i>,
2775 FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
2776 FunctionName = identifier .
2777 FunctionBody = Block .
2781 If the function's <a href="#Function_types">signature</a> declares
2782 result parameters, the function body's statement list must end in
2783 a <a href="#Terminating_statements">terminating statement</a>.
2787 func IndexRune(s string, r rune) int {
2788 for i, c := range s {
2793 // invalid: missing return statement
2798 If the function declaration specifies <a href="#Type_parameter_lists">type parameters</a>,
2799 the function name denotes a <i>type-parameterized function</i>.
2800 Type-parameterized functions must be <a href="#Instantiations">instantiated</a> when they
2805 func min[T constraints.Ordered](x, y T) T {
2814 A function declaration without type parameters may omit the body.
2815 Such a declaration provides the signature for a function implemented outside Go,
2816 such as an assembly routine.
2820 func flushICache(begin, end uintptr) // implemented externally
2823 <h3 id="Method_declarations">Method declarations</h3>
2826 A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
2827 A method declaration binds an identifier, the <i>method name</i>, to a method,
2828 and associates the method with the receiver's <i>base type</i>.
2832 MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
2833 Receiver = Parameters .
2837 The receiver is specified via an extra parameter section preceding the method
2838 name. That parameter section must declare a single non-variadic parameter, the receiver.
2839 Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
2840 pointer to a defined type <code>T</code>, possibly followed by a list of type parameter
2841 names <code>[P1, P2, …]</code> enclosed in square brackets.
2842 <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be
2843 a pointer or interface type and it must be defined in the same package as the method.
2844 The method is said to be <i>bound</i> to its receiver base type and the method name
2845 is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
2850 A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
2851 <a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
2852 If the receiver's value is not referenced inside the body of the method,
2853 its identifier may be omitted in the declaration. The same applies in
2854 general to parameters of functions and methods.
2858 For a base type, the non-blank names of methods bound to it must be unique.
2859 If the base type is a <a href="#Struct_types">struct type</a>,
2860 the non-blank method and field names must be distinct.
2864 Given defined type <code>Point</code>, the declarations
2868 func (p *Point) Length() float64 {
2869 return math.Sqrt(p.x * p.x + p.y * p.y)
2872 func (p *Point) Scale(factor float64) {
2879 bind the methods <code>Length</code> and <code>Scale</code>,
2880 with receiver type <code>*Point</code>,
2881 to the base type <code>Point</code>.
2885 If the receiver base type is a <a href="#Type_declarations">parameterized type</a>, the
2886 receiver specification must declare corresponding type parameters for the method
2887 to use. This makes the receiver type parameters available to the method.
2891 Syntactically, this type parameter declaration looks like an
2892 <a href="#Instantiations">instantiation</a> of the receiver base type, except that
2893 the type arguments are the type parameters being declared, one for each type parameter
2894 of the receiver base type.
2895 The type parameter names do not need to match their corresponding parameter names in the
2896 receiver base type definition, and all non-blank parameter names must be unique in the
2897 receiver parameter section and the method signature.
2898 The receiver type parameter constraints are implied by the receiver base type definition:
2899 corresponding type parameters have corresponding constraints.
2903 type Pair[A, B any] struct {
2908 func (p Pair[A, B]) Swap() Pair[B, A] { return Pair[B, A]{p.b, p.a} }
2909 func (p Pair[First, _]) First() First { return p.a }
2912 <h2 id="Expressions">Expressions</h2>
2915 An expression specifies the computation of a value by applying
2916 operators and functions to operands.
2919 <h3 id="Operands">Operands</h3>
2922 Operands denote the elementary values in an expression. An operand may be a
2923 literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
2924 non-<a href="#Blank_identifier">blank</a> identifier denoting a
2925 <a href="#Constant_declarations">constant</a>,
2926 <a href="#Variable_declarations">variable</a>, or
2927 <a href="#Function_declarations">function</a>,
2928 or a parenthesized expression.
2932 An operand name denoting a <a href="#Function_declarations">type-parameterized function</a>
2933 may be followed by a list of <a href="#Type_arguments">type arguments</a>; the
2934 resulting operand is an <a href="#Instantiations">instantiated</a> function.
2938 The <a href="#Blank_identifier">blank identifier</a> may appear as an
2939 operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
2943 Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
2944 Literal = BasicLit | CompositeLit | FunctionLit .
2945 BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
2946 OperandName = identifier | QualifiedIdent .
2949 <h3 id="Qualified_identifiers">Qualified identifiers</h3>
2952 A <i>qualified identifier</i> is an identifier qualified with a package name prefix.
2953 Both the package name and the identifier must not be
2954 <a href="#Blank_identifier">blank</a>.
2958 QualifiedIdent = PackageName "." identifier .
2962 A qualified identifier accesses an identifier in a different package, which
2963 must be <a href="#Import_declarations">imported</a>.
2964 The identifier must be <a href="#Exported_identifiers">exported</a> and
2965 declared in the <a href="#Blocks">package block</a> of that package.
2969 math.Sin // denotes the Sin function in package math
2972 <h3 id="Composite_literals">Composite literals</h3>
2975 Composite literals construct values for structs, arrays, slices, and maps
2976 and create a new value each time they are evaluated.
2977 They consist of the type of the literal followed by a brace-bound list of elements.
2978 Each element may optionally be preceded by a corresponding key.
2982 CompositeLit = LiteralType LiteralValue .
2983 LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
2984 SliceType | MapType | TypeName .
2985 LiteralValue = "{" [ ElementList [ "," ] ] "}" .
2986 ElementList = KeyedElement { "," KeyedElement } .
2987 KeyedElement = [ Key ":" ] Element .
2988 Key = FieldName | Expression | LiteralValue .
2989 FieldName = identifier .
2990 Element = Expression | LiteralValue .
2994 The LiteralType's underlying type must be a struct, array, slice, or map type
2995 (the grammar enforces this constraint except when the type is given
2997 The types of the elements and keys must be <a href="#Assignability">assignable</a>
2998 to the respective field, element, and key types of the literal type;
2999 there is no additional conversion.
3000 The key is interpreted as a field name for struct literals,
3001 an index for array and slice literals, and a key for map literals.
3002 For map literals, all elements must have a key. It is an error
3003 to specify multiple elements with the same field name or
3004 constant key value. For non-constant map keys, see the section on
3005 <a href="#Order_of_evaluation">evaluation order</a>.
3009 For struct literals the following rules apply:
3012 <li>A key must be a field name declared in the struct type.
3014 <li>An element list that does not contain any keys must
3015 list an element for each struct field in the
3016 order in which the fields are declared.
3018 <li>If any element has a key, every element must have a key.
3020 <li>An element list that contains keys does not need to
3021 have an element for each struct field. Omitted fields
3022 get the zero value for that field.
3024 <li>A literal may omit the element list; such a literal evaluates
3025 to the zero value for its type.
3027 <li>It is an error to specify an element for a non-exported
3028 field of a struct belonging to a different package.
3033 Given the declarations
3036 type Point3D struct { x, y, z float64 }
3037 type Line struct { p, q Point3D }
3045 origin := Point3D{} // zero value for Point3D
3046 line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
3050 For array and slice literals the following rules apply:
3053 <li>Each element has an associated integer index marking
3054 its position in the array.
3056 <li>An element with a key uses the key as its index. The
3057 key must be a non-negative constant
3058 <a href="#Representability">representable</a> by
3059 a value of type <code>int</code>; and if it is typed
3060 it must be of integer type.
3062 <li>An element without a key uses the previous element's index plus one.
3063 If the first element has no key, its index is zero.
3068 <a href="#Address_operators">Taking the address</a> of a composite literal
3069 generates a pointer to a unique <a href="#Variables">variable</a> initialized
3070 with the literal's value.
3074 var pointer *Point3D = &Point3D{y: 1000}
3078 Note that the <a href="#The_zero_value">zero value</a> for a slice or map
3079 type is not the same as an initialized but empty value of the same type.
3080 Consequently, taking the address of an empty slice or map composite literal
3081 does not have the same effect as allocating a new slice or map value with
3082 <a href="#Allocation">new</a>.
3086 p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
3087 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
3091 The length of an array literal is the length specified in the literal type.
3092 If fewer elements than the length are provided in the literal, the missing
3093 elements are set to the zero value for the array element type.
3094 It is an error to provide elements with index values outside the index range
3095 of the array. The notation <code>...</code> specifies an array length equal
3096 to the maximum element index plus one.
3100 buffer := [10]string{} // len(buffer) == 10
3101 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
3102 days := [...]string{"Sat", "Sun"} // len(days) == 2
3106 A slice literal describes the entire underlying array literal.
3107 Thus the length and capacity of a slice literal are the maximum
3108 element index plus one. A slice literal has the form
3116 and is shorthand for a slice operation applied to an array:
3120 tmp := [n]T{x1, x2, … xn}
3125 Within a composite literal of array, slice, or map type <code>T</code>,
3126 elements or map keys that are themselves composite literals may elide the respective
3127 literal type if it is identical to the element or key type of <code>T</code>.
3128 Similarly, elements or keys that are addresses of composite literals may elide
3129 the <code>&T</code> when the element or key type is <code>*T</code>.
3133 [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
3134 [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
3135 [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
3136 map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
3137 map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
3140 [2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
3141 [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
3145 A parsing ambiguity arises when a composite literal using the
3146 TypeName form of the LiteralType appears as an operand between the
3147 <a href="#Keywords">keyword</a> and the opening brace of the block
3148 of an "if", "for", or "switch" statement, and the composite literal
3149 is not enclosed in parentheses, square brackets, or curly braces.
3150 In this rare case, the opening brace of the literal is erroneously parsed
3151 as the one introducing the block of statements. To resolve the ambiguity,
3152 the composite literal must appear within parentheses.
3156 if x == (T{a,b,c}[i]) { … }
3157 if (x == T{a,b,c}[i]) { … }
3161 Examples of valid array, slice, and map literals:
3165 // list of prime numbers
3166 primes := []int{2, 3, 5, 7, 9, 2147483647}
3168 // vowels[ch] is true if ch is a vowel
3169 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
3171 // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
3172 filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
3174 // frequencies in Hz for equal-tempered scale (A4 = 440Hz)
3175 noteFrequency := map[string]float32{
3176 "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
3177 "G0": 24.50, "A0": 27.50, "B0": 30.87,
3182 <h3 id="Function_literals">Function literals</h3>
3185 A function literal represents an anonymous <a href="#Function_declarations">function</a>.
3186 Function literals cannot declare type parameters.
3190 FunctionLit = "func" Signature FunctionBody .
3194 func(a, b int, z float64) bool { return a*b < int(z) }
3198 A function literal can be assigned to a variable or invoked directly.
3202 f := func(x, y int) int { return x + y }
3203 func(ch chan int) { ch <- ACK }(replyChan)
3207 Function literals are <i>closures</i>: they may refer to variables
3208 defined in a surrounding function. Those variables are then shared between
3209 the surrounding function and the function literal, and they survive as long
3210 as they are accessible.
3214 <h3 id="Primary_expressions">Primary expressions</h3>
3217 Primary expressions are the operands for unary and binary expressions.
3225 PrimaryExpr Selector |
3228 PrimaryExpr TypeAssertion |
3229 PrimaryExpr Arguments .
3231 Selector = "." identifier .
3232 Index = "[" Expression "]" .
3233 Slice = "[" [ Expression ] ":" [ Expression ] "]" |
3234 "[" [ Expression ] ":" Expression ":" Expression "]" .
3235 TypeAssertion = "." "(" Type ")" .
3236 Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
3253 <h3 id="Selectors">Selectors</h3>
3256 For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
3257 that is not a <a href="#Package_clause">package name</a>, the
3258 <i>selector expression</i>
3266 denotes the field or method <code>f</code> of the value <code>x</code>
3267 (or sometimes <code>*x</code>; see below).
3268 The identifier <code>f</code> is called the (field or method) <i>selector</i>;
3269 it must not be the <a href="#Blank_identifier">blank identifier</a>.
3270 The type of the selector expression is the type of <code>f</code>.
3271 If <code>x</code> is a package name, see the section on
3272 <a href="#Qualified_identifiers">qualified identifiers</a>.
3276 A selector <code>f</code> may denote a field or method <code>f</code> of
3277 a type <code>T</code>, or it may refer
3278 to a field or method <code>f</code> of a nested
3279 <a href="#Struct_types">embedded field</a> of <code>T</code>.
3280 The number of embedded fields traversed
3281 to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
3282 The depth of a field or method <code>f</code>
3283 declared in <code>T</code> is zero.
3284 The depth of a field or method <code>f</code> declared in
3285 an embedded field <code>A</code> in <code>T</code> is the
3286 depth of <code>f</code> in <code>A</code> plus one.
3290 The following rules apply to selectors:
3295 For a value <code>x</code> of type <code>T</code> or <code>*T</code>
3296 where <code>T</code> is not a pointer or interface type,
3297 <code>x.f</code> denotes the field or method at the shallowest depth
3298 in <code>T</code> where there
3299 is such an <code>f</code>.
3300 If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
3301 with shallowest depth, the selector expression is illegal.
3305 For a value <code>x</code> of type <code>I</code> where <code>I</code>
3306 is an interface type, <code>x.f</code> denotes the actual method with name
3307 <code>f</code> of the dynamic value of <code>x</code>.
3308 If there is no method with name <code>f</code> in the
3309 <a href="#Method_sets">method set</a> of <code>I</code>, the selector
3310 expression is illegal.
3314 As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
3315 pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
3316 (but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
3320 In all other cases, <code>x.f</code> is illegal.
3324 If <code>x</code> is of pointer type and has the value
3325 <code>nil</code> and <code>x.f</code> denotes a struct field,
3326 assigning to or evaluating <code>x.f</code>
3327 causes a <a href="#Run_time_panics">run-time panic</a>.
3331 If <code>x</code> is of interface type and has the value
3332 <code>nil</code>, <a href="#Calls">calling</a> or
3333 <a href="#Method_values">evaluating</a> the method <code>x.f</code>
3334 causes a <a href="#Run_time_panics">run-time panic</a>.
3339 For example, given the declarations:
3365 var t T2 // with t.T0 != nil
3366 var p *T2 // with p != nil and (*p).T0 != nil
3383 q.x // (*(*q).T0).x (*q).x is a valid field selector
3385 p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
3386 p.M1() // ((*p).T1).M1() M1 expects T1 receiver
3387 p.M2() // p.M2() M2 expects *T2 receiver
3388 t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
3392 but the following is invalid:
3396 q.M0() // (*q).M0 is valid but not a field selector
3400 <h3 id="Method_expressions">Method expressions</h3>
3403 If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3404 <code>T.M</code> is a function that is callable as a regular function
3405 with the same arguments as <code>M</code> prefixed by an additional
3406 argument that is the receiver of the method.
3410 MethodExpr = ReceiverType "." MethodName .
3411 ReceiverType = Type .
3415 Consider a struct type <code>T</code> with two methods,
3416 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3417 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3424 func (tv T) Mv(a int) int { return 0 } // value receiver
3425 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3439 yields a function equivalent to <code>Mv</code> but
3440 with an explicit receiver as its first argument; it has signature
3444 func(tv T, a int) int
3448 That function may be called normally with an explicit receiver, so
3449 these five invocations are equivalent:
3456 f1 := T.Mv; f1(t, 7)
3457 f2 := (T).Mv; f2(t, 7)
3461 Similarly, the expression
3469 yields a function value representing <code>Mp</code> with signature
3473 func(tp *T, f float32) float32
3477 For a method with a value receiver, one can derive a function
3478 with an explicit pointer receiver, so
3486 yields a function value representing <code>Mv</code> with signature
3490 func(tv *T, a int) int
3494 Such a function indirects through the receiver to create a value
3495 to pass as the receiver to the underlying method;
3496 the method does not overwrite the value whose address is passed in
3501 The final case, a value-receiver function for a pointer-receiver method,
3502 is illegal because pointer-receiver methods are not in the method set
3507 Function values derived from methods are called with function call syntax;
3508 the receiver is provided as the first argument to the call.
3509 That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
3510 as <code>f(t, 7)</code> not <code>t.f(7)</code>.
3511 To construct a function that binds the receiver, use a
3512 <a href="#Function_literals">function literal</a> or
3513 <a href="#Method_values">method value</a>.
3517 It is legal to derive a function value from a method of an interface type.
3518 The resulting function takes an explicit receiver of that interface type.
3521 <h3 id="Method_values">Method values</h3>
3524 If the expression <code>x</code> has static type <code>T</code> and
3525 <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3526 <code>x.M</code> is called a <i>method value</i>.
3527 The method value <code>x.M</code> is a function value that is callable
3528 with the same arguments as a method call of <code>x.M</code>.
3529 The expression <code>x</code> is evaluated and saved during the evaluation of the
3530 method value; the saved copy is then used as the receiver in any calls,
3531 which may be executed later.
3535 type S struct { *T }
3537 func (t T) M() { print(t) }
3541 f := t.M // receiver *t is evaluated and stored in f
3542 g := s.M // receiver *(s.T) is evaluated and stored in g
3543 *t = 42 // does not affect stored receivers in f and g
3547 The type <code>T</code> may be an interface or non-interface type.
3551 As in the discussion of <a href="#Method_expressions">method expressions</a> above,
3552 consider a struct type <code>T</code> with two methods,
3553 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3554 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3561 func (tv T) Mv(a int) int { return 0 } // value receiver
3562 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3578 yields a function value of type
3586 These two invocations are equivalent:
3595 Similarly, the expression
3603 yields a function value of type
3607 func(float32) float32
3611 As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
3612 using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
3616 As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
3617 using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
3621 f := t.Mv; f(7) // like t.Mv(7)
3622 f := pt.Mp; f(7) // like pt.Mp(7)
3623 f := pt.Mv; f(7) // like (*pt).Mv(7)
3624 f := t.Mp; f(7) // like (&t).Mp(7)
3625 f := makeT().Mp // invalid: result of makeT() is not addressable
3629 Although the examples above use non-interface types, it is also legal to create a method value
3630 from a value of interface type.
3634 var i interface { M(int) } = myVal
3635 f := i.M; f(7) // like i.M(7)
3639 <h3 id="Index_expressions">Index expressions</h3>
3642 A primary expression of the form
3650 denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
3651 The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
3652 The following rules apply:
3656 If <code>a</code> is not a map:
3659 <li>the index <code>x</code> must be of integer type or an untyped constant</li>
3660 <li>a constant index must be non-negative and
3661 <a href="#Representability">representable</a> by a value of type <code>int</code></li>
3662 <li>a constant index that is untyped is given type <code>int</code></li>
3663 <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
3664 otherwise it is <i>out of range</i></li>
3668 For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
3671 <li>a <a href="#Constants">constant</a> index must be in range</li>
3672 <li>if <code>x</code> is out of range at run time,
3673 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3674 <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
3675 <code>a[x]</code> is the element type of <code>A</code></li>
3679 For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
3682 <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
3686 For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
3689 <li>if <code>x</code> is out of range at run time,
3690 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3691 <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
3692 <code>a[x]</code> is the element type of <code>S</code></li>
3696 For <code>a</code> of <a href="#String_types">string type</a>:
3699 <li>a <a href="#Constants">constant</a> index must be in range
3700 if the string <code>a</code> is also constant</li>
3701 <li>if <code>x</code> is out of range at run time,
3702 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3703 <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
3704 <code>a[x]</code> is <code>byte</code></li>
3705 <li><code>a[x]</code> may not be assigned to</li>
3709 For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
3712 <li><code>x</code>'s type must be
3713 <a href="#Assignability">assignable</a>
3714 to the key type of <code>M</code></li>
3715 <li>if the map contains an entry with key <code>x</code>,
3716 <code>a[x]</code> is the map element with key <code>x</code>
3717 and the type of <code>a[x]</code> is the element type of <code>M</code></li>
3718 <li>if the map is <code>nil</code> or does not contain such an entry,
3719 <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
3720 for the element type of <code>M</code></li>
3724 For <code>a</code> of <a href="#Type_parameters">type parameter type</a> <code>P</code>:
3727 <li><code>P</code> must have <a href="#Structure_of_interfaces">specific types</a>.</li>
3728 <li>The index expression <code>a[x]</code> must be valid for values
3729 of all specific types of <code>P</code>.</li>
3730 <li>The element types of all specific types of <code>P</code> must be identical.
3731 In this context, the element type of a string type is <code>byte</code>.</li>
3732 <li>If there is a map type among the specific types of <code>P</code>,
3733 all specific types must be map types, and the respective key types
3734 must be all identical.</li>
3735 <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
3736 or the map element with key <code>x</code> of the type argument
3737 that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
3738 the type of the (identical) element types.</li>
3739 <li><code>a[x]</code> may not be assigned to if the specific types of <code>P</code>
3740 include string types.
3744 Otherwise <code>a[x]</code> is illegal.
3748 An index expression on a map <code>a</code> of type <code>map[K]V</code>
3749 used in an <a href="#Assignments">assignment</a> or initialization of the special form
3759 yields an additional untyped boolean value. The value of <code>ok</code> is
3760 <code>true</code> if the key <code>x</code> is present in the map, and
3761 <code>false</code> otherwise.
3765 Assigning to an element of a <code>nil</code> map causes a
3766 <a href="#Run_time_panics">run-time panic</a>.
3770 <h3 id="Slice_expressions">Slice expressions</h3>
3773 Slice expressions construct a substring or slice from a string, array, pointer
3774 to array, or slice. There are two variants: a simple form that specifies a low
3775 and high bound, and a full form that also specifies a bound on the capacity.
3778 <h4>Simple slice expressions</h4>
3781 For a string, array, pointer to array, or slice <code>a</code>, the primary expression
3789 constructs a substring or slice. The <i>indices</i> <code>low</code> and
3790 <code>high</code> select which elements of operand <code>a</code> appear
3791 in the result. The result has indices starting at 0 and length equal to
3792 <code>high</code> - <code>low</code>.
3793 After slicing the array <code>a</code>
3797 a := [5]int{1, 2, 3, 4, 5}
3802 the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
3812 For convenience, any of the indices may be omitted. A missing <code>low</code>
3813 index defaults to zero; a missing <code>high</code> index defaults to the length of the
3818 a[2:] // same as a[2 : len(a)]
3819 a[:3] // same as a[0 : 3]
3820 a[:] // same as a[0 : len(a)]
3824 If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
3825 <code>(*a)[low : high]</code>.
3829 For arrays or strings, the indices are <i>in range</i> if
3830 <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
3831 otherwise they are <i>out of range</i>.
3832 For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
3833 A <a href="#Constants">constant</a> index must be non-negative and
3834 <a href="#Representability">representable</a> by a value of type
3835 <code>int</code>; for arrays or constant strings, constant indices must also be in range.
3836 If both indices are constant, they must satisfy <code>low <= high</code>.
3837 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3841 Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
3842 the result of the slice operation is a non-constant value of the same type as the operand.
3843 For untyped string operands the result is a non-constant value of type <code>string</code>.
3844 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
3845 and the result of the slice operation is a slice with the same element type as the array.
3849 If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
3850 is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
3851 array with the operand.
3856 s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
3857 s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
3858 s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
3862 <h4>Full slice expressions</h4>
3865 For an array, pointer to array, or slice <code>a</code> (but not a string), the primary expression
3873 constructs a slice of the same type, and with the same length and elements as the simple slice
3874 expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
3875 by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
3876 After slicing the array <code>a</code>
3880 a := [5]int{1, 2, 3, 4, 5}
3885 the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
3894 As for simple slice expressions, if <code>a</code> is a pointer to an array,
3895 <code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
3896 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
3900 The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
3901 otherwise they are <i>out of range</i>.
3902 A <a href="#Constants">constant</a> index must be non-negative and
3903 <a href="#Representability">representable</a> by a value of type
3904 <code>int</code>; for arrays, constant indices must also be in range.
3905 If multiple indices are constant, the constants that are present must be in range relative to each
3907 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3910 <h3 id="Type_assertions">Type assertions</h3>
3913 For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
3914 but not a <a href="#Type_parameters">type parameter</a>, and a type <code>T</code>,
3915 the primary expression
3923 asserts that <code>x</code> is not <code>nil</code>
3924 and that the value stored in <code>x</code> is of type <code>T</code>.
3925 The notation <code>x.(T)</code> is called a <i>type assertion</i>.
3928 More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
3929 that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
3930 to the type <code>T</code>.
3931 In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
3932 otherwise the type assertion is invalid since it is not possible for <code>x</code>
3933 to store a value of type <code>T</code>.
3934 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
3935 of <code>x</code> implements the interface <code>T</code>.
3938 If the type assertion holds, the value of the expression is the value
3939 stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
3940 a <a href="#Run_time_panics">run-time panic</a> occurs.
3941 In other words, even though the dynamic type of <code>x</code>
3942 is known only at run time, the type of <code>x.(T)</code> is
3943 known to be <code>T</code> in a correct program.
3947 var x interface{} = 7 // x has dynamic type int and value 7
3948 i := x.(int) // i has type int and value 7
3950 type I interface { m() }
3953 s := y.(string) // illegal: string does not implement I (missing method m)
3954 r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
3960 A type assertion used in an <a href="#Assignments">assignment</a> or initialization of the special form
3967 var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
3971 yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
3972 if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
3973 the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
3974 No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
3978 <h3 id="Calls">Calls</h3>
3981 Given an expression <code>f</code> of function type
3990 calls <code>f</code> with arguments <code>a1, a2, … an</code>.
3991 Except for one special case, arguments must be single-valued expressions
3992 <a href="#Assignability">assignable</a> to the parameter types of
3993 <code>F</code> and are evaluated before the function is called.
3994 The type of the expression is the result type
3996 A method invocation is similar but the method itself
3997 is specified as a selector upon a value of the receiver type for
4002 math.Atan2(x, y) // function call
4004 pt.Scale(3.5) // method call with receiver pt
4008 If <code>f</code> denotes a parameterized function, it must be
4009 <a href="#Instantiations">instantiated</a> before it can be called
4010 or used as a function value.
4014 In a function call, the function value and arguments are evaluated in
4015 <a href="#Order_of_evaluation">the usual order</a>.
4016 After they are evaluated, the parameters of the call are passed by value to the function
4017 and the called function begins execution.
4018 The return parameters of the function are passed by value
4019 back to the caller when the function returns.
4023 Calling a <code>nil</code> function value
4024 causes a <a href="#Run_time_panics">run-time panic</a>.
4028 As a special case, if the return values of a function or method
4029 <code>g</code> are equal in number and individually
4030 assignable to the parameters of another function or method
4031 <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
4032 will invoke <code>f</code> after binding the return values of
4033 <code>g</code> to the parameters of <code>f</code> in order. The call
4034 of <code>f</code> must contain no parameters other than the call of <code>g</code>,
4035 and <code>g</code> must have at least one return value.
4036 If <code>f</code> has a final <code>...</code> parameter, it is
4037 assigned the return values of <code>g</code> that remain after
4038 assignment of regular parameters.
4042 func Split(s string, pos int) (string, string) {
4043 return s[0:pos], s[pos:]
4046 func Join(s, t string) string {
4050 if Join(Split(value, len(value)/2)) != value {
4051 log.Panic("test fails")
4056 A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
4057 of (the type of) <code>x</code> contains <code>m</code> and the
4058 argument list can be assigned to the parameter list of <code>m</code>.
4059 If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
4060 set contains <code>m</code>, <code>x.m()</code> is shorthand
4061 for <code>(&x).m()</code>:
4070 There is no distinct method type and there are no method literals.
4073 <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
4076 If <code>f</code> is <a href="#Function_types">variadic</a> with a final
4077 parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
4078 the type of <code>p</code> is equivalent to type <code>[]T</code>.
4079 If <code>f</code> is invoked with no actual arguments for <code>p</code>,
4080 the value passed to <code>p</code> is <code>nil</code>.
4081 Otherwise, the value passed is a new slice
4082 of type <code>[]T</code> with a new underlying array whose successive elements
4083 are the actual arguments, which all must be <a href="#Assignability">assignable</a>
4084 to <code>T</code>. The length and capacity of the slice is therefore
4085 the number of arguments bound to <code>p</code> and may differ for each
4090 Given the function and calls
4093 func Greeting(prefix string, who ...string)
4095 Greeting("hello:", "Joe", "Anna", "Eileen")
4099 within <code>Greeting</code>, <code>who</code> will have the value
4100 <code>nil</code> in the first call, and
4101 <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
4105 If the final argument is assignable to a slice type <code>[]T</code> and
4106 is followed by <code>...</code>, it is passed unchanged as the value
4107 for a <code>...T</code> parameter. In this case no new slice is created.
4111 Given the slice <code>s</code> and call
4115 s := []string{"James", "Jasmine"}
4116 Greeting("goodbye:", s...)
4120 within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
4121 with the same underlying array.
4124 <h3 id="Instantiations">Instantiations</h3>
4127 A parameterized function or type is <i>instantiated</i> by substituting type arguments
4128 for the type parameters.
4129 Instantiation proceeds in two phases:
4134 Each type argument is substituted for its corresponding type parameter in the parameterized
4136 This substitution happens across the entire function or type declaration,
4137 including the type parameter list itself and any types in that list.
4141 After substitution, each type argument must <a href="#Interface_types">implement</a>
4142 the <a href="#Type_parameter_lists">constraint</a> (instantiated, if necessary)
4143 of the corresponding type parameter. Otherwise instantiation fails.
4148 Instantiating a type results in a new non-parameterized <a href="#Types">named type</a>;
4149 instantiating a function produces a new non-parameterized function.
4153 type parameter list type arguments after substitution
4155 [P any] int [int any]
4156 [S ~[]E, E any] []int, int [[]int ~[]int, int any]
4157 [P io.Writer] string [string io.Writer] // illegal: string doesn't implement io.Writer
4161 Type arguments may be provided explicitly, or they may be partially or completely
4162 <a href="#Type_inference">inferred</a>.
4163 A partially provided type argument list cannot be empty; there must be at least the
4168 type T[P1 ~int, P2 ~[]P1] struct{ … }
4170 T[] // illegal: at least the first type argument must be present, even if it could be inferred
4171 T[int] // argument for P1 explicitly provided, argument for P2 inferred
4172 T[int, []int] // both arguments explicitly provided
4176 A partial type argument list specifies a prefix of the full list of type arguments, leaving
4177 the remaining arguments to be inferred. Loosely speaking, type arguments may be omitted from
4182 Parameterized types, and parameterized functions that are not <a href="#Calls">called</a>,
4183 require a type argument list for instantiation; if the list is partial, all
4184 remaining type arguments must be inferrable.
4185 Calls to parameterized functions may provide a (possibly partial) type
4186 argument list, or may omit it entirely if the omitted type arguments are
4187 inferrable from the ordinary (non-type) function arguments.
4191 func min[T constraints.Ordered](x, y T) T { … }
4193 f := min // illegal: min must be instantiated when used without being called
4194 minInt := min[int] // minInt has type func(x, y int) int
4195 a := minInt(2, 3) // a has value 2 of type int
4196 b := min[float64](2.0, 3) // b has value 2.0 of type float64
4197 c := min(b, -1) // c has value -1.0 of type float64
4200 <h3 id="Operators">Operators</h3>
4203 Operators combine operands into expressions.
4207 Expression = UnaryExpr | Expression binary_op Expression .
4208 UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
4210 binary_op = "||" | "&&" | rel_op | add_op | mul_op .
4211 rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
4212 add_op = "+" | "-" | "|" | "^" .
4213 mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
4215 unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
4219 Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
4220 For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
4221 unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
4222 For operations involving constants only, see the section on
4223 <a href="#Constant_expressions">constant expressions</a>.
4227 Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
4228 and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
4229 to the type of the other operand.
4233 The right operand in a shift expression must have integer type
4234 or be an untyped constant <a href="#Representability">representable</a> by a
4235 value of type <code>uint</code>.
4236 If the left operand of a non-constant shift expression is an untyped constant,
4237 it is first implicitly converted to the type it would assume if the shift expression were
4238 replaced by its left operand alone.
4245 // The results of the following examples are given for 64-bit ints.
4246 var i = 1<<s // 1 has type int
4247 var j int32 = 1<<s // 1 has type int32; j == 0
4248 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
4249 var m int = 1.0<<s // 1.0 has type int; m == 1<<33
4250 var n = 1.0<<s == j // 1.0 has type int32; n == true
4251 var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
4252 var p = 1<<s == 1<<33 // 1 has type int; p == true
4253 var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
4254 var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
4255 var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
4256 var v float32 = 1<<s // illegal: 1 has type float32, cannot shift
4257 var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
4258 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
4259 var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
4261 // The results of the following examples are given for 32-bit ints,
4262 // which means the shifts will overflow.
4263 var mm int = 1.0<<s // 1.0 has type int; mm == 0
4264 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
4265 var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
4266 var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
4267 var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
4270 <h4 id="Operator_precedence">Operator precedence</h4>
4272 Unary operators have the highest precedence.
4273 As the <code>++</code> and <code>--</code> operators form
4274 statements, not expressions, they fall
4275 outside the operator hierarchy.
4276 As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
4278 There are five precedence levels for binary operators.
4279 Multiplication operators bind strongest, followed by addition
4280 operators, comparison operators, <code>&&</code> (logical AND),
4281 and finally <code>||</code> (logical OR):
4284 <pre class="grammar">
4286 5 * / % << >> & &^
4288 3 == != < <= > >=
4294 Binary operators of the same precedence associate from left to right.
4295 For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
4304 x == y+1 && <-chanInt > 0
4308 <h3 id="Arithmetic_operators">Arithmetic operators</h3>
4310 Arithmetic operators apply to numeric values and yield a result of the same
4311 type as the first operand. The four standard arithmetic operators (<code>+</code>,
4312 <code>-</code>, <code>*</code>, <code>/</code>) apply to integer,
4313 floating-point, and complex types; <code>+</code> also applies to strings.
4314 The bitwise logical and shift operators apply to integers only.
4317 <pre class="grammar">
4318 + sum integers, floats, complex values, strings
4319 - difference integers, floats, complex values
4320 * product integers, floats, complex values
4321 / quotient integers, floats, complex values
4322 % remainder integers
4324 & bitwise AND integers
4325 | bitwise OR integers
4326 ^ bitwise XOR integers
4327 &^ bit clear (AND NOT) integers
4329 << left shift integer << integer >= 0
4330 >> right shift integer >> integer >= 0
4334 <h4 id="Integer_operators">Integer operators</h4>
4337 For two integer values <code>x</code> and <code>y</code>, the integer quotient
4338 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
4343 x = q*y + r and |r| < |y|
4347 with <code>x / y</code> truncated towards zero
4348 (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
4360 The one exception to this rule is that if the dividend <code>x</code> is
4361 the most negative value for the int type of <code>x</code>, the quotient
4362 <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
4363 due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
4371 int64 -9223372036854775808
4375 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
4376 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4377 If the dividend is non-negative and the divisor is a constant power of 2,
4378 the division may be replaced by a right shift, and computing the remainder may
4379 be replaced by a bitwise AND operation:
4383 x x / 4 x % 4 x >> 2 x & 3
4389 The shift operators shift the left operand by the shift count specified by the
4390 right operand, which must be non-negative. If the shift count is negative at run time,
4391 a <a href="#Run_time_panics">run-time panic</a> occurs.
4392 The shift operators implement arithmetic shifts if the left operand is a signed
4393 integer and logical shifts if it is an unsigned integer.
4394 There is no upper limit on the shift count. Shifts behave
4395 as if the left operand is shifted <code>n</code> times by 1 for a shift
4396 count of <code>n</code>.
4397 As a result, <code>x << 1</code> is the same as <code>x*2</code>
4398 and <code>x >> 1</code> is the same as
4399 <code>x/2</code> but truncated towards negative infinity.
4403 For integer operands, the unary operators
4404 <code>+</code>, <code>-</code>, and <code>^</code> are defined as
4408 <pre class="grammar">
4410 -x negation is 0 - x
4411 ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
4412 and m = -1 for signed x
4416 <h4 id="Integer_overflow">Integer overflow</h4>
4419 For unsigned integer values, the operations <code>+</code>,
4420 <code>-</code>, <code>*</code>, and <code><<</code> are
4421 computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
4422 the <a href="#Numeric_types">unsigned integer</a>'s type.
4423 Loosely speaking, these unsigned integer operations
4424 discard high bits upon overflow, and programs may rely on "wrap around".
4427 For signed integers, the operations <code>+</code>,
4428 <code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
4429 overflow and the resulting value exists and is deterministically defined
4430 by the signed integer representation, the operation, and its operands.
4431 Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
4432 A compiler may not optimize code under the assumption that overflow does
4433 not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
4437 <h4 id="Floating_point_operators">Floating-point operators</h4>
4440 For floating-point and complex numbers,
4441 <code>+x</code> is the same as <code>x</code>,
4442 while <code>-x</code> is the negation of <code>x</code>.
4443 The result of a floating-point or complex division by zero is not specified beyond the
4444 IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
4445 occurs is implementation-specific.
4449 An implementation may combine multiple floating-point operations into a single
4450 fused operation, possibly across statements, and produce a result that differs
4451 from the value obtained by executing and rounding the instructions individually.
4452 An explicit floating-point type <a href="#Conversions">conversion</a> rounds to
4453 the precision of the target type, preventing fusion that would discard that rounding.
4457 For instance, some architectures provide a "fused multiply and add" (FMA) instruction
4458 that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
4459 These examples show when a Go implementation can use that instruction:
4463 // FMA allowed for computing r, because x*y is not explicitly rounded:
4467 *p = x*y; r = *p + z
4468 r = x*y + float64(z)
4470 // FMA disallowed for computing r, because it would omit rounding of x*y:
4471 r = float64(x*y) + z
4472 r = z; r += float64(x*y)
4473 t = float64(x*y); r = t + z
4476 <h4 id="String_concatenation">String concatenation</h4>
4479 Strings can be concatenated using the <code>+</code> operator
4480 or the <code>+=</code> assignment operator:
4484 s := "hi" + string(c)
4485 s += " and good bye"
4489 String addition creates a new string by concatenating the operands.
4493 <h3 id="Comparison_operators">Comparison operators</h3>
4496 Comparison operators compare two operands and yield an untyped boolean value.
4499 <pre class="grammar">
4505 >= greater or equal
4509 In any comparison, the first operand
4510 must be <a href="#Assignability">assignable</a>
4511 to the type of the second operand, or vice versa.
4514 The equality operators <code>==</code> and <code>!=</code> apply
4515 to operands that are <i>comparable</i>.
4516 The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
4517 apply to operands that are <i>ordered</i>.
4518 These terms and the result of the comparisons are defined as follows:
4523 Boolean values are comparable.
4524 Two boolean values are equal if they are either both
4525 <code>true</code> or both <code>false</code>.
4529 Integer values are comparable and ordered, in the usual way.
4533 Floating-point values are comparable and ordered,
4534 as defined by the IEEE-754 standard.
4538 Complex values are comparable.
4539 Two complex values <code>u</code> and <code>v</code> are
4540 equal if both <code>real(u) == real(v)</code> and
4541 <code>imag(u) == imag(v)</code>.
4545 String values are comparable and ordered, lexically byte-wise.
4549 Pointer values are comparable.
4550 Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
4551 Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
4555 Channel values are comparable.
4556 Two channel values are equal if they were created by the same call to
4557 <a href="#Making_slices_maps_and_channels"><code>make</code></a>
4558 or if both have value <code>nil</code>.
4562 Interface values are comparable.
4563 Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
4564 and equal dynamic values or if both have value <code>nil</code>.
4568 A value <code>x</code> of non-interface type <code>X</code> and
4569 a value <code>t</code> of interface type <code>T</code> are comparable when values
4570 of type <code>X</code> are comparable and
4571 <code>X</code> implements <code>T</code>.
4572 They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
4573 and <code>t</code>'s dynamic value is equal to <code>x</code>.
4577 Struct values are comparable if all their fields are comparable.
4578 Two struct values are equal if their corresponding
4579 non-<a href="#Blank_identifier">blank</a> fields are equal.
4583 Array values are comparable if values of the array element type are comparable.
4584 Two array values are equal if their corresponding elements are equal.
4589 A comparison of two interface values with identical dynamic types
4590 causes a <a href="#Run_time_panics">run-time panic</a> if values
4591 of that type are not comparable. This behavior applies not only to direct interface
4592 value comparisons but also when comparing arrays of interface values
4593 or structs with interface-valued fields.
4597 Slice, map, and function values are not comparable.
4598 However, as a special case, a slice, map, or function value may
4599 be compared to the predeclared identifier <code>nil</code>.
4600 Comparison of pointer, channel, and interface values to <code>nil</code>
4601 is also allowed and follows from the general rules above.
4605 const c = 3 < 4 // c is the untyped boolean constant true
4610 // The result of a comparison is an untyped boolean.
4611 // The usual assignment rules apply.
4612 b3 = x == y // b3 has type bool
4613 b4 bool = x == y // b4 has type bool
4614 b5 MyBool = x == y // b5 has type MyBool
4618 <h3 id="Logical_operators">Logical operators</h3>
4621 Logical operators apply to <a href="#Boolean_types">boolean</a> values
4622 and yield a result of the same type as the operands.
4623 The right operand is evaluated conditionally.
4626 <pre class="grammar">
4627 && conditional AND p && q is "if p then q else false"
4628 || conditional OR p || q is "if p then true else q"
4633 <h3 id="Address_operators">Address operators</h3>
4636 For an operand <code>x</code> of type <code>T</code>, the address operation
4637 <code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
4638 The operand must be <i>addressable</i>,
4639 that is, either a variable, pointer indirection, or slice indexing
4640 operation; or a field selector of an addressable struct operand;
4641 or an array indexing operation of an addressable array.
4642 As an exception to the addressability requirement, <code>x</code> may also be a
4643 (possibly parenthesized)
4644 <a href="#Composite_literals">composite literal</a>.
4645 If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
4646 then the evaluation of <code>&x</code> does too.
4650 For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
4651 indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
4652 to by <code>x</code>.
4653 If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
4654 will cause a <a href="#Run_time_panics">run-time panic</a>.
4665 *x // causes a run-time panic
4666 &*x // causes a run-time panic
4670 <h3 id="Receive_operator">Receive operator</h3>
4673 For an operand <code>ch</code> of <a href="#Channel_types">channel type</a>,
4674 the value of the receive operation <code><-ch</code> is the value received
4675 from the channel <code>ch</code>. The channel direction must permit receive operations,
4676 and the type of the receive operation is the element type of the channel.
4677 The expression blocks until a value is available.
4678 Receiving from a <code>nil</code> channel blocks forever.
4679 A receive operation on a <a href="#Close">closed</a> channel can always proceed
4680 immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
4681 after any previously sent values have been received.
4688 <-strobe // wait until clock pulse and discard received value
4692 A receive expression used in an <a href="#Assignments">assignment</a> or initialization of the special form
4699 var x, ok T = <-ch
4703 yields an additional untyped boolean result reporting whether the
4704 communication succeeded. The value of <code>ok</code> is <code>true</code>
4705 if the value received was delivered by a successful send operation to the
4706 channel, or <code>false</code> if it is a zero value generated because the
4707 channel is closed and empty.
4711 <h3 id="Conversions">Conversions</h3>
4714 A conversion changes the <a href="#Types">type</a> of an expression
4715 to the type specified by the conversion.
4716 A conversion may appear literally in the source, or it may be <i>implied</i>
4717 by the context in which an expression appears.
4721 An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
4722 where <code>T</code> is a type and <code>x</code> is an expression
4723 that can be converted to type <code>T</code>.
4727 Conversion = Type "(" Expression [ "," ] ")" .
4731 If the type starts with the operator <code>*</code> or <code><-</code>,
4732 or if the type starts with the keyword <code>func</code>
4733 and has no result list, it must be parenthesized when
4734 necessary to avoid ambiguity:
4738 *Point(p) // same as *(Point(p))
4739 (*Point)(p) // p is converted to *Point
4740 <-chan int(c) // same as <-(chan int(c))
4741 (<-chan int)(c) // c is converted to <-chan int
4742 func()(x) // function signature func() x
4743 (func())(x) // x is converted to func()
4744 (func() int)(x) // x is converted to func() int
4745 func() int(x) // x is converted to func() int (unambiguous)
4749 A <a href="#Constants">constant</a> value <code>x</code> can be converted to
4750 type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
4751 by a value of <code>T</code>.
4752 As a special case, an integer constant <code>x</code> can be explicitly converted to a
4753 <a href="#String_types">string type</a> using the
4754 <a href="#Conversions_to_and_from_a_string_type">same rule</a>
4755 as for non-constant <code>x</code>.
4759 Converting a constant to a type that is not a <a href="#Type_parameters">type parameter</a>
4760 yields a typed constant.
4761 Converting a constant to a type parameter yields a non-constant value of that type.
4765 uint(iota) // iota value of type uint
4766 float32(2.718281828) // 2.718281828 of type float32
4767 complex128(1) // 1.0 + 0.0i of type complex128
4768 float32(0.49999999) // 0.5 of type float32
4769 float64(-1e-1000) // 0.0 of type float64
4770 string('x') // "x" of type string
4771 string(0x266c) // "♬" of type string
4772 MyString("foo" + "bar") // "foobar" of type MyString
4773 string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
4774 (*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
4775 int(1.2) // illegal: 1.2 cannot be represented as an int
4776 string(65.0) // illegal: 65.0 is not an integer constant
4780 A non-constant value <code>x</code> can be converted to type <code>T</code>
4781 in any of these cases:
4786 <code>x</code> is <a href="#Assignability">assignable</a>
4790 ignoring struct tags (see below),
4791 <code>x</code>'s type and <code>T</code> are not
4792 <a href="#Type_parameters">type parameters</a> but have
4793 <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
4796 ignoring struct tags (see below),
4797 <code>x</code>'s type and <code>T</code> are pointer types
4798 that are not <a href="#Types">named types</a>,
4799 and their pointer base types are not type parameters but
4800 have identical underlying types.
4803 <code>x</code>'s type and <code>T</code> are both integer or floating
4807 <code>x</code>'s type and <code>T</code> are both complex types.
4810 <code>x</code> is an integer or a slice of bytes or runes
4811 and <code>T</code> is a string type.
4814 <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
4817 <code>x</code> is a slice, <code>T</code> is a pointer to an array,
4818 and the slice and array types have <a href="#Type_identity">identical</a> element types.
4823 Additionally, if <code>T</code> or </code><code>x's</code> type <code>V</code> are type
4824 parameters with <a href="#Structure_of_interfaces">specific types</a>, <code>x</code>
4825 can also be converted to type <code>T</code> if one of the following conditions applies:
4830 Both <code>V</code> and <code>T</code> are type parameters and a value of each
4831 specific type of <code>V</code> can be converted to each specific type
4835 Only <code>V</code> is a type parameter and a value of each
4836 specific type of <code>V</code> can be converted to <code>T</code>.
4839 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
4840 specific type of <code>T</code>.
4845 <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
4846 for identity for the purpose of conversion:
4850 type Person struct {
4859 Name string `json:"name"`
4861 Street string `json:"street"`
4862 City string `json:"city"`
4866 var person = (*Person)(data) // ignoring tags, the underlying types are identical
4870 Specific rules apply to (non-constant) conversions between numeric types or
4871 to and from a string type.
4872 These conversions may change the representation of <code>x</code>
4873 and incur a run-time cost.
4874 All other conversions only change the type but not the representation
4879 There is no linguistic mechanism to convert between pointers and integers.
4880 The package <a href="#Package_unsafe"><code>unsafe</code></a>
4881 implements this functionality under
4882 restricted circumstances.
4885 <h4>Conversions between numeric types</h4>
4888 For the conversion of non-constant numeric values, the following rules apply:
4893 When converting between integer types, if the value is a signed integer, it is
4894 sign extended to implicit infinite precision; otherwise it is zero extended.
4895 It is then truncated to fit in the result type's size.
4896 For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
4897 The conversion always yields a valid value; there is no indication of overflow.
4900 When converting a floating-point number to an integer, the fraction is discarded
4901 (truncation towards zero).
4904 When converting an integer or floating-point number to a floating-point type,
4905 or a complex number to another complex type, the result value is rounded
4906 to the precision specified by the destination type.
4907 For instance, the value of a variable <code>x</code> of type <code>float32</code>
4908 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
4909 but float32(x) represents the result of rounding <code>x</code>'s value to
4910 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
4911 of precision, but <code>float32(x + 0.1)</code> does not.
4916 In all non-constant conversions involving floating-point or complex values,
4917 if the result type cannot represent the value the conversion
4918 succeeds but the result value is implementation-dependent.
4921 <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
4925 Converting a signed or unsigned integer value to a string type yields a
4926 string containing the UTF-8 representation of the integer. Values outside
4927 the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
4931 string(-1) // "\ufffd" == "\xef\xbf\xbd"
4932 string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
4933 type MyString string
4934 MyString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
4939 Converting a slice of bytes to a string type yields
4940 a string whose successive bytes are the elements of the slice.
4943 string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
4944 string([]byte{}) // ""
4945 string([]byte(nil)) // ""
4948 string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
4953 Converting a slice of runes to a string type yields
4954 a string that is the concatenation of the individual rune values
4955 converted to strings.
4958 string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
4959 string([]rune{}) // ""
4960 string([]rune(nil)) // ""
4963 string(MyRunes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
4968 Converting a value of a string type to a slice of bytes type
4969 yields a slice whose successive elements are the bytes of the string.
4972 []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
4973 []byte("") // []byte{}
4975 MyBytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
4980 Converting a value of a string type to a slice of runes type
4981 yields a slice containing the individual Unicode code points of the string.
4984 []rune(MyString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
4985 []rune("") // []rune{}
4987 MyRunes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
4992 <h4 id="Conversions_from_slice_to_array_pointer">Conversions from slice to array pointer</h4>
4995 Converting a slice to an array pointer yields a pointer to the underlying array of the slice.
4996 If the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
4997 a <a href="#Run_time_panics">run-time panic</a> occurs.
5001 s := make([]byte, 2, 4)
5002 s0 := (*[0]byte)(s) // s0 != nil
5003 s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
5004 s2 := (*[2]byte)(s) // &s2[0] == &s[0]
5005 s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
5008 t0 := (*[0]string)(t) // t0 == nil
5009 t1 := (*[1]string)(t) // panics: len([1]string) > len(t)
5011 u := make([]byte, 0)
5012 u0 := (*[0]byte)(u) // u0 != nil
5015 <h3 id="Constant_expressions">Constant expressions</h3>
5018 Constant expressions may contain only <a href="#Constants">constant</a>
5019 operands and are evaluated at compile time.
5023 Untyped boolean, numeric, and string constants may be used as operands
5024 wherever it is legal to use an operand of boolean, numeric, or string type,
5029 A constant <a href="#Comparison_operators">comparison</a> always yields
5030 an untyped boolean constant. If the left operand of a constant
5031 <a href="#Operators">shift expression</a> is an untyped constant, the
5032 result is an integer constant; otherwise it is a constant of the same
5033 type as the left operand, which must be of
5034 <a href="#Numeric_types">integer type</a>.
5038 Any other operation on untyped constants results in an untyped constant of the
5039 same kind; that is, a boolean, integer, floating-point, complex, or string
5041 If the untyped operands of a binary operation (other than a shift) are of
5042 different kinds, the result is of the operand's kind that appears later in this
5043 list: integer, rune, floating-point, complex.
5044 For example, an untyped integer constant divided by an
5045 untyped complex constant yields an untyped complex constant.
5049 const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
5050 const b = 15 / 4 // b == 3 (untyped integer constant)
5051 const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
5052 const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
5053 const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
5054 const d = 1 << 3.0 // d == 8 (untyped integer constant)
5055 const e = 1.0 << 3 // e == 8 (untyped integer constant)
5056 const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
5057 const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
5058 const h = "foo" > "bar" // h == true (untyped boolean constant)
5059 const j = true // j == true (untyped boolean constant)
5060 const k = 'w' + 1 // k == 'x' (untyped rune constant)
5061 const l = "hi" // l == "hi" (untyped string constant)
5062 const m = string(k) // m == "x" (type string)
5063 const Σ = 1 - 0.707i // (untyped complex constant)
5064 const Δ = Σ + 2.0e-4 // (untyped complex constant)
5065 const Φ = iota*1i - 1/1i // (untyped complex constant)
5069 Applying the built-in function <code>complex</code> to untyped
5070 integer, rune, or floating-point constants yields
5071 an untyped complex constant.
5075 const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
5076 const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
5080 Constant expressions are always evaluated exactly; intermediate values and the
5081 constants themselves may require precision significantly larger than supported
5082 by any predeclared type in the language. The following are legal declarations:
5086 const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
5087 const Four int8 = Huge >> 98 // Four == 4 (type int8)
5091 The divisor of a constant division or remainder operation must not be zero:
5095 3.14 / 0.0 // illegal: division by zero
5099 The values of <i>typed</i> constants must always be accurately
5100 <a href="#Representability">representable</a> by values
5101 of the constant type. The following constant expressions are illegal:
5105 uint(-1) // -1 cannot be represented as a uint
5106 int(3.14) // 3.14 cannot be represented as an int
5107 int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
5108 Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
5109 Four * 100 // product 400 cannot be represented as an int8 (type of Four)
5113 The mask used by the unary bitwise complement operator <code>^</code> matches
5114 the rule for non-constants: the mask is all 1s for unsigned constants
5115 and -1 for signed and untyped constants.
5119 ^1 // untyped integer constant, equal to -2
5120 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
5121 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
5122 int8(^1) // same as int8(-2)
5123 ^int8(1) // same as -1 ^ int8(1) = -2
5127 Implementation restriction: A compiler may use rounding while
5128 computing untyped floating-point or complex constant expressions; see
5129 the implementation restriction in the section
5130 on <a href="#Constants">constants</a>. This rounding may cause a
5131 floating-point constant expression to be invalid in an integer
5132 context, even if it would be integral when calculated using infinite
5133 precision, and vice versa.
5137 <h3 id="Order_of_evaluation">Order of evaluation</h3>
5140 At package level, <a href="#Package_initialization">initialization dependencies</a>
5141 determine the evaluation order of individual initialization expressions in
5142 <a href="#Variable_declarations">variable declarations</a>.
5143 Otherwise, when evaluating the <a href="#Operands">operands</a> of an
5144 expression, assignment, or
5145 <a href="#Return_statements">return statement</a>,
5146 all function calls, method calls, and
5147 communication operations are evaluated in lexical left-to-right
5152 For example, in the (function-local) assignment
5155 y[f()], ok = g(h(), i()+x[j()], <-c), k()
5158 the function calls and communication happen in the order
5159 <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
5160 <code><-c</code>, <code>g()</code>, and <code>k()</code>.
5161 However, the order of those events compared to the evaluation
5162 and indexing of <code>x</code> and the evaluation
5163 of <code>y</code> is not specified.
5168 f := func() int { a++; return a }
5169 x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
5170 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
5171 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
5175 At package level, initialization dependencies override the left-to-right rule
5176 for individual initialization expressions, but not for operands within each
5181 var a, b, c = f() + v(), g(), sqr(u()) + v()
5183 func f() int { return c }
5184 func g() int { return a }
5185 func sqr(x int) int { return x*x }
5187 // functions u and v are independent of all other variables and functions
5191 The function calls happen in the order
5192 <code>u()</code>, <code>sqr()</code>, <code>v()</code>,
5193 <code>f()</code>, <code>v()</code>, and <code>g()</code>.
5197 Floating-point operations within a single expression are evaluated according to
5198 the associativity of the operators. Explicit parentheses affect the evaluation
5199 by overriding the default associativity.
5200 In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
5201 is performed before adding <code>x</code>.
5204 <h2 id="Statements">Statements</h2>
5207 Statements control execution.
5212 Declaration | LabeledStmt | SimpleStmt |
5213 GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
5214 FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
5217 SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
5220 <h3 id="Terminating_statements">Terminating statements</h3>
5223 A <i>terminating statement</i> interrupts the regular flow of control in
5224 a <a href="#Blocks">block</a>. The following statements are terminating:
5229 A <a href="#Return_statements">"return"</a> or
5230 <a href="#Goto_statements">"goto"</a> statement.
5231 <!-- ul below only for regular layout -->
5236 A call to the built-in function
5237 <a href="#Handling_panics"><code>panic</code></a>.
5238 <!-- ul below only for regular layout -->
5243 A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
5244 <!-- ul below only for regular layout -->
5249 An <a href="#If_statements">"if" statement</a> in which:
5251 <li>the "else" branch is present, and</li>
5252 <li>both branches are terminating statements.</li>
5257 A <a href="#For_statements">"for" statement</a> in which:
5259 <li>there are no "break" statements referring to the "for" statement, and</li>
5260 <li>the loop condition is absent, and</li>
5261 <li>the "for" statement does not use a range clause.</li>
5266 A <a href="#Switch_statements">"switch" statement</a> in which:
5268 <li>there are no "break" statements referring to the "switch" statement,</li>
5269 <li>there is a default case, and</li>
5270 <li>the statement lists in each case, including the default, end in a terminating
5271 statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
5277 A <a href="#Select_statements">"select" statement</a> in which:
5279 <li>there are no "break" statements referring to the "select" statement, and</li>
5280 <li>the statement lists in each case, including the default if present,
5281 end in a terminating statement.</li>
5286 A <a href="#Labeled_statements">labeled statement</a> labeling
5287 a terminating statement.
5292 All other statements are not terminating.
5296 A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
5297 is not empty and its final non-empty statement is terminating.
5301 <h3 id="Empty_statements">Empty statements</h3>
5304 The empty statement does nothing.
5312 <h3 id="Labeled_statements">Labeled statements</h3>
5315 A labeled statement may be the target of a <code>goto</code>,
5316 <code>break</code> or <code>continue</code> statement.
5320 LabeledStmt = Label ":" Statement .
5321 Label = identifier .
5325 Error: log.Panic("error encountered")
5329 <h3 id="Expression_statements">Expression statements</h3>
5332 With the exception of specific built-in functions,
5333 function and method <a href="#Calls">calls</a> and
5334 <a href="#Receive_operator">receive operations</a>
5335 can appear in statement context. Such statements may be parenthesized.
5339 ExpressionStmt = Expression .
5343 The following built-in functions are not permitted in statement context:
5347 append cap complex imag len make new real
5348 unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice
5356 len("foo") // illegal if len is the built-in function
5360 <h3 id="Send_statements">Send statements</h3>
5363 A send statement sends a value on a channel.
5364 The channel expression must be of <a href="#Channel_types">channel type</a>,
5365 the channel direction must permit send operations,
5366 and the type of the value to be sent must be <a href="#Assignability">assignable</a>
5367 to the channel's element type.
5371 SendStmt = Channel "<-" Expression .
5372 Channel = Expression .
5376 Both the channel and the value expression are evaluated before communication
5377 begins. Communication blocks until the send can proceed.
5378 A send on an unbuffered channel can proceed if a receiver is ready.
5379 A send on a buffered channel can proceed if there is room in the buffer.
5380 A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
5381 A send on a <code>nil</code> channel blocks forever.
5385 ch <- 3 // send value 3 to channel ch
5389 <h3 id="IncDec_statements">IncDec statements</h3>
5392 The "++" and "--" statements increment or decrement their operands
5393 by the untyped <a href="#Constants">constant</a> <code>1</code>.
5394 As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
5395 or a map index expression.
5399 IncDecStmt = Expression ( "++" | "--" ) .
5403 The following <a href="#Assignments">assignment statements</a> are semantically
5407 <pre class="grammar">
5408 IncDec statement Assignment
5414 <h3 id="Assignments">Assignments</h3>
5417 Assignment = ExpressionList assign_op ExpressionList .
5419 assign_op = [ add_op | mul_op ] "=" .
5423 Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
5424 a map index expression, or (for <code>=</code> assignments only) the
5425 <a href="#Blank_identifier">blank identifier</a>.
5426 Operands may be parenthesized.
5433 (k) = <-ch // same as: k = <-ch
5437 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
5438 <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
5439 is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
5440 <code>(y)</code> but evaluates <code>x</code>
5441 only once. The <i>op</i><code>=</code> construct is a single token.
5442 In assignment operations, both the left- and right-hand expression lists
5443 must contain exactly one single-valued expression, and the left-hand
5444 expression must not be the blank identifier.
5449 i &^= 1<<n
5453 A tuple assignment assigns the individual elements of a multi-valued
5454 operation to a list of variables. There are two forms. In the
5455 first, the right hand operand is a single multi-valued expression
5456 such as a function call, a <a href="#Channel_types">channel</a> or
5457 <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
5458 The number of operands on the left
5459 hand side must match the number of values. For instance, if
5460 <code>f</code> is a function returning two values,
5468 assigns the first value to <code>x</code> and the second to <code>y</code>.
5469 In the second form, the number of operands on the left must equal the number
5470 of expressions on the right, each of which must be single-valued, and the
5471 <i>n</i>th expression on the right is assigned to the <i>n</i>th
5472 operand on the left:
5476 one, two, three = '一', '二', '三'
5480 The <a href="#Blank_identifier">blank identifier</a> provides a way to
5481 ignore right-hand side values in an assignment:
5485 _ = x // evaluate x but ignore it
5486 x, _ = f() // evaluate f() but ignore second result value
5490 The assignment proceeds in two phases.
5491 First, the operands of <a href="#Index_expressions">index expressions</a>
5492 and <a href="#Address_operators">pointer indirections</a>
5493 (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
5494 on the left and the expressions on the right are all
5495 <a href="#Order_of_evaluation">evaluated in the usual order</a>.
5496 Second, the assignments are carried out in left-to-right order.
5500 a, b = b, a // exchange a and b
5504 i, x[i] = 1, 2 // set i = 1, x[0] = 2
5507 x[i], i = 2, 1 // set x[0] = 2, i = 1
5509 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
5511 x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
5513 type Point struct { x, y int }
5515 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
5519 for i, x[i] = range x { // set i, x[2] = 0, x[0]
5522 // after this loop, i == 0 and x == []int{3, 5, 3}
5526 In assignments, each value must be <a href="#Assignability">assignable</a>
5527 to the type of the operand to which it is assigned, with the following special cases:
5532 Any typed value may be assigned to the blank identifier.
5536 If an untyped constant
5537 is assigned to a variable of interface type or the blank identifier,
5538 the constant is first implicitly <a href="#Conversions">converted</a> to its
5539 <a href="#Constants">default type</a>.
5543 If an untyped boolean value is assigned to a variable of interface type or
5544 the blank identifier, it is first implicitly converted to type <code>bool</code>.
5548 <h3 id="If_statements">If statements</h3>
5551 "If" statements specify the conditional execution of two branches
5552 according to the value of a boolean expression. If the expression
5553 evaluates to true, the "if" branch is executed, otherwise, if
5554 present, the "else" branch is executed.
5558 IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
5568 The expression may be preceded by a simple statement, which
5569 executes before the expression is evaluated.
5573 if x := f(); x < y {
5575 } else if x > z {
5583 <h3 id="Switch_statements">Switch statements</h3>
5586 "Switch" statements provide multi-way execution.
5587 An expression or type is compared to the "cases"
5588 inside the "switch" to determine which branch
5593 SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
5597 There are two forms: expression switches and type switches.
5598 In an expression switch, the cases contain expressions that are compared
5599 against the value of the switch expression.
5600 In a type switch, the cases contain types that are compared against the
5601 type of a specially annotated switch expression.
5602 The switch expression is evaluated exactly once in a switch statement.
5605 <h4 id="Expression_switches">Expression switches</h4>
5608 In an expression switch,
5609 the switch expression is evaluated and
5610 the case expressions, which need not be constants,
5611 are evaluated left-to-right and top-to-bottom; the first one that equals the
5613 triggers execution of the statements of the associated case;
5614 the other cases are skipped.
5615 If no case matches and there is a "default" case,
5616 its statements are executed.
5617 There can be at most one default case and it may appear anywhere in the
5619 A missing switch expression is equivalent to the boolean value
5624 ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
5625 ExprCaseClause = ExprSwitchCase ":" StatementList .
5626 ExprSwitchCase = "case" ExpressionList | "default" .
5630 If the switch expression evaluates to an untyped constant, it is first implicitly
5631 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
5632 The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
5633 The switch expression type must be <a href="#Comparison_operators">comparable</a>.
5637 If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
5638 to the type of the switch expression.
5639 For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
5640 of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
5644 In other words, the switch expression is treated as if it were used to declare and
5645 initialize a temporary variable <code>t</code> without explicit type; it is that
5646 value of <code>t</code> against which each case expression <code>x</code> is tested
5651 In a case or default clause, the last non-empty statement
5652 may be a (possibly <a href="#Labeled_statements">labeled</a>)
5653 <a href="#Fallthrough_statements">"fallthrough" statement</a> to
5654 indicate that control should flow from the end of this clause to
5655 the first statement of the next clause.
5656 Otherwise control flows to the end of the "switch" statement.
5657 A "fallthrough" statement may appear as the last statement of all
5658 but the last clause of an expression switch.
5662 The switch expression may be preceded by a simple statement, which
5663 executes before the expression is evaluated.
5669 case 0, 1, 2, 3: s1()
5670 case 4, 5, 6, 7: s2()
5673 switch x := f(); { // missing switch expression means "true"
5674 case x < 0: return -x
5686 Implementation restriction: A compiler may disallow multiple case
5687 expressions evaluating to the same constant.
5688 For instance, the current compilers disallow duplicate integer,
5689 floating point, or string constants in case expressions.
5692 <h4 id="Type_switches">Type switches</h4>
5695 A type switch compares types rather than values. It is otherwise similar
5696 to an expression switch. It is marked by a special switch expression that
5697 has the form of a <a href="#Type_assertions">type assertion</a>
5698 using the keyword <code>type</code> rather than an actual type:
5708 Cases then match actual types <code>T</code> against the dynamic type of the
5709 expression <code>x</code>. As with type assertions, <code>x</code> must be of
5710 <a href="#Interface_types">interface type</a>, but not a
5711 <a href="#Type_parameters">type parameter</a>, and each non-interface type
5712 <code>T</code> listed in a case must implement the type of <code>x</code>.
5713 The types listed in the cases of a type switch must all be
5714 <a href="#Type_identity">different</a>.
5718 TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
5719 TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
5720 TypeCaseClause = TypeSwitchCase ":" StatementList .
5721 TypeSwitchCase = "case" TypeList | "default" .
5725 The TypeSwitchGuard may include a
5726 <a href="#Short_variable_declarations">short variable declaration</a>.
5727 When that form is used, the variable is declared at the end of the
5728 TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
5729 In clauses with a case listing exactly one type, the variable
5730 has that type; otherwise, the variable has the type of the expression
5731 in the TypeSwitchGuard.
5735 Instead of a type, a case may use the predeclared identifier
5736 <a href="#Predeclared_identifiers"><code>nil</code></a>;
5737 that case is selected when the expression in the TypeSwitchGuard
5738 is a <code>nil</code> interface value.
5739 There may be at most one <code>nil</code> case.
5743 Given an expression <code>x</code> of type <code>interface{}</code>,
5744 the following type switch:
5748 switch i := x.(type) {
5750 printString("x is nil") // type of i is type of x (interface{})
5752 printInt(i) // type of i is int
5754 printFloat64(i) // type of i is float64
5755 case func(int) float64:
5756 printFunction(i) // type of i is func(int) float64
5758 printString("type is bool or string") // type of i is type of x (interface{})
5760 printString("don't know the type") // type of i is type of x (interface{})
5769 v := x // x is evaluated exactly once
5771 i := v // type of i is type of x (interface{})
5772 printString("x is nil")
5773 } else if i, isInt := v.(int); isInt {
5774 printInt(i) // type of i is int
5775 } else if i, isFloat64 := v.(float64); isFloat64 {
5776 printFloat64(i) // type of i is float64
5777 } else if i, isFunc := v.(func(int) float64); isFunc {
5778 printFunction(i) // type of i is func(int) float64
5780 _, isBool := v.(bool)
5781 _, isString := v.(string)
5782 if isBool || isString {
5783 i := v // type of i is type of x (interface{})
5784 printString("type is bool or string")
5786 i := v // type of i is type of x (interface{})
5787 printString("don't know the type")
5793 The type switch guard may be preceded by a simple statement, which
5794 executes before the guard is evaluated.
5798 The "fallthrough" statement is not permitted in a type switch.
5801 <h3 id="For_statements">For statements</h3>
5804 A "for" statement specifies repeated execution of a block. There are three forms:
5805 The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
5809 ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
5810 Condition = Expression .
5813 <h4 id="For_condition">For statements with single condition</h4>
5816 In its simplest form, a "for" statement specifies the repeated execution of
5817 a block as long as a boolean condition evaluates to true.
5818 The condition is evaluated before each iteration.
5819 If the condition is absent, it is equivalent to the boolean value
5829 <h4 id="For_clause">For statements with <code>for</code> clause</h4>
5832 A "for" statement with a ForClause is also controlled by its condition, but
5833 additionally it may specify an <i>init</i>
5834 and a <i>post</i> statement, such as an assignment,
5835 an increment or decrement statement. The init statement may be a
5836 <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
5837 Variables declared by the init statement are re-used in each iteration.
5841 ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
5842 InitStmt = SimpleStmt .
5843 PostStmt = SimpleStmt .
5847 for i := 0; i < 10; i++ {
5853 If non-empty, the init statement is executed once before evaluating the
5854 condition for the first iteration;
5855 the post statement is executed after each execution of the block (and
5856 only if the block was executed).
5857 Any element of the ForClause may be empty but the
5858 <a href="#Semicolons">semicolons</a> are
5859 required unless there is only a condition.
5860 If the condition is absent, it is equivalent to the boolean value
5865 for cond { S() } is the same as for ; cond ; { S() }
5866 for { S() } is the same as for true { S() }
5869 <h4 id="For_range">For statements with <code>range</code> clause</h4>
5872 A "for" statement with a "range" clause
5873 iterates through all entries of an array, slice, string or map,
5874 or values received on a channel. For each entry it assigns <i>iteration values</i>
5875 to corresponding <i>iteration variables</i> if present and then executes the block.
5879 RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
5883 The expression on the right in the "range" clause is called the <i>range expression</i>,
5884 which may be an array, pointer to an array, slice, string, map, or channel permitting
5885 <a href="#Receive_operator">receive operations</a>.
5886 As with an assignment, if present the operands on the left must be
5887 <a href="#Address_operators">addressable</a> or map index expressions; they
5888 denote the iteration variables. If the range expression is a channel, at most
5889 one iteration variable is permitted, otherwise there may be up to two.
5890 If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
5891 the range clause is equivalent to the same clause without that identifier.
5895 The range expression <code>x</code> is evaluated once before beginning the loop,
5896 with one exception: if at most one iteration variable is present and
5897 <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
5898 the range expression is not evaluated.
5902 Function calls on the left are evaluated once per iteration.
5903 For each iteration, iteration values are produced as follows
5904 if the respective iteration variables are present:
5907 <pre class="grammar">
5908 Range expression 1st value 2nd value
5910 array or slice a [n]E, *[n]E, or []E index i int a[i] E
5911 string s string type index i int see below rune
5912 map m map[K]V key k K m[k] V
5913 channel c chan E, <-chan E element e E
5918 For an array, pointer to array, or slice value <code>a</code>, the index iteration
5919 values are produced in increasing order, starting at element index 0.
5920 If at most one iteration variable is present, the range loop produces
5921 iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
5922 or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
5926 For a string value, the "range" clause iterates over the Unicode code points
5927 in the string starting at byte index 0. On successive iterations, the index value will be the
5928 index of the first byte of successive UTF-8-encoded code points in the string,
5929 and the second value, of type <code>rune</code>, will be the value of
5930 the corresponding code point. If the iteration encounters an invalid
5931 UTF-8 sequence, the second value will be <code>0xFFFD</code>,
5932 the Unicode replacement character, and the next iteration will advance
5933 a single byte in the string.
5937 The iteration order over maps is not specified
5938 and is not guaranteed to be the same from one iteration to the next.
5939 If a map entry that has not yet been reached is removed during iteration,
5940 the corresponding iteration value will not be produced. If a map entry is
5941 created during iteration, that entry may be produced during the iteration or
5942 may be skipped. The choice may vary for each entry created and from one
5943 iteration to the next.
5944 If the map is <code>nil</code>, the number of iterations is 0.
5948 For channels, the iteration values produced are the successive values sent on
5949 the channel until the channel is <a href="#Close">closed</a>. If the channel
5950 is <code>nil</code>, the range expression blocks forever.
5955 The iteration values are assigned to the respective
5956 iteration variables as in an <a href="#Assignments">assignment statement</a>.
5960 The iteration variables may be declared by the "range" clause using a form of
5961 <a href="#Short_variable_declarations">short variable declaration</a>
5963 In this case their types are set to the types of the respective iteration values
5964 and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
5965 statement; they are re-used in each iteration.
5966 If the iteration variables are declared outside the "for" statement,
5967 after execution their values will be those of the last iteration.
5971 var testdata *struct {
5974 for i, _ := range testdata.a {
5975 // testdata.a is never evaluated; len(testdata.a) is constant
5976 // i ranges from 0 to 6
5981 for i, s := range a {
5983 // type of s is string
5989 var val interface{} // element type of m is assignable to val
5990 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
5991 for key, val = range m {
5994 // key == last map key encountered in iteration
5997 var ch chan Work = producer()
6007 <h3 id="Go_statements">Go statements</h3>
6010 A "go" statement starts the execution of a function call
6011 as an independent concurrent thread of control, or <i>goroutine</i>,
6012 within the same address space.
6016 GoStmt = "go" Expression .
6020 The expression must be a function or method call; it cannot be parenthesized.
6021 Calls of built-in functions are restricted as for
6022 <a href="#Expression_statements">expression statements</a>.
6026 The function value and parameters are
6027 <a href="#Calls">evaluated as usual</a>
6028 in the calling goroutine, but
6029 unlike with a regular call, program execution does not wait
6030 for the invoked function to complete.
6031 Instead, the function begins executing independently
6033 When the function terminates, its goroutine also terminates.
6034 If the function has any return values, they are discarded when the
6040 go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
6044 <h3 id="Select_statements">Select statements</h3>
6047 A "select" statement chooses which of a set of possible
6048 <a href="#Send_statements">send</a> or
6049 <a href="#Receive_operator">receive</a>
6050 operations will proceed.
6051 It looks similar to a
6052 <a href="#Switch_statements">"switch"</a> statement but with the
6053 cases all referring to communication operations.
6057 SelectStmt = "select" "{" { CommClause } "}" .
6058 CommClause = CommCase ":" StatementList .
6059 CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
6060 RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
6061 RecvExpr = Expression .
6065 A case with a RecvStmt may assign the result of a RecvExpr to one or
6066 two variables, which may be declared using a
6067 <a href="#Short_variable_declarations">short variable declaration</a>.
6068 The RecvExpr must be a (possibly parenthesized) receive operation.
6069 There can be at most one default case and it may appear anywhere
6070 in the list of cases.
6074 Execution of a "select" statement proceeds in several steps:
6079 For all the cases in the statement, the channel operands of receive operations
6080 and the channel and right-hand-side expressions of send statements are
6081 evaluated exactly once, in source order, upon entering the "select" statement.
6082 The result is a set of channels to receive from or send to,
6083 and the corresponding values to send.
6084 Any side effects in that evaluation will occur irrespective of which (if any)
6085 communication operation is selected to proceed.
6086 Expressions on the left-hand side of a RecvStmt with a short variable declaration
6087 or assignment are not yet evaluated.
6091 If one or more of the communications can proceed,
6092 a single one that can proceed is chosen via a uniform pseudo-random selection.
6093 Otherwise, if there is a default case, that case is chosen.
6094 If there is no default case, the "select" statement blocks until
6095 at least one of the communications can proceed.
6099 Unless the selected case is the default case, the respective communication
6100 operation is executed.
6104 If the selected case is a RecvStmt with a short variable declaration or
6105 an assignment, the left-hand side expressions are evaluated and the
6106 received value (or values) are assigned.
6110 The statement list of the selected case is executed.
6115 Since communication on <code>nil</code> channels can never proceed,
6116 a select with only <code>nil</code> channels and no default case blocks forever.
6121 var c, c1, c2, c3, c4 chan int
6125 print("received ", i1, " from c1\n")
6127 print("sent ", i2, " to c2\n")
6128 case i3, ok := (<-c3): // same as: i3, ok := <-c3
6130 print("received ", i3, " from c3\n")
6132 print("c3 is closed\n")
6134 case a[f()] = <-c4:
6136 // case t := <-c4
6139 print("no communication\n")
6142 for { // send random sequence of bits to c
6144 case c <- 0: // note: no statement, no fallthrough, no folding of cases
6149 select {} // block forever
6153 <h3 id="Return_statements">Return statements</h3>
6156 A "return" statement in a function <code>F</code> terminates the execution
6157 of <code>F</code>, and optionally provides one or more result values.
6158 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6159 are executed before <code>F</code> returns to its caller.
6163 ReturnStmt = "return" [ ExpressionList ] .
6167 In a function without a result type, a "return" statement must not
6168 specify any result values.
6177 There are three ways to return values from a function with a result
6182 <li>The return value or values may be explicitly listed
6183 in the "return" statement. Each expression must be single-valued
6184 and <a href="#Assignability">assignable</a>
6185 to the corresponding element of the function's result type.
6187 func simpleF() int {
6191 func complexF1() (re float64, im float64) {
6196 <li>The expression list in the "return" statement may be a single
6197 call to a multi-valued function. The effect is as if each value
6198 returned from that function were assigned to a temporary
6199 variable with the type of the respective value, followed by a
6200 "return" statement listing these variables, at which point the
6201 rules of the previous case apply.
6203 func complexF2() (re float64, im float64) {
6208 <li>The expression list may be empty if the function's result
6209 type specifies names for its <a href="#Function_types">result parameters</a>.
6210 The result parameters act as ordinary local variables
6211 and the function may assign values to them as necessary.
6212 The "return" statement returns the values of these variables.
6214 func complexF3() (re float64, im float64) {
6220 func (devnull) Write(p []byte) (n int, _ error) {
6229 Regardless of how they are declared, all the result values are initialized to
6230 the <a href="#The_zero_value">zero values</a> for their type upon entry to the
6231 function. A "return" statement that specifies results sets the result parameters before
6232 any deferred functions are executed.
6236 Implementation restriction: A compiler may disallow an empty expression list
6237 in a "return" statement if a different entity (constant, type, or variable)
6238 with the same name as a result parameter is in
6239 <a href="#Declarations_and_scope">scope</a> at the place of the return.
6243 func f(n int) (res int, err error) {
6244 if _, err := f(n-1); err != nil {
6245 return // invalid return statement: err is shadowed
6251 <h3 id="Break_statements">Break statements</h3>
6254 A "break" statement terminates execution of the innermost
6255 <a href="#For_statements">"for"</a>,
6256 <a href="#Switch_statements">"switch"</a>, or
6257 <a href="#Select_statements">"select"</a> statement
6258 within the same function.
6262 BreakStmt = "break" [ Label ] .
6266 If there is a label, it must be that of an enclosing
6267 "for", "switch", or "select" statement,
6268 and that is the one whose execution terminates.
6273 for i = 0; i < n; i++ {
6274 for j = 0; j < m; j++ {
6287 <h3 id="Continue_statements">Continue statements</h3>
6290 A "continue" statement begins the next iteration of the
6291 innermost <a href="#For_statements">"for" loop</a> at its post statement.
6292 The "for" loop must be within the same function.
6296 ContinueStmt = "continue" [ Label ] .
6300 If there is a label, it must be that of an enclosing
6301 "for" statement, and that is the one whose execution
6307 for y, row := range rows {
6308 for x, data := range row {
6309 if data == endOfRow {
6312 row[x] = data + bias(x, y)
6317 <h3 id="Goto_statements">Goto statements</h3>
6320 A "goto" statement transfers control to the statement with the corresponding label
6321 within the same function.
6325 GotoStmt = "goto" Label .
6333 Executing the "goto" statement must not cause any variables to come into
6334 <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
6335 For instance, this example:
6345 is erroneous because the jump to label <code>L</code> skips
6346 the creation of <code>v</code>.
6350 A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
6351 For instance, this example:
6368 is erroneous because the label <code>L1</code> is inside
6369 the "for" statement's block but the <code>goto</code> is not.
6372 <h3 id="Fallthrough_statements">Fallthrough statements</h3>
6375 A "fallthrough" statement transfers control to the first statement of the
6376 next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
6377 It may be used only as the final non-empty statement in such a clause.
6381 FallthroughStmt = "fallthrough" .
6385 <h3 id="Defer_statements">Defer statements</h3>
6388 A "defer" statement invokes a function whose execution is deferred
6389 to the moment the surrounding function returns, either because the
6390 surrounding function executed a <a href="#Return_statements">return statement</a>,
6391 reached the end of its <a href="#Function_declarations">function body</a>,
6392 or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
6396 DeferStmt = "defer" Expression .
6400 The expression must be a function or method call; it cannot be parenthesized.
6401 Calls of built-in functions are restricted as for
6402 <a href="#Expression_statements">expression statements</a>.
6406 Each time a "defer" statement
6407 executes, the function value and parameters to the call are
6408 <a href="#Calls">evaluated as usual</a>
6409 and saved anew but the actual function is not invoked.
6410 Instead, deferred functions are invoked immediately before
6411 the surrounding function returns, in the reverse order
6412 they were deferred. That is, if the surrounding function
6413 returns through an explicit <a href="#Return_statements">return statement</a>,
6414 deferred functions are executed <i>after</i> any result parameters are set
6415 by that return statement but <i>before</i> the function returns to its caller.
6416 If a deferred function value evaluates
6417 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
6418 when the function is invoked, not when the "defer" statement is executed.
6422 For instance, if the deferred function is
6423 a <a href="#Function_literals">function literal</a> and the surrounding
6424 function has <a href="#Function_types">named result parameters</a> that
6425 are in scope within the literal, the deferred function may access and modify
6426 the result parameters before they are returned.
6427 If the deferred function has any return values, they are discarded when
6428 the function completes.
6429 (See also the section on <a href="#Handling_panics">handling panics</a>.)
6434 defer unlock(l) // unlocking happens before surrounding function returns
6436 // prints 3 2 1 0 before surrounding function returns
6437 for i := 0; i <= 3; i++ {
6442 func f() (result int) {
6444 // result is accessed after it was set to 6 by the return statement
6451 <h2 id="Built-in_functions">Built-in functions</h2>
6454 Built-in functions are
6455 <a href="#Predeclared_identifiers">predeclared</a>.
6456 They are called like any other function but some of them
6457 accept a type instead of an expression as the first argument.
6461 The built-in functions do not have standard Go types,
6462 so they can only appear in <a href="#Calls">call expressions</a>;
6463 they cannot be used as function values.
6466 <h3 id="Close">Close</h3>
6469 For a channel <code>c</code>, the built-in function <code>close(c)</code>
6470 records that no more values will be sent on the channel.
6471 It is an error if <code>c</code> is a receive-only channel.
6472 Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
6473 Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
6474 After calling <code>close</code>, and after any previously
6475 sent values have been received, receive operations will return
6476 the zero value for the channel's type without blocking.
6477 The multi-valued <a href="#Receive_operator">receive operation</a>
6478 returns a received value along with an indication of whether the channel is closed.
6482 <h3 id="Length_and_capacity">Length and capacity</h3>
6485 The built-in functions <code>len</code> and <code>cap</code> take arguments
6486 of various types and return a result of type <code>int</code>.
6487 The implementation guarantees that the result always fits into an <code>int</code>.
6490 <pre class="grammar">
6491 Call Argument type Result
6493 len(s) string type string length in bytes
6494 [n]T, *[n]T array length (== n)
6496 map[K]T map length (number of defined keys)
6497 chan T number of elements queued in channel buffer
6498 type parameter see below
6500 cap(s) [n]T, *[n]T array length (== n)
6502 chan T channel buffer capacity
6503 type parameter see below
6507 If the argument type is a <a href="#Type_parameters">type parameter</a> <code>P</code>,
6508 <code>P</code> must have <a href="#Structure of interfaces">specific types</a>, and
6509 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
6510 each specific type of <code>P</code>.
6511 The result is the length (or capacity, respectively) of the argument whose type
6512 corresponds to the type argument with which <code>P</code> was
6513 <a href="#Instantiations">instantiated</a>.
6517 The capacity of a slice is the number of elements for which there is
6518 space allocated in the underlying array.
6519 At any time the following relationship holds:
6523 0 <= len(s) <= cap(s)
6527 The length of a <code>nil</code> slice, map or channel is 0.
6528 The capacity of a <code>nil</code> slice or channel is 0.
6532 The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
6533 <code>s</code> is a string constant. The expressions <code>len(s)</code> and
6534 <code>cap(s)</code> are constants if the type of <code>s</code> is an array
6535 or pointer to an array and the expression <code>s</code> does not contain
6536 <a href="#Receive_operator">channel receives</a> or (non-constant)
6537 <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
6538 Otherwise, invocations of <code>len</code> and <code>cap</code> are not
6539 constant and <code>s</code> is evaluated.
6544 c1 = imag(2i) // imag(2i) = 2.0 is a constant
6545 c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
6546 c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
6547 c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
6548 c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
6553 <h3 id="Allocation">Allocation</h3>
6556 The built-in function <code>new</code> takes a type <code>T</code>,
6557 allocates storage for a <a href="#Variables">variable</a> of that type
6558 at run time, and returns a value of type <code>*T</code>
6559 <a href="#Pointer_types">pointing</a> to it.
6560 The variable is initialized as described in the section on
6561 <a href="#The_zero_value">initial values</a>.
6564 <pre class="grammar">
6573 type S struct { a int; b float64 }
6578 allocates storage for a variable of type <code>S</code>,
6579 initializes it (<code>a=0</code>, <code>b=0.0</code>),
6580 and returns a value of type <code>*S</code> containing the address
6584 <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
6587 The built-in function <code>make</code> takes a type <code>T</code>,
6588 which must be a slice, map or channel type,
6589 optionally followed by a type-specific list of expressions.
6590 It returns a value of type <code>T</code> (not <code>*T</code>).
6591 The memory is initialized as described in the section on
6592 <a href="#The_zero_value">initial values</a>.
6595 <pre class="grammar">
6598 make(T, n) slice slice of type T with length n and capacity n
6599 make(T, n, m) slice slice of type T with length n and capacity m
6601 make(T) map map of type T
6602 make(T, n) map map of type T with initial space for approximately n elements
6604 make(T) channel unbuffered channel of type T
6605 make(T, n) channel buffered channel of type T, buffer size n
6610 Each of the size arguments <code>n</code> and <code>m</code> must be of integer type
6611 or an untyped <a href="#Constants">constant</a>.
6612 A constant size argument must be non-negative and <a href="#Representability">representable</a>
6613 by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
6614 If both <code>n</code> and <code>m</code> are provided and are constant, then
6615 <code>n</code> must be no larger than <code>m</code>.
6616 If <code>n</code> is negative or larger than <code>m</code> at run time,
6617 a <a href="#Run_time_panics">run-time panic</a> occurs.
6621 s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
6622 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
6623 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
6624 s := make([]int, 10, 0) // illegal: len(s) > cap(s)
6625 c := make(chan int, 10) // channel with a buffer size of 10
6626 m := make(map[string]int, 100) // map with initial space for approximately 100 elements
6630 Calling <code>make</code> with a map type and size hint <code>n</code> will
6631 create a map with initial space to hold <code>n</code> map elements.
6632 The precise behavior is implementation-dependent.
6636 <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
6639 The built-in functions <code>append</code> and <code>copy</code> assist in
6640 common slice operations.
6641 For both functions, the result is independent of whether the memory referenced
6642 by the arguments overlaps.
6646 The <a href="#Function_types">variadic</a> function <code>append</code>
6647 appends zero or more values <code>x</code>
6648 to <code>s</code> of type <code>S</code>, which must be a slice type, and
6649 returns the resulting slice, also of type <code>S</code>.
6650 The values <code>x</code> are passed to a parameter of type <code>...T</code>
6651 where <code>T</code> is the <a href="#Slice_types">element type</a> of
6652 <code>S</code> and the respective
6653 <a href="#Passing_arguments_to_..._parameters">parameter passing rules</a> apply.
6654 As a special case, <code>append</code> also accepts a first argument
6655 assignable to type <code>[]byte</code> with a second argument of
6656 string type followed by <code>...</code>. This form appends the
6657 bytes of the string.
6660 <pre class="grammar">
6661 append(s S, x ...T) S // T is the element type of S
6665 If the capacity of <code>s</code> is not large enough to fit the additional
6666 values, <code>append</code> allocates a new, sufficiently large underlying
6667 array that fits both the existing slice elements and the additional values.
6668 Otherwise, <code>append</code> re-uses the underlying array.
6673 s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
6674 s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
6675 s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
6676 s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
6679 t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"}
6682 b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
6686 The function <code>copy</code> copies slice elements from
6687 a source <code>src</code> to a destination <code>dst</code> and returns the
6688 number of elements copied.
6689 Both arguments must have <a href="#Type_identity">identical</a> element type <code>T</code> and must be
6690 <a href="#Assignability">assignable</a> to a slice of type <code>[]T</code>.
6691 The number of elements copied is the minimum of
6692 <code>len(src)</code> and <code>len(dst)</code>.
6693 As a special case, <code>copy</code> also accepts a destination argument assignable
6694 to type <code>[]byte</code> with a source argument of a string type.
6695 This form copies the bytes from the string into the byte slice.
6698 <pre class="grammar">
6699 copy(dst, src []T) int
6700 copy(dst []byte, src string) int
6708 var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
6709 var s = make([]int, 6)
6710 var b = make([]byte, 5)
6711 n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
6712 n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
6713 n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
6717 <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
6720 The built-in function <code>delete</code> removes the element with key
6721 <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
6722 type of <code>k</code> must be <a href="#Assignability">assignable</a>
6723 to the key type of <code>m</code>.
6726 <pre class="grammar">
6727 delete(m, k) // remove element m[k] from map m
6731 If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
6732 does not exist, <code>delete</code> is a no-op.
6736 <h3 id="Complex_numbers">Manipulating complex numbers</h3>
6739 Three functions assemble and disassemble complex numbers.
6740 The built-in function <code>complex</code> constructs a complex
6741 value from a floating-point real and imaginary part, while
6742 <code>real</code> and <code>imag</code>
6743 extract the real and imaginary parts of a complex value.
6746 <pre class="grammar">
6747 complex(realPart, imaginaryPart floatT) complexT
6748 real(complexT) floatT
6749 imag(complexT) floatT
6753 The type of the arguments and return value correspond.
6754 For <code>complex</code>, the two arguments must be of the same
6755 floating-point type and the return type is the complex type
6756 with the corresponding floating-point constituents:
6757 <code>complex64</code> for <code>float32</code> arguments, and
6758 <code>complex128</code> for <code>float64</code> arguments.
6759 If one of the arguments evaluates to an untyped constant, it is first implicitly
6760 <a href="#Conversions">converted</a> to the type of the other argument.
6761 If both arguments evaluate to untyped constants, they must be non-complex
6762 numbers or their imaginary parts must be zero, and the return value of
6763 the function is an untyped complex constant.
6767 For <code>real</code> and <code>imag</code>, the argument must be
6768 of complex type, and the return type is the corresponding floating-point
6769 type: <code>float32</code> for a <code>complex64</code> argument, and
6770 <code>float64</code> for a <code>complex128</code> argument.
6771 If the argument evaluates to an untyped constant, it must be a number,
6772 and the return value of the function is an untyped floating-point constant.
6776 The <code>real</code> and <code>imag</code> functions together form the inverse of
6777 <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
6778 <code>z == Z(complex(real(z), imag(z)))</code>.
6782 If the operands of these functions are all constants, the return
6783 value is a constant.
6787 var a = complex(2, -2) // complex128
6788 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
6789 x := float32(math.Cos(math.Pi/2)) // float32
6790 var c64 = complex(5, -x) // complex64
6791 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
6792 _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
6793 var rl = real(c64) // float32
6794 var im = imag(a) // float64
6795 const c = imag(b) // untyped constant -1.4
6796 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
6799 <h3 id="Handling_panics">Handling panics</h3>
6801 <p> Two built-in functions, <code>panic</code> and <code>recover</code>,
6802 assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
6803 and program-defined error conditions.
6806 <pre class="grammar">
6807 func panic(interface{})
6808 func recover() interface{}
6812 While executing a function <code>F</code>,
6813 an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
6814 terminates the execution of <code>F</code>.
6815 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6816 are then executed as usual.
6817 Next, any deferred functions run by <code>F's</code> caller are run,
6818 and so on up to any deferred by the top-level function in the executing goroutine.
6819 At that point, the program is terminated and the error
6820 condition is reported, including the value of the argument to <code>panic</code>.
6821 This termination sequence is called <i>panicking</i>.
6826 panic("unreachable")
6827 panic(Error("cannot parse"))
6831 The <code>recover</code> function allows a program to manage behavior
6832 of a panicking goroutine.
6833 Suppose a function <code>G</code> defers a function <code>D</code> that calls
6834 <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
6836 When the running of deferred functions reaches <code>D</code>,
6837 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>.
6838 If <code>D</code> returns normally, without starting a new
6839 <code>panic</code>, the panicking sequence stops. In that case,
6840 the state of functions called between <code>G</code> and the call to <code>panic</code>
6841 is discarded, and normal execution resumes.
6842 Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
6843 execution terminates by returning to its caller.
6847 The return value of <code>recover</code> is <code>nil</code> if any of the following conditions holds:
6851 <code>panic</code>'s argument was <code>nil</code>;
6854 the goroutine is not panicking;
6857 <code>recover</code> was not called directly by a deferred function.
6862 The <code>protect</code> function in the example below invokes
6863 the function argument <code>g</code> and protects callers from
6864 run-time panics raised by <code>g</code>.
6868 func protect(g func()) {
6870 log.Println("done") // Println executes normally even if there is a panic
6871 if x := recover(); x != nil {
6872 log.Printf("run time panic: %v", x)
6875 log.Println("start")
6881 <h3 id="Bootstrapping">Bootstrapping</h3>
6884 Current implementations provide several built-in functions useful during
6885 bootstrapping. These functions are documented for completeness but are not
6886 guaranteed to stay in the language. They do not return a result.
6889 <pre class="grammar">
6892 print prints all arguments; formatting of arguments is implementation-specific
6893 println like print but prints spaces between arguments and a newline at the end
6897 Implementation restriction: <code>print</code> and <code>println</code> need not
6898 accept arbitrary argument types, but printing of boolean, numeric, and string
6899 <a href="#Types">types</a> must be supported.
6902 <h2 id="Packages">Packages</h2>
6905 Go programs are constructed by linking together <i>packages</i>.
6906 A package in turn is constructed from one or more source files
6907 that together declare constants, types, variables and functions
6908 belonging to the package and which are accessible in all files
6909 of the same package. Those elements may be
6910 <a href="#Exported_identifiers">exported</a> and used in another package.
6913 <h3 id="Source_file_organization">Source file organization</h3>
6916 Each source file consists of a package clause defining the package
6917 to which it belongs, followed by a possibly empty set of import
6918 declarations that declare packages whose contents it wishes to use,
6919 followed by a possibly empty set of declarations of functions,
6920 types, variables, and constants.
6924 SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
6927 <h3 id="Package_clause">Package clause</h3>
6930 A package clause begins each source file and defines the package
6931 to which the file belongs.
6935 PackageClause = "package" PackageName .
6936 PackageName = identifier .
6940 The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
6948 A set of files sharing the same PackageName form the implementation of a package.
6949 An implementation may require that all source files for a package inhabit the same directory.
6952 <h3 id="Import_declarations">Import declarations</h3>
6955 An import declaration states that the source file containing the declaration
6956 depends on functionality of the <i>imported</i> package
6957 (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
6958 and enables access to <a href="#Exported_identifiers">exported</a> identifiers
6960 The import names an identifier (PackageName) to be used for access and an ImportPath
6961 that specifies the package to be imported.
6965 ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
6966 ImportSpec = [ "." | PackageName ] ImportPath .
6967 ImportPath = string_lit .
6971 The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
6972 to access exported identifiers of the package within the importing source file.
6973 It is declared in the <a href="#Blocks">file block</a>.
6974 If the PackageName is omitted, it defaults to the identifier specified in the
6975 <a href="#Package_clause">package clause</a> of the imported package.
6976 If an explicit period (<code>.</code>) appears instead of a name, all the
6977 package's exported identifiers declared in that package's
6978 <a href="#Blocks">package block</a> will be declared in the importing source
6979 file's file block and must be accessed without a qualifier.
6983 The interpretation of the ImportPath is implementation-dependent but
6984 it is typically a substring of the full file name of the compiled
6985 package and may be relative to a repository of installed packages.
6989 Implementation restriction: A compiler may restrict ImportPaths to
6990 non-empty strings using only characters belonging to
6991 <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
6992 L, M, N, P, and S general categories (the Graphic characters without
6993 spaces) and may also exclude the characters
6994 <code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
6995 and the Unicode replacement character U+FFFD.
6999 Assume we have compiled a package containing the package clause
7000 <code>package math</code>, which exports function <code>Sin</code>, and
7001 installed the compiled package in the file identified by
7002 <code>"lib/math"</code>.
7003 This table illustrates how <code>Sin</code> is accessed in files
7004 that import the package after the
7005 various types of import declaration.
7008 <pre class="grammar">
7009 Import declaration Local name of Sin
7011 import "lib/math" math.Sin
7012 import m "lib/math" m.Sin
7013 import . "lib/math" Sin
7017 An import declaration declares a dependency relation between
7018 the importing and imported package.
7019 It is illegal for a package to import itself, directly or indirectly,
7020 or to directly import a package without
7021 referring to any of its exported identifiers. To import a package solely for
7022 its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
7023 identifier as explicit package name:
7031 <h3 id="An_example_package">An example package</h3>
7034 Here is a complete Go package that implements a concurrent prime sieve.
7042 // Send the sequence 2, 3, 4, … to channel 'ch'.
7043 func generate(ch chan<- int) {
7045 ch <- i // Send 'i' to channel 'ch'.
7049 // Copy the values from channel 'src' to channel 'dst',
7050 // removing those divisible by 'prime'.
7051 func filter(src <-chan int, dst chan<- int, prime int) {
7052 for i := range src { // Loop over values received from 'src'.
7054 dst <- i // Send 'i' to channel 'dst'.
7059 // The prime sieve: Daisy-chain filter processes together.
7061 ch := make(chan int) // Create a new channel.
7062 go generate(ch) // Start generate() as a subprocess.
7065 fmt.Print(prime, "\n")
7066 ch1 := make(chan int)
7067 go filter(ch, ch1, prime)
7077 <h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
7079 <h3 id="The_zero_value">The zero value</h3>
7081 When storage is allocated for a <a href="#Variables">variable</a>,
7082 either through a declaration or a call of <code>new</code>, or when
7083 a new value is created, either through a composite literal or a call
7084 of <code>make</code>,
7085 and no explicit initialization is provided, the variable or value is
7086 given a default value. Each element of such a variable or value is
7087 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
7088 <code>0</code> for numeric types, <code>""</code>
7089 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
7090 This initialization is done recursively, so for instance each element of an
7091 array of structs will have its fields zeroed if no value is specified.
7094 These two simple declarations are equivalent:
7107 type T struct { i int; f float64; next *T }
7112 the following holds:
7122 The same would also be true after
7129 <h3 id="Package_initialization">Package initialization</h3>
7132 Within a package, package-level variable initialization proceeds stepwise,
7133 with each step selecting the variable earliest in <i>declaration order</i>
7134 which has no dependencies on uninitialized variables.
7138 More precisely, a package-level variable is considered <i>ready for
7139 initialization</i> if it is not yet initialized and either has
7140 no <a href="#Variable_declarations">initialization expression</a> or
7141 its initialization expression has no <i>dependencies</i> on uninitialized variables.
7142 Initialization proceeds by repeatedly initializing the next package-level
7143 variable that is earliest in declaration order and ready for initialization,
7144 until there are no variables ready for initialization.
7148 If any variables are still uninitialized when this
7149 process ends, those variables are part of one or more initialization cycles,
7150 and the program is not valid.
7154 Multiple variables on the left-hand side of a variable declaration initialized
7155 by single (multi-valued) expression on the right-hand side are initialized
7156 together: If any of the variables on the left-hand side is initialized, all
7157 those variables are initialized in the same step.
7162 var a, b = f() // a and b are initialized together, before x is initialized
7166 For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
7167 variables are treated like any other variables in declarations.
7171 The declaration order of variables declared in multiple files is determined
7172 by the order in which the files are presented to the compiler: Variables
7173 declared in the first file are declared before any of the variables declared
7174 in the second file, and so on.
7178 Dependency analysis does not rely on the actual values of the
7179 variables, only on lexical <i>references</i> to them in the source,
7180 analyzed transitively. For instance, if a variable <code>x</code>'s
7181 initialization expression refers to a function whose body refers to
7182 variable <code>y</code> then <code>x</code> depends on <code>y</code>.
7188 A reference to a variable or function is an identifier denoting that
7189 variable or function.
7193 A reference to a method <code>m</code> is a
7194 <a href="#Method_values">method value</a> or
7195 <a href="#Method_expressions">method expression</a> of the form
7196 <code>t.m</code>, where the (static) type of <code>t</code> is
7197 not an interface type, and the method <code>m</code> is in the
7198 <a href="#Method_sets">method set</a> of <code>t</code>.
7199 It is immaterial whether the resulting function value
7200 <code>t.m</code> is invoked.
7204 A variable, function, or method <code>x</code> depends on a variable
7205 <code>y</code> if <code>x</code>'s initialization expression or body
7206 (for functions and methods) contains a reference to <code>y</code>
7207 or to a function or method that depends on <code>y</code>.
7212 For example, given the declarations
7220 d = 3 // == 5 after initialization has finished
7230 the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
7231 Note that the order of subexpressions in initialization expressions is irrelevant:
7232 <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
7233 order in this example.
7237 Dependency analysis is performed per package; only references referring
7238 to variables, functions, and (non-interface) methods declared in the current
7239 package are considered. If other, hidden, data dependencies exists between
7240 variables, the initialization order between those variables is unspecified.
7244 For instance, given the declarations
7248 var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
7249 var _ = sideEffect() // unrelated to x, a, or b
7253 type I interface { ab() []int }
7255 func (T) ab() []int { return []int{a, b} }
7259 the variable <code>a</code> will be initialized after <code>b</code> but
7260 whether <code>x</code> is initialized before <code>b</code>, between
7261 <code>b</code> and <code>a</code>, or after <code>a</code>, and
7262 thus also the moment at which <code>sideEffect()</code> is called (before
7263 or after <code>x</code> is initialized) is not specified.
7267 Variables may also be initialized using functions named <code>init</code>
7268 declared in the package block, with no arguments and no result parameters.
7276 Multiple such functions may be defined per package, even within a single
7277 source file. In the package block, the <code>init</code> identifier can
7278 be used only to declare <code>init</code> functions, yet the identifier
7279 itself is not <a href="#Declarations_and_scope">declared</a>. Thus
7280 <code>init</code> functions cannot be referred to from anywhere
7285 A package with no imports is initialized by assigning initial values
7286 to all its package-level variables followed by calling all <code>init</code>
7287 functions in the order they appear in the source, possibly in multiple files,
7288 as presented to the compiler.
7289 If a package has imports, the imported packages are initialized
7290 before initializing the package itself. If multiple packages import
7291 a package, the imported package will be initialized only once.
7292 The importing of packages, by construction, guarantees that there
7293 can be no cyclic initialization dependencies.
7297 Package initialization—variable initialization and the invocation of
7298 <code>init</code> functions—happens in a single goroutine,
7299 sequentially, one package at a time.
7300 An <code>init</code> function may launch other goroutines, which can run
7301 concurrently with the initialization code. However, initialization
7303 the <code>init</code> functions: it will not invoke the next one
7304 until the previous one has returned.
7308 To ensure reproducible initialization behavior, build systems are encouraged
7309 to present multiple files belonging to the same package in lexical file name
7310 order to a compiler.
7314 <h3 id="Program_execution">Program execution</h3>
7316 A complete program is created by linking a single, unimported package
7317 called the <i>main package</i> with all the packages it imports, transitively.
7318 The main package must
7319 have package name <code>main</code> and
7320 declare a function <code>main</code> that takes no
7321 arguments and returns no value.
7329 Program execution begins by initializing the main package and then
7330 invoking the function <code>main</code>.
7331 When that function invocation returns, the program exits.
7332 It does not wait for other (non-<code>main</code>) goroutines to complete.
7335 <h2 id="Errors">Errors</h2>
7338 The predeclared type <code>error</code> is defined as
7342 type error interface {
7348 It is the conventional interface for representing an error condition,
7349 with the nil value representing no error.
7350 For instance, a function to read data from a file might be defined:
7354 func Read(f *File, b []byte) (n int, err error)
7357 <h2 id="Run_time_panics">Run-time panics</h2>
7360 Execution errors such as attempting to index an array out
7361 of bounds trigger a <i>run-time panic</i> equivalent to a call of
7362 the built-in function <a href="#Handling_panics"><code>panic</code></a>
7363 with a value of the implementation-defined interface type <code>runtime.Error</code>.
7364 That type satisfies the predeclared interface type
7365 <a href="#Errors"><code>error</code></a>.
7366 The exact error values that
7367 represent distinct run-time error conditions are unspecified.
7373 type Error interface {
7375 // and perhaps other methods
7379 <h2 id="System_considerations">System considerations</h2>
7381 <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
7384 The built-in package <code>unsafe</code>, known to the compiler
7385 and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
7386 provides facilities for low-level programming including operations
7387 that violate the type system. A package using <code>unsafe</code>
7388 must be vetted manually for type safety and may not be portable.
7389 The package provides the following interface:
7392 <pre class="grammar">
7395 type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
7396 type Pointer *ArbitraryType
7398 func Alignof(variable ArbitraryType) uintptr
7399 func Offsetof(selector ArbitraryType) uintptr
7400 func Sizeof(variable ArbitraryType) uintptr
7402 type IntegerType int // shorthand for an integer type; it is not a real type
7403 func Add(ptr Pointer, len IntegerType) Pointer
7404 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
7408 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
7409 value may not be <a href="#Address_operators">dereferenced</a>.
7410 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be converted to
7411 a type of underlying type <code>Pointer</code> and vice versa.
7412 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
7417 bits = *(*uint64)(unsafe.Pointer(&f))
7419 type ptr unsafe.Pointer
7420 bits = *(*uint64)(ptr(&f))
7426 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
7427 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
7428 as if <code>v</code> was declared via <code>var v = x</code>.
7431 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
7432 <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
7433 or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
7434 If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
7435 without pointer indirections through fields of the struct.
7436 For a struct <code>s</code> with field <code>f</code>:
7440 uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
7444 Computer architectures may require memory addresses to be <i>aligned</i>;
7445 that is, for addresses of a variable to be a multiple of a factor,
7446 the variable's type's <i>alignment</i>. The function <code>Alignof</code>
7447 takes an expression denoting a variable of any type and returns the
7448 alignment of the (type of the) variable in bytes. For a variable
7453 uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
7457 A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
7458 is a type parameter, or if it is an array or struct type containing elements
7459 or fields of variable size. Otherwise the size is <i>constant</i>.
7460 Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
7461 are compile-time <a href="#Constant_expressions">constant expressions</a> of
7462 type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
7463 the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
7468 The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
7469 and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
7470 The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
7471 A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
7472 if it is an untyped constant it is given type <code>int</code>.
7473 The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
7477 The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
7478 and whose length and capacity are <code>len</code>.
7479 <code>Slice(ptr, len)</code> is equivalent to
7483 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
7487 except that, as a special case, if <code>ptr</code>
7488 is <code>nil</code> and <code>len</code> is zero,
7489 <code>Slice</code> returns <code>nil</code>.
7493 The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>.
7494 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
7495 if it is an untyped constant it is given type <code>int</code>.
7496 At run time, if <code>len</code> is negative,
7497 or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
7498 a <a href="#Run_time_panics">run-time panic</a> occurs.
7501 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
7504 For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
7507 <pre class="grammar">
7512 uint32, int32, float32 4
7513 uint64, int64, float64, complex64 8
7518 The following minimal alignment properties are guaranteed:
7521 <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
7524 <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
7525 all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
7528 <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
7529 the alignment of a variable of the array's element type.
7534 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.