2 "Title": "The Go Programming Language Specification - Go 1.18 Draft (incomplete)",
3 "Subtitle": "Version of Feb 8, 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 An <i>integer</i>, <i>floating-point</i>, or <i>complex</i> type
905 represents the set of integer, floating-point, or complex values, respectively.
906 They are collectively called <i>numeric types</i>.
907 The predeclared architecture-independent numeric types are:
910 <pre class="grammar">
911 uint8 the set of all unsigned 8-bit integers (0 to 255)
912 uint16 the set of all unsigned 16-bit integers (0 to 65535)
913 uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
914 uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
916 int8 the set of all signed 8-bit integers (-128 to 127)
917 int16 the set of all signed 16-bit integers (-32768 to 32767)
918 int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
919 int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
921 float32 the set of all IEEE-754 32-bit floating-point numbers
922 float64 the set of all IEEE-754 64-bit floating-point numbers
924 complex64 the set of all complex numbers with float32 real and imaginary parts
925 complex128 the set of all complex numbers with float64 real and imaginary parts
932 The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using
933 <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>.
937 There is also a set of predeclared integer types with implementation-specific sizes:
940 <pre class="grammar">
941 uint either 32 or 64 bits
942 int same size as uint
943 uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value
947 To avoid portability issues all numeric types are <a href="#Type_definitions">defined
948 types</a> and thus distinct except
949 <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
950 <code>rune</code>, which is an alias for <code>int32</code>.
952 are required when different numeric types are mixed in an expression
953 or assignment. For instance, <code>int32</code> and <code>int</code>
954 are not the same type even though they may have the same size on a
955 particular architecture.
958 <h3 id="String_types">String types</h3>
961 A <i>string type</i> represents the set of string values.
962 A string value is a (possibly empty) sequence of bytes.
963 The number of bytes is called the length of the string and is never negative.
964 Strings are immutable: once created,
965 it is impossible to change the contents of a string.
966 The predeclared string type is <code>string</code>;
967 it is a <a href="#Type_definitions">defined type</a>.
971 The length of a string <code>s</code> can be discovered using
972 the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
973 The length is a compile-time constant if the string is a constant.
974 A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a>
975 0 through <code>len(s)-1</code>.
976 It is illegal to take the address of such an element; if
977 <code>s[i]</code> is the <code>i</code>'th byte of a
978 string, <code>&s[i]</code> is invalid.
982 <h3 id="Array_types">Array types</h3>
985 An array is a numbered sequence of elements of a single
986 type, called the element type.
987 The number of elements is called the length of the array and is never negative.
991 ArrayType = "[" ArrayLength "]" ElementType .
992 ArrayLength = Expression .
997 The length is part of the array's type; it must evaluate to a
998 non-negative <a href="#Constants">constant</a>
999 <a href="#Representability">representable</a> by a value
1000 of type <code>int</code>.
1001 The length of array <code>a</code> can be discovered
1002 using the built-in function <a href="#Length_and_capacity"><code>len</code></a>.
1003 The elements can be addressed by integer <a href="#Index_expressions">indices</a>
1004 0 through <code>len(a)-1</code>.
1005 Array types are always one-dimensional but may be composed to form
1006 multi-dimensional types.
1011 [2*N] struct { x, y int32 }
1014 [2][2][2]float64 // same as [2]([2]([2]float64))
1017 <h3 id="Slice_types">Slice types</h3>
1020 A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and
1021 provides access to a numbered sequence of elements from that array.
1022 A slice type denotes the set of all slices of arrays of its element type.
1023 The number of elements is called the length of the slice and is never negative.
1024 The value of an uninitialized slice is <code>nil</code>.
1028 SliceType = "[" "]" ElementType .
1032 The length of a slice <code>s</code> can be discovered by the built-in function
1033 <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during
1034 execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a>
1035 0 through <code>len(s)-1</code>. The slice index of a
1036 given element may be less than the index of the same element in the
1040 A slice, once initialized, is always associated with an underlying
1041 array that holds its elements. A slice therefore shares storage
1042 with its array and with other slices of the same array; by contrast,
1043 distinct arrays always represent distinct storage.
1046 The array underlying a slice may extend past the end of the slice.
1047 The <i>capacity</i> is a measure of that extent: it is the sum of
1048 the length of the slice and the length of the array beyond the slice;
1049 a slice of length up to that capacity can be created by
1050 <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice.
1051 The capacity of a slice <code>a</code> can be discovered using the
1052 built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>.
1056 A new, initialized slice value for a given element type <code>T</code> is
1057 made using the built-in function
1058 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1059 which takes a slice type
1060 and parameters specifying the length and optionally the capacity.
1061 A slice created with <code>make</code> always allocates a new, hidden array
1062 to which the returned slice value refers. That is, executing
1066 make([]T, length, capacity)
1070 produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a>
1071 it, so these two expressions are equivalent:
1075 make([]int, 50, 100)
1080 Like arrays, slices are always one-dimensional but may be composed to construct
1081 higher-dimensional objects.
1082 With arrays of arrays, the inner arrays are, by construction, always the same length;
1083 however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
1084 Moreover, the inner slices must be initialized individually.
1087 <h3 id="Struct_types">Struct types</h3>
1090 A struct is a sequence of named elements, called fields, each of which has a
1091 name and a type. Field names may be specified explicitly (IdentifierList) or
1092 implicitly (EmbeddedField).
1093 Within a struct, non-<a href="#Blank_identifier">blank</a> field names must
1094 be <a href="#Uniqueness_of_identifiers">unique</a>.
1098 StructType = "struct" "{" { FieldDecl ";" } "}" .
1099 FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] .
1100 EmbeddedField = [ "*" ] TypeName .
1108 // A struct with 6 fields.
1112 _ float32 // padding
1119 A field declared with a type but no explicit field name is called an <i>embedded field</i>.
1120 An embedded field must be specified as
1121 a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>,
1122 and <code>T</code> itself may not be
1123 a pointer type. The unqualified type name acts as the field name.
1127 // A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
1129 T1 // field name is T1
1130 *T2 // field name is T2
1131 P.T3 // field name is T3
1132 *P.T4 // field name is T4
1133 x, y int // field names are x and y
1138 The following declaration is illegal because field names must be unique
1144 T // conflicts with embedded field *T and *P.T
1145 *T // conflicts with embedded field T and *P.T
1146 *P.T // conflicts with embedded field T and *T
1151 A field or <a href="#Method_declarations">method</a> <code>f</code> of an
1152 embedded field in a struct <code>x</code> is called <i>promoted</i> if
1153 <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes
1154 that field or method <code>f</code>.
1158 Promoted fields act like ordinary fields
1159 of a struct except that they cannot be used as field names in
1160 <a href="#Composite_literals">composite literals</a> of the struct.
1164 Given a struct type <code>S</code> and a <a href="#Type_definitions">defined type</a>
1165 <code>T</code>, promoted methods are included in the method set of the struct as follows:
1169 If <code>S</code> contains an embedded field <code>T</code>,
1170 the <a href="#Method_sets">method sets</a> of <code>S</code>
1171 and <code>*S</code> both include promoted methods with receiver
1172 <code>T</code>. The method set of <code>*S</code> also
1173 includes promoted methods with receiver <code>*T</code>.
1177 If <code>S</code> contains an embedded field <code>*T</code>,
1178 the method sets of <code>S</code> and <code>*S</code> both
1179 include promoted methods with receiver <code>T</code> or
1185 A field declaration may be followed by an optional string literal <i>tag</i>,
1186 which becomes an attribute for all the fields in the corresponding
1187 field declaration. An empty tag string is equivalent to an absent tag.
1188 The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a>
1189 and take part in <a href="#Type_identity">type identity</a> for structs
1190 but are otherwise ignored.
1195 x, y float64 "" // an empty tag string is like an absent tag
1196 name string "any string is permitted as a tag"
1197 _ [4]byte "ceci n'est pas un champ de structure"
1200 // A struct corresponding to a TimeStamp protocol buffer.
1201 // The tag strings define the protocol buffer field numbers;
1202 // they follow the convention outlined by the reflect package.
1204 microsec uint64 `protobuf:"1"`
1205 serverIP6 uint64 `protobuf:"2"`
1209 <h3 id="Pointer_types">Pointer types</h3>
1212 A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given
1213 type, called the <i>base type</i> of the pointer.
1214 The value of an uninitialized pointer is <code>nil</code>.
1218 PointerType = "*" BaseType .
1227 <h3 id="Function_types">Function types</h3>
1230 A function type denotes the set of all functions with the same parameter
1231 and result types. The value of an uninitialized variable of function type
1232 is <code>nil</code>.
1236 FunctionType = "func" Signature .
1237 Signature = Parameters [ Result ] .
1238 Result = Parameters | Type .
1239 Parameters = "(" [ ParameterList [ "," ] ] ")" .
1240 ParameterList = ParameterDecl { "," ParameterDecl } .
1241 ParameterDecl = [ IdentifierList ] [ "..." ] Type .
1245 Within a list of parameters or results, the names (IdentifierList)
1246 must either all be present or all be absent. If present, each name
1247 stands for one item (parameter or result) of the specified type and
1248 all non-<a href="#Blank_identifier">blank</a> names in the signature
1249 must be <a href="#Uniqueness_of_identifiers">unique</a>.
1250 If absent, each type stands for one item of that type.
1251 Parameter and result
1252 lists are always parenthesized except that if there is exactly
1253 one unnamed result it may be written as an unparenthesized type.
1257 The final incoming parameter in a function signature may have
1258 a type prefixed with <code>...</code>.
1259 A function with such a parameter is called <i>variadic</i> and
1260 may be invoked with zero or more arguments for that parameter.
1266 func(a, _ int, z float32) bool
1267 func(a, b int, z float32) (bool)
1268 func(prefix string, values ...int)
1269 func(a, b int, z float64, opt ...interface{}) (success bool)
1270 func(int, int, float64) (float64, *[]int)
1271 func(n int) func(p *T)
1274 <h3 id="Interface_types">Interface types</h3>
1277 An interface type defines a <i>type set</i>.
1278 A variable of interface type can store a value of any type that is in the type
1279 set of the interface. Such a type is said to <i>implement the interface</i>.
1280 The value of an uninitialized variable of interface type is <code>nil</code>.
1284 InterfaceType = "interface" "{" { InterfaceElem ";" } "}" .
1285 InterfaceElem = MethodElem | TypeElem .
1286 MethodElem = MethodName Signature .
1287 MethodName = identifier .
1288 TypeElem = TypeTerm { "|" TypeTerm } .
1289 TypeTerm = Type | UnderlyingType .
1290 UnderlyingType = "~" Type .
1294 An interface type is specified by a list of <i>interface elements</i>.
1295 An interface element is either a <i>method</i> or a <i>type element</i>,
1296 where a type element is a union of one or more <i>type terms</i>.
1297 A type term is either a single type or a single underlying type.
1301 In its most basic form an interface specifies a (possibly empty) list of methods.
1302 The type set defined by such an interface is the set of types which implement all of
1303 those methods, and the corresponding <a href="#Method_sets">method set</a> consists
1304 exactly of the methods specified by the interface.
1308 // A simple File interface.
1310 Read([]byte) (int, error)
1311 Write([]byte) (int, error)
1317 The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a>
1318 and not <a href="#Blank_identifier">blank</a>.
1324 String() string // illegal: String not unique
1325 _(x int) // illegal: method must have non-blank name
1330 More than one type may implement an interface.
1331 For instance, if two types <code>S1</code> and <code>S2</code>
1336 func (p T) Read(p []byte) (n int, err error)
1337 func (p T) Write(p []byte) (n int, err error)
1338 func (p T) Close() error
1342 (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>)
1343 then the <code>File</code> interface is implemented by both <code>S1</code> and
1344 <code>S2</code>, regardless of what other methods
1345 <code>S1</code> and <code>S2</code> may have or share.
1349 Every type that is a member of the type set of an interface implements that interface.
1350 Any given type may implement several distinct interfaces.
1351 For instance, all types implement the <i>empty interface</i> which stands for the set of all types:
1359 For convenience, the predeclared type <code>any</code> is an alias for the empty interface.
1363 Similarly, consider this interface specification,
1364 which appears within a <a href="#Type_declarations">type declaration</a>
1365 to define an interface called <code>Locker</code>:
1369 type Locker interface {
1376 If <code>S1</code> and <code>S2</code> also implement
1380 func (p T) Lock() { … }
1381 func (p T) Unlock() { … }
1385 they implement the <code>Locker</code> interface as well
1386 as the <code>File</code> interface.
1390 In a slightly more general form
1391 an interface <code>T</code> may use a (possibly qualified) interface type
1392 name <code>E</code> as an interface element. This is called
1393 <i>embedding</i> interface <code>E</code> in <code>T</code>.
1394 The type set of <code>T</code> is the <i>intersection</i> of the type sets
1395 defined by <code>T</code>'s explicitly declared methods and the type sets
1396 of <code>T</code>’s embedded interfaces.
1397 In other words, the type set of <code>T</code> is the set of all types that implement all the
1398 explicitly declared methods of <code>T</code> and also all the methods of
1403 type Reader interface {
1404 Read(p []byte) (n int, err error)
1408 type Writer interface {
1409 Write(p []byte) (n int, err error)
1413 // ReadWriter's methods are Read, Write, and Close.
1414 type ReadWriter interface {
1415 Reader // includes methods of Reader in ReadWriter's method set
1416 Writer // includes methods of Writer in ReadWriter's method set
1421 When embedding interfaces, methods with the
1422 <a href="#Uniqueness_of_identifiers">same</a> names must
1423 have <a href="#Type_identity">identical</a> signatures.
1427 type ReadCloser interface {
1428 Reader // includes methods of Reader in ReadCloser's method set
1429 Close() // illegal: signatures of Reader.Close and Close are different
1434 Finally, in their most general form, an interface element may also be an arbitrary type term
1435 <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>,
1436 or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>.
1437 Together with method specifications, these elements enable the precise
1438 definition of an interface's type set as follows:
1442 <li>The type set of the empty interface is the set of all types.
1445 <li>The type set of a non-empty interface is the intersection of the type sets
1446 of its interface elements.
1449 <li>The type set of a method specification is the set of types
1450 whose method sets include that method.
1453 <li>The type set of a non-interface type term is the set consisting
1457 <li>The type set of a term of the form <code>~T</code>
1458 is the set of types whose underlying type is <code>T</code>.
1461 <li>The type set of a <i>union</i> of terms
1462 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>
1463 is the union of the type sets of the terms.
1468 // An interface representing only the type int.
1473 // An interface representing all types with underlying type int.
1478 // An interface representing all types with underlying type int which implement the String method.
1484 // An interface representing an empty type set: there is no type that is both an int and a string.
1492 In a term of the form <code>~T</code>, the underlying type of <code>T</code>
1493 must be itself, and <code>T</code> cannot be an interface.
1500 ~[]byte // the underlying type of []byte is itself
1501 ~MyInt // illegal: the underlying type of MyInt is not MyInt
1502 ~error // illegal: error is an interface
1507 Union elements denote unions of type sets:
1511 // The Floats interface represents all floating-point types
1512 // (including any named types whose underlying types are
1513 // either float32 or float64).
1514 type Floats interface {
1520 In a union, a term cannot be a type parameter, and the type sets of all
1521 non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty).
1522 Given a type parameter <code>P</code>:
1527 P // illegal: the term P is a type parameter
1528 int | P // illegal: the term P is a type parameter
1529 ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt)
1530 float32 | Floats // overlapping type sets but Floats is an interface
1535 Implementation restriction:
1536 A union with more than one term cannot contain interface types
1537 with non-empty <a href="#Method_sets">method sets</a> or which
1538 are or embed the <a href="#Predeclared_identifiers">predeclared identifier</a>
1539 <code>comparable</code>.
1543 Interfaces that contain non-interface types, terms of the form <code>~T</code>,
1544 or unions may only be used as type constraints, or as elements of other interfaces used
1545 as constraints. They cannot be the types of values or variables, or components of other,
1546 non-interface types.
1550 var x Floats // illegal: Floats is restricted by float32 and float64
1552 var x interface{} = Floats(nil) // illegal
1554 type Floatish struct {
1559 <!-- TODO The rule below needs to be generalized to interface elements.
1560 It should be factored out and generalized to other types
1561 such as arrays and structs which are currently missing such a
1562 rule. See also #5069.
1566 An interface type <code>T</code> may not embed itself
1567 or any interface type that embeds <code>T</code>, recursively.
1571 // illegal: Bad cannot embed itself
1572 type Bad interface {
1576 // illegal: Bad1 cannot embed itself using Bad2
1577 type Bad1 interface {
1580 type Bad2 interface {
1585 <h3 id="Map_types">Map types</h3>
1588 A map is an unordered group of elements of one type, called the
1589 element type, indexed by a set of unique <i>keys</i> of another type,
1590 called the key type.
1591 The value of an uninitialized map is <code>nil</code>.
1595 MapType = "map" "[" KeyType "]" ElementType .
1600 The <a href="#Comparison_operators">comparison operators</a>
1601 <code>==</code> and <code>!=</code> must be fully defined
1602 for operands of the key type; thus the key type must not be a function, map, or
1604 If the key type is an interface type, these
1605 comparison operators must be defined for the dynamic key values;
1606 failure will cause a <a href="#Run_time_panics">run-time panic</a>.
1611 map[*T]struct{ x, y float64 }
1612 map[string]interface{}
1616 The number of map elements is called its length.
1617 For a map <code>m</code>, it can be discovered using the
1618 built-in function <a href="#Length_and_capacity"><code>len</code></a>
1619 and may change during execution. Elements may be added during execution
1620 using <a href="#Assignments">assignments</a> and retrieved with
1621 <a href="#Index_expressions">index expressions</a>; they may be removed with the
1622 <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function.
1625 A new, empty map value is made using the built-in
1626 function <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1627 which takes the map type and an optional capacity hint as arguments:
1631 make(map[string]int)
1632 make(map[string]int, 100)
1636 The initial capacity does not bound its size:
1637 maps grow to accommodate the number of items
1638 stored in them, with the exception of <code>nil</code> maps.
1639 A <code>nil</code> map is equivalent to an empty map except that no elements
1642 <h3 id="Channel_types">Channel types</h3>
1645 A channel provides a mechanism for
1646 <a href="#Go_statements">concurrently executing functions</a>
1648 <a href="#Send_statements">sending</a> and
1649 <a href="#Receive_operator">receiving</a>
1650 values of a specified element type.
1651 The value of an uninitialized channel is <code>nil</code>.
1655 ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .
1659 The optional <code><-</code> operator specifies the channel <i>direction</i>,
1660 <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>,
1661 otherwise it is <i>bidirectional</i>.
1662 A channel may be constrained only to send or only to receive by
1663 <a href="#Assignments">assignment</a> or
1664 explicit <a href="#Conversions">conversion</a>.
1668 chan T // can be used to send and receive values of type T
1669 chan<- float64 // can only be used to send float64s
1670 <-chan int // can only be used to receive ints
1674 The <code><-</code> operator associates with the leftmost <code>chan</code>
1679 chan<- chan int // same as chan<- (chan int)
1680 chan<- <-chan int // same as chan<- (<-chan int)
1681 <-chan <-chan int // same as <-chan (<-chan int)
1682 chan (<-chan int)
1686 A new, initialized channel
1687 value can be made using the built-in function
1688 <a href="#Making_slices_maps_and_channels"><code>make</code></a>,
1689 which takes the channel type and an optional <i>capacity</i> as arguments:
1697 The capacity, in number of elements, sets the size of the buffer in the channel.
1698 If the capacity is zero or absent, the channel is unbuffered and communication
1699 succeeds only when both a sender and receiver are ready. Otherwise, the channel
1700 is buffered and communication succeeds without blocking if the buffer
1701 is not full (sends) or not empty (receives).
1702 A <code>nil</code> channel is never ready for communication.
1706 A channel may be closed with the built-in function
1707 <a href="#Close"><code>close</code></a>.
1708 The multi-valued assignment form of the
1709 <a href="#Receive_operator">receive operator</a>
1710 reports whether a received value was sent before
1711 the channel was closed.
1715 A single channel may be used in
1716 <a href="#Send_statements">send statements</a>,
1717 <a href="#Receive_operator">receive operations</a>,
1718 and calls to the built-in functions
1719 <a href="#Length_and_capacity"><code>cap</code></a> and
1720 <a href="#Length_and_capacity"><code>len</code></a>
1721 by any number of goroutines without further synchronization.
1722 Channels act as first-in-first-out queues.
1723 For example, if one goroutine sends values on a channel
1724 and a second goroutine receives them, the values are
1725 received in the order sent.
1728 <h3 id="Type_parameters">Type parameters</h3>
1731 A <i>type parameter</i> is an (unqualified) type name declared in the
1732 <a href="#Type_parameter_lists">type parameter list</a> of a
1733 <a href="#Function_declarations">function declaration</a> or
1734 <a href="#Type_definitions">type definition</a>; or in the receiver specification
1735 of a <a href="#Method_declarations">method declaration</a> that is associated
1736 with a parameterized type.
1737 A type parameter acts as a place holder for an (as of yet) unknown type in the declaration;
1738 the type parameter is replaced with a <i>type argument</i> upon
1739 <a href="#Instantiations">instantiation</a> of the parameterized function or type.
1743 The properties of a type parameter are determined by its
1744 <a href="#Type_constraints">type constraint</a>.
1747 <h2 id="Properties_of_types_and_values">Properties of types and values</h2>
1749 <h3 id="Type_identity">Type identity</h3>
1752 Two types are either <i>identical</i> or <i>different</i>.
1756 A <a href="#Types">named type</a> is always different from any other type.
1757 Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are
1758 structurally equivalent; that is, they have the same literal structure and corresponding
1759 components have identical types. In detail:
1763 <li>Two array types are identical if they have identical element types and
1764 the same array length.</li>
1766 <li>Two slice types are identical if they have identical element types.</li>
1768 <li>Two struct types are identical if they have the same sequence of fields,
1769 and if corresponding fields have the same names, and identical types,
1771 <a href="#Exported_identifiers">Non-exported</a> field names from different
1772 packages are always different.</li>
1774 <li>Two pointer types are identical if they have identical base types.</li>
1776 <li>Two function types are identical if they have the same number of parameters
1777 and result values, corresponding parameter and result types are
1778 identical, and either both functions are variadic or neither is.
1779 Parameter and result names are not required to match.</li>
1781 <li>Two interface types are identical if they define the same type set.
1784 <li>Two map types are identical if they have identical key and element types.</li>
1786 <li>Two channel types are identical if they have identical element types and
1787 the same direction.</li>
1789 <li>Two <a href="#Instantiations">instantiated</a> types are identical if
1790 their defined types and all type arguments are identical.
1795 Given the declarations
1802 A2 = struct{ a, b int }
1804 A4 = func(A3, float64) *A0
1805 A5 = func(x int, _ float64) *[]string
1809 B2 struct{ a, b int }
1810 B3 struct{ a, c int }
1811 B4 func(int, float64) *B0
1812 B5 func(x int, y float64) *A1
1815 D0[P1, P2 any] struct{ x P1; y P2 }
1816 E0 = D0[int, string]
1821 these types are identical:
1825 A0, A1, and []string
1826 A2 and struct{ a, b int }
1828 A4, func(int, float64) *[]string, and A5
1831 D0[int, string] and E0
1833 struct{ a, b *T5 } and struct{ a, b *T5 }
1834 func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5
1838 <code>B0</code> and <code>B1</code> are different because they are new types
1839 created by distinct <a href="#Type_definitions">type definitions</a>;
1840 <code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code>
1841 are different because <code>B0</code> is different from <code>[]string</code>;
1842 and <code>P1</code> and <code>P2</code> are different because they are different
1844 <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are
1845 different because the former is an <a href="#Instantiations">instantiated</a>
1846 defined type while the latter is a type literal
1847 (but they are still <a href="#Assignability">assignable</a>).
1850 <h3 id="Assignability">Assignability</h3>
1853 A value <code>x</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code>
1854 ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies:
1859 <code>x</code>'s type is identical to <code>T</code>.
1862 <code>x</code>'s type <code>V</code> and <code>T</code> have identical
1863 <a href="#Underlying_types">underlying types</a> and at least one of <code>V</code>
1864 or <code>T</code> is not a <a href="#Types">named type</a>.
1867 <code>x</code>'s type <code>V</code> and <code>T</code> are channel types with
1868 identical element types, <code>V</code> is a bidirectional channel,
1869 and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>.
1872 <code>T</code> is an interface type, but not a type parameter, and
1873 <code>x</code> <a href="#Interface_types">implements</a> <code>T</code>.
1876 <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code>
1877 is a pointer, function, slice, map, channel, or interface type,
1878 but not a type parameter.
1881 <code>x</code> is an untyped <a href="#Constants">constant</a>
1882 <a href="#Representability">representable</a>
1883 by a value of type <code>T</code>.
1888 Additionally, if <code>x's</code> type <code>V</code> or <code>T</code> are type parameters
1889 with <a href="#Structure_of_interfaces">specific types</a>, <code>x</code>
1890 is assignable to a variable of type <code>T</code> if one of the following conditions applies:
1895 <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is
1896 a type parameter, and <code>x</code> is assignable to each specific type of
1900 <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is
1901 a type parameter, and <code>x</code> is assignable to each specific type of
1905 <code>V</code> is a type parameter and <code>T</code> is not a named type,
1906 and values of each specific type of <code>V</code> are assignable
1911 <h3 id="Representability">Representability</h3>
1914 A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i>
1915 by a value of type <code>T</code>,
1916 where <code>T</code> is not a <a href="#Type_parameters">type parameter</a>,
1917 if one of the following conditions applies:
1922 <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>.
1926 <code>T</code> is a <a href="#Numeric_types">floating-point type</a> and <code>x</code> can be rounded to <code>T</code>'s
1927 precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
1928 negative zero further simplified to an unsigned zero. Note that constant values never result
1929 in an IEEE negative zero, NaN, or infinity.
1933 <code>T</code> is a complex type, and <code>x</code>'s
1934 <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code>
1935 are representable by values of <code>T</code>'s component type (<code>float32</code> or
1936 <code>float64</code>).
1941 If <code>T</code> is a type parameter with <a href="#Structure_of_interfaces">specific types</a>,
1942 <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable
1943 by a value of each specific type of <code>T</code>.
1947 x T x is representable by a value of T because
1949 'a' byte 97 is in the set of byte values
1950 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
1951 "foo" string "foo" is in the set of string values
1952 1024 int16 1024 is in the set of 16-bit integers
1953 42.0 byte 42 is in the set of unsigned 8-bit integers
1954 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
1955 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
1956 -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
1957 0i int 0 is an integer value
1958 (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values
1962 x T x is not representable by a value of T because
1964 0 bool 0 is not in the set of boolean values
1965 'a' string 'a' is a rune, it is not in the set of string values
1966 1024 byte 1024 is not in the set of unsigned 8-bit integers
1967 -1 uint16 -1 is not in the set of unsigned 16-bit integers
1968 1.1 int 1.1 is not an integer value
1969 42i float32 (0 + 42i) is not in the set of float32 values
1970 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding
1973 <h3 id="Structure_of_interfaces">Structure of interfaces</h3>
1976 An interface specification which contains <a href="#Interface_types">type elements</a>
1977 defines a (possibly empty) set of <i>specific types</i>.
1978 Loosely speaking, these are the types <code>T</code> that appear in the
1979 interface definition in terms of the form <code>T</code>, <code>~T</code>,
1980 or in unions of such terms.
1984 More precisely, for a given interface, the set of specific types corresponds to
1985 the set 𝑅 of representative types of the interface, if 𝑅 is non-empty and finite.
1986 Otherwise, if 𝑅 is empty or infinite, the interface has <i>no specific types</i>.
1990 For a given interface, type element or type term, the set 𝑅 of representative types is defined as follows:
1994 <li>For an interface with no type elements, 𝑅 is the (infinite) set of all types.
1997 <li>For an interface with type elements,
1998 𝑅 is the intersection of the representative types of its type elements.
2001 <li>For a non-interface type term <code>T</code> or a term of the form <code>~T</code>,
2002 𝑅 is the set consisting of the type <code>T</code>.
2005 <li>For a <i>union</i> of terms
2006 <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>,
2007 𝑅 is the union of the representative types of the terms.
2012 An interface may have specific types even if its <a href="#Interface_types">type set</a>
2017 Examples of interfaces with their specific types:
2021 type Celsius float32
2024 interface{} // no specific types
2025 interface{ int } // int
2026 interface{ ~string } // string
2027 interface{ int|~string } // int, string
2028 interface{ Celsius|Kelvin } // Celsius, Kelvin
2029 interface{ float64|any } // no specific types (union is all types)
2030 interface{ int; m() } // int (but type set is empty because int has no method m)
2031 interface{ ~int; m() } // int (but type set is infinite because many integer types have a method m)
2032 interface{ int; any } // int
2033 interface{ int; string } // no specific types (intersection is empty)
2037 An interface <code>T</code> is called <i>structural</i> if one of the following
2038 conditions is satisfied:
2043 There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a>
2044 of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or
2047 the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a>
2048 with identical element type <code>E</code>, and all directional channels have the same
2054 A structural interface has a <i>structural type</i> which is, depending on the
2055 condition that is satisfied, either:
2060 the type <code>U</code>; or
2063 the type <code>chan E</code> if <code>T</code> contains only bidirectional
2064 channels, or the type <code>chan<- E</code> or <code><-chan E</code>
2065 depending on the direction of the directional channels present.
2070 Examples of structural interfaces with their structural types:
2074 interface{ int } // int
2075 interface{ Celsius|Kelvin } // float32
2076 interface{ ~chan int } // chan int
2077 interface{ ~chan int|~chan<- int } // chan<- int
2078 interface{ ~[]*data; String() string } // []*data
2082 Examples of non-structural interfaces:
2086 interface{} // no single underlying type
2087 interface{ Celsius|float64 } // no single underlying type
2088 interface{ chan int | chan<- string } // channels have different element types
2089 interface{ <-chan int | chan<- int } // directional channels have different directions
2092 <h2 id="Blocks">Blocks</h2>
2095 A <i>block</i> is a possibly empty sequence of declarations and statements
2096 within matching brace brackets.
2100 Block = "{" StatementList "}" .
2101 StatementList = { Statement ";" } .
2105 In addition to explicit blocks in the source code, there are implicit blocks:
2109 <li>The <i>universe block</i> encompasses all Go source text.</li>
2111 <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all
2112 Go source text for that package.</li>
2114 <li>Each file has a <i>file block</i> containing all Go source text
2117 <li>Each <a href="#If_statements">"if"</a>,
2118 <a href="#For_statements">"for"</a>, and
2119 <a href="#Switch_statements">"switch"</a>
2120 statement is considered to be in its own implicit block.</li>
2122 <li>Each clause in a <a href="#Switch_statements">"switch"</a>
2123 or <a href="#Select_statements">"select"</a> statement
2124 acts as an implicit block.</li>
2128 Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>.
2132 <h2 id="Declarations_and_scope">Declarations and scope</h2>
2135 A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a
2136 <a href="#Constant_declarations">constant</a>,
2137 <a href="#Type_declarations">type</a>,
2138 <a href="#Variable_declarations">variable</a>,
2139 <a href="#Function_declarations">function</a>,
2140 <a href="#Labeled_statements">label</a>, or
2141 <a href="#Import_declarations">package</a>.
2142 Every identifier in a program must be declared.
2143 No identifier may be declared twice in the same block, and
2144 no identifier may be declared in both the file and package block.
2148 The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier
2149 in a declaration, but it does not introduce a binding and thus is not declared.
2150 In the package block, the identifier <code>init</code> may only be used for
2151 <a href="#Package_initialization"><code>init</code> function</a> declarations,
2152 and like the blank identifier it does not introduce a new binding.
2156 Declaration = ConstDecl | TypeDecl | VarDecl .
2157 TopLevelDecl = Declaration | FunctionDecl | MethodDecl .
2161 The <i>scope</i> of a declared identifier is the extent of source text in which
2162 the identifier denotes the specified constant, type, variable, function, label, or package.
2166 Go is lexically scoped using <a href="#Blocks">blocks</a>:
2170 <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li>
2172 <li>The scope of an identifier denoting a constant, type, variable,
2173 or function (but not method) declared at top level (outside any
2174 function) is the package block.</li>
2176 <li>The scope of the package name of an imported package is the file block
2177 of the file containing the import declaration.</li>
2179 <li>The scope of an identifier denoting a method receiver, function parameter,
2180 or result variable is the function body.</li>
2182 <li>The scope of an identifier denoting a type parameter of a type-parameterized function
2183 or declared by a method receiver is the function body and all parameter lists of the
2187 <li>The scope of an identifier denoting a type parameter of a parameterized type
2188 begins after the name of the parameterized type and ends at the end
2189 of the TypeSpec.</li>
2191 <li>The scope of a constant or variable identifier declared
2192 inside a function begins at the end of the ConstSpec or VarSpec
2193 (ShortVarDecl for short variable declarations)
2194 and ends at the end of the innermost containing block.</li>
2196 <li>The scope of a type identifier declared inside a function
2197 begins at the identifier in the TypeSpec
2198 and ends at the end of the innermost containing block.</li>
2202 An identifier declared in a block may be redeclared in an inner block.
2203 While the identifier of the inner declaration is in scope, it denotes
2204 the entity declared by the inner declaration.
2208 The <a href="#Package_clause">package clause</a> is not a declaration; the package name
2209 does not appear in any scope. Its purpose is to identify the files belonging
2210 to the same <a href="#Packages">package</a> and to specify the default package name for import
2215 <h3 id="Label_scopes">Label scopes</h3>
2218 Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are
2219 used in the <a href="#Break_statements">"break"</a>,
2220 <a href="#Continue_statements">"continue"</a>, and
2221 <a href="#Goto_statements">"goto"</a> statements.
2222 It is illegal to define a label that is never used.
2223 In contrast to other identifiers, labels are not block scoped and do
2224 not conflict with identifiers that are not labels. The scope of a label
2225 is the body of the function in which it is declared and excludes
2226 the body of any nested function.
2230 <h3 id="Blank_identifier">Blank identifier</h3>
2233 The <i>blank identifier</i> is represented by the underscore character <code>_</code>.
2234 It serves as an anonymous placeholder instead of a regular (non-blank)
2235 identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>,
2236 as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>.
2240 <h3 id="Predeclared_identifiers">Predeclared identifiers</h3>
2243 The following identifiers are implicitly declared in the
2244 <a href="#Blocks">universe block</a>:
2246 <pre class="grammar">
2248 any bool byte comparable
2249 complex64 complex128 error float32 float64
2250 int int8 int16 int32 int64 rune string
2251 uint uint8 uint16 uint32 uint64 uintptr
2260 append cap close complex copy delete imag len
2261 make new panic print println real recover
2264 <h3 id="Exported_identifiers">Exported identifiers</h3>
2267 An identifier may be <i>exported</i> to permit access to it from another package.
2268 An identifier is exported if both:
2271 <li>the first character of the identifier's name is a Unicode upper case
2272 letter (Unicode class "Lu"); and</li>
2273 <li>the identifier is declared in the <a href="#Blocks">package block</a>
2274 or it is a <a href="#Struct_types">field name</a> or
2275 <a href="#MethodName">method name</a>.</li>
2278 All other identifiers are not exported.
2281 <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3>
2284 Given a set of identifiers, an identifier is called <i>unique</i> if it is
2285 <i>different</i> from every other in the set.
2286 Two identifiers are different if they are spelled differently, or if they
2287 appear in different <a href="#Packages">packages</a> and are not
2288 <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same.
2291 <h3 id="Constant_declarations">Constant declarations</h3>
2294 A constant declaration binds a list of identifiers (the names of
2295 the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>.
2296 The number of identifiers must be equal
2297 to the number of expressions, and the <i>n</i>th identifier on
2298 the left is bound to the value of the <i>n</i>th expression on the
2303 ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
2304 ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
2306 IdentifierList = identifier { "," identifier } .
2307 ExpressionList = Expression { "," Expression } .
2311 If the type is present, all constants take the type specified, and
2312 the expressions must be <a href="#Assignability">assignable</a> to that type.
2313 If the type is omitted, the constants take the
2314 individual types of the corresponding expressions.
2315 If the expression values are untyped <a href="#Constants">constants</a>,
2316 the declared constants remain untyped and the constant identifiers
2317 denote the constant values. For instance, if the expression is a
2318 floating-point literal, the constant identifier denotes a floating-point
2319 constant, even if the literal's fractional part is zero.
2323 const Pi float64 = 3.14159265358979323846
2324 const zero = 0.0 // untyped floating-point constant
2327 eof = -1 // untyped integer constant
2329 const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants
2330 const u, v float32 = 0, 3 // u = 0.0, v = 3.0
2334 Within a parenthesized <code>const</code> declaration list the
2335 expression list may be omitted from any but the first ConstSpec.
2336 Such an empty list is equivalent to the textual substitution of the
2337 first preceding non-empty expression list and its type if any.
2338 Omitting the list of expressions is therefore equivalent to
2339 repeating the previous list. The number of identifiers must be equal
2340 to the number of expressions in the previous list.
2341 Together with the <a href="#Iota"><code>iota</code> constant generator</a>
2342 this mechanism permits light-weight declaration of sequential values:
2354 numberOfDays // this constant is not exported
2359 <h3 id="Iota">Iota</h3>
2362 Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier
2363 <code>iota</code> represents successive untyped integer <a href="#Constants">
2364 constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a>
2365 in that constant declaration, starting at zero.
2366 It can be used to construct a set of related constants:
2371 c0 = iota // c0 == 0
2372 c1 = iota // c1 == 1
2373 c2 = iota // c2 == 2
2377 a = 1 << iota // a == 1 (iota == 0)
2378 b = 1 << iota // b == 2 (iota == 1)
2379 c = 3 // c == 3 (iota == 2, unused)
2380 d = 1 << iota // d == 8 (iota == 3)
2384 u = iota * 42 // u == 0 (untyped integer constant)
2385 v float64 = iota * 42 // v == 42.0 (float64 constant)
2386 w = iota * 42 // w == 84 (untyped integer constant)
2389 const x = iota // x == 0
2390 const y = iota // y == 0
2394 By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value:
2399 bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0)
2400 bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1)
2401 _, _ // (iota == 2, unused)
2402 bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3)
2407 This last example exploits the <a href="#Constant_declarations">implicit repetition</a>
2408 of the last non-empty expression list.
2412 <h3 id="Type_declarations">Type declarations</h3>
2415 A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
2416 Type declarations come in two forms: alias declarations and type definitions.
2420 TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
2421 TypeSpec = AliasDecl | TypeDef .
2424 <h4 id="Alias_declarations">Alias declarations</h4>
2427 An alias declaration binds an identifier to the given type.
2431 AliasDecl = identifier "=" Type .
2435 Within the <a href="#Declarations_and_scope">scope</a> of
2436 the identifier, it serves as an <i>alias</i> for the type.
2441 nodeList = []*Node // nodeList and []*Node are identical types
2442 Polar = polar // Polar and polar denote identical types
2447 <h4 id="Type_definitions">Type definitions</h4>
2450 A type definition creates a new, distinct type with the same
2451 <a href="#Types">underlying type</a> and operations as the given type
2452 and binds an identifier, the <i>type name</i>, to it.
2456 TypeDef = identifier [ TypeParameters ] Type .
2460 The new type is called a <i>defined type</i>.
2461 It is <a href="#Type_identity">different</a> from any other type,
2462 including the type it is created from.
2467 Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types
2468 polar Point // polar and Point denote different types
2471 type TreeNode struct {
2472 left, right *TreeNode
2476 type Block interface {
2478 Encrypt(src, dst []byte)
2479 Decrypt(src, dst []byte)
2484 A defined type may have <a href="#Method_declarations">methods</a> associated with it.
2485 It does not inherit any methods bound to the given type,
2486 but the <a href="#Method_sets">method set</a>
2487 of an interface type or of elements of a composite type remains unchanged:
2491 // A Mutex is a data type with two methods, Lock and Unlock.
2492 type Mutex struct { /* Mutex fields */ }
2493 func (m *Mutex) Lock() { /* Lock implementation */ }
2494 func (m *Mutex) Unlock() { /* Unlock implementation */ }
2496 // NewMutex has the same composition as Mutex but its method set is empty.
2499 // The method set of PtrMutex's underlying type *Mutex remains unchanged,
2500 // but the method set of PtrMutex is empty.
2501 type PtrMutex *Mutex
2503 // The method set of *PrintableMutex contains the methods
2504 // Lock and Unlock bound to its embedded field Mutex.
2505 type PrintableMutex struct {
2509 // MyBlock is an interface type that has the same method set as Block.
2514 Type definitions may be used to define different boolean, numeric,
2515 or string types and associate methods with them:
2522 EST TimeZone = -(5 + iota)
2528 func (tz TimeZone) String() string {
2529 return fmt.Sprintf("GMT%+dh", tz)
2534 If the type definition specifies <a href="#Type_parameter_lists">type parameters</a>,
2535 the type name denotes a <i>parameterized type</i>.
2536 Parameterized types must be <a href="#Instantiations">instantiated</a> when they
2541 type List[T any] struct {
2546 type Tree[T constraints.Ordered] struct {
2547 left, right *Tree[T]
2553 The given type cannot be a type parameter in a type definition.
2557 type T[P any] P // illegal: P is a type parameter
2560 type L T // illegal: T is a type parameter declared by the enclosing function
2565 A parameterized type may also have methods associated with it. In this case,
2566 the method receivers must declare the same number of type parameters as
2567 present in the parameterized type definition.
2571 // The method Len returns the number of elements in the linked list l.
2572 func (l *List[T]) Len() int { … }
2575 <h3 id="Type_parameter_lists">Type parameter lists</h3>
2578 A type parameter list declares the <a href="#Type_parameters">type parameters</a>
2579 in a type-parameterized function or type declaration.
2580 The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a>
2581 except that the type parameter names must all be present and the list is enclosed
2582 in square brackets rather than parentheses.
2586 TypeParameters = "[" TypeParamList [ "," ] "]" .
2587 TypeParamList = TypeParamDecl { "," TypeParamDecl } .
2588 TypeParamDecl = IdentifierList TypeConstraint .
2592 Each identifier declares a type parameter.
2593 All non-blank names in the list must be unique.
2594 Each type parameter is a new and different <a href="#Types">named type</a>.
2599 [S interface{ ~[]byte|string }]
2606 Just as each ordinary function parameter has a parameter type, each type parameter
2607 has a corresponding (meta-)type which is called its
2608 <a href="#Type_constraints"><i>type constraint</i></a>.
2612 A parsing ambiguity arises when the type parameter list for a parameterized type
2613 declares a single type parameter with a type constraint of the form <code>*C</code>
2614 or <code>(C)</code> where <code>C</code> is not a (possibly parenthesized)
2615 <a href="#Types">type literal</a>:
2624 In these rare cases, the type parameter declaration is indistinguishable from
2625 the expressions <code>P*C</code> or <code>P(C)</code> and the type declaration
2626 is parsed as an array type declaration.
2627 To resolve the ambiguity, embed the constraint in an interface or use a trailing
2632 type T[P interface{*C}] …
2636 <h4 id="Type_constraints">Type constraints</h4>
2639 A type constraint is an <a href="#Interface_types">interface</a> that defines the
2640 set of permissible type arguments for the respective type parameter and controls the
2641 operations supported by values of that type parameter.
2645 TypeConstraint = TypeElem .
2649 If the constraint is an interface literal containing exactly one embedded type element
2650 <code>interface{E}</code>, in a type parameter list the enclosing <code>interface{ … }</code>
2651 may be omitted for convenience:
2655 [T *P] // = [T interface{*P}]
2656 [T ~int] // = [T interface{~int}]
2657 [T int|string] // = [T interface{int|string}]
2658 type Constraint ~int // illegal: ~int is not inside a type parameter list
2662 We should be able to simplify the rules for comparable or delegate some of them
2663 elsewhere once we have a section that clearly defines how interfaces implement
2664 other interfaces based on their type sets. But this should get us going for now.
2668 The <a href="#Predeclared_identifiers">predeclared</a>
2669 <a href="#Interface_types">interface type</a> <code>comparable</code>
2670 denotes the set of all concrete (non-interface) types that are
2671 <a href="#Comparison_operators">comparable</a>. Specifically,
2672 a type <code>T</code> implements <code>comparable</code> if:
2677 <code>T</code> is not an interface type and <code>T</code> supports the operations
2678 <code>==</code> and <code>!=</code>; or
2681 <code>T</code> is an interface type and each type in <code>T</code>'s
2682 <a href="#Interface_types">type set</a> implements <code>comparable</code>.
2687 Even though interfaces that are not type parameters can be
2688 <a href="#Comparison_operators">compared</a>
2689 (possibly causing a run-time panic) they do not implement
2690 <code>comparable</code>.
2694 int // implements comparable
2695 []byte // does not implement comparable (slices cannot be compared)
2696 interface{} // does not implement comparable (see above)
2697 interface{ ~int | ~string } // type parameter only: implements comparable
2698 interface{ comparable } // type parameter only: implements comparable
2699 interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (not all types in the type set are comparable)
2703 The <code>comparable</code> interface and interfaces that (directly or indirectly) embed
2704 <code>comparable</code> may only be used as type constraints. They cannot be the types of
2705 values or variables, or components of other, non-interface types.
2708 <h3 id="Variable_declarations">Variable declarations</h3>
2711 A variable declaration creates one or more <a href="#Variables">variables</a>,
2712 binds corresponding identifiers to them, and gives each a type and an initial value.
2716 VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
2717 VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
2724 var x, y float32 = -1, -2
2727 u, v, s = 2.0, 3.0, "bar"
2729 var re, im = complexSqrt(-1)
2730 var _, found = entries[name] // map lookup; only interested in "found"
2734 If a list of expressions is given, the variables are initialized
2735 with the expressions following the rules for <a href="#Assignments">assignments</a>.
2736 Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>.
2740 If a type is present, each variable is given that type.
2741 Otherwise, each variable is given the type of the corresponding
2742 initialization value in the assignment.
2743 If that value is an untyped constant, it is first implicitly
2744 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>;
2745 if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>.
2746 The predeclared value <code>nil</code> cannot be used to initialize a variable
2747 with no explicit type.
2751 var d = math.Sin(0.5) // d is float64
2752 var i = 42 // i is int
2753 var t, ok = x.(T) // t is T, ok is bool
2754 var n = nil // illegal
2758 Implementation restriction: A compiler may make it illegal to declare a variable
2759 inside a <a href="#Function_declarations">function body</a> if the variable is
2763 <h3 id="Short_variable_declarations">Short variable declarations</h3>
2766 A <i>short variable declaration</i> uses the syntax:
2770 ShortVarDecl = IdentifierList ":=" ExpressionList .
2774 It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a>
2775 with initializer expressions but no types:
2778 <pre class="grammar">
2779 "var" IdentifierList = ExpressionList .
2784 f := func() int { return 7 }
2785 ch := make(chan int)
2786 r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any
2787 _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate
2791 Unlike regular variable declarations, a short variable declaration may <i>redeclare</i>
2792 variables provided they were originally declared earlier in the same block
2793 (or the parameter lists if the block is the function body) with the same type,
2794 and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new.
2795 As a consequence, redeclaration can only appear in a multi-variable short declaration.
2796 Redeclaration does not introduce a new variable; it just assigns a new value to the original.
2800 field1, offset := nextField(str, 0)
2801 field2, offset := nextField(str, offset) // redeclares offset
2802 a, a := 1, 2 // illegal: double declaration of a or no new variable if a was declared elsewhere
2806 Short variable declarations may appear only inside functions.
2807 In some contexts such as the initializers for
2808 <a href="#If_statements">"if"</a>,
2809 <a href="#For_statements">"for"</a>, or
2810 <a href="#Switch_statements">"switch"</a> statements,
2811 they can be used to declare local temporary variables.
2814 <h3 id="Function_declarations">Function declarations</h3>
2817 Given the importance of functions, this section has always
2818 been woefully underdeveloped. Would be nice to expand this
2823 A function declaration binds an identifier, the <i>function name</i>,
2828 FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
2829 FunctionName = identifier .
2830 FunctionBody = Block .
2834 If the function's <a href="#Function_types">signature</a> declares
2835 result parameters, the function body's statement list must end in
2836 a <a href="#Terminating_statements">terminating statement</a>.
2840 func IndexRune(s string, r rune) int {
2841 for i, c := range s {
2846 // invalid: missing return statement
2851 If the function declaration specifies <a href="#Type_parameter_lists">type parameters</a>,
2852 the function name denotes a <i>type-parameterized function</i>.
2853 Type-parameterized functions must be <a href="#Instantiations">instantiated</a> when they
2858 func min[T constraints.Ordered](x, y T) T {
2867 A function declaration without type parameters may omit the body.
2868 Such a declaration provides the signature for a function implemented outside Go,
2869 such as an assembly routine.
2873 func flushICache(begin, end uintptr) // implemented externally
2876 <h3 id="Method_declarations">Method declarations</h3>
2879 A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>.
2880 A method declaration binds an identifier, the <i>method name</i>, to a method,
2881 and associates the method with the receiver's <i>base type</i>.
2885 MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
2886 Receiver = Parameters .
2890 The receiver is specified via an extra parameter section preceding the method
2891 name. That parameter section must declare a single non-variadic parameter, the receiver.
2892 Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a
2893 pointer to a defined type <code>T</code>, possibly followed by a list of type parameter
2894 names <code>[P1, P2, …]</code> enclosed in square brackets.
2895 <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be
2896 a pointer or interface type and it must be defined in the same package as the method.
2897 The method is said to be <i>bound</i> to its receiver base type and the method name
2898 is visible only within <a href="#Selectors">selectors</a> for type <code>T</code>
2903 A non-<a href="#Blank_identifier">blank</a> receiver identifier must be
2904 <a href="#Uniqueness_of_identifiers">unique</a> in the method signature.
2905 If the receiver's value is not referenced inside the body of the method,
2906 its identifier may be omitted in the declaration. The same applies in
2907 general to parameters of functions and methods.
2911 For a base type, the non-blank names of methods bound to it must be unique.
2912 If the base type is a <a href="#Struct_types">struct type</a>,
2913 the non-blank method and field names must be distinct.
2917 Given defined type <code>Point</code>, the declarations
2921 func (p *Point) Length() float64 {
2922 return math.Sqrt(p.x * p.x + p.y * p.y)
2925 func (p *Point) Scale(factor float64) {
2932 bind the methods <code>Length</code> and <code>Scale</code>,
2933 with receiver type <code>*Point</code>,
2934 to the base type <code>Point</code>.
2938 If the receiver base type is a <a href="#Type_declarations">parameterized type</a>, the
2939 receiver specification must declare corresponding type parameters for the method
2940 to use. This makes the receiver type parameters available to the method.
2944 Syntactically, this type parameter declaration looks like an
2945 <a href="#Instantiations">instantiation</a> of the receiver base type, except that
2946 the type arguments are the type parameters being declared, one for each type parameter
2947 of the receiver base type.
2948 The type parameter names do not need to match their corresponding parameter names in the
2949 receiver base type definition, and all non-blank parameter names must be unique in the
2950 receiver parameter section and the method signature.
2951 The receiver type parameter constraints are implied by the receiver base type definition:
2952 corresponding type parameters have corresponding constraints.
2956 type Pair[A, B any] struct {
2961 func (p Pair[A, B]) Swap() Pair[B, A] { return Pair[B, A]{p.b, p.a} }
2962 func (p Pair[First, _]) First() First { return p.a }
2965 <h2 id="Expressions">Expressions</h2>
2968 An expression specifies the computation of a value by applying
2969 operators and functions to operands.
2972 <h3 id="Operands">Operands</h3>
2975 Operands denote the elementary values in an expression. An operand may be a
2976 literal, a (possibly <a href="#Qualified_identifiers">qualified</a>)
2977 non-<a href="#Blank_identifier">blank</a> identifier denoting a
2978 <a href="#Constant_declarations">constant</a>,
2979 <a href="#Variable_declarations">variable</a>, or
2980 <a href="#Function_declarations">function</a>,
2981 or a parenthesized expression.
2985 An operand name denoting a <a href="#Function_declarations">type-parameterized function</a>
2986 may be followed by a list of <a href="#Instantiations">type arguments</a>; the
2987 resulting operand is an <a href="#Instantiations">instantiated</a> function.
2991 The <a href="#Blank_identifier">blank identifier</a> may appear as an
2992 operand only on the left-hand side of an <a href="#Assignments">assignment</a>.
2996 Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
2997 Literal = BasicLit | CompositeLit | FunctionLit .
2998 BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
2999 OperandName = identifier | QualifiedIdent .
3002 <h3 id="Qualified_identifiers">Qualified identifiers</h3>
3005 A <i>qualified identifier</i> is an identifier qualified with a package name prefix.
3006 Both the package name and the identifier must not be
3007 <a href="#Blank_identifier">blank</a>.
3011 QualifiedIdent = PackageName "." identifier .
3015 A qualified identifier accesses an identifier in a different package, which
3016 must be <a href="#Import_declarations">imported</a>.
3017 The identifier must be <a href="#Exported_identifiers">exported</a> and
3018 declared in the <a href="#Blocks">package block</a> of that package.
3022 math.Sin // denotes the Sin function in package math
3025 <h3 id="Composite_literals">Composite literals</h3>
3028 Composite literals construct values for structs, arrays, slices, and maps
3029 and create a new value each time they are evaluated.
3030 They consist of the type of the literal followed by a brace-bound list of elements.
3031 Each element may optionally be preceded by a corresponding key.
3035 CompositeLit = LiteralType LiteralValue .
3036 LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
3037 SliceType | MapType | TypeName .
3038 LiteralValue = "{" [ ElementList [ "," ] ] "}" .
3039 ElementList = KeyedElement { "," KeyedElement } .
3040 KeyedElement = [ Key ":" ] Element .
3041 Key = FieldName | Expression | LiteralValue .
3042 FieldName = identifier .
3043 Element = Expression | LiteralValue .
3047 The LiteralType's underlying type must be a struct, array, slice, or map type
3048 (the grammar enforces this constraint except when the type is given
3050 The types of the elements and keys must be <a href="#Assignability">assignable</a>
3051 to the respective field, element, and key types of the literal type;
3052 there is no additional conversion.
3053 The key is interpreted as a field name for struct literals,
3054 an index for array and slice literals, and a key for map literals.
3055 For map literals, all elements must have a key. It is an error
3056 to specify multiple elements with the same field name or
3057 constant key value. For non-constant map keys, see the section on
3058 <a href="#Order_of_evaluation">evaluation order</a>.
3062 For struct literals the following rules apply:
3065 <li>A key must be a field name declared in the struct type.
3067 <li>An element list that does not contain any keys must
3068 list an element for each struct field in the
3069 order in which the fields are declared.
3071 <li>If any element has a key, every element must have a key.
3073 <li>An element list that contains keys does not need to
3074 have an element for each struct field. Omitted fields
3075 get the zero value for that field.
3077 <li>A literal may omit the element list; such a literal evaluates
3078 to the zero value for its type.
3080 <li>It is an error to specify an element for a non-exported
3081 field of a struct belonging to a different package.
3086 Given the declarations
3089 type Point3D struct { x, y, z float64 }
3090 type Line struct { p, q Point3D }
3098 origin := Point3D{} // zero value for Point3D
3099 line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x
3103 For array and slice literals the following rules apply:
3106 <li>Each element has an associated integer index marking
3107 its position in the array.
3109 <li>An element with a key uses the key as its index. The
3110 key must be a non-negative constant
3111 <a href="#Representability">representable</a> by
3112 a value of type <code>int</code>; and if it is typed
3113 it must be of <a href="#Numeric_types">integer type</a>.
3115 <li>An element without a key uses the previous element's index plus one.
3116 If the first element has no key, its index is zero.
3121 <a href="#Address_operators">Taking the address</a> of a composite literal
3122 generates a pointer to a unique <a href="#Variables">variable</a> initialized
3123 with the literal's value.
3127 var pointer *Point3D = &Point3D{y: 1000}
3131 Note that the <a href="#The_zero_value">zero value</a> for a slice or map
3132 type is not the same as an initialized but empty value of the same type.
3133 Consequently, taking the address of an empty slice or map composite literal
3134 does not have the same effect as allocating a new slice or map value with
3135 <a href="#Allocation">new</a>.
3139 p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0
3140 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0
3144 The length of an array literal is the length specified in the literal type.
3145 If fewer elements than the length are provided in the literal, the missing
3146 elements are set to the zero value for the array element type.
3147 It is an error to provide elements with index values outside the index range
3148 of the array. The notation <code>...</code> specifies an array length equal
3149 to the maximum element index plus one.
3153 buffer := [10]string{} // len(buffer) == 10
3154 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6
3155 days := [...]string{"Sat", "Sun"} // len(days) == 2
3159 A slice literal describes the entire underlying array literal.
3160 Thus the length and capacity of a slice literal are the maximum
3161 element index plus one. A slice literal has the form
3169 and is shorthand for a slice operation applied to an array:
3173 tmp := [n]T{x1, x2, … xn}
3178 Within a composite literal of array, slice, or map type <code>T</code>,
3179 elements or map keys that are themselves composite literals may elide the respective
3180 literal type if it is identical to the element or key type of <code>T</code>.
3181 Similarly, elements or keys that are addresses of composite literals may elide
3182 the <code>&T</code> when the element or key type is <code>*T</code>.
3186 [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
3187 [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
3188 [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
3189 map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}}
3190 map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"}
3193 [2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}}
3194 [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})}
3198 A parsing ambiguity arises when a composite literal using the
3199 TypeName form of the LiteralType appears as an operand between the
3200 <a href="#Keywords">keyword</a> and the opening brace of the block
3201 of an "if", "for", or "switch" statement, and the composite literal
3202 is not enclosed in parentheses, square brackets, or curly braces.
3203 In this rare case, the opening brace of the literal is erroneously parsed
3204 as the one introducing the block of statements. To resolve the ambiguity,
3205 the composite literal must appear within parentheses.
3209 if x == (T{a,b,c}[i]) { … }
3210 if (x == T{a,b,c}[i]) { … }
3214 Examples of valid array, slice, and map literals:
3218 // list of prime numbers
3219 primes := []int{2, 3, 5, 7, 9, 2147483647}
3221 // vowels[ch] is true if ch is a vowel
3222 vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}
3224 // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
3225 filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}
3227 // frequencies in Hz for equal-tempered scale (A4 = 440Hz)
3228 noteFrequency := map[string]float32{
3229 "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
3230 "G0": 24.50, "A0": 27.50, "B0": 30.87,
3235 <h3 id="Function_literals">Function literals</h3>
3238 A function literal represents an anonymous <a href="#Function_declarations">function</a>.
3239 Function literals cannot declare type parameters.
3243 FunctionLit = "func" Signature FunctionBody .
3247 func(a, b int, z float64) bool { return a*b < int(z) }
3251 A function literal can be assigned to a variable or invoked directly.
3255 f := func(x, y int) int { return x + y }
3256 func(ch chan int) { ch <- ACK }(replyChan)
3260 Function literals are <i>closures</i>: they may refer to variables
3261 defined in a surrounding function. Those variables are then shared between
3262 the surrounding function and the function literal, and they survive as long
3263 as they are accessible.
3267 <h3 id="Primary_expressions">Primary expressions</h3>
3270 Primary expressions are the operands for unary and binary expressions.
3278 PrimaryExpr Selector |
3281 PrimaryExpr TypeAssertion |
3282 PrimaryExpr Arguments .
3284 Selector = "." identifier .
3285 Index = "[" Expression "]" .
3286 Slice = "[" [ Expression ] ":" [ Expression ] "]" |
3287 "[" [ Expression ] ":" Expression ":" Expression "]" .
3288 TypeAssertion = "." "(" Type ")" .
3289 Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .
3306 <h3 id="Selectors">Selectors</h3>
3309 For a <a href="#Primary_expressions">primary expression</a> <code>x</code>
3310 that is not a <a href="#Package_clause">package name</a>, the
3311 <i>selector expression</i>
3319 denotes the field or method <code>f</code> of the value <code>x</code>
3320 (or sometimes <code>*x</code>; see below).
3321 The identifier <code>f</code> is called the (field or method) <i>selector</i>;
3322 it must not be the <a href="#Blank_identifier">blank identifier</a>.
3323 The type of the selector expression is the type of <code>f</code>.
3324 If <code>x</code> is a package name, see the section on
3325 <a href="#Qualified_identifiers">qualified identifiers</a>.
3329 A selector <code>f</code> may denote a field or method <code>f</code> of
3330 a type <code>T</code>, or it may refer
3331 to a field or method <code>f</code> of a nested
3332 <a href="#Struct_types">embedded field</a> of <code>T</code>.
3333 The number of embedded fields traversed
3334 to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
3335 The depth of a field or method <code>f</code>
3336 declared in <code>T</code> is zero.
3337 The depth of a field or method <code>f</code> declared in
3338 an embedded field <code>A</code> in <code>T</code> is the
3339 depth of <code>f</code> in <code>A</code> plus one.
3343 The following rules apply to selectors:
3348 For a value <code>x</code> of type <code>T</code> or <code>*T</code>
3349 where <code>T</code> is not a pointer or interface type,
3350 <code>x.f</code> denotes the field or method at the shallowest depth
3351 in <code>T</code> where there
3352 is such an <code>f</code>.
3353 If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a>
3354 with shallowest depth, the selector expression is illegal.
3358 For a value <code>x</code> of type <code>I</code> where <code>I</code>
3359 is an interface type, <code>x.f</code> denotes the actual method with name
3360 <code>f</code> of the dynamic value of <code>x</code>.
3361 If there is no method with name <code>f</code> in the
3362 <a href="#Method_sets">method set</a> of <code>I</code>, the selector
3363 expression is illegal.
3367 As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a>
3368 pointer type and <code>(*x).f</code> is a valid selector expression denoting a field
3369 (but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>.
3373 In all other cases, <code>x.f</code> is illegal.
3377 If <code>x</code> is of pointer type and has the value
3378 <code>nil</code> and <code>x.f</code> denotes a struct field,
3379 assigning to or evaluating <code>x.f</code>
3380 causes a <a href="#Run_time_panics">run-time panic</a>.
3384 If <code>x</code> is of interface type and has the value
3385 <code>nil</code>, <a href="#Calls">calling</a> or
3386 <a href="#Method_values">evaluating</a> the method <code>x.f</code>
3387 causes a <a href="#Run_time_panics">run-time panic</a>.
3392 For example, given the declarations:
3418 var t T2 // with t.T0 != nil
3419 var p *T2 // with p != nil and (*p).T0 != nil
3436 q.x // (*(*q).T0).x (*q).x is a valid field selector
3438 p.M0() // ((*p).T0).M0() M0 expects *T0 receiver
3439 p.M1() // ((*p).T1).M1() M1 expects T1 receiver
3440 p.M2() // p.M2() M2 expects *T2 receiver
3441 t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls
3445 but the following is invalid:
3449 q.M0() // (*q).M0 is valid but not a field selector
3453 <h3 id="Method_expressions">Method expressions</h3>
3456 If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3457 <code>T.M</code> is a function that is callable as a regular function
3458 with the same arguments as <code>M</code> prefixed by an additional
3459 argument that is the receiver of the method.
3463 MethodExpr = ReceiverType "." MethodName .
3464 ReceiverType = Type .
3468 Consider a struct type <code>T</code> with two methods,
3469 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3470 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3477 func (tv T) Mv(a int) int { return 0 } // value receiver
3478 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3492 yields a function equivalent to <code>Mv</code> but
3493 with an explicit receiver as its first argument; it has signature
3497 func(tv T, a int) int
3501 That function may be called normally with an explicit receiver, so
3502 these five invocations are equivalent:
3509 f1 := T.Mv; f1(t, 7)
3510 f2 := (T).Mv; f2(t, 7)
3514 Similarly, the expression
3522 yields a function value representing <code>Mp</code> with signature
3526 func(tp *T, f float32) float32
3530 For a method with a value receiver, one can derive a function
3531 with an explicit pointer receiver, so
3539 yields a function value representing <code>Mv</code> with signature
3543 func(tv *T, a int) int
3547 Such a function indirects through the receiver to create a value
3548 to pass as the receiver to the underlying method;
3549 the method does not overwrite the value whose address is passed in
3554 The final case, a value-receiver function for a pointer-receiver method,
3555 is illegal because pointer-receiver methods are not in the method set
3560 Function values derived from methods are called with function call syntax;
3561 the receiver is provided as the first argument to the call.
3562 That is, given <code>f := T.Mv</code>, <code>f</code> is invoked
3563 as <code>f(t, 7)</code> not <code>t.f(7)</code>.
3564 To construct a function that binds the receiver, use a
3565 <a href="#Function_literals">function literal</a> or
3566 <a href="#Method_values">method value</a>.
3570 It is legal to derive a function value from a method of an interface type.
3571 The resulting function takes an explicit receiver of that interface type.
3574 <h3 id="Method_values">Method values</h3>
3577 If the expression <code>x</code> has static type <code>T</code> and
3578 <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>,
3579 <code>x.M</code> is called a <i>method value</i>.
3580 The method value <code>x.M</code> is a function value that is callable
3581 with the same arguments as a method call of <code>x.M</code>.
3582 The expression <code>x</code> is evaluated and saved during the evaluation of the
3583 method value; the saved copy is then used as the receiver in any calls,
3584 which may be executed later.
3588 type S struct { *T }
3590 func (t T) M() { print(t) }
3594 f := t.M // receiver *t is evaluated and stored in f
3595 g := s.M // receiver *(s.T) is evaluated and stored in g
3596 *t = 42 // does not affect stored receivers in f and g
3600 The type <code>T</code> may be an interface or non-interface type.
3604 As in the discussion of <a href="#Method_expressions">method expressions</a> above,
3605 consider a struct type <code>T</code> with two methods,
3606 <code>Mv</code>, whose receiver is of type <code>T</code>, and
3607 <code>Mp</code>, whose receiver is of type <code>*T</code>.
3614 func (tv T) Mv(a int) int { return 0 } // value receiver
3615 func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
3631 yields a function value of type
3639 These two invocations are equivalent:
3648 Similarly, the expression
3656 yields a function value of type
3660 func(float32) float32
3664 As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver
3665 using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>.
3669 As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver
3670 using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>.
3674 f := t.Mv; f(7) // like t.Mv(7)
3675 f := pt.Mp; f(7) // like pt.Mp(7)
3676 f := pt.Mv; f(7) // like (*pt).Mv(7)
3677 f := t.Mp; f(7) // like (&t).Mp(7)
3678 f := makeT().Mp // invalid: result of makeT() is not addressable
3682 Although the examples above use non-interface types, it is also legal to create a method value
3683 from a value of interface type.
3687 var i interface { M(int) } = myVal
3688 f := i.M; f(7) // like i.M(7)
3692 <h3 id="Index_expressions">Index expressions</h3>
3695 A primary expression of the form
3703 denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>.
3704 The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively.
3705 The following rules apply:
3709 If <code>a</code> is not a map:
3712 <li>the index <code>x</code> must be of <a href="#Numeric_types">integer type</a> or an untyped constant</li>
3713 <li>a constant index must be non-negative and
3714 <a href="#Representability">representable</a> by a value of type <code>int</code></li>
3715 <li>a constant index that is untyped is given type <code>int</code></li>
3716 <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>,
3717 otherwise it is <i>out of range</i></li>
3721 For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>:
3724 <li>a <a href="#Constants">constant</a> index must be in range</li>
3725 <li>if <code>x</code> is out of range at run time,
3726 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3727 <li><code>a[x]</code> is the array element at index <code>x</code> and the type of
3728 <code>a[x]</code> is the element type of <code>A</code></li>
3732 For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type:
3735 <li><code>a[x]</code> is shorthand for <code>(*a)[x]</code></li>
3739 For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>:
3742 <li>if <code>x</code> is out of range at run time,
3743 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3744 <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of
3745 <code>a[x]</code> is the element type of <code>S</code></li>
3749 For <code>a</code> of <a href="#String_types">string type</a>:
3752 <li>a <a href="#Constants">constant</a> index must be in range
3753 if the string <code>a</code> is also constant</li>
3754 <li>if <code>x</code> is out of range at run time,
3755 a <a href="#Run_time_panics">run-time panic</a> occurs</li>
3756 <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of
3757 <code>a[x]</code> is <code>byte</code></li>
3758 <li><code>a[x]</code> may not be assigned to</li>
3762 For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>:
3765 <li><code>x</code>'s type must be
3766 <a href="#Assignability">assignable</a>
3767 to the key type of <code>M</code></li>
3768 <li>if the map contains an entry with key <code>x</code>,
3769 <code>a[x]</code> is the map element with key <code>x</code>
3770 and the type of <code>a[x]</code> is the element type of <code>M</code></li>
3771 <li>if the map is <code>nil</code> or does not contain such an entry,
3772 <code>a[x]</code> is the <a href="#The_zero_value">zero value</a>
3773 for the element type of <code>M</code></li>
3777 For <code>a</code> of <a href="#Type_parameters">type parameter type</a> <code>P</code>:
3780 <li><code>P</code> must have <a href="#Structure_of_interfaces">specific types</a>.</li>
3781 <li>The index expression <code>a[x]</code> must be valid for values
3782 of all specific types of <code>P</code>.</li>
3783 <li>The element types of all specific types of <code>P</code> must be identical.
3784 In this context, the element type of a string type is <code>byte</code>.</li>
3785 <li>If there is a map type among the specific types of <code>P</code>,
3786 all specific types must be map types, and the respective key types
3787 must be all identical.</li>
3788 <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>,
3789 or the map element with key <code>x</code> of the type argument
3790 that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is
3791 the type of the (identical) element types.</li>
3792 <li><code>a[x]</code> may not be assigned to if the specific types of <code>P</code>
3793 include string types.
3797 Otherwise <code>a[x]</code> is illegal.
3801 An index expression on a map <code>a</code> of type <code>map[K]V</code>
3802 used in an <a href="#Assignments">assignment</a> or initialization of the special form
3812 yields an additional untyped boolean value. The value of <code>ok</code> is
3813 <code>true</code> if the key <code>x</code> is present in the map, and
3814 <code>false</code> otherwise.
3818 Assigning to an element of a <code>nil</code> map causes a
3819 <a href="#Run_time_panics">run-time panic</a>.
3823 <h3 id="Slice_expressions">Slice expressions</h3>
3826 Slice expressions construct a substring or slice from a string, array, pointer
3827 to array, or slice. There are two variants: a simple form that specifies a low
3828 and high bound, and a full form that also specifies a bound on the capacity.
3831 <h4>Simple slice expressions</h4>
3834 For a string, array, pointer to array, or slice <code>a</code>, the primary expression
3842 constructs a substring or slice. The <i>indices</i> <code>low</code> and
3843 <code>high</code> select which elements of operand <code>a</code> appear
3844 in the result. The result has indices starting at 0 and length equal to
3845 <code>high</code> - <code>low</code>.
3846 After slicing the array <code>a</code>
3850 a := [5]int{1, 2, 3, 4, 5}
3855 the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements
3865 For convenience, any of the indices may be omitted. A missing <code>low</code>
3866 index defaults to zero; a missing <code>high</code> index defaults to the length of the
3871 a[2:] // same as a[2 : len(a)]
3872 a[:3] // same as a[0 : 3]
3873 a[:] // same as a[0 : len(a)]
3877 If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for
3878 <code>(*a)[low : high]</code>.
3882 For arrays or strings, the indices are <i>in range</i> if
3883 <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>,
3884 otherwise they are <i>out of range</i>.
3885 For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length.
3886 A <a href="#Constants">constant</a> index must be non-negative and
3887 <a href="#Representability">representable</a> by a value of type
3888 <code>int</code>; for arrays or constant strings, constant indices must also be in range.
3889 If both indices are constant, they must satisfy <code>low <= high</code>.
3890 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3894 Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice,
3895 the result of the slice operation is a non-constant value of the same type as the operand.
3896 For untyped string operands the result is a non-constant value of type <code>string</code>.
3897 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>
3898 and the result of the slice operation is a slice with the same element type as the array.
3902 If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result
3903 is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying
3904 array with the operand.
3909 s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5]
3910 s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
3911 s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
3915 <h4>Full slice expressions</h4>
3918 For an array, pointer to array, or slice <code>a</code> (but not a string), the primary expression
3926 constructs a slice of the same type, and with the same length and elements as the simple slice
3927 expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity
3928 by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0.
3929 After slicing the array <code>a</code>
3933 a := [5]int{1, 2, 3, 4, 5}
3938 the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements
3947 As for simple slice expressions, if <code>a</code> is a pointer to an array,
3948 <code>a[low : high : max]</code> is shorthand for <code>(*a)[low : high : max]</code>.
3949 If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>.
3953 The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>,
3954 otherwise they are <i>out of range</i>.
3955 A <a href="#Constants">constant</a> index must be non-negative and
3956 <a href="#Representability">representable</a> by a value of type
3957 <code>int</code>; for arrays, constant indices must also be in range.
3958 If multiple indices are constant, the constants that are present must be in range relative to each
3960 If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
3963 <h3 id="Type_assertions">Type assertions</h3>
3966 For an expression <code>x</code> of <a href="#Interface_types">interface type</a>,
3967 but not a <a href="#Type_parameters">type parameter</a>, and a type <code>T</code>,
3968 the primary expression
3976 asserts that <code>x</code> is not <code>nil</code>
3977 and that the value stored in <code>x</code> is of type <code>T</code>.
3978 The notation <code>x.(T)</code> is called a <i>type assertion</i>.
3981 More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
3982 that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a>
3983 to the type <code>T</code>.
3984 In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>;
3985 otherwise the type assertion is invalid since it is not possible for <code>x</code>
3986 to store a value of type <code>T</code>.
3987 If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
3988 of <code>x</code> implements the interface <code>T</code>.
3991 If the type assertion holds, the value of the expression is the value
3992 stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false,
3993 a <a href="#Run_time_panics">run-time panic</a> occurs.
3994 In other words, even though the dynamic type of <code>x</code>
3995 is known only at run time, the type of <code>x.(T)</code> is
3996 known to be <code>T</code> in a correct program.
4000 var x interface{} = 7 // x has dynamic type int and value 7
4001 i := x.(int) // i has type int and value 7
4003 type I interface { m() }
4006 s := y.(string) // illegal: string does not implement I (missing method m)
4007 r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader
4013 A type assertion used in an <a href="#Assignments">assignment</a> or initialization of the special form
4020 var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool
4024 yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code>
4025 if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is
4026 the <a href="#The_zero_value">zero value</a> for type <code>T</code>.
4027 No <a href="#Run_time_panics">run-time panic</a> occurs in this case.
4031 <h3 id="Calls">Calls</h3>
4034 Given an expression <code>f</code> of function type
4043 calls <code>f</code> with arguments <code>a1, a2, … an</code>.
4044 Except for one special case, arguments must be single-valued expressions
4045 <a href="#Assignability">assignable</a> to the parameter types of
4046 <code>F</code> and are evaluated before the function is called.
4047 The type of the expression is the result type
4049 A method invocation is similar but the method itself
4050 is specified as a selector upon a value of the receiver type for
4055 math.Atan2(x, y) // function call
4057 pt.Scale(3.5) // method call with receiver pt
4061 If <code>f</code> denotes a parameterized function, it must be
4062 <a href="#Instantiations">instantiated</a> before it can be called
4063 or used as a function value.
4067 In a function call, the function value and arguments are evaluated in
4068 <a href="#Order_of_evaluation">the usual order</a>.
4069 After they are evaluated, the parameters of the call are passed by value to the function
4070 and the called function begins execution.
4071 The return parameters of the function are passed by value
4072 back to the caller when the function returns.
4076 Calling a <code>nil</code> function value
4077 causes a <a href="#Run_time_panics">run-time panic</a>.
4081 As a special case, if the return values of a function or method
4082 <code>g</code> are equal in number and individually
4083 assignable to the parameters of another function or method
4084 <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code>
4085 will invoke <code>f</code> after binding the return values of
4086 <code>g</code> to the parameters of <code>f</code> in order. The call
4087 of <code>f</code> must contain no parameters other than the call of <code>g</code>,
4088 and <code>g</code> must have at least one return value.
4089 If <code>f</code> has a final <code>...</code> parameter, it is
4090 assigned the return values of <code>g</code> that remain after
4091 assignment of regular parameters.
4095 func Split(s string, pos int) (string, string) {
4096 return s[0:pos], s[pos:]
4099 func Join(s, t string) string {
4103 if Join(Split(value, len(value)/2)) != value {
4104 log.Panic("test fails")
4109 A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a>
4110 of (the type of) <code>x</code> contains <code>m</code> and the
4111 argument list can be assigned to the parameter list of <code>m</code>.
4112 If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method
4113 set contains <code>m</code>, <code>x.m()</code> is shorthand
4114 for <code>(&x).m()</code>:
4123 There is no distinct method type and there are no method literals.
4126 <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3>
4129 If <code>f</code> is <a href="#Function_types">variadic</a> with a final
4130 parameter <code>p</code> of type <code>...T</code>, then within <code>f</code>
4131 the type of <code>p</code> is equivalent to type <code>[]T</code>.
4132 If <code>f</code> is invoked with no actual arguments for <code>p</code>,
4133 the value passed to <code>p</code> is <code>nil</code>.
4134 Otherwise, the value passed is a new slice
4135 of type <code>[]T</code> with a new underlying array whose successive elements
4136 are the actual arguments, which all must be <a href="#Assignability">assignable</a>
4137 to <code>T</code>. The length and capacity of the slice is therefore
4138 the number of arguments bound to <code>p</code> and may differ for each
4143 Given the function and calls
4146 func Greeting(prefix string, who ...string)
4148 Greeting("hello:", "Joe", "Anna", "Eileen")
4152 within <code>Greeting</code>, <code>who</code> will have the value
4153 <code>nil</code> in the first call, and
4154 <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second.
4158 If the final argument is assignable to a slice type <code>[]T</code> and
4159 is followed by <code>...</code>, it is passed unchanged as the value
4160 for a <code>...T</code> parameter. In this case no new slice is created.
4164 Given the slice <code>s</code> and call
4168 s := []string{"James", "Jasmine"}
4169 Greeting("goodbye:", s...)
4173 within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code>
4174 with the same underlying array.
4177 <h3 id="Instantiations">Instantiations</h3>
4180 A parameterized function or type is <i>instantiated</i> by substituting <i>type arguments</i>
4181 for the type parameters.
4182 Instantiation proceeds in two phases:
4187 Each type argument is substituted for its corresponding type parameter in the parameterized
4189 This substitution happens across the entire function or type declaration,
4190 including the type parameter list itself and any types in that list.
4194 After substitution, each type argument must <a href="#Interface_types">implement</a>
4195 the <a href="#Type_parameter_lists">constraint</a> (instantiated, if necessary)
4196 of the corresponding type parameter. Otherwise instantiation fails.
4201 Instantiating a type results in a new non-parameterized <a href="#Types">named type</a>;
4202 instantiating a function produces a new non-parameterized function.
4206 type parameter list type arguments after substitution
4208 [P any] int [int any]
4209 [S ~[]E, E any] []int, int [[]int ~[]int, int any]
4210 [P io.Writer] string [string io.Writer] // illegal: string doesn't implement io.Writer
4214 Type arguments may be provided explicitly, or they may be partially or completely
4215 <a href="#Type_inference">inferred</a>.
4216 A partially provided type argument list cannot be empty; there must be at least the
4221 type T[P1 ~int, P2 ~[]P1] struct{ … }
4223 T[] // illegal: at least the first type argument must be present, even if it could be inferred
4224 T[int] // argument for P1 explicitly provided, argument for P2 inferred
4225 T[int, []int] // both arguments explicitly provided
4229 A partial type argument list specifies a prefix of the full list of type arguments, leaving
4230 the remaining arguments to be inferred. Loosely speaking, type arguments may be omitted from
4235 Parameterized types, and parameterized functions that are not <a href="#Calls">called</a>,
4236 require a type argument list for instantiation; if the list is partial, all
4237 remaining type arguments must be inferrable.
4238 Calls to parameterized functions may provide a (possibly partial) type
4239 argument list, or may omit it entirely if the omitted type arguments are
4240 inferrable from the ordinary (non-type) function arguments.
4244 func min[T constraints.Ordered](x, y T) T { … }
4246 f := min // illegal: min must be instantiated when used without being called
4247 minInt := min[int] // minInt has type func(x, y int) int
4248 a := minInt(2, 3) // a has value 2 of type int
4249 b := min[float64](2.0, 3) // b has value 2.0 of type float64
4250 c := min(b, -1) // c has value -1.0 of type float64
4253 <h3 id="Type_inference">Type inference</h3>
4256 Missing type arguments may be <i>inferred</i> by a series of steps, described below.
4257 Each step attempts to use known information to infer additional type arguments.
4258 Type inference stops as soon as all type arguments are known.
4259 After type inference is complete, it is still necessary to substitute all type arguments
4260 for type parameters and verify that each type argument implements the relevant constraint;
4261 it is possible for an inferred type argument to fail to implement a constraint, in which
4262 case instantiation fails.
4266 Type inference is based on
4271 a <a href="#Type_parameter_lists">type parameter list</a>
4274 a substitution map <i>M</i> initialized with the known type arguments, if any
4277 a (possibly empty) list of ordinary function arguments (in case of a function call only)
4282 and then proceeds with the following steps:
4287 apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
4288 to all <i>typed</i> ordinary function arguments
4291 apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
4294 apply function argument type inference to all <i>untyped</i> ordinary function arguments
4295 using the default type for each of the untyped function arguments
4298 apply constraint type inference
4303 If there are no ordinary or untyped function arguments, the respective steps are skipped.
4304 Constraint type inference is skipped if the previous step didn't infer any new type arguments,
4305 but it is run at least once if there are missing type arguments.
4309 The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
4310 The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
4311 If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
4314 <h4 id="Type_unification">Type unification</h3>
4317 Type inference is based on <i>type unification</i>. A single unification step
4318 applies to a <a href="#Type_inference">substitution map</a> and two types, either
4319 or both of which may be or contain type parameters. The substitution map tracks
4320 the known (explicitly provided or already inferred) type arguments: the map
4321 contains an entry <code>P</code> → <code>A</code> for each type
4322 parameter <code>P</code> and corresponding known type argument <code>A</code>.
4323 During unification, known type arguments take the place of their corresponding type
4324 parameters when comparing types. Unification is the process of finding substitution
4325 map entries that make the two types equivalent.
4329 For unification, two types that don't contain any type parameters from the current type
4330 parameter list are <i>equivalent</i>
4331 if they are identical, or if they are channel types that are identical ignoring channel
4332 direction, or if their underlying types are equivalent.
4336 Unification works by comparing the structure of pairs of types: their structure
4337 disregarding type parameters must be identical, and types other than type parameters
4339 A type parameter in one type may match any complete subtype in the other type;
4340 each successful match causes an entry to be added to the substitution map.
4341 If the structure differs, or types other than type parameters are not equivalent,
4346 TODO(gri) Somewhere we need to describe the process of adding an entry to the
4347 substitution map: if the entry is already present, the type argument
4348 values are themselves unified.
4352 For example, if <code>T1</code> and <code>T2</code> are type parameters,
4353 <code>[]map[int]bool</code> can be unified with any of the following:
4357 []map[int]bool // types are identical
4358 T1 // adds T1 → []map[int]bool to substitution map
4359 []T1 // adds T1 → map[int]bool to substitution map
4360 []map[T1]T2 // adds T1 → int and T2 → bool to substitution map
4364 On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
4368 int // int is not a slice
4369 struct{} // a struct is not a slice
4370 []struct{} // a struct is not a map
4371 []map[T1]string // map element types don't match
4375 As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
4376 <code>D</code> and a type literal <code>L</code> are never equivalent,
4377 unification compares the underlying type of <code>D</code> with <code>L</code> instead.
4378 For example, given the defined type
4382 type Vector []float64
4386 and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
4387 <code>[]E</code> and adds an entry <code>E</code> → <code>float64</code> to
4388 the substitution map.
4391 <h4 id="Function_argument_type_inference">Function argument type inference</h3>
4393 <!-- In this section and the section on constraint type inference we start with examples
4394 rather than have the examples follow the rules as is customary elsewhere in spec.
4395 Hopefully this helps building an intuition and makes the rules easier to follow. -->
4398 Function argument type inference infers type arguments from function arguments:
4399 if a function parameter is declared with a type <code>T</code> that uses
4401 <a href="#Type_unification">unifying</a> the type of the corresponding
4402 function argument with <code>T</code> may infer type arguments for the type
4403 parameters used by <code>T</code>.
4407 For instance, given the type-parameterized function
4411 func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
4419 var vector []float64
4420 scaledVector := scale(vector, 42)
4424 the type argument for <code>Number</code> can be inferred from the function argument
4425 <code>vector</code> by unifying the type of <code>vector</code> with the corresponding
4426 parameter type: <code>[]float64</code> and <code>[]Number</code>
4427 match in structure and <code>float64</code> matches with <code>Number</code>.
4428 This adds the entry <code>Number</code> → <code>float64</code> to the
4429 <a href="#Type_unification">substitution map</a>.
4430 Untyped arguments, such as the second function argument <code>42</code> here, are ignored
4431 in the first round of function argument type inference and only considered if there are
4432 unresolved type parameters left.
4436 Function argument type inference can be used when the function has ordinary parameters
4437 whose types are defined using the function's type parameters. Inference happens in two
4438 separate phases; each phase operates on a specific list of (parameter, argument) pairs:
4443 The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
4444 type uses type parameters and where the function argument is <i>typed</i>.
4447 The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
4448 type parameter. In this list, the respective function arguments are untyped.
4453 Any other (parameter, argument) pair is ignored.
4457 By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
4458 (or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
4459 of untyped values are always predeclared non-composite types, they can never match against
4460 a composite type, so it is sufficient to only consider parameter types that are single type
4465 Each list is processed in a separate phase:
4470 In the first phase, the parameter and argument types of each pair in <i>Lt</i>
4471 are unified. If unification succeeds for a pair, it may yield new entries that
4472 are added to the substitution map <i>M</i>. If unification fails, type inference
4476 The second phase considers the entries of list <i>Lu</i>. Type parameters for
4477 which the type argument has already been determined are ignored in this phase.
4478 For each remaining pair, the parameter type (which is a single type parameter) and
4479 the <a href="#Constants">default type</a> of the corresponding untyped argument is
4480 unified. If unification fails, type inference fails.
4489 func min[T constraints.Ordered](x, y T) T
4492 min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
4493 min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
4494 min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
4497 <h4 id="Constraint_type_inference">Constraint type inference</h3>
4500 The next paragraph needs to be updated for the new definition of structural type:
4501 The structural type of an interface is the single underlying type of its type set,
4502 if it exists. But for constraint type inference, if the type set consists of exactly
4503 one type, we want to use that one type (which may be a defined type, different from
4504 its underlying == structural type).
4508 Constraint type inference infers type arguments from already known
4509 type arguments by considering <a href="#Structure_of_interfaces">structural type constraints</a>:
4510 if the structural type <code>T</code> of a structural constraint is parameterized,
4511 <a href="#Type_unification">unifying</a> a known type argument with <code>T</code> may
4512 infer type arguments for other type parameters used by the structural type.
4516 For instance, consider the type parameter list with type parameters <code>List</code> and
4521 [List ~[]Elem, Elem any]
4525 Constraint type inference can deduce the type of <code>Elem</code> from the type argument
4526 for <code>List</code> because <code>Elem</code> is a type parameter in the structural constraint
4527 <code>~[]Elem</code> for <code>List</code>.
4528 If the type argument is <code>Bytes</code>:
4536 unifying the underlying type of <code>Bytes</code> with the structural constraint means
4537 unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
4538 the <a href="#Type_unification">substitution map</a> entry
4539 <code>Elem</code> → <code>byte</code>.
4540 Thus, in this example, constraint type inference can infer the second type argument from the
4545 Generally, constraint type inference proceeds in two phases: Starting with a given
4546 substitution map <i>M</i>
4551 For all type parameters with a structural constraint, unify the type parameter with the structural
4552 type of its constraint. If any unification fails, constraint type inference fails.
4556 At this point, some entries in <i>M</i> may map type parameters to other
4557 type parameters or to types containing type parameters. For each entry
4558 <code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
4559 contains type parameters <code>Q</code> for which there exist entries
4560 <code>Q</code> → <code>B</code> in <i>M</i>, substitute those
4561 <code>Q</code> with the respective <code>B</code> in <code>A</code>.
4562 Stop when no further substitution is possible.
4567 The result of constraint type inference is the final substitution map <i>M</i> from type
4568 parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
4569 appears in any of the <code>A</code>.
4573 For instance, given the type parameter list
4577 [A any, B []C, C *A]
4581 and the single provided type argument <code>int</code> for type parameter <code>A</code>,
4582 the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
4586 In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
4587 with the structural type of their respective constraints. This adds the entries
4588 <code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
4592 At this point there are two entries in <i>M</i> where the right-hand side
4593 is or contains type parameters for which there exists other entries in <i>M</i>:
4594 <code>[]C</code> and <code>*A</code>.
4595 In the second phase, these type parameters are replaced with their respective
4596 types. It doesn't matter in which order this happens. Starting with the state
4597 of <i>M</i> after the first phase:
4601 <code>A</code> → <code>int</code>,
4602 <code>B</code> → <code>[]C</code>,
4603 <code>C</code> → <code>*A</code>
4607 Replace <code>A</code> on the right-hand side of → with <code>int</code>:
4611 <code>A</code> → <code>int</code>,
4612 <code>B</code> → <code>[]C</code>,
4613 <code>C</code> → <code>*int</code>
4617 Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
4621 <code>A</code> → <code>int</code>,
4622 <code>B</code> → <code>[]*int</code>,
4623 <code>C</code> → <code>*int</code>
4627 At this point no further substitution is possible and the map is full.
4628 Therefore, <code>M</code> represents the final map of type parameters
4629 to type arguments for the given type parameter list.
4632 <h3 id="Operators">Operators</h3>
4635 Operators combine operands into expressions.
4639 Expression = UnaryExpr | Expression binary_op Expression .
4640 UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
4642 binary_op = "||" | "&&" | rel_op | add_op | mul_op .
4643 rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
4644 add_op = "+" | "-" | "|" | "^" .
4645 mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .
4647 unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
4651 Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>.
4652 For other binary operators, the operand types must be <a href="#Type_identity">identical</a>
4653 unless the operation involves shifts or untyped <a href="#Constants">constants</a>.
4654 For operations involving constants only, see the section on
4655 <a href="#Constant_expressions">constant expressions</a>.
4659 Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a>
4660 and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a>
4661 to the type of the other operand.
4665 The right operand in a shift expression must have <a href="#Numeric_types">integer type</a>
4666 or be an untyped constant <a href="#Representability">representable</a> by a
4667 value of type <code>uint</code>.
4668 If the left operand of a non-constant shift expression is an untyped constant,
4669 it is first implicitly converted to the type it would assume if the shift expression were
4670 replaced by its left operand alone.
4677 // The results of the following examples are given for 64-bit ints.
4678 var i = 1<<s // 1 has type int
4679 var j int32 = 1<<s // 1 has type int32; j == 0
4680 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33
4681 var m int = 1.0<<s // 1.0 has type int; m == 1<<33
4682 var n = 1.0<<s == j // 1.0 has type int32; n == true
4683 var o = 1<<s == 2<<s // 1 and 2 have type int; o == false
4684 var p = 1<<s == 1<<33 // 1 has type int; p == true
4685 var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift
4686 var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift
4687 var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift
4688 var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift
4689 var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift
4690 var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33
4691 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds
4692 var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33
4694 // The results of the following examples are given for 32-bit ints,
4695 // which means the shifts will overflow.
4696 var mm int = 1.0<<s // 1.0 has type int; mm == 0
4697 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true
4698 var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int
4699 var xx = a[1.0<<s] // 1.0 has type int; xx == a[0]
4700 var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0
4703 <h4 id="Operator_precedence">Operator precedence</h4>
4705 Unary operators have the highest precedence.
4706 As the <code>++</code> and <code>--</code> operators form
4707 statements, not expressions, they fall
4708 outside the operator hierarchy.
4709 As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
4711 There are five precedence levels for binary operators.
4712 Multiplication operators bind strongest, followed by addition
4713 operators, comparison operators, <code>&&</code> (logical AND),
4714 and finally <code>||</code> (logical OR):
4717 <pre class="grammar">
4719 5 * / % << >> & &^
4721 3 == != < <= > >=
4727 Binary operators of the same precedence associate from left to right.
4728 For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>.
4737 x == y+1 && <-chanInt > 0
4741 <h3 id="Arithmetic_operators">Arithmetic operators</h3>
4743 Arithmetic operators apply to numeric values and yield a result of the same
4744 type as the first operand. The four standard arithmetic operators (<code>+</code>,
4745 <code>-</code>, <code>*</code>, <code>/</code>) apply to
4746 <a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and
4747 <a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</.
4748 The bitwise logical and shift operators apply to integers only.
4751 <pre class="grammar">
4752 + sum integers, floats, complex values, strings
4753 - difference integers, floats, complex values
4754 * product integers, floats, complex values
4755 / quotient integers, floats, complex values
4756 % remainder integers
4758 & bitwise AND integers
4759 | bitwise OR integers
4760 ^ bitwise XOR integers
4761 &^ bit clear (AND NOT) integers
4763 << left shift integer << integer >= 0
4764 >> right shift integer >> integer >= 0
4767 <h4 id="Integer_operators">Integer operators</h4>
4770 For two integer values <code>x</code> and <code>y</code>, the integer quotient
4771 <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following
4776 x = q*y + r and |r| < |y|
4780 with <code>x / y</code> truncated towards zero
4781 (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>).
4793 The one exception to this rule is that if the dividend <code>x</code> is
4794 the most negative value for the int type of <code>x</code>, the quotient
4795 <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>)
4796 due to two's-complement <a href="#Integer_overflow">integer overflow</a>:
4804 int64 -9223372036854775808
4808 If the divisor is a <a href="#Constants">constant</a>, it must not be zero.
4809 If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs.
4810 If the dividend is non-negative and the divisor is a constant power of 2,
4811 the division may be replaced by a right shift, and computing the remainder may
4812 be replaced by a bitwise AND operation:
4816 x x / 4 x % 4 x >> 2 x & 3
4822 The shift operators shift the left operand by the shift count specified by the
4823 right operand, which must be non-negative. If the shift count is negative at run time,
4824 a <a href="#Run_time_panics">run-time panic</a> occurs.
4825 The shift operators implement arithmetic shifts if the left operand is a signed
4826 integer and logical shifts if it is an unsigned integer.
4827 There is no upper limit on the shift count. Shifts behave
4828 as if the left operand is shifted <code>n</code> times by 1 for a shift
4829 count of <code>n</code>.
4830 As a result, <code>x << 1</code> is the same as <code>x*2</code>
4831 and <code>x >> 1</code> is the same as
4832 <code>x/2</code> but truncated towards negative infinity.
4836 For integer operands, the unary operators
4837 <code>+</code>, <code>-</code>, and <code>^</code> are defined as
4841 <pre class="grammar">
4843 -x negation is 0 - x
4844 ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x
4845 and m = -1 for signed x
4849 <h4 id="Integer_overflow">Integer overflow</h4>
4852 For unsigned integer values, the operations <code>+</code>,
4853 <code>-</code>, <code>*</code>, and <code><<</code> are
4854 computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
4855 the <a href="#Numeric_types">unsigned integer</a>'s type.
4856 Loosely speaking, these unsigned integer operations
4857 discard high bits upon overflow, and programs may rely on "wrap around".
4861 For signed integers, the operations <code>+</code>,
4862 <code>-</code>, <code>*</code>, <code>/</code>, and <code><<</code> may legally
4863 overflow and the resulting value exists and is deterministically defined
4864 by the signed integer representation, the operation, and its operands.
4865 Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>.
4866 A compiler may not optimize code under the assumption that overflow does
4867 not occur. For instance, it may not assume that <code>x < x + 1</code> is always true.
4871 <h4 id="Floating_point_operators">Floating-point operators</h4>
4874 For floating-point and complex numbers,
4875 <code>+x</code> is the same as <code>x</code>,
4876 while <code>-x</code> is the negation of <code>x</code>.
4877 The result of a floating-point or complex division by zero is not specified beyond the
4878 IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a>
4879 occurs is implementation-specific.
4883 An implementation may combine multiple floating-point operations into a single
4884 fused operation, possibly across statements, and produce a result that differs
4885 from the value obtained by executing and rounding the instructions individually.
4886 An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to
4887 the precision of the target type, preventing fusion that would discard that rounding.
4891 For instance, some architectures provide a "fused multiply and add" (FMA) instruction
4892 that computes <code>x*y + z</code> without rounding the intermediate result <code>x*y</code>.
4893 These examples show when a Go implementation can use that instruction:
4897 // FMA allowed for computing r, because x*y is not explicitly rounded:
4901 *p = x*y; r = *p + z
4902 r = x*y + float64(z)
4904 // FMA disallowed for computing r, because it would omit rounding of x*y:
4905 r = float64(x*y) + z
4906 r = z; r += float64(x*y)
4907 t = float64(x*y); r = t + z
4910 <h4 id="String_concatenation">String concatenation</h4>
4913 Strings can be concatenated using the <code>+</code> operator
4914 or the <code>+=</code> assignment operator:
4918 s := "hi" + string(c)
4919 s += " and good bye"
4923 String addition creates a new string by concatenating the operands.
4927 <h3 id="Comparison_operators">Comparison operators</h3>
4930 Comparison operators compare two operands and yield an untyped boolean value.
4933 <pre class="grammar">
4939 >= greater or equal
4943 In any comparison, the first operand
4944 must be <a href="#Assignability">assignable</a>
4945 to the type of the second operand, or vice versa.
4948 The equality operators <code>==</code> and <code>!=</code> apply
4949 to operands that are <i>comparable</i>.
4950 The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code>
4951 apply to operands that are <i>ordered</i>.
4952 These terms and the result of the comparisons are defined as follows:
4957 Boolean values are comparable.
4958 Two boolean values are equal if they are either both
4959 <code>true</code> or both <code>false</code>.
4963 Integer values are comparable and ordered, in the usual way.
4967 Floating-point values are comparable and ordered,
4968 as defined by the IEEE-754 standard.
4972 Complex values are comparable.
4973 Two complex values <code>u</code> and <code>v</code> are
4974 equal if both <code>real(u) == real(v)</code> and
4975 <code>imag(u) == imag(v)</code>.
4979 String values are comparable and ordered, lexically byte-wise.
4983 Pointer values are comparable.
4984 Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>.
4985 Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal.
4989 Channel values are comparable.
4990 Two channel values are equal if they were created by the same call to
4991 <a href="#Making_slices_maps_and_channels"><code>make</code></a>
4992 or if both have value <code>nil</code>.
4996 Interface values are comparable.
4997 Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types
4998 and equal dynamic values or if both have value <code>nil</code>.
5002 A value <code>x</code> of non-interface type <code>X</code> and
5003 a value <code>t</code> of interface type <code>T</code> are comparable when values
5004 of type <code>X</code> are comparable and
5005 <code>X</code> implements <code>T</code>.
5006 They are equal if <code>t</code>'s dynamic type is identical to <code>X</code>
5007 and <code>t</code>'s dynamic value is equal to <code>x</code>.
5011 Struct values are comparable if all their fields are comparable.
5012 Two struct values are equal if their corresponding
5013 non-<a href="#Blank_identifier">blank</a> fields are equal.
5017 Array values are comparable if values of the array element type are comparable.
5018 Two array values are equal if their corresponding elements are equal.
5023 A comparison of two interface values with identical dynamic types
5024 causes a <a href="#Run_time_panics">run-time panic</a> if values
5025 of that type are not comparable. This behavior applies not only to direct interface
5026 value comparisons but also when comparing arrays of interface values
5027 or structs with interface-valued fields.
5031 Slice, map, and function values are not comparable.
5032 However, as a special case, a slice, map, or function value may
5033 be compared to the predeclared identifier <code>nil</code>.
5034 Comparison of pointer, channel, and interface values to <code>nil</code>
5035 is also allowed and follows from the general rules above.
5039 const c = 3 < 4 // c is the untyped boolean constant true
5044 // The result of a comparison is an untyped boolean.
5045 // The usual assignment rules apply.
5046 b3 = x == y // b3 has type bool
5047 b4 bool = x == y // b4 has type bool
5048 b5 MyBool = x == y // b5 has type MyBool
5052 <h3 id="Logical_operators">Logical operators</h3>
5055 Logical operators apply to <a href="#Boolean_types">boolean</a> values
5056 and yield a result of the same type as the operands.
5057 The right operand is evaluated conditionally.
5060 <pre class="grammar">
5061 && conditional AND p && q is "if p then q else false"
5062 || conditional OR p || q is "if p then true else q"
5067 <h3 id="Address_operators">Address operators</h3>
5070 For an operand <code>x</code> of type <code>T</code>, the address operation
5071 <code>&x</code> generates a pointer of type <code>*T</code> to <code>x</code>.
5072 The operand must be <i>addressable</i>,
5073 that is, either a variable, pointer indirection, or slice indexing
5074 operation; or a field selector of an addressable struct operand;
5075 or an array indexing operation of an addressable array.
5076 As an exception to the addressability requirement, <code>x</code> may also be a
5077 (possibly parenthesized)
5078 <a href="#Composite_literals">composite literal</a>.
5079 If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>,
5080 then the evaluation of <code>&x</code> does too.
5084 For an operand <code>x</code> of pointer type <code>*T</code>, the pointer
5085 indirection <code>*x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed
5086 to by <code>x</code>.
5087 If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>*x</code>
5088 will cause a <a href="#Run_time_panics">run-time panic</a>.
5099 *x // causes a run-time panic
5100 &*x // causes a run-time panic
5104 <h3 id="Receive_operator">Receive operator</h3>
5107 For an operand <code>ch</code> of <a href="#Channel_types">channel type</a>,
5108 the value of the receive operation <code><-ch</code> is the value received
5109 from the channel <code>ch</code>. The channel direction must permit receive operations,
5110 and the type of the receive operation is the element type of the channel.
5111 The expression blocks until a value is available.
5112 Receiving from a <code>nil</code> channel blocks forever.
5113 A receive operation on a <a href="#Close">closed</a> channel can always proceed
5114 immediately, yielding the element type's <a href="#The_zero_value">zero value</a>
5115 after any previously sent values have been received.
5122 <-strobe // wait until clock pulse and discard received value
5126 A receive expression used in an <a href="#Assignments">assignment</a> or initialization of the special form
5133 var x, ok T = <-ch
5137 yields an additional untyped boolean result reporting whether the
5138 communication succeeded. The value of <code>ok</code> is <code>true</code>
5139 if the value received was delivered by a successful send operation to the
5140 channel, or <code>false</code> if it is a zero value generated because the
5141 channel is closed and empty.
5145 <h3 id="Conversions">Conversions</h3>
5148 A conversion changes the <a href="#Types">type</a> of an expression
5149 to the type specified by the conversion.
5150 A conversion may appear literally in the source, or it may be <i>implied</i>
5151 by the context in which an expression appears.
5155 An <i>explicit</i> conversion is an expression of the form <code>T(x)</code>
5156 where <code>T</code> is a type and <code>x</code> is an expression
5157 that can be converted to type <code>T</code>.
5161 Conversion = Type "(" Expression [ "," ] ")" .
5165 If the type starts with the operator <code>*</code> or <code><-</code>,
5166 or if the type starts with the keyword <code>func</code>
5167 and has no result list, it must be parenthesized when
5168 necessary to avoid ambiguity:
5172 *Point(p) // same as *(Point(p))
5173 (*Point)(p) // p is converted to *Point
5174 <-chan int(c) // same as <-(chan int(c))
5175 (<-chan int)(c) // c is converted to <-chan int
5176 func()(x) // function signature func() x
5177 (func())(x) // x is converted to func()
5178 (func() int)(x) // x is converted to func() int
5179 func() int(x) // x is converted to func() int (unambiguous)
5183 A <a href="#Constants">constant</a> value <code>x</code> can be converted to
5184 type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a>
5185 by a value of <code>T</code>.
5186 As a special case, an integer constant <code>x</code> can be explicitly converted to a
5187 <a href="#String_types">string type</a> using the
5188 <a href="#Conversions_to_and_from_a_string_type">same rule</a>
5189 as for non-constant <code>x</code>.
5193 Converting a constant to a type that is not a <a href="#Type_parameters">type parameter</a>
5194 yields a typed constant.
5195 Converting a constant to a type parameter yields a non-constant value of that type.
5199 uint(iota) // iota value of type uint
5200 float32(2.718281828) // 2.718281828 of type float32
5201 complex128(1) // 1.0 + 0.0i of type complex128
5202 float32(0.49999999) // 0.5 of type float32
5203 float64(-1e-1000) // 0.0 of type float64
5204 string('x') // "x" of type string
5205 string(0x266c) // "♬" of type string
5206 MyString("foo" + "bar") // "foobar" of type MyString
5207 string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant
5208 (*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
5209 int(1.2) // illegal: 1.2 cannot be represented as an int
5210 string(65.0) // illegal: 65.0 is not an integer constant
5214 A non-constant value <code>x</code> can be converted to type <code>T</code>
5215 in any of these cases:
5220 <code>x</code> is <a href="#Assignability">assignable</a>
5224 ignoring struct tags (see below),
5225 <code>x</code>'s type and <code>T</code> are not
5226 <a href="#Type_parameters">type parameters</a> but have
5227 <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
5230 ignoring struct tags (see below),
5231 <code>x</code>'s type and <code>T</code> are pointer types
5232 that are not <a href="#Types">named types</a>,
5233 and their pointer base types are not type parameters but
5234 have identical underlying types.
5237 <code>x</code>'s type and <code>T</code> are both integer or floating
5241 <code>x</code>'s type and <code>T</code> are both complex types.
5244 <code>x</code> is an integer or a slice of bytes or runes
5245 and <code>T</code> is a string type.
5248 <code>x</code> is a string and <code>T</code> is a slice of bytes or runes.
5251 <code>x</code> is a slice, <code>T</code> is a pointer to an array,
5252 and the slice and array types have <a href="#Type_identity">identical</a> element types.
5257 Additionally, if <code>T</code> or </code><code>x's</code> type <code>V</code> are type
5258 parameters with <a href="#Structure_of_interfaces">specific types</a>, <code>x</code>
5259 can also be converted to type <code>T</code> if one of the following conditions applies:
5264 Both <code>V</code> and <code>T</code> are type parameters and a value of each
5265 specific type of <code>V</code> can be converted to each specific type
5269 Only <code>V</code> is a type parameter and a value of each
5270 specific type of <code>V</code> can be converted to <code>T</code>.
5273 Only <code>T</code> is a type parameter and <code>x</code> can be converted to each
5274 specific type of <code>T</code>.
5279 <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types
5280 for identity for the purpose of conversion:
5284 type Person struct {
5293 Name string `json:"name"`
5295 Street string `json:"street"`
5296 City string `json:"city"`
5300 var person = (*Person)(data) // ignoring tags, the underlying types are identical
5304 Specific rules apply to (non-constant) conversions between numeric types or
5305 to and from a string type.
5306 These conversions may change the representation of <code>x</code>
5307 and incur a run-time cost.
5308 All other conversions only change the type but not the representation
5313 There is no linguistic mechanism to convert between pointers and integers.
5314 The package <a href="#Package_unsafe"><code>unsafe</code></a>
5315 implements this functionality under
5316 restricted circumstances.
5319 <h4>Conversions between numeric types</h4>
5322 For the conversion of non-constant numeric values, the following rules apply:
5327 When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is
5328 sign extended to implicit infinite precision; otherwise it is zero extended.
5329 It is then truncated to fit in the result type's size.
5330 For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>.
5331 The conversion always yields a valid value; there is no indication of overflow.
5334 When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded
5335 (truncation towards zero).
5338 When converting an integer or floating-point number to a floating-point type,
5339 or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded
5340 to the precision specified by the destination type.
5341 For instance, the value of a variable <code>x</code> of type <code>float32</code>
5342 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
5343 but float32(x) represents the result of rounding <code>x</code>'s value to
5344 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
5345 of precision, but <code>float32(x + 0.1)</code> does not.
5350 In all non-constant conversions involving floating-point or complex values,
5351 if the result type cannot represent the value the conversion
5352 succeeds but the result value is implementation-dependent.
5355 <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4>
5359 Converting a signed or unsigned integer value to a string type yields a
5360 string containing the UTF-8 representation of the integer. Values outside
5361 the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
5365 string(-1) // "\ufffd" == "\xef\xbf\xbd"
5366 string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8"
5367 type MyString string
5368 MyString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5"
5373 Converting a slice of bytes to a string type yields
5374 a string whose successive bytes are the elements of the slice.
5377 string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5378 string([]byte{}) // ""
5379 string([]byte(nil)) // ""
5382 string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø"
5387 Converting a slice of runes to a string type yields
5388 a string that is the concatenation of the individual rune values
5389 converted to strings.
5392 string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5393 string([]rune{}) // ""
5394 string([]rune(nil)) // ""
5397 string(MyRunes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔"
5402 Converting a value of a string type to a slice of bytes type
5403 yields a slice whose successive elements are the bytes of the string.
5406 []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5407 []byte("") // []byte{}
5409 MyBytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
5414 Converting a value of a string type to a slice of runes type
5415 yields a slice containing the individual Unicode code points of the string.
5418 []rune(MyString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}
5419 []rune("") // []rune{}
5421 MyRunes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}
5426 <h4 id="Conversions_from_slice_to_array_pointer">Conversions from slice to array pointer</h4>
5429 Converting a slice to an array pointer yields a pointer to the underlying array of the slice.
5430 If the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array,
5431 a <a href="#Run_time_panics">run-time panic</a> occurs.
5435 s := make([]byte, 2, 4)
5436 s0 := (*[0]byte)(s) // s0 != nil
5437 s1 := (*[1]byte)(s[1:]) // &s1[0] == &s[1]
5438 s2 := (*[2]byte)(s) // &s2[0] == &s[0]
5439 s4 := (*[4]byte)(s) // panics: len([4]byte) > len(s)
5442 t0 := (*[0]string)(t) // t0 == nil
5443 t1 := (*[1]string)(t) // panics: len([1]string) > len(t)
5445 u := make([]byte, 0)
5446 u0 := (*[0]byte)(u) // u0 != nil
5449 <h3 id="Constant_expressions">Constant expressions</h3>
5452 Constant expressions may contain only <a href="#Constants">constant</a>
5453 operands and are evaluated at compile time.
5457 Untyped boolean, numeric, and string constants may be used as operands
5458 wherever it is legal to use an operand of boolean, numeric, or string type,
5463 A constant <a href="#Comparison_operators">comparison</a> always yields
5464 an untyped boolean constant. If the left operand of a constant
5465 <a href="#Operators">shift expression</a> is an untyped constant, the
5466 result is an integer constant; otherwise it is a constant of the same
5467 type as the left operand, which must be of
5468 <a href="#Numeric_types">integer type</a>.
5472 Any other operation on untyped constants results in an untyped constant of the
5473 same kind; that is, a boolean, integer, floating-point, complex, or string
5475 If the untyped operands of a binary operation (other than a shift) are of
5476 different kinds, the result is of the operand's kind that appears later in this
5477 list: integer, rune, floating-point, complex.
5478 For example, an untyped integer constant divided by an
5479 untyped complex constant yields an untyped complex constant.
5483 const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)
5484 const b = 15 / 4 // b == 3 (untyped integer constant)
5485 const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)
5486 const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)
5487 const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)
5488 const d = 1 << 3.0 // d == 8 (untyped integer constant)
5489 const e = 1.0 << 3 // e == 8 (untyped integer constant)
5490 const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)
5491 const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)
5492 const h = "foo" > "bar" // h == true (untyped boolean constant)
5493 const j = true // j == true (untyped boolean constant)
5494 const k = 'w' + 1 // k == 'x' (untyped rune constant)
5495 const l = "hi" // l == "hi" (untyped string constant)
5496 const m = string(k) // m == "x" (type string)
5497 const Σ = 1 - 0.707i // (untyped complex constant)
5498 const Δ = Σ + 2.0e-4 // (untyped complex constant)
5499 const Φ = iota*1i - 1/1i // (untyped complex constant)
5503 Applying the built-in function <code>complex</code> to untyped
5504 integer, rune, or floating-point constants yields
5505 an untyped complex constant.
5509 const ic = complex(0, c) // ic == 3.75i (untyped complex constant)
5510 const iΘ = complex(0, Θ) // iΘ == 1i (type complex128)
5514 Constant expressions are always evaluated exactly; intermediate values and the
5515 constants themselves may require precision significantly larger than supported
5516 by any predeclared type in the language. The following are legal declarations:
5520 const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant)
5521 const Four int8 = Huge >> 98 // Four == 4 (type int8)
5525 The divisor of a constant division or remainder operation must not be zero:
5529 3.14 / 0.0 // illegal: division by zero
5533 The values of <i>typed</i> constants must always be accurately
5534 <a href="#Representability">representable</a> by values
5535 of the constant type. The following constant expressions are illegal:
5539 uint(-1) // -1 cannot be represented as a uint
5540 int(3.14) // 3.14 cannot be represented as an int
5541 int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
5542 Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
5543 Four * 100 // product 400 cannot be represented as an int8 (type of Four)
5547 The mask used by the unary bitwise complement operator <code>^</code> matches
5548 the rule for non-constants: the mask is all 1s for unsigned constants
5549 and -1 for signed and untyped constants.
5553 ^1 // untyped integer constant, equal to -2
5554 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8
5555 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
5556 int8(^1) // same as int8(-2)
5557 ^int8(1) // same as -1 ^ int8(1) = -2
5561 Implementation restriction: A compiler may use rounding while
5562 computing untyped floating-point or complex constant expressions; see
5563 the implementation restriction in the section
5564 on <a href="#Constants">constants</a>. This rounding may cause a
5565 floating-point constant expression to be invalid in an integer
5566 context, even if it would be integral when calculated using infinite
5567 precision, and vice versa.
5571 <h3 id="Order_of_evaluation">Order of evaluation</h3>
5574 At package level, <a href="#Package_initialization">initialization dependencies</a>
5575 determine the evaluation order of individual initialization expressions in
5576 <a href="#Variable_declarations">variable declarations</a>.
5577 Otherwise, when evaluating the <a href="#Operands">operands</a> of an
5578 expression, assignment, or
5579 <a href="#Return_statements">return statement</a>,
5580 all function calls, method calls, and
5581 communication operations are evaluated in lexical left-to-right
5586 For example, in the (function-local) assignment
5589 y[f()], ok = g(h(), i()+x[j()], <-c), k()
5592 the function calls and communication happen in the order
5593 <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>,
5594 <code><-c</code>, <code>g()</code>, and <code>k()</code>.
5595 However, the order of those events compared to the evaluation
5596 and indexing of <code>x</code> and the evaluation
5597 of <code>y</code> is not specified.
5602 f := func() int { a++; return a }
5603 x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
5604 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
5605 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
5609 At package level, initialization dependencies override the left-to-right rule
5610 for individual initialization expressions, but not for operands within each
5615 var a, b, c = f() + v(), g(), sqr(u()) + v()
5617 func f() int { return c }
5618 func g() int { return a }
5619 func sqr(x int) int { return x*x }
5621 // functions u and v are independent of all other variables and functions
5625 The function calls happen in the order
5626 <code>u()</code>, <code>sqr()</code>, <code>v()</code>,
5627 <code>f()</code>, <code>v()</code>, and <code>g()</code>.
5631 Floating-point operations within a single expression are evaluated according to
5632 the associativity of the operators. Explicit parentheses affect the evaluation
5633 by overriding the default associativity.
5634 In the expression <code>x + (y + z)</code> the addition <code>y + z</code>
5635 is performed before adding <code>x</code>.
5638 <h2 id="Statements">Statements</h2>
5641 Statements control execution.
5646 Declaration | LabeledStmt | SimpleStmt |
5647 GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
5648 FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
5651 SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .
5654 <h3 id="Terminating_statements">Terminating statements</h3>
5657 A <i>terminating statement</i> interrupts the regular flow of control in
5658 a <a href="#Blocks">block</a>. The following statements are terminating:
5663 A <a href="#Return_statements">"return"</a> or
5664 <a href="#Goto_statements">"goto"</a> statement.
5665 <!-- ul below only for regular layout -->
5670 A call to the built-in function
5671 <a href="#Handling_panics"><code>panic</code></a>.
5672 <!-- ul below only for regular layout -->
5677 A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement.
5678 <!-- ul below only for regular layout -->
5683 An <a href="#If_statements">"if" statement</a> in which:
5685 <li>the "else" branch is present, and</li>
5686 <li>both branches are terminating statements.</li>
5691 A <a href="#For_statements">"for" statement</a> in which:
5693 <li>there are no "break" statements referring to the "for" statement, and</li>
5694 <li>the loop condition is absent, and</li>
5695 <li>the "for" statement does not use a range clause.</li>
5700 A <a href="#Switch_statements">"switch" statement</a> in which:
5702 <li>there are no "break" statements referring to the "switch" statement,</li>
5703 <li>there is a default case, and</li>
5704 <li>the statement lists in each case, including the default, end in a terminating
5705 statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough"
5711 A <a href="#Select_statements">"select" statement</a> in which:
5713 <li>there are no "break" statements referring to the "select" statement, and</li>
5714 <li>the statement lists in each case, including the default if present,
5715 end in a terminating statement.</li>
5720 A <a href="#Labeled_statements">labeled statement</a> labeling
5721 a terminating statement.
5726 All other statements are not terminating.
5730 A <a href="#Blocks">statement list</a> ends in a terminating statement if the list
5731 is not empty and its final non-empty statement is terminating.
5735 <h3 id="Empty_statements">Empty statements</h3>
5738 The empty statement does nothing.
5746 <h3 id="Labeled_statements">Labeled statements</h3>
5749 A labeled statement may be the target of a <code>goto</code>,
5750 <code>break</code> or <code>continue</code> statement.
5754 LabeledStmt = Label ":" Statement .
5755 Label = identifier .
5759 Error: log.Panic("error encountered")
5763 <h3 id="Expression_statements">Expression statements</h3>
5766 With the exception of specific built-in functions,
5767 function and method <a href="#Calls">calls</a> and
5768 <a href="#Receive_operator">receive operations</a>
5769 can appear in statement context. Such statements may be parenthesized.
5773 ExpressionStmt = Expression .
5777 The following built-in functions are not permitted in statement context:
5781 append cap complex imag len make new real
5782 unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice
5790 len("foo") // illegal if len is the built-in function
5794 <h3 id="Send_statements">Send statements</h3>
5797 A send statement sends a value on a channel.
5798 The channel expression must be of <a href="#Channel_types">channel type</a>,
5799 the channel direction must permit send operations,
5800 and the type of the value to be sent must be <a href="#Assignability">assignable</a>
5801 to the channel's element type.
5805 SendStmt = Channel "<-" Expression .
5806 Channel = Expression .
5810 Both the channel and the value expression are evaluated before communication
5811 begins. Communication blocks until the send can proceed.
5812 A send on an unbuffered channel can proceed if a receiver is ready.
5813 A send on a buffered channel can proceed if there is room in the buffer.
5814 A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>.
5815 A send on a <code>nil</code> channel blocks forever.
5819 ch <- 3 // send value 3 to channel ch
5823 <h3 id="IncDec_statements">IncDec statements</h3>
5826 The "++" and "--" statements increment or decrement their operands
5827 by the untyped <a href="#Constants">constant</a> <code>1</code>.
5828 As with an assignment, the operand must be <a href="#Address_operators">addressable</a>
5829 or a map index expression.
5833 IncDecStmt = Expression ( "++" | "--" ) .
5837 The following <a href="#Assignments">assignment statements</a> are semantically
5841 <pre class="grammar">
5842 IncDec statement Assignment
5848 <h3 id="Assignments">Assignments</h3>
5851 Assignment = ExpressionList assign_op ExpressionList .
5853 assign_op = [ add_op | mul_op ] "=" .
5857 Each left-hand side operand must be <a href="#Address_operators">addressable</a>,
5858 a map index expression, or (for <code>=</code> assignments only) the
5859 <a href="#Blank_identifier">blank identifier</a>.
5860 Operands may be parenthesized.
5867 (k) = <-ch // same as: k = <-ch
5871 An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
5872 <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a>
5873 is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
5874 <code>(y)</code> but evaluates <code>x</code>
5875 only once. The <i>op</i><code>=</code> construct is a single token.
5876 In assignment operations, both the left- and right-hand expression lists
5877 must contain exactly one single-valued expression, and the left-hand
5878 expression must not be the blank identifier.
5883 i &^= 1<<n
5887 A tuple assignment assigns the individual elements of a multi-valued
5888 operation to a list of variables. There are two forms. In the
5889 first, the right hand operand is a single multi-valued expression
5890 such as a function call, a <a href="#Channel_types">channel</a> or
5891 <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>.
5892 The number of operands on the left
5893 hand side must match the number of values. For instance, if
5894 <code>f</code> is a function returning two values,
5902 assigns the first value to <code>x</code> and the second to <code>y</code>.
5903 In the second form, the number of operands on the left must equal the number
5904 of expressions on the right, each of which must be single-valued, and the
5905 <i>n</i>th expression on the right is assigned to the <i>n</i>th
5906 operand on the left:
5910 one, two, three = '一', '二', '三'
5914 The <a href="#Blank_identifier">blank identifier</a> provides a way to
5915 ignore right-hand side values in an assignment:
5919 _ = x // evaluate x but ignore it
5920 x, _ = f() // evaluate f() but ignore second result value
5924 The assignment proceeds in two phases.
5925 First, the operands of <a href="#Index_expressions">index expressions</a>
5926 and <a href="#Address_operators">pointer indirections</a>
5927 (including implicit pointer indirections in <a href="#Selectors">selectors</a>)
5928 on the left and the expressions on the right are all
5929 <a href="#Order_of_evaluation">evaluated in the usual order</a>.
5930 Second, the assignments are carried out in left-to-right order.
5934 a, b = b, a // exchange a and b
5938 i, x[i] = 1, 2 // set i = 1, x[0] = 2
5941 x[i], i = 2, 1 // set x[0] = 2, i = 1
5943 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)
5945 x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5.
5947 type Point struct { x, y int }
5949 x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7
5953 for i, x[i] = range x { // set i, x[2] = 0, x[0]
5956 // after this loop, i == 0 and x == []int{3, 5, 3}
5960 In assignments, each value must be <a href="#Assignability">assignable</a>
5961 to the type of the operand to which it is assigned, with the following special cases:
5966 Any typed value may be assigned to the blank identifier.
5970 If an untyped constant
5971 is assigned to a variable of interface type or the blank identifier,
5972 the constant is first implicitly <a href="#Conversions">converted</a> to its
5973 <a href="#Constants">default type</a>.
5977 If an untyped boolean value is assigned to a variable of interface type or
5978 the blank identifier, it is first implicitly converted to type <code>bool</code>.
5982 <h3 id="If_statements">If statements</h3>
5985 "If" statements specify the conditional execution of two branches
5986 according to the value of a boolean expression. If the expression
5987 evaluates to true, the "if" branch is executed, otherwise, if
5988 present, the "else" branch is executed.
5992 IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .
6002 The expression may be preceded by a simple statement, which
6003 executes before the expression is evaluated.
6007 if x := f(); x < y {
6009 } else if x > z {
6017 <h3 id="Switch_statements">Switch statements</h3>
6020 "Switch" statements provide multi-way execution.
6021 An expression or type is compared to the "cases"
6022 inside the "switch" to determine which branch
6027 SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
6031 There are two forms: expression switches and type switches.
6032 In an expression switch, the cases contain expressions that are compared
6033 against the value of the switch expression.
6034 In a type switch, the cases contain types that are compared against the
6035 type of a specially annotated switch expression.
6036 The switch expression is evaluated exactly once in a switch statement.
6039 <h4 id="Expression_switches">Expression switches</h4>
6042 In an expression switch,
6043 the switch expression is evaluated and
6044 the case expressions, which need not be constants,
6045 are evaluated left-to-right and top-to-bottom; the first one that equals the
6047 triggers execution of the statements of the associated case;
6048 the other cases are skipped.
6049 If no case matches and there is a "default" case,
6050 its statements are executed.
6051 There can be at most one default case and it may appear anywhere in the
6053 A missing switch expression is equivalent to the boolean value
6058 ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
6059 ExprCaseClause = ExprSwitchCase ":" StatementList .
6060 ExprSwitchCase = "case" ExpressionList | "default" .
6064 If the switch expression evaluates to an untyped constant, it is first implicitly
6065 <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>.
6066 The predeclared untyped value <code>nil</code> cannot be used as a switch expression.
6067 The switch expression type must be <a href="#Comparison_operators">comparable</a>.
6071 If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a>
6072 to the type of the switch expression.
6073 For each (possibly converted) case expression <code>x</code> and the value <code>t</code>
6074 of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>.
6078 In other words, the switch expression is treated as if it were used to declare and
6079 initialize a temporary variable <code>t</code> without explicit type; it is that
6080 value of <code>t</code> against which each case expression <code>x</code> is tested
6085 In a case or default clause, the last non-empty statement
6086 may be a (possibly <a href="#Labeled_statements">labeled</a>)
6087 <a href="#Fallthrough_statements">"fallthrough" statement</a> to
6088 indicate that control should flow from the end of this clause to
6089 the first statement of the next clause.
6090 Otherwise control flows to the end of the "switch" statement.
6091 A "fallthrough" statement may appear as the last statement of all
6092 but the last clause of an expression switch.
6096 The switch expression may be preceded by a simple statement, which
6097 executes before the expression is evaluated.
6103 case 0, 1, 2, 3: s1()
6104 case 4, 5, 6, 7: s2()
6107 switch x := f(); { // missing switch expression means "true"
6108 case x < 0: return -x
6120 Implementation restriction: A compiler may disallow multiple case
6121 expressions evaluating to the same constant.
6122 For instance, the current compilers disallow duplicate integer,
6123 floating point, or string constants in case expressions.
6126 <h4 id="Type_switches">Type switches</h4>
6129 A type switch compares types rather than values. It is otherwise similar
6130 to an expression switch. It is marked by a special switch expression that
6131 has the form of a <a href="#Type_assertions">type assertion</a>
6132 using the keyword <code>type</code> rather than an actual type:
6142 Cases then match actual types <code>T</code> against the dynamic type of the
6143 expression <code>x</code>. As with type assertions, <code>x</code> must be of
6144 <a href="#Interface_types">interface type</a>, but not a
6145 <a href="#Type_parameters">type parameter</a>, and each non-interface type
6146 <code>T</code> listed in a case must implement the type of <code>x</code>.
6147 The types listed in the cases of a type switch must all be
6148 <a href="#Type_identity">different</a>.
6152 TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
6153 TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
6154 TypeCaseClause = TypeSwitchCase ":" StatementList .
6155 TypeSwitchCase = "case" TypeList | "default" .
6159 The TypeSwitchGuard may include a
6160 <a href="#Short_variable_declarations">short variable declaration</a>.
6161 When that form is used, the variable is declared at the end of the
6162 TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause.
6163 In clauses with a case listing exactly one type, the variable
6164 has that type; otherwise, the variable has the type of the expression
6165 in the TypeSwitchGuard.
6169 Instead of a type, a case may use the predeclared identifier
6170 <a href="#Predeclared_identifiers"><code>nil</code></a>;
6171 that case is selected when the expression in the TypeSwitchGuard
6172 is a <code>nil</code> interface value.
6173 There may be at most one <code>nil</code> case.
6177 Given an expression <code>x</code> of type <code>interface{}</code>,
6178 the following type switch:
6182 switch i := x.(type) {
6184 printString("x is nil") // type of i is type of x (interface{})
6186 printInt(i) // type of i is int
6188 printFloat64(i) // type of i is float64
6189 case func(int) float64:
6190 printFunction(i) // type of i is func(int) float64
6192 printString("type is bool or string") // type of i is type of x (interface{})
6194 printString("don't know the type") // type of i is type of x (interface{})
6203 v := x // x is evaluated exactly once
6205 i := v // type of i is type of x (interface{})
6206 printString("x is nil")
6207 } else if i, isInt := v.(int); isInt {
6208 printInt(i) // type of i is int
6209 } else if i, isFloat64 := v.(float64); isFloat64 {
6210 printFloat64(i) // type of i is float64
6211 } else if i, isFunc := v.(func(int) float64); isFunc {
6212 printFunction(i) // type of i is func(int) float64
6214 _, isBool := v.(bool)
6215 _, isString := v.(string)
6216 if isBool || isString {
6217 i := v // type of i is type of x (interface{})
6218 printString("type is bool or string")
6220 i := v // type of i is type of x (interface{})
6221 printString("don't know the type")
6227 The type switch guard may be preceded by a simple statement, which
6228 executes before the guard is evaluated.
6232 The "fallthrough" statement is not permitted in a type switch.
6235 <h3 id="For_statements">For statements</h3>
6238 A "for" statement specifies repeated execution of a block. There are three forms:
6239 The iteration may be controlled by a single condition, a "for" clause, or a "range" clause.
6243 ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
6244 Condition = Expression .
6247 <h4 id="For_condition">For statements with single condition</h4>
6250 In its simplest form, a "for" statement specifies the repeated execution of
6251 a block as long as a boolean condition evaluates to true.
6252 The condition is evaluated before each iteration.
6253 If the condition is absent, it is equivalent to the boolean value
6263 <h4 id="For_clause">For statements with <code>for</code> clause</h4>
6266 A "for" statement with a ForClause is also controlled by its condition, but
6267 additionally it may specify an <i>init</i>
6268 and a <i>post</i> statement, such as an assignment,
6269 an increment or decrement statement. The init statement may be a
6270 <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not.
6271 Variables declared by the init statement are re-used in each iteration.
6275 ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
6276 InitStmt = SimpleStmt .
6277 PostStmt = SimpleStmt .
6281 for i := 0; i < 10; i++ {
6287 If non-empty, the init statement is executed once before evaluating the
6288 condition for the first iteration;
6289 the post statement is executed after each execution of the block (and
6290 only if the block was executed).
6291 Any element of the ForClause may be empty but the
6292 <a href="#Semicolons">semicolons</a> are
6293 required unless there is only a condition.
6294 If the condition is absent, it is equivalent to the boolean value
6299 for cond { S() } is the same as for ; cond ; { S() }
6300 for { S() } is the same as for true { S() }
6303 <h4 id="For_range">For statements with <code>range</code> clause</h4>
6306 A "for" statement with a "range" clause
6307 iterates through all entries of an array, slice, string or map,
6308 or values received on a channel. For each entry it assigns <i>iteration values</i>
6309 to corresponding <i>iteration variables</i> if present and then executes the block.
6313 RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .
6317 The expression on the right in the "range" clause is called the <i>range expression</i>,
6318 which may be an array, pointer to an array, slice, string, map, or channel permitting
6319 <a href="#Receive_operator">receive operations</a>.
6320 As with an assignment, if present the operands on the left must be
6321 <a href="#Address_operators">addressable</a> or map index expressions; they
6322 denote the iteration variables. If the range expression is a channel, at most
6323 one iteration variable is permitted, otherwise there may be up to two.
6324 If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>,
6325 the range clause is equivalent to the same clause without that identifier.
6329 The range expression <code>x</code> is evaluated once before beginning the loop,
6330 with one exception: if at most one iteration variable is present and
6331 <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>,
6332 the range expression is not evaluated.
6336 Function calls on the left are evaluated once per iteration.
6337 For each iteration, iteration values are produced as follows
6338 if the respective iteration variables are present:
6341 <pre class="grammar">
6342 Range expression 1st value 2nd value
6344 array or slice a [n]E, *[n]E, or []E index i int a[i] E
6345 string s string type index i int see below rune
6346 map m map[K]V key k K m[k] V
6347 channel c chan E, <-chan E element e E
6352 For an array, pointer to array, or slice value <code>a</code>, the index iteration
6353 values are produced in increasing order, starting at element index 0.
6354 If at most one iteration variable is present, the range loop produces
6355 iteration values from 0 up to <code>len(a)-1</code> and does not index into the array
6356 or slice itself. For a <code>nil</code> slice, the number of iterations is 0.
6360 For a string value, the "range" clause iterates over the Unicode code points
6361 in the string starting at byte index 0. On successive iterations, the index value will be the
6362 index of the first byte of successive UTF-8-encoded code points in the string,
6363 and the second value, of type <code>rune</code>, will be the value of
6364 the corresponding code point. If the iteration encounters an invalid
6365 UTF-8 sequence, the second value will be <code>0xFFFD</code>,
6366 the Unicode replacement character, and the next iteration will advance
6367 a single byte in the string.
6371 The iteration order over maps is not specified
6372 and is not guaranteed to be the same from one iteration to the next.
6373 If a map entry that has not yet been reached is removed during iteration,
6374 the corresponding iteration value will not be produced. If a map entry is
6375 created during iteration, that entry may be produced during the iteration or
6376 may be skipped. The choice may vary for each entry created and from one
6377 iteration to the next.
6378 If the map is <code>nil</code>, the number of iterations is 0.
6382 For channels, the iteration values produced are the successive values sent on
6383 the channel until the channel is <a href="#Close">closed</a>. If the channel
6384 is <code>nil</code>, the range expression blocks forever.
6389 The iteration values are assigned to the respective
6390 iteration variables as in an <a href="#Assignments">assignment statement</a>.
6394 The iteration variables may be declared by the "range" clause using a form of
6395 <a href="#Short_variable_declarations">short variable declaration</a>
6397 In this case their types are set to the types of the respective iteration values
6398 and their <a href="#Declarations_and_scope">scope</a> is the block of the "for"
6399 statement; they are re-used in each iteration.
6400 If the iteration variables are declared outside the "for" statement,
6401 after execution their values will be those of the last iteration.
6405 var testdata *struct {
6408 for i, _ := range testdata.a {
6409 // testdata.a is never evaluated; len(testdata.a) is constant
6410 // i ranges from 0 to 6
6415 for i, s := range a {
6417 // type of s is string
6423 var val interface{} // element type of m is assignable to val
6424 m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
6425 for key, val = range m {
6428 // key == last map key encountered in iteration
6431 var ch chan Work = producer()
6441 <h3 id="Go_statements">Go statements</h3>
6444 A "go" statement starts the execution of a function call
6445 as an independent concurrent thread of control, or <i>goroutine</i>,
6446 within the same address space.
6450 GoStmt = "go" Expression .
6454 The expression must be a function or method call; it cannot be parenthesized.
6455 Calls of built-in functions are restricted as for
6456 <a href="#Expression_statements">expression statements</a>.
6460 The function value and parameters are
6461 <a href="#Calls">evaluated as usual</a>
6462 in the calling goroutine, but
6463 unlike with a regular call, program execution does not wait
6464 for the invoked function to complete.
6465 Instead, the function begins executing independently
6467 When the function terminates, its goroutine also terminates.
6468 If the function has any return values, they are discarded when the
6474 go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)
6478 <h3 id="Select_statements">Select statements</h3>
6481 A "select" statement chooses which of a set of possible
6482 <a href="#Send_statements">send</a> or
6483 <a href="#Receive_operator">receive</a>
6484 operations will proceed.
6485 It looks similar to a
6486 <a href="#Switch_statements">"switch"</a> statement but with the
6487 cases all referring to communication operations.
6491 SelectStmt = "select" "{" { CommClause } "}" .
6492 CommClause = CommCase ":" StatementList .
6493 CommCase = "case" ( SendStmt | RecvStmt ) | "default" .
6494 RecvStmt = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
6495 RecvExpr = Expression .
6499 A case with a RecvStmt may assign the result of a RecvExpr to one or
6500 two variables, which may be declared using a
6501 <a href="#Short_variable_declarations">short variable declaration</a>.
6502 The RecvExpr must be a (possibly parenthesized) receive operation.
6503 There can be at most one default case and it may appear anywhere
6504 in the list of cases.
6508 Execution of a "select" statement proceeds in several steps:
6513 For all the cases in the statement, the channel operands of receive operations
6514 and the channel and right-hand-side expressions of send statements are
6515 evaluated exactly once, in source order, upon entering the "select" statement.
6516 The result is a set of channels to receive from or send to,
6517 and the corresponding values to send.
6518 Any side effects in that evaluation will occur irrespective of which (if any)
6519 communication operation is selected to proceed.
6520 Expressions on the left-hand side of a RecvStmt with a short variable declaration
6521 or assignment are not yet evaluated.
6525 If one or more of the communications can proceed,
6526 a single one that can proceed is chosen via a uniform pseudo-random selection.
6527 Otherwise, if there is a default case, that case is chosen.
6528 If there is no default case, the "select" statement blocks until
6529 at least one of the communications can proceed.
6533 Unless the selected case is the default case, the respective communication
6534 operation is executed.
6538 If the selected case is a RecvStmt with a short variable declaration or
6539 an assignment, the left-hand side expressions are evaluated and the
6540 received value (or values) are assigned.
6544 The statement list of the selected case is executed.
6549 Since communication on <code>nil</code> channels can never proceed,
6550 a select with only <code>nil</code> channels and no default case blocks forever.
6555 var c, c1, c2, c3, c4 chan int
6559 print("received ", i1, " from c1\n")
6561 print("sent ", i2, " to c2\n")
6562 case i3, ok := (<-c3): // same as: i3, ok := <-c3
6564 print("received ", i3, " from c3\n")
6566 print("c3 is closed\n")
6568 case a[f()] = <-c4:
6570 // case t := <-c4
6573 print("no communication\n")
6576 for { // send random sequence of bits to c
6578 case c <- 0: // note: no statement, no fallthrough, no folding of cases
6583 select {} // block forever
6587 <h3 id="Return_statements">Return statements</h3>
6590 A "return" statement in a function <code>F</code> terminates the execution
6591 of <code>F</code>, and optionally provides one or more result values.
6592 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
6593 are executed before <code>F</code> returns to its caller.
6597 ReturnStmt = "return" [ ExpressionList ] .
6601 In a function without a result type, a "return" statement must not
6602 specify any result values.
6611 There are three ways to return values from a function with a result
6616 <li>The return value or values may be explicitly listed
6617 in the "return" statement. Each expression must be single-valued
6618 and <a href="#Assignability">assignable</a>
6619 to the corresponding element of the function's result type.
6621 func simpleF() int {
6625 func complexF1() (re float64, im float64) {
6630 <li>The expression list in the "return" statement may be a single
6631 call to a multi-valued function. The effect is as if each value
6632 returned from that function were assigned to a temporary
6633 variable with the type of the respective value, followed by a
6634 "return" statement listing these variables, at which point the
6635 rules of the previous case apply.
6637 func complexF2() (re float64, im float64) {
6642 <li>The expression list may be empty if the function's result
6643 type specifies names for its <a href="#Function_types">result parameters</a>.
6644 The result parameters act as ordinary local variables
6645 and the function may assign values to them as necessary.
6646 The "return" statement returns the values of these variables.
6648 func complexF3() (re float64, im float64) {
6654 func (devnull) Write(p []byte) (n int, _ error) {
6663 Regardless of how they are declared, all the result values are initialized to
6664 the <a href="#The_zero_value">zero values</a> for their type upon entry to the
6665 function. A "return" statement that specifies results sets the result parameters before
6666 any deferred functions are executed.
6670 Implementation restriction: A compiler may disallow an empty expression list
6671 in a "return" statement if a different entity (constant, type, or variable)
6672 with the same name as a result parameter is in
6673 <a href="#Declarations_and_scope">scope</a> at the place of the return.
6677 func f(n int) (res int, err error) {
6678 if _, err := f(n-1); err != nil {
6679 return // invalid return statement: err is shadowed
6685 <h3 id="Break_statements">Break statements</h3>
6688 A "break" statement terminates execution of the innermost
6689 <a href="#For_statements">"for"</a>,
6690 <a href="#Switch_statements">"switch"</a>, or
6691 <a href="#Select_statements">"select"</a> statement
6692 within the same function.
6696 BreakStmt = "break" [ Label ] .
6700 If there is a label, it must be that of an enclosing
6701 "for", "switch", or "select" statement,
6702 and that is the one whose execution terminates.
6707 for i = 0; i < n; i++ {
6708 for j = 0; j < m; j++ {
6721 <h3 id="Continue_statements">Continue statements</h3>
6724 A "continue" statement begins the next iteration of the
6725 innermost <a href="#For_statements">"for" loop</a> at its post statement.
6726 The "for" loop must be within the same function.
6730 ContinueStmt = "continue" [ Label ] .
6734 If there is a label, it must be that of an enclosing
6735 "for" statement, and that is the one whose execution
6741 for y, row := range rows {
6742 for x, data := range row {
6743 if data == endOfRow {
6746 row[x] = data + bias(x, y)
6751 <h3 id="Goto_statements">Goto statements</h3>
6754 A "goto" statement transfers control to the statement with the corresponding label
6755 within the same function.
6759 GotoStmt = "goto" Label .
6767 Executing the "goto" statement must not cause any variables to come into
6768 <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto.
6769 For instance, this example:
6779 is erroneous because the jump to label <code>L</code> skips
6780 the creation of <code>v</code>.
6784 A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block.
6785 For instance, this example:
6802 is erroneous because the label <code>L1</code> is inside
6803 the "for" statement's block but the <code>goto</code> is not.
6806 <h3 id="Fallthrough_statements">Fallthrough statements</h3>
6809 A "fallthrough" statement transfers control to the first statement of the
6810 next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
6811 It may be used only as the final non-empty statement in such a clause.
6815 FallthroughStmt = "fallthrough" .
6819 <h3 id="Defer_statements">Defer statements</h3>
6822 A "defer" statement invokes a function whose execution is deferred
6823 to the moment the surrounding function returns, either because the
6824 surrounding function executed a <a href="#Return_statements">return statement</a>,
6825 reached the end of its <a href="#Function_declarations">function body</a>,
6826 or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>.
6830 DeferStmt = "defer" Expression .
6834 The expression must be a function or method call; it cannot be parenthesized.
6835 Calls of built-in functions are restricted as for
6836 <a href="#Expression_statements">expression statements</a>.
6840 Each time a "defer" statement
6841 executes, the function value and parameters to the call are
6842 <a href="#Calls">evaluated as usual</a>
6843 and saved anew but the actual function is not invoked.
6844 Instead, deferred functions are invoked immediately before
6845 the surrounding function returns, in the reverse order
6846 they were deferred. That is, if the surrounding function
6847 returns through an explicit <a href="#Return_statements">return statement</a>,
6848 deferred functions are executed <i>after</i> any result parameters are set
6849 by that return statement but <i>before</i> the function returns to its caller.
6850 If a deferred function value evaluates
6851 to <code>nil</code>, execution <a href="#Handling_panics">panics</a>
6852 when the function is invoked, not when the "defer" statement is executed.
6856 For instance, if the deferred function is
6857 a <a href="#Function_literals">function literal</a> and the surrounding
6858 function has <a href="#Function_types">named result parameters</a> that
6859 are in scope within the literal, the deferred function may access and modify
6860 the result parameters before they are returned.
6861 If the deferred function has any return values, they are discarded when
6862 the function completes.
6863 (See also the section on <a href="#Handling_panics">handling panics</a>.)
6868 defer unlock(l) // unlocking happens before surrounding function returns
6870 // prints 3 2 1 0 before surrounding function returns
6871 for i := 0; i <= 3; i++ {
6876 func f() (result int) {
6878 // result is accessed after it was set to 6 by the return statement
6885 <h2 id="Built-in_functions">Built-in functions</h2>
6888 Built-in functions are
6889 <a href="#Predeclared_identifiers">predeclared</a>.
6890 They are called like any other function but some of them
6891 accept a type instead of an expression as the first argument.
6895 The built-in functions do not have standard Go types,
6896 so they can only appear in <a href="#Calls">call expressions</a>;
6897 they cannot be used as function values.
6900 <h3 id="Close">Close</h3>
6903 For a channel <code>c</code>, the built-in function <code>close(c)</code>
6904 records that no more values will be sent on the channel.
6905 It is an error if <code>c</code> is a receive-only channel.
6906 Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>.
6907 Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>.
6908 After calling <code>close</code>, and after any previously
6909 sent values have been received, receive operations will return
6910 the zero value for the channel's type without blocking.
6911 The multi-valued <a href="#Receive_operator">receive operation</a>
6912 returns a received value along with an indication of whether the channel is closed.
6915 <h3 id="Length_and_capacity">Length and capacity</h3>
6918 The built-in functions <code>len</code> and <code>cap</code> take arguments
6919 of various types and return a result of type <code>int</code>.
6920 The implementation guarantees that the result always fits into an <code>int</code>.
6923 <pre class="grammar">
6924 Call Argument type Result
6926 len(s) string type string length in bytes
6927 [n]T, *[n]T array length (== n)
6929 map[K]T map length (number of defined keys)
6930 chan T number of elements queued in channel buffer
6931 type parameter see below
6933 cap(s) [n]T, *[n]T array length (== n)
6935 chan T channel buffer capacity
6936 type parameter see below
6940 If the argument type is a <a href="#Type_parameters">type parameter</a> <code>P</code>,
6941 <code>P</code> must have <a href="#Structure of interfaces">specific types</a>, and
6942 the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for
6943 each specific type of <code>P</code>.
6944 The result is the length (or capacity, respectively) of the argument whose type
6945 corresponds to the type argument with which <code>P</code> was
6946 <a href="#Instantiations">instantiated</a>.
6950 The capacity of a slice is the number of elements for which there is
6951 space allocated in the underlying array.
6952 At any time the following relationship holds:
6956 0 <= len(s) <= cap(s)
6960 The length of a <code>nil</code> slice, map or channel is 0.
6961 The capacity of a <code>nil</code> slice or channel is 0.
6965 The expression <code>len(s)</code> is <a href="#Constants">constant</a> if
6966 <code>s</code> is a string constant. The expressions <code>len(s)</code> and
6967 <code>cap(s)</code> are constants if the type of <code>s</code> is an array
6968 or pointer to an array and the expression <code>s</code> does not contain
6969 <a href="#Receive_operator">channel receives</a> or (non-constant)
6970 <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated.
6971 Otherwise, invocations of <code>len</code> and <code>cap</code> are not
6972 constant and <code>s</code> is evaluated.
6977 c1 = imag(2i) // imag(2i) = 2.0 is a constant
6978 c2 = len([10]float64{2}) // [10]float64{2} contains no function calls
6979 c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls
6980 c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued
6981 c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call
6986 <h3 id="Allocation">Allocation</h3>
6989 The built-in function <code>new</code> takes a type <code>T</code>,
6990 allocates storage for a <a href="#Variables">variable</a> of that type
6991 at run time, and returns a value of type <code>*T</code>
6992 <a href="#Pointer_types">pointing</a> to it.
6993 The variable is initialized as described in the section on
6994 <a href="#The_zero_value">initial values</a>.
6997 <pre class="grammar">
7006 type S struct { a int; b float64 }
7011 allocates storage for a variable of type <code>S</code>,
7012 initializes it (<code>a=0</code>, <code>b=0.0</code>),
7013 and returns a value of type <code>*S</code> containing the address
7017 <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3>
7020 The built-in function <code>make</code> takes a type <code>T</code>,
7021 which must be a slice, map or channel type,
7022 optionally followed by a type-specific list of expressions.
7023 It returns a value of type <code>T</code> (not <code>*T</code>).
7024 The memory is initialized as described in the section on
7025 <a href="#The_zero_value">initial values</a>.
7028 <pre class="grammar">
7031 make(T, n) slice slice of type T with length n and capacity n
7032 make(T, n, m) slice slice of type T with length n and capacity m
7034 make(T) map map of type T
7035 make(T, n) map map of type T with initial space for approximately n elements
7037 make(T) channel unbuffered channel of type T
7038 make(T, n) channel buffered channel of type T, buffer size n
7043 Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>
7044 or an untyped <a href="#Constants">constant</a>.
7045 A constant size argument must be non-negative and <a href="#Representability">representable</a>
7046 by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>.
7047 If both <code>n</code> and <code>m</code> are provided and are constant, then
7048 <code>n</code> must be no larger than <code>m</code>.
7049 If <code>n</code> is negative or larger than <code>m</code> at run time,
7050 a <a href="#Run_time_panics">run-time panic</a> occurs.
7054 s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100
7055 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000
7056 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int
7057 s := make([]int, 10, 0) // illegal: len(s) > cap(s)
7058 c := make(chan int, 10) // channel with a buffer size of 10
7059 m := make(map[string]int, 100) // map with initial space for approximately 100 elements
7063 Calling <code>make</code> with a map type and size hint <code>n</code> will
7064 create a map with initial space to hold <code>n</code> map elements.
7065 The precise behavior is implementation-dependent.
7069 <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3>
7072 The built-in functions <code>append</code> and <code>copy</code> assist in
7073 common slice operations.
7074 For both functions, the result is independent of whether the memory referenced
7075 by the arguments overlaps.
7079 The <a href="#Function_types">variadic</a> function <code>append</code>
7080 appends zero or more values <code>x</code>
7081 to <code>s</code> of type <code>S</code>, which must be a slice type, and
7082 returns the resulting slice, also of type <code>S</code>.
7083 The values <code>x</code> are passed to a parameter of type <code>...T</code>
7084 where <code>T</code> is the <a href="#Slice_types">element type</a> of
7085 <code>S</code> and the respective
7086 <a href="#Passing_arguments_to_..._parameters">parameter passing rules</a> apply.
7087 As a special case, <code>append</code> also accepts a first argument
7088 assignable to type <code>[]byte</code> with a second argument of
7089 string type followed by <code>...</code>. This form appends the
7090 bytes of the string.
7093 <pre class="grammar">
7094 append(s S, x ...T) S // T is the element type of S
7098 If the capacity of <code>s</code> is not large enough to fit the additional
7099 values, <code>append</code> allocates a new, sufficiently large underlying
7100 array that fits both the existing slice elements and the additional values.
7101 Otherwise, <code>append</code> re-uses the underlying array.
7106 s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2}
7107 s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7}
7108 s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
7109 s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
7112 t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"}
7115 b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' }
7119 The function <code>copy</code> copies slice elements from
7120 a source <code>src</code> to a destination <code>dst</code> and returns the
7121 number of elements copied.
7122 Both arguments must have <a href="#Type_identity">identical</a> element type <code>T</code> and must be
7123 <a href="#Assignability">assignable</a> to a slice of type <code>[]T</code>.
7124 The number of elements copied is the minimum of
7125 <code>len(src)</code> and <code>len(dst)</code>.
7126 As a special case, <code>copy</code> also accepts a destination argument assignable
7127 to type <code>[]byte</code> with a source argument of a string type.
7128 This form copies the bytes from the string into the byte slice.
7131 <pre class="grammar">
7132 copy(dst, src []T) int
7133 copy(dst []byte, src string) int
7141 var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
7142 var s = make([]int, 6)
7143 var b = make([]byte, 5)
7144 n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
7145 n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
7146 n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello")
7150 <h3 id="Deletion_of_map_elements">Deletion of map elements</h3>
7153 The built-in function <code>delete</code> removes the element with key
7154 <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The
7155 type of <code>k</code> must be <a href="#Assignability">assignable</a>
7156 to the key type of <code>m</code>.
7159 <pre class="grammar">
7160 delete(m, k) // remove element m[k] from map m
7164 If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code>
7165 does not exist, <code>delete</code> is a no-op.
7169 <h3 id="Complex_numbers">Manipulating complex numbers</h3>
7172 Three functions assemble and disassemble complex numbers.
7173 The built-in function <code>complex</code> constructs a complex
7174 value from a floating-point real and imaginary part, while
7175 <code>real</code> and <code>imag</code>
7176 extract the real and imaginary parts of a complex value.
7179 <pre class="grammar">
7180 complex(realPart, imaginaryPart floatT) complexT
7181 real(complexT) floatT
7182 imag(complexT) floatT
7186 The type of the arguments and return value correspond.
7187 For <code>complex</code>, the two arguments must be of the same
7188 <a href="#Numeric_types">floating-point type</a> and the return type is the
7189 <a href="#Numeric_types">complex type</a>
7190 with the corresponding floating-point constituents:
7191 <code>complex64</code> for <code>float32</code> arguments, and
7192 <code>complex128</code> for <code>float64</code> arguments.
7193 If one of the arguments evaluates to an untyped constant, it is first implicitly
7194 <a href="#Conversions">converted</a> to the type of the other argument.
7195 If both arguments evaluate to untyped constants, they must be non-complex
7196 numbers or their imaginary parts must be zero, and the return value of
7197 the function is an untyped complex constant.
7201 For <code>real</code> and <code>imag</code>, the argument must be
7202 of complex type, and the return type is the corresponding floating-point
7203 type: <code>float32</code> for a <code>complex64</code> argument, and
7204 <code>float64</code> for a <code>complex128</code> argument.
7205 If the argument evaluates to an untyped constant, it must be a number,
7206 and the return value of the function is an untyped floating-point constant.
7210 The <code>real</code> and <code>imag</code> functions together form the inverse of
7211 <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>,
7212 <code>z == Z(complex(real(z), imag(z)))</code>.
7216 If the operands of these functions are all constants, the return
7217 value is a constant.
7221 var a = complex(2, -2) // complex128
7222 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i
7223 x := float32(math.Cos(math.Pi/2)) // float32
7224 var c64 = complex(5, -x) // complex64
7225 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int
7226 _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift
7227 var rl = real(c64) // float32
7228 var im = imag(a) // float64
7229 const c = imag(b) // untyped constant -1.4
7230 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift
7233 <h3 id="Handling_panics">Handling panics</h3>
7235 <p> Two built-in functions, <code>panic</code> and <code>recover</code>,
7236 assist in reporting and handling <a href="#Run_time_panics">run-time panics</a>
7237 and program-defined error conditions.
7240 <pre class="grammar">
7241 func panic(interface{})
7242 func recover() interface{}
7246 While executing a function <code>F</code>,
7247 an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a>
7248 terminates the execution of <code>F</code>.
7249 Any functions <a href="#Defer_statements">deferred</a> by <code>F</code>
7250 are then executed as usual.
7251 Next, any deferred functions run by <code>F's</code> caller are run,
7252 and so on up to any deferred by the top-level function in the executing goroutine.
7253 At that point, the program is terminated and the error
7254 condition is reported, including the value of the argument to <code>panic</code>.
7255 This termination sequence is called <i>panicking</i>.
7260 panic("unreachable")
7261 panic(Error("cannot parse"))
7265 The <code>recover</code> function allows a program to manage behavior
7266 of a panicking goroutine.
7267 Suppose a function <code>G</code> defers a function <code>D</code> that calls
7268 <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code>
7270 When the running of deferred functions reaches <code>D</code>,
7271 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>.
7272 If <code>D</code> returns normally, without starting a new
7273 <code>panic</code>, the panicking sequence stops. In that case,
7274 the state of functions called between <code>G</code> and the call to <code>panic</code>
7275 is discarded, and normal execution resumes.
7276 Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s
7277 execution terminates by returning to its caller.
7281 The return value of <code>recover</code> is <code>nil</code> if any of the following conditions holds:
7285 <code>panic</code>'s argument was <code>nil</code>;
7288 the goroutine is not panicking;
7291 <code>recover</code> was not called directly by a deferred function.
7296 The <code>protect</code> function in the example below invokes
7297 the function argument <code>g</code> and protects callers from
7298 run-time panics raised by <code>g</code>.
7302 func protect(g func()) {
7304 log.Println("done") // Println executes normally even if there is a panic
7305 if x := recover(); x != nil {
7306 log.Printf("run time panic: %v", x)
7309 log.Println("start")
7315 <h3 id="Bootstrapping">Bootstrapping</h3>
7318 Current implementations provide several built-in functions useful during
7319 bootstrapping. These functions are documented for completeness but are not
7320 guaranteed to stay in the language. They do not return a result.
7323 <pre class="grammar">
7326 print prints all arguments; formatting of arguments is implementation-specific
7327 println like print but prints spaces between arguments and a newline at the end
7331 Implementation restriction: <code>print</code> and <code>println</code> need not
7332 accept arbitrary argument types, but printing of boolean, numeric, and string
7333 <a href="#Types">types</a> must be supported.
7336 <h2 id="Packages">Packages</h2>
7339 Go programs are constructed by linking together <i>packages</i>.
7340 A package in turn is constructed from one or more source files
7341 that together declare constants, types, variables and functions
7342 belonging to the package and which are accessible in all files
7343 of the same package. Those elements may be
7344 <a href="#Exported_identifiers">exported</a> and used in another package.
7347 <h3 id="Source_file_organization">Source file organization</h3>
7350 Each source file consists of a package clause defining the package
7351 to which it belongs, followed by a possibly empty set of import
7352 declarations that declare packages whose contents it wishes to use,
7353 followed by a possibly empty set of declarations of functions,
7354 types, variables, and constants.
7358 SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
7361 <h3 id="Package_clause">Package clause</h3>
7364 A package clause begins each source file and defines the package
7365 to which the file belongs.
7369 PackageClause = "package" PackageName .
7370 PackageName = identifier .
7374 The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>.
7382 A set of files sharing the same PackageName form the implementation of a package.
7383 An implementation may require that all source files for a package inhabit the same directory.
7386 <h3 id="Import_declarations">Import declarations</h3>
7389 An import declaration states that the source file containing the declaration
7390 depends on functionality of the <i>imported</i> package
7391 (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>)
7392 and enables access to <a href="#Exported_identifiers">exported</a> identifiers
7394 The import names an identifier (PackageName) to be used for access and an ImportPath
7395 that specifies the package to be imported.
7399 ImportDecl = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
7400 ImportSpec = [ "." | PackageName ] ImportPath .
7401 ImportPath = string_lit .
7405 The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a>
7406 to access exported identifiers of the package within the importing source file.
7407 It is declared in the <a href="#Blocks">file block</a>.
7408 If the PackageName is omitted, it defaults to the identifier specified in the
7409 <a href="#Package_clause">package clause</a> of the imported package.
7410 If an explicit period (<code>.</code>) appears instead of a name, all the
7411 package's exported identifiers declared in that package's
7412 <a href="#Blocks">package block</a> will be declared in the importing source
7413 file's file block and must be accessed without a qualifier.
7417 The interpretation of the ImportPath is implementation-dependent but
7418 it is typically a substring of the full file name of the compiled
7419 package and may be relative to a repository of installed packages.
7423 Implementation restriction: A compiler may restrict ImportPaths to
7424 non-empty strings using only characters belonging to
7425 <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a>
7426 L, M, N, P, and S general categories (the Graphic characters without
7427 spaces) and may also exclude the characters
7428 <code>!"#$%&'()*,:;<=>?[\]^`{|}</code>
7429 and the Unicode replacement character U+FFFD.
7433 Assume we have compiled a package containing the package clause
7434 <code>package math</code>, which exports function <code>Sin</code>, and
7435 installed the compiled package in the file identified by
7436 <code>"lib/math"</code>.
7437 This table illustrates how <code>Sin</code> is accessed in files
7438 that import the package after the
7439 various types of import declaration.
7442 <pre class="grammar">
7443 Import declaration Local name of Sin
7445 import "lib/math" math.Sin
7446 import m "lib/math" m.Sin
7447 import . "lib/math" Sin
7451 An import declaration declares a dependency relation between
7452 the importing and imported package.
7453 It is illegal for a package to import itself, directly or indirectly,
7454 or to directly import a package without
7455 referring to any of its exported identifiers. To import a package solely for
7456 its side-effects (initialization), use the <a href="#Blank_identifier">blank</a>
7457 identifier as explicit package name:
7465 <h3 id="An_example_package">An example package</h3>
7468 Here is a complete Go package that implements a concurrent prime sieve.
7476 // Send the sequence 2, 3, 4, … to channel 'ch'.
7477 func generate(ch chan<- int) {
7479 ch <- i // Send 'i' to channel 'ch'.
7483 // Copy the values from channel 'src' to channel 'dst',
7484 // removing those divisible by 'prime'.
7485 func filter(src <-chan int, dst chan<- int, prime int) {
7486 for i := range src { // Loop over values received from 'src'.
7488 dst <- i // Send 'i' to channel 'dst'.
7493 // The prime sieve: Daisy-chain filter processes together.
7495 ch := make(chan int) // Create a new channel.
7496 go generate(ch) // Start generate() as a subprocess.
7499 fmt.Print(prime, "\n")
7500 ch1 := make(chan int)
7501 go filter(ch, ch1, prime)
7511 <h2 id="Program_initialization_and_execution">Program initialization and execution</h2>
7513 <h3 id="The_zero_value">The zero value</h3>
7515 When storage is allocated for a <a href="#Variables">variable</a>,
7516 either through a declaration or a call of <code>new</code>, or when
7517 a new value is created, either through a composite literal or a call
7518 of <code>make</code>,
7519 and no explicit initialization is provided, the variable or value is
7520 given a default value. Each element of such a variable or value is
7521 set to the <i>zero value</i> for its type: <code>false</code> for booleans,
7522 <code>0</code> for numeric types, <code>""</code>
7523 for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps.
7524 This initialization is done recursively, so for instance each element of an
7525 array of structs will have its fields zeroed if no value is specified.
7528 These two simple declarations are equivalent:
7541 type T struct { i int; f float64; next *T }
7546 the following holds:
7556 The same would also be true after
7563 <h3 id="Package_initialization">Package initialization</h3>
7566 Within a package, package-level variable initialization proceeds stepwise,
7567 with each step selecting the variable earliest in <i>declaration order</i>
7568 which has no dependencies on uninitialized variables.
7572 More precisely, a package-level variable is considered <i>ready for
7573 initialization</i> if it is not yet initialized and either has
7574 no <a href="#Variable_declarations">initialization expression</a> or
7575 its initialization expression has no <i>dependencies</i> on uninitialized variables.
7576 Initialization proceeds by repeatedly initializing the next package-level
7577 variable that is earliest in declaration order and ready for initialization,
7578 until there are no variables ready for initialization.
7582 If any variables are still uninitialized when this
7583 process ends, those variables are part of one or more initialization cycles,
7584 and the program is not valid.
7588 Multiple variables on the left-hand side of a variable declaration initialized
7589 by single (multi-valued) expression on the right-hand side are initialized
7590 together: If any of the variables on the left-hand side is initialized, all
7591 those variables are initialized in the same step.
7596 var a, b = f() // a and b are initialized together, before x is initialized
7600 For the purpose of package initialization, <a href="#Blank_identifier">blank</a>
7601 variables are treated like any other variables in declarations.
7605 The declaration order of variables declared in multiple files is determined
7606 by the order in which the files are presented to the compiler: Variables
7607 declared in the first file are declared before any of the variables declared
7608 in the second file, and so on.
7612 Dependency analysis does not rely on the actual values of the
7613 variables, only on lexical <i>references</i> to them in the source,
7614 analyzed transitively. For instance, if a variable <code>x</code>'s
7615 initialization expression refers to a function whose body refers to
7616 variable <code>y</code> then <code>x</code> depends on <code>y</code>.
7622 A reference to a variable or function is an identifier denoting that
7623 variable or function.
7627 A reference to a method <code>m</code> is a
7628 <a href="#Method_values">method value</a> or
7629 <a href="#Method_expressions">method expression</a> of the form
7630 <code>t.m</code>, where the (static) type of <code>t</code> is
7631 not an interface type, and the method <code>m</code> is in the
7632 <a href="#Method_sets">method set</a> of <code>t</code>.
7633 It is immaterial whether the resulting function value
7634 <code>t.m</code> is invoked.
7638 A variable, function, or method <code>x</code> depends on a variable
7639 <code>y</code> if <code>x</code>'s initialization expression or body
7640 (for functions and methods) contains a reference to <code>y</code>
7641 or to a function or method that depends on <code>y</code>.
7646 For example, given the declarations
7654 d = 3 // == 5 after initialization has finished
7664 the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>.
7665 Note that the order of subexpressions in initialization expressions is irrelevant:
7666 <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization
7667 order in this example.
7671 Dependency analysis is performed per package; only references referring
7672 to variables, functions, and (non-interface) methods declared in the current
7673 package are considered. If other, hidden, data dependencies exists between
7674 variables, the initialization order between those variables is unspecified.
7678 For instance, given the declarations
7682 var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b
7683 var _ = sideEffect() // unrelated to x, a, or b
7687 type I interface { ab() []int }
7689 func (T) ab() []int { return []int{a, b} }
7693 the variable <code>a</code> will be initialized after <code>b</code> but
7694 whether <code>x</code> is initialized before <code>b</code>, between
7695 <code>b</code> and <code>a</code>, or after <code>a</code>, and
7696 thus also the moment at which <code>sideEffect()</code> is called (before
7697 or after <code>x</code> is initialized) is not specified.
7701 Variables may also be initialized using functions named <code>init</code>
7702 declared in the package block, with no arguments and no result parameters.
7710 Multiple such functions may be defined per package, even within a single
7711 source file. In the package block, the <code>init</code> identifier can
7712 be used only to declare <code>init</code> functions, yet the identifier
7713 itself is not <a href="#Declarations_and_scope">declared</a>. Thus
7714 <code>init</code> functions cannot be referred to from anywhere
7719 A package with no imports is initialized by assigning initial values
7720 to all its package-level variables followed by calling all <code>init</code>
7721 functions in the order they appear in the source, possibly in multiple files,
7722 as presented to the compiler.
7723 If a package has imports, the imported packages are initialized
7724 before initializing the package itself. If multiple packages import
7725 a package, the imported package will be initialized only once.
7726 The importing of packages, by construction, guarantees that there
7727 can be no cyclic initialization dependencies.
7731 Package initialization—variable initialization and the invocation of
7732 <code>init</code> functions—happens in a single goroutine,
7733 sequentially, one package at a time.
7734 An <code>init</code> function may launch other goroutines, which can run
7735 concurrently with the initialization code. However, initialization
7737 the <code>init</code> functions: it will not invoke the next one
7738 until the previous one has returned.
7742 To ensure reproducible initialization behavior, build systems are encouraged
7743 to present multiple files belonging to the same package in lexical file name
7744 order to a compiler.
7748 <h3 id="Program_execution">Program execution</h3>
7750 A complete program is created by linking a single, unimported package
7751 called the <i>main package</i> with all the packages it imports, transitively.
7752 The main package must
7753 have package name <code>main</code> and
7754 declare a function <code>main</code> that takes no
7755 arguments and returns no value.
7763 Program execution begins by initializing the main package and then
7764 invoking the function <code>main</code>.
7765 When that function invocation returns, the program exits.
7766 It does not wait for other (non-<code>main</code>) goroutines to complete.
7769 <h2 id="Errors">Errors</h2>
7772 The predeclared type <code>error</code> is defined as
7776 type error interface {
7782 It is the conventional interface for representing an error condition,
7783 with the nil value representing no error.
7784 For instance, a function to read data from a file might be defined:
7788 func Read(f *File, b []byte) (n int, err error)
7791 <h2 id="Run_time_panics">Run-time panics</h2>
7794 Execution errors such as attempting to index an array out
7795 of bounds trigger a <i>run-time panic</i> equivalent to a call of
7796 the built-in function <a href="#Handling_panics"><code>panic</code></a>
7797 with a value of the implementation-defined interface type <code>runtime.Error</code>.
7798 That type satisfies the predeclared interface type
7799 <a href="#Errors"><code>error</code></a>.
7800 The exact error values that
7801 represent distinct run-time error conditions are unspecified.
7807 type Error interface {
7809 // and perhaps other methods
7813 <h2 id="System_considerations">System considerations</h2>
7815 <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
7818 The built-in package <code>unsafe</code>, known to the compiler
7819 and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>,
7820 provides facilities for low-level programming including operations
7821 that violate the type system. A package using <code>unsafe</code>
7822 must be vetted manually for type safety and may not be portable.
7823 The package provides the following interface:
7826 <pre class="grammar">
7829 type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type
7830 type Pointer *ArbitraryType
7832 func Alignof(variable ArbitraryType) uintptr
7833 func Offsetof(selector ArbitraryType) uintptr
7834 func Sizeof(variable ArbitraryType) uintptr
7836 type IntegerType int // shorthand for an integer type; it is not a real type
7837 func Add(ptr Pointer, len IntegerType) Pointer
7838 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
7842 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
7843 value may not be <a href="#Address_operators">dereferenced</a>.
7844 Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be converted to
7845 a type of underlying type <code>Pointer</code> and vice versa.
7846 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
7851 bits = *(*uint64)(unsafe.Pointer(&f))
7853 type ptr unsafe.Pointer
7854 bits = *(*uint64)(ptr(&f))
7860 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
7861 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
7862 as if <code>v</code> was declared via <code>var v = x</code>.
7865 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
7866 <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code>
7867 or <code>*s</code>, and returns the field offset in bytes relative to the struct's address.
7868 If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable
7869 without pointer indirections through fields of the struct.
7870 For a struct <code>s</code> with field <code>f</code>:
7874 uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))
7878 Computer architectures may require memory addresses to be <i>aligned</i>;
7879 that is, for addresses of a variable to be a multiple of a factor,
7880 the variable's type's <i>alignment</i>. The function <code>Alignof</code>
7881 takes an expression denoting a variable of any type and returns the
7882 alignment of the (type of the) variable in bytes. For a variable
7887 uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0
7891 A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code>
7892 is a type parameter, or if it is an array or struct type containing elements
7893 or fields of variable size. Otherwise the size is <i>constant</i>.
7894 Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code>
7895 are compile-time <a href="#Constant_expressions">constant expressions</a> of
7896 type <code>uintptr</code> if their arguments (or the struct <code>s</code> in
7897 the selector expression <code>s.f</code> for <code>Offsetof</code>) are types
7902 The function <code>Add</code> adds <code>len</code> to <code>ptr</code>
7903 and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>.
7904 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
7905 A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>;
7906 if it is an untyped constant it is given type <code>int</code>.
7907 The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply.
7911 The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code>
7912 and whose length and capacity are <code>len</code>.
7913 <code>Slice(ptr, len)</code> is equivalent to
7917 (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
7921 except that, as a special case, if <code>ptr</code>
7922 is <code>nil</code> and <code>len</code> is zero,
7923 <code>Slice</code> returns <code>nil</code>.
7927 The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>.
7928 A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>;
7929 if it is an untyped constant it is given type <code>int</code>.
7930 At run time, if <code>len</code> is negative,
7931 or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero,
7932 a <a href="#Run_time_panics">run-time panic</a> occurs.
7935 <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3>
7938 For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed:
7941 <pre class="grammar">
7946 uint32, int32, float32 4
7947 uint64, int64, float64, complex64 8
7952 The following minimal alignment properties are guaranteed:
7955 <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1.
7958 <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
7959 all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1.
7962 <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
7963 the alignment of a variable of the array's element type.
7968 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.