| <!--{ |
| "Title": "The Go Programming Language Specification", |
| "Subtitle": "Version of May 12, 2022", |
| "Path": "/ref/spec" |
| }--> |
| |
| <h2 id="Introduction">Introduction</h2> |
| |
| <p> |
| This is the reference manual for the Go programming language. |
| The pre-Go1.18 version, without generics, can be found |
| <a href="/doc/go1.17_spec.html">here</a>. |
| For more information and other documents, see <a href="/">golang.org</a>. |
| </p> |
| |
| <p> |
| Go is a general-purpose language designed with systems programming |
| in mind. It is strongly typed and garbage-collected and has explicit |
| support for concurrent programming. Programs are constructed from |
| <i>packages</i>, whose properties allow efficient management of |
| dependencies. |
| </p> |
| |
| <p> |
| The syntax is compact and simple to parse, allowing for easy analysis |
| by automatic tools such as integrated development environments. |
| </p> |
| |
| <h2 id="Notation">Notation</h2> |
| <p> |
| The syntax is specified using a |
| <a href="https://en.wikipedia.org/wiki/Wirth_syntax_notation">variant</a> |
| of Extended Backus-Naur Form (EBNF): |
| </p> |
| |
| <pre class="grammar"> |
| Syntax = { Production } . |
| Production = production_name "=" [ Expression ] "." . |
| Expression = Term { "|" Term } . |
| Term = Factor { Factor } . |
| Factor = production_name | token [ "…" token ] | Group | Option | Repetition . |
| Group = "(" Expression ")" . |
| Option = "[" Expression "]" . |
| Repetition = "{" Expression "}" . |
| </pre> |
| |
| <p> |
| Productions are expressions constructed from terms and the following |
| operators, in increasing precedence: |
| </p> |
| <pre class="grammar"> |
| | alternation |
| () grouping |
| [] option (0 or 1 times) |
| {} repetition (0 to n times) |
| </pre> |
| |
| <p> |
| Lowercase production names are used to identify lexical (terminal) tokens. |
| Non-terminals are in CamelCase. Lexical tokens are enclosed in |
| double quotes <code>""</code> or back quotes <code>``</code>. |
| </p> |
| |
| <p> |
| The form <code>a … b</code> represents the set of characters from |
| <code>a</code> through <code>b</code> as alternatives. The horizontal |
| ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various |
| enumerations or code snippets that are not further specified. The character <code>…</code> |
| (as opposed to the three characters <code>...</code>) is not a token of the Go |
| language. |
| </p> |
| |
| <h2 id="Source_code_representation">Source code representation</h2> |
| |
| <p> |
| Source code is Unicode text encoded in |
| <a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not |
| canonicalized, so a single accented code point is distinct from the |
| same character constructed from combining an accent and a letter; |
| those are treated as two code points. For simplicity, this document |
| will use the unqualified term <i>character</i> to refer to a Unicode code point |
| in the source text. |
| </p> |
| <p> |
| Each code point is distinct; for instance, uppercase and lowercase letters |
| are different characters. |
| </p> |
| <p> |
| Implementation restriction: For compatibility with other tools, a |
| compiler may disallow the NUL character (U+0000) in the source text. |
| </p> |
| <p> |
| Implementation restriction: For compatibility with other tools, a |
| compiler may ignore a UTF-8-encoded byte order mark |
| (U+FEFF) if it is the first Unicode code point in the source text. |
| A byte order mark may be disallowed anywhere else in the source. |
| </p> |
| |
| <h3 id="Characters">Characters</h3> |
| |
| <p> |
| The following terms are used to denote specific Unicode character categories: |
| </p> |
| <pre class="ebnf"> |
| newline = /* the Unicode code point U+000A */ . |
| unicode_char = /* an arbitrary Unicode code point except newline */ . |
| unicode_letter = /* a Unicode code point categorized as "Letter" */ . |
| unicode_digit = /* a Unicode code point categorized as "Number, decimal digit" */ . |
| </pre> |
| |
| <p> |
| In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>, |
| Section 4.5 "General Category" defines a set of character categories. |
| Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo |
| as Unicode letters, and those in the Number category Nd as Unicode digits. |
| </p> |
| |
| <h3 id="Letters_and_digits">Letters and digits</h3> |
| |
| <p> |
| The underscore character <code>_</code> (U+005F) is considered a lowercase letter. |
| </p> |
| <pre class="ebnf"> |
| letter = unicode_letter | "_" . |
| decimal_digit = "0" … "9" . |
| binary_digit = "0" | "1" . |
| octal_digit = "0" … "7" . |
| hex_digit = "0" … "9" | "A" … "F" | "a" … "f" . |
| </pre> |
| |
| <h2 id="Lexical_elements">Lexical elements</h2> |
| |
| <h3 id="Comments">Comments</h3> |
| |
| <p> |
| Comments serve as program documentation. There are two forms: |
| </p> |
| |
| <ol> |
| <li> |
| <i>Line comments</i> start with the character sequence <code>//</code> |
| and stop at the end of the line. |
| </li> |
| <li> |
| <i>General comments</i> start with the character sequence <code>/*</code> |
| and stop with the first subsequent character sequence <code>*/</code>. |
| </li> |
| </ol> |
| |
| <p> |
| A comment cannot start inside a <a href="#Rune_literals">rune</a> or |
| <a href="#String_literals">string literal</a>, or inside a comment. |
| A general comment containing no newlines acts like a space. |
| Any other comment acts like a newline. |
| </p> |
| |
| <h3 id="Tokens">Tokens</h3> |
| |
| <p> |
| Tokens form the vocabulary of the Go language. |
| There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators |
| and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from |
| spaces (U+0020), horizontal tabs (U+0009), |
| carriage returns (U+000D), and newlines (U+000A), |
| is ignored except as it separates tokens |
| that would otherwise combine into a single token. Also, a newline or end of file |
| may trigger the insertion of a <a href="#Semicolons">semicolon</a>. |
| While breaking the input into tokens, |
| the next token is the longest sequence of characters that form a |
| valid token. |
| </p> |
| |
| <h3 id="Semicolons">Semicolons</h3> |
| |
| <p> |
| The formal syntax uses semicolons <code>";"</code> as terminators in |
| a number of productions. Go programs may omit most of these semicolons |
| using the following two rules: |
| </p> |
| |
| <ol> |
| <li> |
| When the input is broken into tokens, a semicolon is automatically inserted |
| into the token stream immediately after a line's final token if that token is |
| <ul> |
| <li>an |
| <a href="#Identifiers">identifier</a> |
| </li> |
| |
| <li>an |
| <a href="#Integer_literals">integer</a>, |
| <a href="#Floating-point_literals">floating-point</a>, |
| <a href="#Imaginary_literals">imaginary</a>, |
| <a href="#Rune_literals">rune</a>, or |
| <a href="#String_literals">string</a> literal |
| </li> |
| |
| <li>one of the <a href="#Keywords">keywords</a> |
| <code>break</code>, |
| <code>continue</code>, |
| <code>fallthrough</code>, or |
| <code>return</code> |
| </li> |
| |
| <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a> |
| <code>++</code>, |
| <code>--</code>, |
| <code>)</code>, |
| <code>]</code>, or |
| <code>}</code> |
| </li> |
| </ul> |
| </li> |
| |
| <li> |
| To allow complex statements to occupy a single line, a semicolon |
| may be omitted before a closing <code>")"</code> or <code>"}"</code>. |
| </li> |
| </ol> |
| |
| <p> |
| To reflect idiomatic use, code examples in this document elide semicolons |
| using these rules. |
| </p> |
| |
| |
| <h3 id="Identifiers">Identifiers</h3> |
| |
| <p> |
| Identifiers name program entities such as variables and types. |
| An identifier is a sequence of one or more letters and digits. |
| The first character in an identifier must be a letter. |
| </p> |
| <pre class="ebnf"> |
| identifier = letter { letter | unicode_digit } . |
| </pre> |
| <pre> |
| a |
| _x9 |
| ThisVariableIsExported |
| αβ |
| </pre> |
| |
| <p> |
| Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>. |
| </p> |
| |
| |
| <h3 id="Keywords">Keywords</h3> |
| |
| <p> |
| The following keywords are reserved and may not be used as identifiers. |
| </p> |
| <pre class="grammar"> |
| break default func interface select |
| case defer go map struct |
| chan else goto package switch |
| const fallthrough if range type |
| continue for import return var |
| </pre> |
| |
| <h3 id="Operators_and_punctuation">Operators and punctuation</h3> |
| |
| <p> |
| The following character sequences represent <a href="#Operators">operators</a> |
| (including <a href="#Assignments">assignment operators</a>) and punctuation: |
| </p> |
| <pre class="grammar"> |
| + & += &= && == != ( ) |
| - | -= |= || < <= [ ] |
| * ^ *= ^= <- > >= { } |
| / << /= <<= ++ = := , ; |
| % >> %= >>= -- ! ... . : |
| &^ &^= ~ |
| </pre> |
| |
| <h3 id="Integer_literals">Integer literals</h3> |
| |
| <p> |
| An integer literal is a sequence of digits representing an |
| <a href="#Constants">integer constant</a>. |
| An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code> |
| for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal, |
| and <code>0x</code> or <code>0X</code> for hexadecimal. |
| A single <code>0</code> is considered a decimal zero. |
| In hexadecimal literals, letters <code>a</code> through <code>f</code> |
| and <code>A</code> through <code>F</code> represent values 10 through 15. |
| </p> |
| |
| <p> |
| For readability, an underscore character <code>_</code> may appear after |
| a base prefix or between successive digits; such underscores do not change |
| the literal's value. |
| </p> |
| <pre class="ebnf"> |
| int_lit = decimal_lit | binary_lit | octal_lit | hex_lit . |
| decimal_lit = "0" | ( "1" … "9" ) [ [ "_" ] decimal_digits ] . |
| binary_lit = "0" ( "b" | "B" ) [ "_" ] binary_digits . |
| octal_lit = "0" [ "o" | "O" ] [ "_" ] octal_digits . |
| hex_lit = "0" ( "x" | "X" ) [ "_" ] hex_digits . |
| |
| decimal_digits = decimal_digit { [ "_" ] decimal_digit } . |
| binary_digits = binary_digit { [ "_" ] binary_digit } . |
| octal_digits = octal_digit { [ "_" ] octal_digit } . |
| hex_digits = hex_digit { [ "_" ] hex_digit } . |
| </pre> |
| |
| <pre> |
| 42 |
| 4_2 |
| 0600 |
| 0_600 |
| 0o600 |
| 0O600 // second character is capital letter 'O' |
| 0xBadFace |
| 0xBad_Face |
| 0x_67_7a_2f_cc_40_c6 |
| 170141183460469231731687303715884105727 |
| 170_141183_460469_231731_687303_715884_105727 |
| |
| _42 // an identifier, not an integer literal |
| 42_ // invalid: _ must separate successive digits |
| 4__2 // invalid: only one _ at a time |
| 0_xBadFace // invalid: _ must separate successive digits |
| </pre> |
| |
| |
| <h3 id="Floating-point_literals">Floating-point literals</h3> |
| |
| <p> |
| A floating-point literal is a decimal or hexadecimal representation of a |
| <a href="#Constants">floating-point constant</a>. |
| </p> |
| |
| <p> |
| A decimal floating-point literal consists of an integer part (decimal digits), |
| a decimal point, a fractional part (decimal digits), and an exponent part |
| (<code>e</code> or <code>E</code> followed by an optional sign and decimal digits). |
| One of the integer part or the fractional part may be elided; one of the decimal point |
| or the exponent part may be elided. |
| An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>. |
| </p> |
| |
| <p> |
| A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code> |
| prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits), |
| and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits). |
| One of the integer part or the fractional part may be elided; the radix point may be elided as well, |
| but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.) |
| An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>. |
| </p> |
| |
| <p> |
| For readability, an underscore character <code>_</code> may appear after |
| a base prefix or between successive digits; such underscores do not change |
| the literal value. |
| </p> |
| |
| <pre class="ebnf"> |
| float_lit = decimal_float_lit | hex_float_lit . |
| |
| decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] | |
| decimal_digits decimal_exponent | |
| "." decimal_digits [ decimal_exponent ] . |
| decimal_exponent = ( "e" | "E" ) [ "+" | "-" ] decimal_digits . |
| |
| hex_float_lit = "0" ( "x" | "X" ) hex_mantissa hex_exponent . |
| hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] | |
| [ "_" ] hex_digits | |
| "." hex_digits . |
| hex_exponent = ( "p" | "P" ) [ "+" | "-" ] decimal_digits . |
| </pre> |
| |
| <pre> |
| 0. |
| 72.40 |
| 072.40 // == 72.40 |
| 2.71828 |
| 1.e+0 |
| 6.67428e-11 |
| 1E6 |
| .25 |
| .12345E+5 |
| 1_5. // == 15.0 |
| 0.15e+0_2 // == 15.0 |
| |
| 0x1p-2 // == 0.25 |
| 0x2.p10 // == 2048.0 |
| 0x1.Fp+0 // == 1.9375 |
| 0X.8p-0 // == 0.5 |
| 0X_1FFFP-16 // == 0.1249847412109375 |
| 0x15e-2 // == 0x15e - 2 (integer subtraction) |
| |
| 0x.p1 // invalid: mantissa has no digits |
| 1p-2 // invalid: p exponent requires hexadecimal mantissa |
| 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent |
| 1_.5 // invalid: _ must separate successive digits |
| 1._5 // invalid: _ must separate successive digits |
| 1.5_e1 // invalid: _ must separate successive digits |
| 1.5e_1 // invalid: _ must separate successive digits |
| 1.5e1_ // invalid: _ must separate successive digits |
| </pre> |
| |
| |
| <h3 id="Imaginary_literals">Imaginary literals</h3> |
| |
| <p> |
| An imaginary literal represents the imaginary part of a |
| <a href="#Constants">complex constant</a>. |
| It consists of an <a href="#Integer_literals">integer</a> or |
| <a href="#Floating-point_literals">floating-point</a> literal |
| followed by the lowercase letter <code>i</code>. |
| The value of an imaginary literal is the value of the respective |
| integer or floating-point literal multiplied by the imaginary unit <i>i</i>. |
| </p> |
| |
| <pre class="ebnf"> |
| imaginary_lit = (decimal_digits | int_lit | float_lit) "i" . |
| </pre> |
| |
| <p> |
| For backward compatibility, an imaginary literal's integer part consisting |
| entirely of decimal digits (and possibly underscores) is considered a decimal |
| integer, even if it starts with a leading <code>0</code>. |
| </p> |
| |
| <pre> |
| 0i |
| 0123i // == 123i for backward-compatibility |
| 0o123i // == 0o123 * 1i == 83i |
| 0xabci // == 0xabc * 1i == 2748i |
| 0.i |
| 2.71828i |
| 1.e+0i |
| 6.67428e-11i |
| 1E6i |
| .25i |
| .12345E+5i |
| 0x1p-2i // == 0x1p-2 * 1i == 0.25i |
| </pre> |
| |
| |
| <h3 id="Rune_literals">Rune literals</h3> |
| |
| <p> |
| A rune literal represents a <a href="#Constants">rune constant</a>, |
| an integer value identifying a Unicode code point. |
| A rune literal is expressed as one or more characters enclosed in single quotes, |
| as in <code>'x'</code> or <code>'\n'</code>. |
| Within the quotes, any character may appear except newline and unescaped single |
| quote. A single quoted character represents the Unicode value |
| of the character itself, |
| while multi-character sequences beginning with a backslash encode |
| values in various formats. |
| </p> |
| |
| <p> |
| The simplest form represents the single character within the quotes; |
| since Go source text is Unicode characters encoded in UTF-8, multiple |
| UTF-8-encoded bytes may represent a single integer value. For |
| instance, the literal <code>'a'</code> holds a single byte representing |
| a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while |
| <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing |
| a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>. |
| </p> |
| |
| <p> |
| Several backslash escapes allow arbitrary values to be encoded as |
| ASCII text. There are four ways to represent the integer value |
| as a numeric constant: <code>\x</code> followed by exactly two hexadecimal |
| digits; <code>\u</code> followed by exactly four hexadecimal digits; |
| <code>\U</code> followed by exactly eight hexadecimal digits, and a |
| plain backslash <code>\</code> followed by exactly three octal digits. |
| In each case the value of the literal is the value represented by |
| the digits in the corresponding base. |
| </p> |
| |
| <p> |
| Although these representations all result in an integer, they have |
| different valid ranges. Octal escapes must represent a value between |
| 0 and 255 inclusive. Hexadecimal escapes satisfy this condition |
| by construction. The escapes <code>\u</code> and <code>\U</code> |
| represent Unicode code points so within them some values are illegal, |
| in particular those above <code>0x10FFFF</code> and surrogate halves. |
| </p> |
| |
| <p> |
| After a backslash, certain single-character escapes represent special values: |
| </p> |
| |
| <pre class="grammar"> |
| \a U+0007 alert or bell |
| \b U+0008 backspace |
| \f U+000C form feed |
| \n U+000A line feed or newline |
| \r U+000D carriage return |
| \t U+0009 horizontal tab |
| \v U+000B vertical tab |
| \\ U+005C backslash |
| \' U+0027 single quote (valid escape only within rune literals) |
| \" U+0022 double quote (valid escape only within string literals) |
| </pre> |
| |
| <p> |
| All other sequences starting with a backslash are illegal inside rune literals. |
| </p> |
| <pre class="ebnf"> |
| rune_lit = "'" ( unicode_value | byte_value ) "'" . |
| unicode_value = unicode_char | little_u_value | big_u_value | escaped_char . |
| byte_value = octal_byte_value | hex_byte_value . |
| octal_byte_value = `\` octal_digit octal_digit octal_digit . |
| hex_byte_value = `\` "x" hex_digit hex_digit . |
| little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit . |
| big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit |
| hex_digit hex_digit hex_digit hex_digit . |
| escaped_char = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) . |
| </pre> |
| |
| <pre> |
| 'a' |
| 'ä' |
| '本' |
| '\t' |
| '\000' |
| '\007' |
| '\377' |
| '\x07' |
| '\xff' |
| '\u12e4' |
| '\U00101234' |
| '\'' // rune literal containing single quote character |
| 'aa' // illegal: too many characters |
| '\xa' // illegal: too few hexadecimal digits |
| '\0' // illegal: too few octal digits |
| '\400' // illegal: octal value over 255 |
| '\uDFFF' // illegal: surrogate half |
| '\U00110000' // illegal: invalid Unicode code point |
| </pre> |
| |
| |
| <h3 id="String_literals">String literals</h3> |
| |
| <p> |
| A string literal represents a <a href="#Constants">string constant</a> |
| obtained from concatenating a sequence of characters. There are two forms: |
| raw string literals and interpreted string literals. |
| </p> |
| |
| <p> |
| Raw string literals are character sequences between back quotes, as in |
| <code>`foo`</code>. Within the quotes, any character may appear except |
| back quote. The value of a raw string literal is the |
| string composed of the uninterpreted (implicitly UTF-8-encoded) characters |
| between the quotes; |
| in particular, backslashes have no special meaning and the string may |
| contain newlines. |
| Carriage return characters ('\r') inside raw string literals |
| are discarded from the raw string value. |
| </p> |
| |
| <p> |
| Interpreted string literals are character sequences between double |
| quotes, as in <code>"bar"</code>. |
| Within the quotes, any character may appear except newline and unescaped double quote. |
| The text between the quotes forms the |
| value of the literal, with backslash escapes interpreted as they |
| are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and |
| <code>\"</code> is legal), with the same restrictions. |
| The three-digit octal (<code>\</code><i>nnn</i>) |
| and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual |
| <i>bytes</i> of the resulting string; all other escapes represent |
| the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>. |
| Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent |
| a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>, |
| <code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent |
| the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character |
| U+00FF. |
| </p> |
| |
| <pre class="ebnf"> |
| string_lit = raw_string_lit | interpreted_string_lit . |
| raw_string_lit = "`" { unicode_char | newline } "`" . |
| interpreted_string_lit = `"` { unicode_value | byte_value } `"` . |
| </pre> |
| |
| <pre> |
| `abc` // same as "abc" |
| `\n |
| \n` // same as "\\n\n\\n" |
| "\n" |
| "\"" // same as `"` |
| "Hello, world!\n" |
| "日本語" |
| "\u65e5本\U00008a9e" |
| "\xff\u00FF" |
| "\uD800" // illegal: surrogate half |
| "\U00110000" // illegal: invalid Unicode code point |
| </pre> |
| |
| <p> |
| These examples all represent the same string: |
| </p> |
| |
| <pre> |
| "日本語" // UTF-8 input text |
| `日本語` // UTF-8 input text as a raw literal |
| "\u65e5\u672c\u8a9e" // the explicit Unicode code points |
| "\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points |
| "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes |
| </pre> |
| |
| <p> |
| If the source code represents a character as two code points, such as |
| a combining form involving an accent and a letter, the result will be |
| an error if placed in a rune literal (it is not a single code |
| point), and will appear as two code points if placed in a string |
| literal. |
| </p> |
| |
| |
| <h2 id="Constants">Constants</h2> |
| |
| <p>There are <i>boolean constants</i>, |
| <i>rune constants</i>, |
| <i>integer constants</i>, |
| <i>floating-point constants</i>, <i>complex constants</i>, |
| and <i>string constants</i>. Rune, integer, floating-point, |
| and complex constants are |
| collectively called <i>numeric constants</i>. |
| </p> |
| |
| <p> |
| A constant value is represented by a |
| <a href="#Rune_literals">rune</a>, |
| <a href="#Integer_literals">integer</a>, |
| <a href="#Floating-point_literals">floating-point</a>, |
| <a href="#Imaginary_literals">imaginary</a>, |
| or |
| <a href="#String_literals">string</a> literal, |
| an identifier denoting a constant, |
| a <a href="#Constant_expressions">constant expression</a>, |
| a <a href="#Conversions">conversion</a> with a result that is a constant, or |
| the result value of some built-in functions such as |
| <code>unsafe.Sizeof</code> applied to <a href="#Package_unsafe">certain values</a>, |
| <code>cap</code> or <code>len</code> applied to |
| <a href="#Length_and_capacity">some expressions</a>, |
| <code>real</code> and <code>imag</code> applied to a complex constant |
| and <code>complex</code> applied to numeric constants. |
| The boolean truth values are represented by the predeclared constants |
| <code>true</code> and <code>false</code>. The predeclared identifier |
| <a href="#Iota">iota</a> denotes an integer constant. |
| </p> |
| |
| <p> |
| In general, complex constants are a form of |
| <a href="#Constant_expressions">constant expression</a> |
| and are discussed in that section. |
| </p> |
| |
| <p> |
| Numeric constants represent exact values of arbitrary precision and do not overflow. |
| Consequently, there are no constants denoting the IEEE-754 negative zero, infinity, |
| and not-a-number values. |
| </p> |
| |
| <p> |
| Constants may be <a href="#Types">typed</a> or <i>untyped</i>. |
| Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>, |
| and certain <a href="#Constant_expressions">constant expressions</a> |
| containing only untyped constant operands are untyped. |
| </p> |
| |
| <p> |
| A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a> |
| or <a href="#Conversions">conversion</a>, or implicitly when used in a |
| <a href="#Variable_declarations">variable declaration</a> or an |
| <a href="#Assignments">assignment</a> or as an |
| operand in an <a href="#Expressions">expression</a>. |
| It is an error if the constant value |
| cannot be <a href="#Representability">represented</a> as a value of the respective type. |
| If the type is a type parameter, the constant is converted into a non-constant |
| value of the type parameter. |
| </p> |
| |
| <p> |
| An untyped constant has a <i>default type</i> which is the type to which the |
| constant is implicitly converted in contexts where a typed value is required, |
| for instance, in a <a href="#Short_variable_declarations">short variable declaration</a> |
| such as <code>i := 0</code> where there is no explicit type. |
| The default type of an untyped constant is <code>bool</code>, <code>rune</code>, |
| <code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code> |
| respectively, depending on whether it is a boolean, rune, integer, floating-point, |
| complex, or string constant. |
| </p> |
| |
| <p> |
| Implementation restriction: Although numeric constants have arbitrary |
| precision in the language, a compiler may implement them using an |
| internal representation with limited precision. That said, every |
| implementation must: |
| </p> |
| |
| <ul> |
| <li>Represent integer constants with at least 256 bits.</li> |
| |
| <li>Represent floating-point constants, including the parts of |
| a complex constant, with a mantissa of at least 256 bits |
| and a signed binary exponent of at least 16 bits.</li> |
| |
| <li>Give an error if unable to represent an integer constant |
| precisely.</li> |
| |
| <li>Give an error if unable to represent a floating-point or |
| complex constant due to overflow.</li> |
| |
| <li>Round to the nearest representable constant if unable to |
| represent a floating-point or complex constant due to limits |
| on precision.</li> |
| </ul> |
| |
| <p> |
| These requirements apply both to literal constants and to the result |
| of evaluating <a href="#Constant_expressions">constant |
| expressions</a>. |
| </p> |
| |
| |
| <h2 id="Variables">Variables</h2> |
| |
| <p> |
| A variable is a storage location for holding a <i>value</i>. |
| The set of permissible values is determined by the |
| variable's <i><a href="#Types">type</a></i>. |
| </p> |
| |
| <p> |
| A <a href="#Variable_declarations">variable declaration</a> |
| or, for function parameters and results, the signature |
| of a <a href="#Function_declarations">function declaration</a> |
| or <a href="#Function_literals">function literal</a> reserves |
| storage for a named variable. |
| |
| Calling the built-in function <a href="#Allocation"><code>new</code></a> |
| or taking the address of a <a href="#Composite_literals">composite literal</a> |
| allocates storage for a variable at run time. |
| Such an anonymous variable is referred to via a (possibly implicit) |
| <a href="#Address_operators">pointer indirection</a>. |
| </p> |
| |
| <p> |
| <i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>, |
| and <a href="#Struct_types">struct</a> types have elements and fields that may |
| be <a href="#Address_operators">addressed</a> individually. Each such element |
| acts like a variable. |
| </p> |
| |
| <p> |
| The <i>static type</i> (or just <i>type</i>) of a variable is the |
| type given in its declaration, the type provided in the |
| <code>new</code> call or composite literal, or the type of |
| an element of a structured variable. |
| Variables of interface type also have a distinct <i>dynamic type</i>, |
| which is the (non-interface) type of the value assigned to the variable at run time |
| (unless the value is the predeclared identifier <code>nil</code>, |
| which has no type). |
| The dynamic type may vary during execution but values stored in interface |
| variables are always <a href="#Assignability">assignable</a> |
| to the static type of the variable. |
| </p> |
| |
| <pre> |
| var x interface{} // x is nil and has static type interface{} |
| var v *T // v has value nil, static type *T |
| x = 42 // x has value 42 and dynamic type int |
| x = v // x has value (*T)(nil) and dynamic type *T |
| </pre> |
| |
| <p> |
| A variable's value is retrieved by referring to the variable in an |
| <a href="#Expressions">expression</a>; it is the most recent value |
| <a href="#Assignments">assigned</a> to the variable. |
| If a variable has not yet been assigned a value, its value is the |
| <a href="#The_zero_value">zero value</a> for its type. |
| </p> |
| |
| |
| <h2 id="Types">Types</h2> |
| |
| <p> |
| A type determines a set of values together with operations and methods specific |
| to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be |
| followed by <a href="#Instantiations">type arguments</a> if the type is generic. |
| A type may also be specified using a <i>type literal</i>, which composes a type |
| from existing types. |
| </p> |
| |
| <pre class="ebnf"> |
| Type = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" . |
| TypeName = identifier | QualifiedIdent . |
| TypeArgs = "[" TypeList [ "," ] "]" . |
| TypeList = Type { "," Type } . |
| TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType | |
| SliceType | MapType | ChannelType . |
| </pre> |
| |
| <p> |
| The language <a href="#Predeclared_identifiers">predeclares</a> certain type names. |
| Others are introduced with <a href="#Type_declarations">type declarations</a> |
| or <a href="#Type_parameter_declarations">type parameter lists</a>. |
| <i>Composite types</i>—array, struct, pointer, function, |
| interface, slice, map, and channel types—may be constructed using |
| type literals. |
| </p> |
| |
| <p> |
| Predeclared types, defined types, and type parameters are called <i>named types</i>. |
| An alias denotes a named type if the type given in the alias declaration is a named type. |
| </p> |
| |
| <h3 id="Boolean_types">Boolean types</h3> |
| |
| <p> |
| A <i>boolean type</i> represents the set of Boolean truth values |
| denoted by the predeclared constants <code>true</code> |
| and <code>false</code>. The predeclared boolean type is <code>bool</code>; |
| it is a <a href="#Type_definitions">defined type</a>. |
| </p> |
| |
| <h3 id="Numeric_types">Numeric types</h3> |
| |
| <p> |
| An <i>integer</i>, <i>floating-point</i>, or <i>complex</i> type |
| represents the set of integer, floating-point, or complex values, respectively. |
| They are collectively called <i>numeric types</i>. |
| The predeclared architecture-independent numeric types are: |
| </p> |
| |
| <pre class="grammar"> |
| uint8 the set of all unsigned 8-bit integers (0 to 255) |
| uint16 the set of all unsigned 16-bit integers (0 to 65535) |
| uint32 the set of all unsigned 32-bit integers (0 to 4294967295) |
| uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615) |
| |
| int8 the set of all signed 8-bit integers (-128 to 127) |
| int16 the set of all signed 16-bit integers (-32768 to 32767) |
| int32 the set of all signed 32-bit integers (-2147483648 to 2147483647) |
| int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807) |
| |
| float32 the set of all IEEE-754 32-bit floating-point numbers |
| float64 the set of all IEEE-754 64-bit floating-point numbers |
| |
| complex64 the set of all complex numbers with float32 real and imaginary parts |
| complex128 the set of all complex numbers with float64 real and imaginary parts |
| |
| byte alias for uint8 |
| rune alias for int32 |
| </pre> |
| |
| <p> |
| The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using |
| <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>. |
| </p> |
| |
| <p> |
| There is also a set of predeclared integer types with implementation-specific sizes: |
| </p> |
| |
| <pre class="grammar"> |
| uint either 32 or 64 bits |
| int same size as uint |
| uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value |
| </pre> |
| |
| <p> |
| To avoid portability issues all numeric types are <a href="#Type_definitions">defined |
| types</a> and thus distinct except |
| <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and |
| <code>rune</code>, which is an alias for <code>int32</code>. |
| Explicit conversions |
| are required when different numeric types are mixed in an expression |
| or assignment. For instance, <code>int32</code> and <code>int</code> |
| are not the same type even though they may have the same size on a |
| particular architecture. |
| |
| |
| <h3 id="String_types">String types</h3> |
| |
| <p> |
| A <i>string type</i> represents the set of string values. |
| A string value is a (possibly empty) sequence of bytes. |
| The number of bytes is called the length of the string and is never negative. |
| Strings are immutable: once created, |
| it is impossible to change the contents of a string. |
| The predeclared string type is <code>string</code>; |
| it is a <a href="#Type_definitions">defined type</a>. |
| </p> |
| |
| <p> |
| The length of a string <code>s</code> can be discovered using |
| the built-in function <a href="#Length_and_capacity"><code>len</code></a>. |
| The length is a compile-time constant if the string is a constant. |
| A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a> |
| 0 through <code>len(s)-1</code>. |
| It is illegal to take the address of such an element; if |
| <code>s[i]</code> is the <code>i</code>'th byte of a |
| string, <code>&s[i]</code> is invalid. |
| </p> |
| |
| |
| <h3 id="Array_types">Array types</h3> |
| |
| <p> |
| An array is a numbered sequence of elements of a single |
| type, called the element type. |
| The number of elements is called the length of the array and is never negative. |
| </p> |
| |
| <pre class="ebnf"> |
| ArrayType = "[" ArrayLength "]" ElementType . |
| ArrayLength = Expression . |
| ElementType = Type . |
| </pre> |
| |
| <p> |
| The length is part of the array's type; it must evaluate to a |
| non-negative <a href="#Constants">constant</a> |
| <a href="#Representability">representable</a> by a value |
| of type <code>int</code>. |
| The length of array <code>a</code> can be discovered |
| using the built-in function <a href="#Length_and_capacity"><code>len</code></a>. |
| The elements can be addressed by integer <a href="#Index_expressions">indices</a> |
| 0 through <code>len(a)-1</code>. |
| Array types are always one-dimensional but may be composed to form |
| multi-dimensional types. |
| </p> |
| |
| <pre> |
| [32]byte |
| [2*N] struct { x, y int32 } |
| [1000]*float64 |
| [3][5]int |
| [2][2][2]float64 // same as [2]([2]([2]float64)) |
| </pre> |
| |
| <h3 id="Slice_types">Slice types</h3> |
| |
| <p> |
| A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and |
| provides access to a numbered sequence of elements from that array. |
| A slice type denotes the set of all slices of arrays of its element type. |
| The number of elements is called the length of the slice and is never negative. |
| The value of an uninitialized slice is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| SliceType = "[" "]" ElementType . |
| </pre> |
| |
| <p> |
| The length of a slice <code>s</code> can be discovered by the built-in function |
| <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during |
| execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a> |
| 0 through <code>len(s)-1</code>. The slice index of a |
| given element may be less than the index of the same element in the |
| underlying array. |
| </p> |
| <p> |
| A slice, once initialized, is always associated with an underlying |
| array that holds its elements. A slice therefore shares storage |
| with its array and with other slices of the same array; by contrast, |
| distinct arrays always represent distinct storage. |
| </p> |
| <p> |
| The array underlying a slice may extend past the end of the slice. |
| The <i>capacity</i> is a measure of that extent: it is the sum of |
| the length of the slice and the length of the array beyond the slice; |
| a slice of length up to that capacity can be created by |
| <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice. |
| The capacity of a slice <code>a</code> can be discovered using the |
| built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>. |
| </p> |
| |
| <p> |
| A new, initialized slice value for a given element type <code>T</code> may be |
| made using the built-in function |
| <a href="#Making_slices_maps_and_channels"><code>make</code></a>, |
| which takes a slice type |
| and parameters specifying the length and optionally the capacity. |
| A slice created with <code>make</code> always allocates a new, hidden array |
| to which the returned slice value refers. That is, executing |
| </p> |
| |
| <pre> |
| make([]T, length, capacity) |
| </pre> |
| |
| <p> |
| produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a> |
| it, so these two expressions are equivalent: |
| </p> |
| |
| <pre> |
| make([]int, 50, 100) |
| new([100]int)[0:50] |
| </pre> |
| |
| <p> |
| Like arrays, slices are always one-dimensional but may be composed to construct |
| higher-dimensional objects. |
| With arrays of arrays, the inner arrays are, by construction, always the same length; |
| however with slices of slices (or arrays of slices), the inner lengths may vary dynamically. |
| Moreover, the inner slices must be initialized individually. |
| </p> |
| |
| <h3 id="Struct_types">Struct types</h3> |
| |
| <p> |
| A struct is a sequence of named elements, called fields, each of which has a |
| name and a type. Field names may be specified explicitly (IdentifierList) or |
| implicitly (EmbeddedField). |
| Within a struct, non-<a href="#Blank_identifier">blank</a> field names must |
| be <a href="#Uniqueness_of_identifiers">unique</a>. |
| </p> |
| |
| <pre class="ebnf"> |
| StructType = "struct" "{" { FieldDecl ";" } "}" . |
| FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] . |
| EmbeddedField = [ "*" ] TypeName . |
| Tag = string_lit . |
| </pre> |
| |
| <pre> |
| // An empty struct. |
| struct {} |
| |
| // A struct with 6 fields. |
| struct { |
| x, y int |
| u float32 |
| _ float32 // padding |
| A *[]int |
| F func() |
| } |
| </pre> |
| |
| <p> |
| A field declared with a type but no explicit field name is called an <i>embedded field</i>. |
| An embedded field must be specified as |
| a type name <code>T</code> or as a pointer to a non-interface type name <code>*T</code>, |
| and <code>T</code> itself may not be |
| a pointer type. The unqualified type name acts as the field name. |
| </p> |
| |
| <pre> |
| // A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4 |
| struct { |
| T1 // field name is T1 |
| *T2 // field name is T2 |
| P.T3 // field name is T3 |
| *P.T4 // field name is T4 |
| x, y int // field names are x and y |
| } |
| </pre> |
| |
| <p> |
| The following declaration is illegal because field names must be unique |
| in a struct type: |
| </p> |
| |
| <pre> |
| struct { |
| T // conflicts with embedded field *T and *P.T |
| *T // conflicts with embedded field T and *P.T |
| *P.T // conflicts with embedded field T and *T |
| } |
| </pre> |
| |
| <p> |
| A field or <a href="#Method_declarations">method</a> <code>f</code> of an |
| embedded field in a struct <code>x</code> is called <i>promoted</i> if |
| <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes |
| that field or method <code>f</code>. |
| </p> |
| |
| <p> |
| Promoted fields act like ordinary fields |
| of a struct except that they cannot be used as field names in |
| <a href="#Composite_literals">composite literals</a> of the struct. |
| </p> |
| |
| <p> |
| Given a struct type <code>S</code> and a <a href="#Types">named type</a> |
| <code>T</code>, promoted methods are included in the method set of the struct as follows: |
| </p> |
| <ul> |
| <li> |
| If <code>S</code> contains an embedded field <code>T</code>, |
| the <a href="#Method_sets">method sets</a> of <code>S</code> |
| and <code>*S</code> both include promoted methods with receiver |
| <code>T</code>. The method set of <code>*S</code> also |
| includes promoted methods with receiver <code>*T</code>. |
| </li> |
| |
| <li> |
| If <code>S</code> contains an embedded field <code>*T</code>, |
| the method sets of <code>S</code> and <code>*S</code> both |
| include promoted methods with receiver <code>T</code> or |
| <code>*T</code>. |
| </li> |
| </ul> |
| |
| <p> |
| A field declaration may be followed by an optional string literal <i>tag</i>, |
| which becomes an attribute for all the fields in the corresponding |
| field declaration. An empty tag string is equivalent to an absent tag. |
| The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a> |
| and take part in <a href="#Type_identity">type identity</a> for structs |
| but are otherwise ignored. |
| </p> |
| |
| <pre> |
| struct { |
| x, y float64 "" // an empty tag string is like an absent tag |
| name string "any string is permitted as a tag" |
| _ [4]byte "ceci n'est pas un champ de structure" |
| } |
| |
| // A struct corresponding to a TimeStamp protocol buffer. |
| // The tag strings define the protocol buffer field numbers; |
| // they follow the convention outlined by the reflect package. |
| struct { |
| microsec uint64 `protobuf:"1"` |
| serverIP6 uint64 `protobuf:"2"` |
| } |
| </pre> |
| |
| <h3 id="Pointer_types">Pointer types</h3> |
| |
| <p> |
| A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given |
| type, called the <i>base type</i> of the pointer. |
| The value of an uninitialized pointer is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| PointerType = "*" BaseType . |
| BaseType = Type . |
| </pre> |
| |
| <pre> |
| *Point |
| *[4]int |
| </pre> |
| |
| <h3 id="Function_types">Function types</h3> |
| |
| <p> |
| A function type denotes the set of all functions with the same parameter |
| and result types. The value of an uninitialized variable of function type |
| is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| FunctionType = "func" Signature . |
| Signature = Parameters [ Result ] . |
| Result = Parameters | Type . |
| Parameters = "(" [ ParameterList [ "," ] ] ")" . |
| ParameterList = ParameterDecl { "," ParameterDecl } . |
| ParameterDecl = [ IdentifierList ] [ "..." ] Type . |
| </pre> |
| |
| <p> |
| Within a list of parameters or results, the names (IdentifierList) |
| must either all be present or all be absent. If present, each name |
| stands for one item (parameter or result) of the specified type and |
| all non-<a href="#Blank_identifier">blank</a> names in the signature |
| must be <a href="#Uniqueness_of_identifiers">unique</a>. |
| If absent, each type stands for one item of that type. |
| Parameter and result |
| lists are always parenthesized except that if there is exactly |
| one unnamed result it may be written as an unparenthesized type. |
| </p> |
| |
| <p> |
| The final incoming parameter in a function signature may have |
| a type prefixed with <code>...</code>. |
| A function with such a parameter is called <i>variadic</i> and |
| may be invoked with zero or more arguments for that parameter. |
| </p> |
| |
| <pre> |
| func() |
| func(x int) int |
| func(a, _ int, z float32) bool |
| func(a, b int, z float32) (bool) |
| func(prefix string, values ...int) |
| func(a, b int, z float64, opt ...interface{}) (success bool) |
| func(int, int, float64) (float64, *[]int) |
| func(n int) func(p *T) |
| </pre> |
| |
| <h3 id="Interface_types">Interface types</h3> |
| |
| <p> |
| An interface type defines a <i>type set</i>. |
| A variable of interface type can store a value of any type that is in the type |
| set of the interface. Such a type is said to |
| <a href="#Implementing_an_interface">implement the interface</a>. |
| The value of an uninitialized variable of interface type is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| InterfaceType = "interface" "{" { InterfaceElem ";" } "}" . |
| InterfaceElem = MethodElem | TypeElem . |
| MethodElem = MethodName Signature . |
| MethodName = identifier . |
| TypeElem = TypeTerm { "|" TypeTerm } . |
| TypeTerm = Type | UnderlyingType . |
| UnderlyingType = "~" Type . |
| </pre> |
| |
| <p> |
| An interface type is specified by a list of <i>interface elements</i>. |
| An interface element is either a <i>method</i> or a <i>type element</i>, |
| where a type element is a union of one or more <i>type terms</i>. |
| A type term is either a single type or a single underlying type. |
| </p> |
| |
| <h4 id="Basic_interfaces">Basic interfaces</h4> |
| |
| <p> |
| In its most basic form an interface specifies a (possibly empty) list of methods. |
| The type set defined by such an interface is the set of types which implement all of |
| those methods, and the corresponding <a href="#Method_sets">method set</a> consists |
| exactly of the methods specified by the interface. |
| Interfaces whose type sets can be defined entirely by a list of methods are called |
| <i>basic interfaces.</i> |
| </p> |
| |
| <pre> |
| // A simple File interface. |
| interface { |
| Read([]byte) (int, error) |
| Write([]byte) (int, error) |
| Close() error |
| } |
| </pre> |
| |
| <p> |
| The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a> |
| and not <a href="#Blank_identifier">blank</a>. |
| </p> |
| |
| <pre> |
| interface { |
| String() string |
| String() string // illegal: String not unique |
| _(x int) // illegal: method must have non-blank name |
| } |
| </pre> |
| |
| <p> |
| More than one type may implement an interface. |
| For instance, if two types <code>S1</code> and <code>S2</code> |
| have the method set |
| </p> |
| |
| <pre> |
| func (p T) Read(p []byte) (n int, err error) |
| func (p T) Write(p []byte) (n int, err error) |
| func (p T) Close() error |
| </pre> |
| |
| <p> |
| (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>) |
| then the <code>File</code> interface is implemented by both <code>S1</code> and |
| <code>S2</code>, regardless of what other methods |
| <code>S1</code> and <code>S2</code> may have or share. |
| </p> |
| |
| <p> |
| Every type that is a member of the type set of an interface implements that interface. |
| Any given type may implement several distinct interfaces. |
| For instance, all types implement the <i>empty interface</i> which stands for the set |
| of all (non-interface) types: |
| </p> |
| |
| <pre> |
| interface{} |
| </pre> |
| |
| <p> |
| For convenience, the predeclared type <code>any</code> is an alias for the empty interface. |
| </p> |
| |
| <p> |
| Similarly, consider this interface specification, |
| which appears within a <a href="#Type_declarations">type declaration</a> |
| to define an interface called <code>Locker</code>: |
| </p> |
| |
| <pre> |
| type Locker interface { |
| Lock() |
| Unlock() |
| } |
| </pre> |
| |
| <p> |
| If <code>S1</code> and <code>S2</code> also implement |
| </p> |
| |
| <pre> |
| func (p T) Lock() { … } |
| func (p T) Unlock() { … } |
| </pre> |
| |
| <p> |
| they implement the <code>Locker</code> interface as well |
| as the <code>File</code> interface. |
| </p> |
| |
| <h4 id="Embedded_interfaces">Embedded interfaces</h4> |
| |
| <p> |
| In a slightly more general form |
| an interface <code>T</code> may use a (possibly qualified) interface type |
| name <code>E</code> as an interface element. This is called |
| <i>embedding</i> interface <code>E</code> in <code>T</code>. |
| The type set of <code>T</code> is the <i>intersection</i> of the type sets |
| defined by <code>T</code>'s explicitly declared methods and the type sets |
| of <code>T</code>’s embedded interfaces. |
| In other words, the type set of <code>T</code> is the set of all types that implement all the |
| explicitly declared methods of <code>T</code> and also all the methods of |
| <code>E</code>. |
| </p> |
| |
| <pre> |
| type Reader interface { |
| Read(p []byte) (n int, err error) |
| Close() error |
| } |
| |
| type Writer interface { |
| Write(p []byte) (n int, err error) |
| Close() error |
| } |
| |
| // ReadWriter's methods are Read, Write, and Close. |
| type ReadWriter interface { |
| Reader // includes methods of Reader in ReadWriter's method set |
| Writer // includes methods of Writer in ReadWriter's method set |
| } |
| </pre> |
| |
| <p> |
| When embedding interfaces, methods with the |
| <a href="#Uniqueness_of_identifiers">same</a> names must |
| have <a href="#Type_identity">identical</a> signatures. |
| </p> |
| |
| <pre> |
| type ReadCloser interface { |
| Reader // includes methods of Reader in ReadCloser's method set |
| Close() // illegal: signatures of Reader.Close and Close are different |
| } |
| </pre> |
| |
| <h4 id="General_interfaces">General interfaces</h4> |
| |
| <p> |
| In their most general form, an interface element may also be an arbitrary type term |
| <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>, |
| or a union of terms <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code>. |
| Together with method specifications, these elements enable the precise |
| definition of an interface's type set as follows: |
| </p> |
| |
| <ul> |
| <li>The type set of the empty interface is the set of all non-interface types. |
| </li> |
| |
| <li>The type set of a non-empty interface is the intersection of the type sets |
| of its interface elements. |
| </li> |
| |
| <li>The type set of a method specification is the set of all non-interface types |
| whose method sets include that method. |
| </li> |
| |
| <li>The type set of a non-interface type term is the set consisting |
| of just that type. |
| </li> |
| |
| <li>The type set of a term of the form <code>~T</code> |
| is the set of all types whose underlying type is <code>T</code>. |
| </li> |
| |
| <li>The type set of a <i>union</i> of terms |
| <code>t<sub>1</sub>|t<sub>2</sub>|…|t<sub>n</sub></code> |
| is the union of the type sets of the terms. |
| </li> |
| </ul> |
| |
| <p> |
| The quantification "the set of all non-interface types" refers not just to all (non-interface) |
| types declared in the program at hand, but all possible types in all possible programs, and |
| hence is infinite. |
| Similarly, given the set of all non-interface types that implement a particular method, the |
| intersection of the method sets of those types will contain exactly that method, even if all |
| types in the program at hand always pair that method with another method. |
| </p> |
| |
| <p> |
| By construction, an interface's type set never contains an interface type. |
| </p> |
| |
| <pre> |
| // An interface representing only the type int. |
| interface { |
| int |
| } |
| |
| // An interface representing all types with underlying type int. |
| interface { |
| ~int |
| } |
| |
| // An interface representing all types with underlying type int that implement the String method. |
| interface { |
| ~int |
| String() string |
| } |
| |
| // An interface representing an empty type set: there is no type that is both an int and a string. |
| interface { |
| int |
| string |
| } |
| </pre> |
| |
| <p> |
| In a term of the form <code>~T</code>, the underlying type of <code>T</code> |
| must be itself, and <code>T</code> cannot be an interface. |
| </p> |
| |
| <pre> |
| type MyInt int |
| |
| interface { |
| ~[]byte // the underlying type of []byte is itself |
| ~MyInt // illegal: the underlying type of MyInt is not MyInt |
| ~error // illegal: error is an interface |
| } |
| </pre> |
| |
| <p> |
| Union elements denote unions of type sets: |
| </p> |
| |
| <pre> |
| // The Float interface represents all floating-point types |
| // (including any named types whose underlying types are |
| // either float32 or float64). |
| type Float interface { |
| ~float32 | ~float64 |
| } |
| </pre> |
| |
| <p> |
| The type <code>T</code> in a term of the form <code>T</code> or <code>~T</code> cannot |
| be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all |
| non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty). |
| Given a type parameter <code>P</code>: |
| </p> |
| |
| <pre> |
| interface { |
| P // illegal: P is a type parameter |
| int | ~P // illegal: P is a type parameter |
| ~int | MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt) |
| float32 | Float // overlapping type sets but Float is an interface |
| } |
| </pre> |
| |
| <p> |
| Implementation restriction: |
| A union (with more than one term) cannot contain the |
| <a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code> |
| or interfaces that specify methods, or embed <code>comparable</code> or interfaces |
| that specify methods. |
| </p> |
| |
| <p> |
| Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type |
| constraints, or as elements of other interfaces used as constraints. |
| They cannot be the types of values or variables, or components of other, |
| non-interface types. |
| </p> |
| |
| <pre> |
| var x Float // illegal: Float is not a basic interface |
| |
| var x interface{} = Float(nil) // illegal |
| |
| type Floatish struct { |
| f Float // illegal |
| } |
| </pre> |
| |
| <p> |
| An interface type <code>T</code> may not embed any type element |
| that is, contains, or embeds <code>T</code>, recursively. |
| </p> |
| |
| <pre> |
| // illegal: Bad cannot embed itself |
| type Bad interface { |
| Bad |
| } |
| |
| // illegal: Bad1 cannot embed itself using Bad2 |
| type Bad1 interface { |
| Bad2 |
| } |
| type Bad2 interface { |
| Bad1 |
| } |
| |
| // illegal: Bad3 cannot embed a union containing Bad3 |
| type Bad3 interface { |
| ~int | ~string | Bad3 |
| } |
| </pre> |
| |
| <h4 id="Implementing_an_interface">Implementing an interface</h4> |
| |
| <p> |
| A type <code>T</code> implements an interface <code>I</code> if |
| </p> |
| |
| <ul> |
| <li> |
| <code>T</code> is not an interface and is an element of the type set of <code>I</code>; or |
| </li> |
| <li> |
| <code>T</code> is an interface and the type set of <code>T</code> is a subset of the |
| type set of <code>I</code>. |
| </li> |
| </ul> |
| |
| <p> |
| A value of type <code>T</code> implements an interface if <code>T</code> |
| implements the interface. |
| </p> |
| |
| <h3 id="Map_types">Map types</h3> |
| |
| <p> |
| A map is an unordered group of elements of one type, called the |
| element type, indexed by a set of unique <i>keys</i> of another type, |
| called the key type. |
| The value of an uninitialized map is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| MapType = "map" "[" KeyType "]" ElementType . |
| KeyType = Type . |
| </pre> |
| |
| <p> |
| The <a href="#Comparison_operators">comparison operators</a> |
| <code>==</code> and <code>!=</code> must be fully defined |
| for operands of the key type; thus the key type must not be a function, map, or |
| slice. |
| If the key type is an interface type, these |
| comparison operators must be defined for the dynamic key values; |
| failure will cause a <a href="#Run_time_panics">run-time panic</a>. |
| </p> |
| |
| <pre> |
| map[string]int |
| map[*T]struct{ x, y float64 } |
| map[string]interface{} |
| </pre> |
| |
| <p> |
| The number of map elements is called its length. |
| For a map <code>m</code>, it can be discovered using the |
| built-in function <a href="#Length_and_capacity"><code>len</code></a> |
| and may change during execution. Elements may be added during execution |
| using <a href="#Assignments">assignments</a> and retrieved with |
| <a href="#Index_expressions">index expressions</a>; they may be removed with the |
| <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function. |
| </p> |
| <p> |
| A new, empty map value is made using the built-in |
| function <a href="#Making_slices_maps_and_channels"><code>make</code></a>, |
| which takes the map type and an optional capacity hint as arguments: |
| </p> |
| |
| <pre> |
| make(map[string]int) |
| make(map[string]int, 100) |
| </pre> |
| |
| <p> |
| The initial capacity does not bound its size: |
| maps grow to accommodate the number of items |
| stored in them, with the exception of <code>nil</code> maps. |
| A <code>nil</code> map is equivalent to an empty map except that no elements |
| may be added. |
| |
| <h3 id="Channel_types">Channel types</h3> |
| |
| <p> |
| A channel provides a mechanism for |
| <a href="#Go_statements">concurrently executing functions</a> |
| to communicate by |
| <a href="#Send_statements">sending</a> and |
| <a href="#Receive_operator">receiving</a> |
| values of a specified element type. |
| The value of an uninitialized channel is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType . |
| </pre> |
| |
| <p> |
| The optional <code><-</code> operator specifies the channel <i>direction</i>, |
| <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>, |
| otherwise it is <i>bidirectional</i>. |
| A channel may be constrained only to send or only to receive by |
| <a href="#Assignments">assignment</a> or |
| explicit <a href="#Conversions">conversion</a>. |
| </p> |
| |
| <pre> |
| chan T // can be used to send and receive values of type T |
| chan<- float64 // can only be used to send float64s |
| <-chan int // can only be used to receive ints |
| </pre> |
| |
| <p> |
| The <code><-</code> operator associates with the leftmost <code>chan</code> |
| possible: |
| </p> |
| |
| <pre> |
| chan<- chan int // same as chan<- (chan int) |
| chan<- <-chan int // same as chan<- (<-chan int) |
| <-chan <-chan int // same as <-chan (<-chan int) |
| chan (<-chan int) |
| </pre> |
| |
| <p> |
| A new, initialized channel |
| value can be made using the built-in function |
| <a href="#Making_slices_maps_and_channels"><code>make</code></a>, |
| which takes the channel type and an optional <i>capacity</i> as arguments: |
| </p> |
| |
| <pre> |
| make(chan int, 100) |
| </pre> |
| |
| <p> |
| The capacity, in number of elements, sets the size of the buffer in the channel. |
| If the capacity is zero or absent, the channel is unbuffered and communication |
| succeeds only when both a sender and receiver are ready. Otherwise, the channel |
| is buffered and communication succeeds without blocking if the buffer |
| is not full (sends) or not empty (receives). |
| A <code>nil</code> channel is never ready for communication. |
| </p> |
| |
| <p> |
| A channel may be closed with the built-in function |
| <a href="#Close"><code>close</code></a>. |
| The multi-valued assignment form of the |
| <a href="#Receive_operator">receive operator</a> |
| reports whether a received value was sent before |
| the channel was closed. |
| </p> |
| |
| <p> |
| A single channel may be used in |
| <a href="#Send_statements">send statements</a>, |
| <a href="#Receive_operator">receive operations</a>, |
| and calls to the built-in functions |
| <a href="#Length_and_capacity"><code>cap</code></a> and |
| <a href="#Length_and_capacity"><code>len</code></a> |
| by any number of goroutines without further synchronization. |
| Channels act as first-in-first-out queues. |
| For example, if one goroutine sends values on a channel |
| and a second goroutine receives them, the values are |
| received in the order sent. |
| </p> |
| |
| <h2 id="Properties_of_types_and_values">Properties of types and values</h2> |
| |
| <h3 id="Underlying_types">Underlying types</h3> |
| |
| <p> |
| Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code> |
| is one of the predeclared boolean, numeric, or string types, or a type literal, |
| the corresponding underlying type is <code>T</code> itself. |
| Otherwise, <code>T</code>'s underlying type is the underlying type of the |
| type to which <code>T</code> refers in its declaration. |
| For a type parameter that is the underlying type of its |
| <a href="#Type_constraints">type constraint</a>, which is always an interface. |
| </p> |
| |
| <pre> |
| type ( |
| A1 = string |
| A2 = A1 |
| ) |
| |
| type ( |
| B1 string |
| B2 B1 |
| B3 []B1 |
| B4 B3 |
| ) |
| |
| func f[P any](x P) { … } |
| </pre> |
| |
| <p> |
| The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>, |
| and <code>B2</code> is <code>string</code>. |
| The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>. |
| The underlying type of <code>P</code> is <code>interface{}</code>. |
| </p> |
| |
| <h3 id="Core_types">Core types</h3> |
| |
| <p> |
| Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the |
| <a href="#Underlying_types">underlying type</a> of <code>T</code>. |
| </p> |
| |
| <p> |
| An interface <code>T</code> has a core type if one of the following |
| conditions is satisfied: |
| </p> |
| |
| <ol> |
| <li> |
| There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a> |
| of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or |
| </li> |
| <li> |
| the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a> |
| with identical element type <code>E</code>, and all directional channels have the same |
| direction. |
| </li> |
| </ol> |
| |
| <p> |
| No other interfaces have a core type. |
| </p> |
| |
| <p> |
| The core type of an interface is, depending on the condition that is satisfied, either: |
| </p> |
| |
| <ol> |
| <li> |
| the type <code>U</code>; or |
| </li> |
| <li> |
| the type <code>chan E</code> if <code>T</code> contains only bidirectional |
| channels, or the type <code>chan<- E</code> or <code><-chan E</code> |
| depending on the direction of the directional channels present. |
| </li> |
| </ol> |
| |
| <p> |
| By definition, a core type is never a <a href="#Type_definitions">defined type</a>, |
| <a href="#Type_parameter_declarations">type parameter</a>, or |
| <a href="#Interface_types">interface type</a>. |
| </p> |
| |
| <p> |
| Examples of interfaces with core types: |
| </p> |
| |
| <pre> |
| type Celsius float32 |
| type Kelvin float32 |
| |
| interface{ int } // int |
| interface{ Celsius|Kelvin } // float32 |
| interface{ ~chan int } // chan int |
| interface{ ~chan int|~chan<- int } // chan<- int |
| interface{ ~[]*data; String() string } // []*data |
| </pre> |
| |
| <p> |
| Examples of interfaces without core types: |
| </p> |
| |
| <pre> |
| interface{} // no single underlying type |
| interface{ Celsius|float64 } // no single underlying type |
| interface{ chan int | chan<- string } // channels have different element types |
| interface{ <-chan int | chan<- int } // directional channels have different directions |
| </pre> |
| |
| <h3 id="Type_identity">Type identity</h3> |
| |
| <p> |
| Two types are either <i>identical</i> or <i>different</i>. |
| </p> |
| |
| <p> |
| A <a href="#Types">named type</a> is always different from any other type. |
| Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are |
| structurally equivalent; that is, they have the same literal structure and corresponding |
| components have identical types. In detail: |
| </p> |
| |
| <ul> |
| <li>Two array types are identical if they have identical element types and |
| the same array length.</li> |
| |
| <li>Two slice types are identical if they have identical element types.</li> |
| |
| <li>Two struct types are identical if they have the same sequence of fields, |
| and if corresponding fields have the same names, and identical types, |
| and identical tags. |
| <a href="#Exported_identifiers">Non-exported</a> field names from different |
| packages are always different.</li> |
| |
| <li>Two pointer types are identical if they have identical base types.</li> |
| |
| <li>Two function types are identical if they have the same number of parameters |
| and result values, corresponding parameter and result types are |
| identical, and either both functions are variadic or neither is. |
| Parameter and result names are not required to match.</li> |
| |
| <li>Two interface types are identical if they define the same type set. |
| </li> |
| |
| <li>Two map types are identical if they have identical key and element types.</li> |
| |
| <li>Two channel types are identical if they have identical element types and |
| the same direction.</li> |
| |
| <li>Two <a href="#Instantiations">instantiated</a> types are identical if |
| their defined types and all type arguments are identical. |
| </li> |
| </ul> |
| |
| <p> |
| Given the declarations |
| </p> |
| |
| <pre> |
| type ( |
| A0 = []string |
| A1 = A0 |
| A2 = struct{ a, b int } |
| A3 = int |
| A4 = func(A3, float64) *A0 |
| A5 = func(x int, _ float64) *[]string |
| |
| B0 A0 |
| B1 []string |
| B2 struct{ a, b int } |
| B3 struct{ a, c int } |
| B4 func(int, float64) *B0 |
| B5 func(x int, y float64) *A1 |
| |
| C0 = B0 |
| D0[P1, P2 any] struct{ x P1; y P2 } |
| E0 = D0[int, string] |
| ) |
| </pre> |
| |
| <p> |
| these types are identical: |
| </p> |
| |
| <pre> |
| A0, A1, and []string |
| A2 and struct{ a, b int } |
| A3 and int |
| A4, func(int, float64) *[]string, and A5 |
| |
| B0 and C0 |
| D0[int, string] and E0 |
| []int and []int |
| struct{ a, b *B5 } and struct{ a, b *B5 } |
| func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5 |
| </pre> |
| |
| <p> |
| <code>B0</code> and <code>B1</code> are different because they are new types |
| created by distinct <a href="#Type_definitions">type definitions</a>; |
| <code>func(int, float64) *B0</code> and <code>func(x int, y float64) *[]string</code> |
| are different because <code>B0</code> is different from <code>[]string</code>; |
| and <code>P1</code> and <code>P2</code> are different because they are different |
| type parameters. |
| <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are |
| different because the former is an <a href="#Instantiations">instantiated</a> |
| defined type while the latter is a type literal |
| (but they are still <a href="#Assignability">assignable</a>). |
| </p> |
| |
| <h3 id="Assignability">Assignability</h3> |
| |
| <p> |
| A value <code>x</code> of type <code>V</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code> |
| ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies: |
| </p> |
| |
| <ul> |
| <li> |
| <code>V</code> and <code>T</code> are identical. |
| </li> |
| <li> |
| <code>V</code> and <code>T</code> have identical |
| <a href="#Underlying_types">underlying types</a> |
| but are not type parameters and at least one of <code>V</code> |
| or <code>T</code> is not a <a href="#Types">named type</a>. |
| </li> |
| <li> |
| <code>V</code> and <code>T</code> are channel types with |
| identical element types, <code>V</code> is a bidirectional channel, |
| and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>. |
| </li> |
| <li> |
| <code>T</code> is an interface type, but not a type parameter, and |
| <code>x</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>. |
| </li> |
| <li> |
| <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code> |
| is a pointer, function, slice, map, channel, or interface type, |
| but not a type parameter. |
| </li> |
| <li> |
| <code>x</code> is an untyped <a href="#Constants">constant</a> |
| <a href="#Representability">representable</a> |
| by a value of type <code>T</code>. |
| </li> |
| </ul> |
| |
| <p> |
| Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code> |
| is assignable to a variable of type <code>T</code> if one of the following conditions applies: |
| </p> |
| |
| <ul> |
| <li> |
| <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is |
| a type parameter, and <code>x</code> is assignable to each type in |
| <code>T</code>'s type set. |
| </li> |
| <li> |
| <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is |
| a type parameter, and <code>x</code> is assignable to each type in |
| <code>T</code>'s type set. |
| </li> |
| <li> |
| <code>V</code> is a type parameter and <code>T</code> is not a named type, |
| and values of each type in <code>V</code>'s type set are assignable |
| to <code>T</code>. |
| </li> |
| </ul> |
| |
| <h3 id="Representability">Representability</h3> |
| |
| <p> |
| A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i> |
| by a value of type <code>T</code>, |
| where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>, |
| if one of the following conditions applies: |
| </p> |
| |
| <ul> |
| <li> |
| <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>. |
| </li> |
| |
| <li> |
| <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 |
| precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE |
| negative zero further simplified to an unsigned zero. Note that constant values never result |
| in an IEEE negative zero, NaN, or infinity. |
| </li> |
| |
| <li> |
| <code>T</code> is a complex type, and <code>x</code>'s |
| <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code> |
| are representable by values of <code>T</code>'s component type (<code>float32</code> or |
| <code>float64</code>). |
| </li> |
| </ul> |
| |
| <p> |
| If <code>T</code> is a type parameter, |
| <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable |
| by a value of each type in <code>T</code>'s type set. |
| </p> |
| |
| <pre> |
| x T x is representable by a value of T because |
| |
| 'a' byte 97 is in the set of byte values |
| 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers |
| "foo" string "foo" is in the set of string values |
| 1024 int16 1024 is in the set of 16-bit integers |
| 42.0 byte 42 is in the set of unsigned 8-bit integers |
| 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers |
| 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values |
| -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0 |
| 0i int 0 is an integer value |
| (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values |
| </pre> |
| |
| <pre> |
| x T x is not representable by a value of T because |
| |
| 0 bool 0 is not in the set of boolean values |
| 'a' string 'a' is a rune, it is not in the set of string values |
| 1024 byte 1024 is not in the set of unsigned 8-bit integers |
| -1 uint16 -1 is not in the set of unsigned 16-bit integers |
| 1.1 int 1.1 is not an integer value |
| 42i float32 (0 + 42i) is not in the set of float32 values |
| 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding |
| </pre> |
| |
| <h3 id="Method_sets">Method sets</h3> |
| |
| <p> |
| The <i>method set</i> of a type determines the methods that can be |
| <a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type. |
| Every type has a (possibly empty) method set associated with it: |
| </p> |
| |
| <ul> |
| <li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all |
| <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>. |
| </li> |
| |
| <li> |
| The method set of a pointer to a defined type <code>T</code> |
| (where <code>T</code> is neither a pointer nor an interface) |
| is the set of all methods declared with receiver <code>*T</code> or <code>T</code>. |
| </li> |
| |
| <li>The method set of an <a href="#Interface_types">interface type</a> is the intersection |
| of the method sets of each type in the interface's <a href="#Interface_types">type set</a> |
| (the resulting method set is usually just the set of declared methods in the interface). |
| </li> |
| </ul> |
| |
| <p> |
| Further rules apply to structs (and pointer to structs) containing embedded fields, |
| as described in the section on <a href="#Struct_types">struct types</a>. |
| Any other type has an empty method set. |
| </p> |
| |
| <p> |
| In a method set, each method must have a |
| <a href="#Uniqueness_of_identifiers">unique</a> |
| non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>. |
| </p> |
| |
| <h2 id="Blocks">Blocks</h2> |
| |
| <p> |
| A <i>block</i> is a possibly empty sequence of declarations and statements |
| within matching brace brackets. |
| </p> |
| |
| <pre class="ebnf"> |
| Block = "{" StatementList "}" . |
| StatementList = { Statement ";" } . |
| </pre> |
| |
| <p> |
| In addition to explicit blocks in the source code, there are implicit blocks: |
| </p> |
| |
| <ol> |
| <li>The <i>universe block</i> encompasses all Go source text.</li> |
| |
| <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all |
| Go source text for that package.</li> |
| |
| <li>Each file has a <i>file block</i> containing all Go source text |
| in that file.</li> |
| |
| <li>Each <a href="#If_statements">"if"</a>, |
| <a href="#For_statements">"for"</a>, and |
| <a href="#Switch_statements">"switch"</a> |
| statement is considered to be in its own implicit block.</li> |
| |
| <li>Each clause in a <a href="#Switch_statements">"switch"</a> |
| or <a href="#Select_statements">"select"</a> statement |
| acts as an implicit block.</li> |
| </ol> |
| |
| <p> |
| Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>. |
| </p> |
| |
| |
| <h2 id="Declarations_and_scope">Declarations and scope</h2> |
| |
| <p> |
| A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a |
| <a href="#Constant_declarations">constant</a>, |
| <a href="#Type_declarations">type</a>, |
| <a href="#Type_parameter_declarations">type parameter</a>, |
| <a href="#Variable_declarations">variable</a>, |
| <a href="#Function_declarations">function</a>, |
| <a href="#Labeled_statements">label</a>, or |
| <a href="#Import_declarations">package</a>. |
| Every identifier in a program must be declared. |
| No identifier may be declared twice in the same block, and |
| no identifier may be declared in both the file and package block. |
| </p> |
| |
| <p> |
| The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier |
| in a declaration, but it does not introduce a binding and thus is not declared. |
| In the package block, the identifier <code>init</code> may only be used for |
| <a href="#Package_initialization"><code>init</code> function</a> declarations, |
| and like the blank identifier it does not introduce a new binding. |
| </p> |
| |
| <pre class="ebnf"> |
| Declaration = ConstDecl | TypeDecl | VarDecl . |
| TopLevelDecl = Declaration | FunctionDecl | MethodDecl . |
| </pre> |
| |
| <p> |
| The <i>scope</i> of a declared identifier is the extent of source text in which |
| the identifier denotes the specified constant, type, variable, function, label, or package. |
| </p> |
| |
| <p> |
| Go is lexically scoped using <a href="#Blocks">blocks</a>: |
| </p> |
| |
| <ol> |
| <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li> |
| |
| <li>The scope of an identifier denoting a constant, type, variable, |
| or function (but not method) declared at top level (outside any |
| function) is the package block.</li> |
| |
| <li>The scope of the package name of an imported package is the file block |
| of the file containing the import declaration.</li> |
| |
| <li>The scope of an identifier denoting a method receiver, function parameter, |
| or result variable is the function body.</li> |
| |
| <li>The scope of an identifier denoting a type parameter of a function |
| or declared by a method receiver begins after the name of the function |
| and ends at the end of the function body.</li> |
| |
| <li>The scope of an identifier denoting a type parameter of a type |
| begins after the name of the type and ends at the end |
| of the TypeSpec.</li> |
| |
| <li>The scope of a constant or variable identifier declared |
| inside a function begins at the end of the ConstSpec or VarSpec |
| (ShortVarDecl for short variable declarations) |
| and ends at the end of the innermost containing block.</li> |
| |
| <li>The scope of a type identifier declared inside a function |
| begins at the identifier in the TypeSpec |
| and ends at the end of the innermost containing block.</li> |
| </ol> |
| |
| <p> |
| An identifier declared in a block may be redeclared in an inner block. |
| While the identifier of the inner declaration is in scope, it denotes |
| the entity declared by the inner declaration. |
| </p> |
| |
| <p> |
| The <a href="#Package_clause">package clause</a> is not a declaration; the package name |
| does not appear in any scope. Its purpose is to identify the files belonging |
| to the same <a href="#Packages">package</a> and to specify the default package name for import |
| declarations. |
| </p> |
| |
| |
| <h3 id="Label_scopes">Label scopes</h3> |
| |
| <p> |
| Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are |
| used in the <a href="#Break_statements">"break"</a>, |
| <a href="#Continue_statements">"continue"</a>, and |
| <a href="#Goto_statements">"goto"</a> statements. |
| It is illegal to define a label that is never used. |
| In contrast to other identifiers, labels are not block scoped and do |
| not conflict with identifiers that are not labels. The scope of a label |
| is the body of the function in which it is declared and excludes |
| the body of any nested function. |
| </p> |
| |
| |
| <h3 id="Blank_identifier">Blank identifier</h3> |
| |
| <p> |
| The <i>blank identifier</i> is represented by the underscore character <code>_</code>. |
| It serves as an anonymous placeholder instead of a regular (non-blank) |
| identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>, |
| as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>. |
| </p> |
| |
| |
| <h3 id="Predeclared_identifiers">Predeclared identifiers</h3> |
| |
| <p> |
| The following identifiers are implicitly declared in the |
| <a href="#Blocks">universe block</a>: |
| </p> |
| <pre class="grammar"> |
| Types: |
| any bool byte comparable |
| complex64 complex128 error float32 float64 |
| int int8 int16 int32 int64 rune string |
| uint uint8 uint16 uint32 uint64 uintptr |
| |
| Constants: |
| true false iota |
| |
| Zero value: |
| nil |
| |
| Functions: |
| append cap close complex copy delete imag len |
| make new panic print println real recover |
| </pre> |
| |
| <h3 id="Exported_identifiers">Exported identifiers</h3> |
| |
| <p> |
| An identifier may be <i>exported</i> to permit access to it from another package. |
| An identifier is exported if both: |
| </p> |
| <ol> |
| <li>the first character of the identifier's name is a Unicode uppercase |
| letter (Unicode character category Lu); and</li> |
| <li>the identifier is declared in the <a href="#Blocks">package block</a> |
| or it is a <a href="#Struct_types">field name</a> or |
| <a href="#MethodName">method name</a>.</li> |
| </ol> |
| <p> |
| All other identifiers are not exported. |
| </p> |
| |
| <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3> |
| |
| <p> |
| Given a set of identifiers, an identifier is called <i>unique</i> if it is |
| <i>different</i> from every other in the set. |
| Two identifiers are different if they are spelled differently, or if they |
| appear in different <a href="#Packages">packages</a> and are not |
| <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same. |
| </p> |
| |
| <h3 id="Constant_declarations">Constant declarations</h3> |
| |
| <p> |
| A constant declaration binds a list of identifiers (the names of |
| the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>. |
| The number of identifiers must be equal |
| to the number of expressions, and the <i>n</i>th identifier on |
| the left is bound to the value of the <i>n</i>th expression on the |
| right. |
| </p> |
| |
| <pre class="ebnf"> |
| ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) . |
| ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] . |
| |
| IdentifierList = identifier { "," identifier } . |
| ExpressionList = Expression { "," Expression } . |
| </pre> |
| |
| <p> |
| If the type is present, all constants take the type specified, and |
| the expressions must be <a href="#Assignability">assignable</a> to that type, |
| which must not be a type parameter. |
| If the type is omitted, the constants take the |
| individual types of the corresponding expressions. |
| If the expression values are untyped <a href="#Constants">constants</a>, |
| the declared constants remain untyped and the constant identifiers |
| denote the constant values. For instance, if the expression is a |
| floating-point literal, the constant identifier denotes a floating-point |
| constant, even if the literal's fractional part is zero. |
| </p> |
| |
| <pre> |
| const Pi float64 = 3.14159265358979323846 |
| const zero = 0.0 // untyped floating-point constant |
| const ( |
| size int64 = 1024 |
| eof = -1 // untyped integer constant |
| ) |
| const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants |
| const u, v float32 = 0, 3 // u = 0.0, v = 3.0 |
| </pre> |
| |
| <p> |
| Within a parenthesized <code>const</code> declaration list the |
| expression list may be omitted from any but the first ConstSpec. |
| Such an empty list is equivalent to the textual substitution of the |
| first preceding non-empty expression list and its type if any. |
| Omitting the list of expressions is therefore equivalent to |
| repeating the previous list. The number of identifiers must be equal |
| to the number of expressions in the previous list. |
| Together with the <a href="#Iota"><code>iota</code> constant generator</a> |
| this mechanism permits light-weight declaration of sequential values: |
| </p> |
| |
| <pre> |
| const ( |
| Sunday = iota |
| Monday |
| Tuesday |
| Wednesday |
| Thursday |
| Friday |
| Partyday |
| numberOfDays // this constant is not exported |
| ) |
| </pre> |
| |
| |
| <h3 id="Iota">Iota</h3> |
| |
| <p> |
| Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier |
| <code>iota</code> represents successive untyped integer <a href="#Constants"> |
| constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a> |
| in that constant declaration, starting at zero. |
| It can be used to construct a set of related constants: |
| </p> |
| |
| <pre> |
| const ( |
| c0 = iota // c0 == 0 |
| c1 = iota // c1 == 1 |
| c2 = iota // c2 == 2 |
| ) |
| |
| const ( |
| a = 1 << iota // a == 1 (iota == 0) |
| b = 1 << iota // b == 2 (iota == 1) |
| c = 3 // c == 3 (iota == 2, unused) |
| d = 1 << iota // d == 8 (iota == 3) |
| ) |
| |
| const ( |
| u = iota * 42 // u == 0 (untyped integer constant) |
| v float64 = iota * 42 // v == 42.0 (float64 constant) |
| w = iota * 42 // w == 84 (untyped integer constant) |
| ) |
| |
| const x = iota // x == 0 |
| const y = iota // y == 0 |
| </pre> |
| |
| <p> |
| By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value: |
| </p> |
| |
| <pre> |
| const ( |
| bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0) |
| bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1) |
| _, _ // (iota == 2, unused) |
| bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3) |
| ) |
| </pre> |
| |
| <p> |
| This last example exploits the <a href="#Constant_declarations">implicit repetition</a> |
| of the last non-empty expression list. |
| </p> |
| |
| |
| <h3 id="Type_declarations">Type declarations</h3> |
| |
| <p> |
| A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>. |
| Type declarations come in two forms: alias declarations and type definitions. |
| </p> |
| |
| <pre class="ebnf"> |
| TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) . |
| TypeSpec = AliasDecl | TypeDef . |
| </pre> |
| |
| <h4 id="Alias_declarations">Alias declarations</h4> |
| |
| <p> |
| An alias declaration binds an identifier to the given type. |
| </p> |
| |
| <pre class="ebnf"> |
| AliasDecl = identifier "=" Type . |
| </pre> |
| |
| <p> |
| Within the <a href="#Declarations_and_scope">scope</a> of |
| the identifier, it serves as an <i>alias</i> for the type. |
| </p> |
| |
| <pre> |
| type ( |
| nodeList = []*Node // nodeList and []*Node are identical types |
| Polar = polar // Polar and polar denote identical types |
| ) |
| </pre> |
| |
| |
| <h4 id="Type_definitions">Type definitions</h4> |
| |
| <p> |
| A type definition creates a new, distinct type with the same |
| <a href="#Types">underlying type</a> and operations as the given type |
| and binds an identifier, the <i>type name</i>, to it. |
| </p> |
| |
| <pre class="ebnf"> |
| TypeDef = identifier [ TypeParameters ] Type . |
| </pre> |
| |
| <p> |
| The new type is called a <i>defined type</i>. |
| It is <a href="#Type_identity">different</a> from any other type, |
| including the type it is created from. |
| </p> |
| |
| <pre> |
| type ( |
| Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types |
| polar Point // polar and Point denote different types |
| ) |
| |
| type TreeNode struct { |
| left, right *TreeNode |
| value any |
| } |
| |
| type Block interface { |
| BlockSize() int |
| Encrypt(src, dst []byte) |
| Decrypt(src, dst []byte) |
| } |
| </pre> |
| |
| <p> |
| A defined type may have <a href="#Method_declarations">methods</a> associated with it. |
| It does not inherit any methods bound to the given type, |
| but the <a href="#Method_sets">method set</a> |
| of an interface type or of elements of a composite type remains unchanged: |
| </p> |
| |
| <pre> |
| // A Mutex is a data type with two methods, Lock and Unlock. |
| type Mutex struct { /* Mutex fields */ } |
| func (m *Mutex) Lock() { /* Lock implementation */ } |
| func (m *Mutex) Unlock() { /* Unlock implementation */ } |
| |
| // NewMutex has the same composition as Mutex but its method set is empty. |
| type NewMutex Mutex |
| |
| // The method set of PtrMutex's underlying type *Mutex remains unchanged, |
| // but the method set of PtrMutex is empty. |
| type PtrMutex *Mutex |
| |
| // The method set of *PrintableMutex contains the methods |
| // Lock and Unlock bound to its embedded field Mutex. |
| type PrintableMutex struct { |
| Mutex |
| } |
| |
| // MyBlock is an interface type that has the same method set as Block. |
| type MyBlock Block |
| </pre> |
| |
| <p> |
| Type definitions may be used to define different boolean, numeric, |
| or string types and associate methods with them: |
| </p> |
| |
| <pre> |
| type TimeZone int |
| |
| const ( |
| EST TimeZone = -(5 + iota) |
| CST |
| MST |
| PST |
| ) |
| |
| func (tz TimeZone) String() string { |
| return fmt.Sprintf("GMT%+dh", tz) |
| } |
| </pre> |
| |
| <p> |
| If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>, |
| the type name denotes a <i>generic type</i>. |
| Generic types must be <a href="#Instantiations">instantiated</a> when they |
| are used. |
| </p> |
| |
| <pre> |
| type List[T any] struct { |
| next *List[T] |
| value T |
| } |
| </pre> |
| |
| <p> |
| In a type definition the given type cannot be a type parameter. |
| </p> |
| |
| <pre> |
| type T[P any] P // illegal: P is a type parameter |
| |
| func f[T any]() { |
| type L T // illegal: T is a type parameter declared by the enclosing function |
| } |
| </pre> |
| |
| <p> |
| A generic type may also have <a href="#Method_declarations">methods</a> associated with it. |
| In this case, the method receivers must declare the same number of type parameters as |
| present in the generic type definition. |
| </p> |
| |
| <pre> |
| // The method Len returns the number of elements in the linked list l. |
| func (l *List[T]) Len() int { … } |
| </pre> |
| |
| <h3 id="Type_parameter_declarations">Type parameter declarations</h3> |
| |
| <p> |
| A type parameter list declares the <i>type parameters</i> of a generic function or type declaration. |
| The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a> |
| except that the type parameter names must all be present and the list is enclosed |
| in square brackets rather than parentheses. |
| </p> |
| |
| <pre class="ebnf"> |
| TypeParameters = "[" TypeParamList [ "," ] "]" . |
| TypeParamList = TypeParamDecl { "," TypeParamDecl } . |
| TypeParamDecl = IdentifierList TypeConstraint . |
| </pre> |
| |
| <p> |
| All non-blank names in the list must be unique. |
| Each name declares a type parameter, which is a new and different <a href="#Types">named type</a> |
| that acts as a place holder for an (as of yet) unknown type in the declaration. |
| The type parameter is replaced with a <i>type argument</i> upon |
| <a href="#Instantiations">instantiation</a> of the generic function or type. |
| </p> |
| |
| <pre> |
| [P any] |
| [S interface{ ~[]byte|string }] |
| [S ~[]E, E any] |
| [P Constraint[int]] |
| [_ any] |
| </pre> |
| |
| <p> |
| Just as each ordinary function parameter has a parameter type, each type parameter |
| has a corresponding (meta-)type which is called its |
| <a href="#Type_constraints"><i>type constraint</i></a>. |
| </p> |
| |
| <p> |
| A parsing ambiguity arises when the type parameter list for a generic type |
| declares a single type parameter <code>P</code> with a constraint <code>C</code> |
| such that the text <code>P C</code> forms a valid expression: |
| </p> |
| |
| <pre> |
| type T[P *C] … |
| type T[P (C)] … |
| type T[P *C|Q] … |
| … |
| </pre> |
| |
| <p> |
| In these rare cases, the type parameter list is indistinguishable from an |
| expression and the type declaration is parsed as an array type declaration. |
| To resolve the ambiguity, embed the constraint in an |
| <a href="#Interface_types">interface</a> or use a trailing comma: |
| </p> |
| |
| <pre> |
| type T[P interface{*C}] … |
| type T[P *C,] … |
| </pre> |
| |
| <p> |
| Type parameters may also be declared by the receiver specification |
| of a <a href="#Method_declarations">method declaration</a> associated |
| with a generic type. |
| </p> |
| |
| <!-- |
| This section needs to explain if and what kind of cycles are permitted |
| using type parameters in a type parameter list. |
| --> |
| |
| <h4 id="Type_constraints">Type constraints</h4> |
| |
| <p> |
| A type constraint is an <a href="#Interface_types">interface</a> that defines the |
| set of permissible type arguments for the respective type parameter and controls the |
| operations supported by values of that type parameter. |
| </p> |
| |
| <pre class="ebnf"> |
| TypeConstraint = TypeElem . |
| </pre> |
| |
| <p> |
| If the constraint is an interface literal of the form <code>interface{E}</code> where |
| <code>E</code> is an embedded type element (not a method), in a type parameter list |
| the enclosing <code>interface{ … }</code> may be omitted for convenience: |
| </p> |
| |
| <pre> |
| [T []P] // = [T interface{[]P}] |
| [T ~int] // = [T interface{~int}] |
| [T int|string] // = [T interface{int|string}] |
| type Constraint ~int // illegal: ~int is not inside a type parameter list |
| </pre> |
| |
| <!-- |
| We should be able to simplify the rules for comparable or delegate some of them |
| elsewhere since we have a section that clearly defines how interfaces implement |
| other interfaces based on their type sets. But this should get us going for now. |
| --> |
| |
| <p> |
| The <a href="#Predeclared_identifiers">predeclared</a> |
| <a href="#Interface_types">interface type</a> <code>comparable</code> |
| denotes the set of all non-interface types that are |
| <a href="#Comparison_operators">comparable</a>. Specifically, |
| a type <code>T</code> implements <code>comparable</code> if: |
| </p> |
| |
| <ul> |
| <li> |
| <code>T</code> is not an interface type and <code>T</code> supports the operations |
| <code>==</code> and <code>!=</code>; or |
| </li> |
| <li> |
| <code>T</code> is an interface type and each type in <code>T</code>'s |
| <a href="#Interface_types">type set</a> implements <code>comparable</code>. |
| </li> |
| </ul> |
| |
| <p> |
| Even though interfaces that are not type parameters can be |
| <a href="#Comparison_operators">compared</a> |
| (possibly causing a run-time panic) they do not implement |
| <code>comparable</code>. |
| </p> |
| |
| <pre> |
| int // implements comparable |
| []byte // does not implement comparable (slices cannot be compared) |
| interface{} // does not implement comparable (see above) |
| interface{ ~int | ~string } // type parameter only: implements comparable |
| interface{ comparable } // type parameter only: implements comparable |
| interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (not all types in the type set are comparable) |
| </pre> |
| |
| <p> |
| The <code>comparable</code> interface and interfaces that (directly or indirectly) embed |
| <code>comparable</code> may only be used as type constraints. They cannot be the types of |
| values or variables, or components of other, non-interface types. |
| </p> |
| |
| <h3 id="Variable_declarations">Variable declarations</h3> |
| |
| <p> |
| A variable declaration creates one or more <a href="#Variables">variables</a>, |
| binds corresponding identifiers to them, and gives each a type and an initial value. |
| </p> |
| |
| <pre class="ebnf"> |
| VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) . |
| VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) . |
| </pre> |
| |
| <pre> |
| var i int |
| var U, V, W float64 |
| var k = 0 |
| var x, y float32 = -1, -2 |
| var ( |
| i int |
| u, v, s = 2.0, 3.0, "bar" |
| ) |
| var re, im = complexSqrt(-1) |
| var _, found = entries[name] // map lookup; only interested in "found" |
| </pre> |
| |
| <p> |
| If a list of expressions is given, the variables are initialized |
| with the expressions following the rules for <a href="#Assignments">assignments</a>. |
| Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>. |
| </p> |
| |
| <p> |
| If a type is present, each variable is given that type. |
| Otherwise, each variable is given the type of the corresponding |
| initialization value in the assignment. |
| If that value is an untyped constant, it is first implicitly |
| <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>; |
| if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>. |
| The predeclared value <code>nil</code> cannot be used to initialize a variable |
| with no explicit type. |
| </p> |
| |
| <pre> |
| var d = math.Sin(0.5) // d is float64 |
| var i = 42 // i is int |
| var t, ok = x.(T) // t is T, ok is bool |
| var n = nil // illegal |
| </pre> |
| |
| <p> |
| Implementation restriction: A compiler may make it illegal to declare a variable |
| inside a <a href="#Function_declarations">function body</a> if the variable is |
| never used. |
| </p> |
| |
| <h3 id="Short_variable_declarations">Short variable declarations</h3> |
| |
| <p> |
| A <i>short variable declaration</i> uses the syntax: |
| </p> |
| |
| <pre class="ebnf"> |
| ShortVarDecl = IdentifierList ":=" ExpressionList . |
| </pre> |
| |
| <p> |
| It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a> |
| with initializer expressions but no types: |
| </p> |
| |
| <pre class="grammar"> |
| "var" IdentifierList "=" ExpressionList . |
| </pre> |
| |
| <pre> |
| i, j := 0, 10 |
| f := func() int { return 7 } |
| ch := make(chan int) |
| r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any |
| _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate |
| </pre> |
| |
| <p> |
| Unlike regular variable declarations, a short variable declaration may <i>redeclare</i> |
| variables provided they were originally declared earlier in the same block |
| (or the parameter lists if the block is the function body) with the same type, |
| and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new. |
| As a consequence, redeclaration can only appear in a multi-variable short declaration. |
| Redeclaration does not introduce a new variable; it just assigns a new value to the original. |
| The non-blank variable names on the left side of <code>:=</code> |
| must be <a href="#Uniqueness_of_identifiers">unique</a>. |
| </p> |
| |
| <pre> |
| field1, offset := nextField(str, 0) |
| field2, offset := nextField(str, offset) // redeclares offset |
| x, y, x := 1, 2, 3 // illegal: x repeated on left side of := |
| </pre> |
| |
| <p> |
| Short variable declarations may appear only inside functions. |
| In some contexts such as the initializers for |
| <a href="#If_statements">"if"</a>, |
| <a href="#For_statements">"for"</a>, or |
| <a href="#Switch_statements">"switch"</a> statements, |
| they can be used to declare local temporary variables. |
| </p> |
| |
| <h3 id="Function_declarations">Function declarations</h3> |
| |
| <!-- |
| Given the importance of functions, this section has always |
| been woefully underdeveloped. Would be nice to expand this |
| a bit. |
| --> |
| |
| <p> |
| A function declaration binds an identifier, the <i>function name</i>, |
| to a function. |
| </p> |
| |
| <pre class="ebnf"> |
| FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] . |
| FunctionName = identifier . |
| FunctionBody = Block . |
| </pre> |
| |
| <p> |
| If the function's <a href="#Function_types">signature</a> declares |
| result parameters, the function body's statement list must end in |
| a <a href="#Terminating_statements">terminating statement</a>. |
| </p> |
| |
| <pre> |
| func IndexRune(s string, r rune) int { |
| for i, c := range s { |
| if c == r { |
| return i |
| } |
| } |
| // invalid: missing return statement |
| } |
| </pre> |
| |
| <p> |
| If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>, |
| the function name denotes a <i>generic function</i>. |
| A generic function must be <a href="#Instantiations">instantiated</a> before it can be |
| called or used as a value. |
| </p> |
| |
| <pre> |
| func min[T ~int|~float64](x, y T) T { |
| if x < y { |
| return x |
| } |
| return y |
| } |
| </pre> |
| |
| <p> |
| A function declaration without type parameters may omit the body. |
| Such a declaration provides the signature for a function implemented outside Go, |
| such as an assembly routine. |
| </p> |
| |
| <pre> |
| func flushICache(begin, end uintptr) // implemented externally |
| </pre> |
| |
| <h3 id="Method_declarations">Method declarations</h3> |
| |
| <p> |
| A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>. |
| A method declaration binds an identifier, the <i>method name</i>, to a method, |
| and associates the method with the receiver's <i>base type</i>. |
| </p> |
| |
| <pre class="ebnf"> |
| MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] . |
| Receiver = Parameters . |
| </pre> |
| |
| <p> |
| The receiver is specified via an extra parameter section preceding the method |
| name. That parameter section must declare a single non-variadic parameter, the receiver. |
| Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a |
| pointer to a defined type <code>T</code>, possibly followed by a list of type parameter |
| names <code>[P1, P2, …]</code> enclosed in square brackets. |
| <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be |
| a pointer or interface type and it must be defined in the same package as the method. |
| The method is said to be <i>bound</i> to its receiver base type and the method name |
| is visible only within <a href="#Selectors">selectors</a> for type <code>T</code> |
| or <code>*T</code>. |
| </p> |
| |
| <p> |
| A non-<a href="#Blank_identifier">blank</a> receiver identifier must be |
| <a href="#Uniqueness_of_identifiers">unique</a> in the method signature. |
| If the receiver's value is not referenced inside the body of the method, |
| its identifier may be omitted in the declaration. The same applies in |
| general to parameters of functions and methods. |
| </p> |
| |
| <p> |
| For a base type, the non-blank names of methods bound to it must be unique. |
| If the base type is a <a href="#Struct_types">struct type</a>, |
| the non-blank method and field names must be distinct. |
| </p> |
| |
| <p> |
| Given defined type <code>Point</code> the declarations |
| </p> |
| |
| <pre> |
| func (p *Point) Length() float64 { |
| return math.Sqrt(p.x * p.x + p.y * p.y) |
| } |
| |
| func (p *Point) Scale(factor float64) { |
| p.x *= factor |
| p.y *= factor |
| } |
| </pre> |
| |
| <p> |
| bind the methods <code>Length</code> and <code>Scale</code>, |
| with receiver type <code>*Point</code>, |
| to the base type <code>Point</code>. |
| </p> |
| |
| <p> |
| If the receiver base type is a <a href="#Type_declarations">generic type</a>, the |
| receiver specification must declare corresponding type parameters for the method |
| to use. This makes the receiver type parameters available to the method. |
| Syntactically, this type parameter declaration looks like an |
| <a href="#Instantiations">instantiation</a> of the receiver base type: the type |
| arguments must be identifiers denoting the type parameters being declared, one |
| for each type parameter of the receiver base type. |
| The type parameter names do not need to match their corresponding parameter names in the |
| receiver base type definition, and all non-blank parameter names must be unique in the |
| receiver parameter section and the method signature. |
| The receiver type parameter constraints are implied by the receiver base type definition: |
| corresponding type parameters have corresponding constraints. |
| </p> |
| |
| <pre> |
| type Pair[A, B any] struct { |
| a A |
| b B |
| } |
| |
| func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B |
| func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair |
| </pre> |
| |
| <h2 id="Expressions">Expressions</h2> |
| |
| <p> |
| An expression specifies the computation of a value by applying |
| operators and functions to operands. |
| </p> |
| |
| <h3 id="Operands">Operands</h3> |
| |
| <p> |
| Operands denote the elementary values in an expression. An operand may be a |
| literal, a (possibly <a href="#Qualified_identifiers">qualified</a>) |
| non-<a href="#Blank_identifier">blank</a> identifier denoting a |
| <a href="#Constant_declarations">constant</a>, |
| <a href="#Variable_declarations">variable</a>, or |
| <a href="#Function_declarations">function</a>, |
| or a parenthesized expression. |
| </p> |
| |
| <pre class="ebnf"> |
| Operand = Literal | OperandName [ TypeArgs ] | "(" Expression ")" . |
| Literal = BasicLit | CompositeLit | FunctionLit . |
| BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit . |
| OperandName = identifier | QualifiedIdent . |
| </pre> |
| |
| <p> |
| An operand name denoting a <a href="#Function_declarations">generic function</a> |
| may be followed by a list of <a href="#Instantiations">type arguments</a>; the |
| resulting operand is an <a href="#Instantiations">instantiated</a> function. |
| </p> |
| |
| <p> |
| The <a href="#Blank_identifier">blank identifier</a> may appear as an |
| operand only on the left-hand side of an <a href="#Assignments">assignment</a>. |
| </p> |
| |
| <p> |
| Implementation restriction: A compiler need not report an error if an operand's |
| type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty |
| <a href="#Interface_types">type set</a>. Functions with such type parameters |
| cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead |
| to an error at the instantiation site. |
| </p> |
| |
| <h3 id="Qualified_identifiers">Qualified identifiers</h3> |
| |
| <p> |
| A <i>qualified identifier</i> is an identifier qualified with a package name prefix. |
| Both the package name and the identifier must not be |
| <a href="#Blank_identifier">blank</a>. |
| </p> |
| |
| <pre class="ebnf"> |
| QualifiedIdent = PackageName "." identifier . |
| </pre> |
| |
| <p> |
| A qualified identifier accesses an identifier in a different package, which |
| must be <a href="#Import_declarations">imported</a>. |
| The identifier must be <a href="#Exported_identifiers">exported</a> and |
| declared in the <a href="#Blocks">package block</a> of that package. |
| </p> |
| |
| <pre> |
| math.Sin // denotes the Sin function in package math |
| </pre> |
| |
| <h3 id="Composite_literals">Composite literals</h3> |
| |
| <p> |
| Composite literals construct new composite values each time they are evaluated. |
| They consist of the type of the literal followed by a brace-bound list of elements. |
| Each element may optionally be preceded by a corresponding key. |
| </p> |
| |
| <pre class="ebnf"> |
| CompositeLit = LiteralType LiteralValue . |
| LiteralType = StructType | ArrayType | "[" "..." "]" ElementType | |
| SliceType | MapType | TypeName . |
| LiteralValue = "{" [ ElementList [ "," ] ] "}" . |
| ElementList = KeyedElement { "," KeyedElement } . |
| KeyedElement = [ Key ":" ] Element . |
| Key = FieldName | Expression | LiteralValue . |
| FieldName = identifier . |
| Element = Expression | LiteralValue . |
| </pre> |
| |
| <p> |
| The LiteralType's <a href="#Core_types">core type</a> <code>T</code> |
| must be a struct, array, slice, or map type |
| (the syntax enforces this constraint except when the type is given |
| as a TypeName). |
| The types of the elements and keys must be <a href="#Assignability">assignable</a> |
| to the respective field, element, and key types of type <code>T</code>; |
| there is no additional conversion. |
| The key is interpreted as a field name for struct literals, |
| an index for array and slice literals, and a key for map literals. |
| For map literals, all elements must have a key. It is an error |
| to specify multiple elements with the same field name or |
| constant key value. For non-constant map keys, see the section on |
| <a href="#Order_of_evaluation">evaluation order</a>. |
| </p> |
| |
| <p> |
| For struct literals the following rules apply: |
| </p> |
| <ul> |
| <li>A key must be a field name declared in the struct type. |
| </li> |
| <li>An element list that does not contain any keys must |
| list an element for each struct field in the |
| order in which the fields are declared. |
| </li> |
| <li>If any element has a key, every element must have a key. |
| </li> |
| <li>An element list that contains keys does not need to |
| have an element for each struct field. Omitted fields |
| get the zero value for that field. |
| </li> |
| <li>A literal may omit the element list; such a literal evaluates |
| to the zero value for its type. |
| </li> |
| <li>It is an error to specify an element for a non-exported |
| field of a struct belonging to a different package. |
| </li> |
| </ul> |
| |
| <p> |
| Given the declarations |
| </p> |
| <pre> |
| type Point3D struct { x, y, z float64 } |
| type Line struct { p, q Point3D } |
| </pre> |
| |
| <p> |
| one may write |
| </p> |
| |
| <pre> |
| origin := Point3D{} // zero value for Point3D |
| line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x |
| </pre> |
| |
| <p> |
| For array and slice literals the following rules apply: |
| </p> |
| <ul> |
| <li>Each element has an associated integer index marking |
| its position in the array. |
| </li> |
| <li>An element with a key uses the key as its index. The |
| key must be a non-negative constant |
| <a href="#Representability">representable</a> by |
| a value of type <code>int</code>; and if it is typed |
| it must be of <a href="#Numeric_types">integer type</a>. |
| </li> |
| <li>An element without a key uses the previous element's index plus one. |
| If the first element has no key, its index is zero. |
| </li> |
| </ul> |
| |
| <p> |
| <a href="#Address_operators">Taking the address</a> of a composite literal |
| generates a pointer to a unique <a href="#Variables">variable</a> initialized |
| with the literal's value. |
| </p> |
| |
| <pre> |
| var pointer *Point3D = &Point3D{y: 1000} |
| </pre> |
| |
| <p> |
| Note that the <a href="#The_zero_value">zero value</a> for a slice or map |
| type is not the same as an initialized but empty value of the same type. |
| Consequently, taking the address of an empty slice or map composite literal |
| does not have the same effect as allocating a new slice or map value with |
| <a href="#Allocation">new</a>. |
| </p> |
| |
| <pre> |
| p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0 |
| p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0 |
| </pre> |
| |
| <p> |
| The length of an array literal is the length specified in the literal type. |
| If fewer elements than the length are provided in the literal, the missing |
| elements are set to the zero value for the array element type. |
| It is an error to provide elements with index values outside the index range |
| of the array. The notation <code>...</code> specifies an array length equal |
| to the maximum element index plus one. |
| </p> |
| |
| <pre> |
| buffer := [10]string{} // len(buffer) == 10 |
| intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6 |
| days := [...]string{"Sat", "Sun"} // len(days) == 2 |
| </pre> |
| |
| <p> |
| A slice literal describes the entire underlying array literal. |
| Thus the length and capacity of a slice literal are the maximum |
| element index plus one. A slice literal has the form |
| </p> |
| |
| <pre> |
| []T{x1, x2, … xn} |
| </pre> |
| |
| <p> |
| and is shorthand for a slice operation applied to an array: |
| </p> |
| |
| <pre> |
| tmp := [n]T{x1, x2, … xn} |
| tmp[0 : n] |
| </pre> |
| |
| <p> |
| Within a composite literal of array, slice, or map type <code>T</code>, |
| elements or map keys that are themselves composite literals may elide the respective |
| literal type if it is identical to the element or key type of <code>T</code>. |
| Similarly, elements or keys that are addresses of composite literals may elide |
| the <code>&T</code> when the element or key type is <code>*T</code>. |
| </p> |
| |
| <pre> |
| [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}} |
| [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}} |
| [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}} |
| map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}} |
| map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"} |
| |
| type PPoint *Point |
| [2]*Point{{1.5, -3.5}, {}} // same as [2]*Point{&Point{1.5, -3.5}, &Point{}} |
| [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})} |
| </pre> |
| |
| <p> |
| A parsing ambiguity arises when a composite literal using the |
| TypeName form of the LiteralType appears as an operand between the |
| <a href="#Keywords">keyword</a> and the opening brace of the block |
| of an "if", "for", or "switch" statement, and the composite literal |
| is not enclosed in parentheses, square brackets, or curly braces. |
| In this rare case, the opening brace of the literal is erroneously parsed |
| as the one introducing the block of statements. To resolve the ambiguity, |
| the composite literal must appear within parentheses. |
| </p> |
| |
| <pre> |
| if x == (T{a,b,c}[i]) { … } |
| if (x == T{a,b,c}[i]) { … } |
| </pre> |
| |
| <p> |
| Examples of valid array, slice, and map literals: |
| </p> |
| |
| <pre> |
| // list of prime numbers |
| primes := []int{2, 3, 5, 7, 9, 2147483647} |
| |
| // vowels[ch] is true if ch is a vowel |
| vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true} |
| |
| // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1} |
| filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1} |
| |
| // frequencies in Hz for equal-tempered scale (A4 = 440Hz) |
| noteFrequency := map[string]float32{ |
| "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83, |
| "G0": 24.50, "A0": 27.50, "B0": 30.87, |
| } |
| </pre> |
| |
| |
| <h3 id="Function_literals">Function literals</h3> |
| |
| <p> |
| A function literal represents an anonymous <a href="#Function_declarations">function</a>. |
| Function literals cannot declare type parameters. |
| </p> |
| |
| <pre class="ebnf"> |
| FunctionLit = "func" Signature FunctionBody . |
| </pre> |
| |
| <pre> |
| func(a, b int, z float64) bool { return a*b < int(z) } |
| </pre> |
| |
| <p> |
| A function literal can be assigned to a variable or invoked directly. |
| </p> |
| |
| <pre> |
| f := func(x, y int) int { return x + y } |
| func(ch chan int) { ch <- ACK }(replyChan) |
| </pre> |
| |
| <p> |
| Function literals are <i>closures</i>: they may refer to variables |
| defined in a surrounding function. Those variables are then shared between |
| the surrounding function and the function literal, and they survive as long |
| as they are accessible. |
| </p> |
| |
| |
| <h3 id="Primary_expressions">Primary expressions</h3> |
| |
| <p> |
| Primary expressions are the operands for unary and binary expressions. |
| </p> |
| |
| <pre class="ebnf"> |
| PrimaryExpr = |
| Operand | |
| Conversion | |
| MethodExpr | |
| PrimaryExpr Selector | |
| PrimaryExpr Index | |
| PrimaryExpr Slice | |
| PrimaryExpr TypeAssertion | |
| PrimaryExpr Arguments . |
| |
| Selector = "." identifier . |
| Index = "[" Expression "]" . |
| Slice = "[" [ Expression ] ":" [ Expression ] "]" | |
| "[" [ Expression ] ":" Expression ":" Expression "]" . |
| TypeAssertion = "." "(" Type ")" . |
| Arguments = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" . |
| </pre> |
| |
| |
| <pre> |
| x |
| 2 |
| (s + ".txt") |
| f(3.1415, true) |
| Point{1, 2} |
| m["foo"] |
| s[i : j + 1] |
| obj.color |
| f.p[i].x() |
| </pre> |
| |
| |
| <h3 id="Selectors">Selectors</h3> |
| |
| <p> |
| For a <a href="#Primary_expressions">primary expression</a> <code>x</code> |
| that is not a <a href="#Package_clause">package name</a>, the |
| <i>selector expression</i> |
| </p> |
| |
| <pre> |
| x.f |
| </pre> |
| |
| <p> |
| denotes the field or method <code>f</code> of the value <code>x</code> |
| (or sometimes <code>*x</code>; see below). |
| The identifier <code>f</code> is called the (field or method) <i>selector</i>; |
| it must not be the <a href="#Blank_identifier">blank identifier</a>. |
| The type of the selector expression is the type of <code>f</code>. |
| If <code>x</code> is a package name, see the section on |
| <a href="#Qualified_identifiers">qualified identifiers</a>. |
| </p> |
| |
| <p> |
| A selector <code>f</code> may denote a field or method <code>f</code> of |
| a type <code>T</code>, or it may refer |
| to a field or method <code>f</code> of a nested |
| <a href="#Struct_types">embedded field</a> of <code>T</code>. |
| The number of embedded fields traversed |
| to reach <code>f</code> is called its <i>depth</i> in <code>T</code>. |
| The depth of a field or method <code>f</code> |
| declared in <code>T</code> is zero. |
| The depth of a field or method <code>f</code> declared in |
| an embedded field <code>A</code> in <code>T</code> is the |
| depth of <code>f</code> in <code>A</code> plus one. |
| </p> |
| |
| <p> |
| The following rules apply to selectors: |
| </p> |
| |
| <ol> |
| <li> |
| For a value <code>x</code> of type <code>T</code> or <code>*T</code> |
| where <code>T</code> is not a pointer or interface type, |
| <code>x.f</code> denotes the field or method at the shallowest depth |
| in <code>T</code> where there is such an <code>f</code>. |
| If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a> |
| with shallowest depth, the selector expression is illegal. |
| </li> |
| |
| <li> |
| For a value <code>x</code> of type <code>I</code> where <code>I</code> |
| is an interface type, <code>x.f</code> denotes the actual method with name |
| <code>f</code> of the dynamic value of <code>x</code>. |
| If there is no method with name <code>f</code> in the |
| <a href="#Method_sets">method set</a> of <code>I</code>, the selector |
| expression is illegal. |
| </li> |
| |
| <li> |
| As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a> |
| pointer type and <code>(*x).f</code> is a valid selector expression denoting a field |
| (but not a method), <code>x.f</code> is shorthand for <code>(*x).f</code>. |
| </li> |
| |
| <li> |
| In all other cases, <code>x.f</code> is illegal. |
| </li> |
| |
| <li> |
| If <code>x</code> is of pointer type and has the value |
| <code>nil</code> and <code>x.f</code> denotes a struct field, |
| assigning to or evaluating <code>x.f</code> |
| causes a <a href="#Run_time_panics">run-time panic</a>. |
| </li> |
| |
| <li> |
| If <code>x</code> is of interface type and has the value |
| <code>nil</code>, <a href="#Calls">calling</a> or |
| <a href="#Method_values">evaluating</a> the method <code>x.f</code> |
| causes a <a href="#Run_time_panics">run-time panic</a>. |
| </li> |
| </ol> |
| |
| <p> |
| For example, given the declarations: |
| </p> |
| |
| <pre> |
| type T0 struct { |
| x int |
| } |
| |
| func (*T0) M0() |
| |
| type T1 struct { |
| y int |
| } |
| |
| func (T1) M1() |
| |
| type T2 struct { |
| z int |
| T1 |
| *T0 |
| } |
| |
| func (*T2) M2() |
| |
| type Q *T2 |
| |
| var t T2 // with t.T0 != nil |
| var p *T2 // with p != nil and (*p).T0 != nil |
| var q Q = p |
| </pre> |
| |
| <p> |
| one may write: |
| </p> |
| |
| <pre> |
| t.z // t.z |
| t.y // t.T1.y |
| t.x // (*t.T0).x |
| |
| p.z // (*p).z |
| p.y // (*p).T1.y |
| p.x // (*(*p).T0).x |
| |
| q.x // (*(*q).T0).x (*q).x is a valid field selector |
| |
| p.M0() // ((*p).T0).M0() M0 expects *T0 receiver |
| p.M1() // ((*p).T1).M1() M1 expects T1 receiver |
| p.M2() // p.M2() M2 expects *T2 receiver |
| t.M2() // (&t).M2() M2 expects *T2 receiver, see section on Calls |
| </pre> |
| |
| <p> |
| but the following is invalid: |
| </p> |
| |
| <pre> |
| q.M0() // (*q).M0 is valid but not a field selector |
| </pre> |
| |
| |
| <h3 id="Method_expressions">Method expressions</h3> |
| |
| <p> |
| If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>, |
| <code>T.M</code> is a function that is callable as a regular function |
| with the same arguments as <code>M</code> prefixed by an additional |
| argument that is the receiver of the method. |
| </p> |
| |
| <pre class="ebnf"> |
| MethodExpr = ReceiverType "." MethodName . |
| ReceiverType = Type . |
| </pre> |
| |
| <p> |
| Consider a struct type <code>T</code> with two methods, |
| <code>Mv</code>, whose receiver is of type <code>T</code>, and |
| <code>Mp</code>, whose receiver is of type <code>*T</code>. |
| </p> |
| |
| <pre> |
| type T struct { |
| a int |
| } |
| func (tv T) Mv(a int) int { return 0 } // value receiver |
| func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver |
| |
| var t T |
| </pre> |
| |
| <p> |
| The expression |
| </p> |
| |
| <pre> |
| T.Mv |
| </pre> |
| |
| <p> |
| yields a function equivalent to <code>Mv</code> but |
| with an explicit receiver as its first argument; it has signature |
| </p> |
| |
| <pre> |
| func(tv T, a int) int |
| </pre> |
| |
| <p> |
| That function may be called normally with an explicit receiver, so |
| these five invocations are equivalent: |
| </p> |
| |
| <pre> |
| t.Mv(7) |
| T.Mv(t, 7) |
| (T).Mv(t, 7) |
| f1 := T.Mv; f1(t, 7) |
| f2 := (T).Mv; f2(t, 7) |
| </pre> |
| |
| <p> |
| Similarly, the expression |
| </p> |
| |
| <pre> |
| (*T).Mp |
| </pre> |
| |
| <p> |
| yields a function value representing <code>Mp</code> with signature |
| </p> |
| |
| <pre> |
| func(tp *T, f float32) float32 |
| </pre> |
| |
| <p> |
| For a method with a value receiver, one can derive a function |
| with an explicit pointer receiver, so |
| </p> |
| |
| <pre> |
| (*T).Mv |
| </pre> |
| |
| <p> |
| yields a function value representing <code>Mv</code> with signature |
| </p> |
| |
| <pre> |
| func(tv *T, a int) int |
| </pre> |
| |
| <p> |
| Such a function indirects through the receiver to create a value |
| to pass as the receiver to the underlying method; |
| the method does not overwrite the value whose address is passed in |
| the function call. |
| </p> |
| |
| <p> |
| The final case, a value-receiver function for a pointer-receiver method, |
| is illegal because pointer-receiver methods are not in the method set |
| of the value type. |
| </p> |
| |
| <p> |
| Function values derived from methods are called with function call syntax; |
| the receiver is provided as the first argument to the call. |
| That is, given <code>f := T.Mv</code>, <code>f</code> is invoked |
| as <code>f(t, 7)</code> not <code>t.f(7)</code>. |
| To construct a function that binds the receiver, use a |
| <a href="#Function_literals">function literal</a> or |
| <a href="#Method_values">method value</a>. |
| </p> |
| |
| <p> |
| It is legal to derive a function value from a method of an interface type. |
| The resulting function takes an explicit receiver of that interface type. |
| </p> |
| |
| <h3 id="Method_values">Method values</h3> |
| |
| <p> |
| If the expression <code>x</code> has static type <code>T</code> and |
| <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>, |
| <code>x.M</code> is called a <i>method value</i>. |
| The method value <code>x.M</code> is a function value that is callable |
| with the same arguments as a method call of <code>x.M</code>. |
| The expression <code>x</code> is evaluated and saved during the evaluation of the |
| method value; the saved copy is then used as the receiver in any calls, |
| which may be executed later. |
| </p> |
| |
| <pre> |
| type S struct { *T } |
| type T int |
| func (t T) M() { print(t) } |
| |
| t := new(T) |
| s := S{T: t} |
| f := t.M // receiver *t is evaluated and stored in f |
| g := s.M // receiver *(s.T) is evaluated and stored in g |
| *t = 42 // does not affect stored receivers in f and g |
| </pre> |
| |
| <p> |
| The type <code>T</code> may be an interface or non-interface type. |
| </p> |
| |
| <p> |
| As in the discussion of <a href="#Method_expressions">method expressions</a> above, |
| consider a struct type <code>T</code> with two methods, |
| <code>Mv</code>, whose receiver is of type <code>T</code>, and |
| <code>Mp</code>, whose receiver is of type <code>*T</code>. |
| </p> |
| |
| <pre> |
| type T struct { |
| a int |
| } |
| func (tv T) Mv(a int) int { return 0 } // value receiver |
| func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver |
| |
| var t T |
| var pt *T |
| func makeT() T |
| </pre> |
| |
| <p> |
| The expression |
| </p> |
| |
| <pre> |
| t.Mv |
| </pre> |
| |
| <p> |
| yields a function value of type |
| </p> |
| |
| <pre> |
| func(int) int |
| </pre> |
| |
| <p> |
| These two invocations are equivalent: |
| </p> |
| |
| <pre> |
| t.Mv(7) |
| f := t.Mv; f(7) |
| </pre> |
| |
| <p> |
| Similarly, the expression |
| </p> |
| |
| <pre> |
| pt.Mp |
| </pre> |
| |
| <p> |
| yields a function value of type |
| </p> |
| |
| <pre> |
| func(float32) float32 |
| </pre> |
| |
| <p> |
| As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver |
| using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(*pt).Mv</code>. |
| </p> |
| |
| <p> |
| As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver |
| using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>. |
| </p> |
| |
| <pre> |
| f := t.Mv; f(7) // like t.Mv(7) |
| f := pt.Mp; f(7) // like pt.Mp(7) |
| f := pt.Mv; f(7) // like (*pt).Mv(7) |
| f := t.Mp; f(7) // like (&t).Mp(7) |
| f := makeT().Mp // invalid: result of makeT() is not addressable |
| </pre> |
| |
| <p> |
| Although the examples above use non-interface types, it is also legal to create a method value |
| from a value of interface type. |
| </p> |
| |
| <pre> |
| var i interface { M(int) } = myVal |
| f := i.M; f(7) // like i.M(7) |
| </pre> |
| |
| |
| <h3 id="Index_expressions">Index expressions</h3> |
| |
| <p> |
|