| <!-- title The Go Programming Language Specification --> |
| <!-- subtitle Version of December 2, 2010 --> |
| |
| <!-- |
| TODO |
| [ ] need language about function/method calls and parameter passing rules |
| [ ] last paragraph of #Assignments (constant promotion) should be elsewhere |
| and mention assignment to empty interface. |
| [ ] need to say something about "scope" of selectors? |
| [ ] clarify what a field name is in struct declarations |
| (struct{T} vs struct {T T} vs struct {t T}) |
| [ ] need explicit language about the result type of operations |
| [ ] may want to have some examples for the types of shift operations |
| [ ] should string(1<<s) and float(1<<s) be valid? |
| [ ] should probably write something about evaluation order of statements even |
| though obvious |
| [ ] review language on implicit dereferencing |
| [ ] clarify what it means for two functions to be "the same" when comparing them |
| --> |
| |
| |
| <h2 id="Introduction">Introduction</h2> |
| |
| <p> |
| This is a reference manual for the Go programming language. For |
| more information and other documents, see <a href="http://golang.org/">http://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. The existing implementations use a traditional |
| compile/link model to generate executable binaries. |
| </p> |
| |
| <p> |
| The grammar is compact and regular, allowing for easy analysis by |
| automatic tools such as integrated development environments. |
| </p> |
| |
| <h2 id="Notation">Notation</h2> |
| <p> |
| The syntax is specified using Extended Backus-Naur Form (EBNF): |
| </p> |
| |
| <pre class="grammar"> |
| Production = production_name "=" Expression "." . |
| Expression = Alternative { "|" Alternative } . |
| Alternative = Term { Term } . |
| Term = 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> |
| Lower-case production names are used to identify lexical tokens. |
| Non-terminals are in CamelCase. Lexical symbols 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. |
| </p> |
| |
| <h2 id="Source_code_representation">Source code representation</h2> |
| |
| <p> |
| Source code is Unicode text encoded in |
| <a href="http://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 term <i>character</i> to refer to a Unicode code point. |
| </p> |
| <p> |
| Each code point is distinct; for instance, upper and lower case 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> |
| |
| <h3 id="Characters">Characters</h3> |
| |
| <p> |
| The following terms are used to denote specific Unicode character classes: |
| </p> |
| <pre class="ebnf"> |
| unicode_char = /* an arbitrary Unicode code point */ . |
| unicode_letter = /* a Unicode code point classified as "Letter" */ . |
| unicode_digit = /* a Unicode code point classified as "Digit" */ . |
| </pre> |
| |
| <p> |
| In <a href="http://www.unicode.org/versions/Unicode5.2.0/">The Unicode Standard 5.2</a>, |
| Section 4.5 General Category-Normative |
| defines a set of character categories. Go treats |
| those characters in category Lu, Ll, Lt, Lm, or Lo as Unicode letters, |
| and those in 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 letter. |
| </p> |
| <pre class="ebnf"> |
| letter = unicode_letter | "_" . |
| decimal_digit = "0" ... "9" . |
| 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> |
| There are two forms of comments: |
| </p> |
| |
| <ol> |
| <li> |
| <i>Line comments</i> start with the character sequence <code>//</code> |
| and stop at the end of the line. A line comment acts like a newline. |
| </li> |
| <li> |
| <i>General comments</i> start with the character sequence <code>/*</code> |
| and continue through the character sequence <code>*/</code>. A general |
| comment that spans multiple lines acts like a newline, otherwise it acts |
| like a space. |
| </li> |
| </ol> |
| |
| <p> |
| Comments do not nest. |
| </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 delimiters</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 grammar 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> |
| <p> |
| When the input is broken into tokens, a semicolon is automatically inserted |
| into the token stream at the end of a non-blank line if the line's final |
| token is |
| </p> |
| <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="#Character_literals">character</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_Delimiters">operators and delimiters</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_Delimiters">Operators and Delimiters</h3> |
| |
| <p> |
| The following character sequences represent <a href="#Operators">operators</a>, delimiters, and other special tokens: |
| </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>0</code> for octal, <code>0x</code> or |
| <code>0X</code> for hexadecimal. In hexadecimal literals, letters |
| <code>a-f</code> and <code>A-F</code> represent values 10 through 15. |
| </p> |
| <pre class="ebnf"> |
| int_lit = decimal_lit | octal_lit | hex_lit . |
| decimal_lit = ( "1" ... "9" ) { decimal_digit } . |
| octal_lit = "0" { octal_digit } . |
| hex_lit = "0" ( "x" | "X" ) hex_digit { hex_digit } . |
| </pre> |
| |
| <pre> |
| 42 |
| 0600 |
| 0xBadFace |
| 170141183460469231731687303715884105727 |
| </pre> |
| |
| <h3 id="Floating-point_literals">Floating-point literals</h3> |
| <p> |
| A floating-point literal is a decimal representation of a |
| <a href="#Constants">floating-point constant</a>. |
| It has an integer part, a decimal point, a fractional part, |
| and an exponent part. The integer and fractional part comprise |
| decimal digits; the exponent part is an <code>e</code> or <code>E</code> |
| followed by an optionally signed decimal exponent. One of the |
| integer part or the fractional part may be elided; one of the decimal |
| point or the exponent may be elided. |
| </p> |
| <pre class="ebnf"> |
| float_lit = decimals "." [ decimals ] [ exponent ] | |
| decimals exponent | |
| "." decimals [ exponent ] . |
| decimals = decimal_digit { decimal_digit } . |
| exponent = ( "e" | "E" ) [ "+" | "-" ] decimals . |
| </pre> |
| |
| <pre> |
| 0. |
| 72.40 |
| 072.40 // == 72.40 |
| 2.71828 |
| 1.e+0 |
| 6.67428e-11 |
| 1E6 |
| .25 |
| .12345E+5 |
| </pre> |
| |
| <h3 id="Imaginary_literals">Imaginary literals</h3> |
| <p> |
| An imaginary literal is a decimal representation of the imaginary part of a |
| <a href="#Constants">complex constant</a>. |
| It consists of a |
| <a href="#Floating-point_literals">floating-point literal</a> |
| or decimal integer followed |
| by the lower-case letter <code>i</code>. |
| </p> |
| <pre class="ebnf"> |
| imaginary_lit = (decimals | float_lit) "i" . |
| </pre> |
| |
| <pre> |
| 0i |
| 011i // == 11i |
| 0.i |
| 2.71828i |
| 1.e+0i |
| 6.67428e-11i |
| 1E6i |
| .25i |
| .12345E+5i |
| </pre> |
| |
| |
| <h3 id="Character_literals">Character literals</h3> |
| |
| <p> |
| A character literal represents an <a href="#Constants">integer constant</a>, |
| typically a Unicode code point, as one or more characters enclosed in single |
| quotes. Within the quotes, any character may appear except single |
| quote and newline. A single quoted character represents 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 represented |
| 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 character literals) |
| \" U+0022 double quote (valid escape only within string literals) |
| </pre> |
| <p> |
| All other sequences starting with a backslash are illegal inside character literals. |
| </p> |
| <pre class="ebnf"> |
| char_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' |
| </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 |
| <code>``</code>. Within the quotes, any character is legal except |
| back quote. The value of a raw string literal is the |
| string composed of the uninterpreted characters between the quotes; |
| in particular, backslashes have no special meaning and the string may |
| span multiple lines. |
| </p> |
| <p> |
| Interpreted string literals are character sequences between double |
| quotes <code>""</code>. The text between the quotes, |
| which may not span multiple lines, forms the |
| value of the literal, with backslash escapes interpreted as they |
| are in character literals (except that <code>\'</code> is illegal and |
| <code>\"</code> is legal). 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 } "`" . |
| interpreted_string_lit = `"` { unicode_value | byte_value } `"` . |
| </pre> |
| |
| <pre> |
| `abc` // same as "abc" |
| `\n |
| \n` // same as "\\n\n\\n" |
| "\n" |
| "" |
| "Hello, world!\n" |
| "日本語" |
| "\u65e5本\U00008a9e" |
| "\xff\u00FF" |
| </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 character 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>integer constants</i>, |
| <i>floating-point constants</i>, <i>complex constants</i>, |
| and <i>string constants</i>. Integer, floating-point, |
| and complex constants are |
| collectively called <i>numeric constants</i>. |
| </p> |
| |
| <p> |
| A constant value is represented by an |
| <a href="#Integer_literals">integer</a>, |
| <a href="#Floating-point_literals">floating-point</a>, |
| <a href="#Imaginary_literals">imaginary</a>, |
| <a href="#Character_literals">character</a>, or |
| <a href="#String_literals">string</a> literal, |
| an identifier denoting a constant, |
| a <a href="#Constant_expressions">constant expression</a>, or |
| the result value of some built-in functions such as |
| <code>unsafe.Sizeof</code> applied to any value, |
| <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>cmplx</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 values of arbitrary precision and do not overflow. |
| </p> |
| |
| <p> |
| Constants may be <a href="#Types">typed</a> or untyped. |
| 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 accurately represented as a value of the respective type. |
| For instance, <code>3.0</code> can be given any integer or any |
| floating-point type, while <code>2147483648.0</code> (equal to <code>1<<31</code>) |
| can be given the types <code>float32</code>, <code>float64</code>, or <code>uint32</code> but |
| not <code>int32</code> or <code>string</code>. |
| </p> |
| |
| <p> |
| There are no constants denoting the IEEE-754 infinity and not-a-number values, |
| but the <a href="/pkg/math/"><code>math</code> package</a>'s |
| <a href="/pkg/math/#Inf">Inf</a>, |
| <a href="/pkg/math/#NaN">NaN</a>, |
| <a href="/pkg/math/#IsInf">IsInf</a>, and |
| <a href="/pkg/math/#IsNaN">IsNaN</a> |
| functions return and test for those values at run time. |
| </p> |
| |
| <p> |
| Implementation restriction: A compiler may implement numeric constants by choosing |
| an internal representation with at least twice as many bits as any machine type; |
| for floating-point values, both the mantissa and exponent must be twice as large. |
| </p> |
| |
| |
| <h2 id="Types">Types</h2> |
| |
| <p> |
| A type determines the set of values and operations specific to values of that |
| type. A type may be specified by a (possibly qualified) <i>type name</i> |
| (§<a href="#Qualified_identifiers">Qualified identifier</a>, §<a href="#Type_declarations">Type declarations</a>) or a <i>type literal</i>, |
| which composes a new type from previously declared types. |
| </p> |
| |
| <pre class="ebnf"> |
| Type = TypeName | TypeLit | "(" Type ")" . |
| TypeName = QualifiedIdent . |
| TypeLit = ArrayType | StructType | PointerType | FunctionType | InterfaceType | |
| SliceType | MapType | ChannelType . |
| </pre> |
| |
| <p> |
| Named instances of the boolean, numeric, and string types are |
| <a href="#Predeclared_identifiers">predeclared</a>. |
| <i>Composite types</i>—array, struct, pointer, function, |
| interface, slice, map, and channel types—may be constructed using |
| type literals. |
| </p> |
| |
| <p> |
| Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code> |
| is a predeclared type 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 |
| <a href="#Type_declarations">type declaration</a>. |
| </p> |
| |
| <pre> |
| type T1 string |
| type T2 T1 |
| type T3 []T1 |
| type T4 T3 |
| </pre> |
| |
| <p> |
| The underlying type of <code>string</code>, <code>T1</code>, and <code>T2</code> |
| is <code>string</code>. The underlying type of <code>[]T1</code>, <code>T3</code>, |
| and <code>T4</code> is <code>[]T1</code>. |
| </p> |
| |
| <p> |
| A type may have a <i>method set</i> associated with it |
| (§<a href="#Interface_types">Interface types</a>, §<a href="#Method_declarations">Method declarations</a>). |
| The method set of an <a href="#Interface_types">interface type</a> is its interface. |
| The method set of any other named type <code>T</code> |
| consists of all methods with receiver type <code>T</code>. |
| The method set of the corresponding pointer type <code>*T</code> |
| is the set of all methods with receiver <code>*T</code> or <code>T</code> |
| (that is, it also contains the method set of <code>T</code>). |
| Any other type has an empty method set. |
| In a method set, each method must have a unique name. |
| </p> |
| <p> |
| The <i>static type</i> (or just <i>type</i>) of a variable is the |
| type defined by its declaration. Variables of interface type |
| also have a distinct <i>dynamic type</i>, which |
| is the actual type of the value stored in the variable at run-time. |
| The dynamic type may vary during execution but is always |
| <a href="#Assignability">assignable</a> |
| to the static type of the interface variable. For non-interface |
| types, the dynamic type is always the static type. |
| </p> |
| |
| |
| <h3 id="Boolean_types">Boolean types</h3> |
| |
| 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>. |
| |
| |
| <h3 id="Numeric_types">Numeric types</h3> |
| |
| <p> |
| A <i>numeric type</i> represents sets of integer or floating-point values. |
| 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 familiar alias for uint8 |
| </pre> |
| |
| <p> |
| The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using |
| <a href="http://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>. |
| </p> |
| |
| <p> |
| There is also a set of predeclared numeric types with implementation-specific sizes: |
| </p> |
| |
| <pre class="grammar"> |
| uint either 32 or 64 bits |
| int either 32 or 64 bits |
| float either 32 or 64 bits |
| complex real and imaginary parts have type float |
| 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 distinct except |
| <code>byte</code>, which is an alias for <code>uint8</code>. |
| 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. |
| Strings behave like arrays of bytes but are immutable: once created, |
| it is impossible to change the contents of a string. |
| The predeclared string type is <code>string</code>. |
| |
| <p> |
| The elements of strings have type <code>byte</code> and may be |
| accessed using the usual <a href="#Indexes">indexing operations</a>. It is |
| illegal to take the address of such an element; if |
| <code>s[i]</code> is the <i>i</i>th byte of a |
| string, <code>&s[i]</code> is invalid. The length of string |
| <code>s</code> can be discovered using the built-in function |
| <code>len</code>. The length is a compile-time constant if <code>s</code> |
| is a string literal. |
| </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 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 and must be a |
| <a href="#Constant_expressions">constant expression</a> that evaluates to a non-negative |
| integer value. The length of array <code>a</code> can be discovered |
| using the built-in function <a href="#Length_and_capacity"><code>len(a)</code></a>. |
| The elements can be indexed by integer |
| indices 0 through the <code>len(a)-1</code> (§<a href="#Indexes">Indexes</a>). |
| 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 reference to a contiguous segment of an array and |
| contains a numbered sequence of elements from that array. A slice |
| type denotes the set of all slices of arrays of its element type. |
| The value of an uninitialized slice is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| SliceType = "[" "]" ElementType . |
| </pre> |
| |
| <p> |
| Like arrays, slices are indexable and have a length. The length of a |
| slice <code>s</code> can be discovered by the built-in function |
| <a href="#Length_and_capacity"><code>len(s)</code></a>; unlike with arrays it may change during |
| execution. The elements can be addressed by integer indices 0 |
| through <code>len(s)-1</code> (§<a href="#Indexes">Indexes</a>). 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 `slicing' a new |
| one from the original slice (§<a href="#Slices">Slices</a>). |
| 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> is |
| 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: |
| </p> |
| |
| <pre> |
| make([]T, length) |
| make([]T, length, capacity) |
| </pre> |
| |
| <p> |
| The <code>make()</code> call 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 slicing it, so these two examples |
| result in the same slice: |
| </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 lengths may vary dynamically. |
| Moreover, the inner slices must be allocated individually (with <code>make</code>). |
| </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 (AnonymousField). |
| Within a struct, non-<a href="#Blank_identifier">blank</a> field names must |
| be unique. |
| </p> |
| |
| <pre class="ebnf"> |
| StructType = "struct" "{" { FieldDecl ";" } "}" . |
| FieldDecl = (IdentifierList Type | AnonymousField) [ Tag ] . |
| AnonymousField = [ "*" ] TypeName . |
| Tag = string_lit . |
| </pre> |
| |
| <pre> |
| // An empty struct. |
| struct {} |
| |
| // A struct with 6 fields. |
| struct { |
| x, y int |
| u float |
| _ float // padding |
| A *[]int |
| F func() |
| } |
| </pre> |
| |
| <p> |
| A field declared with a type but no explicit field name is an <i>anonymous field</i> |
| (colloquially called an embedded field). |
| Such a field type 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 anonymous fields of type 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 anonymous field *T and *P.T |
| *T // conflicts with anonymous field T and *P.T |
| *P.T // conflicts with anonymous field T and *T |
| } |
| </pre> |
| |
| <p> |
| Fields and methods (§<a href="#Method_declarations">Method declarations</a>) of an anonymous field are |
| promoted to be ordinary fields and methods of the struct (§<a href="#Selectors">Selectors</a>). |
| The following rules apply for a struct type named <code>S</code> and |
| a type named <code>T</code>: |
| </p> |
| <ul> |
| <li>If <code>S</code> contains an anonymous field <code>T</code>, the |
| method set of <code>S</code> includes the method set of <code>T</code>. |
| </li> |
| |
| <li>If <code>S</code> contains an anonymous field <code>*T</code>, the |
| method set of <code>S</code> includes the method set of <code>*T</code> |
| (which itself includes the method set of <code>T</code>). |
| </li> |
| |
| <li>If <code>S</code> contains an anonymous field <code>T</code> or |
| <code>*T</code>, the method set of <code>*S</code> includes the |
| method set of <code>*T</code> (which itself includes the method |
| set of <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. The tags are made |
| visible through a <a href="#Package_unsafe">reflection interface</a> |
| but are otherwise ignored. |
| </p> |
| |
| <pre> |
| // A struct corresponding to the TimeStamp protocol buffer. |
| // The tag strings define the protocol buffer field numbers. |
| struct { |
| microsec uint64 "field 1" |
| serverIP6 uint64 "field 2" |
| process string "field 3" |
| } |
| </pre> |
| |
| <h3 id="Pointer_types">Pointer types</h3> |
| |
| <p> |
| A pointer type denotes the set of all pointers to variables 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> |
| *int |
| *map[string] *chan 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; 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 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) |
| func() int |
| func(prefix string, values ...int) |
| func(a, b int, z float) bool |
| func(a, b int, z float) (bool) |
| func(a, b int, z float, opt ...interface{}) (success bool) |
| func(int, int, float) (float, *[]int) |
| func(n int) func(p *T) |
| </pre> |
| |
| |
| <h3 id="Interface_types">Interface types</h3> |
| |
| <p> |
| An interface type specifies a <a href="#Types">method set</a> called its <i>interface</i>. |
| A variable of interface type can store a value of any type with a method set |
| that is any superset of the interface. Such a type is said to |
| <i>implement the interface</i>. |
| The value of an uninitialized variable of interface type is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| InterfaceType = "interface" "{" { MethodSpec ";" } "}" . |
| MethodSpec = MethodName Signature | InterfaceTypeName . |
| MethodName = identifier . |
| InterfaceTypeName = TypeName . |
| </pre> |
| |
| <p> |
| As with all method sets, in an interface type, each method must have a unique name. |
| </p> |
| |
| <pre> |
| // A simple File interface |
| interface { |
| Read(b Buffer) bool |
| Write(b Buffer) bool |
| Close() |
| } |
| </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(b Buffer) bool { return ... } |
| func (p T) Write(b Buffer) bool { return ... } |
| func (p T) Close() { ... } |
| </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> |
| A type implements any interface comprising any subset of its methods |
| and may therefore implement several distinct interfaces. For |
| instance, all types implement the <i>empty interface</i>: |
| </p> |
| |
| <pre> |
| interface{} |
| </pre> |
| |
| <p> |
| Similarly, consider this interface specification, |
| which appears within a <a href="#Type_declarations">type declaration</a> |
| to define an interface called <code>Lock</code>: |
| </p> |
| |
| <pre> |
| type Lock 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>Lock</code> interface as well |
| as the <code>File</code> interface. |
| </p> |
| <p> |
| An interface may contain an interface type name <code>T</code> |
| in place of a method specification. |
| The effect is equivalent to enumerating the methods of <code>T</code> explicitly |
| in the interface. |
| </p> |
| |
| <pre> |
| type ReadWrite interface { |
| Read(b Buffer) bool |
| Write(b Buffer) bool |
| } |
| |
| type File interface { |
| ReadWrite // same as enumerating the methods in ReadWrite |
| Lock // same as enumerating the methods in Lock |
| Close() |
| } |
| </pre> |
| |
| <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 comparison operators <code>==</code> and <code>!=</code> |
| (§<a href="#Comparison_operators">Comparison operators</a>) must be fully defined |
| for operands of the key type; thus the key type must not be a struct, array 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 float } |
| 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(m)</code></a> |
| and may change during execution. Values may be added and removed |
| during execution using special forms of <a href="#Assignments">assignment</a>. |
| </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. |
| </p> |
| |
| <h3 id="Channel_types">Channel types</h3> |
| |
| <p> |
| A channel provides a mechanism for two concurrently executing functions |
| to synchronize execution and communicate by passing a value of a |
| specified element type. |
| The value of an uninitialized channel is <code>nil</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| ChannelType = ( "chan" [ "<-" ] | "<-" "chan" ) ElementType . |
| </pre> |
| |
| <p> |
| The <code><-</code> operator specifies the channel <i>direction</i>, |
| <i>send</i> or <i>receive</i>. If no direction is given, the channel is |
| <i>bi-directional</i>. |
| A channel may be constrained only to send or only to receive by |
| <a href="#Conversions">conversion</a> or <a href="#Assignments">assignment</a>. |
| </p> |
| |
| <pre> |
| chan T // can be used to send and receive values of type T |
| chan<- float // can only be used to send floats |
| <-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 capacity 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 greater than zero, the channel is asynchronous: provided the |
| buffer is not full, sends can succeed without blocking. If the capacity is zero |
| or absent, the communication succeeds only when both a sender and receiver are ready. |
| </p> |
| |
| <p> |
| A channel may be closed and tested for closure with the built-in functions |
| <a href="#Close_and_closed"><code>close</code> and <code>closed</code></a>. |
| </p> |
| |
| <h2 id="Properties_of_types_and_values">Properties of types and values</h2> |
| |
| <h3 id="Type_identity">Type identity</h3> |
| |
| <p> |
| Two types are either <i>identical</i> or <i>different</i>. |
| </p> |
| |
| <p> |
| Two named types are identical if their type names originate in the same |
| type <a href="#Declarations_and_scope">declaration</a>. |
| A named and an unnamed type are always different. Two unnamed types are identical |
| if the corresponding type literals are identical, that is, if 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. |
| Two anonymous fields are considered to have the same name. Lower-case 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 have the same set of methods |
| with the same names and identical function types. Lower-case method names from |
| different packages are always different. The order of the methods is irrelevant.</li> |
| |
| <li>Two map types are identical if they have identical key and value types.</li> |
| |
| <li>Two channel types are identical if they have identical value types and |
| the same direction.</li> |
| </ul> |
| |
| <p> |
| Given the declarations |
| </p> |
| |
| <pre> |
| type ( |
| T0 []string |
| T1 []string |
| T2 struct { a, b int } |
| T3 struct { a, c int } |
| T4 func(int, float) *T0 |
| T5 func(x int, y float) *[]string |
| ) |
| </pre> |
| |
| <p> |
| these types are identical: |
| </p> |
| |
| <pre> |
| T0 and T0 |
| []int and []int |
| struct { a, b *T5 } and struct { a, b *T5 } |
| func(x int, y float) *[]string and func(int, float) (result *[]string) |
| </pre> |
| |
| <p> |
| <code>T0</code> and <code>T1</code> are different because they are named types |
| with distinct declarations; <code>func(int, float) *T0</code> and |
| <code>func(x int, y float) *[]string</code> are different because <code>T0</code> |
| is different from <code>[]string</code>. |
| </p> |
| |
| |
| <h3 id="Assignability">Assignability</h3> |
| |
| <p> |
| A value <code>x</code> is <i>assignable</i> to a variable of type <code>T</code> |
| ("<code>x</code> is assignable to <code>T</code>") in any of these cases: |
| </p> |
| |
| <ul> |
| <li> |
| <code>x</code>'s type is identical to <code>T</code>. |
| </li> |
| <li> |
| <code>x</code>'s type <code>V</code> and <code>T</code> have identical |
| <a href="#Types">underlying types</a> and at least one of <code>V</code> |
| or <code>T</code> is not a named type. |
| </li> |
| <li> |
| <code>T</code> is an interface type and |
| <code>x</code> <a href="#Interface_types">implements</a> <code>T</code>. |
| </li> |
| <li> |
| <code>x</code> is a bidirectional channel value, <code>T</code> is a channel type, |
| <code>x</code>'s type <code>V</code> and <code>T</code> have identical element types, |
| and at least one of <code>V</code> or <code>T</code> is not a named type. |
| </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. |
| </li> |
| <li> |
| <code>x</code> is an untyped <a href="#Constants">constant</a> representable |
| by a value of type <code>T</code>. |
| </li> |
| </ul> |
| |
| <p> |
| If <code>T</code> is a struct type, either all fields of <code>T</code> |
| must be <a href="#Exported_identifiers">exported</a>, or the assignment must be in |
| the same package in which <code>T</code> is declared. |
| In other words, a struct value can be assigned to a struct variable only if |
| every field of the struct may be legally assigned individually by the program. |
| </p> |
| |
| <p> |
| Any value may be assigned to the <a href="#Blank_identifier">blank identifier</a>. |
| </p> |
| |
| |
| <h2 id="Blocks">Blocks</h2> |
| |
| <p> |
| A <i>block</i> is a sequence of declarations and statements within matching |
| brace brackets. |
| </p> |
| |
| <pre class="ebnf"> |
| Block = "{" { 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 <code>if</code>, <code>for</code>, and <code>switch</code> |
| statement is considered to be in its own implicit block.</li> |
| |
| <li>Each clause in a <code>switch</code> or <code>select</code> 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 declaration binds a non-<a href="#Blank_identifier">blank</a> |
| identifier to a constant, type, variable, function, or package. |
| 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> |
| |
| <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, or package. |
| </p> |
| |
| <p> |
| Go is lexically scoped using blocks: |
| </p> |
| |
| <ol> |
| <li>The scope of a predeclared identifier is the universe block.</li> |
| |
| <li>The scope of an identifier denoting a constant, type, variable, |
| or function declared at top level (outside any function) is the |
| package block.</li> |
| |
| <li>The scope of an imported package identifier is the file block |
| of the file containing the import declaration.</li> |
| |
| <li>The scope of an identifier denoting a function parameter or |
| result variable is the function body.</li> |
| |
| <li>The scope of a constant or variable identifier declared |
| inside a function begins at the end of the ConstSpec or VarSpec |
| 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 <code>break</code>, <code>continue</code>, and <code>goto</code> |
| statements (§<a href="#Break_statements">Break statements</a>, §<a href="#Continue_statements">Continue statements</a>, §<a href="#Goto_statements">Goto statements</a>). |
| 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="Predeclared_identifiers">Predeclared identifiers</h3> |
| |
| <p> |
| The following identifiers are implicitly declared in the universe block: |
| </p> |
| <pre class="grammar"> |
| Basic types: |
| bool byte complex64 complex128 float32 float64 |
| int8 int16 int32 int64 string uint8 uint16 uint32 uint64 |
| |
| Architecture-specific convenience types: |
| complex float int uint uintptr |
| |
| Constants: |
| true false iota |
| |
| Zero value: |
| nil |
| |
| Functions: |
| append cap close closed cmplx copy 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 |
| using a <a href="#Qualified_identifiers">qualified identifier</a>. An identifier |
| is exported if both: |
| </p> |
| <ol> |
| <li>the first character of the identifier's name is a Unicode upper case letter (Unicode class "Lu"); and</li> |
| <li>the identifier is declared in the <a href="#Blocks">package block</a> or denotes a field or method of a type |
| declared in that block.</li> |
| </ol> |
| <p> |
| All other identifiers are not exported. |
| </p> |
| |
| |
| <h3 id="Blank_identifier">Blank identifier</h3> |
| |
| <p> |
| The <i>blank identifier</i>, represented by the underscore character <code>_</code>, may be used in a declaration like |
| any other identifier but the declaration does not introduce a new binding. |
| </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. |
| 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 float = 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 declaration. |
| 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>. It is reset to 0 whenever the reserved word <code>const</code> |
| appears in the source and increments after each <a href="#ConstSpec">ConstSpec</a>. |
| It can be used to construct a set of related constants: |
| </p> |
| |
| <pre> |
| const ( // iota is reset to 0 |
| c0 = iota // c0 == 0 |
| c1 = iota // c1 == 1 |
| c2 = iota // c2 == 2 |
| ) |
| |
| const ( |
| a = 1 << iota // a == 1 (iota has been reset) |
| b = 1 << iota // b == 2 |
| c = 1 << iota // c == 4 |
| ) |
| |
| const ( |
| u = iota * 42 // u == 0 (untyped integer constant) |
| v float = iota * 42 // v == 42.0 (float constant) |
| w = iota * 42 // w == 84 (untyped integer constant) |
| ) |
| |
| const x = iota // x == 0 (iota has been reset) |
| const y = iota // y == 0 (iota has been reset) |
| </pre> |
| |
| <p> |
| Within an ExpressionList, the value of each <code>iota</code> is the same because |
| it is only incremented after each ConstSpec: |
| </p> |
| |
| <pre> |
| const ( |
| bit0, mask0 = 1 << iota, 1 << iota - 1 // bit0 == 1, mask0 == 0 |
| bit1, mask1 // bit1 == 2, mask1 == 1 |
| _, _ // skips iota == 2 |
| bit3, mask3 // bit3 == 8, mask3 == 7 |
| ) |
| </pre> |
| |
| <p> |
| This last example exploits the implicit repetition 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 new type |
| that has the same <a href="#Types">underlying type</a> as |
| an existing type. The new type is <a href="#Type_identity">different</a> from |
| the existing type. |
| </p> |
| |
| <pre class="ebnf"> |
| TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) . |
| TypeSpec = identifier Type . |
| </pre> |
| |
| <pre> |
| type IntArray [16]int |
| |
| type ( |
| Point struct { x, y float64 } |
| Polar Point |
| ) |
| |
| type TreeNode struct { |
| left, right *TreeNode |
| value *Comparable |
| } |
| |
| type Cipher interface { |
| BlockSize() int |
| Encrypt(src, dst []byte) |
| Decrypt(src, dst []byte) |
| } |
| </pre> |
| |
| <p> |
| The declared type does not inherit any <a href="#Method_declarations">methods</a> |
| bound to the existing type, but the <a href="#Types">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 *PrintableMutex contains the methods |
| // Lock and Unlock bound to its anonymous field Mutex. |
| type PrintableMutex struct { |
| Mutex |
| } |
| |
| // MyCipher is an interface type that has the same method set as Cipher. |
| type MyCipher Cipher |
| </pre> |
| |
| <p> |
| A type declaration may be used to define a different boolean, numeric, or string |
| type and attach methods to it: |
| </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> |
| |
| |
| <h3 id="Variable_declarations">Variable declarations</h3> |
| |
| <p> |
| A variable declaration creates a variable, binds an identifier to it and |
| gives it a type and optionally 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 float |
| var k = 0 |
| var x, y float = -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 |
| by assigning the expressions to the variables (§<a href="#Assignments">Assignments</a>) |
| in order; all expressions must be consumed and all variables initialized from them. |
| Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>. |
| </p> |
| |
| <p> |
| If the type is present, each variable is given that type. |
| Otherwise, the types are deduced from the assignment |
| of the expression list. |
| </p> |
| |
| <p> |
| If the type is absent and the corresponding expression evaluates to an |
| untyped <a href="#Constants">constant</a>, the type of the declared variable |
| is <code>bool</code>, <code>int</code>, <code>float</code>, or <code>string</code> |
| respectively, depending on whether the value is a boolean, integer, |
| floating-point, or string constant: |
| </p> |
| |
| <pre> |
| var b = true // t has type bool |
| var i = 0 // i has type int |
| var f = 3.0 // f has type float |
| var s = "OMDB" // s has type string |
| </pre> |
| |
| <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 a shorthand for a regular variable declaration 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(fd) // os.Pipe() returns two values |
| _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate |
| </pre> |
| |
| <p> |
| Unlike regular variable declarations, a short variable declaration may redeclare variables provided they |
| were originally declared in the same block 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. |
| </p> |
| |
| <pre> |
| field1, offset := nextField(str, 0) |
| field2, offset := nextField(str, offset) // redeclares offset |
| </pre> |
| |
| <p> |
| Short variable declarations may appear only inside functions. |
| In some contexts such as the initializers for <code>if</code>, |
| <code>for</code>, or <code>switch</code> statements, |
| they can be used to declare local temporary variables (§<a href="#Statements">Statements</a>). |
| </p> |
| |
| <h3 id="Function_declarations">Function declarations</h3> |
| |
| <p> |
| A function declaration binds an identifier to a function (§<a href="#Function_types">Function types</a>). |
| </p> |
| |
| <pre class="ebnf"> |
| FunctionDecl = "func" identifier Signature [ Body ] . |
| Body = Block . |
| </pre> |
| |
| <p> |
| A function declaration may omit the body. Such a declaration provides the |
| signature for a function implemented outside Go, such as an assembly routine. |
| </p> |
| |
| <pre> |
| func min(x int, y int) int { |
| if x < y { |
| return x |
| } |
| return y |
| } |
| |
| func flushICache(begin, end uintptr) // implemented externally |
| </pre> |
| |
| <h3 id="Method_declarations">Method declarations</h3> |
| |
| <p> |
| A method is a function with a <i>receiver</i>. |
| A method declaration binds an identifier to a method. |
| </p> |
| <pre class="ebnf"> |
| MethodDecl = "func" Receiver MethodName Signature [ Body ] . |
| Receiver = "(" [ identifier ] [ "*" ] BaseTypeName ")" . |
| BaseTypeName = identifier . |
| </pre> |
| |
| <p> |
| The receiver type must be of the form <code>T</code> or <code>*T</code> where |
| <code>T</code> is a type name. <code>T</code> is called the |
| <i>receiver base type</i> or just <i>base type</i>. |
| The base type must not be a pointer or interface type and must be |
| declared in the same package as the method. |
| The method is said to be <i>bound</i> to the base type |
| and is visible only within selectors for that type |
| (§<a href="#Type_declarations">Type declarations</a>, §<a href="#Selectors">Selectors</a>). |
| </p> |
| |
| <p> |
| Given 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'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> |
| The type of a method is the type of a function with the receiver as first |
| argument. For instance, the method <code>Scale</code> has type |
| </p> |
| |
| <pre> |
| func(p *Point, factor float64) |
| </pre> |
| |
| <p> |
| However, a function declared this way is not a method. |
| </p> |
| |
| |
| <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. |
| </p> |
| |
| <pre class="ebnf"> |
| Operand = Literal | QualifiedIdent | MethodExpr | "(" Expression ")" . |
| Literal = BasicLit | CompositeLit | FunctionLit . |
| BasicLit = int_lit | float_lit | imaginary_lit | char_lit | string_lit . |
| </pre> |
| |
| |
| <h3 id="Qualified_identifiers">Qualified identifiers</h3> |
| |
| <p> |
| A qualified identifier is a non-<a href="#Blank_identifier">blank</a> identifier qualified by a package name prefix. |
| </p> |
| |
| <pre class="ebnf"> |
| QualifiedIdent = [ PackageName "." ] identifier . |
| </pre> |
| |
| <p> |
| A qualified identifier accesses an identifier in a separate package. |
| The identifier must be <a href="#Exported_identifiers">exported</a> by that |
| package, which means that it must begin with a Unicode upper case letter. |
| </p> |
| |
| <pre> |
| math.Sin |
| </pre> |
| |
| <!--- |
| <p> |
| <span class="alert">TODO: Unify this section with Selectors - it's the same syntax.</span> |
| </p> |
| ---> |
| |
| <h3 id="Composite_literals">Composite literals</h3> |
| |
| <p> |
| Composite literals construct values for structs, arrays, slices, and maps |
| and create a new value each time they are evaluated. |
| They consist of the type of the value |
| followed by a brace-bound list of composite elements. An element may be |
| a single expression or a key-value pair. |
| </p> |
| |
| <pre class="ebnf"> |
| CompositeLit = LiteralType LiteralValue . |
| LiteralType = StructType | ArrayType | "[" "..." "]" ElementType | |
| SliceType | MapType | TypeName . |
| LiteralValue = "{" [ ElementList [ "," ] ] "}" . |
| ElementList = Element { "," Element } . |
| Element = [ Key ":" ] Value . |
| Key = FieldName | ElementIndex . |
| FieldName = identifier . |
| ElementIndex = Expression . |
| Value = Expression | LiteralValue . |
| </pre> |
| |
| <p> |
| The LiteralType must be a struct, array, slice, or map type |
| (the grammar enforces this constraint except when the type is given |
| as a TypeName). |
| The types of the expressions must be <a href="#Assignability">assignable</a> |
| to the respective field, element, and key types of the LiteralType; |
| there is no additional conversion. |
| The key is interpreted as a field name for struct literals, |
| an index expression 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. |
| </p> |
| |
| <p> |
| For struct literals the following rules apply: |
| </p> |
| <ul> |
| <li>A key must be a field name declared in the LiteralType. |
| </li> |
| <li>A literal 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>A literal 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 constant integer expression. |
| </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> |
| Taking the address of a composite literal (§<a href="#Address_operators">Address operators</a>) |
| generates a unique pointer to an instance of the literal's value. |
| </p> |
| <pre> |
| var pointer *Point3D = &Point3D{y: 1000} |
| </pre> |
| |
| <p> |
| The length of an array literal is the length specified in the LiteralType. |
| 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 a shortcut for a slice operation applied to an array literal: |
| </p> |
| |
| <pre> |
| [n]T{x1, x2, ... xn}[0 : n] |
| </pre> |
| |
| <p> |
| Within a composite literal of array, slice, or map type <code>T</code>, |
| elements that are themselves composite literals may elide the respective |
| literal type if it is identical to the element type of <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}} |
| </pre> |
| |
| <p> |
| A parsing ambiguity arises when a composite literal using the |
| TypeName form of the LiteralType appears between the |
| <a href="#Keywords">keyword</a> and the opening brace of the block of an |
| "if", "for", or "switch" statement, because the braces surrounding |
| the expressions in the literal are confused with those introducing |
| the block of statements. To resolve the ambiguity in this rare case, |
| 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, 11, 13, 17, 19, 991} |
| |
| // 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]float{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1} |
| filter := [10]float{-1, 4: -0.1, -0.1, 9: -1} |
| |
| // frequencies in Hz for equal-tempered scale (A4 = 440Hz) |
| noteFrequency := map[string]float{ |
| "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 function. |
| It consists of a specification of the function type and a function body. |
| </p> |
| |
| <pre class="ebnf"> |
| FunctionLit = FunctionType Body . |
| </pre> |
| |
| <pre> |
| func(a, b int, z float) 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 } (reply_chan) |
| </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 | |
| BuiltinCall | |
| PrimaryExpr Selector | |
| PrimaryExpr Index | |
| PrimaryExpr Slice | |
| PrimaryExpr TypeAssertion | |
| PrimaryExpr Call . |
| |
| Selector = "." identifier . |
| Index = "[" Expression "]" . |
| Slice = "[" [ Expression ] ":" [ Expression ] "]" . |
| TypeAssertion = "." "(" Type ")" . |
| Call = "(" [ ArgumentList [ "," ] ] ")" . |
| ArgumentList = ExpressionList [ "..." ] . |
| </pre> |
| |
| |
| <pre> |
| x |
| 2 |
| (s + ".txt") |
| f(3.1415, true) |
| Point{1, 2} |
| m["foo"] |
| s[i : j + 1] |
| obj.color |
| math.Sin |
| f.p[i].x() |
| </pre> |
| |
| |
| <h3 id="Selectors">Selectors</h3> |
| |
| <p> |
| A primary expression of the form |
| </p> |
| |
| <pre> |
| x.f |
| </pre> |
| |
| <p> |
| denotes the field or method <code>f</code> of the value denoted by <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 expression is the type of <code>f</code>. |
| </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 anonymous field of |
| <code>T</code>. |
| The number of anonymous 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 anonymous 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 an 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 one <code>f</code> with shallowest depth, the selector |
| expression is illegal. |
| </li> |
| <li> |
| For a variable <code>x</code> of type <code>I</code> or <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 value assigned |
| to <code>x</code> if there is such a method. |
| If no value or <code>nil</code> was assigned to <code>x</code>, <code>x.f</code> is illegal. |
| </li> |
| <li> |
| In all other cases, <code>x.f</code> is illegal. |
| </li> |
| </ol> |
| <p> |
| Selectors automatically dereference pointers to structs. |
| If <code>x</code> is a pointer to a struct, <code>x.y</code> |
| is shorthand for <code>(*x).y</code>; if the field <code>y</code> |
| is also a pointer to a struct, <code>x.y.z</code> is shorthand |
| for <code>(*(*x).y).z</code>, and so on. |
| If <code>x</code> contains an anonymous field of type <code>*A</code>, |
| where <code>A</code> is also a struct type, |
| <code>x.f</code> is a shortcut for <code>(*x.A).f</code>. |
| </p> |
| <p> |
| For example, given the declarations: |
| </p> |
| |
| <pre> |
| type T0 struct { |
| x int |
| } |
| |
| func (recv *T0) M0() |
| |
| type T1 struct { |
| y int |
| } |
| |
| func (recv T1) M1() |
| |
| type T2 struct { |
| z int |
| T1 |
| *T0 |
| } |
| |
| func (recv *T2) M2() |
| |
| var p *T2 // with p != nil and p.T1 != nil |
| </pre> |
| |
| <p> |
| one may write: |
| </p> |
| |
| <pre> |
| p.z // (*p).z |
| p.y // ((*p).T1).y |
| p.x // (*(*p).T0).x |
| |
| p.M2 // (*p).M2 |
| p.M1 // ((*p).T1).M1 |
| p.M0 // ((*p).T0).M0 |
| </pre> |
| |
| |
| <!--- |
| <span class="alert"> |
| TODO: Specify what happens to receivers. |
| </span> |
| ---> |
| |
| |
| <h3 id="Indexes">Indexes</h3> |
| |
| <p> |
| A primary expression of the form |
| </p> |
| |
| <pre> |
| a[x] |
| </pre> |
| |
| <p> |
| denotes the element of the array, slice, string or map <code>a</code> indexed by <code>x</code>. |
| The value <code>x</code> is called the |
| <i>index</i> or <i>map key</i>, respectively. The following |
| rules apply: |
| </p> |
| |
| <p> |
| For <code>a</code> of type <code>A</code> or <code>*A</code> |
| where <code>A</code> is an <a href="#Array_types">array type</a>, |
| or for <code>a</code> of type <code>S</code> where <code>S</code> is a <a href="#Slice_types">slice type</a>: |
| </p> |
| <ul> |
| <li><code>x</code> must be an integer value and <code>0 <= x < len(a)</code></li> |
| <li><code>a[x]</code> is the array element at index <code>x</code> and the type of |
| <code>a[x]</code> is the element type of <code>A</code></li> |
| <li>if the index <code>x</code> is out of range, |
| a <a href="#Run_time_panics">run-time panic</a> occurs</li> |
| </ul> |
| |
| <p> |
| For <code>a</code> of type <code>T</code> |
| where <code>T</code> is a <a href="#String_types">string type</a>: |
| </p> |
| <ul> |
| <li><code>x</code> must be an integer value and <code>0 <= x < len(a)</code></li> |
| <li><code>a[x]</code> is the byte at index <code>x</code> and the type of |
| <code>a[x]</code> is <code>byte</code></li> |
| <li><code>a[x]</code> may not be assigned to</li> |
| <li>if the index <code>x</code> is out of range, |
| a <a href="#Run_time_panics">run-time panic</a> occurs</li> |
| </ul> |
| |
| <p> |
| For <code>a</code> of type <code>M</code> |
| where <code>M</code> is a <a href="#Map_types">map type</a>: |
| </p> |
| <ul> |
| <li><code>x</code>'s type must be |
| <a href="#Assignability">assignable</a> |
| to the key type of <code>M</code></li> |
| <li>if the map contains an entry with key <code>x</code>, |
| <code>a[x]</code> is the map value with key <code>x</code> |
| and the type of <code>a[x]</code> is the value type of <code>M</code></li> |
| <li>if the map does not contain such an entry, |
| <code>a[x]</code> is the <a href="#The_zero_value">zero value</a> |
| for the value type of <code>M</code></li> |
| </ul> |
| |
| <p> |
| Otherwise <code>a[x]</code> is illegal. |
| </p> |
| |
| <p> |
| An index expression on a map <code>a</code> of type <code>map[K]V</code> |
| may be used in an assignment or initialization of the special form |
| </p> |
| |
| <pre> |
| v, ok = a[x] |
| v, ok := a[x] |
| var v, ok = a[x] |
| </pre> |
| |
| <p> |
| where the result of the index expression is a pair of values with types |
| <code>(V, bool)</code>. In this form, the value of <code>ok</code> is |
| <code>true</code> if the key <code>x</code> is present in the map, and |
| <code>false</code> otherwise. The value of <code>v</code> is the value |
| <code>a[x]</code> as in the single-result form. |
| </p> |
| |
| <p> |
| Similarly, if an assignment to a map has the special form |
| </p> |
| |
| <pre> |
| a[x] = v, ok |
| </pre> |
| |
| <p> |
| and boolean <code>ok</code> has the value <code>false</code>, |
| the entry for key <code>x</code> is deleted from the map; if |
| <code>ok</code> is <code>true</code>, the construct acts like |
| a regular assignment to an element of the map. |
| </p> |
| |
| |
| <h3 id="Slices">Slices</h3> |
| |
| <p> |
| For a string, array, or slice <code>a</code>, the primary expression |
| </p> |
| |
| <pre> |
| a[low : high] |
| </pre> |
| |
| <p> |
| constructs a substring or slice. The index expressions <code>low</code> and |
| <code>high</code> select which elements appear in the result. The result has |
| indexes starting at 0 and length equal to |
| <code>high</code> - <code>low</code>. |
| After slicing the array <code>a</code> |
| </p> |
| |
| <pre> |
| a := [5]int{1, 2, 3, 4, 5} |
| s := a[1:4] |
| </pre> |
| |
| <p> |
| the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements |
| </p> |
| |
| <pre> |
| s[0] == 2 |
| s[1] == 3 |
| s[2] == 4 |
| </pre> |
| |
| <p> |
| For convenience, any of the index expressions may be omitted. A missing <code>low</code> |
| index defaults to zero; a missing <code>high</code> index defaults to the length of the |
| sliced operand: |
| </p> |
| |
| <pre> |
| a[2:] // same a[2 : len(a)] |
| a[:3] // same as a[0 : 3] |
| a[:] // same as a[0 : len(a)] |
| </pre> |
| |
| <p> |
| For arrays or strings, the indexes <code>low</code> and <code>high</code> must |
| satisfy 0 <= <code>low</code> <= <code>high</code> <= length; for |
| slices, the upper bound is the capacity rather than the length. |
| </p> |
| |
| <p> |
| If the sliced operand is a string or slice, the result of the slice operation |
| is a string or slice of the same type. |
| If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a> |
| and the result of the slice operation is a slice with the same element type as the array. |
| </p> |
| |
| |
| <h3 id="Type_assertions">Type assertions</h3> |
| |
| <p> |
| For an expression <code>x</code> of <a href="#Interface_types">interface type</a> |
| and a type <code>T</code>, the primary expression |
| </p> |
| |
| <pre> |
| x.(T) |
| </pre> |
| |
| <p> |
| asserts that <code>x</code> is not <code>nil</code> |
| and that the value stored in <code>x</code> is of type <code>T</code>. |
| The notation <code>x.(T)</code> is called a <i>type assertion</i>. |
| </p> |
| <p> |
| More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts |
| that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a> |
| to the type <code>T</code>. |
| If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type |
| of <code>x</code> implements the interface <code>T</code> (§<a href="#Interface_types">Interface types</a>). |
| </p> |
| <p> |
| If the type assertion holds, the value of the expression is the value |
| stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false, |
| a <a href="#Run_time_panics">run-time panic</a> occurs. |
| In other words, even though the dynamic type of <code>x</code> |
| is known only at run-time, the type of <code>x.(T)</code> is |
| known to be <code>T</code> in a correct program. |
| </p> |
| <p> |
| If a type assertion is used in an assignment or initialization of the form |
| </p> |
| |
| <pre> |
| v, ok = x.(T) |
| v, ok := x.(T) |
| var v, ok = x.(T) |
| </pre> |
| |
| <p> |
| the result of the assertion is a pair of values with types <code>(T, bool)</code>. |
| If the assertion holds, the expression returns the pair <code>(x.(T), true)</code>; |
| otherwise, the expression returns <code>(Z, false)</code> where <code>Z</code> |
| is the <a href="#The_zero_value">zero value</a> for type <code>T</code>. |
| No run-time panic occurs in this case. |
| The type assertion in this construct thus acts like a function call |
| returning a value and a boolean indicating success. (§<a href="#Assignments">Assignments</a>) |
| </p> |
| |
| |
| <h3 id="Calls">Calls</h3> |
| |
| <p> |
| Given an expression <code>f</code> of function type |
| <code>F</code>, |
| </p> |
| |
| <pre> |
| f(a1, a2, ... an) |
| </pre> |
| |
| <p> |
| calls <code>f</code> with arguments <code>a1, a2, ... an</code>. |
| Except for one special case, arguments must be single-valued expressions |
| <a href="#Assignability">assignable</a> to the parameter types of |
| <code>F</code> and are evaluated before the function is called. |
| The type of the expression is the result type |
| of <code>F</code>. |
| A method invocation is similar but the method itself |
| is specified as a selector upon a value of the receiver type for |
| the method. |
| </p> |
| |
| <pre> |
| math.Atan2(x, y) // function call |
| var pt *Point |
| pt.Scale(3.5) // method call with receiver pt |
| </pre> |
| |
| <p> |
| As a special case, if the return parameters of a function or method |
| <code>g</code> are equal in number and individually |
| assignable to the parameters of another function or method |
| <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code> |
| will invoke <code>f</code> after binding the return values of |
| <code>g</code> to the parameters of <code>f</code> in order. The call |
| of <code>f</code> must contain no parameters other than the call of <code>g</code>. |
| If <code>f</code> has a final <code>...</code> parameter, it is |
| assigned the return values of <code>g</code> that remain after |
| assignment of regular parameters. |
| </p> |
| |
| <pre> |
| func Split(s string, pos int) (string, string) { |
| return s[0:pos], s[pos:] |
| } |
| |
| func Join(s, t string) string { |
| return s + t |
| } |
| |
| if Join(Split(value, len(value)/2)) != value { |
| log.Crash("test fails") |
| } |
| </pre> |
| |
| <p> |
| A method call <code>x.m()</code> is valid if the method set of |
| (the type of) <code>x</code> contains <code>m</code> and the |
| argument list can be assigned to the parameter list of <code>m</code>. |
| If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method |
| set contains <code>m</code>, <code>x.m()</code> is shorthand |
| for <code>(&x).m()</code>: |
| </p> |
| |
| <pre> |
| var p Point |
| p.Scale(3.5) |
| </pre> |
| |
| <p> |
| There is no distinct method type and there are no method literals. |
| </p> |
| |
| <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3> |
| |
| <p> |
| If <code>f</code> is variadic with final parameter type <code>...T</code>, |
| then within the function the argument is equivalent to a parameter of type |
| <code>[]T</code>. At each call of <code>f</code>, the argument |
| passed to the final parameter is |
| a new slice of type <code>[]T</code> whose successive elements are |
| the actual arguments, which all must be <a href="#Assignability">assignable</a> |
| to the type <code>T</code>. The length of the slice is therefore the number of |
| arguments bound to the final parameter and may differ for each call site. |
| </p> |
| |
| <p> |
| Given the function and call |
| </p> |
| <pre> |
| func Greeting(prefix string, who ... string) |
| Greeting("hello:", "Joe", "Anna", "Eileen") |
| </pre> |
| |
| <p> |
| within <code>Greeting</code>, <code>who</code> will have the value |
| <code>[]string{"Joe", "Anna", "Eileen"}</code> |
| </p> |
| |
| <p> |
| If the final argument is assignable to a slice type <code>[]T</code>, it may be |
| passed unchanged as the value for a <code>...T</code> parameter if the argument |
| is followed by <code>...</code>. In this case no new slice is created. |
| </p> |
| |
| <p> |
| Given the slice <code>s</code> and call |
| </p> |
| |
| <pre> |
| s := []string{"James", "Jasmine"} |
| Greeting("goodbye:", s...) |
| </pre> |
| |
| <p> |
| within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code> |
| with the same underlying array. |
| </p> |
| |
| |
| <h3 id="Operators">Operators</h3> |
| |
| <p> |
| Operators combine operands into expressions. |
| </p> |
| |
| <pre class="ebnf"> |
| Expression = UnaryExpr | Expression binary_op UnaryExpr . |
| UnaryExpr = PrimaryExpr | unary_op UnaryExpr . |
| |
| binary_op = log_op | com_op | rel_op | add_op | mul_op . |
| log_op = "||" | "&&" . |
| com_op = "<-" . |
| rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" . |
| add_op = "+" | "-" | "|" | "^" . |
| mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" . |
| |
| unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" . |
| </pre> |
| |
| <p> |
| Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>. |
| For other binary operators, the operand types must be <a href="#Type_identity">identical</a> |
| unless the operation involves channels, shifts, or untyped <a href="#Constants">constants</a>. |
| For operations involving constants only, see the section on |
| <a href="#Constant_expressions">constant expressions</a>. |
| </p> |
| |
| <p> |
| In a channel send, the first operand is always a channel and the second |
| must be a value <a href="#Assignability">assignable</a> |
| to the channel's element type. |
| </p> |
| |
| <p> |
| Except for shift operations, |
| if one operand is an untyped <a href="#Constants">constant</a> |
| and the other operand is not, the constant is <a href="#Conversions">converted</a> |
| to the type of the other operand. |
| </p> |
| |
| <p> |
| The right operand in a shift operation must have unsigned integer type |
| or be an untyped constant that can be converted to unsigned integer type. |
| </p> |
| |
| <p> |
| If the left operand of a non-constant shift operation is an untyped constant, |
| the type of constant is what it would be if the shift operation were replaced by |
| the left operand alone. |
| </p> |
| |
| <pre> |
| var s uint = 33 |
| var i = 1<<s // 1 has type int |
| var j = int32(1<<s) // 1 has type int32; j == 0 |
| var u = uint64(1<<s) // 1 has type uint64; u == 1<<33 |
| var f = float(1<<s) // illegal: 1 has type float, cannot shift |
| var g = float(1<<33) // legal; 1<<33 is a constant shift operation; g == 1<<33 |
| </pre> |
| |
| <h3 id="Operator_precedence">Operator precedence</h3> |
| <p> |
| Unary operators have the highest precedence. |
| As the <code>++</code> and <code>--</code> operators form |
| statements, not expressions, they fall |
| outside the operator hierarchy. |
| As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>. |
| <p> |
| There are six precedence levels for binary operators. |
| Multiplication operators bind strongest, followed by addition |
| operators, comparison operators, <code><-</code> (channel send), |
| <code>&&</code> (logical and), and finally <code>||</code> (logical or): |
| </p> |
| |
| <pre class="grammar"> |
| Precedence Operator |
| 6 * / % << >> & &^ |
| 5 + - | ^ |
| 4 == != < <= > >= |
| 3 <- |
| 2 && |
| 1 || |
| </pre> |
| |
| <p> |
| Binary operators of the same precedence associate from left to right. |
| For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>. |
| </p> |
| |
| <pre> |
| +x |
| 23 + 3*x[i] |
| x <= f() |
| ^a >> b |
| f() || g() |
| x == y+1 && <-chan_ptr > 0 |
| </pre> |
| |
| |
| <h3 id="Arithmetic_operators">Arithmetic operators</h3> |
| <p> |
| Arithmetic operators apply to numeric values and yield a result of the same |
| type as the first operand. The four standard arithmetic operators (<code>+</code>, |
| <code>-</code>, <code>*</code>, <code>/</code>) apply to integer, |
| floating-point, and complex types; <code>+</code> also applies |
| to strings. All other arithmetic operators apply to integers only. |
| </p> |
| |
| <pre class="grammar"> |
| + sum integers, floats, complex values, strings |
| - difference integers, floats, complex values |
| * product integers, floats, complex values |
| / quotient integers, floats, complex values |
| % remainder integers |
| |
| & bitwise and integers |
| | bitwise or integers |
| ^ bitwise xor integers |
| &^ bit clear (and not) integers |
| |
| << left shift integer << unsigned integer |
| >> right shift integer >> unsigned integer |
| </pre> |
| |
| <p> |
| Strings can be concatenated using the <code>+</code> operator |
| or the <code>+=</code> assignment operator: |
| </p> |
| |
| <pre> |
| s := "hi" + string(c) |
| s += " and good bye" |
| </pre> |
| |
| <p> |
| String addition creates a new string by concatenating the operands. |
| </p> |
| <p> |
| For integer values, <code>/</code> and <code>%</code> satisfy the following relationship: |
| </p> |
| |
| <pre> |
| (a / b) * b + a % b == a |
| </pre> |
| |
| <p> |
| with <code>(a / b)</code> truncated towards zero. |
| </p> |
| |
| <pre> |
| x y x / y x % y |
| 5 3 1 2 |
| -5 3 -1 -2 |
| 5 -3 -1 2 |
| -5 -3 1 -2 |
| </pre> |
| |
| <p> |
| If the divisor is zero, a <a href="#Run_time_panics">run-time panic</a> occurs. |
| If the dividend is positive and the divisor is a constant power of 2, |
| the division may be replaced by a right shift, and computing the remainder may |
| be replaced by a bitwise "and" operation: |
| </p> |
| |
| <pre> |
| x x / 4 x % 4 x >> 2 x & 3 |
| 11 2 3 2 3 |
| -11 -2 -3 -3 1 |
| </pre> |
| |
| <p> |
| The shift operators shift the left operand by the shift count specified by the |
| right operand. They implement arithmetic shifts if the left operand is a signed |
| integer and logical shifts if it is an unsigned integer. The shift count must |
| be an unsigned integer. There is no upper limit on the shift count. Shifts behave |
| as if the left operand is shifted <code>n</code> times by 1 for a shift |
| count of <code>n</code>. |
| As a result, <code>x << 1</code> is the same as <code>x*2</code> |
| and <code>x >> 1</code> is the same as |
| <code>x/2</code> but truncated towards negative infinity. |
| </p> |
| |
| <p> |
| For integer operands, the unary operators |
| <code>+</code>, <code>-</code>, and <code>^</code> are defined as |
| follows: |
| </p> |
| |
| <pre class="grammar"> |
| +x is 0 + x |
| -x negation is 0 - x |
| ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x |
| and m = -1 for signed x |
| </pre> |
| |
| <p> |
| For floating-point numbers, |
| <code>+x</code> is the same as <code>x</code>, |
| while <code>-x</code> is the negation of <code>x</code>. |
| The result of a floating-point division by zero is not specified beyond the |
| IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a> |
| occurs is implementation-specific. |
| </p> |
| |
| <h3 id="Integer_overflow">Integer overflow</h3> |
| |
| <p> |
| For unsigned integer values, the operations <code>+</code>, |
| <code>-</code>, <code>*</code>, and <code><<</code> are |
| computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of |
| the unsigned integer's type |
| (§<a href="#Numeric_types">Numeric types</a>). Loosely speaking, these unsigned integer operations |
| discard high bits upon overflow, and programs may rely on ``wrap around''. |
| </p> |
| <p> |
| For signed integers, the operations <code>+</code>, |
| <code>-</code>, <code>*</code>, and <code><<</code> may legally |
| overflow and the resulting value exists and is deterministically defined |
| by the signed integer representation, the operation, and its operands. |
| No exception is raised as a result of overflow. A |
| compiler may not optimize code under the assumption that overflow does |
| not occur. For instance, it may not assume that <code>x < x + 1</code> is always true. |
| </p> |
| |
| |
| <h3 id="Comparison_operators">Comparison operators</h3> |
| |
| <p> |
| Comparison operators compare two operands and yield a value of type <code>bool</code>. |
| </p> |
| |
| <pre class="grammar"> |
| == equal |
| != not equal |
| < less |
| <= less or equal |
| > greater |
| >= greater or equal |
| </pre> |
| |
| <p> |
| The operands must be <i>comparable</i>; that is, the first operand |
| must be <a href="#Assignability">assignable</a> |
| to the type of the second operand, or vice versa. |
| </p> |
| <p> |
| The operators <code>==</code> and <code>!=</code> apply |
| to operands of all types except arrays and structs. |
| All other comparison operators apply only to integer, floating-point |
| and string values. The result of a comparison is defined as follows: |
| </p> |
| |
| <ul> |
| <li> |
| Integer values are compared in the usual way. |
| </li> |
| <li> |
| Floating point values are compared as defined by the IEEE-754 |
| standard. |
| </li> |
| <li> |
| Two complex values <code>u</code>, <code>v</code> are |
| equal if both <code>real(u) == real(v)</code> and |
| <code>imag(u) == imag(v)</code>. |
| </li> |
| <li> |
| String values are compared byte-wise (lexically). |
| </li> |
| <li> |
| Boolean values are equal if they are either both |
| <code>true</code> or both <code>false</code>. |
| </li> |
| <li> |
| Pointer values are equal if they point to the same location |
| or if both are <code>nil</code>. |
| </li> |
| <li> |
| Function values are equal if they refer to the same function |
| or if both are <code>nil</code>. |
| </li> |
| <li> |
| A slice value may only be compared to <code>nil</code>. |
| </li> |
| <li> |
| Channel and map values are equal if they were created by the same call to <code>make</code> |
| (§<a href="#Making_slices_maps_and_channels">Making slices, maps, and channels</a>) |
| or if both are <code>nil</code>. |
| </li> |
| <li> |
| Interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types and |
| equal dynamic values or if both are <code>nil</code>. |
| </li> |
| <li> |
| An interface value <code>x</code> is equal to a non-interface value |
| <code>y</code> if the dynamic type of <code>x</code> is identical to |
| the static type of <code>y</code> and the dynamic value of <code>x</code> |
| is equal to <code>y</code>. |
| </li> |
| <li> |
| A pointer, function, slice, channel, map, or interface value is equal |
| to <code>nil</code> if it has been assigned the explicit value |
| <code>nil</code>, if it is uninitialized, or if it has been assigned |
| another value equal to <code>nil</code>. |
| </li> |
| </ul> |
| |
| |
| <h3 id="Logical_operators">Logical operators</h3> |
| |
| <p> |
| Logical operators apply to <a href="#Boolean_types">boolean</a> values |
| and yield a result of the same type as the operands. |
| The right operand is evaluated conditionally. |
| </p> |
| |
| <pre class="grammar"> |
| && conditional and p && q is "if p then q else false" |
| || conditional or p || q is "if p then true else q" |
| ! not !p is "not p" |
| </pre> |
| |
| |
| <h3 id="Address_operators">Address operators</h3> |
| |
| <p> |
| The address-of operator <code>&</code> generates the address of its operand, |
| which must be <i>addressable</i>, |
| that is, either a variable, pointer indirection, or slice indexing |
| operation; |
| or a field selector of an addressable struct operand; |
| or an array indexing operation of an addressable array. |
| Given an operand of pointer type, the pointer indirection |
| operator <code>*</code> retrieves the value pointed |
| to by the operand. |
| </p> |
| |
| <pre> |
| &x |
| &a[f(2)] |
| *p |
| *pf(x) |
| </pre> |
| |
| <h3 id="Communication_operators">Communication operators</h3> |
| |
| <p> |
| The term <i>channel</i> means "value of <a href="#Channel_types">channel type</a>". |
| </p> |
| <p> |
| The send operation uses the binary operator "<-", which operates on |
| a channel and a value (expression): |
| </p> |
| |
| <pre> |
| ch <- 3 |
| </pre> |
| |
| <p> |
| The send operation sends the value on the channel. Both the channel |
| and the expression are evaluated before communication begins. |
| Communication blocks until the send can proceed, at which point the |
| value is transmitted on the channel. |
| A send on an unbuffered channel can proceed if a receiver is ready. |
| A send on a buffered channel can proceed if there is room in the buffer. |
| </p> |
| <p> |
| If the send operation appears in an expression context, the value |
| of the expression is a boolean and the operation is non-blocking. |
| The value of the boolean reports true if the communication succeeded, |
| false if it did not. (The channel and |
| the expression to be sent are evaluated regardless.) |
| These two examples are equivalent: |
| </p> |
| |
| <pre> |
| ok := ch <- 3 |
| if ok { print("sent") } else { print("not sent") } |
| |
| if ch <- 3 { print("sent") } else { print("not sent") } |
| </pre> |
| |
| <p> |
| In other words, if the program tests the value of a send operation, |
| the send is non-blocking and the value of the expression is the |
| success of the operation. If the program does not test the value, |
| the operation blocks until it succeeds. |
| </p> |
| <p> |
| The receive operation uses the prefix unary operator "<-". |
| The value of the expression is the value received, whose type |
| is the element type of the channel. |
| </p> |
| |
| <pre> |
| <-ch |
| </pre> |
| |
| <p> |
| The expression blocks until a value is available, which then can |
| be assigned to a variable or used like any other expression. |
| If the receive expression does not save the value, the value is |
| discarded. |
| </p> |
| |
| <pre> |
| v1 := <-ch |
| v2 = <-ch |
| f(<-ch) |
| <-strobe // wait until clock pulse |
| </pre> |
| |
| <p> |
| If a receive expression is used in an assignment or initialization of the form |
| </p> |
| |
| <pre> |
| x, ok = <-ch |
| x, ok := <-ch |
| var x, ok = <-ch |
| </pre> |
| |
| <p> |
| the receive operation becomes non-blocking. |
| If the operation can proceed, the boolean variable |
| <code>ok</code> will be set to <code>true</code> |
| and the value stored in <code>x</code>; otherwise |
| <code>ok</code> is set |
| to <code>false</code> and <code>x</code> is set to the |
| zero value for its type (§<a href="#The_zero_value">The zero value</a>). |
| </p> |
| |
| <p> |
| Except in a communications clause of a <a href="#Select_statements">select statement</a>, |
| sending or receiving from a <code>nil</code> channel causes a |
| <a href="#Run_time_panics">run-time panic</a>. |
| </p> |
| |
| <!--- |
| <p> |
| <span class="alert">TODO: Probably in a separate section, communication semantics |
| need to be presented regarding send, receive, select, and goroutines.</span> |
| </p> |
| ---> |
| |
| <h3 id="Method_expressions">Method expressions</h3> |
| |
| <p> |
| If <code>M</code> is in the method set 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 = TypeName | "(" "*" TypeName ")" . |
| </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 float) float { 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 three invocations are equivalent: |
| </p> |
| |
| <pre> |
| t.Mv(7) |
| T.Mv(t, 7) |
| f := T.Mv; f(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 float) float |
| </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">closure</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="Conversions">Conversions</h3> |
| |
| <p> |
| Conversions are expressions of the form <code>T(x)</code> |
| where <code>T</code> is a type and <code>x</code> is an expression |
| that can be converted to type <code>T</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| Conversion = Type "(" Expression ")" . |
| </pre> |
| |
| <p> |
| If the type starts with an operator it must be parenthesized: |
| </p> |
| |
| <pre> |
| *Point(p) // same as *(Point(p)) |
| (*Point)(p) // p is converted to (*Point) |
| <-chan int(c) // same as <-(chan int(c)) |
| (<-chan int)(c) // c is converted to (<-chan int) |
| </pre> |
| |
| <p> |
| A value <code>x</code> can be converted to type <code>T</code> in any |
| of these cases: |
| </p> |
| |
| <ul> |
| <li> |
| <code>x</code> is <a href="#Assignability">assignable</a> |
| to <code>T</code>. |
| </li> |
| <li> |
| <code>x</code>'s type and <code>T</code> have identical |
| <a href="#Types">underlying types</a>. |
| </li> |
| <li> |
| <code>x</code>'s type and <code>T</code> are unnamed pointer types |
| and their pointer base types have identical underlying types. |
| </li> |
| <li> |
| <code>x</code>'s type and <code>T</code> are both integer or floating |
| point types. |
| </li> |
| <li> |
| <code>x</code>'s type and <code>T</code> are both complex types. |
| </li> |
| <li> |
| <code>x</code> is an integer or has type <code>[]byte</code> or |
| <code>[]int</code> and <code>T</code> is a string type. |
| </li> |
| <li> |
| <code>x</code> is a string and <code>T</code> is <code>[]byte</code> or |
| <code>[]int</code>. |
| </li> |
| </ul> |
| |
| <p> |
| Specific rules apply to conversions between numeric types or to and from |
| a string type. |
| These conversions may change the representation of <code>x</code> |
| and incur a run-time cost. |
| All other conversions only change the type but not the representation |
| of <code>x</code>. |
| </p> |
| |
| <h4>Conversions between numeric types</h4> |
| <ol> |
| <li> |
| When converting between integer types, if the value is a signed integer, it is |
| sign extended to implicit infinite precision; otherwise it is zero extended. |
| It is then truncated to fit in the result type's size. |
| For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>. |
| The conversion always yields a valid value; there is no indication of overflow. |
| </li> |
| <li> |
| When converting a floating-point number to an integer, the fraction is discarded |
| (truncation towards zero). |
| </li> |
| <li> |
| When converting an integer or floating-point number to a floating-point type, |
| or a complex number to another complex type, the result value is rounded |
| to the precision specified by the destination type. |
| For instance, the value of a variable <code>x</code> of type <code>float32</code> |
| may be stored using additional precision beyond that of an IEEE-754 32-bit number, |
| but float32(x) represents the result of rounding <code>x</code>'s value to |
| 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits |
| of precision, but <code>float32(x + 0.1)</code> does not. |
| </li> |
| </ol> |
| |
| <p> |
| In all conversions involving floating-point or complex values, |
| if the result type cannot represent the value the conversion |
| succeeds but the result value is |
| implementation-dependent. |
| </p> |
| |
| <h4>Conversions to and from a string type</h4> |
| |
| <ol> |
| <li> |
| Converting a signed or unsigned integer value to a string type yields a |
| string containing the UTF-8 representation of the integer. Values outside |
| the range of valid Unicode code points are converted to <code>"\uFFFD"</code>. |
| |
| <pre> |
| string('a') // "a" |
| string(-1) // "\ufffd" == "\xef\xbf\xbd " |
| string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8" |
| type MyString string |
| MyString(0x65e5) // "\u65e5" == "æ—¥" == "\xe6\x97\xa5" |
| </pre> |
| </li> |
| |
| <li> |
| Converting a value of type <code>[]byte</code> (or |
| the equivalent <code>[]uint8</code>) to a string type yields a |
| string whose successive bytes are the elements of the slice. If |
| the slice value is <code>nil</code>, the result is the empty string. |
| |
| <pre> |
| string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø" |
| </pre> |
| </li> |
| |
| <li> |
| Converting a value of type <code>[]int</code> to a string type yields |
| a string that is the concatenation of the individual integers |
| converted to strings. If the slice value is <code>nil</code>, the |
| result is the empty string. |
| <pre> |
| string([]int{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔" |
| </pre> |
| </li> |
| |
| <li> |
| Converting a value of a string type to <code>[]byte</code> (or <code>[]uint8</code>) |
| yields a slice whose successive elements are the bytes of the string. |
| If the string is empty, the result is <code>[]byte(nil)</code>. |
| |
| <pre> |
| []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'} |
| </pre> |
| </li> |
| |
| <li> |
| Converting a value of a string type to <code>[]int</code> yields a |
| slice containing the individual Unicode code points of the string. |
| If the string is empty, the result is <code>[]int(nil)</code>. |
| <pre> |
| []int(MyString("白鵬翔")) // []int{0x767d, 0x9d6c, 0x7fd4} |
| </pre> |
| </li> |
| </ol> |
| |
| <p> |
| There is no linguistic mechanism to convert between pointers and integers. |
| The package <a href="#Package_unsafe"><code>unsafe</code></a> |
| implements this functionality under |
| restricted circumstances. |
| </p> |
| |
| <h3 id="Constant_expressions">Constant expressions</h3> |
| |
| <p> |
| Constant expressions may contain only <a href="#Constants">constant</a> |
| operands and are evaluated at compile-time. |
| </p> |
| |
| <p> |
| Untyped boolean, numeric, and string constants may be used as operands |
| wherever it is legal to use an operand of boolean, numeric, or string type, |
| respectively. Except for shift operations, if the operands of a binary operation |
| are an untyped integer constant and an untyped floating-point constant, |
| the integer constant is converted to an untyped floating-point constant |
| (relevant for <code>/</code> and <code>%</code>). |
| Similarly, |
| untyped integer or floating-point constants may be used as operands |
| wherever it is legal to use an operand of complex type; |
| the integer or floating point constant is converted to a |
| complex constant with a zero imaginary part. |
| </p> |
| |
| <p> |
| Applying an operator to untyped constants results in an untyped |
| constant of the same kind (that is, a boolean, integer, floating-point, |
| complex, or string constant), except for |
| <a href="#Comparison_operators">comparison operators</a>, which result in |
| a constant of type <code>bool</code>. |
| </p> |
| |
| <p> |
| Imaginary literals are untyped complex constants (with zero real part) |
| and may be combined in binary |
| operations with untyped integer and floating-point constants; the |
| result is an untyped complex constant. |
| Complex constants are always constructed from |
| constant expressions involving imaginary |
| literals or constants derived from them, or calls of the built-in function |
| <a href="#Complex_numbers"><code>cmplx</code></a>. |
| </p> |
| |
| <pre> |
| const Σ = 1 - 0.707i |
| const Δ = Σ + 2.0e-4 - 1/1i |
| const Φ = iota * 1i |
| const iΓ = cmplx(0, Γ) |
| </pre> |
| |
| <p> |
| Constant expressions are always evaluated exactly; intermediate values and the |
| constants themselves may require precision significantly larger than supported |
| by any predeclared type in the language. The following are legal declarations: |
| </p> |
| |
| <pre> |
| const Huge = 1 << 100 |
| const Four int8 = Huge >> 98 |
| </pre> |
| |
| <p> |
| The values of <i>typed</i> constants must always be accurately representable as values |
| of the constant type. The following constant expressions are illegal: |
| </p> |
| |
| <pre> |
| uint(-1) // -1 cannot be represented as a uint |
| int(3.14) // 3.14 cannot be represented as an int |
| int64(Huge) // 1<<100 cannot be represented as an int64 |
| Four * 300 // 300 cannot be represented as an int8 |
| Four * 100 // 400 cannot be represented as an int8 |
| </pre> |
| |
| <p> |
| The mask used by the unary bitwise complement operator <code>^</code> matches |
| the rule for non-constants: the mask is all 1s for unsigned constants |
| and -1 for signed and untyped constants. |
| </p> |
| |
| <pre> |
| ^1 // untyped integer constant, equal to -2 |
| uint8(^1) // error, same as uint8(-2), out of range |
| ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE) |
| int8(^1) // same as int8(-2) |
| ^int8(1) // same as -1 ^ int8(1) = -2 |
| </pre> |
| |
| <!--- |
| <p> |
| <span class="alert"> |
| TODO: perhaps ^ should be disallowed on non-uints instead of assuming twos complement. |
| Also it may be possible to make typed constants more like variables, at the cost of fewer |
| overflow etc. errors being caught. |
| </span> |
| </p> |
| ---> |
| |
| <h3 id="Order_of_evaluation">Order of evaluation</h3> |
| |
| <p> |
| When evaluating the elements of an assignment or expression, |
| all function calls, method calls and |
| communication operations are evaluated in lexical left-to-right |
| order. |
| </p> |
| |
| <p> |
| For example, in the assignment |
| </p> |
| <pre> |
| y[f()], ok = g(h(), i() + x[j()], <-c), k() |
| </pre> |
| <p> |
| the function calls and communication happen in the order |
| <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>, |
| <code><-c</code>, <code>g()</code>, and <code>k()</code>. |
| However, the order of those events compared to the evaluation |
| and indexing of <code>x</code> and the evaluation |
| of <code>y</code> is not specified. |
| </p> |
| |
| <p> |
| Floating-point operations within a single expression are evaluated according to |
| the associativity of the operators. Explicit parentheses affect the evaluation |
| by overriding the default associativity. |
| In the expression <code>x + (y + z)</code> the addition <code>y + z</code> |
| is performed before adding <code>x</code>. |
| </p> |
| |
| <h2 id="Statements">Statements</h2> |
| |
| <p> |
| Statements control execution. |
| </p> |
| |
| <pre class="ebnf"> |
| Statement = |
| Declaration | LabeledStmt | SimpleStmt | |
| GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt | |
| FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt | |
| DeferStmt . |
| |
| SimpleStmt = EmptyStmt | ExpressionStmt | IncDecStmt | Assignment | ShortVarDecl . |
| </pre> |
| |
| |
| <h3 id="Empty_statements">Empty statements</h3> |
| |
| <p> |
| The empty statement does nothing. |
| </p> |
| |
| <pre class="ebnf"> |
| EmptyStmt = . |
| </pre> |
| |
| |
| <h3 id="Labeled_statements">Labeled statements</h3> |
| |
| <p> |
| A labeled statement may be the target of a <code>goto</code>, |
| <code>break</code> or <code>continue</code> statement. |
| </p> |
| |
| <pre class="ebnf"> |
| LabeledStmt = Label ":" Statement . |
| Label = identifier . |
| </pre> |
| |
| <pre> |
| Error: log.Crash("error encountered") |
| </pre> |
| |
| |
| <h3 id="Expression_statements">Expression statements</h3> |
| |
| <p> |
| Function calls, method calls, and channel operations |
| can appear in statement context. |
| </p> |
| |
| |
| <pre class="ebnf"> |
| ExpressionStmt = Expression . |
| </pre> |
| |
| <pre> |
| f(x+y) |
| <-ch |
| </pre> |
| |
| |
| <h3 id="IncDec_statements">IncDec statements</h3> |
| |
| <p> |
| The "++" and "--" statements increment or decrement their operands |
| by the untyped <a href="#Constants">constant</a> <code>1</code>. |
| As with an assignment, the operand must be <a href="#Address_operators">addressable</a> |
| or a map index expression. |
| </p> |
| |
| <pre class="ebnf"> |
| IncDecStmt = Expression ( "++" | "--" ) . |
| </pre> |
| |
| <p> |
| The following <a href="#Assignments">assignment statements</a> are semantically |
| equivalent: |
| </p> |
| |
| <pre class="grammar"> |
| IncDec statement Assignment |
| x++ x += 1 |
| x-- x -= 1 |
| </pre> |
| |
| |
| <h3 id="Assignments">Assignments</h3> |
| |
| <pre class="ebnf"> |
| Assignment = ExpressionList assign_op ExpressionList . |
| |
| assign_op = [ add_op | mul_op ] "=" . |
| </pre> |
| |
| <p> |
| Each left-hand side operand must be <a href="#Address_operators">addressable</a>, |
| a map index expression, |
| or the <a href="#Blank_identifier">blank identifier</a>. |
| </p> |
| |
| <pre> |
| x = 1 |
| *p = f() |
| a[i] = 23 |
| k = <-ch |
| </pre> |
| |
| <p> |
| An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code> |
| <code>y</code> where <i>op</i> is a binary arithmetic operation is equivalent |
| to <code>x</code> <code>=</code> <code>x</code> <i>op</i> |
| <code>y</code> but evaluates <code>x</code> |
| only once. The <i>op</i><code>=</code> construct is a single token. |
| In assignment operations, both the left- and right-hand expression lists |
| must contain exactly one single-valued expression. |
| </p> |
| |
| <pre> |
| a[i] <<= 2 |
| i &^= 1<<n |
| </pre> |
| |
| <p> |
| A tuple assignment assigns the individual elements of a multi-valued |
| operation to a list of variables. There are two forms. In the |
| first, the right hand operand is a single multi-valued expression |
| such as a function evaluation or <a href="#Channel_types">channel</a> or |
| <a href="#Map_types">map</a> operation or a <a href="#Type_assertions">type assertion</a>. |
| The number of operands on the left |
| hand side must match the number of values. For instance, if |
| <code>f</code> is a function returning two values, |
| </p> |
| |
| <pre> |
| x, y = f() |
| </pre> |
| |
| <p> |
| assigns the first value to <code>x</code> and the second to <code>y</code>. |
| The <a href="#Blank_identifier">blank identifier</a> provides a |
| way to ignore values returned by a multi-valued expression: |
| </p> |
| |
| <pre> |
| x, _ = f() // ignore second value returned by f() |
| </pre> |
| |
| <p> |
| In the second form, the number of operands on the left must equal the number |
| of expressions on the right, each of which must be single-valued, and the |
| <i>n</i>th expression on the right is assigned to the <i>n</i>th |
| operand on the left. |
| The expressions on the right are evaluated before assigning to |
| any of the operands on the left, but otherwise the evaluation |
| order is unspecified beyond <a href="#Order_of_evaluation">the usual rules</a>. |
| </p> |
| |
| <pre> |
| a, b = b, a // exchange a and b |
| </pre> |
| |
| <p> |
| In assignments, each value must be |
| <a href="#Assignability">assignable</a> to the type of the |
| operand to which it is assigned. If an untyped <a href="#Constants">constant</a> |
| is assigned to a variable of interface type, the constant is <a href="#Conversions">converted</a> |
| to type <code>bool</code>, <code>int</code>, <code>float</code>, |
| <code>complex</code> or <code>string</code> |
| respectively, depending on whether the value is a boolean, integer, floating-point, |
| complex, or string constant. |
| </p> |
| |
| |
| <h3 id="If_statements">If statements</h3> |
| |
| <p> |
| "If" statements specify the conditional execution of two branches |
| according to the value of a boolean expression. If the expression |
| evaluates to true, the "if" branch is executed, otherwise, if |
| present, the "else" branch is executed. A missing condition |
| is equivalent to <code>true</code>. |
| </p> |
| |
| <pre class="ebnf"> |
| IfStmt = "if" [ SimpleStmt ";" ] [ Expression ] Block [ "else" Statement ] . |
| </pre> |
| |
| <pre> |
| if x > 0 { |
| return true; |
| } |
| </pre> |
| |
| <p> |
| The expression may be preceded by a simple statement, which |
| executes before the expression is evaluated. |
| </p> |
| |
| <pre> |
| if x := f(); x < y { |
| return x |
| } else if x > z { |
| return z |
| } else { |
| return y |
| } |
| </pre> |
| |
| |
| <h3 id="Switch_statements">Switch statements</h3> |
| |
| <p> |
| "Switch" statements provide multi-way execution. |
| An expression or type specifier is compared to the "cases" |
| inside the "switch" to determine which branch |
| to execute. |
| </p> |
| |
| <pre class="ebnf"> |
| SwitchStmt = ExprSwitchStmt | TypeSwitchStmt . |
| </pre> |
| |
| <p> |
| There are two forms: expression switches and type switches. |
| In an expression switch, the cases contain expressions that are compared |
| against the value of the switch expression. |
| In a type switch, the cases contain types that are compared against the |
| type of a specially annotated switch expression. |
| </p> |
| |
| <h4 id="Expression_switches">Expression switches</h4> |
| |
| <p> |
| In an expression switch, |
| the switch expression is evaluated and |
| the case expressions, which need not be constants, |
| are evaluated left-to-right and top-to-bottom; the first one that equals the |
| switch expression |
| triggers execution of the statements of the associated case; |
| the other cases are skipped. |
| If no case matches and there is a "default" case, |
| its statements are executed. |
| There can be at most one default case and it may appear anywhere in the |
| "switch" statement. |
| A missing switch expression is equivalent to |
| the expression <code>true |