This document is not ready for external review, it is under active development. Any part may change substantially as design progresses. ----- - -Contents ----- +
+
A package is built from one or more source files, each of which consists of a package specifier followed by declarations. There are no statements at the top level of a file. - +
By convention, the package called "main" is the starting point for execution. It contains a function, also called "main", that is the first function invoked by the run time system after initialization (if a source file within the program contains a function "init()", that function will be executed before "main.main()" is called). - +
Source files can be compiled separately (without the source code of packages they depend on), but not independently (the compiler does check dependencies by consulting the symbol information in compiled packages). -Modularity, identifiers and scopes ----- +
In particular, all identifiers occurring in a package are either declared explicitly within the package, arise from an import declaration, or belong to a small set of predeclared identifiers (such as "string"). - +
Scoping follows the usual rules: The scope of an identifier declared within a ``block'' generally extends from the declaration of the identifier to the end of the block. An identifier shadows identifiers with the same name declared in outer scopes. Within a scope, an identifier can be declared at most once. - +
Identifiers may be ``internal'' or ``exported''. Internal identifiers are only accessible to files belonging to the package in which they are declared. External identifiers are accessible to other packages. -Typing, polymorphism, and object-orientation ----- +
Object-oriented programming is supported by interface types. Different interface types are independent of each other and no explicit hierarchy is required (such as single or @@ -341,25 +361,24 @@ multiple inheritance explicitly specified through respective type declarations). Interface types only define a set of methods that a corresponding implementation must provide. Thus interface and implementation are strictly separated. - +
An interface is implemented by associating methods with types. If a type defines all methods of an interface, it implements that interface and thus can be used where that interface is required. Unless used through a variable of interface type, methods can always be statically bound (they are not ``virtual''), and invoking them incurs no extra run-time overhead compared to ordinary functions. - +
Go has no explicit notion of classes, sub-classes, or inheritance. These concepts are trivially modeled in Go through the use of functions, structures, embedding of types, associated methods, and interfaces. - +
Go has no explicit notion of type parameters or templates. Instead, containers (such as stacks, lists, etc.) are implemented through the use of abstract operations on interface types. -Pointers and garbage collection ----- +
For example, when calling a function with a struct, the struct is passed by value, possibly by making a copy. To pass a reference, one must explicitly pass a pointer to the struct. On the other hand, when calling a function with @@ -385,160 +403,163 @@ pointer to the map; thus the map contents are not copied when a map is assigned to a variable. -Multithreading and channels ----- +
+Production = production_name "=" Expression .
+Expression = Alternative { "|" Alternative } .
+Alternative = Term { Term } .
+Term = production_name | token [ "..." token ] | Group | Option | Repetition .
+Group = "(" Expression ")" .
+Option = "[" Expression ")" .
+Repetition = "{" Expression "}" .
+
Productions are expressions constructed from terms and the following operators:
- | separates alternatives (least binding strength)
- () groups
- [] specifies an option (0 or 1 times)
- {} specifies repetition (0 to n times)
+
+| separates alternatives (least binding strength)
+() groups
+[] specifies an option (0 or 1 times)
+{} specifies repetition (0 to n times)
+
Lower-case production names are used to identify productions that cannot
be broken by white space or comments; they are tokens. Other production
names are in CamelCase.
-
+Tokens (lexical symbols) are enclosed in double quotes '''' (the double quote symbol is written as ''"''). - +
The form "a ... b" represents the set of characters from "a" through "b" as alternatives. - +
Where possible, recursive productions are used to express evaluation order and operator precedence syntactically (for instance for expressions). - +
A production may be referenced from various places in this document but is usually defined close to its first use. Productions and code examples are indented. ----- +
Tokenization follows the usual rules. Source text is case-sensitive. - +
White space is blanks, newlines, carriage returns, or tabs. - +
Comments are // to end of line or /* */ without nesting and are treated as white space. - +
Some Unicode characters (e.g., the character U+00E4) may be representable in two forms, as a single code point or as two code points. For simplicity of implementation, Go treats these as distinct characters: each Unicode code point is a single character in Go. -Characters ----- +
+
+letter = unicode_letter | "_" . +decimal_digit = "0" ... "9" . +octal_digit = "0" ... "7" . +hex_digit = "0" ... "9" | "A" ... "F" | "a" ... "f" . ++
+identifier = letter { letter | decimal_digit } .
+
Exported identifiers (§Exported identifiers) start with a capital_letter.
-
- a
- _x9
- ThisVariableIsExported
- αβ
-
++a +_x9 +ThisVariableIsExported +αβ +Some identifiers are predeclared (§Predeclared identifiers). -Numeric literals ----- +
+int_lit = decimal_int | octal_int | hex_int .
+decimal_int = ( "1" ... "9" ) { decimal_digit } .
+octal_int = "0" { octal_digit } .
+hex_int = "0" ( "x" | "X" ) hex_digit { hex_digit } .
+
- int_lit = decimal_int | octal_int | hex_int .
- decimal_int = ( "1" ... "9" ) { decimal_digit } .
- octal_int = "0" { octal_digit } .
- hex_int = "0" ( "x" | "X" ) hex_digit { hex_digit } .
-
- 42
- 0600
- 0xBadFace
- 170141183460469231731687303715884105727
++42 +0600 +0xBadFace +170141183460469231731687303715884105727 +A floating point literal represents a mathematically ideal floating point constant of arbitrary precision, or 'ideal float'. - float_lit = - decimals "." [ decimals ] [ exponent ] | - decimals exponent | - "." decimals [ exponent ] . - decimals = decimal_digit { decimal_digit } . - exponent = ( "e" | "E" ) [ "+" | "-" ] decimals . +
+float_lit =
+ decimals "." [ decimals ] [ exponent ] |
+ decimals exponent |
+ "." decimals [ exponent ] .
+decimals = decimal_digit { decimal_digit } .
+exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .
+
- 0.
- 2.71828
- 1.e+0
- 6.67428e-11
- 1E6
- .25
- .12345E+5
++0. +2.71828 +1.e+0 +6.67428e-11 +1E6 +.25 +.12345E+5 +Numeric literals are unsigned. A negative constant is formed by applying the unary prefix operator "-" (§Arithmetic operators). - +
An 'ideal number' is either an 'ideal int' or an 'ideal float'. - +
Only when an ideal number (or an arithmetic expression formed solely from ideal numbers) is bound to a variable or used in an expression or constant of fixed-size integers or floats it is required to fit @@ -546,85 +567,88 @@ a particular size. In other words, ideal numbers and arithmetic upon them are not subject to overflow; only use of them in assignments or expressions involving fixed-size numbers may cause overflow, and thus an error (§Expressions). - +
Implementation restriction: A compiler may implement ideal numbers by choosing a "sufficiently large" internal representation of such numbers. -Character and string literals ----- +
+
+escaped_char = "\" ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | "\" | "'" | """ ) . +A unicode_value takes one of four forms: - -* The UTF-8 encoding of a Unicode code point. Since Go source +
+
An octal_byte_value contains three octal digits. A hex_byte_value contains two hexadecimal digits. (Note: This differs from C but is simpler.) - +
It is erroneous for an octal_byte_value to represent a value larger than 255. (By construction, a hex_byte_value cannot.) - +
A character literal is a form of unsigned integer constant. Its value is that of the Unicode code point represented by the text between the quotes. - 'a' - 'ä' - '本' - '\t' - '\000' - '\007' - '\377' - '\x07' - '\xff' - '\u12e4' - '\U00101234' +
+'a' +'ä' +'本' +'\t' +'\000' +'\007' +'\377' +'\x07' +'\xff' +'\u12e4' +'\U00101234' +String literals come in two forms: double-quoted and back-quoted. Double-quoted strings have the usual properties; back-quoted strings do not interpret backslashes at all. - string_lit = raw_string_lit | interpreted_string_lit . - raw_string_lit = "`" { unicode_char } "`" . - interpreted_string_lit = """ { unicode_value | byte_value } """ . +
+string_lit = raw_string_lit | interpreted_string_lit .
+raw_string_lit = "`" { unicode_char } "`" .
+interpreted_string_lit = """ { unicode_value | byte_value } """ .
+
A string literal has type "string" (§Strings). Its value is constructed
by taking the byte values formed by the successive elements of the
@@ -640,34 +664,40 @@ the value 255, while the second contains a single byte of value 255.
The same rules apply to raw string literals, except the contents are
uninterpreted UTF-8.
- `abc`
- `\n`
- "hello, world\n"
- "\n"
- ""
- "Hello, world!\n"
- "日本語"
- "\u65e5本\U00008a9e"
- "\xff\u00FF"
++`abc` +`\n` +"hello, world\n" +"\n" +"" +"Hello, world!\n" +"日本語" +"\u65e5本\U00008a9e" +"\xff\u00FF" +These examples all represent the same string: - "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 +
+"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 +Adjacent strings separated only by whitespace (including comments) are concatenated into a single string. The following two lines represent the same string: - "Alea iacta est." - "Alea " /* The die */ `iacta est` /* is cast */ "." +
+"Alea iacta est." +"Alea " /* The die */ `iacta est` /* is cast */ "." +The language does not canonicalize Unicode text or evaluate combining forms. The text of source code is passed uninterpreted. - +
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 @@ -675,131 +705,143 @@ point), and will appear as two code points if placed in a string literal. -Operators and delimitors ----- +
++ & += &= && == != ( )
+- | -= |= || < <= [ ]
+* ^ *= ^= <- > >= { }
+/ << /= <<= ++ = := , ;
+% >> %= >>= -- ! ... . :
+
-Reserved words
-----
++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 ++
+Declaration = ConstDecl | TypeDecl | VarDecl | FunctionDecl | MethodDecl . +Every identifier in a program must be declared; some identifiers, such as "int" and "true", are predeclared (§Predeclared identifiers). - +
The ``scope'' of an identifier is the extent of source text within which the identifier denotes the bound entity. No identifier may be declared twice in a single scope. Go is lexically scoped: An identifier denotes the entity it is bound to only within the scope of the identifier. - +
For instance, for a variable named "x", the scope of identifier "x" is the extent of source text within which "x" denotes that particular variable. It is illegal to declare another identifier "x" within the same scope. - +
The scope of an identifier depends on the entity declared. The scope for an identifier always excludes scopes redeclaring the identifier in nested blocks. An identifier declared in a nested block is said to ``shadow'' the same identifier declared in an outer block. - 1. The scope of predeclared identifiers is the entire source file. +
+bool, byte, uint8, uint16, uint32, uint64, int8, int16, int32, int64, +float32, float64, string +A set of platform-specific convenience types: - uint, int, float, uintptr +
+uint, int, float, uintptr +The predeclared constants: - true, false, iota, nil +
+true, false, iota, nil +The predeclared functions (note: this list is likely to change): - cap(), convert(), len(), make(), new(), panic(), panicln(), print(), println(), typeof(), ... +
+cap(), convert(), len(), make(), new(), panic(), panicln(), print(), println(), typeof(), ... +-Exported identifiers ----- +
All other identifiers are ``internal''; they are only visible in files belonging to the same package which declares them. - +
+ TODO: This should be made clearer. For instance, function-local identifiers are never exported, but non-global fields/methods may be exported. + -Const declarations ----- +
+ConstDecl = "const" ( ConstSpec | "(" [ ConstSpecList ] ")" ) .
+ConstSpecList = ConstSpec { ";" ConstSpec } [ ";" ] .
+ConstSpec = IdentifierList [ CompleteType ] [ "=" ExpressionList ] .
- IdentifierList = identifier { "," identifier } .
- ExpressionList = Expression { "," Expression } .
+IdentifierList = identifier { "," identifier } .
+ExpressionList = Expression { "," Expression } .
+
A constant declaration binds a list of identifiers (the names of the constants)
to the values of a list of constant expressions. The number of identifiers must
@@ -811,14 +853,16 @@ the type of all constants is the type specified, and the types of all
expressions in ExpressionList must be assignment-compatible with the
constant type.
- const Pi float64 = 3.14159265358979323846
- const E = 2.718281828
- const (
- size int64 = 1024;
- eof = -1;
- )
- const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo"
- const u, v float = 0, 3 // u = 0.0, v = 3.0
++const Pi float64 = 3.14159265358979323846 +const E = 2.718281828 +const ( + size int64 = 1024; + eof = -1; +) +const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo" +const u, v float = 0, 3 // u = 0.0, v = 3.0 +As a special case, within a parenthesized "const" declaration list the ExpressionList may be omitted from any but the first declaration. Such an empty @@ -827,20 +871,22 @@ non-empty ExpressionList in the same "const" declaration list. That is, omitting the list of expressions is 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 "iota" constant generator implicit repetition of ExpressionLists permit light-weight declaration of enumerated values (§Iota): - const ( - Sunday = iota; - Monday; - Tuesday; - Wednesday; - Thursday; - Friday; - Partyday; - numberOfDays; // this constant in not exported - ) +
+const ( + Sunday = iota; + Monday; + Tuesday; + Wednesday; + Thursday; + Friday; + Partyday; + numberOfDays; // this constant in not exported +) +The initializing expression for a numeric constant is evaluated using the principles described in the section on numeric literals: @@ -849,8 +895,10 @@ to a variable. Intermediate values, and the constants themselves, may require precision significantly larger than any concrete type in the language. Thus the following is legal: - const Huge = 1 << 100; - const Four int8 = Huge >> 98; +
+const Huge = 1 << 100; +const Four int8 = Huge >> 98; +A given numeric constant expression is, however, defined to be either an integer or a floating point value, depending on the syntax @@ -859,12 +907,14 @@ nature of the arithmetic operations depends on the type of the values; for example, 3/2 is an integer division yielding 1, while 3./2. is a floating point division yielding 1.5. Thus - const x = 3./2. + 3/2; +
+const x = 3./2. + 3/2; +yields a floating point constant of value 2.5 (1.5 + 1); its constituent expressions are evaluated using different rules for division. - +
If the type is missing from a numeric constant declaration, the constant represents a value of abitrary precision, either integer or floating point, determined by the type of the initializing expression. Such @@ -876,8 +926,7 @@ It is erroneous to assign a value with a non-zero fractional part to an integer, or if the assignment would overflow or underflow. -Iota ----- +
+const ( // iota is set to 0 + enum0 = iota; // sets enum0 to 0, etc. + enum1 = iota; + enum2 = iota +) - const ( - a = 1 << iota; // a == 1 (iota has been reset) - b = 1 << iota; // b == 2 - c = 1 << iota; // c == 4 - ) +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 (ideal integer) - v float = iota * 42; // v == 42.0 (float) - w = iota * 42; // w == 84 (ideal integer) - ) +const ( + u = iota * 42; // u == 0 (ideal integer) + v float = iota * 42; // v == 42.0 (float) + w = iota * 42; // w == 84 (ideal integer) +) - const x = iota; // x == 0 (iota has been reset) - const y = iota; // y == 0 (iota has been reset) +const x = iota; // x == 0 (iota has been reset) +const y = iota; // y == 0 (iota has been reset) +Within an ExpressionList, the value of all "iota"'s is the same because "iota" is only incremented at each semicolon: - const ( - base0, mask0 int64 = 1 << iota, i << iota - 1; // base0 == 1, mask0 = 0 - base1, mask1 int64 = 1 << iota, i << iota - 1; // base1 == 2, mask1 = 1 - base2, mask2 int64 = 1 << iota, i << iota - 1; // base2 == 4, mask2 = 3 - ) +
+const ( + base0, mask0 int64 = 1 << iota, i << iota - 1; // base0 == 1, mask0 = 0 + base1, mask1 int64 = 1 << iota, i << iota - 1; // base1 == 2, mask1 = 1 + base2, mask2 int64 = 1 << iota, i << iota - 1; // base2 == 4, mask2 = 3 +) +Since the ExpressionList in constant declarations repeats implicitly if omitted, some of the examples above can be abbreviated: - const ( - enum0 = iota; - enum1; - enum2 - ) +
+const ( + enum0 = iota; + enum1; + enum2 +) - const ( - a = 1 << iota; - b; - c; - ) +const ( + a = 1 << iota; + b; + c; +) - const ( - u = iota * 42; - v float; - w; - ) +const ( + u = iota * 42; + v float; + w; +) - const ( - base0, mask0 int64 = 1 << iota, i << iota - 1; - base1, mask1 int64; - base2, mask2 int64; - ) +const ( + base0, mask0 int64 = 1 << iota, i << iota - 1; + base1, mask1 int64; + base2, mask2 int64; +) +-Type declarations ----- +
+TypeDecl = "type" ( TypeSpec | "(" [ TypeSpecList ] ")" ) .
+TypeSpecList = TypeSpec { ";" TypeSpec } [ ";" ] .
+TypeSpec = identifier Type .
+
A struct or interface type may be forward-declared (§Struct types,
§Interface types). A forward-declared type is incomplete (§Types)
until it is fully declared. The full declaration must must follow
within the same block containing the forward declaration.
- type IntArray [16] int
+
+type IntArray [16] int
- type (
- Point struct { x, y float };
- Polar Point
- )
+type (
+ Point struct { x, y float };
+ Polar Point
+)
- type TreeNode struct {
- left, right *TreeNode;
- value Point;
- }
-
- type Comparable interface {
- cmp(Comparable) int
- }
+type TreeNode struct {
+ left, right *TreeNode;
+ value Point;
+}
+
+type Comparable interface {
+ cmp(Comparable) int
+}
+
-Variable declarations
-----
+
+VarDecl = "var" ( VarSpec | "(" [ VarSpecList ] ")" ) .
+VarSpecList = VarSpec { ";" VarSpec } [ ";" ] .
+VarSpec = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) .
+
- var i int
- var U, V, W float
- var k = 0
- var x, y float = -1.0, -2.0
- var (
- i int;
- u, v, s = 2.0, 3.0, "bar"
- )
++var i int +var U, V, W float +var k = 0 +var x, y float = -1.0, -2.0 +var ( + i int; + u, v, s = 2.0, 3.0, "bar" +) +If the expression list is present, it must have the same number of elements as there are variables in the variable specification. - +
If the variable type is omitted, an initialization expression (or expression list) must be present, and the variable type is the type of the expression value (in case of a list of variables, the variables assume the types of the corresponding expression values). - +
If the variable type is omitted, and the corresponding initialization expression is a constant expression of abstract int or floating point type, the type of the variable is "int" or "float" respectively: - var i = 0 // i has int type - var f = 3.1415 // f has float type +
+var i = 0 // i has int type +var f = 3.1415 // f has float type +The syntax - SimpleVarDecl = IdentifierList ":=" ExpressionList . +
+SimpleVarDecl = IdentifierList ":=" ExpressionList . +is shorthand for - "var" IdentifierList = ExpressionList . +
+"var" IdentifierList = ExpressionList . +- i, j := 0, 10; - f := func() int { return 7; } - ch := new(chan int); +
+i, j := 0, 10;
+f := func() int { return 7; }
+ch := new(chan int);
+
Also, in some contexts such as "if", "for", or "switch" statements,
this construct can be used to declare local temporary variables.
+A type may be specified by a type name (§Type declarations) or a type literal. A type literal is a syntactic construct that explicitly specifies the composition of a new type in terms of other (already declared) types. - Type = TypeName | TypeLit . - TypeName = QualifiedIdent. - TypeLit = - ArrayType | StructType | PointerType | FunctionType | InterfaceType | - SliceType | MapType | ChannelType . +
+Type = TypeName | TypeLit . +TypeName = QualifiedIdent. +TypeLit = + ArrayType | StructType | PointerType | FunctionType | InterfaceType | + SliceType | MapType | ChannelType . +Some types are predeclared and denoted by their type names; these are called ``basic types''. Generally (except for strings) they are not composed of more elementary types; instead they model elementary machine data types. - +
All other types are called ``composite types'; they are composed from other (basic or composite) types and denoted by their type names or by type literals. There are arrays, structs, pointers, functions, interfaces, slices, maps, and channels. - +
At a given point in the source code, a type may be ``complete'' or ''incomplete''. Array and struct types are complete when they are fully declared. All other types are always complete (although their components, such as the base @@ -1062,51 +1131,53 @@ type of a pointer type, may be incomplete). Incomplete types are subject to usag restrictions; for instance the type of a variable must be complete where the variable is declared. - CompleteType = Type . +
+CompleteType = Type . +The ``interface'' of a type is the set of methods bound to it (§Method declarations). The interface of a pointer type is the interface of the pointer base type (§Pointer types). All types have an interface; if they have no methods associated with them, their interface is called the ``empty'' interface. - +
The ``static type'' (or simply ``type'') of a variable is the type defined by the variable's declaration. The ``dynamic type'' of a variable is the actual type of the value stored in a variable at run-time. Except for variables of interface type, the dynamic type of a variable is always its static type. - +
Variables of interface type may hold values with different dynamic types during execution. However, its dynamic type is always compatible with the static type of the interface variable (§Interface types). -Basic types ----- +
+byte same as uint8 (for convenience) - 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) +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) +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 valid IEEE-754 32-bit floating point numbers - float64 the set of all valid IEEE-754 64-bit floating point numbers +float32 the set of all valid IEEE-754 32-bit floating point numbers +float64 the set of all valid IEEE-754 64-bit floating point numbers +Integer types are represented in the usual binary format; the value of an n-bit integer is n bits wide. A negative signed integer is represented @@ -1123,15 +1194,17 @@ its corresponding unsigned type without loss). Additionally, Go declares a set of platform-specific numeric types for convenience: - uint at least 32 bits, at most the size of the largest uint type - int at least 32 bits, at most the size of the largest int type - float at least 32 bits, at most the size of the largest float type - uintptr smallest uint type large enough to store the uninterpreted - bits of a pointer value +
+uint at least 32 bits, at most the size of the largest uint type +int at least 32 bits, at most the size of the largest int type +float at least 32 bits, at most the size of the largest float type +uintptr smallest uint type large enough to store the uninterpreted + bits of a pointer value +For instance, int might have the same size as int32 on a 32-bit architecture, or int64 on a 64-bit architecture. - +
Except for "byte", which is an alias for "uint8", all numeric types are different from each other to avoid portability issues. Conversions are required when different numeric types are mixed in an expression or assignment. @@ -1139,42 +1212,42 @@ For instance, "int32" and "int" are not the same type even though they may have the same size on a particular platform. -Booleans ----- +
+
+a [3]byte; a[0] = 'a'; a[1] = 'b'; a[2] = 'c'; string(a) == "abc"; ++
+ArrayType = "[" ArrayLength "]" ElementType . +ArrayLength = Expression . +ElementType = CompleteType . +The array length and its value are part of the array type. The array length must be a constant expression (§Constant expressions) that evaluates to an integer value >= 0. - +
The number of elements of an array "a" can be discovered using the built-in function - len(a) +
+len(a) +The length of arrays is known at compile-time, and the result of a call to "len(a)" is a compile-time constant. - [32]byte - [2*N] struct { x, y int32 } - [1000]*float64 +
+[32]byte
+[2*N] struct { x, y int32 }
+[1000]*float64
+
Assignment compatibility: Arrays can be assigned to variables of equal type
and to slice variables with equal element type. When assigning to a slice
@@ -1208,8 +1287,7 @@ variable, the array is not copied but a slice comprising the entire array
is created.
-Struct types
-----
+
+StructType = "struct" [ "{" [ FieldDeclList ] "}" ] .
+FieldDeclList = FieldDecl { ";" FieldDecl } [ ";" ] .
+FieldDecl = (IdentifierList CompleteType | [ "*" ] TypeName) [ Tag ] .
+Tag = StringLit .
+
- // An empty struct.
- struct {}
+
+// An empty struct.
+struct {}
- // A struct with 5 fields.
- struct {
- x, y int;
- u float;
- A *[]int;
- F func();
- }
+// A struct with 5 fields.
+struct {
+ x, y int;
+ u float;
+ A *[]int;
+ F func();
+}
+
A struct may contain ``anonymous fields'', which are declared with a type
but no explicit field identifier. An anonymous field type must be specified as
a type name "T", or as a pointer to a type name ``*T'', and T itself may not be
a pointer or interface type. The unqualified type name acts as the field identifier.
- // A struct with four anonymous fields of type T1, *T2, P.T3 and *P.T4
- struct {
- T1; // the field name is T1
- *T2; // the field name is T2
- P.T3; // the field name is the unqualified type name T3
- *P.T4; // the field name is the unqualified type name T4
- x, y int;
- }
+
+// A struct with four anonymous fields of type T1, *T2, P.T3 and *P.T4
+struct {
+ T1; // the field name is T1
+ *T2; // the field name is T2
+ P.T3; // the field name is the unqualified type name T3
+ *P.T4; // the field name is the unqualified type name T4
+ x, y int;
+}
+
The unqualified type name of an anonymous field must not conflict with the
field identifier (or unqualified type name for an anonymous field) of any
other field within the struct. The following declaration is illegal:
- 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
- }
+
+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
+}
+
Fields and methods (§Method declarations) of an anonymous field become directly
accessible as fields and methods of the struct without the need to provide the
type name of the respective anonymous field (§Selectors).
-
+A field declaration may be followed by an optional string literal tag which becomes an ``attribute'' for all the identifiers in the corresponding field declaration. The tags are available via the reflection library but are ignored otherwise. A tag may contain arbitrary application-specific information. - // A struct corresponding to the EventIdMessage protocol buffer. - // The tag strings contain the protocol buffer field tags. - struct { - time_usec uint64 "1"; - server_ip uint32 "2"; - process_id uint32 "3"; - } +
+// A struct corresponding to the EventIdMessage protocol buffer.
+// The tag strings contain the protocol buffer field tags.
+struct {
+ time_usec uint64 "1";
+ server_ip uint32 "2";
+ process_id uint32 "3";
+}
+
Forward declaration:
A struct type consisting of only the reserved word "struct" may be used in
a type declaration; it declares an incomplete struct type (§Type declarations).
This allows the construction of mutually recursive types such as:
- type S2 struct // forward declaration of S2
- type S1 struct { s2 *S2 }
- type S2 struct { s1 *S1 }
+
+type S2 struct // forward declaration of S2
+type S1 struct { s2 *S2 }
+type S2 struct { s1 *S1 }
+
Assignment compatibility: Structs are assignment compatible to variables of
equal type only.
-Pointer types
-----
++PointerType = "*" BaseType . +BaseType = Type . +- *int - map[string] chan +
+*int +map[string] chan +The pointer base type may be denoted by an identifier referring to an incomplete type (§Types), possibly declared via a forward declaration. This allows the construction of recursive and mutually recursive types such as: - type S struct { s *S } +
+type S struct { s *S }
- type S2 struct // forward declaration of S2
- type S1 struct { s2 *S2 }
- type S2 struct { s1 *S1 }
+type S2 struct // forward declaration of S2
+type S1 struct { s2 *S2 }
+type S2 struct { s1 *S1 }
+
Assignment compatibility: A pointer is assignment compatible to a variable
of pointer type, only if both types are equal.
-
+Comparisons: A variable of pointer type can be compared against "nil" with the operators "==" and "!=" (§Comparison operators). The variable is "nil" only if "nil" is assigned explicitly to the variable (§Assignments), or if the variable has not been modified since creation (§Program initialization and execution). - +
Two variables of equal pointer type can be tested for equality with the operators "==" and "!=" (§Comparison operators). The pointers are equal if they point to the same location. @@ -1327,23 +1422,24 @@ if they point to the same location. Pointer arithmetic of any kind is not permitted. -Function types ----- +
+FunctionType = "func" Signature .
+Signature = "(" [ ParameterList ] ")" [ Result ] .
+ParameterList = ParameterDecl { "," ParameterDecl } .
+ParameterDecl = [ IdentifierList ] ( Type | "..." ) .
+Result = Type | "(" ParameterList ")" .
+
In ParameterList, the parameter names (IdentifierList) either must all be
present, or all be absent. If the parameters are named, each name stands
for one parameter of the specified type. If the parameters are unnamed, each
type stands for one parameter of that type.
-
+For the last incoming parameter only, instead of a parameter type one may write "...". The ellipsis indicates that the last parameter stands for an arbitrary number of additional arguments of any type (including @@ -1351,58 +1447,67 @@ no additional arguments). If the parameters are named, the identifier list immediately preceding "..." must contain only one identifier (the name of the last parameter). - func () - func (x int) - func () int - func (string, float, ...) - func (a, b int, z float) bool - func (a, b int, z float) (bool) - func (a, b int, z float, opt ...) (success bool) - func (int, int, float) (float, *[]int) +
+func () +func (x int) +func () int +func (string, float, ...) +func (a, b int, z float) bool +func (a, b int, z float) (bool) +func (a, b int, z float, opt ...) (success bool) +func (int, int, float) (float, *[]int) +If the result type of a function is itself a function type, the result type must be parenthesized to resolve a parsing ambiguity: - func (n int) (func (p* T)) +
+func (n int) (func (p* T)) +Assignment compatibility: A function can be assigned to a function variable only if both function types are equal. - +
Comparisons: A variable of function type can be compared against "nil" with the operators "==" and "!=" (§Comparison operators). The variable is "nil" only if "nil" is assigned explicitly to the variable (§Assignments), or if the variable has not been modified since creation (§Program initialization and execution). - +
Two variables of equal function type can be tested for equality with the operators "==" and "!=" (§Comparison operators). The variables are equal if they refer to the same function. -Interface types ----- +
+InterfaceType = "interface" [ "{" [ MethodSpecList ] "}" ] .
+MethodSpecList = MethodSpec { ";" MethodSpec } [ ";" ] .
+MethodSpec = IdentifierList Signature | TypeName .
+
- // An interface specifying a basic File type.
- interface {
- Read, Write (b Buffer) bool;
- Close ();
- }
+
+// An interface specifying a basic File type.
+interface {
+ Read, Write (b Buffer) bool;
+ Close ();
+}
+
Any type (including interface types) whose interface has, possibly as a
subset, the complete set of methods of an interface I is said to implement
interface I. For instance, if two types S1 and S2 have the methods
- func (p T) Read(b Buffer) bool { return ... }
- func (p T) Write(b Buffer) bool { return ... }
- func (p T) Close() { ... }
+
+func (p T) Read(b Buffer) bool { return ... }
+func (p T) Write(b Buffer) bool { return ... }
+func (p T) Close() { ... }
+
(where T stands for either S1 or S2) then the File interface is
implemented by both S1 and S2, regardless of what other methods
@@ -1410,59 +1515,69 @@ S1 and S2 may have or share.
All types implement the empty interface:
- interface {}
+
+interface {}
+
In general, a type implements an arbitrary number of interfaces.
For instance, consider the interface
- type Lock interface {
- Lock, Unlock ();
- }
+
+type Lock interface {
+ Lock, Unlock ();
+}
+
If S1 and S2 also implement
- func (p T) Lock() { ... }
- func (p T) Unlock() { ... }
+
+func (p T) Lock() { ... }
+func (p T) Unlock() { ... }
+
they implement the Lock interface as well as the File interface.
-
+An interface may contain a type name T in place of a method specification. T must denote another, complete (and not forward-declared) interface type. Using this notation is equivalent to enumerating the methods of T explicitly in the interface containing T. - type ReadWrite interface { - Read, Write (b Buffer) bool; - } +
+type ReadWrite interface {
+ Read, Write (b Buffer) bool;
+}
- type File interface {
- ReadWrite; // same as enumerating the methods in ReadWrite
- Lock; // same as enumerating the methods in Lock
- Close();
- }
+type File interface {
+ ReadWrite; // same as enumerating the methods in ReadWrite
+ Lock; // same as enumerating the methods in Lock
+ Close();
+}
+
Forward declaration:
A interface type consisting of only the reserved word "interface" may be used in
a type declaration; it declares an incomplete interface type (§Type declarations).
This allows the construction of mutually recursive types such as:
- type T2 interface
- type T1 interface {
- foo(T2) int;
- }
- type T2 interface {
- bar(T1) int;
- }
+
+type T2 interface
+type T1 interface {
+ foo(T2) int;
+}
+type T2 interface {
+ bar(T1) int;
+}
+
Assignment compatibility: A value can be assigned to an interface variable
if the static type of the value implements the interface or if the value is "nil".
-
+Comparisons: A variable of interface type can be compared against "nil" with the operators "==" and "!=" (§Comparison operators). The variable is "nil" only if "nil" is assigned explicitly to the variable (§Assignments), or if the variable has not been modified since creation (§Program initialization and execution). - +
Two variables of interface type can be tested for equality with the operators "==" and "!=" (§Comparison operators) if both variables have the same static type. They are equal if both their dynamic types and values are @@ -1470,8 +1585,7 @@ equal. If the dynamic types are equal but the values do not support comparison, a run-time error occurs. -Slice types ----- +
+SliceType = "[" "]" ElementType . +Syntactically and semantically, arrays and slices look and behave very similarly, but with one important difference: A slice is a descriptor @@ -1488,58 +1604,72 @@ refer to different (and possibly overlapping) segments of the same underlying array. Thus, with respect to the underlying array, slices behave like references. In contrast, two different variables of array type always denote two different arrays. - +
For slices, the actual array underlying the slice may extend past the current slice length; the maximum length a slice may assume is called its capacity. The capacity of any slice "a" can be discovered using the built-in function - cap(a) +
+cap(a) +and the following relationship between "len()" and "cap()" holds: - 0 <= len(a) <= cap(a) +
+0 <= len(a) <= cap(a) +The value of an uninitialized slice is "nil", and its length and capacity are 0. A new, initialized slice value for a given element type T is made using the built-in function "make", which takes a slice type and parameters specifying the length and optionally the capacity: - make([]T, length) - make([]T, length, capacity) +
+make([]T, length) +make([]T, length, capacity) +The "make()" call allocates a new underlying array to which the returned slice value refers. More precisely, calling "make" - make([]T, length, capacity) +
+make([]T, length, capacity) +is effectively the same as allocating an array and slicing it - new([capacity]T)[0 : length] +
+new([capacity]T)[0 : length] +Assignment compatibility: Slices are assignment compatible to variables of the same type. - +
Indexing: Given a (pointer to) a slice variable "a", a slice element is specified with an index operation: - a[i] +
+a[i] +This denotes the slice element at index "i". "i" must be within bounds, -that is "0 <= i < len(a)". - +that is "0 <= i < len(a)". +
Slicing: Given a a slice variable "a", a sub-slice is created with a slice operation: - a[i : j] +
+a[i : j] +This creates the sub-slice consisting of the elements "a[i]" through "a[j - 1]" (that is, excluding "a[j]"). The values "i" and "j" must satisfy the condition -"0 <= i <= j <= cap(a)". The length of the new slice is "j - i". The capacity of +"0 <= i <= j <= cap(a)". The length of the new slice is "j - i". The capacity of the slice is "cap(a) - i"; thus if "i" is 0, the slice capacity does not change as a result of a slice operation. The type of a sub-slice is the same as the type of the slice. Unlike the capacity, the length of a sub-slice may be larger than the length of the original slice. - +
Comparisons: A variable of slice type can be compared against "nil" with the operators "==" and "!=" (§Comparison operators). The variable is "nil" only if "nil" is assigned explicitly to the variable (§Assignments), or @@ -1547,8 +1677,7 @@ if the variable has not been modified since creation (§Program initialization and execution). -Map types ----- +
+MapType = "map" "[" KeyType "]" ValueType . +KeyType = CompleteType . +ValueType = CompleteType . +The comparison operators "==" and "!=" (§Comparison operators) must be defined for operands of the key type; thus the key type must be a basic, pointer, @@ -1566,30 +1697,36 @@ interface, or channel type. If the key type is an interface type, the dynamic key types must support these comparison operators. In this case, inserting a map value with a key that does not support testing for equality is a run-time error. - +
Upon creation, a map is empty and values may be added and removed during execution. - map [string] int - map [*T] struct { x, y float } - map [string] interface {} +
+map [string] int
+map [*T] struct { x, y float }
+map [string] interface {}
+
The length of a map "m" can be discovered using the built-in function
- len(m)
++len(m) +The value of an uninitialized map is "nil". A new, empty map value for given map type M is made using the built-in function "make" which takes the map type and an optional capacity as arguments: - my_map := make(M, 100); +
+my_map := make(M, 100); +The map capacity is an allocation hint for more efficient incremental growth of the map. - +
Assignment compatibility: A map type is assignment compatible to a variable of map type only if both types are equal. - +
Comparisons: A variable of map type can be compared against "nil" with the operators "==" and "!=" (§Comparison operators). The variable is "nil" only if "nil" is assigned explicitly to the variable (§Assignments), or @@ -1597,56 +1734,60 @@ if the variable has not been modified since creation (§Program initialization and execution). -Channel types ----- +
+ChannelType = Channel | SendChannel | RecvChannel . +Channel = "chan" ValueType . +SendChannel = "chan" "<-" ValueType . +RecvChannel = "<-" "chan" ValueType . +Upon creation, a channel can be used both to send and to receive. By conversion or assignment, a channel may be constrained only to send or to receive. This constraint is called a channel's ``direction''; either bi-directional (unconstrained), send, or receive. - chan T // can send and receive values of type T - chan <- float // can only be used to send floats - <-chan int // can only receive ints +
+chan T // can send and receive values of type T +chan <- float // can only be used to send floats +<-chan int // can only receive ints +The value of an uninitialized channel is "nil". A new, initialized channel value for a given element type T is made using the built-in function "make", which takes the channel type and an optional capacity as arguments: - my_chan = make(chan int, 100); +
+my_chan = make(chan int, 100); +The capacity sets the size of the buffer in the communication channel. If the capacity is greater than zero, the channel is asynchronous and, provided the buffer is not full, sends can succeed without blocking. If the capacity is zero, the communication succeeds only when both a sender and receiver are ready. - +
Assignment compatibility: A value of type channel can be assigned to a variable of type channel only if a) both types are equal (§Type equality), or b) both have equal channel value types and the value is a bidirectional channel. - +
Comparisons: A variable of channel type can be compared against "nil" with the operators "==" and "!=" (§Comparison operators). The variable is "nil" only if "nil" is assigned explicitly to the variable (§Assignments), or if the variable has not been modified since creation (§Program initialization and execution). - +
Two variables of channel type can be tested for equality with the operators "==" and "!=" (§Comparison operators) if both variables have the same ValueType. They are equal if both values were created by the same "make" call (§Making slices, maps, and channels). -Type equality ----- +
Structural type equality (equality for short) is defined by these rules: - +
Two type names denote equal types if the types in the corresponding declarations are equal. Two type literals specify equal types if they have the same literal structure and corresponding components have equal types. Loosely speaking, two types are equal if their values have the same layout in memory. More precisely: - - - Two array types are equal if they have equal element types and if they +
+
Type identity is defined by these rules: - +
Two type names denote identical types if they originate in the same type declaration. Two type literals specify identical types if they have the same literal structure and corresponding components have identical types. More precisely: - - - Two array types are identical if they have identical element types and if +
+
Finally, two types are different if they are not structurally equal. (By definition, they cannot be identical, either). For instance, given the declarations - 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 - ) +
+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
+)
+
these are some types that are equal
- T0 and T0
- T0 and []string
- T2 and T3
- T4 and T5
- T3 and struct { a int; int }
+
+T0 and T0
+T0 and []string
+T2 and T3
+T4 and T5
+T3 and struct { a int; int }
+
and these are some types that are identical
- T0 and T0
- []int and []int
- struct { a, b *T5 } and struct { a, b *T5 }
+
+T0 and T0
+[]int and []int
+struct { a, b *T5 } and struct { a, b *T5 }
+
As an example, "T0" and "T1" are equal but not identical because they have
different declarations.
+The type of a constant expression may be an ideal number. The type of such expressions is implicitly converted into the 'expected numeric type' required for the expression. The conversion is legal if the (ideal) expression value is a member of the set represented by the expected numeric type. In all other cases, and specifically if the expected type is not a numeric type, the expression is erroneous. - +
For instance, if the expected numeric type is a uint32, any ideal number which fits into a uint32 without loss of precision can be legally converted. Thus, the values 991, 42.0, and 1e9 are ok, but -1, 3.14, or 1e100 are not. @@ -1782,19 +1931,19 @@ should be ideal number, because for arrays, it is a constant. --> -Operands ----- +
+Operand = Literal | QualifiedIdent | "(" Expression ")" .
+Literal = BasicLit | CompositeLit | FunctionLit .
+BasicLit = int_lit | float_lit | char_lit | StringLit .
+StringLit = string_lit { string_lit } .
+
-Constants
-----
++ TODO(gri) expand this section. + - QualifiedIdent = { PackageName "." } identifier . - PackageName = identifier . +
+QualifiedIdent = { PackageName "." } identifier .
+PackageName = identifier .
+
-Composite literals
-----
+
+CompositeLit = LiteralType "(" [ ( ExpressionList | ExprPairList ) [ "," ] ] ")" .
+LiteralType = Type | "[" "..." "]" ElementType .
+ExprPairList = ExprPair { "," ExprPair } .
+ExprPair = Expression ":" Expression .
+
The LiteralType must be an struct, array, slice, or map type.
The types of the expressions must match the respective field, element, and
key types of the LiteralType; there is no automatic type conversion.
Composite literals are values of the type specified by LiteralType; that is
a new value is created every time the literal is evaluated. To get
-a pointer to the literal, the address operator "&" must be used.
-
+a pointer to the literal, the address operator "&" must be used.
+Given - type Rat struct { num, den int } - type Num struct { r Rat; f float; s string } +
+type Rat struct { num, den int }
+type Num struct { r Rat; f float; s string }
+
one can write
- pi := Num(Rat(22, 7), 3.14159, "pi");
++pi := Num(Rat(22, 7), 3.14159, "pi"); +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 @@ -1849,48 +2006,63 @@ It is an error to provide more elements than specified in LiteralType. The notation "..." may be used in place of the length expression to denote a length equal to the number of elements in the literal. - buffer := [10]string(); // len(buffer) == 10 - primes := [6]int(2, 3, 5, 7, 9, 11); // len(primes) == 6 - days := [...]string("sat", "sun"); // len(days) == 2 +
+buffer := [10]string(); // len(buffer) == 10
+primes := [6]int(2, 3, 5, 7, 9, 11); // len(primes) == 6
+days := [...]string("sat", "sun"); // len(days) == 2
+
A slice literal is a slice describing the entire underlying array literal.
Thus, the length and capacity of a slice literal is the number of elements
provided in the literal. A slice literal of the form
- []T(x1, x2, ... xn)
++[]T(x1, x2, ... xn) +is essentially a shortcut for a slice operation applied to an array literal: - [n]T(x1, x2, ... xn)[0 : n] +
+[n]T(x1, x2, ... xn)[0 : n] +Map literals are similar except the elements of the expression list are key-value pairs separated by a colon: - m := map[string]int("good": 0, "bad": 1, "indifferent": 7); +
+m := map[string]int("good": 0, "bad": 1, "indifferent": 7);
+
+
TODO: Consider adding helper syntax for nested composites
(avoids repeating types but complicates the spec needlessly.)
+
-Function literals
-----
+
+FunctionLit = "func" Signature Block .
+Block = "{" [ StatementList ] "}" .
+
The type of a function literal is the function type specified.
- func (a, b int, z float) bool { return a*b < int(z); }
+
+func (a, b int, z float) bool { return a*b < int(z); }
+
A function literal can be assigned to a variable of the
corresponding function type, or invoked directly.
- f := func(x, y int) int { return x + y; }
- func(ch chan int) { ch <- ACK; } (reply_chan)
+
+f := func(x, y int) int { return x + y; }
+func(ch chan int) { ch <- ACK; } (reply_chan)
+
Function literals are "closures": they may refer to variables
defined in a surrounding function. Those variables are then shared between
@@ -1898,181 +2070,202 @@ the surrounding function and the function literal, and they survive as long
as they are accessible in any way.
-Primary expressions
-----
+
+PrimaryExpr =
+ Operand |
+ PrimaryExpr Selector |
+ PrimaryExpr Index |
+ PrimaryExpr Slice |
+ PrimaryExpr TypeGuard |
+ PrimaryExpr Call .
- Selector = "." identifier .
- Index = "[" Expression "]" .
- Slice = "[" Expression ":" Expression "]" .
- TypeGuard = "." "(" Type ")" .
- Call = "(" [ ExpressionList ] ")" .
+Selector = "." identifier .
+Index = "[" Expression "]" .
+Slice = "[" Expression ":" Expression "]" .
+TypeGuard = "." "(" Type ")" .
+Call = "(" [ ExpressionList ] ")" .
+
- 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()
++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() +-Selectors ----- +
+x.f +denotes the field or method f of the value denoted by x (or of *x if x is of pointer type). The identifier f is called the (field or method) ``selector''. - +
A selector f may denote a field f declared in a type T, or it may refer to a field f declared in a nested anonymous field of T. Analogously, f may denote a method f of T, or it may refer to a method f of the type of a nested anonymous field of T. The number of anonymous fields traversed to get to the field or method is called its ``depth'' in T. - +
More precisely, the depth of a field or method f declared in T is zero. The depth of a field or method f declared anywhere inside an anonymous field A declared in T is the depth of f in A plus one. - +
The following rules apply to selectors: - +
1) For a value x of type T or *T where T is not an interface type, x.f denotes the field or method at the shallowest depth in T where there is such an f. The type of x.f is the type of the field or method f. If there is not exactly one f with shallowest depth, the selector expression is illegal. - +
2) For a variable x of type I or *I where I is an interface type, x.f denotes the actual method with name f of the value assigned to x if there is such a method. The type of x.f is the type of the method f. If no value or nil was assigned to x, x.f is illegal. - +
3) In all other cases, x.f is illegal. - +
Thus, selectors automatically dereference pointers as necessary. For instance, for an x of type *T where T declares an f, x.f is a shortcut for (*x).f. Furthermore, for an x of type T containing an anonymous field A declared as *A inside T, and where A contains a field f, x.f is a shortcut for (*x.A).f (assuming that the selector is legal in the first place). - +
The following examples illustrate selector use in more detail. Given the declarations: - type T0 struct { - x int; - } +
+type T0 struct {
+ x int;
+}
- func (recv *T0) M0()
+func (recv *T0) M0()
- type T1 struct {
- y int;
- }
+type T1 struct {
+ y int;
+}
- func (recv T1) M1()
+func (recv T1) M1()
- type T2 struct {
- z int;
- T1;
- *T0;
- }
+type T2 struct {
+ z int;
+ T1;
+ *T0;
+}
- func (recv *T2) M2()
+func (recv *T2) M2()
- var p *T2; // with p != nil and p.T1 != nil
+var p *T2; // with p != nil and p.T1 != nil
+
one can write:
- p.z // (*p).z
- p.y // ((*p).T1).y
- p.x // (*(*p).T0).x
++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 +p.M2 // (*p).M2 +p.M1 // ((*p).T1).M1 +p.M0 // ((*p).T0).M0 ++ TODO: Specify what happens to receivers. + -Indexes ----- +
+a[x] +denotes the array or map element x. The value x is called the ``array index'' or ``map key'', respectively. The following rules apply: - +
For a of type A or *A where A is an array type (§Array types): - - - x must be an integer value and 0 <= x < len(a) - - a[x] is the array element at index x and the type of a[x] +
+
For a of type *M, where M is a map type (§Map types): - - - x must be of the same type as the key type of M +
+
+ TODO: Need to expand map rules for assignments of the form v, ok = m[k]. + -Slices ----- +
+a := [4]int(1, 2, 3, 4); +s := a[1:3]; +the slice "s" has type "[]int", length 2, and elements - s[0] == 2 - s[1] == 3 +
+s[0] == 2 +s[1] == 3 +The index values in the slice must be in bounds for the original array (or string) and the slice length must be non-negative. - +
If the sliced operand is a string, the result of the slice operation is another string (§String types). If the sliced operand is an array or slice, the result of the slice operation is a slice (§Slice types). -Type guards ----- +
+x.(T) +asserts that the value stored in "x" is an element of type "T" (§Types). The notation ".(T)" is called a ``type guard'', and "x.(T)" is called a ``guarded expression''. The type of "x" must be an interface type. - +
More precisely, if "T" is not an interface type, the expression asserts that the dynamic type of "x" is identical to the type "T" (§Types). If "T" is an interface type, the expression asserts that the dynamic type @@ -2080,17 +2273,19 @@ of T implements the interface "T" (§Interface types). Because it can be verified statically, a type guard in which the static type of "x" implements the interface "T" is illegal. The type guard is said to succeed if the assertion holds. - +
If the type guard succeeds, the value of the guarded expression is the value stored in "x" and its type is "T". If the type guard fails, a run-time exception occurs. In other words, even though the dynamic type of "x" is only known at run-time, the type of the guarded expression "x.(T)" is known to be "T" in a correct program. - +
As a special form, if a guarded expression is used in an assignment - v, ok = x.(T) - v, ok := x.(T) +
+v, ok = x.(T) +v, ok := x.(T) +the result of the guarded expression is a pair of values with types "(T, bool)". If the type guard succeeds, the expression returns the pair "(x.(T), true)"; @@ -2098,58 +2293,72 @@ that is, the value stored in "x" (of type "T") is assigned to "v", and "ok" is set to true. If the type guard fails, the value in "v" is set to the initial value for the type of "v" (§Program initialization and execution), and "ok" is set to false. No run-time exception occurs in this case. - +
+ TODO add examples + -Calls ----- +
+p() +to call the function. - +
A method is called using the notation - receiver.method() +
+receiver.method() +where receiver is a value of the receiver type of the method. - +
For instance, given a *Point variable pt, one may call - pt.Scale(3.5) +
+pt.Scale(3.5) +The type of a method is the type of a function with the receiver as first argument. For instance, the method "Scale" has type - (p *Point, factor float) +
+(p *Point, factor float) +However, a function declared this way is not a method. - +
There is no distinct method type and there are no method literals. -Parameter passing ----- +
+*struct {
+ arg(0) typeof(arg(0));
+ arg(1) typeof(arg(1));
+ arg(2) typeof(arg(2));
+ ...
+ arg(n-1) typeof(arg(n-1));
+}
+
where the "arg(i)"'s correspond to the actual arguments passed in place
of the "..." parameter (the parameter and type names are for illustration
@@ -2160,11 +2369,15 @@ function instead of the actual arguments.
For instance, consider the function
- func f(x int, s string, f_extra ...)
++func f(x int, s string, f_extra ...) +and the call - f(42, "foo", 3.14, true, []int(1, 2, 3)) +
+f(42, "foo", 3.14, true, []int(1, 2, 3)) +Upon invocation, the parameters "3.14", "true", and "[]int(1, 2, 3)" are wrapped into a struct and the pointer to the struct is passed to f. @@ -2173,145 +2386,169 @@ The dynamic type of "f_extra" is the type of the value assigned to it upon invocation (the field names "arg0", "arg1", "arg2" are made up for illustration only, they are not accessible via reflection): - *struct { - arg0 float; - arg1 bool; - arg2 []int; - } +
+*struct {
+ arg0 float;
+ arg1 bool;
+ arg2 []int;
+}
+
The values of the fields "arg0", "arg1", and "arg2" are "3.14", "true",
and "[]int(1, 2, 3)".
-
+As a special case, if a function passes a "..." parameter as the argument for a "..." parameter of a function, the parameter is not wrapped again into a struct. Instead it is passed along unchanged. For instance, the function f may call a function g with declaration - func g(x int, g_extra ...) +
+func g(x int, g_extra ...) +as - g(x, f_extra); +
+g(x, f_extra); +Inside g, the value stored in g_extra is the same as the value stored in f_extra. -Operators ----- +
+Expression = UnaryExpr | Expression binaryOp 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 = "*" | "/" | "%" | "<<" | ">>" | "&" . +binary_op = log_op | com_op | rel_op | add_op | mul_op . +log_op = "||" | "&&" . +com_op = "<-" . +rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" . +add_op = "+" | "-" | "|" | "^" . +mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" . - unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" . +unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" . +The operand types in binary operations must be equal, with the following exceptions: - - - If one operand has numeric type and the other operand is +
+
There are six precedence levels for binary operators: multiplication operators bind strongest, followed by addition operators, comparison operators, communication operators, -"&&" (logical and), and finally "||" (logical or) with the +"&&" (logical and), and finally "||" (logical or) with the lowest precedence: - Precedence Operator - 6 * / % << >> & - 5 + - | ^ - 4 == != < <= > >= - 3 <- - 2 && - 1 || +
+Precedence Operator + 6 * / % << >> & + 5 + - | ^ + 4 == != < <= > >= + 3 <- + 2 && + 1 || +Binary operators of the same precedence associate from left to right. For instance, "x / y / z" stands for "(x / y) / z". - +
Examples - +x - 23 + 3*x[i] - x <= f() - ^a >> b - f() || g() - x == y + 1 && <-chan_ptr > 0 +
++x +23 + 3*x[i] +x <= f() +^a >> b +f() || g() +x == y + 1 && <-chan_ptr > 0 +-Arithmetic operators ----- - +
Arithmetic operators apply to numeric types and yield a result of the same type as the first operand. The four standard arithmetic operators ("+", "-", "*", "/") apply to both integer and floating point types, while "+" also applies to strings and arrays; all other arithmetic operators apply to integer types only. - + sum integers, floats, strings, arrays - - difference integers, floats - * product integers, floats - / quotient integers, floats - % remainder integers - - & bitwise and integers - | bitwise or integers - ^ bitwise xor integers - - << left shift integer << unsigned integer - >> right shift integer >> unsigned integer +
++ sum integers, floats, strings, arrays +- difference integers, floats +* product integers, floats +/ quotient integers, floats +% remainder integers + +& bitwise and integers +| bitwise or integers +^ bitwise xor integers + +<< left shift integer << unsigned integer +>> right shift integer >> unsigned integer +Strings can be concatenated using the "+" operator (or the "+=" assignment): - s := "hi" + string(c) +
+s := "hi" + string(c) +String addition creates a new string by copying the elements. - +
For integer values, "/" and "%" satisfy the following relationship: - (a / b) * b + a % b == a +
+(a / b) * b + a % b == a +and - (a / b) is "truncated towards zero". +
+(a / b) is "truncated towards zero". +Examples: - x y x / y x % y - 5 3 1 2 - -5 3 -1 -2 - 5 -3 -1 2 - -5 -3 1 -2 +
+ x y x / y x % y + 5 3 1 2 +-5 3 -1 -2 + 5 -3 -1 2 +-5 -3 1 -2 +Note that if the dividend is positive and the divisor is a constant power of 2, the division may be replaced by a left shift, and computing the remainder may be replaced by a bitwise "and" operation: - x x / 4 x % 4 x >> 2 x & 3 - 11 2 3 2 3 - -11 -2 -3 -3 1 +
+ x x / 4 x % 4 x >> 2 x & 3 + 11 2 3 2 3 +-11 -2 -3 -3 1 +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 @@ -2324,29 +2561,29 @@ Specifically, "x << 1" is the same as "x*2"; and "x >> 1" is the same as For integer operands, the unary operators "+", "-", and "^" are defined as follows: - +x is 0 + x - -x negation is 0 - x - ^x bitwise complement is m ^ x with m = "all bits set to 1" +
++x is 0 + x +-x negation is 0 - x +^x bitwise complement is m ^ x with m = "all bits set to 1" +-Integer overflow ----- +
+For signed integers, the operations "+", "-", "*", and "<<" 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. As a consequence, a compiler may not optimize code under the assumption that overflow does -not occur. For instance, it may not assume that "x < x + 1" is always true. +not occur. For instance, it may not assume that "x < x + 1" is always true. -Comparison operators ----- +
+== equal +!= not equal +< less +<= less or equal +> greater +>= greater or equal +Strings are compared byte-wise (lexically). - +
Booleans are equal if they are either both "true" or both "false". - +
Pointers are equal if they point to the same value. - +
Interface, slice, map, and channel types can be compared for equality according to the rules specified in the section on §Interface types, §Slice types, §Map types, and §Channel types, respectively. -Logical operators ----- +
+&& 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" +-Address operators ----- +
+TODO: This text needs to be cleaned up and go elsewhere, there are no address operators involved. - + +
Methods are a form of function, and a method ``value'' has a function type. Consider the type T with method M: - type T struct { - a int; - } - func (tp *T) M(a int) int; - var t *T; +
+type T struct {
+ a int;
+}
+func (tp *T) M(a int) int;
+var t *T;
+
To construct the value of method M, one writes
- t.M
++t.M +using the variable t (not the type T). -TODO: It makes perfect sense to be able to say T.M (in fact, it makes more +TODO: It makes perfect sense to be able to say T.M (in fact, it makes more sense then t.M, since only the type T is needed to find the method M, i.e., its address). TBD. + The expression t.M is a function value with type - func (t *T, a int) int +
+func (t *T, a int) int +and may be invoked only as a function, not as a method: - var f func (t *T, a int) int; - f = t.M; - x := f(t, 7); +
+var f func (t *T, a int) int; +f = t.M; +x := f(t, 7); +Note that one does not write t.f(7); taking the value of a method demotes it to a function. @@ -2425,231 +2674,288 @@ it to a function. In general, given type T with method M and variable t of type T, the method invocation - t.M(args) +
+t.M(args) +is equivalent to the function call - (t.M)(t, args) +
+(t.M)(t, args) ++ TODO: should probably describe the effect of (t.m) under §Expressions if t.m denotes a method: Effect is as described above, converts into function. - + +
If T is an interface type, the expression t.M does not determine which underlying type's M is called until the point of the call itself. Thus given T1 and T2, both implementing interface I with method M, the sequence - var t1 *T1; - var t2 *T2; - var i I = t1; - m := i.M; - m(t2, 7); +
+var t1 *T1; +var t2 *T2; +var i I = t1; +m := i.M; +m(t2, 7); +will invoke t2.M() even though m was constructed with an expression involving t1. Effectively, the value of m is a function literal - func (recv I, a int) { - recv.M(a); - } +
+func (recv I, a int) {
+ recv.M(a);
+}
+
that is automatically created.
-
++ TODO: Document implementation restriction: It is illegal to take the address -of a result parameter (e.g.: func f() (x int, p *int) { return 2, &x }). +of a result parameter (e.g.: func f() (x int, p *int) { return 2, &x }). (TBD: is it an implementation restriction or fact?) + - -Communication operators ----- +
Here the term "channel" means "variable of type chan". - +
The built-in function "make" makes a new channel value: - ch := make(chan int) +
+ch := make(chan int) +An optional argument to "make()" specifies a buffer size for an asynchronous channel; if absent or zero, the channel is synchronous: - sync_chan := make(chan int) - buffered_chan := make(chan int, 10) +
+sync_chan := make(chan int) +buffered_chan := make(chan int, 10) +-The send operation uses the binary operator "<-", which operates on +The send operation uses the binary operator "<-", which operates on a channel and a value (expression): - ch <- 3 +
+ch <- 3 +In this form, the send operation is an (expression) statement that 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. - +
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. These two examples are equivalent: - ok := ch <- 3; - if ok { print("sent") } else { print("not sent") } +
+ok := ch <- 3;
+if ok { print("sent") } else { print("not sent") }
- if ch <- 3 { print("sent") } else { print("not sent") }
+if ch <- 3 { print("sent") } else { print("not sent") }
+
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.
-
++ TODO: Adjust the above depending on how we rule on the ok semantics. For instance, does the sent expression get evaluated if ok is false? - -The receive operation uses the prefix unary operator "<-". + +
+The receive operation uses the prefix unary operator "<-". The value of the expression is the value received: - <-ch +
+<-ch +The expression blocks until a value is available, which then can be assigned to a variable or used like any other expression: - v1 := <-ch - v2 = <-ch - f(<-ch) +
+v1 := <-ch +v2 = <-ch +f(<-ch) +If the receive expression does not save the value, the value is discarded: - <-strobe // wait until clock pulse +
+<-strobe // wait until clock pulse +If a receive expression is used in a tuple assignment of the form - x, ok = <-ch; // or: x, ok := <-ch +
+x, ok = <-ch; // or: x, ok := <-ch +the receive operation becomes non-blocking, and the boolean variable "ok" will be set to "true" if the receive operation succeeded, and set to "false" otherwise. -Constant expressions ----- +
+len(a) if a is an array (as opposed to an array slice) ++ TODO: Complete this list as needed. - + +
Constant expressions can be evaluated at compile time. +
+Statement = + Declaration | LabelDecl | EmptyStat | + SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat | + FallthroughStat | Block | IfStat | SwitchStat | SelectStat | ForStat | + DeferStat . - SimpleStat = - ExpressionStat | IncDecStat | Assignment | SimpleVarDecl . +SimpleStat = + ExpressionStat | IncDecStat | Assignment | SimpleVarDecl . +Statements in a statement list are separated by semicolons, which can be omitted in some cases as expressed by the OptSemicolon production. - StatementList = Statement { OptSemicolon Statement } . +
+StatementList = Statement { OptSemicolon Statement } .
+
A semicolon may be omitted immediately following:
-
- - a closing parenthesis ")" ending a list of declarations (§Declarations and scope rules)
- - a closing brace "}" ending a type declaration (§Type declarations)
- - a closing brace "}" ending a block (including switch and select statements)
- - a label declaration (§Label declarations)
++
+EmptyStat = . +-Expression statements ----- +
+ExpressionStat = Expression . +- f(x+y) +
+f(x+y) ++ TODO: specify restrictions. 6g only appears to allow calls here. + -IncDec statements ----- +
+IncDecStat = Expression ( "++" | "--" ) . +The following assignment statements (§Assignments) are semantically equivalent: - IncDec statement Assignment - x++ x += 1 - x-- x -= 1 +
+IncDec statement Assignment +x++ x += 1 +x-- x -= 1 +Both operators apply to integer and floating point types only. - +
Note that increment and decrement are statements, not expressions. For instance, "x++" cannot be used as an operand in an expression. -Assignments ----- +
+Assignment = ExpressionList assign_op ExpressionList . +- assign_op = [ add_op | mul_op ] "=" . +
+assign_op = [ add_op | mul_op ] "=" . +The left-hand side must be an l-value such as a variable, pointer indirection, or an array index. - x = 1 - *p = f() - a[i] = 23 - k = <-ch +
+x = 1 +*p = f() +a[i] = 23 +k = <-ch +As in C, arithmetic binary operators can be combined with assignments: - j <<= 2 +
+j <<= 2 +A tuple assignment assigns the individual elements of a multi-valued operation, such as function evaluation or some channel and map operations, into individual variables. For instance, a tuple assignment such as - v1, v2, v3 = e1, e2, e3 +
+v1, v2, v3 = e1, e2, e3 +assigns the expressions e1, e2, e3 to temporaries and then assigns the temporaries to the variables v1, v2, v3. Thus - a, b = b, a +
+a, b = b, a +exchanges the values of a and b. The tuple assignment - x, y = f() +
+x, y = f() +calls the function f, which must return two values, and assigns them to x and y. As a special case, retrieving a value from a map, when written as a two-element @@ -2657,42 +2963,51 @@ tuple assignment, assign a value and a boolean. If the value is present in the m the value is assigned and the second, boolean variable is set to true. Otherwise, the variable is unchanged, and the boolean value is set to false. - value, present = map_var[key] +
+value, present = map_var[key] +To delete a value from a map, use a tuple assignment with the map on the left and a false boolean expression as the second expression on the right, such as: - map_var[key] = value, false +
+map_var[key] = value, false +In assignments, the type of the expression must match the type of the left-hand side. -If statements ----- +
+IfStat = "if" [ [ SimpleStat ] ";" ] [ Expression ] Block [ "else" Statement ] . +- if x > 0 { - return true; - } +
+if x > 0 {
+ return true;
+}
+
An "if" statement may include the declaration of a single temporary variable.
The scope of the declared variable extends to the end of the if statement, and
the variable is initialized once before the statement is entered.
- if x := f(); x < y {
- return x;
- } else if x > z {
- return z;
- } else {
- return y;
- }
+
+if x := f(); x < y {
+ return x;
+} else if x > z {
+ return z;
+} else {
+ return y;
+}
+
-Switch statements
-----
+
+SwitchStat = "switch" [ [ SimpleStat ] ";" ] [ Expression ] "{" { CaseClause } "}" .
+CaseClause = SwitchCase ":" [ StatementList ] .
+SwitchCase = "case" ExpressionList | "default" .
+
There can be at most one default case in a switch statement. In a case clause,
the last statement only may be a fallthrough statement ($Fallthrough statement).
It indicates that the control should flow from the end of this case clause to
the first statement of the next clause.
-
+Each case clause effectively acts as a block for scoping purposes ($Declarations and scope rules). - +
The expressions do not need to be constants. They will be evaluated top to bottom until the first successful non-default case is reached. If none matches and there is a default case, the statements of the default case are executed. - switch tag { - default: s3() - case 0, 1: s1() - case 2: s2() - } +
+switch tag {
+default: s3()
+case 0, 1: s1()
+case 2: s2()
+}
+
A switch statement may include the declaration of a single temporary variable.
The scope of the declared variable extends to the end of the switch statement, and
the variable is initialized once before the switch is entered.
- switch x := f(); true {
- case x < 0: return -x
- default: return x
- }
+
+switch x := f(); true {
+case x < 0: return -x
+default: return x
+}
+
Cases do not fall through unless explicitly marked with a "fallthrough" statement.
- switch a {
- case 1:
- b();
- fallthrough
- case 2:
- c();
- }
+
+switch a {
+case 1:
+ b();
+ fallthrough
+case 2:
+ c();
+}
+
If the expression is omitted, it is equivalent to "true".
- switch {
- case x < y: f1();
- case x < z: f2();
- case x == 4: f3();
- }
+
+switch {
+case x < y: f1();
+case x < z: f2();
+case x == 4: f3();
+}
+
-For statements
-----
++ForStat = "for" [ Condition | ForClause | RangeClause ] Block . +Condition = Expression . +In its simplest form, a for statement specifies the repeated execution of a block as long as a condition evaluates to true. The condition is evaluated before each iteration. The type of the condition expression must be boolean. If the condition is absent, it is equivalent to "true". - for a < b { - a *= 2 - } +
+for a < b {
+ a *= 2
+}
+
A for statement with a for clause is also controlled by its condition, but
additionally it may specify an init and post statement, such as an assignment,
an increment or decrement statement. The init statement may also be a (simple)
variable declaration; no variables can be declared in the post statement.
- ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
- InitStat = SimpleStat .
- PostStat = SimpleStat .
++ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] . +InitStat = SimpleStat . +PostStat = SimpleStat . +For instance, one may declare an iteration variable in the init statement: - for i := 0; i < 10; i++ { - f(i) - } +
+for i := 0; i < 10; i++ {
+ f(i)
+}
+
If present, the init statement is executed once before commencing the iteration;
the post statement is executed after each execution of the statement block (and
only if the block was executed). The scope of any variable declared in the init
statement ends with the end of the for statement block ($Declarations and scope
rules, Rule 3).
-
+The init and post statement as well as the condition may be omitted; however if either the init or post statement are present, the separating semicolons must be present. If the condition is absent, it is equivalent to "true". The following statements are equivalent: - for ; cond ; { S() } is the same as for cond { S() } - for true { S() } is the same as for { S() } +
+for ; cond ; { S() } is the same as for cond { S() }
+for true { S() } is the same as for { S() }
+
Alternatively, a for statement may be controlled by a range clause. A
range clause specifies iteration through all entries of an array or map.
@@ -2827,7 +3160,9 @@ of iteration variables - and then executes the block. Iteration terminates
when all entries have been processed, or if the for statement is terminated
early, for instance by a break or return statement.
- RangeClause = IdentifierList ( "=" | ":=" ) "range" Expression .
++RangeClause = IdentifierList ( "=" | ":=" ) "range" Expression . +The type of the right-hand expression in the range clause must be an array or map, or a pointer to an array or map. If it is a pointer, it must not be nil. @@ -2837,29 +3172,31 @@ map key, and the second variable, if present, is set to the corresponding array element or map value. The types of the array index (int) and element, or of the map key and value respectively, must be assignment-compatible to the iteration variables. - +
The iteration variables may be declared by the range clause (":="), in which case their scope ends at the end of the for statement block ($Declarations and scope rules, Rule 3). In this case their types are the array index and element, or the map key and value types, respectively. - var a [10]string; - m := map[string]int("mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6); - - for i, s := range a { - // type of i is int - // type of s is string - // s == a[i] - g(i, s) - } +
+var a [10]string;
+m := map[string]int("mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6);
- var key string;
- var val interface {}; // value type of m is assignment-compatible to val
- for key, value = range m {
- h(key, value)
- }
- // key == last map key encountered in iteration
- // val == map[key]
+for i, s := range a {
+ // type of i is int
+ // type of s is string
+ // s == a[i]
+ g(i, s)
+}
+
+var key string;
+var val interface {}; // value type of m is assignment-compatible to val
+for key, value = range m {
+ h(key, value)
+}
+// key == last map key encountered in iteration
+// val == map[key]
+
If map entries that have not yet been processed are deleted during iteration,
they will not be processed. If map entries are inserted during iteration, the
@@ -2870,38 +3207,42 @@ the values of those variables for the current iteration; it does not affect any
subsequent iterations.
-Go statements
-----
++GoStat = "go" Expression . +Unlike with a regular function call, program execution does not wait for the invoked function to complete. - go Server() - go func(ch chan <- bool) { for { sleep(10); ch <- true; }} (c) +
+go Server()
+go func(ch chan <- bool) { for { sleep(10); ch <- true; }} (c)
+
-Select statements
-----
+
+SelectStat = "select" "{" { CommClause } "}" .
+CommClause = CommCase ":" [ StatementList ] .
+CommCase = "case" ( SendExpr | RecvExpr) | "default" .
+SendExpr = Expression "<-" Expression .
+RecvExpr = [ Expression ( "=" | ":=" ) ] "<-" Expression .
+
Each communication clause acts as a block for the purpose of scoping
(§Declarations and scope rules).
-
+For all the send and receive expressions in the select statement, the channel expression is evaluated. Any values that appear on the right hand side of send expressions are also @@ -2912,171 +3253,196 @@ if not, the statement blocks until one of the communications can complete. The channels and send expressions are not re-evaluated. A channel pointer may be nil, which is equivalent to that case not being present in the select statement. - +
Since all the channels and send expressions are evaluated, any side effects in that evaluation will occur for all the communications in the select. - +
If the channel sends or receives an interface type, its communication can proceed only if the type of the communication clause matches that of the dynamic value to be exchanged. - +
If multiple cases can proceed, a uniform fair choice is made regarding which single communication will execute. - +
The receive case may declare a new variable (via a ":=" assignment). The scope of such variables begins immediately after the variable identifier and ends at the end of the respective "select" case (that is, before the next "case", "default", or closing brace). - var c, c1, c2 chan int; - var i1, i2 int; +
+var c, c1, c2 chan int;
+var i1, i2 int;
+select {
+case i1 = <-c1:
+ print("received ", i1, " from c1\n");
+case c2 <- i2:
+ print("sent ", i2, " to c2\n");
+default:
+ print("no communication\n");
+}
+
+for { // send random sequence of bits to c
select {
- case i1 = <-c1:
- print("received ", i1, " from c1\n");
- case c2 <- i2:
- print("sent ", i2, " to c2\n");
- default:
- print("no communication\n");
+ case c <- 0: // note: no statement, no fallthrough, no folding of cases
+ case c <- 1:
}
+}
- for { // send random sequence of bits to c
- select {
- case c <- 0: // note: no statement, no fallthrough, no folding of cases
- case c <- 1:
- }
- }
-
- var ca chan interface {};
- var i int;
- var f float;
- select {
- case i = <-ca:
- print("received int ", i, " from ca\n");
- case f = <-ca:
- print("received float ", f, " from ca\n");
- }
+var ca chan interface {};
+var i int;
+var f float;
+select {
+case i = <-ca:
+ print("received int ", i, " from ca\n");
+case f = <-ca:
+ print("received float ", f, " from ca\n");
+}
+
+
TODO: Make semantics more precise.
+
-Return statements
-----
++ReturnStat = "return" [ ExpressionList ] . +There are two ways to return values from a function. The first is to explicitly list the return value or values in the return statement: - func simple_f() int { - return 2; - } +
+func simple_f() int {
+ return 2;
+}
+
A function may return multiple values.
The syntax of the return clause in that case is the same as
that of a parameter list; in particular, names must be provided for
the elements of the return value.
- func complex_f1() (re float, im float) {
- return -7.0, -4.0;
- }
+
+func complex_f1() (re float, im float) {
+ return -7.0, -4.0;
+}
+
A second method to return values
is to use those names within the function as variables
to be assigned explicitly; the return statement will then provide no
values:
- func complex_f2() (re float, im float) {
- re = 7.0;
- im = 4.0;
- return;
- }
+
+func complex_f2() (re float, im float) {
+ re = 7.0;
+ im = 4.0;
+ return;
+}
+
-Break statements
-----
++BreakStat = "break" [ identifier ]. +If there is an identifier, it must be a label marking an enclosing for, switch, or select statement, and that is the one whose execution terminates. - L: for i < n { - switch i { - case 5: break L - } +
+L: for i < n {
+ switch i {
+ case 5: break L
}
+}
+
-Continue statements
-----
++ContinueStat = "continue" [ identifier ]. +The optional identifier is analogous to that of a break statement. -Label declarations ----- +
+LabelDecl = identifier ":" . +Example: - Error: +
+Error: +-Goto statements ----- +
+GotoStat = "goto" identifier . +- goto Error +
+goto Error +Executing the goto statement must not cause any variables to come into scope that were not already in scope at the point of the goto. For instance, this example: - goto L; // BAD - v := 3; - L: +
+goto L; // BAD +v := 3; +L: +is erroneous because the jump to label L skips the creation of v. -Fallthrough statements ----- +
+FallthroughStat = "fallthrough" . +-Defer statements ----- +
+DeferStat = "defer" Expression . +The expression must be a function or method call. Each time the defer statement executes, the parameters to the function call are evaluated and saved anew but the @@ -3085,45 +3451,50 @@ the defer statement returns, but after its return value (if any) is evaluated, each deferred function is executed with its saved parameters. Deferred functions are executed in LIFO order. - lock(l); - defer unlock(l); // unlocking happens before surrounding function returns +
+lock(l);
+defer unlock(l); // unlocking happens before surrounding function returns
- // prints 3 2 1 0 before surrounding function returns
- for i := 0; i <= 3; i++ {
- defer fmt.Print(i);
- }
+// prints 3 2 1 0 before surrounding function returns
+for i := 0; i <= 3; i++ {
+ defer fmt.Print(i);
+}
+
++FunctionDecl = "func" identifier Signature [ Block ] . +- func min(x int, y int) int { - if x < y { - return x; - } - return y; +
+func min(x int, y int) int {
+ if x < y {
+ return x;
}
+ return y;
+}
+
A function declaration without a block serves as a forward declaration:
- func MakeNode(left, right *Node) *Node
++func MakeNode(left, right *Node) *Node +Implementation restrictions: Functions can only be declared at the global level. A function must be declared or forward-declared before it can be invoked. -Method declarations ----- +
+MethodDecl = "func" Receiver identifier Signature [ Block ] .
+Receiver = "(" [ identifier ] [ "*" ] TypeName ")" .
+
All methods bound to a receiver base type must have the same receiver type:
Either all receiver types are pointers to the base type or they are the base
-type. (TODO: This restriction can be relaxed at the cost of more complicated
+type.
+(TODO: This restriction can be relaxed at the cost of more complicated
assignment rules to interface types).
+
For instance, given type Point, the declarations
- func (p *Point) Length() float {
- return Math.sqrt(p.x * p.x + p.y * p.y);
- }
+
+func (p *Point) Length() float {
+ return Math.sqrt(p.x * p.x + p.y * p.y);
+}
- func (p *Point) Scale(factor float) {
- p.x = p.x * factor;
- p.y = p.y * factor;
- }
+func (p *Point) Scale(factor float) {
+ p.x = p.x * factor;
+ p.y = p.y * factor;
+}
+
bind the methods "Length" and "Scale" to the receiver base type "Point".
@@ -3160,117 +3537,133 @@ Method declarations may appear anywhere after the declaration of the receiver
base type and may be forward-declared.
-Predeclared functions
-----
++
+Call Argument type Result -Length and capacity ----- +len(s) string, *string string length (in bytes) + [n]T, *[n]T array length (== n) + []T, *[]T slice length + map[K]T, *map[K]T map length + chan T number of elements in channel buffer - Call Argument type Result - - len(s) string, *string string length (in bytes) - [n]T, *[n]T array length (== n) - []T, *[]T slice length - map[K]T, *map[K]T map length - chan T number of elements in channel buffer - - cap(s) []T, *[]T capacity of s - map[K]T, *map[K]T capacity of s - chan T channel buffer capacity +cap(s) []T, *[]T capacity of s + map[K]T, *map[K]T capacity of s + chan T channel buffer capacity ++ TODO: confirm len() and cap() for channels + +
The type of the result is always "int" and the implementation guarantees that the result always fits into an "int". - +
The capacity of a slice or map is the number of elements for which there is space allocated in the underlying array (for a slice) or map. For a slice "s", at any time the following relationship holds: - 0 <= len(s) <= cap(s) +
+0 <= len(s) <= cap(s) +-Conversions ----- +
+T(value) +where "T" is the type name of an arithmetic type or string (§Basic types), and "value" is the value of an expression which can be converted to a value of result type "T". - +
The following conversion rules apply: - +
1) Between integer types. If the value is a signed quantity, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type size. For example, uint32(int8(0xFF)) is 0xFFFFFFFF. The conversion always yields a valid value; there is no signal for overflow. - +
2) Between integer and floating point types, or between floating point types. To avoid overdefining the properties of the conversion, for now it is defined as a ``best effort'' conversion. The conversion always succeeds but the value may be a NaN or other problematic -result. TODO: clarify? - +result. TODO: clarify? +
3) Strings permit two special conversions. - +
3a) Converting an integer value yields a string containing the UTF-8 representation of the integer. - string(0x65e5) // "\u65e5" +
+string(0x65e5) // "\u65e5" +3b) Converting an array of uint8s yields a string whose successive bytes are those of the array. (Recall byte is a synonym for uint8.) - string([]byte('h', 'e', 'l', 'l', 'o')) // "hello" +
+string([]byte('h', 'e', 'l', 'l', 'o')) // "hello"
+
There is no linguistic mechanism to convert between pointers
and integers. A library may be provided under restricted circumstances
to acccess this conversion in low-level code.
-
++ TODO: Do we allow interface/ptr conversions in this form or do they have to be written as type guards? (§Type guards) + -Allocation ----- +
+new(T) +For instance - type S struct { a int; b float } - new(S) +
+type S struct { a int; b float }
+new(S)
+
dynamically allocates memory for a variable of type S, initializes it
(a=0, b=0.0), and returns a value of type *S pointing to that variable.
-
++ TODO Once this has become clearer, connect new() and make() (new() may be explained by make() and vice versa). + - -Making slices, maps, and channels ----- +
+make(T [, optional list of expressions]) +For instance - make(map[string] int) +
+make(map[string] int) +creates a new map value and initializes it to an empty map. The only defined parameters affect sizes for allocating slices, maps, and buffered channels: - s := make([]int, 10, 100); # slice with len(s) == 10, cap(s) == 100 - c := make(chan int, 10); # channel with a buffer size of 10 - m := make(map[string] int, 100); # map with initial space for 100 elements +
+s := make([]int, 10, 100); # slice with len(s) == 10, cap(s) == 100 +c := make(chan int, 10); # channel with a buffer size of 10 +m := make(map[string] int, 100); # map with initial space for 100 elements ++ TODO Once this has become clearer, connect new() and make() (new() may be explained by make() and vice versa). + +
+Package = PackageClause { ImportDecl [ ";" ] } { Declaration [ ";" ] } .
+
The source text following the package clause acts like a block for scoping
purposes ($Declarations and scope rules).
-
+Every source file identifies the package to which it belongs. The file must begin with a package clause. - PackageClause = "package" PackageName . +
+PackageClause = "package" PackageName . - package Math +package Math +A package can gain access to exported identifiers from another package through an import declaration: - ImportDecl = "import" ( ImportSpec | "(" [ ImportSpecList ] ")" ) . - ImportSpecList = ImportSpec { ";" ImportSpec } [ ";" ] . - ImportSpec = [ "." | PackageName ] PackageFileName . - PackageFileName = StringLit . +
+ImportDecl = "import" ( ImportSpec | "(" [ ImportSpecList ] ")" ) .
+ImportSpecList = ImportSpec { ";" ImportSpec } [ ";" ] .
+ImportSpec = [ "." | PackageName ] PackageFileName .
+PackageFileName = StringLit .
+
An import statement makes the exported top-level identifiers of the named
package file accessible to this package.
-
+In the following discussion, assume we have a package in the file "/lib/math", called package "math", which exports the identifiers "Sin" and "Cos" denoting the respective trigonometric functions. - +
In the general form, with an explicit package name, the import statement declares that package name as an identifier whose contents are the exported elements of the imported package. For instance, after - import M "/lib/math" +
+import M "/lib/math" +the contents of the package /lib/math can be accessed by "M.Sin", "M.Cos", etc. - +
In its simplest form, with no package name, the import statement implicitly uses the imported package name itself as the local package name. After - import "/lib/math" +
+import "/lib/math" +the contents are accessible by "math.Sin", "math.Cos". - +
Finally, if instead of a package name the import statement uses an explicit period, the contents of the imported package are added to the current package. After - import . "/lib/math" +
+import . "/lib/math" +the contents are accessible by "Sin" and "Cos". In this instance, it is an error if the import introduces name conflicts. - +
Here is a complete example Go package that implements a concurrent prime sieve: - package main - - // Send the sequence 2, 3, 4, ... to channel 'ch'. - func generate(ch chan <- int) { - for i := 2; ; i++ { - ch <- i // Send 'i' to channel 'ch'. +
+package main
+
+// Send the sequence 2, 3, 4, ... to channel 'ch'.
+func generate(ch chan <- int) {
+ for i := 2; ; i++ {
+ ch <- i // Send 'i' to channel 'ch'.
+ }
+}
+
+// Copy the values from channel 'in' to channel 'out',
+// removing those divisible by 'prime'.
+func filter(in chan <- int, out *<-chan int, prime int) {
+ for {
+ i := <-in; // Receive value of new variable 'i' from 'in'.
+ if i % prime != 0 {
+ out <- i // Send 'i' to channel 'out'.
}
}
-
- // Copy the values from channel 'in' to channel 'out',
- // removing those divisible by 'prime'.
- func filter(in chan <- int, out *<-chan int, prime int) {
- for {
- i := <-in; // Receive value of new variable 'i' from 'in'.
- if i % prime != 0 {
- out <- i // Send 'i' to channel 'out'.
- }
- }
- }
-
- // The prime sieve: Daisy-chain filter processes together.
- func sieve() {
- ch := make(chan int); // Create a new channel.
- go generate(ch); // Start generate() as a subprocess.
- for {
- prime := <-ch;
- print(prime, "\n");
- ch1 := make(chan int);
- go filter(ch, ch1, prime);
- ch = ch1
- }
- }
-
- func main() {
- sieve()
- }
+}
+// The prime sieve: Daisy-chain filter processes together.
+func sieve() {
+ ch := make(chan int); // Create a new channel.
+ go generate(ch); // Start generate() as a subprocess.
+ for {
+ prime := <-ch;
+ print(prime, "\n");
+ ch1 := make(chan int);
+ go filter(ch, ch1, prime);
+ ch = ch1
+ }
+}
-----
+func main() {
+ sieve()
+}
+
-Program initialization and execution
-----
+These two simple declarations are equivalent: - var i int; - var i int = 0; +
+var i int; +var i int = 0; +After - type T struct { i int; f float; next *T }; - t := new(T); +
+type T struct { i int; f float; next *T };
+t := new(T);
+
the following holds:
- t.i == 0
- t.f == 0.0
- t.next == nil
++t.i == 0 +t.f == 0.0 +t.next == nil +A package with no imports is initialized by assigning initial values to @@ -3435,125 +3852,136 @@ all its global variables in declaration order and then calling any init() functions defined in its source. Since a package may contain more than one source file, there may be more than one init() function, but only one per source file. - +
Initialization code may contain "go" statements, but the functions they invoke do not begin execution until initialization of the entire program is complete. Therefore, all initialization code is run in a single thread of execution. - +
Furthermore, an "init()" function cannot be referred to from anywhere in a program. In particular, "init()" cannot be called explicitly, nor can a pointer to "init" be assigned to a function variable). - +
If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package P, P will be initialized only once. - +
The importing of packages, by construction, guarantees that there can be no cyclic dependencies in initialization. - +
A complete program, possibly created by linking multiple packages, must have one package called main, with a function - func main() { ... } +
+func main() { ... }
+
defined. The function main.main() takes no arguments and returns no
value.
-
+Program execution begins by initializing the main package and then invoking main.main(). - +
When main.main() returns, the program exits. +
The package "unsafe" provides (at least) the following package interface: - package unsafe +
+package unsafe - const Maxalign int +const Maxalign int - type Pointer *any +type Pointer *any - func Alignof(variable any) int - func Offsetof(selector any) int - func Sizeof(variable any) int +func Alignof(variable any) int +func Offsetof(selector any) int +func Sizeof(variable any) int +The pseudo type "any" stands for any Go type; "any" is not a type generally available in Go programs. - +
Any pointer type as well as values of type "uintptr" can be converted into an "unsafe.Pointer" and vice versa. - +
The function "Sizeof" takes an expression denoting a variable of any type and returns the size of the variable in bytes. - +
The function "Offsetof" takes a selector (§Selectors) denoting a struct field of any type and returns the field offset in bytes relative to the struct address. Specifically, the following condition is satisfied for a struct "s" with field "f": - uintptr(unsafe.Pointer(&s)) + uintptr(unsafe.Offsetof(s.f)) == - uintptr(unsafe.Pointer(&s.f)) +
+uintptr(unsafe.Pointer(&s)) + uintptr(unsafe.Offsetof(s.f)) == +uintptr(unsafe.Pointer(&s.f)) +Computer architectures may impose restrictions on the memory addresses accessed directly by machine instructions. A common such restriction is the requirement for such addresses to be ``aligned''; that is, addresses must be a multiple of a factor, the ``alignment''. The alignment depends on the type of datum accessed. - +
The function "Alignof" takes an expression denoting a variable of any type and returns the alignment of the variable in bytes. The following alignment condition is satisfied for a variable "x": - uintptr(unsafe.Pointer(&x)) % uintptr(unsafe.Alignof(x)) == 0 +
+uintptr(unsafe.Pointer(&x)) % uintptr(unsafe.Alignof(x)) == 0 +The maximum alignment is given by the constant "unsafe.Maxalign". It usually corresponds to the value of "unsafe.Sizeof(x)" for a variable "x" of the largest arithmetic type (8 for a float64), but may be smaller on systems that have less stringent alignment restrictions or are space constrained. - +
The results of calls to "unsafe.Alignof", "unsafe.Offsetof", and "unsafe.Sizeof" are compile-time constants. -Size and alignment guarantees ----- +
+type size in bytes - byte, uint8, int8 1 - uint16, int16 2 - uint32, int32, float32 4 - uint64, int64, float64 8 +byte, uint8, int8 1 +uint16, int16 2 +uint32, int32, float32 4 +uint64, int64, float64 8 +A Go compiler guarantees the following minimal alignment properties: +
+