The Go Programming Language Specification (DRAFT) ---- Robert Griesemer, Rob Pike, Ken Thompson ---- (October 24, 2008) This document is a semi-formal specification of the Go systems programming language. This document is not ready for external review, it is under active development. Any part may change substantially as design progresses. Contents ---- Introduction Notation Source code representation Characters Letters and digits Vocabulary Identifiers Numeric literals Character and string literals Operators and delimitors Reserved words Declarations and scope rules Const declarations Type declarations Variable declarations Export declarations Types Basic types Arithmetic types Booleans Strings Array types Struct types Pointer types Map types Channel types Function types Interface types Expressions Operands Constants Qualified identifiers Iota Composite Literals Function Literals Primary expressions Selectors Indexes Slices Type guards Calls Operators Arithmetic operators Comparison operators Logical operators Address operators Communication operators Constant expressions Statements Label declarations Expression statements IncDec statements Assignments If statements Switch statements For statements Range statements Go statements Select statements Return statements Break statements Continue statements Label declaration Goto statements Function declarations Method declarations Predeclared functions Length and capacity Conversions Allocation Packages Program initialization and execution ---- Introduction ---- Notation ---- The syntax is specified using Parameterized Extended Backus-Naur Form (PEBNF). Specifically, 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) The syntax of PEBNF can be expressed in itself: Production = production_name [ Parameters ] "=" Expression . Parameters = "<" production_name { "," production_name } ">" . Expression = Alternative { "|" Alternative } . Alternative = Term { Term } . Term = production_name [ Arguments ] | token [ "..." token ] | Group | Option | Repetition . Arguments = "<" Expression { "," Expression } ">" . Group = "(" Expression ")" . Option = "[" Expression ")" . Repetition = "{" Expression "}" . Lower-case production names are used to identify productions that cannot be broken by white space or comments; they are usually 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. Productions can be parameterized. To get the actual production the parameter is substituted with the argument provided where the production name is used. For instance, there are various forms of semicolon-separated lists in the grammar. The parameterized production for such lists is: List

= P { ";" P } [ ";" ] . In this case, P stands for the actual list element. 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. Source code representation ---- Source code is Unicode text encoded in UTF-8. 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. Characters ---- In the grammar we use the notation utf8_char to refer to an arbitrary Unicode code point encoded in UTF-8. We use non_ascii to refer to the subset of "utf8_char" code points with values >= 128. Letters and digits ---- letter = "A" ... "Z" | "a" ... "z" | "_" | non_ascii. decimal_digit = "0" ... "9" . octal_digit = "0" ... "7" . hex_digit = "0" ... "9" | "A" ... "F" | "a" ... "f" . All non-ASCII code points are considered letters; digits are always ASCII. Vocabulary ---- Tokens make up the vocabulary of the Go language. They consist of identifiers, numbers, strings, operators, and delimitors. Identifiers ---- An identifier is a name for a program entity such as a variable, a type, a function, etc. identifier = letter { letter | decimal_digit } . a _x ThisIsVariable9 αβ Some identifiers are predeclared (§Declarations). Numeric literals ---- An integer literal represents a mathematically ideal integer constant of arbitrary precision, or 'ideal int'. 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 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 . 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 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 ---- Character and string literals are almost the same as in C, with the following differences: - The encoding is UTF-8 - `` strings exist; they do not interpret backslashes - Octal character escapes are always 3 digits ("\077" not "\77") - Hexadecimal character escapes are always 2 digits ("\x07" not "\x7") The rules are: char_lit = "'" ( unicode_value | byte_value ) "'" . unicode_value = utf8_char | little_u_value | big_u_value | escaped_char . byte_value = octal_byte_value | hex_byte_value . octal_byte_value = "\" octal_digit octal_digit octal_digit . hex_byte_value = "\" "x" hex_digit hex_digit . little_u_value = "\" "u" hex_digit hex_digit hex_digit hex_digit . big_u_value = "\" "U" hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit . escaped_char = "\" ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | "\" | "'" | """ ) . A unicode_value takes one of four forms: * The UTF-8 encoding of a Unicode code point. Since Go source text is in UTF-8, this is the obvious translation from input text into Unicode characters. * The usual list of C backslash escapes: "\n", "\t", etc. Within a character or string literal, only the corresponding quote character is a legal escape (this is not explicitly reflected in the above syntax). * A `little u' value, such as "\u12AB". This represents the Unicode code point with the corresponding hexadecimal value. It always has exactly 4 hexadecimal digits. * A `big U' value, such as "\U00101234". This represents the Unicode code point with the corresponding hexadecimal value. It always has exactly 8 hexadecimal digits. Some values that can be represented this way are illegal because they are not valid Unicode code points. These include values above 0x10FFFF and surrogate halves. 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' 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 = "`" { utf8_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 literal. For byte_values, these are the literal bytes; for unicode_values, these are the bytes of the UTF-8 encoding of the corresponding Unicode code points. Note that "\u00FF" and "\xFF" are different strings: the first contains the two-byte UTF-8 expansion of 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" 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 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 */ "." 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 point), and will appear as two code points if placed in a string literal. Operators and delimitors ---- The following special character sequences serve as operators or delimitors: + & += &= && == != ( ) - | -= |= || < <= [ ] * ^ *= ^= <- > >= { } / << /= <<= ++ = := , ; % >> %= >>= -- ! ... . : Reserved words ---- The following words are reserved and must not be used as identifiers: break default func interface select case else go map struct chan export goto package switch const fallthrough if range type continue for import return var Declarations and scope rules ---- A declaration ``binds'' an identifier to a language entity (such as a package, constant, type, struct field, variable, parameter, result, function, method) and specifies properties of that entity such as its type. Declaration = [ "export" ] ( ConstDecl | TypeDecl | VarDecl | FunctionDecl | MethodDecl ) . Except for function, method and abbreviated variable declarations (using ":="), all declarations follow the same pattern. There is either a single declaration of the form P, or an optional semicolon-separated list of declarations of the form P surrounded by parentheses: Decl

= P | "(" [ List

] ")" . List

= P { ";" P } [ ";" ] . Every identifier in a program must be declared; some identifiers, such as "int" and "true", are predeclared. 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. 2. The scope of an identifier denoting a type, function or package extends textually from the point of the identifier in the declaration to the end of the innermost surrounding block. 3. The scope of a constant or variable extends textually from after the declaration to the end of the innermost surrounding block. 4. The scope of a parameter or result identifier is the body of the corresponding function. 5. The scope of a field or method identifier is selectors for the corresponding type containing the field or method (§Selectors). 6. The scope of a label is the body of the innermost surrounding function and does not intersect with any non-label scope. Thus, each function has its own private label scope. An entity is said to be ``local'' to its scope. Declarations in the package scope are ``global'' declarations. Global declarations optionally may be marked for export with the reserved word "export". Local declarations can never be exported. Identifiers declared in exported declarations (and no other identifiers) are made visible to clients of this package, that is, other packages that import this package. If the declaration defines a type, the type structure is exported as well. In particular, if the declaration defines a new "struct" or "interface" type, all structure fields and all structure and interface methods are exported also. export const pi float = 3.14159265 export func Parse(source string); Note that at the moment the old-style export via ExportDecl is still supported. TODO: Eventually we need to be able to restrict visibility of fields and methods. (gri) The default should be no struct fields and methods are automatically exported. Export should be identifier-based: an identifier is either exported or not, and thus visible or not in importing package. TODO: Need some text with respect to QualifiedIdents. QualifiedIdent = [ PackageName "." ] identifier . PackageName = identifier . The following identifiers are predeclared: - all basic types: bool, uint8, uint16, uint32, uint64, int8, int16, int32, int64, float32, float64, float80, string - and their alias types: byte, ushort, uint, ulong, short, int, long, float, double, ptrint - the predeclared constants: true, false, iota, nil - the predeclared functions (note: this list is likely to change): cap(), convert(), len(), new(), panic(), print(), typeof(), ... TODO(gri) We should think hard about reducing the alias type list to: byte, uint, int, float, ptrint (note that for instance the C++ style guide is explicit about not using short, long, etc. because their sizes are unknown in general). Const declarations ---- A constant declaration binds an identifier to the value of a constant expression (§Constant expressions). ConstDecl = "const" Decl . ConstSpec = identifier [ CompleteType ] [ "=" Expression ] . const pi float = 3.14159265 const e = 2.718281828 const ( one int = 1; two = 3 ) The constant expression may be omitted, in which case the expression is the last expression used after the reserved word "const". If no such expression exists, the constant expression cannot be omitted. Together with the "iota" constant generator (§Iota), implicit repetition permits light-weight declaration of enumerated values: const ( Sunday = iota; Monday; Tuesday; Wednesday; Thursday; Friday; Partyday; ) The initializing expression of a constant may contain only other constants. This is illegal: var i int = 10; const c = i; // error The initializing expression for a numeric constant is evaluated using the principles described in the section on numeric literals: constants are mathematical values given a size only upon assignment 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; var 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 of the literals it comprises (123 vs. 1.0e4). This is because the 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; 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 specified, the resulting constant has the named type. If the type is missing from the constant declaration, the constant represents a value of abitrary precision, either integer or floating point, determined by the type of the initializing expression. Such a constant may be assigned to any variable that can represent its value accurately, regardless of type. For instance, 3 can be assigned to any int variable but also to any floating point variable, while 1e12 can be assigned to a float32, float64, or even int64. It is erroneous to assign a value with a non-zero fractional part to an integer, or if the assignment would overflow or underflow. Type declarations ---- A type declaration specifies a new type and binds an identifier to it. TypeDecl = "type" Decl . 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 ( Point struct { x, y float }; Polar Point ) type TreeNode struct { left, right *TreeNode; value Point; } type Comparable interface { cmp(Comparable) int } Variable declarations ---- A variable declaration creates a variable, binds an identifier to it and gives it a type. It may optionally give the variable an initial value. The variable type must be a complete type (§Types). In some forms of declaration the type of the initial value defines the type of the variable. VarDecl = "var" Decl . VarSpec = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) . IdentifierList = identifier { "," identifier } . ExpressionList = Expression { "," Expression } . var i int var u, v, w float var k = 0 var x, y float = -1.0, -2.0 var ( i int; u, v = 2.0, 3.0 ) 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 The syntax SimpleVarDecl = identifier ":=" Expression . is shorthand for var identifier = Expression. i := 0 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. Export declarations ---- Global identifiers may be exported, thus making the exported identifier visible outside the package. Another package may then import the identifier to use it. Export declarations must only appear at the global level of a source file and can name only globally-visible identifiers. That is, one can export global functions, types, and so on but not local variables or structure fields. Exporting an identifier makes the identifier visible externally to the package. If the identifier represents a type, it must be a complete type (§Types) and the type structure is exported as well. The exported identifiers may appear later in the source than the export directive itself, but it is an error to specify an identifier not declared anywhere in the source file containing the export directive. ExportDecl = "export" ExportIdentifier { "," ExportIdentifier } . ExportIdentifier = QualifiedIdent . export sin, cos export math.abs TODO: complete this section TODO: export as a mechanism for public and private struct fields? Types ---- A type specifies the set of values that variables of that type may assume, and the operators that are applicable. There are basic types and composite types. Basic types are predeclared. Composite types are arrays, maps, channels, structures, functions, pointers, and interfaces. They are constructed from other (basic or composite) types. Type = TypeName | ArrayType | ChannelType | InterfaceType | FunctionType | MapType | StructType | PointerType . TypeName = QualifiedIdent. Types may be ``complete'' or ''incomplete''. Basic, pointer, function and interface types are always complete (although their components, such as the base type of a pointer type, may be incomplete). All other types are complete when they are fully declared. Incomplete types are subject to usage restrictions; for instance the type of a variable must be complete where the variable is declared. 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. TODO: Since methods are added one at a time, the interface of a type may be different at different points in the source text. Thus, static checking may give different results then dynamic checking which is problematic. Need to resolve. 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 runtime. 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 ---- Go defines a number of basic types, referred to by their predeclared type names. These include traditional arithmetic types, booleans, and strings. Arithmetic types ---- uint8 the set of all unsigned 8-bit integers uint16 the set of all unsigned 16-bit integers uint32 the set of all unsigned 32-bit integers uint64 the set of all unsigned 64-bit integers int8 the set of all signed 8-bit integers, in 2's complement int16 the set of all signed 16-bit integers, in 2's complement int32 the set of all signed 32-bit integers, in 2's complement int64 the set of all signed 64-bit integers, in 2's complement 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 float80 the set of all valid IEEE-754 80-bit floating point numbers Additionally, Go declares several platform-specific type aliases; the bit width of these types is ``natural'' for the respective types for the given platform. For instance, int is usually the same as int32 on a 32-bit architecture, or int64 on a 64-bit architecture. The integer sizes are defined such that short is at least 16 bits, int is at least 32 bits, and long is at least 64 bits (and ditto for the unsigned equivalents). Also, the sizes are such that short <= int <= long. Similarly, float is at least 32 bits, double is at least 64 bits, and the sizes have float <= double. byte alias for uint8 ushort uint16 <= ushort <= uint uint uint32 <= uint <= ulong ulong uint64 <= ulong short int16 <= short <= int int int32 <= int <= long long int64 <= long float float32 <= float <= double double float64 <= double An arithmetic type ``ptrint'' is also defined. It is an unsigned integer type that is the smallest natural integer type of the machine large enough to store the uninterpreted bits of a pointer value. Generally, programmers should use these types rather than the explicitly sized types to maximize portability. Booleans ---- The type "bool" comprises the truth values true and false, which are available through the two predeclared constants, "true" and "false". Strings ---- The string type represents the set of string values (strings). Strings behave like arrays of bytes, with the following properties: - They are immutable: after creation, it is not possible to change the contents of a string. - No internal pointers: it is illegal to create a pointer to an inner element of a string. - They can be indexed: given string "s1", "s1[i]" is a byte value. - They can be concatenated: given strings "s1" and "s2", "s1 + s2" is a value combining the elements of "s1" and "s2" in sequence. - Known length: the length of a string "s1" can be obtained by calling "len(s1)". The length of a string is the number of bytes within. Unlike in C, there is no terminal NUL byte. - Creation 1: a string can be created from an integer value by a conversion; the result is a string containing the UTF-8 encoding of that code point (§Conversions). "string('x')" yields "x"; "string(0x1234)" yields the equivalent of "\u1234" - Creation 2: a string can by created from an array of integer values (maybe just array of bytes) by a conversion (§Conversions): a [3]byte; a[0] = 'a'; a[1] = 'b'; a[2] = 'c'; string(a) == "abc"; Array types ---- An array is a composite type consisting of a number of elements all of the same type, called the element type. The number of elements of an array is called its length; it is always positive (including zero). The elements of an array are designated by indices which are integers between 0 and the length - 1. An array type specifies the array element type and an optional array length which must be a compile-time constant expression of a (signed or unsigned) int type. If present, the array length and its value is part of the array type. The element type must be a complete type (§Types). If the length is present in the declaration, the array is called ``fixed array''; if the length is absent, the array is called ``open array''. ArrayType = "[" [ ArrayLength ] "]" ElementType . ArrayLength = Expression . ElementType = CompleteType . Type equality: Two array types are equal only if both have the same element type and if both are either fixed arrays with the same array length, or both are open arrays. The length of an array "a" can be discovered using the built-in function len(a) If "a" is a fixed array, the length is known at compile-time and "len(a)" can be evaluated to a compile-time constant. If "a" is an open array, then "len(a)" will only be known at run-time. The amount of space actually allocated to hold the array data may be larger then the current array length; this maximum array length is called the array capacity. The capacity of an array "a" can be discovered using the built-in function cap(a) and the following relationship between "len()" and "cap()" holds: 0 <= len(a) <= cap(a) Allocation: An open array may only be used as a function parameter type, or as element type of a pointer type. There are no other variables (besides parameters), struct or map fields of open array type; they must be pointers to open arrays. For instance, an open array may have a fixed array element type, but a fixed array must not have an open array element type (though it may have a pointer to an open array). Thus, for now, there are only ``one-dimensional'' open arrays. The following are legal array types: [32] byte [2*N] struct { x, y int32 } [1000]*[] float64 [] int [][1024] byte Variables of fixed arrays may be declared statically: var a [32] byte var m [1000]*[] float64 Static and dynamic arrays may be allocated dynamically via the built-in function "new()" which takes an array type and zero or one array lengths as parameters, depending on the number of open arrays in the type: new([32] byte) // *[32] byte new([]int, 100); // *[100] int new([][1024] byte, 4); // *[4][1024] byte Assignment compatibility: Fixed arrays are assignment compatible to variables of the same type, or to open arrays with the same element type. Open arrays may only be assigned to other open arrays with the same element type. For the variables: var fa, fb [32] int var fc [64] int var pa, pb *[] int var pc *[][32] int the following assignments are legal, and cause the respective array elements to be copied: fa = fb; pa = pb; *pa = *pb; fa = *pc[7]; *pa = fa; *pb = fc; *pa = *pc[11]; The following assignments are illegal: fa = *pa; // cannot assign open array to fixed array *pc[7] = *pa; // cannot assign open array to fixed array fa = fc; // different fixed array types *pa = *pc; // different element types of open arrays Array indexing: Given a (pointer to an) array variable "a", an array element is specified with an array index operation: a[i] This selects the array element at index "i". "i" must be within array bounds, that is "0 <= i < len(a)". Array slicing: Given a (pointer to an) array variable "a", a sub-array is specified with an array slice operation: a[i : j] This selects the sub-array consisting of the elements "a[i]" through "a[j - 1]" (exclusive "a[j]"). "i" must be within array bounds, and "j" must satisfy "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 array capacity does not change as a result of a slice operation. An array slice is always an open array. Note that a slice operation does not ``crop'' the underlying array, it only provides a new ``view'' to an array. If the capacity of an array is larger then its length, slicing can be used to ``grow'' an array: // allocate an open array of bytes with length i and capacity 100 i := 10; a := new([] byte, 100) [0 : i]; // grow the array by n bytes, with i + n <= 100 a = a[0 : i + n]; TODO: Expand on details of slicing and assignment, especially between pointers to arrays and arrays. Struct types ---- A struct is a composite type consisting of a fixed number of elements, called fields, with possibly different types. A struct type declares an identifier and type for each field. Within a struct type no field identifier may be declared twice and all field types must be complete types (§Types). StructType = "struct" [ "{" [ List ] "}" ] . FieldDecl = IdentifierList CompleteType | TypeName . // An empty struct. struct {} // A struct with 5 fields. struct { x, y int; u float; a *[]int; f *(); } 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 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; } 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 } 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). 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 equality: Two struct types are equal only if both have the same number of fields in the same order, corresponding fields are either both named or anonymous, and the corresponding field types are equal. Specifically, field names don't have to match. Assignment compatibility: Structs are assignment compatible to variables of equal type only. Pointer types ---- A pointer type denotes the set of all pointers to variables of a given type, called the ``base type'' of the pointer, and the value "nil". PointerType = "*" BaseType . BaseType = Type . *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 S2 struct // forward declaration of S2 type S1 struct { s2 *S2 } type S2 struct { s1 *S1 } Type equality: Two pointer types are equal only if both have equal base types. Assignment compatibility: A pointer is assignment compatible to a variable of pointer type, only if both types are equal. Pointer arithmetic of any kind is not permitted. Map types ---- A map is a composite type consisting of a variable number of entries called (key, value) pairs. For a given map, the keys and values must each be of a specific complete type (§Types) called the key and value type, respectively. Upon creation, a map is empty and values may be added and removed during execution. The number of entries in a map is called its length. MapType = "map" "[" KeyType "]" ValueType . KeyType = CompleteType . ValueType = CompleteType . map [string] int map [struct { pid int; name string }] *chan Buffer map [string] any The length of a map "m" can be discovered using the built-in function len(m) Allocation: A map may only be used as a base type of a pointer type. There are no variables, parameters, array, struct, or map fields of map type, only of pointers to maps. Type equivalence: Two map types are equal only if both have equal key and value types. Assignment compatibility: A pointer to a map type is assignment compatible to a variable of pointer to map type only if both types are equal. Channel types ---- A channel provides a mechanism for two concurrently executing functions to synchronize execution and exchange values of a specified type. This type must be a complete type (§Types). Upon creation, a channel can be used both to send and to receive. By conversion or assignment, a 'full' channel may be constrained only to send or to receive. Such a restricted channel is called a 'send channel' or a 'receive channel'. ChannelType = FullChannel | SendChannel | RecvChannel . FullChannel = "chan" ValueType . SendChannel = "chan" "<-" ValueType . RecvChannel = "<-" "chan" ValueType . chan T // a channel that can exchange values of type T chan <- float // a channel that can only be used to send floats <-chan int // a channel that can receive only ints Channel variables always have type pointer to channel. It is an error to attempt to use a channel value and in particular to dereference a channel pointer. var ch *chan int; ch = new(chan int); // new returns type *chan int TODO(gri): Do we need the channel conversion? It's enough to just keep the assignment rule. Function types ---- A function type denotes the set of all functions with the same parameter and result types. FunctionType = "(" [ 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 no additional arguments). If the parameters are named, the identifier list immediately preceding "..." must contain only one identifier (the name of the last parameter). () (x int) () int (string, float, ...) (a, b int, z float) bool (a, b int, z float) (bool) (a, b int, z float, opt ...) (success bool) (int, int, float) (float, *[]int) A variable can hold only a pointer to a function, not a function value. In particular, v := func() {} creates a variable of type *(). To call the function referenced by v, one writes v(). It is illegal to dereference a function pointer. Type equality: Two function types are equal if both have the same number of parameters and result values and if corresponding parameter and result types are equal. In particular, the parameter and result names are ignored for the purpose of type equivalence. Assignment compatibility: A function pointer can be assigned to a function (pointer) variable only if both function types are equal. Interface types ---- Type interfaces may be specified explicitly by interface types. An interface type denotes the set of all types that implement at least the set of methods specified by the interface type, and the value "nil". InterfaceType = "interface" [ "{" [ List ] "}" ] . MethodSpec = identifier FunctionType . // A basic file interface. interface { Read(b Buffer) bool; Write(b Buffer) bool; Close(); } Any type 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() { ... } (where T stands for either S1 or S2) then the File interface is implemented by both S1 and S2, regardless of what other methods S1 and S2 may have or share. All types implement the empty interface: interface {} In general, a type implements an arbitrary number of interfaces. For instance, if we have type Lock interface { lock(); unlock(); } and S1 and S2 also implement func (p T) lock() { ... } func (p T) unlock() { ... } they implement the Lock interface as well as the File interface. 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 equivalence: Two interface types are equal only if both declare the same number of methods with the same names, and corresponding (by name) methods have the same function types. 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". Expressions ---- An expression specifies the computation of a value via the application of operators and function invocations on operands. An expression has a value and a type. 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. If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), the behavior is undefined. For instance, the behavior of integer under- or overflow is not defined. Operands ---- Operands denote the elementary values in an expression. Operand = Literal | QualifiedIdent | "(" Expression ")" . Literal = BasicLit | CompositeLit | FunctionLit . BasicLit = int_lit | float_lit | char_lit | string_lit . Constants ---- An operand is called ``constant'' if it is a literal of a basic type (including the predeclared constants "true" and "false", and the values denoted by "iota"), the predeclared constant "nil", or a parenthesized constant expression (§Constant expressions). Constants have values that are known at compile-time. Qualified identifiers ---- TODO(gri) write this section Iota ---- Within a declaration, the predeclared operand "iota" represents successive elements of an integer sequence. It is reset to zero whenever the reserved word "const" introduces a new declaration and increments as each identifier is declared. For instance, "iota" can be used to construct a set of related constants: const ( enum0 = iota; // sets enum0 to 0, etc. enum1 = iota; enum2 = iota ) const ( a = 1 << iota; // sets a to 1 (iota has been reset) b = 1 << iota; // sets b to 2 c = 1 << iota; // sets c to 4 ) const x = iota; // sets x to 0 const y = iota; // sets y to 0 Since the expression in constant declarations repeats implicitly if omitted, the first two examples above can be abbreviated: const ( enum0 = iota; // sets enum0 to 0, etc. enum1; enum2 ) const ( a = 1 << iota; // sets a to 1 (iota has been reset) b; // sets b to 2 c; // sets c to 4 ) Composite Literals ---- Literals for composite data structures consist of the type of the value followed by a braced expression list for array and structure literals, or a list of expression pairs for map literals. CompositeLit = LiteralType "{" [ ( ExpressionList | ExprPairList ) [ "," ] ] "}" . LiteralType = TypeName | ArrayType | MapType | StructType . ExprPairList = ExprPair { "," ExprPair } . ExprPair = Expression ":" Expression . If LiteralType is a TypeName, the denoted type must be an array, map, or structure. The types of the expressions must match the respective key, element, and field types of the literal type; 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. Implementation restriction: Currently, map literals are pointers to maps. Given type Rat struct { num, den int }; type Num struct { r Rat; f float; s string }; we can write pi := Num{Rat{22, 7}, 3.14159, "pi"}; Array literals are always fixed arrays: If no array length is specified in LiteralType, the array length is the number of elements provided in the composite literal. Otherwise the array length is the length specified in LiteralType. In the latter case, fewer elements than the array length may be provided in the literal, and the missing elements are set to the appropriate zero value for the array element type. It is an error to provide more elements then specified in LiteralType. buffer := [10]string{}; // len(buffer) == 10 primes := [6]int{2, 3, 5, 7, 9, 11}; // len(primes) == 6 weekenddays := &[]string{"sat", "sun"}; // len(weekenddays) == 2 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}; TODO: Consider adding helper syntax for nested composites (avoids repeating types but complicates the spec needlessly.) Function Literals ---- A function literal represents an anonymous function. It consists of a specification of the function type and the function body. The parameter and result types of the function type must all be complete types (§Types). FunctionLit = "func" FunctionType Block . Block = "{" [ StatementList ] "}" . The type of a function literal is a pointer to the function type. 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 pointer type, or invoked directly. f := func(x, y int) int { return x + y; } func(ch *chan int) { ch <- ACK; } (reply_chan) Implementation restriction: A function literal can reference only its parameters, global variables, and variables declared within the function literal. Primary expressions ---- PrimaryExpr = Operand | PrimaryExpr Selector | PrimaryExpr Index | PrimaryExpr Slice | PrimaryExpr TypeGuard | PrimaryExpr Call . Selector = "." identifier . Index = "[" Expression "]" . Slice = "[" Expression ":" Expression "]" . TypeGuard = "." "(" QualifiedIdent ")" . Call = "(" [ ExpressionList ] ")" . x 2 (s + ".txt") f(3.1415, true) Point(1, 2) new([]int, 100) m["foo"] s[i : j + 1] obj.color Math.sin f.p[i].x() Selectors ---- A primary expression of the form 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; } func (recv *T0) M0() type T1 struct { y int; } func (recv T1) M1() type T2 struct { z int; T1; *T0; } func (recv *T2) M2() var p *T2; // with p != nil and p.T1 != nil we can write: 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 TODO: Specify what happens to receivers. Indexes ---- A primary expression of the form 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] is the element type of A 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 and the map must contain an entry with key x - a[x] is the map value with key x and the type of a[x] is the value type of M Otherwise a[x] is illegal. TODO: Need to expand map rules for assignments of the form v, ok = m[k]. Slices ---- Strings and arrays can be ``sliced'' to construct substrings or subarrays. The index expressions in the slice select which elements appear in the result. The result has indexes starting at 0 and length equal to the difference in the index values in the slice. After a := []int(1,2,3,4) slice := a[1:3] The array ``slice'' has length two and elements slice[0] == 2 slice[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. Slices are new arrays (or strings) storing copies of the elements, so changes to the elements of the slice do not affect the original. In the example, a subsequent assignment to element 0, slice[0] = 5 would have no effect on ``a''. Type guards ---- TODO: write this section Calls ---- Given a function pointer, one writes p() to call the function. A method is called using the notation receiver.method() where receiver is a value of the receive type of the method. For instance, given a *Point variable pt, one may call 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) However, a function declared this way is not a method. There is no distinct method type and there are no method literals. Operators ---- Operators combine operands into expressions. 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 = "*" | "/" | "%" | "<<" | ">>" | "&" . 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 an ideal number, the ideal number is converted to match the type of the other operand (§Expression). - If both operands are ideal numbers, the conversion is to ideal floats if one of the operands is an ideal float (relevant for "/" and "%"). - The right operand in a shift operation must be always be an unsigned int (or an ideal number that can be safely converted into an unsigned int) (§Arithmetic operators). Unary operators have the highest precedence. They are evaluated from right to left. Note that "++" and "--" are outside the unary operator hierachy (they are statements) and they apply to the operand on the left. Specifically, "*p++" means "(*p)++" in Go (as opposed to "*(p++)" in C). 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 lowest precedence: 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 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 Strings and arrays can be concatenated using the "+" operator (or via the "+=" assignment): s := "hi" + string(c) a += []int{5, 6, 7} String and array addition creates a new array or string by copying the elements. For integer values, "/" and "%" satisfy the following relationship: (a / b) * b + a % b == a and (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 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 The shift operators shift the left operand by the shift count specified by the right operand. They implement arithmetic shifts if the left operand is a signed integer, and logical shifts if it is an unsigned integer. The shift count must be an unsigned integer. There is no upper limit on the shift count. It is as if the left operand is shifted "n" times by 1 for a shift count of "n". The unary operators "+", "-", and "^" are defined as follows: +x is 0 + x -x negation is 0 - x ^x bitwise complement is -1 ^ x Comparison operators ---- Comparison operators yield a boolean result. All comparison operators apply to strings and numeric types. The operators "==" and "!=" also apply to boolean values, pointer and interface types (including the value "nil"). == equal != not equal < less <= less or equal > greater >= greater or equal Strings are compared byte-wise (lexically). Pointers are equal if they point to the same value. Interfaces are equal if both their dynamic types and values are equal. For a value "v" of interface type, "v == nil" is true only if the predeclared constant "nil" is assigned explicitly to "v" (§Assignments), or "v" has not been modified since creation (§Program initialization and execution). Logical operators ---- Logical operators apply to boolean operands and yield a boolean result. The right operand is evaluated conditionally. && 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: Need to talk about unary "*", clean up section below. Given a function f, declared as func f(a int) int; taking the address of f with the expression &f creates a pointer to the function that may be stored in a value of type pointer to function: var fp *(a int) int = &f; The function pointer may be invoked with the usual syntax; no explicit indirection is required: fp(7) Methods are a form of function, and the address of a method has the type pointer to function. Consider the type T with method M: type T struct { a int; } func (tp *T) M(a int) int; var t *T; To construct the address of method M, we write &t.M using the variable t (not the type T). The expression is a pointer to a function, with type *(t *T, a int) int and may be invoked only as a function, not a method: var f *(t *T, a int) int; f = &t.M; x := f(t, 7); Note that one does not write t.f(7); taking the address of a method demotes it to a function. In general, given type T with method M and variable t of type *T, the method invocation t.M(args) is equivalent to the function call (&t.M)(t, args) 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 interface M, the sequence var t1 *T1; var t2 *T2; var i I = t1; m := &i.M; m(t2); will invoke t2.M() even though m was constructed with an expression involving t1. Communication operators ---- The syntax presented above covers communication operations. This section describes their form and function. Here the term "channel" means "variable of type *chan". A channel is created by allocating it: ch := new(chan int) An optional argument to new() specifies a buffer size for an asynchronous channel; if absent or zero, the channel is synchronous: sync_chan := new(chan int) buffered_chan := new(chan int, 10) The send operation uses the binary operator "<-", which operates on a channel and a value (expression): 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") } 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 value of the expression is the value received: <-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) If the receive expression does not save the value, the value is discarded: <-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 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 ---- A constant expression is an expression whose operands are all constants (§Constants). Additionally, the result of the predeclared functions below (with appropriate arguments) is also constant: len(a) if a is a fixed array TODO: Complete this list as needed. Constant expressions can be evaluated at compile time. Statements ---- Statements control execution. Statement = Declaration | LabelDecl | EmptyStat | SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat | FallthroughStat | Block | IfStat | SwitchStat | SelectStat | ForStat | RangeStat . 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 } . 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) In all other cases a semicolon is required to separate two statements. Since there is an empty statement, a statement list can always be ``terminated'' with a semicolon. Label declarations ---- TODO write this section Empty statements ---- The empty statement does nothing. EmptyStat = . Expression statements ---- ExpressionStat = Expression . f(x+y) TODO: specify restrictions. 6g only appears to allow calls here. IncDec statements ---- The "++" and "--" statements increment or decrement their operands by the (ideal) constant value 1. IncDecStat = Expression ( "++" | "--" ) . The following assignment statements (§Assignments) are semantically equivalent: 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 ] "=" . 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 As in C, arithmetic binary operators can be combined with assignments: 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 assigns the expressions e1, e2, e3 to temporaries and then assigns the temporaries to the variables v1, v2, v3. Thus a, b = b, a exchanges the values of a and b. The tuple assignment 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 tuple assignment, assign a value and a boolean. If the value is present in the map, 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] 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 In assignments, the type of the expression must match the type of the left-hand side. If statements ---- If statements specify the conditional execution of two branches; the "if" and the "else" branch. If Expression evaluates to true, the "if" branch is executed. Otherwise the "else" branch is executed if present. If Condition is omitted, it is equivalent to true. IfStat = "if" [ [ Simplestat ] ";" ] [ Expression ] Block [ "else" Statement ] . 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; } Switch statements ---- Switches provide multi-way execution. SwitchStat = "switch" [ [ Simplestat ] ";" ] [ Expression ] "{" { CaseClause } "}" . CaseClause = Case [ StatementList ] . Case = ( "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() } 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 } Cases do not fall through unless explicitly marked with a "fallthrough" statement. 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(); } For statements ---- For statements are a combination of the "for" and "while" loops of C. ForStat = "for" [ Condition | ForClause ] Block . ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] . InitStat = SimpleStat . Condition = Expression . PostStat = SimpleStat . A SimpleStat is a simple statement such as an assignment, a SimpleVarDecl, or an increment or decrement statement. Therefore one may declare a loop variable in the init statement. for i := 0; i < 10; i++ { print(i, "\n") } A for statement with just a condition executes until the condition becomes false. Thus it is the same as C's while statement. for a < b { a *= 2 } If the condition is absent, it is equivalent to "true". for { f() } Range statements ---- Range statements are a special control structure for iterating over the contents of arrays and maps. RangeStat = "range" IdentifierList ":=" RangeExpression Block . RangeExpression = Expression . A range expression must evaluate to an array, map or string. The identifier list must contain either one or two identifiers. If the range expression is a map, a single identifier is declared to range over the keys of the map; two identifiers range over the keys and corresponding values. For arrays and strings, the behavior is analogous for integer indices (the keys) and array elements (the values). a := []int(1, 2, 3); m := [string]map int("fo",2, "foo",3, "fooo",4) range i := a { f(a[i]); } range i, v := a { f(v); } range k, v := m { assert(len(k) == v); } TODO: is this right? Go statements ---- A go statement starts the execution of a function as an independent concurrent thread of control within the same address space. The expression must evaluate into a function call. 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) Select statements ---- A select statement chooses which of a set of possible communications will proceed. It looks similar to a switch statement but with the cases all referring to communication operations. SelectStat = "select" "{" { CommClause } "}" . CommClause = CommCase [ StatementList ] . CommCase = ( "default" | ( "case" ( SendExpr | RecvExpr) ) ) ":" . 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 evaluated. If any of the resulting channels can proceed, one is chosen and the corresponding communication and statements are evaluated. Otherwise, if there is a default case, that executes; 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; 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 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"); } TODO: Make semantics more precise. Return statements ---- A return statement terminates execution of the containing function and optionally provides a result value or values to the caller. 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; } 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; } 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; } Break statements ---- Within a for or switch statement, a break statement terminates execution of the innermost for or switch statement. BreakStat = "break" [ identifier ]. If there is an identifier, it must be the label name of an enclosing for or switch statement, and that is the one whose execution terminates. L: for i < n { switch i { case 5: break L } } Continue statements ---- Within a for loop a continue statement begins the next iteration of the loop at the post statement. ContinueStat = "continue" [ identifier ]. The optional identifier is analogous to that of a break statement. Label declaration ---- A label declaration serves as the target of a goto, break or continue statement. LabelDecl = identifier ":" . Error: Goto statements ---- A goto statement transfers control to the corresponding label statement. GotoStat = "goto" identifier . 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: is erroneous because the jump to label L skips the creation of v. Fallthrough statements ---- A fallthrough statement transfers control to the first statement of the next case clause in a switch statement (§Switch statements). It may only be used in a switch statement, and only as the last statement in a case clause of the switch statement. FallthroughStat = "fallthrough" . Function declarations ---- A function declaration binds an identifier to a function. Functions contain declarations and statements. They may be recursive. Except for forward declarations (see below), the parameter and result types of the function type must all be complete types (§Type declarations). FunctionDecl = "func" identifier FunctionType [ Block ] . 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 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 ---- A method declaration is a function declaration with a receiver. The receiver is the first parameter of the method, and the receiver type must be specified as a type name, or as a pointer to a type name. The type specified by the type name is called ``receiver base type''. The receiver base type must be a type declared in the current file, and it must not be a pointer type. The method is said to be ``bound'' to the receiver base type; specifically it is declared within the scope of that type (§Type declarations). MethodDecl = "func" Receiver identifier FunctionType [ 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 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) 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". Method declarations may appear anywhere after the declaration of the receiver base type and may be forward-declared. Predeclared functions ---- cap convert len new panic print typeof TODO: (gri) suggests that we should consider assert() as a built-in function. It is like panic, but takes a boolean guard as first argument. (rsc also thinks this is a good idea). Length and capacity ---- The predeclared function "len()" takes a value of type string, array or map type, or of pointer to array or map type, and returns the length of the string in bytes, or the number of array of map elements, respectively. The predeclared function "cap()" takes a value of array or pointer to array type and returns the number of elements for which there is space allocated in the array. For an array "a", at any time the following relationship holds: 0 <= len(a) <= cap(a) TODO(gri) Change this and the following sections to use a table indexed by functions and parameter types instead of lots of prose. Conversions ---- Conversions syntactically look like function calls of the form 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 we define it as a ``best effort'' conversion. The conversion always succeeds but the value may be a NaN or other problematic 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" 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" 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 ---- The built-in function "new()" takes a type "T", optionally followed by a type-specific list of expressions. It allocates memory for a variable of type "T" and returns a pointer of type "*T" to that variable. The memory is initialized as described in the section on initial values. new(type, [optional list of expressions]) For instance 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. The only defined parameters affect sizes for allocating arrays, buffered channels, and maps. ap := new([]int, 10); # a pointer to an open array of 10 ints c := new(chan int, 10); # a pointer to a channel with a buffer size of 10 m := new(map[string] int, 100); # a pointer to a map with initial space for 100 elements Packages ---- A package is a package clause, optionally followed by import declarations, followed by a series of declarations. 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 . package Math A package can gain access to exported items from another package through an import declaration: ImportDecl = "import" Decl . ImportSpec = [ "." | PackageName ] PackageFileName . An import statement makes the exported contents of the named package file accessible in this package. In the following discussion, assume we have a package in the file "/lib/math", called package Math, which exports functions sin and cos. 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" the contents of the package /lib/math can be accessed by M.cos, M.sin, 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" 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" 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'. } } // 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 := new(chan int); // Create a new channel. go Generate(ch); // Start Generate() as a subprocess. for { prime := <-ch; print(prime, "\n"); ch1 := new(chan int); go Filter(ch, ch1, prime); ch = ch1 } } func main() { Sieve() } Program initialization and execution ---- When memory is allocated to store a value, either through a declaration or "new()", and no explicit initialization is provided, the memory is given a default initialization. Each element of such a value is set to the ``zero'' for that type: "false" for booleans, "0" for integers, "0.0" for floats, '''' for strings, and "nil" for pointers and interfaces. This intialization is done recursively, so for instance each element of an array of integers will be set to 0 if no other value is specified. These two simple declarations are equivalent: var i int; var i int = 0; After 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 A package with no imports is initialized by assigning initial values to 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 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() { ... } 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. TODO: is there a way to override the default for package main or the default for the function name main.main?