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More spec progress:
- language for selectors and array/map indices - formal description of syntax notation used - factor out common productions to better show symmetries in grammar R=r DELTA=113 (77 added, 13 deleted, 23 changed) OCL=16853 CL=16865
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Robert Griesemer
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132
doc/go_spec.txt
132
doc/go_spec.txt
@ -4,7 +4,7 @@ The Go Programming Language Specification (DRAFT)
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Robert Griesemer, Rob Pike, Ken Thompson
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----
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(October 8, 2008)
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(October 9, 2008)
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This document is a semi-formal specification of the Go systems
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@ -179,30 +179,50 @@ Introduction
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Notation
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----
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The syntax is specified using Extended Backus-Naur Form (EBNF).
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In particular:
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The syntax is specified using Parameterized Extended Backus-Naur Form (PEBNF).
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Specifically, productions are expressions constructed from terms and the
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following operators:
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- | separates alternatives (least binding strength)
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- () groups
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- [] specifies an option (0 or 1 times)
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- {} specifies repetition (0 to n times)
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Lexical symbols are enclosed in double quotes '''' (the
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The syntax of PEBNF can be expressed in itself:
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Production = production_name [ Parameters ] "=" Expression .
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Parameters = "<" production_name { "," production_name } ">" .
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Expression = Alternative { "|" Alternative } .
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Alternative = Term { Term } .
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Term = production_name [ Arguments ] | token | Group | Option | Repetition .
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Arguments = "<" Expression { "," Expression } ">" .
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Group = "(" Expression ")" .
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Option = "[" Expression ")" .
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Repetition = "{" Expression "}" .
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Lower-case production names are used to identify productions that cannot
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be broken by white space or comments; they are usually tokens. Other
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production names are in CamelCase.
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Tokens (lexical symbols) are enclosed in double quotes '''' (the
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double quote symbol is written as ''"'').
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The form "a ... b" represents the set of characters from "a" through "b" as
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alternatives.
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Productions can be parameterized. To get the actual production the parameter is
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substituted with the argument provided where the production name is used. For
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instance, there are various forms of semicolon-separated lists in the grammar.
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The parameterized production for such lists is:
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List<P> = P { ";" P } [ ";" ] .
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In this case, P stands for the actual list element.
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A production may be referenced from various places in this document
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but is usually defined close to its first use. Productions and code
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examples are indented.
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Lower-case production names are used to identify productions that cannot
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be broken by white space or comments; they are usually tokens. Other
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productions are in CamelCase.
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Productions with names ending in List never produces the empty phrase.
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For instance, an ExpressionList always contains at least one expression.
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Source code representation
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@ -479,6 +499,14 @@ function, method) and specifies properties of that entity such as its type.
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[ "export" ]
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( ConstDecl | TypeDecl | VarDecl | FunctionDecl | MethodDecl ) .
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Except for function, method and abbreviated variable declarations (using ":="),
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all declarations follow the same pattern. There is either a single declaration
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of the form P, or an optional semicolon-separated list of declarations of the
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form P surrounded by parentheses:
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Decl<P> = P | "(" [ List<P> ] ")" .
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List<P> = P { ";" P } [ ";" ] .
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Every identifier in a program must be declared; some identifiers, such as "int"
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and "true", are predeclared.
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@ -577,10 +605,8 @@ Const declarations
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A constant declaration binds an identifier to the value of a constant
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expression (§Constant expressions).
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ConstDecl = "const" ( ConstSpec | "(" [ ConstSpecList ] ")" ) .
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ConstSpec = identifier [ CompleteType ] "=" Expression .
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ConstSpecList = ConstSpec { ";" ConstSpecOptExpr } [ ";" ] .
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ConstSpecOptExpr = identifier [ Type ] [ "=" Expression ] .
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ConstDecl = "const" Decl<ConstSpec> .
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ConstSpec = identifier [ CompleteType ] [ "=" Expression ] .
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const pi float = 3.14159265
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const e = 2.718281828
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@ -655,9 +681,8 @@ Type declarations
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A type declaration specifies a new type and binds an identifier to it.
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TypeDecl = "type" ( TypeSpec | "(" [ TypeSpecList ] ")" ).
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TypeDecl = "type" Decl<TypeSpec> .
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TypeSpec = identifier Type .
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TypeSpecList = TypeSpec { ";" TypeSpec } [ ";" ] .
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A struct or interface type may be forward-declared (§Struct types,
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§Interface types). A forward-declared type is incomplete (§Types)
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@ -690,9 +715,8 @@ The variable type must be a complete type (§Types).
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In some forms of declaration the type of the initial value defines the type
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of the variable.
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VarDecl = "var" ( VarSpec | "(" [ VarSpecList ] ")" ) .
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VarDecl = "var" Decl<VarSpec> .
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VarSpec = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) .
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VarSpecList = VarSpec { ";" VarSpec } [ ";" ] .
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IdentifierList = identifier { "," identifier } .
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ExpressionList = Expression { "," Expression } .
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@ -1048,8 +1072,7 @@ an identifier and type for each field. Within a struct type no field
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identifier may be declared twice and all field types must be complete
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types (§Types).
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StructType = "struct" [ "{" [ FieldList ] "}" ] .
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FieldList = FieldDecl { ";" FieldDecl } [ ";" ] .
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StructType = "struct" [ "{" [ List<FieldDecl> ] "}" ] .
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FieldDecl = IdentifierList CompleteType | TypeName .
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// An empty struct.
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@ -1242,8 +1265,7 @@ Type interfaces may be specified explicitly by interface types.
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An interface type denotes the set of all types that implement at least
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the set of methods specified by the interface type, and the value "nil".
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InterfaceType = "interface" [ "{" [ MethodList ] "}" ] .
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MethodList = MethodSpec { ";" MethodSpec } [ ";" ] .
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InterfaceType = "interface" [ "{" [ List<MethodSpec> ] "}" ] .
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MethodSpec = identifier FunctionType .
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// A basic file interface.
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@ -1502,23 +1524,66 @@ Primary expressions
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Selectors
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----
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Given a pointer p to a struct, one writes
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p.f
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to access field f of the struct.
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A primary expression of the form
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TODO: Complete this section:
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- type rules
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- conflict resolution rules for anonymous fields
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x.f
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denotes the field or method f of the value denoted by x (or of *x if
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x is of pointer type). The identifier f is called the ``selector''.
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The following rules apply:
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For x of type S or *S where S is a struct type (§Struct types):
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1) If f is declared as a (named or anonymous) field of S then x.f denotes
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that field.
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2) If f is declared (or forward-declared) as a method of S textually
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before x.f then x.f denotes that method and x becomes the receiver
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of f.
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3) Otherwise, if there is single anononymous field A of S such that
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x.A.f denotes a valid field according to 1) or 2), then x.f is
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a shortcut for x.A.f, and x.A becomes the receiver of f.
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If there is none or more then one anonymous field of S satisfying
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this criterion, x.f is illegal.
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For x of type I or *I where I is an interface type (§Interface types):
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- If f is a method declared in I then x.f denotes the actual method with
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name f of the value assigned to the variable x and x becomes the receiver
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of f. If no value or nil was assigned to x, x.f is illegal.
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Otherwise, x.f is illegal.
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Indexes
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----
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Given an array or map pointer, one writes
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p[i]
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to access an element.
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A primary expression of the form
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TODO: Complete this section:
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a[x]
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denotes the array or map element x. The value x is called the
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``array index'' or ``map key'', respectively. The following
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rules apply:
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For a of type A or *A where A is an array type (§Array types):
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- x must be an integer value and 0 <= x < len(a)
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- a[x] is the array element at index x and the type of a[x]
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is the element type of A
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For a of type *M, where M is a map type (§Map types):
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- x must be of the same type as the key type of M
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and the map must contain an entry with key x
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- a[x] is the map value with key x and the type of a[x]
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is the value type of M
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Otherwise a[x] is illegal.
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TODO: Need to expand map rules for assignments of the form v, ok = m[k].
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Slices
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@ -2582,9 +2647,8 @@ The file must begin with a package clause.
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A package can gain access to exported items from another package
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through an import declaration:
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ImportDecl = "import" ( ImportSpec | "(" ImportSpecList ")" ) .
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ImportDecl = "import" Decl<ImportSpec> .
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ImportSpec = [ "." | PackageName ] PackageFileName .
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ImportSpecList = ImportSpec { ";" ImportSpec } [ ";" ] .
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An import statement makes the exported contents of the named
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package file accessible in this package.
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