2.20 The Syntax in BNF

This section contains the definition of the GAP syntax in Backus-Naur form.

A BNF is a set of rules, whose left side is the name of a syntactical construct. Those names are enclosed in angle brackets and written in italics. The right side of each rule contains a possible form for that syntactic construct. Each right side may contain names of other syntactic constructs, again enclosed in angle brackets and written in italics, or character sequences that must occur literally; they are written in typewriter style.

Furthermore each righthand side can contain the following metasymbols written in boldface. If the right hand side contains forms separated by a pipe symbol (|) this means that one of the possible forms can occur. If a part of a form is enclosed in square brackets ([ ]) this means that this part is optional, i.e. might be present or missing. If part of the form is enclosed in curly braces ({ }) this means that the part may occur arbitrarily often, or possibly be missing.

Permutation := Expr kill Ident := a|...|z|A|...|Z|_ {a|...|z|A|...|Z|0|...|9|_}
Var := Ident
| Var . Ident
| Var . ( Expr )
| Var [ Expr ]
| Var { Expr }
| Var ( [ Expr { , Expr } ] )
List := [ [ Expr ] {, [ Expr ] } ]
| [ Expr [, Expr ] .. Expr ]
Record := rec( [ Ident := Expr {, Ident := Expr } ] )
Permutation := ( Expr {, Expr } ) { ( Expr {, Expr } ) }
Function := function ( [ Ident {, Ident } ] )
[ local Ident {, Ident } ; ]
Statements
end
Char := ' any character '
String := " { any character } "
Int := 0|1|...|9 { 0|1|...|9 }
Atom := Int
| Var
| ( Expr )
| Permutation
| Char
| String
| Function
| List
| Record
Factor := {+|-} Atom [ ^ {+|-} Atom ]
Term := Factor { *|/|mod Factor }
Arith := Term { +|- Term }
Rel := { not } Arith { =|<>|<|>|<=|>=|in Arith }
And := Rel { and Rel }
Log := And { or And }
Expr := Log
| Var [ - Log ]
Statement := Expr
| Var := Expr
| if Expr then Statements
{ elif Expr then Statements }
[ else Statements ] fi
| for Var in Expr do Statements od
| while Expr do Statements od
| repeat Statements until Expr
| return [ Expr ]
| quit
Statements := { Statement ; }
| ;

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