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C(?Input, ?Token, ?Rest)

Specifies how DCG grammar rules get a token from their input.
Input
A List or a variable.
Token
A term or a variable.
Rest
A List or a variable.

Description

This predicate is only useful in connection with definite clause grammar rules (DCG). There is a global default definition of 'C'/3 which consists of the single clause 'C'([Token|Rest], Token, Rest). The Input argument represents the parsing input before consuming Token and Rest is the input after consuming Token.

DCGs normally operate on lists of tokens. However, by redefining 'C'/3, it is possible to let them manipulate other data structures. The example shows how to parse directly from an I/O stream.

Modes and Determinism

Fail Conditions

Fails if Input is not a non-empty list

Examples

[eclipse 1]: [user].
 sentence --> noun, [is], adjective.
 noun --> [prolog] ; [lisp].
 adjective --> [boring] ; [great].
user       compiled traceable 560 bytes in 0.05 seconds

yes.
[eclipse 2]: phrase(sentence, [prolog,is,great], []).

yes.
[eclipse 3]: [user].
:- local 'C'/3.       % to avoid a redefinition warning
'C'(Stream-Pos0, Token, Stream-Pos1) :-
        seek(Stream, Pos0),
        read_string(Stream, " ", _, TokenString),
        atom_string(Token, TokenString),
        at(Stream, Pos1).
user       compiled traceable 308 bytes in 0.00 seconds

yes.
[eclipse 4]: open(string("prolog is great"), read, S),
             phrase(sentence, S-0, S-End).
S = 9
End = 15     More? (;)
yes.

See Also

phrase / 3