The following example of a definite clause grammar defines in a formal way the traditional mapping of simple English sentences into formulae of classical logic. By way of illustration, if the sentence
Every man that lives loves a woman.
is parsed as a sentence by the call
| ?- phrase(sentence(P), [every,man,that,lives,loves,a,woman]).
then P will get instantiated to
all(X):(man(X)&lives(X) => exists(Y):(woman(Y)&loves(X,Y)))
where :
, &
and =>
are infix operators defined
by
:- op(900, xfx, =>). :- op(800, xfy, &). :- op(550, xfy, :). /* predefined */
The grammar follows:
sentence(P) --> noun_phrase(X, P1, P), verb_phrase(X, P1). noun_phrase(X, P1, P) --> determiner(X, P2, P1, P), noun(X, P3), rel_clause(X, P3, P2). noun_phrase(X, P, P) --> name(X). verb_phrase(X, P) --> trans_verb(X, Y, P1), noun_phrase(Y, P1, P). verb_phrase(X, P) --> intrans_verb(X, P). rel_clause(X, P1, P1&P2) --> [that], verb_phrase(X, P2). rel_clause(_, P, P) --> []. determiner(X, P1, P2, all(X):(P1=>P2)) --> [every]. determiner(X, P1, P2, exists(X):(P1&P2)) --> [a]. noun(X, man(X)) --> [man]. noun(X, woman(X)) --> [woman]. name(john) --> [john]. trans_verb(X, Y, loves(X,Y)) --> [loves]. intrans_verb(X, lives(X)) --> [lives].