/*
    sentence/2
    sentence(+S,-D)

    The predicate sentence returns a derivation if the list of tokens
    has a valid derivation according to the grammar rules.

    This definite clause grammar is written without the --> macro.

    Warren Sack <wsack@ucsc.edu>
    May 2017
	
*/

append([],List,List).
append([First|Rest],List2,[First|List3]) :- append(Rest,List2,List3).

sentence(Sentence,sentence(NP,VP)) :-
       append(NounPhrase,VerbPhrase,Sentence),
       noun_phrase(NounPhrase,NP),
       verb_phrase(VerbPhrase,VP).
noun_phrase(NounPhrase,noun_phrase(A,N)) :-
       append(Article,Noun,NounPhrase),
       article(Article,A),
       noun(Noun,N).
verb_phrase(VerbPhrase,verb_phrase(V,NP)) :-
       append(Verb,NounPhrase,VerbPhrase),
       verb(Verb,V),
       noun_phrase(NounPhrase,NP).
article([a],article(a)).
article([the],article(the)).
noun([man],noun(man)).
noun([ball],noun(ball)).
verb([hit],verb(hit)).
verb([took],verb(took)).