% GPL(C) Mohsin Ahmed, http://www.cs.albany.edu/~mosh 
% SYNOPSIS: English sentence parser.
% Usage: see end of this file.

% This is the i/o part -- rwl(S) reads a sentence as a list S.

rwl(Ws):-get0(C),rwl(C,Ws).
rwl(C,[W|Ws]):-wc(C),rw(C,W,C1),rwl(C1,Ws).
rwl(C,Ws):-fc(C),get0(C1),rwl(C1,Ws).
rwl(C,[]):-eow(C).
rw(C,W,C1):-wcs(C,Cs,C1),name(W,Cs).
wcs(C,[C|Cs],C0):-wc(C),!,get0(C1),wcs(C1,Cs,C0).
wcs(C,[],C):-not(wc(C)).
wc(C):-97 =< C, C =<  122.
wc(C):-65 =< C, C =< 90.
wc(95).
fc(32).
eow(46).

% The grammar.

sentence --> sentence(X).
sentence(X) --> noun_phrase(X), verb_phrase(X).
noun_phrase(X) --> determiner(X), noun(X).
verb_phrase(X) --> verb(X), noun_phrase(X).

determiner(_) --> [the].
determiner(_) --> [a].

noun(singular) --> [man].
noun(singular) --> [apple].
noun(plural) --> [men].
noun(plural) --> [apples].

verb(singular) --> [eats].
verb(singular) --> [sings].
verb(singular) --> [sees].
verb(plural) --> [eat].
verb(plural) --> [sing].
verb(plural) --> [see].

go :-
	write('please type a sentence: '),
	rwl(S),
	write(S),nl,
	test(S).

test(S):-
	sentence(S,[]), !,
	write('Yes, it is a sentence.'), nl.
test(S):-
	write('No, it is not a sentence.'), nl.