Re: DCG

Thanks, Richard - the paper you reference says '
	1. Find all of the strings in L that start with c.
 ' as if that's trivial.

Here's a broken example. This runs forever because it tries appending first one, then two, then three, unbound
variables out to infinity.

I'm looking for a solution for any definition of sentence. I'd take a solution for any definition of
sentence over some known alphabet. The example grammar has a finite maximum length, that's just
this example.

% next_token(+Prefix, -NextList)
% Prefix is a list and NextList is the potential next tokens
% in sentence.
% So next_token([give], [a, the, ball, toy, tom, helen]).
%
% This runs forever
next_token(Prefix, NextList) :-
setof(Next, a_next_token(Prefix, Next), NextList).

a_next_token(Prefix, Next) :-
vocabulary(V),
member(Next, V),
append([Prefix, [Next], _], ANextList),
phrase(sentence, ANextList).

% imperative sentence of questionable semantics
sentence -->  imperative_sentence.

imperative_sentence -->
intransitive_verb,
object_phrase.
imperative_sentence -->
transitive_verb(V),
object_phrase,
matching_preposition(V),
object_phrase.


intransitive_verb -->
[X],
{
    intransitive_verbs(List),
    memberchk(X, List)
}.

intransitive_verbs([drink,eat]).

transitive_verb(X) -->
[X],
{
    transitive_verbs(List),
    memberchk(X, List)
}.

transitive_verbs([give,take,throw,make]).

matching_preposition(V) -->
[Prep],
{
    memberchk(V/Prep, [give/to, take/from, throw/to, make/for])
}.

object_phrase --> object.
object_phrase --> [a], object.
object_phrase --> [the], object.

object --> [ball].
object --> [toy].
object --> [tom].
object --> [helen].

vocabulary([ball,toy,tom,helen,a,the,give,to,take,from,throw,make,for,eat,drink]).