Question

I have a predicate “lookupOptions” which returns one by one some lists (Menus).

I’m trying to get it to satisfy the case of multiple inputs. I can return a single set of options as follows, by reading the head of the “list_places” list.

find_options(Restaurant,Town,Menu) :- lookupOptions(Restaurant,H,Menu), list_places(Town,[H|T])

But, I’m not able to get it to iterate.

I have tried a lot of things, these were my best efforts so far.

a) standard enough iteration, but it wont resolve …

doStuff(X,[],_).
doStuff(Restaurant,[H|T],_):-  lookupOptions(Resturant,H,_), doStuff(Restaurant,T,_).
find_options(Restaurant,Town,Menu) :- doStuff(Restaurant,[H|T],Menu), list_places(Town,[H|T]).

b) expanding the goal predicate …

find_options(_,Town,[H|T],_)
find_options(Restaurant,Town,Menu) :- find_options(Restaurant,Town,[],Menu).
find_options(Restaurant,Town,X,Menu) :- list_places(Town,X).
find_options(Restaurant,Town,[H|T],Menu) :- lookupOptions(Restaurant,[H],Menu), find_options(Restaurant,Town,T,Menu).

Would either of these work ? if the pattern was written correctly. Or if there was an appropriate cut put in place?

Any help most appreciated …

Answer 1

It’s no clear on what you want iterate. Prolog uses backtracking to examine all alternatives, then you should start backtracking if you are after some alternative, or use the all solutions family.

Now I think you want simply declare there could be more find_options(Restaurant,Town,Menu). Then try replacing the head match [H|T] with this:

find_options(Restaurant,Town,Menu) :-
   lookupOptions(Restaurant,H,Menu),
   list_places(Town, Places),
   member(H, Places).

BTW T is a singleton in your original rule. This could be a hint for the need of generalize it.