Question

I have the following prolog code:

equiAngularTriangle(T) :-
    equiLateralTriangle(T).

equiLateralTriangle(T) :-
    equiAngularTriangle(T).

Is there a way to keep the interpreter from asking the same question twice? For instance, if I ask equiAngularTriangle(t), then it’s going to ask equiLateralTriangle(t), then ask equiLateralTriangle(t), but it should know not to pursue that last one again, because the same question is on the “query stack”.

Is there an option or some special syntax that lets Prolog behave the way I want it to?

Answer 1

If the prolog implementation supports tabling or you are using XSB then you could use it and get the desired behaviour.

You could also add a state argument:

%State = [Checked_for_equiAngular, Checked_for_equiLateral]

equiAngularTriangle(T, [_,false]) :-
    equiLateralTriangle(T, [true,true]).

equiLateralTriangle(T, [false,_]) :-
    equiAngularTriangle(T, [true,true]).

of course you will need to modify the rest of the clauses.

The last (and best imo) option is to rewrite your predicates. I guess that your code will be similar to this example:

ang(T):-
  foo(T).
ang(T):-
  lat(T).

lat(T):-
  bar(T).
lat(T):-
  ang(T).

so you could simply write:

ang(T):-
  foo(T).
ang(T):-
  bar(T).

lat(T):-
  ang(T).

normally you will use some wrapper predicates if instead of foo(T) you had foo1(T),foo2(T) etc