Question

I have something like this in the database.

flight( tk1, ist, esb, 40 ).
flight( tk2, ist, mlx, 90 ).

And I have written a predicate called test.

test([S], L1):-
    findall( C, (flight( _, S, C, _ )), L1 ).

The output when I write test([ist], X). is X = [esb, mlx]. So this is true but when I write test(X, [esb, mlx]). it is false. Why is it false? Should I write another predicate for this?

Answer 1

Well, as you can see, findall/3 isn’t necessarily supposed to be used with a bound last parameter, for example in SWI-Prolog, in its description we can read:

findall( +Template, :Goal, -Bag )

which indicates that Bag must be free when called. Some predicates sadly don’t work in all possible ways!

Though, you can easily write another predicate to test that:

reverse_test( Cs, Ss ) :-
    findall( S, ( flight( _, S, C, _ ), member(C, Cs) ), Ss ).

BTW, since your relation is clearly not a bijection between Ss and Cs, you shouldn’t expect the result to be reverses (test(A, R), reverse_test(R, A2) won’t result in A == A2)