Prolog: Check predicate against every item in the list

Question

Basically, I want to be able to check to see if at least one value in a list satisfies some predicate.

What I have so far:

need(x,y).
check_list(X,[H|T]) :- 
   need(H,X).

And so this works fine so long as I only have one value in the list. I'm not sure how to make it check the other values. When I try and use recursion I eventually find an element that satisfies the second predicate but it then goes back up the stack which will eventually cause it to be false.How can I make it 'break' essentially?

Answer 1

The backtracking you are seeing during recursion is Prolog attempting to find more ways for the predicate to succeed. This is a fundamental Prolog behavior and is what makes it useful. It seeks to find all of the solutions.

In your case, you only want to confirm one solution to the problem of, An element in the list that meets a specific criterion. For this, you could use a cut:

check_list(X, [H|_]) :-
    need(X, H), !.    % Don't backtrack after success
check_list(X, [_|T]) :-
    check_list(X, T).

Or you could use once/1 which is specifically designed to handle cases where you only want a single solution:

check_list(X, [H|_]) :-
    need(X, H).
check_list(X, [_|T]) :-
    check_list(X, T).

check_list_once(X, L) :- once(check_list(X, L)).