Make Prolog return one solution and stop showing the query option

Question

I'm new to prolog, and am experimenting with how to get it to stop querying after it finds one answer. I'm using this code:

member1(L,[L|_]).                    
member1(L,[_|RS]) :- member1(L,RS),!.      

The result is:

| ?- member1(3,[3,2,3]).

true ? a

yes

I'm lost as to how I could get Prolog to stop printing "true ?" and just print "yes" instead. I've tried using if/else construct and the format function but it still prints "true ?". Any ideas?

Answer 1

You're cutting the wrong place. Cut after the base condition, which says, "once the base is met, don't backtrack any more":

member1(L,[L|_]) :- !.                   
member1(L,[_|RS]) :- member1(L,RS).    

If-then does work for me, perhaps you implemented it different? (on swi-prolog)

member1(X,[Y|RS]) :-
    ( X = Y         -> true
    ; member1(X,RS) -> true
    ; false
    ) .

Swi also has the predicate once/1.

edited to account for the error pointed out by false.