Prolog - Getting different results by yielding

Question

I'm trying to write a prolog program which needs to have a predicate that does returns minimum variable every time it called. But i need it in such a way that if you yield the function, it should return next minimum value. For example:

min([5,3,2,7],B).
B = 2 ;
B = 3 ;
B = 5 ;
B = 7 ;
false.

Any ideas?

Answer 1

min(L, Min) :- sort(L, S), member(Min, S).

will do for your example data. More generally, setof/3 will allow to build a sorted list of solutions to a generic goal.

edit: without sorting, a somewhat involved solution based on select/3:

min(L, M) :-
    select(T, L, R),
    ( maplist('<'(T), R), (M = T ; min(R, M)) ).

yields

?- min([2,6,1],X).
X = 1 ;
X = 2 ;
X = 6 ;
false.

Answer 2

In general, you cannot rely on the order of solutions, in a somewhat weird system member might be implemented differently:

member2(X,[_|Xs]) :-
    member2(X,Xs).
member2(X,[X|_]).

min(L, Min) :- 
    sort(L, S), 
    member2(Min, S).

yielding:

?- min([2,1,4,3,2],X).
X = 4 ;
X = 3 ;
X = 2 ;
X = 1.

it becomes reliable if the order is made explicit by putting the original list into relation with a list of the solutions in correct order. In your case this amounts to the sort predicate (as long as you accept that double entries are not reported twice), for a general terminating predicate, this can be done by using setof/2 and determining the order in a seperate step, as CapelliC already mentioned.