Sum only the positive elements in a list in prolog

Question

How do i only sum all the positive numbers?

sumlist([],0).
sumlist([H|T],N):-sumlist(T,N1),N is N1 +H.

?- sumlist([-2,2,3],N).
   N = 3, unexpected.
   N = 5. % expected

Answer 1

Another method:

plus_positive(N, Sum, Sum1) :-
    (   N > 0 ->
        plus(N, Sum, Sum1)
    ;   Sum1 = Sum
    ).
    
sum_positive(Lst, Sum) :-
    foldl(plus_positive, Lst, 0, Sum).

Result in swi-prolog:

?- sum_positive([-2, 2, -4, 3, -8], Sum).
Sum = 5.