Why does Prolog skip all further if statements in recursion if one of them fails?

Question

The point of the program is to calculate the sum of all even numbers in a list of integers.

    is_even(Q):- Q mod 2 =:= 0.  

    sum_even([],0).
    sum_even([A|L],X) :-  sum_even(L,X1) -> is_even(A) -> X is X1 + A .

Whenever the is_even predicate succeeds there is no problem, it normally goes back to calculate the sum. However when the number is not even and is_even checks it, it fails, goes back to the recursion and fails everything that follows, doesn't even check if the number is even anymore and just returns false. In a list full of even numbers it works as intended, it returns the sum of all numbers in the list. This here is the trace of the code

Answer 1

Using an accumulator with tail-end recursion is fastest:

is_even(N) :-
    N mod 2 =:= 0.

sum_even(Lst, Sum) :-
    sum_even_(Lst, 0, Sum).

sum_even_([], Sum, Sum).
sum_even_([H|T], Upto, Sum) :-
    (   is_even(H) ->
        Upto1 is Upto + H
    ;   Upto1 = Upto
    ),
    sum_even_(T, Upto1, Sum).


sum_even_slow([], 0).
sum_even_slow([H|T], Sum) :-
    sum_even_slow(T, Sum0),
    (   is_even(H) ->
        Sum is Sum0 + H
    ;   Sum = Sum0
    ).

Performance comparison in swi-prolog:

?- numlist(1, 1_000_000, L), time(sum_even(L, Sum)).
% 4,000,002 inferences, 0.765 CPU in 0.759 seconds (101% CPU, 5228211 Lips)
Sum = 250000500000.

?- numlist(1, 1_000_000, L), time(sum_even_slow(L, Sum)).
% 4,000,001 inferences, 4.062 CPU in 4.023 seconds (101% CPU, 984755 Lips)
Sum = 250000500000.