Non-determinism in Prolog simply refers to a predicate having potentially more than one solution. Clearly, member/2
is such a predicate. This does not mean that you have to be worried about your computation becoming unpredictable. Prolog has a well-defined computation rule which essentially says that alternative solutions are explored in a depth-first, left-to-right manner. Thus your goal member(X,[1,2,3,4])
will generate solutions to X
in the expected order 1,2,3,4.
Sorting the list [1,2,3,4] will not make any difference, as it is already sorted (according to Prolog's standard term order).
A word of caution about forall/2
: some Prologs define this, but it is probably less useful than you imagine, because it is not really a "loop". You can use it in your example because you only perform a print side effect in each iteration. For most other purposes, you should familiarize yourself with recursive patterns like
print_list([]).
print_list([X|Xs]) :- print(X), print_list(Xs).