10.26.3.1 Determinate Tests

Determinate tests are tests that must succeed exactly once, leaving no choicepoints behind. The test-body supplies proper values for the input arguments and verifies the output arguments. Verification can use test-options or be explicit in the test-body. The tests in the example below are equivalent.

     test(add1) :-
             A is 1 + 2,
             A =:= 3.
     
     test(add2, [true(A =:= 3)]) :-
             A is 1 + 2.

The test engine verifies that the test-body does not leave a choicepoint. We illustrate this using the test below:

     test(member1) :-
             member(b, [a,b,c]).

Although this test succeeds, member/2 leaves a choicepoint behind, which is reported by the test harness. To make the test silent, use one of the alternatives below.

     test(member2) :-
             memberchk(b, [a,b,c]).
     
     test(member3) :-
             member(b, [a,b,c]), !.
     
     test(member4, [nondet]) :-
             member(b, [a,b,c]).