2.2.1 Testing deterministic predicates

Deterministic predicates are predicates that must succeed exactly once and, for well behaved predicates, leave no choicepoints. Typically they have zero or more input- and zero or more output arguments. The test goal supplies proper values for the input arguments and verifies the output arguments. Verification can use test-options or be explicit in the body. The tests in the example below are equivalent.

test(add) :-
        A is 1 + 2,
        A =:= 3.

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

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

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

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

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

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