13.5.4 Example

Here is an example file:

     :- load_files(library(detcheck),
                   [when(compile_time), if(changed)]).
     parent(abe, rob).
     parent(abe, sam).
     parent(betty, rob).
     parent(betty, sam).
     is_parent(Parent) :- parent(Parent, _).

The determinacy checker notices that the first arguments of clauses 1 and 2 have the same principal functor, and similarly for clauses 3 and 4. It reports:

     * Non-determinate: user:parent/2 (clause 1)
     *     Indexing cannot distinguish this from clause 2.
     * Non-determinate: user:parent/2 (clause 3)
     *     Indexing cannot distinguish this from clause 4.

In fact, parent/2 should be nondeterminate, so we should add the declaration

     :- nondet parent/2.

before the clauses for parent/2. If run again after modifying file, the determinacy checker prints:

     * Non-determinate: user:is_parent/1 (clause 1)
     *     This clause calls user:parent/2, which may be nondeterminate.

It no longer complains about parent/2 being nondeterminate, since this is declared. But now it notices that because parent/2 is nondeterminate, then so is is_parent/1.