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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
:- parent/2 is nondet.
before the clauses for parent/2. If run again after modifying file, then
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.