9.7.6 What is Detected

As mentioned earlier, it is not in general possible to find exactly which places in a program will lead to nondeterminacy. The determinacy checker gives predicates the benefit of the doubt: when it is possible that a predicate will be determinate, it will not be reported. The checker will only report places in your program that will be nondeterminate regardless of which arguments are bound. Despite this, the checker catches most unwanted nondeterminacy in practice.

The determinacy checker looks for the following sources of nondeterminacy:

• Multiple clauses that cannot be distinguished by the principal functor of the first arguments, and are not made determinate with an explicit cut, fail/0, false/0, or raise_exception/1. First argument indexing is not considered for multifile predicates, because another file may have a clause for this predicate with the same principal functor of its first argument.
• A clause with a disjunction not forced to be determinate by a cut, fail/0, false/0, or raise_exception/1 in each arm of the disjunction but the last, or where the whole disjunction is followed by a cut, fail/0, false/0, or raise_exception/1.
• A clause that calls something known to be nondeterminate, other than when it is followed by a cut, fail/0, false/0, or raise_exception/1, or where it appears in the condition of an if-then-else construct. Known nondeterminate predicates include hooks and those declared nondeterminate or dynamic (since they can be modified, dynamic predicates are assumed to be nondeterminate), plus the following built-in predicates:
  • - absolute_file_name/3, when the options list contains solutions(all).
  • - atom_concat/3, when the first two arguments are variables not appearing earlier in the clause (including the clause head).
  • - bagof/3, when the second argument contains any variables not appearing earlier in the clause (including the clause head).
  • - clause/[2,3].
  • - current_op/3, when any argument contains any variables not appearing earlier in the clause (including the clause head).
  • - current_key/2, when the second argument contains any variables not appearing earlier in the clause (including the clause head).
  • - current_predicate/2, when the second argument contains any variables not appearing earlier in the clause (including the clause head).
  • - length/2, when both arguments are variables not appearing earlier in the clause (including the clause head).
  • - predicate_property/2, when either argument contains any variables not appearing earlier in the clause (including the clause head).
  • - recorded/3.
  • - repeat/0.
  • - retract/1.
  • - setof/3, when the second argument contains any variables not appearing earlier in the clause (including the clause head).
  • - source_file/[1,2] when the last argument contains any variables not appearing earlier in the clause (including the clause head).
  • - sub_atom/5, when at least two of the second, fourth and fifth arguments are variables not appearing earlier in the clause (including the clause head).