9.7.3 Checker Output

The output of the determinacy checker is quite simple. For each clause containing unexpected nondeterminacy, a single line is printed showing the module, name, arity, and clause number (counting from 1). The form of the information is:

* Non-determinate: module:name/arity (clause number)

A second line for each nondeterminate clause indicates the cause of the nondeterminacy. The recognized causes are:

• The clause contains a disjunction that is not forced to be determinate with a cut or by ending the clause with a call to fail/0 or throw/1.
• The clause calls a nondeterminate predicate. In this case the predicate is named.
• There is a later clause for the same predicate whose first argument has the same principal functor (or one of the two clauses has a variable for the first argument), and this clause does not contain a cut or end with a call to fail/0 or throw/1. In this case, the clause number of the other clause is mentioned.
• If the predicate is multifile, then clause indexing is not considered sufficient to ensure determinacy. This is because other clauses may be added to the predicate in other files, so the determinacy checker cannot be sure it has seen all the clauses for the predicate. It is good practice to include a cut (or fail) in every clause of a multifile predicate.

The determinacy checker also occasionally prints warnings when declarations are made too late in the file or not at all. For example, if you include a dynamic, nondet, or discontiguous declaration for a predicate after some clauses for that predicate, or if you put a dynamic or nondet declaration for a predicate after a clause that includes a call to that predicate, then the determinacy checker may have missed some nondeterminacy in your program. The checker also detects undeclared discontiguous predicates, which may also have undetected nondeterminacy. Finally, the checker looks for goals in your program that indicate that predicates are dynamic; if no dynamic declaration for those predicates exists, then you will be warned.

These warnings take the following form:

! warning: predicate module:name/arity is property.
!          Some nondeterminacy may have been missed.
!          Add (or move) the directive
!              :- property module:name/arity.
!          near the top of this file.