9.7.2 Declaring Nondeterminacy

Some predicates are intended to be nondeterminate. By declaring intended nondeterminacy, you avoid warnings about predicates you intend to be nondeterminate. Equally importantly, you also inform the determinacy checker about nondeterminate predicates. It uses this information to identify unwanted nondeterminacy.

Nondeterminacy is declared by putting a declaration of the form

:- name/arity is nondet.

using an is/2-declaration (see lib-is_directives), or the legacy form

:- nondet name/arity.

in your source file. This is similar to a dynamic or discontiguous declaration. You may have multiple is or nondet declarations, and a single declaration may mention several predicates, separating them by commas.

Similarly, a predicate P/N may be classified as nondeterminate by the checker, whereas in reality it is determinate. This may happen e.g. if P/N calls a dynamic predicate that in reality never has more than one clause. To prevent false alarms arising from this, you can inform the checker about determinate predicates by declarations of the form:

:- name/arity is det.

using an is/2-declaration (see lib-is_directives), or the legacy form

:- det name/arity.

If you wish to include the legacy det and nondet declarations in your file and you plan to use the stand-alone determinacy checker, then you must include the line

:- load_files(library(nondetdecl),
              [when(compile_time), if(changed)]).

near the top of each file that contains such declarations. If you instead use the recommended is/2-declarations, or the integrated determinacy checker, then you do not need (and should not have) this line.