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prologswi-prologoccurs-check

How to enable the occurs check in all unifications in SWI-Prolog?

According to Wikipedia:

Implementations offering sound unification for all unifications are Qu-Prolog and Strawberry Prolog and (optionally, via a runtime flag): XSB, SWI-Prolog and Tau Prolog.

However, when I do apropos(occur) it only finds unify_with_occurs_check/2. The man page doesn't mention "occur" either. How can the occurs check be enabled for all unifications in SWI-Prolog?

Solution

  • In the section on Environment Control, it lists the flags of the system. The occurs_check flag is the one that controls occurs checks for unification.

    You can set the flag with:

    :- set_prolog_flag(occurs_check, true).

    For example:

    ?- X = f(X).
X = f(X).

?- set_prolog_flag(occurs_check, true).
true.

?- X = f(X).
false.

    So first it unifies X with f(X). If we later set the occurs_check to true, then if we try to unify X again with f(X), it fails.