A relaxed condition for avoiding the occur-check

• Published in: Theoretical computer science Vol. 975; p. 114107
• Main Author: Drabent, Włodzimierz
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 09.10.2023
• Subjects: Delays/coroutining; Logic programming; Modes; Occur-check; Selection rule; SLD-resolution; Unification; Modes; Logic programming; Unification; Occur-check; Selection rule; Delays/coroutining; SLD-resolution
• ISSN: 0304-3975
• DOI: 10.1016/j.tcs.2023.114107

We generalize the notion of “not subject to occur-check” (NSTO), on which most known results on avoiding the occur-check in logic programming are based. NSTO means that unification is performed only on such pairs of atoms for which the occur-check never succeeds in any run of a nondeterministic unification algorithm. Here we show that “any run” can be weakened to “some run”. We present some related sufficient conditions under which the occur-check may be safely omitted. We show examples for which the proposed approach provides more general results than the approaches based on well-moded and nicely moded programs (this includes cases to which the latter approaches are inapplicable). We additionally present a sufficient condition based on NSTO, working for arbitrary selection rules. •We deal with sufficient conditions for avoiding the occur-check in unification.•Such conditions have been based on a notion of “not subject to occur-check” (NSTO).•We propose a less restrictive version of NSTO.•It widens the class of cases dealt with.•Also, it leads to simpler sufficient conditions than NSTO, for some cases.