Pruning infinite failure branches in programs with occur-check

Abstract

We propose an improved execution model for Prolog with occurcheck. Prolog's incompleteness is reduced in the case of difference-lists and similar data structures which are crucial for practical programming tasks. We show that adding the occur-check to the Prolog-system is not sufficient for using difference-lists in a reliable manner. In fact, most programs where the occur-check fails end up looping infinitely. By propagating the subterm annotations of variables via abstract interpretation we avoid the creation of these undesirable infinite SLD-trees. In addition, our extension provides a better framework for transformations of programs that use difference-lists that cannot be realized in the common framework of infinite trees.