Extracting Determinacy in Logic Programs

Unnecessary backtracking is a principal source of ineeciency in Prolog execution. In order to avoid the overhead of the general backtracking mechanism, determinate programs should be executed deterministically. Achieving this requires an automatic means of extracting de-terminacy. We present a method in which determinacy is inferred from the success conditions and the calling contexts of predicates. Unlike previous approaches that rely on cuts or use a limited notion of test predicates, we propagate the conditions to detect determinacy, enabling us to handle a much larger class of programs. Since the conditions are propagated explicitly, the power of the method can be readily increased by increasing the expressiveness of these conditions. Moreover, our approach provides a uniform treatment of calling contexts and success conditions, and clearly spells out their roles in inferring determinacy. We also describe how de-terminacy information can be combined with state-of-the-art indexing techniques to eeciently realize the beneets of determinism.