Incremental Tabling for Query-Driven Propagation of Logic Program Updates

摘要

We foster a novel implementation technique for logic program updates, which exploits incremental tabling in logic programming - using XSB Prolog to that effect. Propagation of updates of fluents is controlled by initially keeping any fluent updates pending in the database. And, on the initiative of queries, making active just those updates up to the timestamp of an actual query, by performing incremental assertions of the pending ones. These assertions, in turn, automatically trigger system-implemented incremental bottom-up tabling of other fluents (or their negated complements), with respect to a predefined overall upper time limit, in order to avoid runaway iteration. The frame problem can then be dealt with by inspecting a table for the latest time a fluent is known to be assuredly true, i.e., the latest time it is not supervened by its negated complement, relative to the given q'uery time. To do so, we adopt the dual program transformation for defining and helping propagate, also incrementally and bottom-up, the negated complement of a fluent, in order to establish whether a fluent is still true at some time point, or rather if its complement is. The use of incremental tabling in this approach affords us a form of controlled, but automatic, system level truth-maintenance, up to some actual query time. Consequently, propagation of update side-effects need not employ top-down recursion or bottom-up iteration through a logically defined frame axiom, but can be dealt with by the mechanics of the underlying world. Our approach thus reconciles high-level top-down deliberative reasoning about a query, with autonomous low-level bottom-up world reactivity to ongoing updates, and it might be adopted elsewhere for reasoning in logic.