On Applying Program Transformation to Implement Suspension-Based Tabling in Prolog

A common approach used to include tabling support into existing Prolog systems is to modify and extend the low-level engine. Although this approach is ideal for run-time efficiency, it is not easily portable to other Prolog systems as engine level modifications are rather complex and time consuming. A different approach is to apply source level transformations to a tabled program and then use external tabling primitives to provide direct control over the search strategy. We propose a suspension-based tabling mechanism based on program transformation that uses the C interface of the Yap Prolog system to implement the tabling primitives. The program transformation module is fully written in Prolog. The tabling primitives module implements a local scheduling search strategy and uses tries to implement the table space. Suspension is implemented by leaving the continuation call for the current computation in the table entry corresponding to the variant call being suspended. During this process and as further new answers are found, they are stored in their tables and returned to all variant calls by calling the previously stored continuation calls. We ran our approach against the YapTab system that implements tabling support at the low-level engine. YapTab also implements a suspension-based mechanism, uses tries to implement the table space and is implemented on top of Yap. This is thus a first and fair comparison between the approach of supporting tabling at the low-level engine and the approach of supporting tabling by applying source level transformations coupled with tabling primitives.