A new data structure for implementing extensions to Prolog

Abstract

We propose a new data structure for implementing delayed computations in Prolog as efficiently as commonly employed solutions, together with the optimal memory management needed for this data structure. It appears that this data structure (and the associated memory management) is also useful for implementing other extensions where there is a need for reversibly modifying terms other than in the leaves, e.g. functional extensions.

We first recall that optimal memory management in Prolog is possible only when considering the binding states in a precise manner.

Then we describe the classical implementation of delayed computations, along with the new memory management problems involved. The conclusion is that a correct implementation, w.r.t memory management, is only possible when introducing a new data structure which is known by the garbage collector.

A memory management machine, called MALI, has been realized in our group following the principles described, and offering such a data structure, called the attributed variable. On top of this machine, we have implemented a number of extensions to Prolog.

In the course of our experiments, we have verified that the initial choice in the design of MALI, i.e. to make it independent from the procedural semantics of Prolog, was a good choice. Indeed, we have been able to implement various logic programming systems extending the so-called “pure-Prolog”, ranging from PrologII to λ-Prolog and fair proof strategies.