Prolog Program Transformations and Meta-Interpreters

Abstract

Partial Evaluation in Prolog (generally associated to Folding/Unfolding techniques) is a code optimization technique, which transforms a readable and structured program in a more efficient program (that is to say better adapted to the interpreter or compiler). These programs are Prolog programs semantically equivalent with respect to the least Herbrand model. Our context is the operational equivalence of programs, and not the efficiency of program transformations.

We introduce first the notion of Strong Operational Equivalence (s.-o.e.), and then we propose to study strong operational equivalence preserving transformations. Strong operational equivalence means that the solution order is preserved and, when using inference as complexity unit, the runtime of any solution in the transformed program is linearly dependent on the runtime of the corresponding solution in the original program. This equivalence allows us to consider programs including non-logical features. We will restrict our study to Cut and Read/Write predicates and our framework is the standard resolution (depth first, leftmost atom).

Then, two meta-interpreters are studied. Using the Vanilla meta-interpreter and a new meta-interpreter (that we introduce and call quasiiterative meta-interpreter), we show that any Prolog program ? can be transformed to a meta-program (that is, an instance of these metainterpreters) which is strongly operationally equivalent to the original one. The structure of the quasi-iterative meta-interpreter is the simpliest one known up to date, that is, composed of three unit clauses and one right linear recursive rule.

partially supported by the G.D.R. Greco de la Programmation