An Application of Rational Trees in a Logic Programming Interpreter for a Procedural Language

Abstract

We describe here a simple application of rational trees to the implementation of an interpreter for a procedural language written in a logic programming language. This is possible in languages designed to support rational trees (such as Prolog II and its descendants), but also in traditional Prolog, whose data structures are initially based on Herbrand terms, but in which implementations often omit the occurs check needed to avoid the creation of infinite data structures. We provide code implementing two interpreters, one of which needs non-occurs-check unification, which makes it faster (and more economic). We provide experimental data supporting this, and we argue that rational trees are interesting enough as to receive thorough support inside the language.