ELAN - Introduction

This description of the language ELAN is inspired by the paper [33] presented at the Second Workshop on Rewriting Logic and its Applications, Pont-A-Mousson (France), 1998.

The ELAN system  [2] provides an environment for specifying and prototyping deduction systems in a language based on rules controlled by strategies. Its purpose is to support the design of theorem provers, logic programming languages, constraints solvers and decision procedures and to offer a modular framework for studying their combination.

ELAN takes from functional programming the concept of abstract data types and the function evaluation principle based on rewriting. But rewriting is inherently non-deterministic since several rules can be applied at different positions in a same term, and in ELAN, a computation may have several results. This aspect is taken into account through choice operations and a backtracking capability. One of the main originality of the language is to provide strategy constructors to specify whether a function call returns several, at-least one or only one result. This declarative handling of non-determinism is part of a strategy language allowing the programmer to specify the control on rules application. This is in contrast to many existing rewriting-based languages where the term reduction strategy is hard-wired and not accessible to the designer of an application. The strategy language offers primitives for sequential composition, iteration, deterministic and non-deterministic choices of elementary strategies that are labelled rules. From these primitives, more complex strategies can be expressed. In addition the user can introduce new strategy operators and define them by rewrite rules. Evaluation of strategy application is itself based on rewriting. So the simple and well-known paradigm of rewriting provides both the logical framework in which deduction systems can be expressed and combined, and the evaluation mechanism of the language.

The purpose of these pages is to summarize ELAN features, library and environment and to provide a guide to the literature on the language.