Actors as a special case of concurrent constraint (logic) programming

Abstract

Saraswat recently introduced the framework of concurrent constraint programming [14]. The essence of the framework is that computations consist of concurrent agents interacting by communicating constraints. Several concurrent constraint programming languages have been defined. They differ in the kinds of constraints that can be used as well as the kinds of operations on constraints which are available. In this paper we introduce a very simple concurrent constraint language we call Lucy, designed to closely mimic the actor model of computation. Agents can communicate only by the posting of constraints upon bags (un-ordered collections possibly with duplicate elements). This very impoverished concurrent constraint language is a syntactic subset of Janus, a concurrent constraint language which closely resembles concurrent logic programming languages such as Guarded Horn Clauses [21], Strand [5], Parlog [2] and Flat Concurrent Prolog [13]. By identifying the subset of Janus which is an actor language, we elucidate the relationship between actors and concurrent logic programming (and its generalization as concurrent constraint programming). Lucy is best not thought of as a unification of logic and constraint programming with actor and object-oriented programming, but as the missing link between these programming language genera.