Concurrent Constraint Programming

Abstract

Concurrent Constraint Programming introduces a new and rich class of programming languages based on the notion of computing with partial information, or constraints, that synthesize and extend work on concurrent logic programming and that offer a promising approach for treating thorny issues in the semantics of concurrent, nondeterministic programming languages. Saraswat develops an elegant and semantically tractable framework for computing with constraints, emphasizing their importance for communication and control in concurrent, programming languages. He describes the basic paradigm, illustrates its structure, discusses various augmentations, gives a simple implementation of a concrete language, and specifies its connections with other formalisms. In this framework, concurrently executing agents communicate by placing and checking constraints on shared variables in a common store. The major form of concurrency control in the system is through the operations of Atomic Tell—an agent may instantaneously place constraints only if they are consistent with constraints that have already been placed—and Blocking Ask—an agent must block when it checks a constraint that is not yet known to hold. Other operations at a finer granularity of atomicity are also presented. Saraswat introduces and develops the concurrent constraint family of programming languages based on these ideas, shows how various constraint systems can naturally realize data structures common in computer science, and presents a formal operational semantics for many languages in the concurrent constraint family. In addition, he provides a concrete realization of the paradigm on a sequential machine by presenting a compiler for the concurrent constraint language Herbrand and demonstrates a number of constraint-based concurrent programming techniques that lead to novel presentations of algorithms for many concurrent programming problems.

Soft concurrent constraint programming

“…The concurrent constraint (cc) programming paradigm [Saraswat 1993] concerns the behavior of a set of concurrent agents with a shared store, which is a conjunction of constraints. Each computation step possibly adds new • S. Bistarelli et al constraints to the store.…”

“…In a program P = F.A, A is the initial agent, to be executed in the context of the set of declarations F . This corresponds to the language considered in [Saraswat 1993], which allows only guarded nondeterminism.…”

“…A formal treatment of the cc semantics can be found in [Saraswat 1993] and Boer and Palamidessi [1991]. Also, a denotational semantics of deterministic cc programs, based on closure operators, can be found in Saraswat [1993].…”

“…Also, a denotational semantics of deterministic cc programs, based on closure operators, can be found in Saraswat [1993]. A more complete survey on several concurrent paradigms is given also in de Boer and Palamidessi [1994].…”

“…The concurrent constraint (cc) paradigm [Saraswat 1993] is a very interesting computational framework which merges together constraint solving and concurrency. The main idea is to choose a constraint system and use constraints to model communication and synchronization among concurrent agents.…”