EVENTS Sponsored by ERCIM
ERCIM News No.35 - October 1998

HLCL'98 and EXPRESS’98

by Uwe Nestmann and Ilaria.Castellani

EXPRESS’98 was the 5th International Workshop on Expressiveness in Concurrency. These workshops, the first four of which were held as meetings of the European HCM network EXPRESS (1994-1997), focused on the comparison of different computational models and paradigms on the basis of their expressive power. HLCL’98, the 3rd International Workshop on High-Level Concurrent Languages, was intended to bring together active researchers involved in the design, development, foundations, and applications of high-level concurrent programming languages and models.

Both events took place this year as pre- and post-satellite workshops to the 9th International Conference on Concurrency Theory, CONCUR’98, which was organized this year by INRIA Sophia-Antipolis in Nice, France. Around 60 researchers attended the satellite events.


The EXPRESS workshops aim at bringing together researchers interested in the relations between various formal systems, particularly in the field of Concurrency. More specifically, they focus on the comparison between programming concepts (such as concurrent, functional, imperative, logic and object-oriented programming) and mathematical models of computation (such as process algebras, Petri nets, event structures, modal logics, rewrite systems etc.) on the basis of their relative expressive power.

The EXPRESS’98 workshop started with an invited talk by Matthew Hennessy, reviewing some recently proposed location calculi. These are extensions of name-passing calculi where names may also represent sites, thus allowing for the modelling of process migration. The following two presentations focussed on the comparison of the expressiveness of several name-passing calculi inspired by the pi-calculus and proposed new mathematical concepts such as that of ‘extended subset’. The second morning session was more eclectic but not less interesting, devoted to expressiveness issues in process algebras with data and rule formats for higher-order languages. After lunch, the participants reconvened for the second invited talk by P. S. Thiagarajan, who gave an account on the extension of results from the classical automata theory to a model for distributed systems. Following two talks on reactive probabilistic processes and local event structures, there was a session on the relative expressive power of different dialects of the language LINDA. Jos Baeten gave a lively concluding talk comparing two known semantics for process algebras on the basis of their capacity to express deadlocked behaviours.


Programming models should be simple, practical, high-level, and well founded. These qualities allow rigorous language specifications and support both formal and informal reasoning about programs. For concurrent and distributed systems, recent research on programming models has driven the design of several encouraging programming languages including Erlang, versions of ML, like CML and Facile and Haskell, as well as languages explicitly designed for concurrency or distribution such as Obliq, Oz, Pict, and the Join-Calculus language. Although the motivations behind the design of these languages are diverse (ranging from the development of graphical user interfaces and multi-agent systems to constraint, real-time, and distributed programming), suitable foundations have turned out to be quite similar in style and technique, often based on variants of well-known calculi for mobile processes.

After the invited talk (by James E. White, initiator of Telescript and chief technology officer at General Magic Inc., USA) with the provocative title Why networks aren’t programmable (or, why programs aren’t protocols), the program was first devoted to the foundations of safe distributed programming with mobile agents. Thereafter, presentations on implementation solutions, static type systems, and object-oriented and the functional paradigms in the setting of concurrent programming rounded off the high-quality programme. Apart from the high standard of the presentations, much of the success of this workshop was generated by contrasting the traditionally rather technical audience with an industrialist invited speaker, which opened up very lively discussions on possible future developments and expectations, for both the theoretical and practical aspects of high-level concurrent and distributed languages.

The proceedings of both events will be published by Elsevier Science as Numbers 2 and 3 of Volume 16 of ‘Electronic Notes in Theoretical Computer Science’.

Please contact:

Ilaria Castellani - INRIA
Tel: +33 4 9238 7640

Uwe Nestmann - BRICS/DANIT
Tel: +45 96 35 8892

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