The theme of this paper is profunctors, and their centrality and ubiquity in understanding concurrent computation. Profunctors (a.k.a. distributors, or bimodules) are a generalisation of relations to categories. Here they are first presented and motivated via spans of event structures, and the semantics of nondeterministic dataflow. Profunctors are shown to play a key role in relating models for concurrency and to support an interpretation as higher-order processes (where input and output may be processes). Two recent directions of research are described. One is concerned with a language and computational interpretation for profunctors. This addresses the duality between input and output in profunctors. The other is to investigate general spans of event structures (the spans can be viewed as special profunctors) to give causal semantics to higher-order processes. For this it is useful to generalise event structures to allow events which “persist.”

Structural Operational Semantics (SOS) provides a framework to give an operational semantics to programming and specification languages, which, because of its intuitive appeal and flexibility, has found considerable application in the theory of concurrent processes. Even though SOS is widely used in programming language semantics at large, some of its most interesting theoretical developments have taken place within concurrency theory. In particular, SOS has been successfully applied as a formal tool to establish results that hold for whole classes of process description languages. The concept of rule format has played a major role in the development of this general theory of process description languages, and several such formats have been proposed in the research literature. This chapter presents an exposition of existing rule formats, and of the rich body of results that are guaranteed to hold for any process description language whose SOS is within one of these formats. As far as possible, the theory is developed for SOS with features like predicates and negative premises.

The expressive power of process algebras is investigated in a general setting of structural operational semantics. The notion of an effective operational semantics is introduced and it is observed that no effective operational semantics for an enumerable language can specify all effective process graphs up to trace equivalence. A natural class of Plotkin style SOS specifications is identified, containing the guarded versions of calculi like CCS, SCCS, Meije and ACP, and it is proved that any specification in this class induces an effective operational semantics. Using techniques introduced by Bloom, it is shown that for the guarded versions of CCS-like calculi, there is a double exponential bound on the speed with which the number of outgoing transitions in a state can grow. As a corollary of this result it follows that two expressiveness results of De Simone for Meije and SCCS depend in a fundamental way on the use of unguarded recursion. A final result of this paper is that all operators definable via a finite number of rules in a format due to De Simone, are derived operators in the simple process calculus PC. 1991 Mathematics Subject Classification: 68Q05, 68Q10, 68Q55, 68Q75, 03D20. 1991 CR Categories: D.3.1, D.3.3, F.1.1, F.1.2, F.3.2, F.4.1. Keywords & Phrases: process algebra, PC, labeled transition systems, process graphs, effective process graphs, effective operational semantics, structural operational semantics, expressiveness, bisimulation equivalence, trace equivalence, action transducers. Notes: Most of this work was carried out while the author was at the MIT Laboratory for Computer Science, supported by ONR contract N00014-85-K-0168. Part of this work took place in the context of the ESPRIT Basic Research Action 7166, CONCUR2. This p...

A many-sorted variant, called stalk format, of the single sorted tyft-format for transition system specifications, introduced by Groote and Vaandrager, is proposed. The stalk format is shown to be a convenient formalism to express continuation-style transition systems for languages incorporating concepts as, e.g., process creation or backtracking, for which the existing formats seem less adequate. It is proved, extending a similar result for the single sorted case, that for an appropriate generalization of (strong) bisimilarity for the present many-sorted setting, bisimulation with respect to a transition system specification in stalk format, is a congruence. It is argued that the several conditions, required for the type of transition system specification put forward in the paper, can not be relaxed without loosing this congruence. Finally, the present format is compared with several existing ones in the literature, viz. De Simone-, GSOS- and pure tyft-format. Keywords and phrases se...

process algebra.

We show how to use a wide class of Structural Operational Semantics specifications for Truly Concurrent Semantics, whereas the classical approach is rather based on interleaving models. The models we consider are Asynchronous Transition Systems and their behaviors are defined by means of a classical truly concurrent bisimulation : ST-bisimulation. However, the definition of ST-bisimulation over Asynchronous Transition Systems is new and makes this equivalence better understood. Moreover, we explain how our result holds of many other truly concurrent bisimulations of the literature. Introduction Plotkin has clearly formalized the technics of Structural Operational Semantics specification as a general method for defining semantics of programming languages [Plo81]. Milner [Mil81], [Mil89] introduced his calculus, CCS, by using this approach. Next, Groote and Vaandrager in [GV89], [Gro90] have considerably enriched the field by deriving transition systems [Kel76] modulo strong bisim...