Communicating clauses are proposed as an extension to contextual logic programming aiming at specifying the synchronous communication between agents, described here as units. The expressiveness of the extended framework is argued through the coding of producer/consumer schemes and several applications combining the logic and object-oriented styles of programming. Operational and declarative semantics are designed for the new framework. The operational semantics rests on a derivation relation stating how agents can be evaluated under contextual and synchronization constraints. The declarative semantics extend the classical model and fixed point theory to take these constraints into account. As suggested, an effort has been made to keep these semantics in the main streams of logic programming semantics. However, the contextual and synchronization constraints raise new problems for which fresh solutions are proposed. 1 Introduction Recently, contexts have been introduced in logic program...

In this report a comparative semantics is given for a language L p containing probabilistic and non-deterministic choice. The effects of interpreting these operators as local or global are investigated. For three of the possible combinations an operational model and a denotational model are given and compared. First models for local probabilistic choice and local non-deterministic choice are given using a generative approach. By adjusting these models slightly models for global probability and local non-determinism are obtained. Finally models for local probability and global non-determinism are presented using a stratified approach. For use with the denotational models a construction of a complete ultra-metric space of finite multisets is given. 1 Introduction The goal of this paper is to construct comparative semantics for a language combining non-determinism and probabilistic choice. The main interest is the interplay between these two concepts. Since many of the interesting proper...

Well-known metric spaces for modelling finitely branching and image finite systems are shown to be (the carrier of) terminal coalgebras. Introduction In the area of metric semantics, various metric structures have been proposed to model a wide spectrum of programming notions (see, e.g., [BV96]). In this paper, we focus on metric structures for modelling nondeterministic systems which may give rise to both terminating and nonterminating computations. The systems we have in mind are labelled transition systems [Kel76]. A large variety of programming notions can be modelled by means of these systems (see, e.g., [Plo81]). The models we consider are linear (cf. [Pnu85]). In these models, the locations in a computation where a nondeterministic choice is made are not visible. These linear models are usually contrasted with branching models (cf. [Gla90]). In those models, the positions in the computation where a nondeterministic choice is made are administrated. Typical examples of linear me...

Sets of pomsets are frequently used as a true-concurrency, linear-time semantics of Petri nets. For a Petri net N , let P(N ), a set of pomsets, denote this pomset behaviour of N , and let P := fP(N) j N is a Petri netg denote the class of pomset behaviours of Petri nets, a generalization of L, the class of all Petri net languages. We present here an algebraic characterization for P, similar to the known algebraic characterizations for L. 1 Introduction We assume the reader is familiar with Petri nets and to some extent with the definition of their interleaving behaviour, Petri net languages. Interleaving languages of Petri nets, consisting of linear words of firing sequences or their labellings, have been intensively studied. Famous results are found e.g. in [3], [4], [5], or [7]. Especially Hack [4] has proved several characterizations for interleaving behaviours. He distinguishes between terminal behaviours where a firing sequence must reach some predefined terminal marking, and pr...

. The classification of properties of concurrent programs into safety and liveness was first proposed by Lamport [20]. Since then several characterizations of hierarchies of properties have been given, see e.g. [4, 18, 8, 19]; this includes syntactic characterizations (in terms classes of formulas of logics such as the linear temporal logic) as well as extensional (as sets of computations in some abstract domain). The latter often admits a topological characterization with respect to the natural topologies of the domain of computations. We introduce a general notion of a linear time model of computation which consists of partial and completed computations satisfying certain axioms. The model is endowed with a natural topology. We show that the usual topologies on strings, Mazurkiewicz traces and pomsets arise as special cases. We then introduce a hierarchy of properties including safety, liveness, guarantee, response and persistence properties, and show that our definition ...

ness of an Interleaving Semantics for Action Refinement J.I. den Hartog 1 , E.P. de Vink 1 and J.W. de Bakker 1;2 Abstract For an abstract programming language with action refinement both an operational and a denotational semantics are given. The operational semantics is based on an SOS-style transition system specification involving syntactical refinement sequences. The denotational semantics is an interleaving model which uses semantical refinement `environments'. It identifies those statements which are equal under all refinements. The denotational model is shown to be fully abstract with respect to the operational one. The underlying metric machinery is exploited to obtain this full abstractness result. Usually, action refinement is treated either in a model with some form of true concurrency, or, when an interleaving model is applied, by assuming that the refining statements are atomized. We argue that an interleaving model without such atomization is attractive as well. 1 I...

This paper continues a research on universal contexts and semantics for Petri nets started by Nielsen, Priese and Sassone (1995). We consider generalized, labelled Petri nets N where some transitions and places are distinguished as public. They form the interface of N that may communicate with a Petri net context. An elementary calculus, 8, is introduced in which one can construct any Petri net with an interface from trivial constants (single places, single tiansitions) by drawing arcs, adding tokens, and hiding public places and transitions. We prove the existence of a universal context U such that two Petri nets behave the same in any context if their behaviour is equal in the universal context. Let B(U[N]) be the behaviour of N embedded in its universal context, where B may be the interleaving language, step-language, or true-concurrent pomset-language. In any of these cases, B(U[N]) (in contrast to B(N)) turns out to be a compositional semantics of N with respect to the algebra 8. In addition, the interleavingand step-semantics are just special cases of the true-concurrency pomset-semantics. @ 1998-