We study two encodings of the asynchronous #-calculus with input-guarded choice into its choice-free fragment. One encoding is divergence-free, but refines the atomic commitment of choice into gradual commitment. The other preserves atomicity, but introduces divergence. The divergent encoding is fully abstract with respect to weak bisimulation, but the more natural divergence-free encoding is not. Instead, we show that it is fully abstract with respect to coupled simulation, a slightly coarser---but still coinductively defined---equivalence that does not enforce bisimilarity of internal branching decisions. The correctness proofs for the two choice encodings introduce a novel proof technique exploiting the properties of explicit decodings from translations to source terms.

The asynchronous pi-calculus is considered the basis of experimental programming languages (or proposal of programming languages) like Pict, Join, and Blue calculus. However, at a closer inspection, these languages are based on an even simpler calculus, called Local (L), where: (a) only the output capability of names may be transmitted; (b) there is no matching or similar constructs for testing equality between names. We study the basic operational and algebraic theory of Lpi. We focus on bisimulation-based behavioural equivalences, precisely on barbed congruence. We prove two coinductive characterisations of barbed congruence in Lpi, and some basic algebraic laws. We then show applications of this theory, including: the derivability of delayed input; the correctness of an optimisation of the encoding of call-by-name lambda-calculus; the validity of some laws for Join.

The -calculus with synchronous output and mixed-guarded choices is strictly more expressive than the -calculus with asynchronous output and no choice. As a corollary, Palamidessi recently proved that there is no fully compositional encoding from the former into the latter that preserves divergence-freedom and symmetries. This paper shows

communication primitives acting across ambient boundaries. Expressiveness is achieved at the price of communication interferences on message reception whose resolution requires synchronisation of activities at multiple, distributed locations. We study a variant of BA aimed at controlling communication interferences as well as mobility ones. Our calculus draws inspiration from Safe Ambients (SA) (with passwords) and modifies the communication mechanism of BA. Expressiveness is maintained through a new form of co-capability that at the same time registers incoming agents with the receiver ambient and performs access control.

Process algebra semantics can be categorised into non-interleaving semantics, where parallel composition is considered a primitive operator, and interleaving semantics, where concurrency is reduced to sequentiality plus nondeterminism. The former have an appealing intuitive justification, but the latter are mathematically more tractable. This paper addresses the study of non-interleaving semantics in the framework of process algebras for mobile systems, like -calculus [MPW92, Mil91]. We focus on location bisimulation ( ` ), in our opinion one of the most convincing non-interleaving equivalences, which aims to describe the spatial dependencies on processes. We introduce ` in -calculus following the definition for CCS given in [BCHK91b]. Our main contribution is to show that in -calculus ` can be expressed, or implemented, within the ordinary interleaving observation equivalence [Mil89, MPW92] by means of a fairly simple and fully abstract encoding. Thus, we can take advantage of the...

) David Sands y Department of Computing, Imperial College 180 Queens Gate, London SW7 2BZ email: ds@uk.ac.ic.doc Abstract In this paper we address the technical foundations essential to the aim of providing a semantic basis for the formal treatment of relative efficiency in functional languages. For a general class of "functional" computation systems, we define a family of improvement preorderings which express, in a variety of ways, when one expression is more efficient than another. The main results of this paper build on Howe's study of equality in lazy computation systems, and are concerned with the question of when a given improvement relation is subject to the usual forms of (in)equational reasoning (so that, for example, we can improve an expression by improving any sub-expression). For a general class of computation systems we establish conditions on the operators of the language which guarantee that an improvement relation is a precongruence. In addition, for...

We study the behavioural theory of Cardelli and Gordon's Mobile Ambients. We give an LTS based operational semantics, and a labelled bisimulation based equivalence that coincides with reduction barbed congruence. We also provide two up-to proof techniques that we use to prove a set of algebraic laws, including the perfect firewall equation.

In a previous paper we had dened the notion of an eciency preorder for concurrent systems. In this paper, we present a coarser relation, called the elaboration preorder, which is ner than observational equivalence. Further, this preorder is incomparable with the almostweak bismulation preorder of Milner and Sangiorgi. In particular, the elaboration preorder is preserved under all contexts except summation. The largest precongruence contained in it, which we call conformance, is obtained by the usual means and a complete axiomatization for conformance of nite processes is given. The paper ends with an example to show the use of this relation. 1 Introduction In [1] the eciency preorder was dened on processes and a proof system was given and shown to be complete for nite processes. It was shown that it was possible to compare eciencies of dierent implementations of the same specication with little extra eort than that required to prove their correctness. However, the eciency preo...

A testing scenario in the sense of De Nicola and Hennessy is developed to measure the worst-case efficiency of asynchronous systems using dense time. For all three variants considered, it is shown that one can equivalently use discrete time; in the discrete versions, one variant coincides with an approach based on discrete time in [Vog95b], and thus we can clarify the assumptions behind this approach. The resulting testing-preorders are characterized with some kind of refusal traces and shown to satisfy some properties that make them attractive as faster-than relations. The three testing-preorders are incomparable in general, but for some interesting classes of systems implications are shown. 1 Introduction In the testing approach of [DNH84], reactive systems are compared by embedding them -- with a parallel composition operator k -- in arbitrary test environments. One variant of testing (must-testing) considers the worst-case behaviour: a system N performs successfully in an environm...

A general approach for defining behavioural preorders over process terms as the maximal pre--congruences induced by basic observables is examined. Three different observables, that provide information about the initial communication capabilities of processes and about the possibility that processes get engaged in divergent computations, will be considered. We show that the pre--congruences induced by our basic observables coincide with intuitive and/or widely studied behavioural preorders. In particular, we retrieve in our setting the must preorder of De Nicola and Hennessy and the fair/should preorder introduced by Cleaveland and Natarajan and by Brinksma, Rensink and Vogler. A new form of testing preorder, which we call safe--must, also emerges. The alternative characterizations we offer shed light on the differences between these preorders, and on the role played in their definition by tests for divergence. 1 Introduction In the classical theory of functional programming, the point...