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Chapter 3    Management

3.1  Consortium level

The main management of the project is done at INRIA, Rocquencourt, and at ICL.

The following activities at the consortium level have taken place during the second year of the CONFER-2 working group:

Two workshops have been held. The first took place in April 1998 in Brighton near University of Sussex with 20 presentations and 30 participants, and was organised by Alan Jeffrey and Matthew Hennessy. The second workshop took place in September 1998 near INRIA Sophia-Antipolis in Nice with 14 presentations and 30 participants. It was organised by Gérard Boudol.

In 1998, the site of Warwick left our working group; a new site, Oxford University, is now participating to our group. This is due to David Walker's move from Warwick to Oxford.

As specified in §2.6 of the Working Group Programme, the vast majority of the activity of CONFER-2 is devoted to the organisation of 2 workshops per year. The cost statement forms will strengthen this distribution of the activity, although some of the funds are still reserved for travel expenses between sites.

3.2  Università di Bologna

3.2.1  Research

The research activity in Bologna, organized along the main task of the Working Group, has been the following:

In this area, we have been involved both in the analysis of bisimulation in the framework of the join-calculus and the study of a novel class-based concurrent calculus -- the objective join-calculus.

Analysis of Bisimulation We have mainly investigated the theory of bisimulation in the join-calculus [BFL97]. For this purpose, we introduce a refined operational model that makes interactions with the environment explicit. In this framework we show that the so-called ``locality property'' strongly affects the treatment of bisimulation, and provides a semantics simpler than other proposals in the literature. We also propose several formulations of bisimulation and we establish that all formulations yield the same equivalence, which is finer than barbed congruence. Bisimulation and barbed congruence coincide in the presence of name-testing.

Inheritance in the join-calculus In [FLMR98] we present an elementary class-based calculus of concurrent objects obtained from the join-calculus by introducing primitive record structure. We provide inheritance as the combination of a few operators to assemble objects from partial join-calculus definitions. As usual, method definitions and private fields of a parent class can be reused, overridden, or extended. As expected with concurrent objects, inheritance admits some but not all refinements of the synchronization patterns of the parent class.

Logics for Concurrency and the l-calculus
Our research in this field has been mainly focused on the theory of optimal sharing in b-reduction. In particular, we proved that the notion of optimal ``parallel'' reduction, as formalized by Lèvy, is not elementary recursive [AM98]. The result should not be understood in a negative sense: it just says that the notion of optimal sharing cannot have a simple implementation since, essentially, all the computational work can be done via sharing. Actually, the result is a straightforward consequence of the fact that every term of the simply typed lambda calculus can be essentially reduced in a linear number of ``parallel'' (i.e. sharable) b-reductions.

Programming Languages
We proceeded in the development of our implementation of Lamping's graph reduction algorithm for optimal reduction of functional languages (the Bologna Optimal Higher-order Machine [AG98]). We have completed a new ``light'' version, essentially inspired by Girard's Light Linear Logic.

3.2.2  Workshops, Travels and Visits

Travels Andrea Asperti and Luca Padovani participated at the Third CONFER-2 Workshop held in Brighton, April 1998.

Cosimo Laneve participated at the Fourth CONFER-2 Workshop held in Nice, September 1998.

Visits Vincent Danos visited University of Bologna for two weeks in July 1998. He gave a course on game semantics and he worked with Andrea Asperti on this issue.

Cosimo Laneve visited INRIA Rocquencourt in July 1998, where he worked with Cédric Fournet, Luc Maranget and Didier Remy on ``Concurrent Class-Based Languages''.

3.2.3  Publications

A. Asperti, H.Mairson Optimal b-reduction is not elementary recursive. Proc. of the twenty-fifth Annual ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages (POPL'98).

A. Asperti, S.Guerrini The Optimal Implementation of Functional Programming Languages. in ``Cambridge Tracts in Theoretical Computer Science'' Series, Cambridge University Press, October 1998.

M. Boreale, C. Fournet, C. Laneve: Bisimulations in the Join-Calculus. In Proc. Programming Concepts and Methods, Procomet '98, New York, U.S.A., 1998.

C. Fournet, C. Laneve, L. Maranget, D. Rèmy: Inheritance in the Join-Calculus. Draft, October 1998.

3.3  University of Cambridge

3.3.1  Research

Work at Cambridge has proceeded on several fronts, mainly in the areas Foundational Models, Calculi, and Programming Languages. Most items lie in two or three areas.

3.3.2  Personnel and exchanges

Researchers and PhD students associated with CONFER-2 at Cambridge include Gian Luca Cattani, Philippa Gardner, James Leifer, Robin Milner, Michael Norrish, Mark Sawle, Peter Sewell, Alistair Turnbull, Asis Unyapoth, Lucian Wischik and Pawel Wojciechowski.

During 97-98 Leifer visited INRIA-Sophia Antipolis, Gardner visited INRIA-Rocquencourt and Sewell visited Edinburgh. Leifer and Sewell attended CONFER-2 workshops in Sussex and Nice.

3.3.3  Publications

A. Barber, P.A. Gardner, M. Hasegawa and G. Plotkin, From Action Calculi to Linear Logic. European Association of Computer Science Logic, Springer-Verlag, 1998; conference article appears in BRICS Notes Series NS-97-1, 1997.

G. L. Cattani, M. Fiore and G. Winskel. A Theory of Recursive Domains with Applications to Concurrency. Proceedings of the Thirteenth Annual IEEE Symposium on Logic in Computer Science, LICS '98, 214--225.

G. L. Cattani, A. J. Power and G. Winskel. A Categorical Axiomatics for Bisimulation In Proceedings of CONCUR '98: Concurrency Theory (Nice). LNCS 1466, pages 581--596, September 1998.

P.A. Gardner and M. Hasegawa. Higher-order and Reflexive Action Calculi: their type-theory and models submitted to Information and Computation, 1998.

James Leifer. Bisimulation Congruences for Action Calculi. Thesis submitted to Trinity College, Cambridge, 1998.

Robin Milner. Communicating and Mobile Systems: the Pi Calculus. Forthcoming (CUP Jan 1999)

A. J. Power, G. L. Cattani and G. Winskel. A Representation Result for Free Cocompletions. BRICS Technical report RS-98-21, Department of Computer Science, University of Aarhus, September 1998. Submitted for publication.

Peter Sewell, Pawe T. Wojciechowski, and Benjamin C. Pierce. Location independence for mobile agents. In Proceedings of the Workshop on Internet Programming Languages (Chicago), May 1998.

Peter Sewell, Pawe T. Wojciechowski, and Benjamin C. Pierce. Location-independent communication for mobile agents: a two-level architecture. Submitted for publication.

Peter Sewell. Global/local subtyping and capability inference for a distributed p-calculus. In Proceedings of ICALP '98: International Colloquium on Automata, Languages and Programming (Aarhus). LNCS 1443, pages 695--706. Springer-Verlag, July 1998.

Peter Sewell. From rewrite rules to bisimulation congruences. Technical Report 444, University of Cambridge, June 1998.

Peter Sewell. From rewrite rules to bisimulation congruences. In Proceedings of CONCUR '98: Concurrency Theory (Nice). LNCS 1466, pages 269--284, September 1998.

3.4  CWI

3.4.1  Research Directions

J.W. Klop and I. Bethke have worked on ``origin tracking'', both in the framework of first-order term rewriting and lambda calculus. For first-order rewriting using this technique a proof was given of the theorem of Huet and Levy on normalization of needed reductions. This technique is in the literature also called ``dynamic dependence tracking'' and is used for applications such as program slicing and error recovery. For lambda calculus a corresponding notion of origin tracking using Levy's labeled lambda calculus was formulated and used to give a new proof of the classical Genericity Lemma in lambda calculus.

3.4.2  Workshops, Travels and Visits

J.W. Klop attended the conference RTA 99 in Tsukuba, Japan, March 30 - April 1, 1998, where he gave an invited talk 'Descendants and Origins in Term Rewriting'.

J.W. Klop and I. Bethke attended the CONFER-2 Workshop in Brighton, April 13-16, 1998.

F. van Raamsdonk attended the CONFER-2 Workshop in Nice, September 14-15, 1998.

3.5  University of Edinburgh

3.5.1  Research directions

Abramsky, Honda and McCusker used game semantics to give the first fully abstract model for a language with higher-order functions and general reference types---a substantial fragment of ``real'' languages such as SML and SCHEME. Their paper appeared in LiCS `98.

Abramsky and McCusker wrote a survey paper based on Abramsky's lectures at Marktoberdorf `97 on the use of game semantics to give a precise account of the ``space'' of programming languages between purely functional languages, and languages extended with features such as references and non-local control constructs. This account is highly structured: the various language extensions are accounted for by relaxing some of the constraints on strategies imposed to capture the purely functional discipline, and the extended setting is related to the more restricted one by means of a factorization theorem. This paper will appear in the Proceedings of Marktoberdorf `97.

Abramsky and Melliès have developed a concurrent version of game semantics where player and opponent may play many tokens at each move, in a truly concurrent fashion. Contrary to traditional ``sequential'' games where interaction between strategies proceeds along one active thread, interaction in concurrent games involves many threads evolving concurrently, with the possibility of delays and synchronisations. This modelling ability of concurrent games allows to describe the whole of Linear Logic, and not just fragments as previously in sequential game semantics. Finally, a Full Completeness result for the Multiplicative-Additive fragment of Linear Logic was obtained for the model. The work was presented on various occasions, at the Linear Logic Workshop in Luminy in April 1998, at the MFPS conference at Queen Mary and Westfield College, London in May 1998, in September 1998 at the Linear workshop in Tende and at the CONFER-2 workshop in Nice. A paper containing the Full Completeness result was submitted to LiCS'99.

Melliès presented his result of stability in Axiomatic Rewriting Systems [AHM98] at the LiCS'98 conference held in Indianapolis, US.

3.5.2  Workshops, Travels and Visits

Travels Paul-André Melliès participated at the CONFER-2 Workshops held in Brighton and Nice.

3.5.3  Publications

P.-A. Melliès, Axiomatic Rewriting Theory IV A stability theorem in Rewriting Theory, Proceedings of the 14th Annual Symposium on Logic in Computer Science, Indianapolis, pp. 287-298, 1998. LICS'98

S. Abramsky, K. Honda and G. McCusker, A fully abstract game semantics for general references, Proceedings of the 14th Annual Symposium on Logic in Computer Science, Indianapolis, pp. 334--344, 1998. LICS '98

3.6  ENS

3.6.1  Research Directions

Game semantics has been the main topic of investigation at ENS.

In [BDER98], a first bridge between dynamic semantics and static semantics of functional languages is given. Specifically, it is shown that there is a lax functor from a variant of the AJM games model where morphisms are strategies [BDER97] to a variant of plain relational semantics which the authors call ``bipolarized pointed relations'' and where morphisms are relations. Both models are models for classical linear logic without weakening. Intuitively, that functor works in two steps: it first goes from the modeling of a strategy in terms of moves to a modeling in terms of positions, and then forgets the time ordering of positions.

This paper together with [DHR96] sets up a neat connection between operational semantics and denotational semantics.

One direction of further investigation might be the construction of that intermediate games model based on positions, the existence of which the functor above suggests. But additional structure seems to be needed, such as a coherence relation over positions.

Di Cosmo, Loddo, and Nicolet have introduced in [DCLN98] a semantics of Logic Programming based on classical Game Theory, which is proven to be sound and complete w.r.t. the traditional operational semantics and Negation as Failure. This approach opens the way to a better understanding of the mechanisms at work in parallel implementations of logic programs and in the operational semantics of logic programs with negative goals.

3.6.2  Publications

Patrick Baillot, Vincent Danos, Thomas Ehrhard, and Laurent Regnier. Believe it or not, AJM's games model is a model of linear logic. In Proceedings of Logic in Computer Science '97. IEEE, 1997.

Patrick Baillot, Vincent Danos, Thomas Ehrhard, and Laurent Regnier. Timeless games. In Proceedings of Computer Science Logic '98. Springer LNCS 1414, 1998.

Roberto Di Cosmo, Jean-Vincent Loddo, and Stéphane Nicolet. A game semantics foundation for logic programming. In Catuscia Palamidessi, Hugh Glaser, and Karl Meinke, editors, PLILP'98, volume 1490, pages 355--373, 1998.

Vincent Danos, Hugo Herbelin, and Laurent Regnier. Games semantics and abstract machines. In Proceeding of Logic in Computer Science '96, 1996.

3.7  France Telecom -- CNET

3.7.1  Research

In 1998, CNET investigated some aspects of low-level security in distributed programming languages, such as protecting hosts against potentially hostile mobile code in open environments. A popular solution involves performing static analysis on executable code.

Using the Coq proof-assistant, P. Brisset has studied the feasibility of a certified byte-code verifier for a small subset of the Java architecture while C. Charras investigated Carnegie Mellon University's Proof-Carrying Code technique. Security properties of implementations of higher-order concurrent process calculi could be formally verified using similar approaches.

3.7.2  Workshops

CNET participated to the two CONFER-2 workshops in Brighton and Nice. Pascal Brisset presented his results at the Nice Workshop, September 98.

3.8  ICL

3.8.1  Research Directions

The role of ICL in CONFER-2 is, through our work on designing programming languages and advanced end-user applications, to be serving as a ``laboratory'' in which theoretical results are transferred into industrial usage.

In the First Year Report of CONFER-2 we reported on a new research programme, called the ICL GTD1 framework in Agents and Mobility. This framework has now been chosen as the example of how ICL sets up frameworks in general, hence it has been renamed to the ICL framework in Agents and Mobility. The programme itself still encourages ICL businesses to take part in the research process from an early stage to ensure they are prepared for knowledge and technology transfer as results emerge. Part of the framework is running a virtual laboratory in collaboration with Fujitsu Laboratories (recently Fujitsu have become the sole owner of ICL). The participation in CONFER-2 is an integral part of this framework.

Of particular interest to CONFER-2 is recent work on transferring ideas from the Facile system to the Java world via the mlj (ML to Java Virtual Machine code) compiler from Persimmon Research ( We have developed interfaces for the Facile concurrency primitives and are currently looking into how various packages for Mobile Agents in Java can be used to implement (some of) the constructs for distributed computing from Facile. Recently discussions of this work with Nick Benton from Microsoft Research and Andrew Kennedy from Persimmon Research have taken place in the context of the EU working group APPSEM 26142.

Work on understanding the foundation for mobile agent programming and mobile/distributed computing support in general is continuing to influence the development of ICL's technical strategy in this area.

In addition to the technical work relevant to CONFER-2 Lone Leth and Bent Thomsen have assisted Jean-Jacques Levy in the management of the working group.

3.8.2  Exchange of Personnel

Lone Leth and Bent Thomsen participated in the 3rd CONFER-2 workshop in Brighton (University of Sussex) 14.04.98-16.04.98.

Lone Leth participated in the 4th CONFER-2 workshop in Nice (INRIA) 14.09.98-15.09.98. Raja Nagarajan from Imperial College also attended this workshop on behalf of ICL.

3.8.3  Publications

In the second year of CONFER-2 the group at ICL involved in the working group has produced two relevant publications:

3.9  INRIA-Rocquencourt

3.9.1  Research directions

INRIA Rocquencourt has worked in the following directions.

3.9.2  Persons and exchanges

INRIA Rocquencourt participants are Jean-Jacques Lévy, Fabrice le Fessant, Cédric Fournet, Georges Gonthier, Luc Maranget, and Didier Rémy. A new PhD student, Sylvain Conchon, joined our group this year.

Carolina Lavatelli left INRIA in December. Cédric Fournet passed his PhD in November, and works now at Microsoft Research laboratory in Cambridge.

From Cambridge, we had one-week visits by Luca Cardelli in April and by Philippa Gardner in September, and from Bologna a one-week visit by Cosimo Laneve in July. We also had a visit by Silvano Dal-Zilio.

3.9.3  Publications

Martín Abadi, Cédric Fournet, and Georges Gonthier. Secure implementation of channel abstractions. In Thirteenth Symposium on Logic in Computer Science (LICS '98, Indianapolis), June 1998, pages 105--116.

Martín Abadi, Cédric Fournet, and Georges Gonthier. Secure implementation of channel abstractions. Manuscript, full version of this paper, on the Web at, 1998.

Michele Boreale, Cédric Fournet, and Cosimo Laneve. Bisimulations in the join-calculus. In IFIP Working Conference on Programming Concepts, Methods and Calculi (PROCOMET' 98). IFIP, June 1998.

Cédric Fournet and Georges Gonthier. A hierarchy of equivalences for asynchronous calculi. In Proc. of ICALP'98, July 1998.

Fabrice Le Fessant, Ian Piumarta, and Marc Shapiro. An implementation for complete asynchronous distributed garbage collection. In Proceedings of SIGPLAN'98 Conference on Programming Languages Design and Implementation, ACM SIGPLAN Notices, Montreal, June 1998. ACM Press.

Carolina Lavatelli. Deadlock sensitive types for lambda calculus with resources. In 8th Conference FST & TCS 1998, Chennai, India, 1998.

Fabrice Le Fessant and Luc Maranget. Compiling join-patterns. In Uwe Nestmann and Benjamin C. Pierce, editors, HLCL '98: High-Level Concurrent Languages (Nice, France, September 12, 1998), volume 16.3 of entcs. Elsevier Science Publishers, 1998.

3.10  INRIA-Sophia

3.10.1  Research directions

The work conducted this year by the Sophia group can be divided into a part on semantics and expressiveness, and a part on objects and mobility.

3.10.2  Semantics and expressiveness

Milner has shown that the p-calculus can encode functions. Work by Boudol then showed that Milner's encodings can be seen as CPS (continuation passing style) transformations. In [STR98] Sangiorgi examines this issue in details, analysing different translations of the l-calculus into p-calculus and their relationship with CPS transformations. This work is also a tutorial paper on encodings of l-calculus into p-calculus and on the equality among functions that is induced by their representation as p-calculus processes. This question is also analysed by Boudol [Bou98], who shows that, in the case of call-by-name l-calculus, the same equality among functions can also be obtained from a CPS transformation.

Massimo Merro [ME98] has shown that the asynchronous p-calculus is powerful enough to encode asynchronous variants of Chi, Update, and Fusion calculi, which are modifications of the p-calculus (Update and Fusion were developed within CONFER by the Swedish group) in which communications have a more global scope.

Recently there has been a growing interest in asynchronous languages and process calculi, where message emission is non-blocking. An important reference for this class of languages is the asynchronous pi-calculus, introduced by Honda and Tokoro and independently by Boudol, whose descriptive power has now been largely demonstrated. Castellani and Hennessy [CH98] study testing semantics for asynchronous process calculi. They examine an asynchronous version of CCS with internal and external choice operators. On this language asynchronous versions of the may and must testing preorders originally introduced by De Nicola and Hennessy are defined. Alternative characterisations for these preorders are given, which are contextual in nature. An equational characterisation for both preorders on the finite fragment of the language is also presented.

Boreale and Sangiorgi [BS97] study barbed equivalence in name-passing languages where there is no matching construct for testing equality between names. They concentrate on the p-calculus with capability types and subtypes, of which the untyped p-calculus without matching is a special case. The techniques introduced are used to prove some process equalities that fail in the ordinary p-calculus.

The asynchronous p-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 p, where: (a) only the output capability of names may be transmitted; (b) there is no matching or similar constructs for testing equality between names. In Local p, as reported by Sangiorgi and Merro [MeSa98], a number of useful equalities that are not sound in the standard p-calculus, do become sound. Proof techniques for proving such equalities are studied in [MeSa98].

3.10.3  Objects and mobility

In [KlSa98] Kleist and Sangiorgi give an interpretation of Abadi and Cardelli's first-order Imperative Object Calculus into a typed p-calculus. The interpretation validates the subtyping relation and the typing judgements of the Object Calculus, and is computationally adequate. The proof of computational adequacy makes use of (a p-calculus version) of ready simulation, and of a factorisation of the interpretation into a functional part and a very simple imperative part. The interpretation can be used to compare and contrast the Imperative and the Functional Object Calculi, and to prove properties about them, within a unified framework.

This work is being continued by Hüttel, Kleist, Merro, and Nestman [HKMN98] where a modelisations of the core of Cardelli's Obliq language is studied. The additional important problem here is to model the primitives that allow one to program the migration of objects, and to show their correctness.

Silvano Dal-Zilio [S98b] uses the modelisation of Abadi and Cardelli objects in the blue calculus extended with a very simple system of localities. It is shown that two migration behaviours can be defined, namely those of bouncing and quiet objects. These migration control abstractions are defined separately from the other aspects of the object definition and can easily be reused, thus providing more flexibility in the definition of "migration constraints". The interpretation of objects in the blue calculus is described in detail in [S98a].

3.10.4  Participation to Workshops

R. Amadio, I. Castellani, M. Merro and S. Dal-Zilio attended the CONFER-2 workshop in Brighton.

R. Amadio, G. Boudol, I. Castellani, M. Merro, D. Sangiorgi and S. Dal-Zilio attended the CONFER-2 workshop in Nice.

3.10.5  Publications

R. Amadio, S. Coupet-Grimal, « Analysis of a guard condition in type theory (extended abstract) », in : Proc. FOSSACS, ETAPS 98, Springer Lect. Notes in Comp. Sci. 1378, p. 48--62, 1998.

R. Amadio, S. Prasad, « Modelling IP mobility », in : Proc. CONCUR98, Springer Lect. Notes in Comp. Sci. 1466, p. 301--316, 1998.

M. Boreale, D. Sangiorgi, « Bisimulation in name-passing calculi without matching », in : thirteen LICS Conf., IEEE Computer Society Press, 1998.

G. Boudol, « On the semantics of the call-by-name CPS-transform », available at the author's web page, 1998, submitted.

I. Castellani, M. Hennessy, « Testing theories for asynchronous languages », in : Proceedings FST-TCS'98, 1998. To appear

H. Hüttel, J. Kleist, M. Merro, and U. Nestmann. Migration = cloning ; aliasing. To be presented at Sixth Workshop on Foundations of Object-Oriented Languages (FOOL 6), 1999.

J. Kleist, D. Sangiorgi, « Imperative Objects and Mobile Processes », in : Proc. IFIP Working Conference on Programming Concepts and Methods (PROCOMET'98), North-Holland, 1998.

M. Merro, D. Sangiorgi, « On asynchrony in name-passing calculi », in : 25th ICALP, LNCS, 1443, Springer Verlag, 1998.

M. Merro, « On the Expressiveness of Chi, Update and Fusion Calculi », Electronic Notes in Theoretical Computer Science Volume 16 , Issue 2


S. Dal-Zilio, « Objets concurrents dans un p-calcul applicatif », in : accepté par les Journées Francophones des Langages Applicatifs, 1998.

S. Dal-Zilio, « Quiet and Bouncing Objects: Two Migration Abstractions in a Simple Distributed Blue Calculus », in : 1st International Workshop on Semantics of Objects as Processes, BRICS Notes Series, 1998.

D. Sangiorgi, « Interpreting functions as pi-calculus processes: a tutorial », rapport de recherche NoRR-3470, INRIA-Sophia Antipolis, 1998.

3.11  KTH

3.11.1  Research directions

We have developed the fusion calculus which simplifies and extends the pi-calculus.

The fusion calculus contains the polyadic pi-calculus as a proper subcalculus and thus inherits all its expressive power. In addition fusion contains actions akin to updating a shared state, and a scoping construct for bounding their effects. Therefore it is easier to represent computational models with shared state, including concurrent constraint formalisms. It is also easy to represent the so called strong reduction strategies in the lambda-calculus, involving reduction under abstraction. In the pi-calculus these tasks require elaborate encodings.

The fusion calculus simplifies the pi-calculus by reducing the number of binding operators and the number of bisimulation equivalences, and by making input and output symmetric like in pure CCS. We attain a calculus where concepts from other models of computation are more easily expressed than in the pi-calculus, thereby taking a step towards a unified yet simple model of computation.

We have developed a broad foundational theory of the fusion calculus. We defined its labelled and unlabelled operational semantics, and treated strong and weak bisimulation equivalences for both semantics in some detail, including complete axiom systems for finite terms. The equivalences were given symbolic characterizations, leading to algorithms and an automatic tool for equivalence checking. We demonstrated the expressive power of the fusion calculus to give simple encodings of foundational calculi for functional and concurrent constraint programming.

3.11.2  Persons and exchanges

During 1998, the following persons have been active within CONFER-2: Joachim Parrow and Björn Victor (Uppsala University). Parrow attended the CONFER workshop in Brighton, and Victor attended the workshop in Nice.

3.11.3  Publications

J. Parrow and B. Victor. The fusion calculus: Expressiveness and symmetry in mobile processes. In Proceedings of LICS '98, 1998.

J. Parrow and B. Victor. The tau-laws of fusion. In D. Sangiorgi and R. de Simone, eds, Proceedings of CONCUR '98, volume 1466 of LNCS, pages 99--114. Springer, Sept. 1998.

B. Victor. The Fusion Calculus: Expressiveness and Symmetry in Mobile Processes. PhD thesis, Department of Computer Systems, Uppsala University, June 1998. Available as report DoCS 98/98.

B. Victor and J. Parrow. Concurrent constraints in the fusion calculus. In K. G. Larsen, S. Skyum and G. Winskel, eds, Proceedings of ICALP '98, volume 1443 of LNCS, pages 455--469. Springer, July 1998.

3.12  Università di Pisa

3.12.1  Outline

The major line of research of the group at the Dipartimento di Informatica, Università di Pisa, has been in the areas of Calculi, Foundational Models and Abstract Machines and Programming Languages.

3.12.2  Perspective

We are interested to further develop theoretical foundations of HD-automata and the corresponding verification techniques. One aspect that we intend to study is the development of verification methods for checking observational (bisimulation) semantics for classes of mobile calculi.

The format of the tile rules is essential in proving important properties of the system. For instance, a main proof method is given by the ability to vertically or horizontally decompose a derivable tile into more basic derivable tiles. This decomposition is driven by a decomposition of the effect or of the initial configuration, which in turn is possible if every rule (in a certain class) has an atomic initial configuration or an atomic effect. Among the properties provable in this way there are also important results such as ``bisimulation is a congruence'', as shown in for the tile system for CCS. It would be interesting to develop similar results for general tile systems, thus exploiting the powerful rule induction intrinsic in the tile model.

On the application side, we would like to mention the graphical orientation of tiles. While no graphical support for tiles is currently available, we think that the (natural) wire-and-box representation of tiles is (after some training) expressive and suggestive. It could be useful for users with a limited theoretical background, or for applications where simplicity and flexibility is essential, such as software architectures.

We intend to further investigate notions of types for the KLAIM language. We aim at demonstrating that network coordination policies based on typing information can be systematically used to guarantee that well--typed KLAIM processes always enjoy classes of security properties.

3.12.3  Persons

The group at Pisa involved in the project consists of Ugo Montanari, Gianluigi Ferrari, Marco Pistore and Roberto Bruni.

Two PhD-students, Marco Pistore and Roberto Bruni, both under the supervision of Ugo Montanari, are going to present their PhD thesis on topics which are closely related to the aims of the CONFER-2 Working Group.

3.12.4  Networking

Dates Place Participant
28/3/98 Lisboa (Portugal) ETAPS'98 Conf. Montanari U.
13/4/98 Brighton (UK) CONFER-2 Workshop Ferrari G.
13/4/98 Brighton (UK) CONFER-2 Workshop Montanari U.
13/4/98 Brighton (UK) CONFER-2 Workshop Pistore M.
6/9/98 Nice (France) CONCUR'98 & CONFER-2 Pistore M.
6/9/98 Nice (France) CONCUR'98 Montanari U.
13/9/98 Nice (France) CONFER-2 Montanari U.

3.12.5  Publications

  1. Bettini, L., De Nicola, R., Ferrari, G., Pugliese, R. Interactive Mobile Agents in XKLAIM. WETICE'98, Proceedings, IEEE, 1998, to appear.

  2. Bruni, R., A Logic for Modular Descriptions of Asynchronous and Synchronized Concurrent Systems (abstract) in Proceedings 2nd International Workshop on Rewriting Logic and its Applications, (Kirchner C., Kirchner H., Editors), ENTCS 15 (1998).

  3. Bruni R., Gadducci F., and Montanari U. Normal Forms for Partitions and Relations in Proceedings 13th Workshop on Algebraic Development Techniques, LNCS, to appear.

  4. Bruni, R. and Montanari, U. Zero-Safe Nets: The Individual Token Approach in Proceedings 12th Workshop on Algebraic Development Techniques, (Parisi-Presicce F., Editor), LNCS 1376, pp. 122--140 (1998).

  5. Bruni R., Meseguer J. and Montanari U. Process and Term Tile Logic Technical Report SRI-CSL-98-06, SRI International (July 1998). Also Technical Report TR-98-09, Department of Computer Science, University of Pisa (1998).

  6. Bruni R., Meseguer J. and Montanari U. Internal Strategies in a Rewriting Implementation of Tile Systems in Proceedings 2nd International Workshop on Rewriting Logic and its Applications, (Kirchner C., Kirchner H., Editors), ENTCS 15 (1998).

  7. Bruni R., Meseguer J. and Montanari U. Implementing Tile Systems: Some Examples from Process Calculi in Proceedings 6th Italian Conference on Theoretical Computer Science, World Scientific, 1998.

  8. Bruni R., Meseguer J., Montanari U. and Sassone V., A Comparison of Petri Net Semantics under the Collective Token Philosophy in Proceedings 4th Asian Computing Science Conference, (Hsiang J., Ohori A., Editors), LNCS, to appear.

  9. Bruni, R., Montanari, U., Zero-Safe Nets: Comparing the Collective and Individual Token Information and Computation, to appear.

  10. De Nicola, R., Ferrari, G., Pugliese, R., Klaim: a Kernel Language for Agents Interaction and Mobility. IEEE Transactions on Software Engineering, Vol.24(5):315-330, May, 1998.

  11. Ferrari,G., Gnesi, S., Montanari, U., Pistore, M. and Ristori, G. Verifying Mobile Processes in the HAL Environment, In Proc. CAV'98, LNCS 1427. Springer Verlag, 1998.

  12. Ferrari, G. and Montanari, U., Tile Formats for Located and Mobile Systems, To appear in Information and Computation, 1998.

  13. Honsell, F., Lenisa, M., Montanari, U., Pistore, M. Final Semantics for the pi-Calculus, In Proc. PROCOMET'98, 1998.

  14. Hirsch, D., Inverardi, P., and Montanari, U., Modeling Software Architectures and Styles with Graph Grammars and Constraint Solving, to appear in Proceedings WICSA 98, 1998.

  15. Montanari, U. and Meseguer, J., Mapping Tile Logic into Rewriting Logic, in: Francesco Parisi-Presicce, Ed., Recent Trends in Algebraic Development Techniques, Spinger LNCS 1376, 1998, pp. 62-91. i

  16. Montanari , U. and Pistore, M. An Introduction to History Dependent Automata In Proc. HOOTS'98, ENTCS 10. Elsevier Science, 1998.

  17. Montanari , U. and Talcott, C., Can Actors and pi-Agents Live Together?, in: Andrew Gordon, Andrew Pitts and Carolyn Talcott, Eds, Second Workshop on Higher-Order Operational Techniques in Semantics (HOOTS II), ENTCS, Vol. 10, 1998.

3.13  University of Sussex

3.13.1  Research directions

Asynchronous Mobile Processes In this area we have worked on two lines of research. The first concerns the expressivity of the various programming constructs which comprise typical mobile calculi. Specifically we have investigated a particular finite set of "combinators" which are sufficiently powerful to describe all (asynchronous) pi-calculus terms. Moreover we have shown that this set is minimal, in the sense that if any one combinator is omitted then there are pi-calculus terms which are no longer expressible. The research is reported in [Yos98]

In the second line we have proposed a new operational model for asynchronous processes, based on certain collapsings of traditional transition systems. This leads to a new class of processes, which we call ``asynchronously regular processes''. Using the new models we provide two appealing abstract characterisations of asynchronous bisimulation equivalence, namely, as spans of open maps and as a winning strategies for a bisimulation game. Also, by exploiting the coincidence of finite graphs with regular processes we see that bisimulation is polynomial time decidable over our class of asynchronously regular processes. The details are given in [Rat98].

Types for Distributed Processes Here we have continued our line of research started in the previous report period of CONFER-2. In [RH98] we propose a distributed version of the pi-calculus, in which mobile agents may migrate from site to site in a distributed environment. The novelty here is the use of a system of types, which may be used to manage access to resources, such as memory or communication channels. We prove that if all agents are well-typed then, in some intuitive sense, no agent can have access to resources unless they have been specifically granted permission. This work is continued in [HR98], where we now assume that there may be malicious agents, agents which do not respect the type system. By introducing a system of dynamic type-checking, where incoming agents are checked below allowed access to a site, we show that good site, which respect the type system, can not be harmed by malicious sites.

3.13.2  Personnel and Exchanges

The personnel at the University of Sussex who were actively involved in CONFER-2 related research during the period covered by the progress report are Matthew Hennessy, Julian Rathke, James Riely and Nobuko Yoshida.

3.13.3  Publications

Matthew Hennessy and James Riely. Type-safe execution of mobile agents in anonymous networks (extended abstract). In Proceedings of 4th Workshop on Mobile Onbject Systems, Brussels, Belgium, July 1998. Full version available as Computer Science Technical Report 03/98. Available from

J. Rathke. Resource based models for asynchrony. In Proc. FOSSACS'98, International Conference on Foundations of Software Science and Computation Structures, Lisbon. Springer-Verlag, 1998.

James Riely and Matthew Hennessy. Resource access control in systems of mobile agents (extended abstract). In Proceedings of 3rd International Workshop on High-Level Concurrent Languages, Nice, France, September 1998. Full version available as Computer Science Technical Report 02/98. Available from

Nobuko Yoshida. Minimality and separation results on asynchronous mobile processes: representability theorem by concurrent combinators (extended abstract). In Proceedings of CONCUR98, Nice, France, September 1998. Full version available as Computer Science Technical Report 05/98. Available from

3.14  University of Warwick, Oxford University

3.14.1  Research activity

Work has been done on theory of p-calculus, and the final versions of papers on semantic definition of concurrent object-oriented programming languages and correctness of program-transformation rules have been published in journals.

In the monadic p-calculus an interaction between processes involves the transmission of a single name, while in the polyadic p-calculus a tuple of names is passed in an interaction. Communication of tuples of names is expressible in the monadic calculus. Moreover, although passing of an n-tuple requires n+1 monadic interactions, only the first is semantically significant: in the first action a private link is established between the communicating processes; the subsequent semantically-inert communications transfer the names of the tuple from sender to receiver via that link.

In the polyadic calculus there is a pressing need for some kind of typing discipline, as among the processes are terms where the components disagree on the length of the tuple to be passed. Even on well-typed polyadic processes, however, the translation to monadic processes is not fully abstract when barbed congruence is adopted as process equivalence. Equivalence of the monadic translations of well-typed polyadic processes implies equivalence of the polyadic processes themselves, but the converse does not hold, the reason being, briefly, that there are monadic contexts that do not conform to the interaction regime that underlies the translation.

In [QW98] a typing system for monadic processes that captures the interaction regime underlying the translation is introduced and used to obtain a full-abstraction result: two well-typed polyadic processes are barbed congruent if and only if their monadic translations are barbed equivalent in all well-typed monadic contexts. The monadic typing system combines and extends ideas present in other typing systems for p-calculus processes.

The papers [LW98] and [PW98] give a detailed account of a theory of partial confluence of mobile processes, together with applications of the theory to correctness of transformation rules for concurrent object-oriented programs.

The main theorems of [PW98] concern systems which can be thought of as composed from two components: a questioner, and an answerer which may process questions concurrently. One result gives conditions under which a system of this kind is behaviourally indistinguishable from the corresponding system in which the answerer may handle only one question at a time. This result is used in conjunction with a semantic definition for a concurrent object-oriented programming language by translation to a mobile-process calculus to show the interchangeability in an arbitrary program context of two class definitions which prescribe binary-tree symbol-table data structures, one supporting concurrent operations, the other sequential.

In [LW98], this work is extended to show the soundness of transformation rules for concurrent object-oriented programs which increase the scope for concurrent activity within the system prescribed by a program without altering its observable behaviour. The rules allow one object to release another from a rendezvous before it has finished all the associated activity, and allow one object to delegate to a second object the responsibility for returning a result to a third object.

3.14.2  Workshops and site visits

Working-Group funds were used to support the participation in the Brighton workshop in April 1998 of Guy McCusker, Corin Pitcher, and David Walker. They also supported David Walker's participation in the Sophia Antipolis workshop in September 1998 which was combined with a short site visit to INRIA where the main focus of interaction was joint work on p-calculus with Davide Sangiorgi.

3.14.3  Publications

P. Quaglia and D. Walker, On encoding pp in mp, in Proceedings of Foundations of Software Technology and Theoretical Computer Science, December 1998, Springer-Verlag Lecture Notes in Computer Science, to appear.

X. Liu and D. Walker, Partial confluence of processes and systems of objects, Theoretical Computer Science 206 (1998) 127--162.

A. Philippou and D. Walker, On transformations of concurrent object programs, Theoretical Computer Science 195 (1998) 259--289.

Group Technical Directorate

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