Team Sardes

Overall Objectives
Scientific Foundations
Application Domains
New Results
Contracts and Grants with Industry
Other Grants and Activities

Section: New Results

Languages and foundations

Participants : Damien Pous, Alan Schmitt, Jean-Bernard Stefani, Thomas Braibant, Sergueï Lenglet, Michaël Lienhardt, Claudio Mezzina.

Process algebra

The goal of this work is to study process algebraic foundations for component-based programming. Because of the inherently higher-order character of dynamic configuration operations (modelled e.g. by the passivation construct of the Kell calculus [106] ), we are led to study new techniques for proving program equivalence in higher-order calculi, to develop new forms of bisimulation for characterizing barbed congruence, the natural form of program equivalence in process calculi [96] , and to study the expressivity of different constructs in higher-order calculi.

We have refined results obtained last year on the definition of bisimulations for higher-order calculi with passivation. The study of normal bisimulations for such calculi was published at FOSSACS [29] . We have shown that even in a simple calculus with passivation, there is no direct equivalent to the notion of normal bisimulation developed by Sangiorgi for the higher-order $ \pi$ -calculus [104] . The definition of a new semantics for higher-order process calculi, called complementary semantics, resulting in the first characterization of weak contextual congruence for calculi with passivation, was presented at CONCUR [28] . The PhD dissertation of Sergueiï Lenglet, to be defended in January 2010, shows how to apply these techniques to the Kell calculus.

In parallel, we have continued the study of the expressivity of higher-order calculi. In particular, during the six month visit of Jorge A. Perez (a PhD student of Davide Sangiorgi), we have shown that it is impossible to find a compositional encoding of the bi-adic higher-order pi-calculus in the monadic higher-order pi calculus. We have also started to work on such expressiveness results in presence of passivation, and will continue to do so with Cinzia di Giusto who is starting an 18 months postdoc in January 2010.

Together with Daniel Hirschkoff from the Plume team at ENS Lyon, we have studied bisimilarity in a fragment of CCS that contains only prefix, parallel composition, synchronisation and a limited form of replication [48] . The characterisation is not an axiomatisation, but is instead presented as a rewriting system. Our method allows us to derive a new congruence result in the $ \pi$ -calculus: congruence holds in the sub-calculus that does not include restriction nor sum, and features a limited form of replication. This work has been submitted for publication.

Proof assistant

As part of a general work towards proof-assistant-based tools for verifying distributed systems and distributed abstract machines, we have implemented and proved correct a decision procedure for the equational theory of Kleene algebra [22] . This required a formalisation of finite automata theory algorithms, and raised several issues about writing efficient code in Coq. This work also raised an original question about the ability to add and remove types to various non commutative algebraic structures [40] . This work is the subject of the PhD thesis of Thomas Braibant.

Type systems

A new type system for the assemblage of communicating components has been defined, using a slightly simplified semantics for the routing of messages. This small modification allows us to have a decidable type inference, which is a strong improvement on our previous type system [90] . We have implemented the type inference algorithm and tested it with Fractal [62] and Click [85] configurations. This work has been presented at FORTE/FMOODS [30] , and is part of the PhD thesis of Michaël Lienhardt.

In the setting of our collaboration with Nabil Layaïda and Pierre Genevès of the WAM project team, we have extended our previous work on an efficient static analysis tool for XML paths and types [72] to now include counting queries. This work has been submitted for publication.


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