Analyse et conception de systèmes

Date et lieu : le jeudi à 14h, salle 1007, Sophie Germain

Responsable : Jean Krivine

Mardi 02 mai 2017 · 11h00 · Salle 3052

Joachim Breitner (University of Pennsylvania) · Who needs theorem provers when we have compilers?

After decades of work on functional programming and on interactive theorem proving, a Haskell programmer who wants to include simple equational proofs in his programs, e.g. that some Monad laws hold, is still most likely to simply do the derivation as comments in the file, as all the advanced powerful proving tools are inconvenient.

But one powerful tool capable of doing (some of) these proofs is hidden in plain sight: GHC, the Haskell compiler! Its optimization machinery, in particular the simplifier, can prove many simple equations all by himself, simply by compiling both sides and noting that the result is the same. Furthermore, and crucially to make this approach applicable to more complicated equations, the compiler can be instructed to do almost arbitrary symbolic term rewritings by using Rewrite Rules.

In this rather hands-on talk I will show a small GHC plugin that I can use to prove the monad laws for a non-trivial functor. I am looking forward to a discussion of the merits, limits and capabilities of this approach.


Jeudi 18 mai 2017 · 14h30 · Salle 3052

Giovanni Bernardi (IRIF) · Full-abstraction for Must Testing Preorders

The client Must preorder relates tests (clients) instead of processes (servers). The existing characterisation of this preorder is unsatisfactory for it relies on the notion of usable clients which, in turn, are defined using an existential quantification over the servers that ensure client satisfaction. In this talk we characterise the set of usable clients for finite-branching LTSs, and give a sound and complete decision procedure for it. We also provide a novel coinductive characterisation of the client preorder, which we use to argue that the preorder is decidable, thus proving a claim made by Bernardi and Hennessy in 2013.

Jeudi 08 juin 2017 · 14h00 · Salle 3052

Sergio Rajsbaum (Universidad Nacional Autonoma de Mexico) · A simplicial complex model of dynamic epistemic logic for fault-tolerant distributed computing

The usual epistemic S5 model for multi-agent systems is a Kripke graph, whose edges are labeled with the agents that do not distinguish between two states. We propose to uncover the higher dimensional information implicit in the Kripke graph, by using as a model its dual, a chromatic simplicial complex. For each state of the Kripke model there is a facet in the complex, with one vertex per agent. If an edge (u,v) is labeled with a set of agents S, the facets corresponding to u and v intersect in a simplex consisting of one vertex for each agent of S. Then we use dynamic epistemic logic to study how the simplicial complex epistemic model changes after the agents communicate with each other. We show that there are topological invariants preserved from the initial epistemic complex to the epistemic complex after an action model is applied, that depend on how reliable the communication is. In turn these topological properties determine the knowledge that the agents may gain after the communication happens.

We choose distributed computing as a case study to work out in detail the dynamic epistemic simplicial complex theory. The reason is that distributed computability has been studied using combinatorial topology, where the set of all possible executions in a distributed system is represented by a simplicial complex. We establish a formal, categorical equivalence between Kripke models and simplicial complex epistemic models.

In one direction, the connection provides a dynamic epistemic logic semantics to distributed computability, opening the possibility of reasoning about knowledge change in distributed computing. In the other direction, the connection allows to bring in the topological invariants known in distributed computing, to dynamic epistemic logic, and in particular show that knowledge gained after an epistemic action model is intimately related to higher dimensional topological properties.


Jeudi 01 décembre 2016 · 10h30 · Salle 1007

Julien Lange (Imperial College) · Building Graphical Choreographies From Communicating Machines: Principles and Applications

Graphical choreographies, or global graphs, are general multiparty session specifications featuring expressive constructs such as forking, merging, and joining for representing application-level protocols. Global graphs can be directly translated into modelling notations such as BPMN and UML. In the first part of the talk, I will first present an algorithm whereby a global graph can be constructed from asynchronous interactions represented by communicating finite-state machines (CFSMs); and a sound characterisation of a subset of safe CFSMs from which global graphs can be constructed. In the second part, I will outline a few recent applications of this work to communicating timed automata and the Go programming language.

Jeudi 23 juin 2016 · 14h30 · Salle 1007

Elisabeth Remy (Institut de Mathématiques de Luminy) · Analyse qualitative des réseaux de régulation génétiques

Les modèles d’interactions Booléens ont été introduits dans le contexte des réseaux de gènes par S. Kauffman dans la fin des années 60. On considère ici une variante, les modèles logiques, qui reposent sur un formalisme qualitatif avec mise à jour asynchrone des systèmes dynamiques discrets associés (R. Thomas, 1973). L’expression d'un gène y est représentée par une variable discrète et l’évolution du réseau est contrôlée par un système d'équations logiques. A partir de ce système peut être extrait un graphe de régulation : il s’agit d’un graphe orienté signé, dont les noeuds représentent les gènes, et les arcs les régulations entre gènes. On y distingue deux types de régulations : les activations (interactions positives) et les inhibitions (interactions négatives). Ces modèles génèrent des dynamiques de taille exponentielle, et nous faisons face à des problèmes d’explosion combinatoire. Ainsi, nous cherchons à caractériser des propriétés de la dynamique (attracteurs, atteignabilité,…) sans avoir à générer le graphe de transition d’états, qui représente l’ensemble des trajectoires possibles. Une méthode consiste à réduire le graphe, et étudier la dynamique réduite. L’opération de réduction crée des modifications potentiellement importantes dans la dynamique, il est donc nécessaire de bien caractériser ses propriétés de (non-)conservation. Nous verrons aussi comment identifier les attracteurs et quantifier leurs bassins d’attraction à l’aide de méthodes de Monte-Carlo adaptées. Enfin, nous mettrons en valeur ces méthodes d'analyse à travers l’étude d’un modèle concernant la tumorigénèse du cancer de vessie.

Jeudi 26 mai 2016 · 14h30 · Salle 1008

Ralf Treinen (IRIF) · Towards the verification of file tree transformations - the Colis project

This talk describes a recently started ANR project named Colis (http://colis.irif.univ-paris-diderot.fr/), which has the goal of developing techniques and tools for the formal verification of shell scripts. More specifically, our goal is to verify the transformation of a file system tree described by so-called debian maintainer scripts. These scripts, often written in the Posix shell language, are part of the software packages in the Debian GNU/Linux distribution.

A possible example of a program specification is absence of execution error under certain initial conditions. Automatic program verification even for this kind of specification is a challenging task. In case of Debian maintainer scripts we are faced with even more challenging properties like idempotency of scripts (required by policy), or commutation of scripts.

The project is still in the beginning, so there are no results yet to present. However, I will explain why I think that the case of Debian maintainer scripts is very interesting for program verification : some aspects of scripts (POSIX shell, manipulation of a complex data structure) make the problem very difficult, while other aspects of the Debian case are likely to make the problem easier than the task of verifying any arbitary shell script.


Jeudi 07 avril 2016 · 14h30 · Salle UFR

Tobias Heindel (University of Copehagen) · Computing means and moments of occurrence counts: rule-based modeling meets adaptive uniformization and finite state projection

The talk presents recent results on how to solve the general problem of computing the time evolution of means and moments of ``occurence counts in rule-based models; computability is in the sense of Weihrauch. Roughly, established techniques, namely adaptive uniformization and finite state projection, can be reused – provided that occurrence counts (and their powers) are bounded by a polynomial in the ``size of the rule-based system and the ``size'' of the system does not explode. The most interesting results can be obtained for context-free systems, which can be thought of as branching processes equipped with structure; for these systems we have PTIME computability.

All results will be exemplified for the case of string rewriting, aiming for a widest possible audience of computer scientists, assuming only basic knowledge of continous time Markov chains with countable state space. Computing the time evolution of mean word counts in stochastic string rewriting is already highly non-trivial, even if it is the most basic example. The GEM OF THE TALK is the computation of mean word counts for context-free grammars!