Jorge A. Pérez

On the expressiveness and decidability of higher-order process calculi

In higher-order process calculi the values exchanged in communications may contain processes. A core calculus of higher-order concurrency is studied; it has only the operators necessary to express higher-order communications: input prefix, process output, and parallel composition. By exhibiting a nearly deterministic encoding of Minsky machines, the calculus is shown to be Turing complete and therefore its termination problem is undecidable. Strong bisimilarity, however, is shown to be decidable. Further, the main forms of strong bisimilarity for higher-order processes (higher-order bisimilarity, context bisimilarity, normal bisimilarity, barbed congruence) coincide. They also coincide with their asynchronous versions. A sound and complete axiomatization of bisimilarity is given. Finally, bisimilarity is shown to become undecidable if at least four static (i.e., top-level) restrictions are added to the calculus.

Behavioral polymorphism and parametricity in session-based communication

We investigate a notion of behavioral genericity in the context of session type disciplines. To this end, we develop a logically motivated theory of parametric polymorphism, reminiscent of the Girard-Reynolds polymorphic λ-calculus, but casted in the setting of concurrent processes. In our theory, polymorphism accounts for the exchange of abstract communication protocols and dynamic instantiation of heterogeneous interfaces, as opposed to the exchange of data types and dynamic instantiation of individual message types. Our polymorphic session-typed process language satisfies strong forms of type preservation and global progress, is strongly normalizing, and enjoys a relational parametricity principle. Combined, our results confer strong correctness guarantees for communicating systems. In particular, parametricity is key to derive non-trivial results about internal protocol independence, a concurrent analogous of representation independence, and non-interference properties of modular, distributed systems.

Linear logical relations for session-based concurrency

In prior work we proposed an interpretation of intuitionistic linear logic propositions as session types for concurrent processes. The type system obtained from the interpretation ensures fundamental properties of session-based typed disciplines--most notably, type preservation, session fidelity, and global progress. In this paper, we complement and strengthen these results by developing a theory of logical relations. Our development is based on, and is remarkably similar to, that for functional languages, extended to an (intuitionistic) linear type structure. A main result is that well-typed processes always terminate (strong normalization). We also introduce a notion of observational equivalence for session-typed processes. As applications, we prove that all proof conversions induced by the logic interpretation actually express observational equivalences, and explain how type isomorphisms resulting from linear logic equivalences are realized by coercions between interface types of session-based concurrent systems.

On the expressiveness of polyadic and synchronous communication in higher-order process calculi

Higher-order process calculi are calculi in which processes can be communicated. We study the expressiveness of strictly higher-order process calculi, and focus on two issues well-understood for first-order calculi but not in the higher-order setting: synchronous vs. asynchronous communication and polyadic vs. monadic communication. First, and similarly to the first-order setting, synchronous process-passing is shown to be encodable into asynchronous processpassing. Then, the absence of name-passing is shown to induce a hierarchy of higher-order process calculi based on the arity of polyadic communication, thus revealing a striking point of contrast with respect to first-order calculi. Finally, the passing of abstractions (i.e., functions from processes to processes) is shown to be more expressive than process-passing alone.

Disciplined structured communications with consistent runtime adaptation

Session types offer a powerful type-theoretic foundation for the analysis of structured communications, as commonly found in service-oriented systems. They are defined upon core programming calculi which offer only limited support for expressing adaptation and evolvability requirements. This is unfortunate, as service-oriented systems are increasingly being deployed upon highly dynamic infrastructures in which such requirements are central concerns. In previous work, we developed a process calculi framework of adaptable processes, in which concurrent processes can be replaced, suspended, or discarded at runtime. In this paper, we propose a session types discipline for a calculus with adaptable processes. Our framework offers an alternative for integrating runtime adaptation mechanisms in the analysis of structured communications. We show that well-typed processes enjoy consistency: communicating behavior is never interrupted by evolvability actions.

Towards Global and Local Types for Adaptation

Choreographies allow designers to specify the protocols followed by participants of a distributed interaction. In this context, adaptation may be necessary to respond to external requests or to better suit a changing environment (a self-update). Adapting the behavior of a participant requires to update in a coordinated way possibly all the participants interacting with him. We propose a language able to describe a choreography together with its adaptation strategies, and we discuss the main issues that have to be solved to enable adaptation on a participant code dealing with many interleaved protocols.

An Overview of FORCES: An INRIA Project on Declarative Formalisms for Emergent Systems

The FORCES project aims at providing robust and declarative formalisms for analyzing systems in the emerging areas of Security Protocols, Biological Systems and Multimedia Semantic Interaction . This short paper describes FORCESs motivations, results and future research directions.

On the Expressiveness of Forwarding in Higher-Order Communication

In higher-order process calculi the values exchanged in communications may contain processes. There are only two capabilities for received processes: execution and forwarding. Here we propose a limited form of forwarding: output actions can only communicate the parallel composition of statically known closed processes and processes received through previously executed input actions. We study the expressiveness of a higher-order process calculus featuring this style of communication. Our main result shows that in this calculus termination is decidable while convergence is undecidable.

Non-determinism and Probabilities in Timed Concurrent Constraint Programming

A timed concurrent constraint process calculus with probabilistic and non-deterministic choices is proposed. We outline the rationale of an operational semantics for the calculus. The semantics ensures consistent interactions between both kinds of choices and is indispensable for the definition of logic-based verification capabilities over system specifications.

Multiparty Session Types Within a Canonical Binary Theory, and Beyond

A widespread approach to software service analysis uses session types. Very different type theories for binary and multiparty protocols have been developed; establishing precise connections between them remains an open problem. We present the first formal relation between two existing theories of binary and multiparty session types: a binary system rooted in linear logic, and a multiparty system based on automata theory. Our results enable the analysis of multiparty protocols using a much simpler type theory for binary protocols, ensuring protocol fidelity and deadlock-freedom. As an application, we offer the first theory of multiparty session types with behavioral genericity. This theory is natural and powerful; its analysis techniques reuse results for binary session types.