Reversibility, allowing one to execute a program not only forwards as usual, but also backwards, has emerged as a fundamental concept in computing, with applications ranging from debugging and fault tolerance to biological and quantum systems. CCSK, a reversible extension of CCS, is a paradigmatic model of reversible concurrent computation. In this paper, we investigate the encodability of CCSK into classical forward-only concurrent models. We establish a separation theorem showing that there is no basic, success-sensitive encoding of CCSK into CCS or the π-calculus, highlighting the strong impact of reversibility on expressive power. We then present an encoding of CCSK processes with only top-level parallel composition into the internal π-calculus, correct up to strong bisimilarity. We also identify a fundamental limitation: no parallel-preserving encoding of CCSK (with arbitrary parallel composition) into the π-calculus can be correct up to strong bisimilarity. Finally, we provide a parallel-preserving encoding correct under a weaker behavioural correspondence: weak mutual simulation. Our findings extend the literature of encodability results to reversible process calculi.

Reversible debuggers help programmers to find the causes of misbehaviours in concurrent programs more quickly, by executing a program backwards from the point where a misbehaviour was observed, and looking for the bug(s) that caused it. Reversible debuggers can be founded on the well-studied theory of causal-consistent reversibility, which only allows one to undo an action provided that its consequences, if any, are undone beforehand. Causal-consistent reversibility yields more efficient debugging by reducing the number of states to be explored when looking backwards. Till now, causal-consistent reversibility has never considered time, which is a key aspect in real-world applications. Here, we study the interplay between reversibility and time in concurrent systems via a process algebra. The Temporal Process Language (TPL) by Hennessy and Regan is a well-understood extension of CCS with discrete-time and a timeout operator. We define revTPL, a reversible extension of TPL, and we show that it satisfies the properties expected from a causal-consistent reversible calculus. We show that, alternatively, revTPL can be interpreted as an extension of reversible CCS with time.

Multiparty Session Types (MPST) is a typing discipline for communication protocols. It ensures the absence of communication errors and deadlocks for well-typed communicating processes. The state-of-the-art implementations of the MPST theory rely on (1) runtime linearity checks to ensure correct usage of communication channels and (2) external domain-specific languages for specifying and verifying multiparty protocols. To overcome these limitations, we propose a library for programming with global combinators - a set of functions for writing and verifying multiparty protocols in OCaml. Local behaviours for all processes in a protocol are inferred at once from a global combinator. We formalise global combinators and prove a sound realisability of global combinators - a well-typed global combinator derives a set of local types, by which typed endpoint programs can ensure type and communication safety. Our approach enables fully-static verification and implementation of the whole protocol, from the protocol specification to the process implementations, to happen in the same language. We compare our implementation to untyped and continuation-passing style implementations, and demonstrate its expressiveness by implementing a plethora of protocols. We show our library can interoperate with existing libraries and services, implementing DNS (Domain Name Service) protocol and the OAuth (Open Authentication) protocol.

We propose session-ocaml, a novel library for session-typed concurrent/distributed programming in OCaml. Our technique solely relies on parametric polymorphism, which can encode core session type structures with strong static guarantees. Our key ideas are: (1) polarised session types, which give an alternative formulation of duality enabling OCaml to automatically infer an appropriate session type in a session with a reasonable notational overhead; and (2) a parameterised monad with a data structure called ‘slots’ manipulated with lenses, which can statically enforce session linearity including delegations. We introduce a notational extension to enhance the session linearity for integrating the session types into the functional programming style. We show applications of session-ocaml to a travel agency use case and an SMTP protocol implementation. Furthermore, we evaluate the performance of on a number of benchmarks.

Interrupts are important aspects of real-time embedded systems to handle events in time. When there exist nested interrupts in a real-time system, and an urgent interrupt is allowed to preempt the current interrupt handling, the design and analysis of the system become difficult due to the lack of appropriate behavioral models. This paper proposes a compositional model for nested interrupts and an analysis named environmental simulation. We present a new kind of timed transition system, named controller automata, to treat interrupts. Together with an interrupt environment modeled as a timed automaton, and a scheduler as a timed automaton with semaphores, the system behaviors with nested interrupts are realized by a sequence of transitions with time. Although various verification problems for this model are undecidable in general, it is shown that the reachability of error states is practically solvable with our implementation of the environmental simulation by Maude.

We propose a compositional analysis method for synchronization flow in AIBO OPEN-R programs based on communicating processes. Concurrent objects of AIBO programs with the OPEN-RAPI are synchronized by two types of signals: ready and notify. Focusing on these signals, we describe abstract behavior of AIBO programs in the pi-calculus preserving the source code structure. We model-check deadlock freeness and interactions order of AIBO programs based on this abstract behavior. Since a primary issue in model-checking is the state space explosion in the behavioral model, we present a decomposing method to reduce the combination of states of concurrent objects. Since our translation to the pi-calculus preserves the syntactical structure of source code, when a counter-example is pointed out, our method enables not only to detect the violation of property of the whole system, but also to point out which component may cause the violation. We developed a prototype translator from AIBO OPEN-R programs to abstract description in the pi-calculus. We show that an application of our decomposing method enables to practically model- check properties by existing tools in two examples of AIBO programs.