Universal Coinductive Characterisations of Process Semantics

We present a theoretical framework which allows to define in a uniform way coinductive characterisations of nearly any semantic preorder or equivalence between processes, by means of simulations up-to and bisimulations up-to. In particular, all the semantics in the linear time-branching time spectrum are covered. Constrained simulations, that generalise plain simulations by including a constraint that all the pairs of related processes must satisfy, are the key to obtain such a general framework. We provide a simple axiomatisation of any constrained simulation preorder and also for the corresponding equivalence. These axiomatizations allow us to prove in a uniform way that each constrained simulation preorder (equivalence) defines a class of process preorders (equivalences) which share commons properties, like the possibility of giving coinductive characterisations for all of them, or the existence of a canonical preorder inducing each of these equivalences

Expressive Logics for Coinductive Predicates

The classical Hennessy-Milner theorem says that two states of an image-finite transition system are bisimilar if and only if they satisfy the same formulas in a certain modal logic. In this paper we study this type of result in a general context, moving from transition systems to coalgebras and from bisimilarity to coinductive predicates. We formulate when a logic fully characterises a coinductive predicate on coalgebras, by providing suitable notions of adequacy and expressivity, and give sufficient conditions on the semantics. The approach is illustrated with logics characterising similarity, divergence and a behavioural metric on automata

Distributive Laws for Monotone Specifications

Turi and Plotkin introduced an elegant approach to structural operational semantics based on universal coalgebra, parametric in the type of syntax and the type of behaviour. Their framework includes abstract GSOS, a categorical generalisation of the classical GSOS rule format, as well as its categorical dual, coGSOS. Both formats are well behaved, in the sense that each specification has a unique model on which behavioural equivalence is a congruence. Unfortunately, the combination of the two formats does not feature these desirable properties. We show that monotone specifications - that disallow negative premises - do induce a canonical distributive law of a monad over a comonad, and therefore a unique, compositional interpretation.Comment: In Proceedings EXPRESS/SOS 2017, arXiv:1709.0004

Coinduction up to in a fibrational setting

Bisimulation up-to enhances the coinductive proof method for bisimilarity, providing efficient proof techniques for checking properties of different kinds of systems. We prove the soundness of such techniques in a fibrational setting, building on the seminal work of Hermida and Jacobs. This allows us to systematically obtain up-to techniques not only for bisimilarity but for a large class of coinductive predicates modelled as coalgebras. By tuning the parameters of our framework, we obtain novel techniques for unary predicates and nominal automata, a variant of the GSOS rule format for similarity, and a new categorical treatment of weak bisimilarity

(Bi)Simulations Up-to Characterise Process Semantics

We define (bi)simulations up-to a preorder and show how we can use them to provide a coinductive, (bi)simulation-like, characterisation of semantic (equivalences) preorders for processes. In particular, we can apply our results to all the semantics in the linear time-branching time spectrum that are defined by preorders coarser than the ready simulation preorder. The relation between bisimulations up-to and simulations up-to allows us to find some new relations between the equivalences that define the semantics and the corresponding preorders. In particular, we have shown that the simulation up-to an equivalence relation is a canonical preorder whose kernel is the given equivalence relation. Since all of these canonical preorders are defined in an homogeneous way, we can prove properties for them in a generic way. As an illustrative example of this technique, we generate an axiomatic characterisation of each of these canonical preorders, that is obtained simply by adding a single axiom to the axiomatization of the original equivalence relation. Thus we provide an alternative axiomatization for any axiomatizable preorder in the linear time-branching time spectrum, whose correctness and completeness can be proved once and for all. Although we first prove, by induction, our results for finite processes, then we see, by using continuity arguments, that they are also valid for infinite (finitary) processes

Constrained simulations, nested simulation semantics and counting bisimulations

Nested simulations define an interesting hierarchy of semantic preorders and equivalences in which every semantics refines the previous one and it is refined by the following. This nested nature provides a fruitful framework for the study of the formal meaning and the properties of concurrent processes. In this paper we present the notion of constrained simulation that, although rather simple, allows us to find general results for a wide family of semantics. In particular, we provide an axiomatization for both the preorder and the equivalence induced by any constrained simulation. Nested simulations are constrained simulations and therefore our results can be instantiated directly to them. Besides, constrained simulations suggest the definition of a new family of semantics, generalised nested simulation semantics, constructed over the base of any order relation, instead of plain simulation. Finally, we conclude the study of the (generalised) nested semantics defining a generalisation of bisimulation relations, counting bisimulation, that allows us to define a characterisation of nested semantics in terms of a bisimulation-like game

Characteristic Bisimulation for Higher-Order Session Processes

Characterising contextual equivalence is a long-standing issue for higher-order (process) languages. In the setting of a higher-order pi-calculus with sessions, we develop characteristic bisimilarity, a typed bisimilarity which fully characterises contextual equivalence. To our knowledge, ours is the first characterisation of its kind. Using simple values inhabiting (session) types, our approach distinguishes from untyped methods for characterising contextual equivalence in higher-order processes: we show that observing as inputs only a precise finite set of higher-order values suffices to reason about higher-order session processes. We demonstrate how characteristic bisimilarity can be used to justify optimisations in session protocols with mobile code communication