2006 was a special year for both mathematical logic and computer science, as it celebrated Gödel's centenary. Although Gödel's work was mainly concerned with mathematics and metamathematics, the crucial role it had in the foundation of modern theoretical computer science is undeniable: for instance, one only has to remember Gödel's contributions to the birth of recursion theory as well as his part in the debate in the nineteen thirties on the subject of the Church Thesis.

This paper analyses the interpretation of mathematical entities in the formalisations of languages. Four case studies are considered, covering both denotational and axiomatic approaches. We argue that the usual interpretation consists of direct statements about the language concepts and, occasionally, about the real world; some problems of this approach are discussed. Applying results from philosophical studies into the structure of scientific theories, we propose an alternative interpretation of mathematical entities as statements defining constructed concepts, which can be employed in turn as theoretical models of the language concepts and the world. Though this approach requires us to write hypotheses claiming some similarity between the models and the represented subjects, we reason that it is more adequate for formalising certain languages.

This special issue of Mathematical Structures in Computer Science is devoted to recent work in subtyping. When subtyping was first proposed, it presented a new vehicle for understanding programming languages, together with challenging theoretical issues. The papers in this special issue include a new approach to the decidability of subtyping, a metatheoretic investigation of transitivity of coercive subtyping for parametrised dependent types, and applications of subtyping to the classic programming language concerns of flow analysis and typing for distributed systems. We believe that the scope of the papers demonstrates convincingly that the theory and practice of subtyping continue to be extended in novel and interesting ways.

The integration of first-order and higher-order paradigms has been one of the main challenges in the design of both declarative programming languages and proof environments. It has led to the development of new computation models and new logical frameworks, which have been obtained by enriching first-order rewriting with higher-order capabilities or by adding algebraic features to the λ-calculus.

Isomorphisms of types are computational witnesses of logical equivalence with additional properties. The types/formulas A and B are isomorphic if there are functions (in a certain formalism) f : A → B and g : B → A such that g ○ f and f ○ g are equal in a certain sense to the identity on A and B, respectively. Typical such formalisms are extensions of simply typed λ-calculus, with βη-convertibility as equality relation. Another view of a pair of functions f : A → B and g : B → A (besides establishing the logical equivalence of A and B) is that f is invertible with left-inverse g, and it is then natural to relax the above symmetric condition to just g ○ f being equal to the identity on A. In this situation, A is called a retract of B, which is thus a natural generalisation of the notion of an isomorphism, while both these notions are refinements of the concept of logical equivalence in operational terms, that is, in terms of computable functions.

We present a new kind of ambient calculus in which the open capability is replaced by direct mobility of generic processes. The calculus comes equipped with a labelled transition system in which types play a major role: this system allows us to show interesting algebraic laws. As usual, types express the communication, access and mobility properties of the modelled system, and inferred types express the minimal constraints required for the system to be well behaved.

This paper studies the equational theory of bisimulation equivalence over the process algebra BCCSP extended with the priority operator of Baeten, Bergstra and Klop. We prove that, in the presence of an infinite set of actions, bisimulation equivalence has no finite, sound, ground-complete equational axiomatisation over that language. This negative result applies even if the syntax is extended with an arbitrary collection of auxiliary operators, and motivates the study of axiomatisations using equations with action predicates as conditions. In the presence of an infinite set of actions, it is shown that, in general, bisimulation equivalence has no finite, sound, ground-complete axiomatisation consisting of equations with action predicates as conditions over the language studied in this paper. Finally, sufficient conditions on the priority structure over actions are identified that lead to a finite, ground-complete axiomatisation of bisimulation equivalence using equations with action predicates as conditions.

Curry-style system F, that is, system F with no explicit types in terms, may be viewed as a core presentation of polymorphism from the point of view of programming languages.

This paper gives a characterisation of type isomorphisms for this language using a game model, whose intuitions come from both the syntax and the game semantics universe. The model is composed of an untyped part to interpret terms, a notion of arena to interpret types and a typed part to express the fact that an untyped strategy σ plays on an arena A.

By analysing isomorphisms in the model, we prove that the equational system corresponding to type isomorphisms for Curry-style system F is the extension of the equational system for Church-style isomorphisms with a new, non-trivial equation: ∀X.AA[∀Y.Y/X] if X appears only positively in A.

This paper presents a short survey of some recent approaches relating two different areas, viz. deterministic chaos and computability. Chaos in classical physics may be approached by dynamical (equationally determined) systems or stochastic ones (as random processes). However, randomness has also been effectively modelled using recursion theoretic tools by P. Martin-Löf. We recall its connections to Kolmogorov complexity and show some applications to dynamical systems. This allows us to introduce results that connect well-established notions of entropy and algorithmic information.

Pure Pattern Type Systems (P2TS) combine the frameworks and capabilities of rewriting and λ-calculus within a unified setting. Their type systems, which are adapted from Barendregt's λ-cube, are especially interesting from a logical point of view. Until now, strong normalisation, which is an essential property for logical soundness, has only been conjectured: in this paper, we give a positive answer for the simply-typed system and the dependently-typed system.

The proof is based on a translation of terms and types from P2TS into the λ-calculus. First, we deal with untyped terms, ensuring that reductions are faithfully mimicked in the λ-calculus. For this, we rely on an original encoding of the pattern matching capability of P2TS into the System Fω.

Then we show how to translate types: the expressive power of System Fω is needed in order to fully reproduce the original typing judgments of P2TS. We prove that the encoding is correct with respect to reductions and typing, and we conclude with the strong normalisation of simply-typed P2TS terms. The strong normalisation with dependent types is in turn obtained by an intermediate translation into simply-typed terms.

We introduce a notion of reducibility of representations of topological spaces and study some basic properties of this notion for domain representations.

A representation reduces to another if its representing map factors through the other representation. Reductions form a pre-order on representations. A spectrum is a class of representations divided by the equivalence relation induced by reductions. We establish some basic properties of spectra, such as, non-triviality. Equivalent representations represent the same set of functions on the represented space.

Within a class of representations, a representation is universal if all representations in the class reduce to it. We show that notions of admissibility, considered both for domains and within Weihrauch's TTE, are universality concepts in the appropriate spectra. Viewing TTE representations as domain representations, the reduction notion here is a natural generalisation of the one from TTE.

To illustrate the framework, we consider some domain representations of real numbers and show that the usual interval domain representation, which is universal among dense representations, does not reduce to various Cantor domain representations. On the other hand, however, we show that a substructure of the interval domain more suitable for efficient computation of operations is equivalent to the usual interval domain with respect to reducibility.

We present an algorithm for deciding polarised higher-order subtyping without bounded quantification. Constructors are identified not only modulo β, but also η. We give a direct proof of completeness, without constructing a model or establishing a strong normalisation theorem. Inductive and coinductive types are enriched with a notion of size and the subtyping calculus is extended to account for the inclusions arising between the sized types.

A common way to show the termination of the union of two abstract reduction systems, provided both systems terminate, is to prove that they enjoy a specific property (some sort of ‘commutation’ for instance). This specific property is actually used to show that, for the union not to terminate, one of the systems must itself be non-terminating, which leads to a contradiction. Unfortunately, the property may be impossible to prove because some of the objects that are reduced do not enjoy an adequate form.

Hence the purpose of this paper is threefold:

• – First, it introduces an operator enabling us to insert a reduction step on such an object, and therefore to change its shape, while still preserving the ability to use the property. Of course, some new properties will need to be verified.

• – Second, as an instance of our technique, the operator is applied to relax a well-known lemma stating the termination of the union of two termination abstract reduction systems.

• – Finally, this lemma is applied in a peculiar and then in a more general way to show the termination of some lambda calculi with inductive types augmented with specific reductions dealing with:

  1. (i) copies of inductive types;

  2. (ii) the representation of symmetric groups.

We propose an imperative version of the Rewriting Calculus, a calculus based on pattern matching, pattern abstraction and side effects, which we call iRho.

We formulate both a static and big-step call-by-value operational semantics of iRho. The operational semantics is deterministic, and immediately suggests how an interpreter for the calculus may be built. The static semantics is given using a first-order type system based on a form of product types, which can be assigned to term-like structures (that is, records).

The calculus is à la Church, that is, pattern abstractions are decorated with the types of the free variables of the pattern.

iRho is a good candidate for the core of a pattern-matching imperative language, where a (monomorphic) typed store can be safely manipulated and where fixed points are built into the language itself.

Properties such as determinism of the interpreter and subject-reduction have been completely checked using a machine-assisted approach with the Coq proof assistant. Progress and decidability of type checking are proved using pen and paper.

The three classical process algebras CCS, CSP and ACP present several differences in their respective technical machinery. This is due, not only to the difference in their operators, but also to the terminology and ‘way of thinking’ of the community that has been (and still is) working with them. In this paper we will first discuss these differences and try to clarify the different usage of terminology and concepts. Then, as a result of this discussion, we define a generic process algebra where each of the basic mechanisms of the three process algebras (including minimal fixpoint based unguarded recursion) is expressed by an operator, and which can be used as an underlying common language. We show an example of the advantages of adopting such a language instead of one of the three more specialised algebras: producing a complete axiomatisation for Milner's observational congruence in the presence of (unguarded) recursion and static operators. More precisely, we provide a syntactical characterisation (allowing as many terms as possible) for the equations involved in recursion operators, which guarantees that transition systems generated by the operational semantics are finite state. Conversely, we show that every process admits a specification in terms of such a restricted form of recursion. We then present an axiomatisation that is ground complete over such a restricted signature. Notably, we also show that the two standard axioms of Milner for weakly unguarded recursion can be expressed using a single axiom only.

Behavioural equivalences are a means of establishing whether computing systems possess the same properties. The specific set of properties that are preserved by a specific behavioural equivalence clearly depends on how the system behaviour is observed and can usually be characterised by means of a modal logic. In this paper we consider three different approaches to the definition of behavioural equivalences – bisimulation, testing and trace – applied to three different classes of systems – non-deterministic, probabilistic and Markovian – and we survey the nine resulting modal logic characterisations, each of which stems from the Hennessy–Milner logic. We then compare the nine characterisations with respect to the logical operators, in order to emphasise the differences between the three approaches in the definition of behavioural equivalences and the regularities within each of them. In the probabilistic and Markovian cases we also address the issue of whether the probabilistic and temporal aspects should be treated in a local or global way and consequently whether the modal logic interpretation should be qualitative or quantitative.

In this paper we characterise the equivalential reducts of classical and intuitionistic logics over a language with two propositional variables. We then investigate the size of the fraction of the tautologies of these logics against all formulas. Some methods from complex analysis are used to achieve this goal.

We present a novel approach to computing the context-sensitive flow of values through procedures and data structures. Our approach combines and extends techniques from two seemingly disparate areas: polymorphic subtyping and interprocedural dataflow analysis based on context-free language reachability. The resulting technique offers several advantages over previous approaches: it works directly on higher-order programs; provides demand-driven interprocedural queries; and improves the asymptotic complexity of a known algorithm based on polymorphic subtyping from O(n8) to O(n3) for computing all queries. For intra-procedural flow restricted to equivalence classes, our algorithm yields linear inter-procedural flow queries.

We incorporate the idea of coercive subtyping, a theory of abbreviation for dependent type theories, into the polymorphic type system in functional programming languages. The traditional type system with let-polymorphism is extended with argument coercions and function coercions, and a corresponding type inference algorithm is presented and proved to be sound and complete.

In recent decades, qualitative approaches to probabilistic uncertainty have received more and more attention. We propose a characterisation of partial preference orders through a uniform axiomatic treatment of a variety of qualitative uncertainty notions. To this end, we prove a representation result that connects qualitative notions of partial uncertainty to their numerical counterparts. We describe an executable specification, in the declarative framework of Answer Set Programming, that constitutes the core engine for qualitative management of uncertainty. Some basic reasoning tasks are also identified.