Abstract In Hyland et al. (1980), Hyland, Johnstone and Pitts introduced the notion of tripos for the purpose of organizing the construction of realizability toposes in a way that generalizes the construction of localic toposes from complete Heyting algebras. In Pitts (2002), one finds a generalization of this notion eliminating an unnecessary assumption of Hyland et al. (1980). The aim of this paper is to characterize triposes over a base topos ${\cal S}$ in terms of so-called constant objects functors from ${\cal S}$ to some elementary topos. Our characterization is slightly different from the one in Pitts’s PhD Thesis (Pitts, 1981) and motivated by the fibered view of geometric morphisms as described in Streicher (2020). In particular, we discuss the question whether triposes over Set giving rise to equivalent toposes are already equivalent as triposes.

Abstract I recall how Martin Hofmann and I found the groupoid model of type theory in the early 1990s.

The basic notions of quantum mechanics are formulated in terms of separable infinite dimensional Hilbert space $\mathcal{H}$. In terms of the Hilbert lattice $\mathcal{L}$ of closed linear subspaces of $\mathcal{H}$ the notions of state and observable can be formulated as kinds of measures as in [21]. The aim of this paper is to show that there is a good notion of computability for these data structures in the sense of Weihrauch's Type Two Effectivity (TTE) [26]. Instead of explicitly exhibiting admissible representations for the data types under consideration we show that they do live within the category $\mathbf{QCB}_0$ which is equivalent to the category $\mathbf{AdmRep}$ of admissible representations and continuously realizable maps between them. For this purpose in case of observables we have to replace measures by valuations which allows us to prove an effective version of von Neumann's Spectral Theorem.

We study a classical realizability model (in the sense of J.-L. Krivine) arising from a model of untyped lambda calculus in coherence spaces. We show that this model validates countable choice using bar recursion and bar induction.

In a sequence of papers (Krivine 2001; Krivine 2003; Krivine 2009), J.-L. Krivine introduced his notion of classical realisability for classical second-order logic and Zermelo–Fraenkel set theory. Moreover, in more recent work (Krivine 2008), he has considered forcing constructions on top of it with the ultimate aim of providing a realisability interpretation for the axiom of choice. The aim of the current paper is to show how Krivine's classical realisability can be understood as an instance of the categorical approach to realisability as started by Martin Hyland in Hyland (1982) and described in detail in van Oosten (2008). Moreover, we will give an intuitive explanation of the iteration of realisability as described in Krivine (2008).

This special issue of Mathematical Structures in Computer Science contains six papers from the Workshop on Domains IX held at the University of Sussex (Brighton), on 22–24 September 2008. This was the ninth event in the long tradition of Domains workshops, which started in Darmstadt in 1994. Since then, workshops have been organised in Braunschweig (1996), Munich (1997), Siegen (1998), Darmstadt again (1999, 2004), Birmingham (2002) and Novosibirsk (2007).

This is the second part of a special issue in honour of Klaus Keimel – the first part appeared in Mathematical Structures in Computer Science16 (2). This second part consists of a single paper by John Longley, which could not be included in the earlier issue for reasons of size.

In late August 2004, some 60 mathematicians and computer scientists gathered in Darmstadt for the seventh Workshop Domains, to mark the 65th birthday of Professor Klaus Keimel and his retirement from his position at the Technical University Darmstadt. The papers in this volume were selected from submissions that were received in response to a call issued to participants during the meeting, and to a wider community afterwards.

In this note we show that quotients of countably based spaces (qcb spaces) and topological predomains, as introduced by M. Schröder and A. Simpson, are not closed under sobrification. As a consequence, replete topological predomains need not be sober, that is, in general, repletion is not given by sobrification. Our counterexample also shows that a certain tentative ‘equaliser construction’ of repletion fails for qcb spaces.Our results also extend to the more general class of core compactly generated spaces.

We show that real-number computations in the interval-domain environment are ‘inherently parallel’ in a precise mathematical sense. We do this by reducing computations of the weak parallel-or operation on the Sierpinski domain to computations of the addition operation on the interval domain.

The main contribution of this paper is a formal characterization of recursive object specifications based on a denotational untyped semantics of the object calculus and the discussion of existence of those (recursive) specifications. The semantics is then applied to prove soundness of a programming logic for the object calculus and to suggest possible extensions. For the purposes of this discussion we use an informal logic of predomains in order to avoid any commitment to a particular syntax of specification logic.

In standard realizability one works with respect to an untyped universe of realizers called a partial combinatory algebra (pca). It is well known that a pca [Ascr ] gives rise to a categorical model of impredicative type theory via the category Asm([Ascr ]) of assemblies over [Ascr ] or the realizability topos over [Ascr ]. Recently, John Longley introduced a typed version of pca's (Longley 1999b). The above mentioned construction of categorical models extends to the typed case. However, in general these are no longer impredicative. We show that for a typed pca [Tscr ] the ensuing models are impredicative if and only if [Tscr ] has a universal type U. Such a type U can be endowed with the structure of an untyped pca such that U and [Tscr ] induce equivalent realizability models: in other words, a typed pca [Tscr ] with a universal type is essentially untyped. Thus, a posteriori it turns out that nothing is lost by restricting to (untyped) pca's as far as realizability models of impredicative type theories are concerned. For instance, we show that for a typed pca [Tscr ] the fibred category of discrete families in Asm([Tscr ]) is small if and only if [Tscr ] has a universal type. As the category of ¬¬-separated objects of the modified realizability topos is equivalent to Asm([Tscr ]) for an appropriate typed pca [Tscr ] without a universal type, it follows that the discrete families in the subcategory of ¬¬-separated objects of the modified realizability topos do not provide a model of polymorphic λ-calculus.

We construct fully abstract realisability models of PCF. In particular, we prove a variant of the Longley-Phoa Conjecture by showing that the realisability model over an untyped /spl lambda/-calculus with arithmetic is fully abstract for PCF. Further we consider the extension of our results to a general sequential functional programming language SFPL giving rise to universal realisability models for SFPL.

One of the goals of this paper is to demonstrate that denotational semantics is useful for operational issues like implementation of functional languages by abstract machines. This is exemplified in a tutorial way by studying the case of extensional untyped call-by-name λ-calculus with Felleisen's control operator [Cscr ]. We derive the transition rules for an abstract machine from a continuation semantics which appears as a generalization of the ¬¬-translation known from logic. The resulting abstract machine appears as an extension of Krivine's machine implementing head reduction. Though the result, namely Krivine's machine, is well known our method of deriving it from continuation semantics is new and applicable to other languages (as e.g. call-by-value variants). Further new results are that Scott's D ∞ -models are all instances of continuation models. Moreover, we extend our continuation semantics to Parigot's λμ-calculus from which we derive an extension of Krivine's machine for λμ-calculus. The relation between continuation semantics and the abstract machines is made precise by proving computational adequacy results employing an elegant method introduced by Pitts.

The partial real line is the continuous domain of compact real intervals ordered by reverse inclusion. The idea is that singleton intervals represent total real numbers, and that the remaining intervals represent partial real numbers. The partial real line has been used to model exact real number computation in the framework of the programming language Real PCF. We introduce induction principles and recursion schemes for the partial unit interval, which allows us to verify that Real PCF programs meet their specification. The theory is based on a domain-equation-like presentation of the partial unit interval, which we refer to as a biquotient of a bifree algebra.

We show that a certain simple call-by-name continuation semantics of Parigot's /spl lambda//sub /spl mu//-calculus (1992) is complete. More precisely, for every /spl lambda//spl mu/-theory we construct a cartesian closed category such that the ensuing continuation-style interpretation of /spl lambda//sub /spl mu//, which maps terms to functions sending abstract continuations to responses, is full and faithful. Thus, any /spl lambda//sub /spl mu//-category in the sense of is isomorphic to a continuation model derived from a cartesian-closed category of continuations.

We give a semantical proof that every term of a combinator version of system F has a normal form. As the argument is entirely formalisable in an impredicative constructive type theory a reduction-free normalisation algorithm can be extracted from this. The proof is presented as the construction of a model of the calculus inside a category of presheaves. Its definition is given entirely in terms of the internal language.

We give a model of intensional Martin-Lof type theory based on groupoids and fibrations of groupoids in which identity types may contain two distinct elements which are not even prepositionally equal. This shows that the principle of uniqueness of identity proofs is not derivable in the syntax.>

In a PCF-like call-by-name typed λ-calculus with a minimal fixpoint operator, ‘parallelor’, Plotkin's ‘continuous existential quantifier’ ∃ and recursive types together with constructors and destructors, all computable objects can be denoted by terms of the programming language. According to A. Meyer's terminology (cf. Meyer (1988)), such a programming language is called universal in the sense that any extension of it must be conservative, as all computable objects can already be expressed by program terms. As a byproduct, we get that, in principle, recursive types could be totally avoided, as they appear as syntactically expressible retracts of the non-recursive type → , where and are the flat domains of natural numbers and boolean values, respectively.

Based on a categorical semantics for impredicative calculi of dependent types we prove several dependence and independence results. Especially we prove that there exists a model where all usual syntactical concepts can be interpreted with only one exception: in the model the strong sum of a family of propositions indexed over a proposition need not be isomorphic to a proposition again. The method of proof consists of restricting the set of propositions in the well known PERw model due to E. Moggi. The subsets of PERw considered in this paper are inspired by the subset ExpO of PERw introduced by Freyd et al. Finally we show that a weak and a strong notion of sub-locally-cartesian-closed-category coincide under rather mild completeness conditions.

An attempt is made to relate various notions of duality used in mathematics with the denotational semantics of linear logic. The author proposes a naive semantics for linear logic that, in a certain sense, generalizes various notions such as finite-dimensional vector spaces, topological spaces, and J.-Y. Girard's (1987) coherence spaces. A game consists of a set of vectors (or strategies), a set of forms (or co-strategies) and an evaluation bracket. This is enough to interpret the connectives of full propositional linear logic, including exponentials.>