The first system of intersection types, Coppo and Dezani [3], extended simple types to include intersections and added intersection introduction and elimination rules (( $\wedge$ I) and ( $\wedge$ E)) to the type assignment system. The major advantage of these new types was that they were invariant under β-equality, later work by Barendregt, Coppo and Dezani [1], extended this to include an (η) rule which gave types invariant under βη-reduction. Urzyczyn proved in [6] that for both these systems it is (...) undecidable whether a given intersection type is empty. Kurata and Takahashi however have shown in [5] that this emptiness problem is decidable for the sytem including (η), but without ( $\wedge$ I). The aim of this paper is to classify intersection type systems lacking some of ( $\wedge$ I), ( $\wedge$ E) and (η), into equivalence classes according to their strength in typing λ-terms and also according to their strength in possessing inhabitants. This classification is used in a later paper to extend the above (un)decidability results to two of the five inhabitation-equivalence classes. This later paper also shows that the systems in two more of these classes have decidable inhabitation problems and develops algorithms to find such inhabitants. (shrink)

We present a version of system Fω, called image, in which the layer of type constructors is essentially the traditional one of Fω, whereas provability of types is classical. The proof-term calculus accounting for the classical reasoning is a variant of Barbanera and Berardi’s symmetric λ-calculus.We prove that the whole calculus is strongly normalising. For the layer of type constructors, we use Tait and Girard’s reducibility method combined with orthogonality techniques. For the layer of terms, we use Barbanera and Berardi’s (...) method based on a symmetric notion of reducibility candidate. We prove that orthogonality does not capture the fixpoint construction of symmetric candidates.We establish the consistency of image, and relate the calculus to the traditional system Fω, also when the latter is extended with axioms for classical logic. (shrink)

In [gallier], general results (due to Coppo, Dezani and Veneri) relating properties of pure lambda terms and their typability in some systems with conjunctive types are proved in a uniform way by using the reducibility method.This paper gives a very short proof of the same results (actually, one of them is a bit stronger) using purely arithmetical methods.

We introduce a new type system called “System ST” , based on subtyping, and prove the basic property of the system. We show the extraordinary expressive power of the system which leads us to think that it could be a good candidate for doing both proof and extraction of programs.

This paper studies intersection and union type assignment for the calculus , a proof-term syntax for Gentzen’s classical sequent calculus, with the aim of defining a type-based semantics, via setting up a system that is closed under conversion. We will start by investigating what the minimal requirements are for a system, for to be complete ; this coincides with System , the notion defined in Dougherty et al. [18]; however, we show that this system is not sound , so our (...) goal cannot be achieved. We will then show that System is also not complete, but can recover from this by presenting System as an extension of and showing that it satisfies completeness; it still lacks soundness. We show how to restrict so that it satisfies soundness as well by limiting the applicability of certain type assignment rules, but only when limiting reduction to call-by-name or call-by-value reduction; in restricting the system this way, we sacrifice completeness. These results when combined show that, with respect to full reduction, it is not possible to define a sound and complete intersection and union type assignment system for. (shrink)

In this paper we consider a type system with a universal type $omega$ where any term (whether open or closed, $beta$-normalising or not) has type $omega$. We provide this type system with a realisability semantics where an atomic type is interpreted as the set of $lambda$-terms saturated by a certain relation. The variation of the saturation relation gives a number of interpretations to each type. We show the soundness and completeness of our semantics and that for different notions of saturation (...) (based on weak head reduction and normal $beta$-reduction) we obtain the same interpretation for types. Since the presence of $omega$ prevents typability and realisability from coinciding and creates extra difficulties in characterizing the interpretation of a type, we define a class ${mathbb U}^+$ of the so-called positive types (where $omega$ can only occur at specific positions). We show that if a term inhabits a positive type, then this term is $beta$-normalisable and reduces to a closed term. In other words, positive types can be used to represent abstract data types. The completeness theorem for ${mathbb U}^+$ becomes interesting indeed since it establishes a perfect equivalence between typable terms and terms that inhabit a type. In other words, typability and realisability coincide on ${mathbb U}^+$. We give a number of examples to explain the intuition behind the definition of ${mathbb U}^+$ and to show that this class cannot be extended while keeping its desired properties. (shrink)

It was observed by Curry that when λ-terms can be assigned types, for example, simple types, these terms have nice properties . Coppo, Dezani, and Veneri, introduced type systems using conjunctive types, and showed that several important classes of terms can be characterized according to the shape of the types that can be assigned to these terms. For example, the strongly normalizable terms, the normalizable terms, and the terms having head-normal forms, can be characterized in some systems and Ω. The (...) proofs use variants of the method of reducibility. In this paper, we present a uniform approach for proving several meta-theorems relating properties of λ-terms and their typability in the systems and Ω. Our proofs use a new and more modular version of the reducibility method. As an application of our metatheorems, we show how the characterizations obtained by Coppo, Dezani, Veneri, and Pottinger, can be easily rederived. We also characterize the terms that have weak head-normal forms, which appears to be new. We conclude by stating a number of challenging open problems regarding possible generalizations of the realizability method. (shrink)