. We present a denition of untyped -terms using a heterogeneous datatype, i.e. an inductively dened operator. This operator can be extended to a Kleisli triple, which is a concise way to verify the substitution laws for -calculus. We also observe that repetitions in the denition of the monad as well as in the proofs can be avoided by using well-founded recursion and induction instead of structural induction. We extend the construction to the simply typed -calculus using dependent types, and show that this is an instance of a generalization of Kleisli triples. The proofs for the untyped case have been checked using the LEGO system. Keywords. Type Theory, inductive types, -calculus, category theory. 1 Introduction The metatheory of substitution for -calculi is interesting maybe because it seems intuitively obvious but becomes quite intricate if we take a closer look. [Hue92] states seven formal properties of substitution which are then used to prove a general substitution theor...

This article The purpose of this paper is to introduce b, a simply typed -calculus that supports type-based recursive definitions. Although heavily inspired from previous work by Giménez (Giménez 1998) and closely related to recent work by Amadio and Coupet (Amadio and Coupet-Grimal 1998), the technical machinery behind our system puts a slightly different emphasis on the interpretation of types. More precisely, we formalize the notion of type-based termination using a restricted form of type dependency (a.k.a. indexed types), as popularized by (Xi and Pfenning 1998; Xi and Pfenning 1999). This leads to a simple and intuitive system which is robust under several extensions, such as mutually inductive datatypes and mutually recursive function definitions; however, such extensions are not treated in the paper

this paper but which have some intriguing connections to some of our results and techniques, are [32] and [20]. We believe that the concept of prelogical relation would have a beneficial impact on the presentation and understanding of their results

. A new treatment of data refinement in typed lambda calculus is proposed, based on pre-logical relations [HS99] rather than logical relations as in [Ten94], and incorporating a constructive element. Constructive data refinement is shown to have desirable properties, and a substantial example of refinement is presented. 1 Introduction Various treatments of data refinement in the context of typed lambda calculus, beginning with Tennent's in [Ten94], have used logical relations to formalize the intuitive notion of refinement. This work has its roots in [Hoa72], which proposes that the correctness of a concrete version of an abstract program be verified using an invariant on the domain of concrete values together with a function mapping concrete values (that satisfy the invariant) to abstract values. In algebraic terms, what is required is a homomorphism from a subalgebra of the concrete algebra to the abstract algebra. A strictly more general method is to take a homomorphic relatio...

We present a new strong normalisation proof for a λ-calculus with interleaving strictly positive inductive types λ^µ which avoids the use of impredicative reasoning, i.e., the theorem of Knaster-Tarski. Instead it only uses predicative, i.e., strictly positive inductive definitions on the metalevel. To achieve this we show that every strictly positive operator on types gives rise to an operator on saturated sets which is not only monotone but also (deterministically) set based -- a concept introduced by Peter Aczel in the context of intuitionistic set theory. We also extend this to coinductive types using greatest fixpoints of strictly monotone

The problem of defining iteration for higher-order nested datatypes of arbitrary (finite) rank is solved within the framework of System F ω of higher-order parametric polymorphism. The proposed solution heavily relies on a general notion of monotonicity as opposed to a syntactic criterion on the shape of the type constructors such as positivity or even being polynomial. Its use is demonstrated for some rank-2 heterogeneous/nested datatypes such as powerlists and de Bruijn terms with explicit substitutions. An important feature is the availability of an iterative definition of the mapping operation (the functoriality) for those rank-1 type transformers (i. e., functions from types to types) arising as least fixed-points of monotone rank-2 type transformers. Strong normalization is shown by an embedding into F ω. The results dualize to greatest fixed-points, hence to coinductive constructors with coiteration.

This article studies the implementation of inductive and coinductive constructors of higher kinds (higher-order nested datatypes) in typed term rewriting, with emphasis on the choice of the iteration and coiteration constructions to support as primitive. We propose and compare several well-behaved extensions of System with some form of iteration and coiteration uniform in all kinds. In what we call Mendler-style systems, the iterator and coiterator have a computational behavior similar to the general recursor, but their types guarantee termination. In conventional-style systems, monotonicity witnesses are used for a notion of monotonicity de ned uniformly for all kinds. Our most expressive systems GMIt of generalized Mendler resp. conventional (co)iteration encompass Martin, Gibbons and Bailey's ecient folds for rank-2 inductive types. Strong normalization of all systems considered is proved by providing an embedding of the basic Mendler-style system MIt into System F .

The new concept of lambda calculi with monotone inductive types is introduced by help of motivations drawn from Tarski's xed-point theorem (in preorder theory) and initial algebras and initial recursive algebras from category theory. They are intended to serve as formalisms for studying iteration and primitive recursion on general inductively given structures. Special accent is put on the behaviour of the rewrite rules motivated by the categorical approach, most notably on the question of strong normalization (i. e., the impossibility of an innite sequence of successive rewrite steps). It is shown that this key property hinges on the concrete formulation. The canonical system of monotone inductive types, where monotonicity is expressed by a monotonicity witness being a term expressing monotonicity through its type, enjoys strong normalization shown by an embedding into the traditional system of non-interleaving positive inductive types which, however, has to be enriched...

Abstract. We give a semantic footing to the fold/build syntax of programming with inductive types, covering shortcut deforestation, based on a universal property. Specifically, we give a semantics for inductive types based on limits of algebra structure forgetting functors and show that it is equivalent to the usual initial algebra semantics. We also give a similar semantic account of the augment generalization of build and of the unfold/destroy syntax of coinductive types. 1

Tarski proved that monotone operators on complete lattices have a least xed-point which is simply given by the inmum of all pre- xed-points. This theorem is the basis of a general understanding of induction. The computational counterpart of induction is iteration. Classically, one may derive (full) primitive recursion from iteration. However, this translation is computationally not acceptable. We introduce some polymorphic lambda calculi suitable for studying those phenomena. Led by the lattice-theoretic motivation we even arrive at a surprisingly simple reduction of arbitrary monotone inductive types (the lambda calculus representation of those least xed-points) to syntactically monotone (i. e., positive) inductive types|reduction in the sense of higher-order term rewrite systems which is simulation of computation. 1 Introduction and Overview The aim is to give a notion of inductive types which is as general as possible and draws its motivation from Tarski's theorem. ...