Combinatory Reduction Systems, or CRSs for short, were designed to combine the usual first-order format of term rewriting with the presence of bound variables as in pure -calculus and various typed -calculi. Bound variables are also present in many other rewrite systems, such as systems with simplification rules for proof normalization. The original idea of CRSs is due to Aczel, who introduced a restricted class of CRSs and, under the assumption of orthogonality, proved confluence. Orthogonality means that the rules are non-ambiguous (no overlap leading to a critical pair) and left-linear (no global comparison of terms necessary). We introduce the class of orthogonal CRSs, illustrated with many examples, discuss its expressive power, and give an outline of a short proof of confluence. This proof is a direct generalization of Aczel's original proof, which is close to the well-known confluence proof for -calculus by Tait and Martin-Lof. There is a well-known connection between the para...

This paper describes a method of proving strong normalization based on an extension of the conservation theorem. We introduce a structural notion of reduction that we call fi S , and we prove that any -term that has a fi I fi S-normal form is strongly fi-normalizable. We show how to use this result to prove the strong normalization of different typed -calculi.

Abstract. In a previous work, we proved that an important part of the Calculus of Inductive Constructions (CIC), the basis of the Coq proof assistant, can be seen as a Calculus of Algebraic Constructions (CAC), an extension of the Calculus of Constructions with functions and predicates defined by higher-order rewrite rules. In this paper, we prove that almost all CIC can be seen as a CAC, and that it can be further extended with non-strictly positive types and inductive-recursive types together with non-free constructors and pattern-matching on defined symbols. 1.

We consider reductions in Orthogonal Expression Reduction Systems (OERS), that is, Orthogonal Term Rewriting Systems with bound variables and substitutions, as in the -calculus. We design a strategy that for any given term t constructs a longest reduction starting from t if t is strongly normalizable, and constructs an infinite reduction otherwise. The Conservation Theorem for OERSs follows easily from the properties of the strategy. We develop a method for computing the length of a longest reduction starting from a strongly normalizable term. We study properties of pure substitutions and several kinds of similarity of redexes. We apply these results to construct an algorithm for computing lengths of longest reductions in strongly persistent OERSs that does not require actual transformation of the input term. As a corollary, we have an algorithm for computing lengths of longest developments in OERSs. 1 Introduction A strategy is perpetual if, given a term t, it constructs an infinit...

A redex family is a set of redexes which are `created in the same way'. Families specify which redexes should be shared in any so-called optimal implementation of a rewriting system. We formalise the notion of family for orthogonal higher-order term rewriting systems (OHRSs). In order to comfort our formalisation of the intuitive concept of family, we actually provide three conceptually different formalisations, via labelling, extraction and zigzag and show them to be equivalent. This generalises the results known from literature and gives a firm theoretical basis for the optimal implementation of OHRSs. 1. Introduction A computation of a result is optimal if its cost is minimal among all computations of the result. Taking rewrite steps as computational units the cost of a rewrite sequence is simply its length. Given a rewrite system the question then is: does an effective optimal strategy exist for it? In the case of lambda calculus, a discouraging result was obtained in [BBKV76]: th...

In a previous work, we proved that an important part of the Calculus of Inductive Constructions (CIC), the basis of the Coq proof assistant, can be seen as a Calculus of Algebraic Constructions (CAC), an extension of the Calculus of Constructions with functions and predicates defined by higher-order rewrite rules. In this paper, we prove that almost all CIC can be seen as a CAC, and that it can be further extended with non-strictly positive types and inductive-recursive types together with non-free constructors and pattern-matching on defined symbols. 1.

We introduce Context-sensitive Conditional Expression Reduction Systems (CERS) by extending and generalizing the notion of conditional TRS to the higher order case. We justify our framework in two ways. First, we define orthogonality for CERSs and show that the usual results for orthogonal systems (finiteness of developments, confluence, permutation equivalence) carry over immediately. This can be used e.g. to infer confluence from the subject reduction property in several typed -calculi possibly enriched with pattern-matching definitions. Second, we express several proof and transition systems as CERSs. In particular, we give encodings of Hilbert-style proof systems, Gentzen-style sequent-calculi, rewrite systems with rule priorities, and the ß-calculus into CERSs. This last encoding is an (important) example of real context-sensitive rewriting. 1 Introduction A term rewriting system is a pair consisting of an alphabet and a set of rewrite rules. The alphabet is used freely to gene...

The paper develops notation for strings of abstracters in typed lambda calculus, and shows how to treat them more or less as single abstracters. 0 1991 Academic Press. Inc. 1.

. We design a strategy that for any given term t in an Orthogonal Term Rewriting System (OTRS) constructs a longest reduction starting from t if t is strongly normalizable, and constructs an infinite reduction otherwise. For some classes of OTRSs the strategy is easily computable. We develop a method for finding the least upper bound of lengths of reductions starting from a strongly normalizable term. We give also some applications of our results. 1 Introduction It is shown in O'Donnell [12] that the innermost strategy is perpetual for orthogonal term rewriting systems (OTRSs). That is, contraction of innermost redexes gives an infinite reduction of a given term whenever such a reduction exists. In fact, a strategy that only contracts redexes that do not erase any other redex is perpetual. Moreover, one can even reduce redexes whose erased arguments are strongly normalizable (Klop [10]). For the lambda-calculus, a more subtle perpetual strategy was invented in Barendregt et al. [1]. H...

Two notions of reduction for terms of the λ-calculus are introduced and the question of whether a λ-term is β-strongly normalizing is reduced to the question of whether a λ-term is merely normalizing under one of the notions of reduction. This gives a method to prove strong β-normalization for typed λ-calculi. Instead of the usual semantic proof style based on Tait's realizability or Girard's "candidats de réductibilité", termination can be proved using a decreasing metric over a well-founded ordering. This proof method is applied to the simply-typed λ-calculus and the system of intersection types, giving the first non-semantic proof for a polymorphic extension of the λ-calculus.