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Combinatory Reduction Systems: introduction and survey
 THEORETICAL COMPUTER SCIENCE
, 1993
"... Combinatory Reduction Systems, or CRSs for short, were designed to combine the usual firstorder format of term rewriting with the presence of bound variables as in pure λcalculus and various typed calculi. Bound variables are also present in many other rewrite systems, such as systems with simpl ..."
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Cited by 84 (9 self)
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Combinatory Reduction Systems, or CRSs for short, were designed to combine the usual firstorder format of term rewriting with the presence of bound variables as in pure λcalculus 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 nonambiguous (no overlap leading to a critical pair) and leftlinear (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 wellknown confluence proof for λcalculus by Tait and MartinLof. There is a wellknown connection between the para...
Higherorder narrowing
 PROC. NINTH ANNUAL IEEE SYMPOSIUM ON LOGIC IN COMPUTER SCIENCE
, 1994
"... We introduce several approaches for solving higherorder equational problems by higherorder narrowing and give first completeness results. The results apply to higherorder functionallogic programming languages and to higherorder unification modulo a higherorder equational theory. We lift the ge ..."
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Cited by 20 (8 self)
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We introduce several approaches for solving higherorder equational problems by higherorder narrowing and give first completeness results. The results apply to higherorder functionallogic programming languages and to higherorder unification modulo a higherorder equational theory. We lift the general notion of firstorder narrowing to socalled higherorder patterns and argue that the full higherorder case is problematic. Integrating narrowing into unification, called lazy narrowing, can avoid these problems and can be adapted to the full higherorder case. For the secondorder case, we develop a version where the needed secondorder unification remains decidable. Finally we discuss a method that combines both approaches by using narrowing on higherorder patterns with full higherorder constraints.
Comparing Combinatory Reduction Systems and HigherOrder Rewrite Systems
, 1993
"... In this paper two formats of higherorder rewriting are compared: Combinatory Reduction Systems introduced by Klop [Klo80] and Higherorder Rewrite Systems defined by Nipkow [Nipa]. Although it always has been obvious that both formats are closely related to each other, up to now the exact relations ..."
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Cited by 18 (3 self)
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In this paper two formats of higherorder rewriting are compared: Combinatory Reduction Systems introduced by Klop [Klo80] and Higherorder Rewrite Systems defined by Nipkow [Nipa]. Although it always has been obvious that both formats are closely related to each other, up to now the exact relationship between them has not been clear. This was an unsatisfying situation since it meant that proofs for much related frameworks were given twice. We present two translations, one from Combinatory Reduction Systems into HigherOrder Rewrite Systems and one vice versa, based on a detailed comparison of both formats. Since the translations are very `neat' in the sense that the rewrite relation is preserved and (almost) reflected, we can conclude that as far as rewrite theory is concerned, Combinatory Reduction Systems and HigherOrder Rewrite Systems are equivalent, the only difference being that Combinatory Reduction Systems employ a more `lazy' evaluation strategy. Moreover, due to this result...
Developing Developments
, 1994
"... Confluence of orthogonal rewriting systems can be proved using the Finite Developments Theorem. We present, in a general setting, several adaptations of this proof method for obtaining confluence of `not quite' orthogonal systems. 1. Introduction Rewriting as studied here is based on the analogy ..."
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Cited by 17 (2 self)
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Confluence of orthogonal rewriting systems can be proved using the Finite Developments Theorem. We present, in a general setting, several adaptations of this proof method for obtaining confluence of `not quite' orthogonal systems. 1. Introduction Rewriting as studied here is based on the analogy: rewriting = substitution + rules. This analogy is useful since it enables a clearcut distinction between the `designer' defined substition process, i.e. management of resources, and the `user' defined rewrite rules, of rewriting systems. For example, application of the `user' defined term rewriting rule 2 \Theta x ! x + x to the term 2 \Theta 3 gives rise to the duplication of the term 3 in the result 3 + 3. How this duplication is actually performed (for example, using sharing) depends on the `designer's' implementation of substitution. This decomposition has been shown useful in [OR94, Oos94] in the case of firstorder term rewriting systems (TRSs, [DJ90, Klo92]) and higherorder term r...
Relative Normalization in Deterministic Residual Structures
 In: Proc. of the 19 th International Colloquium on Trees in Algebra and Programming, CAAP'96, Springer LNCS
, 1996
"... . This paper generalizes the Huet and L'evy theory of normalization by neededness to an abstract setting. We define Stable Deterministic Residual Structures (SDRS) and Deterministic Family Structures (DFS) by axiomatizing some properties of the residual relation and the family relation on redexes in ..."
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Cited by 17 (13 self)
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. This paper generalizes the Huet and L'evy theory of normalization by neededness to an abstract setting. We define Stable Deterministic Residual Structures (SDRS) and Deterministic Family Structures (DFS) by axiomatizing some properties of the residual relation and the family relation on redexes in an Abstract Rewriting System. We present two proofs of the Relative Normalization Theorem, one for SDRSs for regular stable sets, and another for DFSs for all stable sets of desirable `normal forms'. We further prove the Relative Optimality Theorem for DFSs. We extend this result to deterministic Computation Structures which are deterministic Event Structures with an extra relation expressing selfessentiality. 1 Introduction A normalizable term, in a rewriting system, may have an infinite reduction, so it is important to have a normalizing strategy which enables one to construct reductions to normal form. It is well known that the leftmostoutermost strategy is normalizing in the calc...
HigherOrder Families
 In International Conference on Rewriting Techniques and Applications '96, LNCS
, 1996
"... A redex family is a set of redexes which are `created in the same way'. Families specify which redexes should be shared in any socalled optimal implementation of a rewriting system. We formalise the notion of family for orthogonal higherorder term rewriting systems (OHRSs). In order to comfort our ..."
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Cited by 15 (2 self)
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A redex family is a set of redexes which are `created in the same way'. Families specify which redexes should be shared in any socalled optimal implementation of a rewriting system. We formalise the notion of family for orthogonal higherorder 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...
Termination Proofs for Higherorder Rewrite Systems
 IN 1ST INTERNATIONAL WORKSHOP ON HIGHERORDER ALGEBRA, LOGIC AND TERM REWRITING
, 1994
"... This paper deals with termination proofs for HigherOrder Rewrite Systems (HRSs), introduced in [12]. This formalism combines the computational aspects of term rewriting and simply typed lambda calculus. The result is a proof technique for the termination of a HRS, similar to the proof technique "Te ..."
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Cited by 13 (0 self)
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This paper deals with termination proofs for HigherOrder Rewrite Systems (HRSs), introduced in [12]. This formalism combines the computational aspects of term rewriting and simply typed lambda calculus. The result is a proof technique for the termination of a HRS, similar to the proof technique "Termination by interpretation in a wellfounded monotone algebra", described in [8, 19]. The resulting technique is as follows: Choose a higherorder algebra with operations for each function symbol in the HRS, equipped with some wellfounded partial ordering. The operations must be strictly monotonic in this ordering. This choice generates a model for the HRS. If the choice can be made in such a way that for each rule the interpretation of the left hand side is greater than the interpretation of the right hand side, then the HRS is terminating. At the end of the paper some applications of this technique are given, which show that this technique is natural and can easily be applied.
Contextsensitive Conditional Expression Reduction Systems
 In Proc. of the International Workshop on Graph Rewriting and Computation, SEGRAGRA'95
, 1995
"... We introduce Contextsensitive 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 ..."
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Cited by 12 (4 self)
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We introduce Contextsensitive 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 patternmatching definitions. Second, we express several proof and transition systems as CERSs. In particular, we give encodings of Hilbertstyle proof systems, Gentzenstyle sequentcalculi, rewrite systems with rule priorities, and the ßcalculus into CERSs. This last encoding is an (important) example of real contextsensitive 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...
Relative Normalization in Orthogonal Expression Reduction Systems
 In: Proc. of the 4 th International workshop on Conditional (and Typed) Term Rewriting Systems, CTRS'94, Springer LNCS
, 1994
"... . We study reductions in orthogonal (leftlinear and nonambiguous) Expression Reduction Systems, a formalism for Term Rewriting Systems with bound variables and substitutions. To generalise the normalization theory of Huet and L'evy, we introduce the notion of neededness with respect to a set of r ..."
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Cited by 11 (10 self)
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. We study reductions in orthogonal (leftlinear and nonambiguous) Expression Reduction Systems, a formalism for Term Rewriting Systems with bound variables and substitutions. To generalise the normalization theory of Huet and L'evy, we introduce the notion of neededness with respect to a set of reductions \Pi or a set of terms S so that each existing notion of neededness can be given by specifying \Pi or S. We imposed natural conditions on S, called stability, that are sufficient and necessary for each term not in Snormal form (i.e., not in S) to have at least one Sneeded redex, and repeated contraction of Sneeded redexes in a term t to lead to an Snormal form of t whenever there is one. Our relative neededness notion is based on tracing (open) components, which are occurrences of contexts not containing any bound variable, rather than tracing redexes or subterms. 1 Introduction Since a normalizable term, in a rewriting system, may have an infinite reduction, it is important to...