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Stable computational semantics of conflictfree rewrite systems (Draft). Available at http://www.sys.uea.ac.uk/~zurab
, 2000
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Innocent Game Models of Untyped λCalculus
 Theoretical Computer Science
, 2000
"... We present a new denotational model for the untyped calculus, using the techniques of game semantics. The strategies used are innocent in the sense of Hyland and Ong [9] and Nickau [17], but the traditional distinction between \question" and \answer" moves is removed. We rst construct mod ..."
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We present a new denotational model for the untyped calculus, using the techniques of game semantics. The strategies used are innocent in the sense of Hyland and Ong [9] and Nickau [17], but the traditional distinction between \question" and \answer" moves is removed. We rst construct models D and DREC as global sections of a reexive object in the categories A and A REC of arenas and innocent and recursive innocent strategies respectively. We show that these are sensible algebras but are neither extensional nor universal. We then introduce a new representation of innocent strategies in an economical form. We show a strong connexion between the economical form of the denotation of a term in the game models and a variablefree form of the Nakajima tree of the term. Using this we show that the denable elements of DREC are precisely what we call eectively almosteverywhere copycat (EAC) strategies. The category A EAC with these strategies as morphisms gives rise to a model D...
Infinite λcalculus and Types
, 1998
"... Recent work on infinitary versions of the lambda calculus has shown that the infinite lambda calculus can be a useful tool to study the unsolvable terms of the classical lambda calculus. Working in the framework of the intersection type disciplines, we devise a type assignment system such that two t ..."
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Recent work on infinitary versions of the lambda calculus has shown that the infinite lambda calculus can be a useful tool to study the unsolvable terms of the classical lambda calculus. Working in the framework of the intersection type disciplines, we devise a type assignment system such that two terms are equal in the infinite lambda calculus iff they can be assigned the same types in any basis. A novel feature of the system is the presence of a type constant to denote the set of all terms of order zero, and the possibility of applying a type to another type. We prove a completeness and an approximation theorem for our system. Our results can be considered as a first step towards the goal of giving a denotational semantics for the lambda calculus which is suited for the study of the unsolvable terms. However some noncontinuity phenomena of the infinite lambda calculus make a full realization of this idea (namely the construction of a filter model) a quite difficult task.
Khasidashvili Z. An abstract Böhmnormalization. in
 Proc. WRS’02, Electronic Notes in Computer Science, Elsevier Science B.V
, 2002
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Types for Trees
 IFIP 1996
, 1996
"... We introduce a type assignment system which is parametric with respect to five families of trees obtained by evaluating terms (Bohm trees, LevyLongo trees, ...). Then we prove, in an (almost) uniform way, that each type assignment system fully describes the observational equivalences induced by th ..."
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We introduce a type assignment system which is parametric with respect to five families of trees obtained by evaluating terms (Bohm trees, LevyLongo trees, ...). Then we prove, in an (almost) uniform way, that each type assignment system fully describes the observational equivalences induced by the corresponding tree representation of terms. More precisely, for each family of trees two terms have the same tree if and only if they get assigned the same types in the corresponding type assignment system.
Problem 19
"... Abstract. A closed λterm M is easy if, for any other closed term N, the lambda theory generated by M = N is consistent, while it is simple easy if, given an arbitrary intersection type τ, one can find a suitable preorder on types which allows to derive τ for M. Simple easiness implies easiness. Th ..."
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Abstract. A closed λterm M is easy if, for any other closed term N, the lambda theory generated by M = N is consistent, while it is simple easy if, given an arbitrary intersection type τ, one can find a suitable preorder on types which allows to derive τ for M. Simple easiness implies easiness. The question whether easiness implies simple easiness constitutes Problem 19 in the TLCA list of open problems. In this paper we negatively answer the question providing a nonempty cor.e. (complement of a recursively enumerable) set of easy, but non simple easy, λterms. Key words: Lambda calculus, easy terms, simple easy terms, filter models 1
Under consideration for publication in J. Functional Programming 1 Finally Tagless, Partially Evaluated Tagless Staged Interpreters for Simpler Typed Languages
"... We have built the first family of tagless interpretations for a higherorder typed object language in a typed metalanguage (Haskell or ML) that require no dependent types, generalized algebraic data types, or postprocessing to eliminate tags. The statically typepreserving interpretations include an ..."
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We have built the first family of tagless interpretations for a higherorder typed object language in a typed metalanguage (Haskell or ML) that require no dependent types, generalized algebraic data types, or postprocessing to eliminate tags. The statically typepreserving interpretations include an evaluator, a compiler (or staged evaluator), a partial evaluator, and callbyname and callbyvalue CPS transformers. Our principal technique is to encode de Bruijn or higherorder abstract syntax using combinator functions rather than data constructors. In other words, we represent object terms not in an initial algebra but using the coalgebraic structure of the λcalculus. Our representation also simulates inductive maps from types to types, which are required for typed partial evaluation and CPS transformations. Our encoding of an object term abstracts uniformly over the family of ways to interpret it, yet statically assures that the interpreters never get stuck. This family of interpreters thus demonstrates again that it is useful to abstract over higherkinded types. It should also be possible to define languages with a highly refined syntactic type structure. Ideally, such a treatment should be metacircular, in the sense that the type structure used in the defined language should be adequate for the defining language. (Reynolds 1972) 1
On the equational consistency of ordertheoretic models of the λcalculus
"... Answering a question by Honsell and Plotkin, we show that there are two equations between λterms, the socalled subtractive equations, consistent with λcalculus but not satisfied in any partially ordered model with bottom element. We also relate the subtractive equations to the open problem of the ..."
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Answering a question by Honsell and Plotkin, we show that there are two equations between λterms, the socalled subtractive equations, consistent with λcalculus but not satisfied in any partially ordered model with bottom element. We also relate the subtractive equations to the open problem of the orderincompleteness of λcalculus.