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Representations of stream processors using nested fixed points
 Logical Methods in Computer Science
"... Abstract. We define representations of continuous functions on infinite streams of discrete values, both in the case of discretevalued functions, and in the case of streamvalued functions. We define also an operation on the representations of two continuous functions between streams that yields a ..."
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Abstract. We define representations of continuous functions on infinite streams of discrete values, both in the case of discretevalued functions, and in the case of streamvalued functions. We define also an operation on the representations of two continuous functions between streams that yields a representation of their composite. In the case of discretevalued functions, the representatives are wellfounded (finitepath) trees of a certain kind. The underlying idea can be traced back to Brouwer’s justification of barinduction, or to Kreisel and Troelstra’s elimination of choicesequences. In the case of streamvalued functions, the representatives are nonwellfounded trees pieced together in a coinductive fashion from wellfounded trees. The definition requires an alternating fixpoint construction of some ubiquity.
Semicontinuous sized types and termination
 In Zoltán Ésik, editor, Computer Science Logic, 20th International Workshop, CSL 2006, 15th Annual Conference of the EACSL
"... Abstract. Some typebased approaches to termination use sized types: an ordinal bound for the size of a data structure is stored in its type. A recursive function over a sized type is accepted if it is visible in the type system that recursive calls occur just at a smaller size. This approach is onl ..."
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Abstract. Some typebased approaches to termination use sized types: an ordinal bound for the size of a data structure is stored in its type. A recursive function over a sized type is accepted if it is visible in the type system that recursive calls occur just at a smaller size. This approach is only sound if the type of the recursive function is admissible, i.e., depends on the size index in a certain way. To explore the space of admissible functions in the presence of higherkinded data types and impredicative polymorphism, a semantics is developed where sized types are interpreted as functions from ordinals into sets of strongly normalizing terms. It is shown that upper semicontinuity of such functions is a sufficient semantic criterion for admissibility. To provide a syntactical criterion, a calculus for semicontinuous functions is developed. 1.
(Co)iteration for higherorder nested datatypes
 POSTCONF. PROC. OF IST WG TYPES 2ND ANN. MEETING, TYPES'02, LECT. NOTES IN COMPUT. SCI
, 2003
"... The problem of defining iteration for higherorder nested datatypes of arbitrary (finite) rank is solved within the framework of System F ω of higherorder parametric polymorphism. The proposed solution heavily relies on a general notion of monotonicity as opposed to a syntactic criterion on the sh ..."
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The problem of defining iteration for higherorder nested datatypes of arbitrary (finite) rank is solved within the framework of System F ω of higherorder 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 rank2 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 rank1 type transformers (i. e., functions from types to types) arising as least fixedpoints of monotone rank2 type transformers. Strong normalization is shown by an embedding into F ω. The results dualize to greatest fixedpoints, hence to coinductive constructors with coiteration.
Interactive programs and weakly final coalgebras (extended version
 Dependently typed programming, number 04381 in Dagstuhl Seminar Proceedings, 2004. Available via http://drops.dagstuhl.de/opus
"... GR/S30450/01. 2 A. Setzer, P. Hancock 1 Introduction According to MartinL"of [19]: "... I do not think that the search for logically ever more satisfactory high level programming languages can stop short of anything but ..."
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GR/S30450/01. 2 A. Setzer, P. Hancock 1 Introduction According to MartinL"of [19]: "... I do not think that the search for logically ever more satisfactory high level programming languages can stop short of anything but
Guarded Induction and Weakly Final Coalgebras in Dependent Type Theory
, 2004
"... We introduce concepts for representing interactive programs in dependent type theory. The representation uses a monad, as in Haskell. We consider two versions, one, in which the interface with the real world is fixed, and another one, in which the interface varies depending on previous interactions. ..."
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We introduce concepts for representing interactive programs in dependent type theory. The representation uses a monad, as in Haskell. We consider two versions, one, in which the interface with the real world is fixed, and another one, in which the interface varies depending on previous interactions. We then generalise the monadic construction to polynomial functors. Then we look at rules needed in order to introduce weakly final coalgebras in dependent type theory. We arrive at the notion of coiteration, and investigate its relationship to guarded induction. Finally we explore the relationship between state dependent coalgebras and bisimulation.
ObjectOriented Programming in Dependent Type Theory
"... Abstract: We introduce basic concepts from objectoriented programming into dependent type theory based on the idea of modelling objects as interactive programs. We consider methods, interfaces, and the interaction between a fixed number of objects, including selfreferential method calls. We introd ..."
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Abstract: We introduce basic concepts from objectoriented programming into dependent type theory based on the idea of modelling objects as interactive programs. We consider methods, interfaces, and the interaction between a fixed number of objects, including selfreferential method calls. We introduce a monad like syntax for developing objects in dependent type theory. 1.1
Type Fusion
"... Fusion is an indispensable tool in the arsenal of techniques for program derivation. Less wellknown, but equally valuable is type fusion, which states conditions for fusing an application of a functor with an initial algebra to form another initial algebra. We provide a novel proof of type fusion b ..."
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Fusion is an indispensable tool in the arsenal of techniques for program derivation. Less wellknown, but equally valuable is type fusion, which states conditions for fusing an application of a functor with an initial algebra to form another initial algebra. We provide a novel proof of type fusion based on adjoint folds and discuss several applications: type firstification, type specialisation and tabulation. 1.
CMCS’03 Preliminary Version Computable Functions on Final Coalgebras
"... This paper tackles computability issues on final coalgebras and tries to shed light on the following two questions: First, which functions on final coalgebras are computable? Second, which formal system allows us to define all computable functions on final coalgebras? In particular, we give a defini ..."
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This paper tackles computability issues on final coalgebras and tries to shed light on the following two questions: First, which functions on final coalgebras are computable? Second, which formal system allows us to define all computable functions on final coalgebras? In particular, we give a definition of computability on final coalgebras, deriving from the theory of effective domains. We then establish the admissibility of coinductive definitions and of a generalised µoperator. This gives rise to a formal system, in which every term denotes a computable function. 1
M4M 2007 Continuous Functions on Final Coalgebras
"... It can be traced back to Brouwer that continuous functions of type StrA → B, where StrA is the type of infinite streams over elements of A, can be represented by well founded, Abranching trees whose leafs are elements of B. This paper generalises the above correspondence to functions defined on fin ..."
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It can be traced back to Brouwer that continuous functions of type StrA → B, where StrA is the type of infinite streams over elements of A, can be represented by well founded, Abranching trees whose leafs are elements of B. This paper generalises the above correspondence to functions defined on final coalgebras for powerseries functors on the category of sets and functions. While our main technical contribution is the characterisation of all continuous functions, defined on a final coalgebra and taking values in a discrete space by means of inductive types, a methodological point is that these inductive types are most conveniently formulated in a framework of dependent type theory.
Background
"... We investigate CPS translatability of typed λcalculi with inductive and coinductive types. We show that tenable Plotkinstyle callbyname CPS translations exist for simply typed λcalculi with a natural number type and stream types and, more generally, with arbitrary positive inductive and coinduc ..."
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We investigate CPS translatability of typed λcalculi with inductive and coinductive types. We show that tenable Plotkinstyle callbyname CPS translations exist for simply typed λcalculi with a natural number type and stream types and, more generally, with arbitrary positive inductive and coinductive types. These translations also work in the presence of control operators and generalize for dependently typed calculi where caselike eliminations are only allowed in nondependent forms. No translation is possible along the same lines for small Σtypes and sum types with dependent case.