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Polytypic Values Possess Polykinded Types
, 2000
"... A polytypic value is one that is defined by induction on the structure of types. In Haskell the type structure is described by the socalled kind system, which distinguishes between manifest types like the type of integers and functions on types like the list type constructor. Previous approaches to ..."
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Cited by 107 (20 self)
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A polytypic value is one that is defined by induction on the structure of types. In Haskell the type structure is described by the socalled kind system, which distinguishes between manifest types like the type of integers and functions on types like the list type constructor. Previous approaches to polytypic programming were restricted in that they only allowed to parameterize values by types of one fixed kind. In this paper we show how to define values that are indexed by types of arbitrary kinds. It appears that these polytypic values possess types that are indexed by kinds. We present several examples that demonstrate that the additional exibility is useful in practice. One paradigmatic example is the mapping function, which describes the functorial action on arrows. A single polytypic definition yields mapping functions for datatypes of arbitrary kinds including first and higherorder functors. Polytypic values enjoy polytypic properties. Using kindindexed logical relations we prove...
A New Approach to Generic Functional Programming
 In The 27th Annual ACM SIGPLANSIGACT Symposium on Principles of Programming Languages
, 1999
"... This paper describes a new approach to generic functional programming, which allows us to define functions generically for all datatypes expressible in Haskell. A generic function is one that is defined by induction on the structure of types. Typical examples include pretty printers, parsers, and co ..."
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Cited by 96 (13 self)
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This paper describes a new approach to generic functional programming, which allows us to define functions generically for all datatypes expressible in Haskell. A generic function is one that is defined by induction on the structure of types. Typical examples include pretty printers, parsers, and comparison functions. The advanced type system of Haskell presents a real challenge: datatypes may be parameterized not only by types but also by type constructors, type definitions may involve mutual recursion, and recursive calls of type constructors can be arbitrarily nested. We show that despite this complexitya generic function is uniquely defined by giving cases for primitive types and type constructors (such as disjoint unions and cartesian products). Given this information a generic function can be specialized to arbitrary Haskell datatypes. The key idea of the approach is to model types by terms of the simply typed calculus augmented by a family of recursion operators. While co...
Efficient Generalized Folds
, 1999
"... Fold operators capture a common recursion pattern over algebraic datatypes. A fold essentially replaces constructors by functions. However, if the datatype is parameterized, the corresponding fold operates on polymorphic functions which severely limits its applicability. In order to overcome this li ..."
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Cited by 11 (0 self)
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Fold operators capture a common recursion pattern over algebraic datatypes. A fold essentially replaces constructors by functions. However, if the datatype is parameterized, the corresponding fold operates on polymorphic functions which severely limits its applicability. In order to overcome this limitation R. Bird and R. Paterson (Bird & Paterson, 1999b) have proposed socalled generalized folds. We show how to define a variation of these folds by induction on the structure of datatype definitions. Unfortunately, for some datatypes generalized folds are less efficient than one would expect. We identify the source of inefficiency and explain how to remedy this shortcoming. While conceptually simple, our approach places high demands on the type system: it requires polymorphic recursion, rank2 types, and a strong form of type constructor polymorphism. 1 Introduction Fold operators are in every functional programmer's toolbox. In essence, a fold operator replaces constructors by functi...
Initial algebra semantics is enough
 Proceedings, Typed Lambda Calculus and Applications
, 2007
"... Abstract. Initial algebra semantics is a cornerstone of the theory of modern functional programming languages. For each inductive data type, it provides a fold combinator encapsulating structured recursion over data of that type, a Church encoding, a build combinator which constructs data of that ty ..."
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Cited by 8 (5 self)
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Abstract. Initial algebra semantics is a cornerstone of the theory of modern functional programming languages. For each inductive data type, it provides a fold combinator encapsulating structured recursion over data of that type, a Church encoding, a build combinator which constructs data of that type, and a fold/build rule which optimises modular programs by eliminating intermediate data of that type. It has long been thought that initial algebra semantics is not expressive enough to provide a similar foundation for programming with nested types. Specifically, the folds have been considered too weak to capture commonly occurring patterns of recursion, and no Church encodings, build combinators, or fold/build rules have been given for nested types. This paper overturns this conventional wisdom by solving all of these problems. 1
Perfect Trees and BitReversal Permutations
, 1999
"... A famous algorithm is the Fast Fourier Transform, or FFT. An ecient iterative version of the FFT algorithm performs as a rst step a bitreversal permutation of the input list. The bitreversal permutation swaps elements whose indices have binary representations that are the reverse of each other. U ..."
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A famous algorithm is the Fast Fourier Transform, or FFT. An ecient iterative version of the FFT algorithm performs as a rst step a bitreversal permutation of the input list. The bitreversal permutation swaps elements whose indices have binary representations that are the reverse of each other. Using an amortized approach this operation can be made to run in linear time on a randomaccess machine. An intriguing question is whether a lineartime implementation is also feasible on a pointer machine, that is in a purely functional setting. We show that the answer to this question is in the armative. In deriving a solution we employ several advanced programming language concepts such as nested datatypes, associated fold and unfold operators, rank2 types, and polymorphic recursion.
Theoretical Foundations for Practical ‘Totally Functional Programming’
, 2007
"... Interpretation is an implicit part of today’s programming; it has great power but is overused and has
significant costs. For example, interpreters are typically significantly hard to understand and hard
to reason about. The methodology of “Totally Functional Programming” (TFP) is a reasoned
attempt ..."
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Interpretation is an implicit part of today’s programming; it has great power but is overused and has
significant costs. For example, interpreters are typically significantly hard to understand and hard
to reason about. The methodology of “Totally Functional Programming” (TFP) is a reasoned
attempt to redress the problem of interpretation. It incorporates an awareness of the undesirability
of interpretation with observations that definitions and a certain style of programming appear to
offer alternatives to it. Application of TFP is expected to lead to a number of significant outcomes,
theoretical as well as practical. Primary among these are novel programming languages to lessen or
eliminate the use of interpretation in programming, leading to betterquality software. However,
TFP contains a number of lacunae in its current formulation, which hinder development of these
outcomes. Among others, formal semantics and typesystems for TFP languages are yet to be
discovered, the means to reduce interpretation in programs is to be determined, and a detailed
explication is needed of interpretation, definition, and the differences between the two. Most
important of all however is the need to develop a complete understanding of the nature of
interpretation. In this work, suitable typesystems for TFP languages are identified, and guidance
given regarding the construction of appropriate formal semantics. Techniques, based around the
‘fold’ operator, are identified and developed for modifying programs so as to reduce the amount of
interpretation they contain. Interpretation as a means of languageextension is also investigated.
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Finally, the nature of interpretation is considered. Numerous hypotheses relating to it considered in
detail. Combining the results of those analyses with discoveries from elsewhere in this work leads
to the proposal that interpretation is not, in fact, symbolbased computation, but is in fact something
more fundamental: computation that varies with input. We discuss in detail various implications of
this characterisation, including its practical application. An often moreuseful property, ‘inherent
interpretiveness’, is also motivated and discussed in depth. Overall, our inquiries act to give
conceptual and theoretical foundations for practical TFP.
Substitution in nonwellfounded . . .
 ELECTRONIC NOTES IN THEORETICAL COMPUTER SCIENCE 82 NO. 1 (2003)
, 2003
"... Inspired from the recent developments in theories of nonwellfounded syntax (coinductively defined languages) and of syntax with binding operators, the structure of algebras of wellfounded and nonwellfounded terms is studied for a very general notion of signature permitting both simple variable bin ..."
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Inspired from the recent developments in theories of nonwellfounded syntax (coinductively defined languages) and of syntax with binding operators, the structure of algebras of wellfounded and nonwellfounded terms is studied for a very general notion of signature permitting both simple variable binding operators as well as operators of explicit substitution. This is done in an extensional mathematical setting of initial algebras and final coalgebras of endofunctors on a functor category. In the nonwellfounded case, the fundamental operation of substitution is more beneficially defined in terms of primitive corecursion than coiteration.
HIGHERORDER THEORIES ANDRÉ HIRSCHOWITZ AND MARCO MAGGESI
, 704
"... Abstract. We extend our approach to abstract syntax (with binding constructions) through modules and linearity. First we give a new general definition of arity, yielding the companion notion of signature. Then we obtain a modularity result as requested by [GU03]: in our setting, merging two extensio ..."
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Abstract. We extend our approach to abstract syntax (with binding constructions) through modules and linearity. First we give a new general definition of arity, yielding the companion notion of signature. Then we obtain a modularity result as requested by [GU03]: in our setting, merging two extensions of syntax corresponds to building an amalgamated sum. Finally we define a natural notion of equation concerning a signature and prove the existence of an initial semantics for a socalled representable signature equipped with a set of equations. 1.
Haskell Programming with Nested Types: A Principled Approach †
"... Abstract. Initial algebra semantics is one of the cornerstones of the theory of modern functional programming languages. For each inductive data type, it provides a Church encoding for that type, a build combinator which constructs data of that type, a fold combinator which encapsulates structured r ..."
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Abstract. Initial algebra semantics is one of the cornerstones of the theory of modern functional programming languages. For each inductive data type, it provides a Church encoding for that type, a build combinator which constructs data of that type, a fold combinator which encapsulates structured recursion over data of that type, and a fold/build rule which optimises modular programs by eliminating from them data constructed using the build combinator, and immediately consumed using the fold combinator, for that type. It has long been thought that initial algebra semantics is not expressive enough to provide a similar foundation for programming with nested types in Haskell. Specifically, the standard folds derived from initial algebra semantics have been considered too weak to capture commonly occurring patterns of recursion over data of nested types in Haskell, and no build combinators or fold/build rules have until now been defined for nested types. This paper shows that standard folds are, in fact, sufficiently expressive for programming with nested types in Haskell. It also defines build combinators and fold/build fusion rules for nested types. It thus shows how initial algebra semantics provides a principled, expressive, and elegant foundation for programming with nested types in Haskell. 1.