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39
Data types à la carte
"... This paper describes a technique for assembling both data types and functions from isolated individual components. We also explore how the same technology can be used to combine free monads and, as a result, structure Haskell’s monolithic IO monad. 1 ..."
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This paper describes a technique for assembling both data types and functions from isolated individual components. We also explore how the same technology can be used to combine free monads and, as a result, structure Haskell’s monolithic IO monad. 1
Type Checking with Open Type Functions
"... We report on an extension of Haskell with open typelevel functions and equality constraints that unifies earlier work on GADTs, functional dependencies, and associated types. The contribution of the paper is that we identify and characterise the key technical challenge of entailment checking; and w ..."
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Cited by 52 (23 self)
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We report on an extension of Haskell with open typelevel functions and equality constraints that unifies earlier work on GADTs, functional dependencies, and associated types. The contribution of the paper is that we identify and characterise the key technical challenge of entailment checking; and we give a novel, decidable, sound, and complete algorithm to solve it, together with some practicallyimportant variants. Our system is implemented in GHC, and is already in active use.
Factorising Folds for Faster Functions
"... The worker/wrapper transformation is a general technique for improving the performance of recursive programs by changing their types. The previous formalisation (Gill & Hutton, 2009) was based upon a simple fixed point semantics of recursion. In this article we develop a more structured approach ..."
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Cited by 13 (9 self)
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The worker/wrapper transformation is a general technique for improving the performance of recursive programs by changing their types. The previous formalisation (Gill & Hutton, 2009) was based upon a simple fixed point semantics of recursion. In this article we develop a more structured approach, based upon initial algebra semantics. In particular, we show how the worker/wrapper transformation can be applied to programs defined using the structured pattern of recursion captured by fold operators, and illustrate our new technique with a number of examples.
Extending type inference to variational programs
, 2012
"... Through the use of conditional compilation and related tools, many software projects can be used to generate a huge number of related programs. The problem of typing such variational software is difficult. The bruteforce strategy of generating all variants and typing each one individually is (1) us ..."
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Cited by 11 (5 self)
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Through the use of conditional compilation and related tools, many software projects can be used to generate a huge number of related programs. The problem of typing such variational software is difficult. The bruteforce strategy of generating all variants and typing each one individually is (1) usually infeasible for efficiency reasons and (2) produces results that do not map well to the underlying variational program. Recent research has focused mainly on efficiency and addressed only the problem of type checking. In this work we tackle the more general problem of variational type inference and introduce variational types to represent the result of typing a variational program. We introduce the variational lambda calculus (VLC) as a formal foundation for research on typing variational programs. We define a type system for VLC in which VLC expressions are mapped to correspondingly variational types. We show that the type system is correct by proving that the typing of expressions is preserved over the process of variation elimination, which eventually results in a plain lambda calculus expression and its corresponding type. We identify a set of equivalence rules for variational types and prove that the type unification problem modulo these equivalence rules is unitary and decidable; we also present a sound and complete unification algorithm. Based on the unification algorithm, the variational type inference algorithm is an extension of algorithm W. We show that it is sound and complete and computes principal types. We also consider the extension of VLC with sum types, a necessary feature for supporting variational data types, and demonstrate that the previous theoretical results also hold under this extension. Finally, we characterize the complexity of variational type inference and demonstrate the efficiency gains over the bruteforce strategy.
Relational Parametricity for Higher Kinds
"... Abstract. Reynolds ’ notion of relational parametricity has been extremely influential and well studied for polymorphic type theories such as System F. The extension of relational parametricity to higher kinded polymorphism, which allows quantification over type operators as well as types, has not b ..."
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Abstract. Reynolds ’ notion of relational parametricity has been extremely influential and well studied for polymorphic type theories such as System F. The extension of relational parametricity to higher kinded polymorphism, which allows quantification over type operators as well as types, has not been as well studied. In this paper we give a model of relational parametricity for System F ω and investigate some of its consequences. 1
Fibrational Induction Rules for Initial Algebras ⋆
"... Abstract. This paper provides an induction rule that can be used to prove properties of data structures whose types are inductive, i.e., are carriers of initial algebras of functors. Our results are semantic in nature and are inspired by Hermida and Jacobs ’ elegant algebraic formulation of inductio ..."
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Cited by 6 (1 self)
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Abstract. This paper provides an induction rule that can be used to prove properties of data structures whose types are inductive, i.e., are carriers of initial algebras of functors. Our results are semantic in nature and are inspired by Hermida and Jacobs ’ elegant algebraic formulation of induction for polynomial data types. Our contribution is to derive, under slightly different assumptions, an induction rule that is generic over all inductive types, polynomial or not. Our induction rule is generic over the kinds of properties to be proved as well: like Hermida and Jacobs, we work in a general fibrational setting and so can accommodate very general notions of properties on inductive types rather than just those of particular syntactic forms. We establish the correctness of our generic induction rule by reducing induction to iteration. We show how our rule can be instantiated to give induction rules for the data types of rose trees, finite hereditary sets, and hyperfunctions. The former lies outside the scope of Hermida and Jacobs ’ work because it is not polynomial; as far as we are aware, no induction rules have been known to exist for the latter two in a general fibrational framework. Our instantiation for hyperfunctions underscores the value of working in the general fibrational setting since this data type cannot be interpreted as a set. 1
Towards Modular Compilers for Effects
"... Compilers are traditionally factorised into a number of separate phases, such as parsing, type checking, code generation, etc. However, there is another potential factorisation that has received comparatively little attention: the treatment of separate language features, such as mutable state, inpu ..."
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Cited by 4 (2 self)
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Compilers are traditionally factorised into a number of separate phases, such as parsing, type checking, code generation, etc. However, there is another potential factorisation that has received comparatively little attention: the treatment of separate language features, such as mutable state, input/output, exceptions, concurrency and so forth. In this article we focus on the problem of modular compilation, in which the aim is to develop compilers for separate language features independently, which can then be combined as required. We summarise our progress to date, issues that have arisen, and further work.
Initial Algebra Semantics for Cyclic Sharing Structures
"... Abstract. Terms are a concise representation of tree structures. Since they can be naturally defined by an inductive type, they offer data structures in functional programming and mechanised reasoning with useful principles such as structural induction and structural recursion. In the case of graphs ..."
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Abstract. Terms are a concise representation of tree structures. Since they can be naturally defined by an inductive type, they offer data structures in functional programming and mechanised reasoning with useful principles such as structural induction and structural recursion. In the case of graphs or ”treelike ” structures – trees involving cycles and sharing – however, it is not clear what kind of inductive structures exists and how we can faithfully assign a term representation of them. In this paper we propose a simple term syntax for cyclic sharing structures that admits structural induction and recursion principles. We show that the obtained syntax is directly usable in the functional language Haskell, as well as ordinary data structures such as lists and trees. To achieve this goal, we use categorical approach to initial algebra semantics in a presheaf category. That approach follows the line of Fiore, Plotkin and Turi’s models of abstract syntax with variable binding. 1
C.: Generic fibrational induction
 Log. Meth. Comput. Sci
, 2012
"... Abstract. This paper provides an induction rule that can be used to prove properties of data structures whose types are inductive, i.e., are carriers of initial algebras of functors. Our results are semantic in nature and are inspired by Hermida and Jacobs ’ elegant algebraic formulation of inducti ..."
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Cited by 4 (2 self)
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Abstract. This paper provides an induction rule that can be used to prove properties of data structures whose types are inductive, i.e., are carriers of initial algebras of functors. Our results are semantic in nature and are inspired by Hermida and Jacobs ’ elegant algebraic formulation of induction for polynomial data types. Our contribution is to derive, under slightly different assumptions, a sound induction rule that is generic over all inductive types, polynomial or not. Our induction rule is generic over the kinds of properties to be proved as well: like Hermida and Jacobs, we work in a general fibrational setting and so can accommodate very general notions of properties on inductive types rather than just those of a particular syntactic form. We establish the soundness of our generic induction rule by reducing induction to iteration. We then show how our generic induction rule can be instantiated to give induction rules for the data types of rose trees, finite hereditary sets, and hyperfunctions. The first of these lies outside the scope of Hermida and Jacobs’ work because it is not polynomial, and as far as we are aware, no induction rules have been known to exist for the second and third in a general fibrational framework. Our instantiation for hyperfunctions underscores the value of working in the general fibrational setting since this data type cannot be interpreted as a set. 1.