Functional and Logic Programming

Volume 6009 of the series Lecture Notes in Computer Science pp 56-71

Haskell Type Constraints Unleashed

  • Dominic OrchardAffiliated withLancaster UniversityComputer Laboratory, University of Cambridge
  • , Tom SchrijversAffiliated withCarnegie Mellon UniversityKatholieke Universiteit Leuven

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The popular Glasgow Haskell Compiler extends the Haskell 98 type system with several powerful features, leading to an expressive language of type terms. In contrast, constraints over types have received much less attention, creating an imbalance in the expressivity of the type system. In this paper, we rectify the imbalance, transferring familiar type-level constructs, synonyms and families, to the language of constraints, providing a symmetrical set of features at the type-level and constraint-level. We introduce constraint synonyms and constraint families, and illustrate their increased expressivity for improving the utility of polymorphic EDSLs in Haskell, amongst other examples. We provide a discussion of the semantics of the new features relative to existing type system features and similar proposals, including details of termination.