Type-checking multi-parameter type classes

  • Authors:

  • Dominic Duggan;John Ophel

  • Affiliations:

  • Department of Computer Science, Stevens Institute of Technology, Castle Point on the Hudson, Hoboken, NJ 07030, USA;Department of Computer Science and Computer Engineering, La Trobe University, Bundoora, Victoria 3083, Australia

  • Venue:
  • Journal of Functional Programming
  • Year:
  • 2002

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Abstract

Type classes are a novel combination of parametric polymorphism and constrained types. Although most implementations restrict type classes to be single-parameter, the generalization to multi-parameter type classes has gained increasing attention. A problem with multi-parameter type classes is the increased possibilities they introduce for ambiguity in inferred types, impacting their usefulness in many practical situations. A new type-checking strategy, domain-driven unifying resolution, is identified as an approach to solve these problems. Domain-driven unifying resolution is simple, efficient, and practically useful. However, even with severe restrictions on instance definitions, it is not possible to guarantee that type-checking with unifying resolution terminates. This is in contrast with the naive generalization of single parameter resolution strategies. Domain-driven unifying resolution is guaranteed to terminate if the type class constraints are satisfiable; however satisfiability is undecidable even with severe restrictions on instance definitions. These results shed some light on ambiguity problems with multi-parameter type classes.