Multi-parameter type classes

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Brief Explanation

Classes take one or more arguments, becoming relations between types.

References

•  Type classes: exploring the design space by Simon Peyton Jones, Mark Jones and Erik Meijer, Haskell Workshop 1997.

•  GHC documentation

Pros

• offered by GHC and Hugs for years
• express more interesting relationships, as in the two occurrences in the base package of the  Haskell hierarchical libraries:

  class HasBounds a => IArray a e
  class (HasBounds a, Monad m) => MArray a e m
  

  The instances supplied do not overlap.

Cons

• not very usable without FlexibleInstances or UndecidableInstances
• often lead to OverlappingInstances and ambiguity, though these can sometimes be avoided using FunctionalDependencies or AssociatedTypes. Examples include container and monad classes.