Multi-parameter type classes

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Brief Explanation

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

The base package of the  Haskell hierarchical libraries contains only two such classes:

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

There are FlexibleInstances

instance IArray Array e
instance IArray UArray Bool
instance IArray UArray Char
...

but no OverlappingInstances.

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

Cons

• usually require 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.