Multi-parameter type classes

Brief Explanation

Classes take one or more arguments, becoming relations between types. See also the dilemma they cause.

The mtl package of the  Haskell hierarchical libraries utilizes this feature. The base package 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

Tickets

#49

add multi parameter type classes

#90

solve the MultiParamTypeClassDilemma

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.