[Haskell-cafe] Contexts for type family instances

[Max Bolingbroke]

On 12 December 2010 12:26, Stephen Tetley <stephen.tetley at gmail.com> wrote:
>> type instance (DUnit a ~ DUnit b) => DUnit (a,b)           = DUnit a

Requires UndecidableInstances but should work:

"""
{-# LANGUAGE TypeFamilies #-}

type family DUnit a :: *

data Point u = P2 u u
type instance DUnit (Point u) = u

type instance DUnit (a,b) = GuardEq (DUnit a) (DUnit b)

type family GuardEq a b :: *
type instance GuardEq a a = a
"""

More realistically, you will have to write functions that
produce/consume DUnit using type classes so you can pattern match on
the "a" of "DUnit a". You could just have all your instances for
"DUnit (a, b)" require (DUnit a ~ DUnit b):

"""
class Consume a where
  consume :: DUnit a -> Foo
instance (DUnit a ~ DUnit b) => Consume (a, b) where
  consume a = undefined
"""

Cheers,
Max