[Haskell-cafe] Restrictions on associated types for classes

[Miguel Mitrofanov]

{-# LANGUAGE GADTs, TypeFamilies #-}
module Assoc where
data EqD k where EqD :: Eq k => EqD k
class MyClass k where
     data AssociatedType k :: *
     evidence :: AssociatedType k -> EqD (AssociatedType k)
eq :: MyClass k => AssociatedType k -> AssociatedType k -> Bool
-- eq k1 k2 = k1 == k2 -- doesn't work
eq k1 k2 = case evidence k1 of EqD -> k1 == k2 -- works fine

On 17 Dec 2009, at 16:37, Stephen Lavelle wrote:

> class MyClass k where
>  type AssociatedType k :: *