[Haskell-cafe] redefining a type class for one instance?

12 Apr 2011

      I am trying as an experiment to write something as follows:

data Wrapper a b where
  Pure :: (a -> b) -> Wrapper a b
  Nullable :: (Maybe a -> b) -> Wrapper a b

class Liftable a b where
  liftTo :: (b -> c) -> a b c

instance Liftable Wrapper a where
  liftTo = Pure

instance Liftable Wrapper (Maybe a) where
  liftTo = Nullable

Obviously this code (with suitable extensions enabled) complains of
overlapping type instances, but is there a way to accomplish
redefining a class instance for one particular group of types while
having it defined for all others? (Maybe a vs. a in this instance)

-- 
Edward Amsden
Student
Computer Science
Rochester Institute of Technology
www.edwardamsden.com