Magicloud Magiclouds wrote:
So I think I got what you guys meant, I limited ClassB to only H. Then how to archive my requirement, that from and to only return items that instanced ClassB?
If you are willing to go beyond Haskell98 (or Haskell2010), you can use a multi-parameter class. Enable the extension: {-# LANGUAGE MultiParamTypeClasses #-} An then, instead of class (ClassA a) => ClassC a where from :: (ClassB b) => a -> [b] to :: (ClassB c) => a -> [c] you say class (ClassA a, ClassB b) => ClassC a b c where from :: c -> [b] to :: c -> [a] This means that for each triple of concrete types (a,b,c) that you wish to be an instance of ClassC, you must provide an instance declaration, e.g. instance ClassC Test H H where from = ...whatever... to = ...whatever... Now you have the fixed type H in the instance declaration and not a universally quantified type variable. Cheers Ben