On Fri, Jan 22, 2010 at 05:08:15PM +0100, Andrew U. Frank wrote:
i encounter often a problem when i have a class with some operations (say class X with push) applied to a type A b. I then wrap A in a type A_sup, with some more type parameters and i cannot write a instance of class A_sup because i have a kind mismatch. any suggestions? (reordering of the type parameters of A_sup is not a solution, because another class operates on this parameter)
here a simplistic case (i know that A could be reduced to [], my real cases are more complicated).
data A b = A b [b]
data Asup x ab y = Asup x ab y
class X a b where push :: b -> a b -> a b
This is a little strange. Are you sure you don't want either class X a where push :: b -> a b -> a b or class X a b where push :: b -> a -> a ? The second one might help you with your subsequent problem (although I didn't understand quite what you were trying to do there). Regards, Reid Barton