[Haskell-cafe] classes with types which are wrapped in

Reid Barton rwbarton at math.harvard.edu
Fri Jan 22 11:25:41 EST 2010

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


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).

Reid Barton

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