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

Daniel Fischer daniel.is.fischer at web.de
Fri Jan 22 13:30:16 EST 2010

Am Freitag 22 Januar 2010 17:08:15 schrieb Andrew U. Frank:
> 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

That doesn't match its use below, perhaps 

data Asup x ab y = Asup x (A ab) y

(matches better with A_2)
or use Asup Char (A Int) Float below?

> class X a b where
>     push :: b -> a b -> a b

How about type families?

class Y a where
    type Pushy a :: *
    push :: Pushy a -> a -> a

instance Y (A Int) where
    type Pushy (A Int) = Int
    push b' (A b bs) = A b' (b:bs)

instance Y (Asup Char (A Int) Float) where
    type Pushy (Asup Char (A Int) Float) = Int
    push b' (Asup a b c) = Asup a (push b' b) c

> instance X A Int where
>     push b' (A b bs) = A b' (b:bs)
> instance X Asup Char Int Float where
>     push b' (Asup a b c) = Asup a (push b' b) c

Missing instance X Int Int, but Int has wrong kind for that :)

> -- this does not compile because the number of type arguments for X is
> wrong.
> if i try with a type
> type A_2 b = Asup Char (A b) Float
> instance X A_2 Int where
>     push b' (Asup a b c) = Asup a (push b' b) c
> (and --TypeSynonymInstances) i get:
> Type synonym `A_2' should have 1 argument, but has been given 0
>     In the instance declaration for `X A_2 Int'
> what is the solution? thank you!
> andrew

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