Kevin Atkinson wrote:
>Could someone tell me why this is not allowed and if there is a possible
>workaround. (I know this example seems pointless however it is a real
>concern as having bulk types in the context is something I would really
>like to do.)
Here's the not allowed bit:
>class Listable c a where
> toList :: c a -> [a]
The kind of c in the above class declaration is * -> *. That is, unary type
constructors can be provided as actual arguments in instance declarations.
>class Find c a b where
> find :: a -> c a b -> b
The kind of c in this class declaration is * -> * -> *. That is, binary type
constructors can be provided as actual arguments in instance declarations.
>instance (Listable c (a,b)) => Find c a b where
> find a c = lookup a (toList c)
Here you are trying to use c with two distinct kinds.
Dan Russell
Kingston University
