Ivan Lazar Miljenovic wrote:
I'm trying to update container-classes to duplicate the pre-existing
classes defined in the Prelude (Functor, etc.) and am trying to get my
approach on how to have functions/classes that work on types of kind *
(e.g. Bytestring) as well as kind * -> * (e.g. lists), as my previous
approach didn't work.
To define the connection, I've copied the style set out by Ganesh's
rmonad ( http://hackage.haskell.org/package/rmonad ) package:
,----
| -- | Indicates what kind of value may be stored within a type. Once
| -- superclass constraints are available, the @v@ parameter will
| -- become an associated type.
The comment above tells enough.
| class Stores c v | c -> v
|
| data family Constraints :: (* -> *) -> * -> *
|
| class (Stores (c v) v) => Suitable c v where
| constraints :: Constraints c v
`----
This works. However, what doesn't work is when I try to use these
classes to re-define Functor:
,----
| class (Stores c v) => Mappable c v where
And where is functional dependency?
| rigidMap :: (v -> v) -> c -> c
|
| class (Mappable (c v) v) => Functor c where
| fmap :: (Suitable c a, Suitable c b) => (a -> b) -> c a -> c b
`----
GHC doesn't like this definition of Functor; the only way to get it to
work is to make `v' a parameter of the Functor class as well, even
though it doesn't actually get used.
Why is this? Is there any way around it?
(Limitations like this are making me more and more consider dumping this
whole concept as it's just becoming a pain, but more importantly rather
boring :s)
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
