On 19/08/10 14:26, Ivan Lazar Miljenovic wrote:
,----
| -- | Indicates what kind of value may be stored within a type. Once
| -- superclass constraints are available, the @v@ parameter will
| -- become an associated type.
| 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
| 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
`----
I'm no typing expert... but how do you expect the type-checker to be
able to infer a type for 'v' here? Even with the fundep from Stores, 'c
v' should determine 'v', but what value is the type-checker going to
pick for 'v'? It's not mentioned again anywhere else, and it is needed
in order to pick the Mappable instance.
What would you do with the Mappable instance anyway? You can only use
it if type 'a' is the same as type 'b' (and then I think *this* is type
you'd want for 'v'), and I don't see that any of your other constraints
or machinery can help you to identify when this is the case.
Hope that helps your thinking,
Neil.
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