[Haskell-cafe] Having a connection between kind * and kind * -> *

[Ivan Lazar Miljenovic]

Miguel Mitrofanov <miguelimo38 at yandex.ru> writes:

> 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.

About what?

The point of the Stores class is that I'm aiming at only having to
explicitly state the constraint (in this case the functional dependency)
once for all associated classes.

>
>> | 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?

In Stores.  If I put an explicit Functional Dependency here as well, it
doesn't make a difference.

>
>> |   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)
>>

-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com