2008/12/15 Mario Blazevic <mblaze...@stilo.com>
> Alexander Dunlap wrote:
>
>> The problem is that y is not mentioned in the signature of wrapper.
>> When you call wrapper x, there could be many different instances of
>> Container x y with the same x, so GHC doesn't know which version to
>> call.
>>
>
>
> I guess I see it now. However, if the explicit 'Container x y =>'
> context couldn't fix the y to use for instantiation of Container x y, I
> don't see any way to fix it. And if there is no way to call wrapper in any
> context, the class declaration itself is illegal and GHC should have
> reported the error much sooner. Should I create a ticket?
>
Please do not create a ticket. Such a typeclass is legitimate, but not
useful alone or with functional dependencies. It is useful with Type
Families though, so celebrate!
Thomas
----- START CODE ----
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleInstances #-}
import Data.Maybe
class Container x where
type Contains x
wrapper :: x -> Bool
unwrap :: x -> Contains x
rewrap :: Contains x -> x
liftWrap :: Container x => (Contains x -> Contains x) -> x -> x
liftWrap f x = (if wrapper x then rewrap . f . unwrap else id) x
instance Container (Maybe x) where
type Contains (Maybe x) = x
wrapper = isJust
unwrap = fromJust
rewrap = Just
main = print (liftWrap (succ :: Int -> Int) (Just 1 :: Maybe Int))
-----
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