2008/12/15 Mario Blazevic
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)) -----