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))
-----