MPTC and canonical wrappers

[treeowl]

I've never been the biggest fan of single-parameter classes with buried type families. Could we add an MPTC version of PrimUnlifted?

class (PrimUnlifted a, Unlifted a ~ ua) => PU ua a
instance (PrimUnlifted a, Unlifted a ~ ua) => PU ua a

That thinking leads me to the possibility of a class for types that have a "canonical wrapper":

class PrimUnlifted (Lifted ua) => PrimUnliftedC ua where
  type Lifted ua :: Type
instance PrimUnliftedC (Array# a) where
  type Lifted (Array# a) = Array a
instance PrimUnliftedC (MVar# a) where
  type Lifted (MVar# a) = MVar a
instance PrimUnliftedC (Weak# a) where
  type Lifted (Weak# a) = Weak a

toUnliftedC# :: forall a ua. (PrimUnliftedC ua, ua ~  Unlifted a) => a -> ua
toUnliftedC# = toUnlifted#

fromUnliftedC# :: forall ua a. (PrimUnliftedC ua, ua ~ Unlifted a) => ua -> a
fromUnliftedC# = fromUnlifted#

class (PU ua a, Lifted ua ~ a) => PUC ua a
instance (PU ua a, Lifted ua ~ a) => PUC ua a

toUnliftedC# :: PUC ua a => a -> ua
toUnliftedlC# = toUnlifted#

fromUnliftedC# :: PUC ua a => ua -> a
fromUnliftedC# = fromUnlifted#

For many types, unfortunately, there's no good answer to the canonical instance question. For example, we can only pick one of the following instances:

instance PrimUnliftedC (MutVar# s a) where
  type Lifted (MutVar# s a) = STRef s a
instance s ~ RealWorld => PrimUnliftedC (MutVar# s a) where
  type Lifted  (MutVar# s a) = IORef a

That leads to the possibility of PrimUnliftedC being an empty class with just one instance, and having Lifted as an independent type family.

Bleh.