[Haskell-cafe] Possible to automatically determine typeclass membership?

Is it possible in Haskell + GHC extensions to use reflection techniques to determine typeclass membership? I'm thinking of things like the following: Idea 1:

data MaybeEq a = NoEq a | Eq a => HasEq a checkEq :: (some typeclass constraint) => a -> MaybeEq a

(such that checkEq x returns HasEq x if and only if x has an Eq instance.) Idea 2:

data HTrue data HFalse

class MaybeEq a b | a -> b

instance Eq a => MaybeEq a HTrue instance (otherwise) => MaybeEq a HFalse

-- ryan