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

[Ryan Ingram]

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