[Haskell-cafe] Current research on overlapping/closed type families?

[Ryan Ingram]

What's the status of overlapping/closed type families?  I'm interested
in something like the following, which can currently be implemented in
GHC with Oleg-magic using functional dependencies, but cannot, to my
knowledge, be implemented with type families:

data HTrue = HTrue
data HFalse = HFalse

type family IsFunction f

{- not legal in GHC6.10 -}
type instances
   IsFunction (a -> b) = HTrue
   IsFunction a = HFalse

  -- ryan