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
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