[Haskell-cafe] Type families and polymorphism

Why does compiling the following program give an error?

{-# LANGUAGE TypeFamilies, RankNTypes #-}

type family TF a

identity :: (forall a. TF a) -> (forall a. TF a) identity x = x

GHC 6.10.3 gives me: Couldn't match expected type `TF a1' against inferred type `TF a' In the expression: x In the definition of `identity': identity x = x -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.