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See the source at

http://hpaste.org/43297/ghc7_problem_with_rankntypes_a

It works on ghc6 but not on ghc 7

The type error message is

ghc7rankn.hs:23:20:
    Couldn't match expected type `forall a'1.
                                  x a'1 -> Const (Const b a') a'1'
                with actual type `forall a'1. x1 a'1 -> Const b a'1'
    Expected type: (forall a'1. x a'1 -> Const (Const b a') a'1)
                   -> f x a
                   -> Const (Const b a') a
      Actual type: (forall a'1. x1 a'1 -> Const b a'1)
                   -> f1 x1 a1
                   -> Const b a1
    In the first argument of `mcata', namely `f'
    In the first argument of `f', namely `(mcata f)'