[GHC] #10326: ImpredicativeTypes: Unable to use in Functor etc..

[GHC]

#10326: ImpredicativeTypes: Unable to use in Functor etc..
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              Reporter:              |             Owner:
  j80JjBjVNRMajmA                    |            Status:  new
                  Type:  bug         |         Milestone:
              Priority:  normal      |           Version:  7.10.1
             Component:  Compiler    |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 I want to use values of RankNTypes within Functors/Monads/... just like
 normal values.

 The following code shows, that I cannot lift a function `ReturnNull ->
 ReturnNull` into a functor the way I would like to (e.g. `liftId2`).
 However, using another version that seems equivalent and that is allowed
 by GHC, `liftId3`, does not allow me to pipe the result to `override`. How
 can I do this?

 {{{#!hs
 {-# LANGUAGE ImpredicativeTypes #-}

 type ReturnNull = forall m. Monad m => m ()

 id2 :: ReturnNull -> ReturnNull
 id2 = id

 testId2 :: ReturnNull
 testId2 = id2 $ return ()

 liftId :: Functor f => f ReturnNull -> f ReturnNull
 liftId = fmap id

 -- rejected
 liftId2 :: Functor f => f ReturnNull -> f ReturnNull
 liftId2 = fmap id2

 liftId3 :: (Monad m,Functor f) => f ReturnNull -> f (m ())
 liftId3 = fmap id2

 override :: Functor f => f ReturnNull -> f ()
 override = fmap $ const ()

 testLift :: Functor f => f ReturnNull -> f ()
 testLift = override . liftId

 --rejected
 testLift3 :: Functor f => f ReturnNull -> f ()
 testLift3 = override . liftId3
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10326>
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