#10326: ImpredicativeTypes: Unable to use in Functor etc.. -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10326 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler