[GHC] #4347: Asymmetry of (impredicative) instantiation/higher rank types

[GHC]

#4347: Asymmetry of (impredicative) instantiation/higher rank types
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    Reporter:  dolio                    |        Owner:                           
        Type:  bug                      |       Status:  new                      
    Priority:  normal                   |    Milestone:  7.4.1                    
   Component:  Compiler (Type checker)  |      Version:  7.1                      
    Keywords:                           |     Testcase:                           
   Blockedby:                           |   Difficulty:                           
          Os:  Linux                    |     Blocking:                           
Architecture:  x86_64 (amd64)           |      Failure:  GHC rejects valid program
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Comment(by carlhowells):

 Yes, I have a very small example:

 {{{
 {-# LANGUAGE ImpredicativeTypes #-}

 import Control.Concurrent.MVar
 import System.Random

 dumbTrick :: IO (forall a. Random a => (forall g. RandomGen g => g -> (a,
 g)) -> IO a)
 dumbTrick = do
     gen <- newMVar =<< newStdGen
     let apply :: forall b. Random b => (forall g'. RandomGen g' => g' ->
 (b, g')) -> IO b
         apply f = modifyMVar gen $ (\(a, b) -> return (b, a)) . f
     return apply
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4347#comment:6>
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