#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>
GHC <http://www.haskell.org/ghc/>
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