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{-# OPTIONS -XImpredicativeTypes -fno-warn-deprecated-flags #-}
module ShouldFail where
import Control.Concurrent
-- Attempt to put a polymorphic value in an MVar
-- Fails, but the error message is worth keeping an eye on
--
-- Actually (Dec 06) it succeeds now
--
-- In GHC 7.0 it fails again! (and rightly so)
--
-- With the Visible Type Application patch, this succeeds again.
--
-- Sept 16: fails again as it should
foo = do var <- newEmptyMVar :: IO (MVar (forall a. Show a => a -> String))
putMVar var (show :: forall b. Show b => b -> String)
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