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import Control.Concurrent
import Control.Exception
import Control.Monad
import System.IO.Unsafe
main :: IO ()
main = do
-- evaluate lock -- adding this line fixes the problem
fin1 <- newEmptyMVar
fin2 <- newEmptyMVar
forkIO $ ping >>= putMVar fin1
forkIO $ ping >>= putMVar fin2
takeMVar fin1
takeMVar fin2
{-# NOINLINE lock #-}
lock :: MVar ()
lock = unsafePerformIO $ newMVar ()
ping = do
() <- takeMVar lock
putMVar lock ()
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