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import Control.Exception
import Control.Concurrent
import GHC.Conc
import Foreign.StablePtr
main :: IO ()
main = do
tv <- atomically $ newTVar True
_ <- newStablePtr tv
t <- mask_ $ forkIO (blockSTM tv)
killThread t
check b = if b then return () else retry
blockSTM :: TVar Bool -> IO ()
blockSTM tv = do
atomically $ do
v <- readTVar tv
check $ not v
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