I recently upgraded to ghc 7.0.2 from 6.12.3, along with the Haskell platform, and noticed that the following code no longer works as expected:

waitFor tvar = atomically $ do count <- readTVar tvar check (count == 0)

worker tchan tvar = loop where loop = do putStrLn "checking" finished <- atomically $ isEmptyTChan tchan threadDelay 50000 if finished then atomically $ do val <- readTVar tvar writeTVar tvar $ (subtract 1) val else (atomically $ readTChan tchan) >> loop

test = do tchan <- newTChanIO pure $ forM_ [1..5] $ writeTChan tchan -- THIS LINE tvar <- newTVarIO 1 forkIO $ worker tchan tvar waitFor tvar putStrLn "DONE"

You did not include your imports, but I assume that's Control.Applicative.pure. If so, that line should never do anything. pure for IO is the same as return - you are never running the STM action that line calculates. It's the same as if you had written

test = do tchan <- newTChanIO let neverGetsUsed = forM_ [1..5] $ writeTChan tchan -- THIS LINE tvar <- newTVarIO 1 forkIO $ worker tchan tvar waitFor tvar putStrLn "DONE"

Brandon