The documentation of withMVar (in Control.Concurrent.MVar module of base package) says: {-| 'withMVar' is an exception-safe wrapper for operating on the contents of an 'MVar'. This operation is exception-safe: it will replace the original contents of the 'MVar' if an exception is raised (see "Control.Exception"). However, it is only atomic if there are no other producers for this 'MVar'.-}withMVar :: MVar a -> (a -> IO b) -> IO bwithMVar m io = mask $ \restore -> do a <- takeMVar m b <- restore (io a) `onException` putMVar m a putMVar m a return b Can someone shed some light on what is meant by the statement - "However, it is only atomic if there are no other producers for this 'MVar'."? I hope this is the right mailing list for this question. Thanks, Harendra