{-|
  '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 b
withMVar m io =
  mask $ \restore -> do
    a <- takeMVar m
    b <- restore (io a) `onException` putMVar m a
    putMVar m a
    return b