MVarExample.hs

twitter_user :: MVar (Maybe AuthUser)
twitter_user = unsafePerformIO (newMVar Nothing)

setTwitterUser :: IO ()
setTwitterUser = do
   putStr "User name: "
   u <- getLine
   putStr "User password: "
   p <- getLine
   modifyMVar_ twitter_user (\ _ -> return $ Just (AuthUser u p)) 

tweet :: String -> IO ()
tweet s = do
   r <- readMVar twitter_user
   case r of
     Nothing -> do
       putStrLn "Unable to tweet, no user set - run 'setTwitterUser'" -- "
       return ()
     Just au -> do
       runTM au (update s Nothing)
       return ()