talk :: MVar State -> String -> Handle -> IO ()
talk mvar address h = do
    line <- hGetLine h
    state <- takeMVar mvar
    putStrLn line
    let result = parseLine state address line
    let newState = fst result
    let answer = snd result
    putMVar mvar newState
    case answer of
      Just value -> hPutStrLn h value
      Nothing -> return ()
    talk mvar address h