gistfile1.hs

type MockIO = State MockIOData
    
instance CharIO MockIO where
    getCh = do
        d <- get
        case d of
            MockIOData (i:is) o -> do
                put $ MockIOData is o
                return $ Just i
            MockIOData [] _ ->
                return Nothing
    putCh c = modify $ \s -> s{moOutput = moOutput s ++ [c]}