maybeIO.hs

newtype MaybeIO a = MaybeIO (IO (Maybe a))

runMaybeIO :: MaybeIO a -> IO (Maybe a)
runMaybeIO (MaybeIO x) = x

hole = undefined
data Hole = Hole

instance Monad MaybeIO where
    return x = MaybeIO (return (Just x))
    (MaybeIO x) >>= k  = MaybeIO $ do
        y <- x
        case y of Just z  -> let (MaybeIO z') = k z in z'
                  Nothing -> return Nothing

readMaybe :: (Read a) => String -> Maybe a
readMaybe x = case reads x of
                  [(name,"")] -> Just name
                  _           -> Nothing

maybeIONumber :: MaybeIO Int
maybeIONumber = MaybeIO $ do
    x <- getLine
    return $ readMaybe x

maybeIONumber' :: MaybeIO Int
maybeIONumber' = liftFromIOToMaybeIO getLine
             >>= liftFromMaybeToMaybeIO . readMaybe

maybeIONumberWithPlus :: Int -> MaybeIO Int
maybeIONumberWithPlus x = liftFromIOToMaybeIO getLine
                      >>= liftFromMaybeToMaybeIO . readMaybe

liftFromIOToMaybeIO :: IO a -> MaybeIO a
liftFromIOToMaybeIO x = MaybeIO $ do
    y <- x
    return (return y)

liftFromMaybeToMaybeIO :: Maybe a -> MaybeIO a
liftFromMaybeToMaybeIO = MaybeIO . return