Forever/MaybeT Demo

ForeverDemo.hs

maybeBind :: IO (Maybe a) -> (a -> IO (Maybe b)) -> IO (Maybe b)

maybeBind x f  =
    x >>= \a ->
        case a of
        Just y -> f y
        Nothing -> return Nothing

getLineMaybe :: IO (Maybe String)
getLineMaybe = getLine >>= \line -> return $ if null line then Nothing else Just line

program1 :: IO (Maybe ())
program1 = getLineMaybe `maybeBind` \line -> fmap Just (putStrLn $ "result: " ++ (map toUpper line))

foreverMaybe :: IO (Maybe a) -> IO (Maybe b)
foreverMaybe m = m `maybeBind` \_ -> foreverMaybe m

fmap (const ()) (foreverMaybe program1)