maybet.hs

import Control.Monad.Trans.Maybe
import Control.Monad.IO.Class
import Data.List

env :: [(String, Int)]
env = [("x", 1), ("y", 2), ("z", 3)]

main :: IO ()
main = do
    result <- runMaybeT $ do
        n <- mb $ lookup "x" env
        q <- liftIO $ getLine
        mb $ if q /= "q" then Just q else Nothing
    putStrLn $ maybe "quit!" id $ result
    where
    mb = MaybeT . pure