すごい Haskell 10 章の逆ポーランド電卓 Maybe を使ったエラー処理をしてる版

rpn.hs

import Data.String
import Control.Monad
import Data.Maybe

main :: IO ()
main = interactEachLines $ (fromMaybe "Error!") . rpn

interactEachLines :: (String -> String) -> IO ()
interactEachLines f = interact $ unlines . (map f) . lines

rpn :: String -> Maybe String
rpn s = do
    [ss] <- (foldM rpnOneStep []) $ words s
    return $ show ss

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

rpnOneStep :: [Integer] -> String -> Maybe [Integer]
rpnOneStep (x:y:ys) "*" = return $ (y*x):ys
rpnOneStep (x:y:ys) "/" = return $ (y `div` x):ys
rpnOneStep (x:y:ys) "+" = return $ (y+x):ys
rpnOneStep (x:y:ys) "-" = return $ (y-x):ys
rpnOneStep (0:y:_:ys) "?" = return $ y:ys
rpnOneStep (_:_:z:ys) "?" = return $ z:ys
rpnOneStep stack num = liftM (:stack) (readMaybe num)