monadic_rpn.hs

import Data.List
import Control.Monad

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

foldingFunction :: [Double] -> String -> Maybe [Double]
foldingFunction (x:y:ys) "*" = return ((y * x):ys)
foldingFunction (x:y:ys) "+" = return ((y + x):ys)
foldingFunction (x:y:ys) "-" = return ((y - x):ys)
foldingFunction xs numberString = liftM (:xs) (readMaybe numberString) -- liftM = fmap

solveRPN :: String -> Maybe Double
solveRPN st = do
  [result] <- foldM foldingFunction [] (words st)
  return result

main = do
  print $ solveRPN "1 2 * 4 +" -- Just 6
  print $ solveRPN "1 2 * 4" -- Nothing
  print $ solveRPN "1 8 foo" -- Nothing