rom2num in Haskell

import Control.Monad
import Data.List

rom2num :: String -> Maybe Integer
rom2num roman = return roman >>= liftM (groupBy (<)) . sequence . map romanLetter >>= liftM sum . sequence . map parseRoll
  where
    parseRoll [ num ]      = Just num
    parseRoll [ pre, num ] = Just (num - pre)
    parseRoll _            = Nothing
    romanLetter x = lookup x $ zip "IVXLCDM" [1, 5, 10, 50, 100, 500, 1000]