Reverse Polish Notation

rpn.hs

calculate :: [String] -> Maybe Double
calculate [] = Nothing
calculate a = calculateHelper a []
    where
        calculateHelper [] [a] = Just a
        calculateHelper ("+":items) (x:y:rest) = calculateHelper items ((y+x):rest)
        calculateHelper ("-":items) (x:y:rest) = calculateHelper items ((y-x):rest)
        calculateHelper ("*":items) (x:y:rest) = calculateHelper items ((y*x):rest)
        calculateHelper ("/":items) (x:y:rest) = calculateHelper items ((y/x):rest)
        calculateHelper (a:items) rest = case (readMaybe a :: Maybe Double) of
            Just a -> calculateHelper items (a:rest)
            Nothing -> Nothing
        calculateHelper _ _ = Nothing

parts = words "10 4 3 + 2 * -"