Simple RPN calculator. Takes the program from command-line arguments.

rpn.hs

module Main where

import System.Environment (getArgs)
import Control.Applicative
import Data.Maybe
import Text.Read

main :: IO ()
main = do
    program <- parse . unwords <$> getArgs
    putStrLn . unwords . reverse . fmap show $ eval program

type Val = Integer

type Op = Val -> Val -> Val

data Instr = Push Val | Exec Op

parse :: String -> [Instr]
parse = mapMaybe parseWord . words
  where
    parseWord w = (Push <$> readMaybe w) <|> (Exec <$> lookup w ops)

ops :: [(String, Op)]
ops =
  [ ("+", (+))
  , ("-", (-))
  , ("*", (*))
  ]

eval :: [Instr] -> [Val]
eval prog = eval' prog []

eval' :: [Instr] -> [Val] -> [Val]
eval' []     vs = vs
eval' (i:is) vs = case i of
  Push v -> eval' is (v:vs)
  Exec op -> case vs of
    x:y:rest -> eval' is ((op x y):rest)
    _ -> error "Not enough arguments!"