Haskell RPN calculator

rpn.hs

module Main where

import System.Environment

runRPN :: (Fractional a, Read a) => String -> [a]
runRPN = foldl doToken [] . words
    where doToken (x:y:zs) "+" = (y+x):zs
          doToken (x:y:zs) "-" = (y-x):zs
          doToken (x:y:zs) "*" = (y*x):zs
          doToken (x:y:zs) "/" = (y/x):zs
          doToken a        n   = read n:a

main :: IO ()
main = do
  args <- getArgs
  print $ head $ runRPN $ unwords args