reverse-polish-notation

rpn.hs

import System.Environment

main = do
  expr <- getArgs
  print $ head $ foldl rpn [] expr

rpn :: [Double] -> String -> [Double]
rpn []           a             = [read a]
rpn xs@[_]       a             = (read a) : xs
rpn xs@(x:y:xs') a | a == "+"  = (x + y)  : xs'
                   | a == "-"  = (x - y)  : xs'
                   | a == "*"  = (x * y)  : xs'
                   | a == "/"  = (x / y)  : xs'
                   | otherwise = (read a) : xs