Tiny polish notation arithmetic interpreter in Haskell.

arithmetic.hs

{- Based on Miran Lipovaca's RPN solver from LYAH. -}

import Control.Monad (forever)

apply :: String -> [Double] -> [Double]
apply "*"   (x:y:ys)  = (x * y):ys
apply "+"   (x:y:ys)  = (x + y):ys
apply "-"   (x:y:ys)  = (x - y):ys
apply "/"   (x:y:ys)  = (x / y):ys
apply "exp" (x:y:ys)  = (x ** y):ys
apply numberString xs = (read numberString):xs

eval :: String -> Double
eval = head . foldr apply [] . words

main :: IO ()
main = forever $ putStr "> " >> getLine >>= \e -> print (eval e)