rpn.hs

main = do
    print $ (rpn "1 2 + 10 * 13 52 - 12 /")

rpn :: (Read a, Fractional a) => String -> a
rpn input = rpnProcess (words input) []

rpnProcess :: (Read a, Fractional a) => [String] -> [a] -> a
rpnProcess [] stack            = head stack
rpnProcess (x:inputTail) stack = if x `elem` ["+", "-", "*", "/"] then
                                    rpnProcess inputTail (action x stack)
                                 else
                                    rpnProcess inputTail ((read x):stack)

action :: (Read a, Fractional a) => String -> [a] -> [a]
action "+" (a:b:tail) = (a + b):tail
action "-" (a:b:tail) = (a - b):tail
action "*" (a:b:tail) = (a * b):tail
action "/" (a:b:tail) = (a / b):tail