20-153814.hs

import Data.List.Split
import Control.Monad.State

rpn str = rpnPrime (splitOn " " str) []

rpnPrime ("+":t) (y:x:zs) = rpnPrime t $ x + y:zs
rpnPrime (n:t) zs = rpnPrime t $ read n:zs
rpnPrime [] [x] = x

rpnM str = evalState (rpnPrimeM $ splitOn " " str) []

rpnPrimeM :: [String] -> State [Int] Int
rpnPrimeM ("+":t) = do
    (y:x:zs) <- get
    put $ x + y:zs
    rpnPrimeM t
rpnPrimeM (n:t) = do
    zs <- get
    put $ read n:zs
    rpnPrimeM t
rpnPrimeM [] = do
    [x] <- get
    return x

main = do
    print $ rpnM "1"
    print $ rpnM "1 2 +"
    print $ rpnM "1 2 3 + +"
    print $ rpnM "1 2 + 3 +"
    print $ rpnM "1 2 + 3 4 + +"