20-133423.hs

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

-- 状態は引数としては明示的に書かない
pn str = fst $ pnPrime $ splitOn " " str
pnPrime ("+":s0) =
    let (a, s1) = pnPrime s0
        (b, s2) = pnPrime s1
    in (a + b, s2)
pnPrime (x:xs) = ((read x:: Int), xs)

-- evalStateで値を取り出す
pnM str = evalState pnPrimeM $ splitOn " " str
pnPrimeM :: State [String] Int
pnPrimeM = do
    -- 状態をxsに更新する
    (x:xs) <- get
    put xs
    case x of
        "+" -> do
               -- ただの<-で値を取り出す
               a <- pnPrimeM
               b <- pnPrimeM
               return $ a + b
        _   -> do
               return $ read x

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