sumWithIOAndST.hs

import Control.Monad    (forM_)
import Control.Monad.ST (runST)
import Data.IORef 
import Data.STRef

-- dirty, can't get rid of IO
sumIO :: [Int] -> IO Int
sumIO xs = do
    acc <- newIORef 0
    forM_ xs (modifyIORef acc . (+))
    readIORef acc

-- still a bit dirty, yet pure
sumST :: [Int] -> Int
sumST xs = runST $ do
    acc <- newSTRef 0
    forM_ xs (modifySTRef acc . (+))
    readSTRef acc