ST monad example

MonadST.hs

import Control.Monad
import Control.Monad.ST
import Data.STRef

imperativeSum :: Num a => [a] -> a
imperativeSum xs = runST $ do
    n <- newSTRef 0             -- Create an STRef (place in memory to store values)
 
    forM_ xs $ \x -> do         -- For each element of xs ..
        modifySTRef n (+x)      -- add it to what we have in n.
 
    readSTRef n  

main :: IO ()
main = print $ imperativeSum [1..10]