Doubler.hs

import Control.Monad
import Data.IORef

doubler :: IORef Int -> IO ()
doubler i = do
  newValue <- readIORef i
  writeIORef i (2 * newValue)

main :: IO ()
main = do
  a <- newIORef 1
  let xs = [a, a, a]
  mapM_ doubler xs
  mapM_ (readIORef >=> print) xs