Accumulating string values across in a map

incr.hs

import Data.IORef
import qualified Data.Map as M
import Control.Monad(forever)

prompt msg = putStr msg >> getLine

main = do
  ref <- newIORef M.empty
  forever $ do
    str <- prompt "enter a string:"
    modifyIORef ref $ M.insertWith (+) str 1
    print =<< readIORef ref