在Haskell中处理全局标志的正确方法

import Data.IORef

main :: IO ()
main = do
  flag <- newIORef False
  modifyIORef flag not
  readIORef flag >>= print

import Control.Concurrent.MVar

main :: IO ()
main = do
  flag <- newMVar False
  modifyMVar_ flag $ \current -> do
    let newFlag = not current
    print newFlag
    return newFlag