-- Russian Doll principle in haskell
module Main where

import Control.Concurrent.MVar
import Control.Monad (join)

main :: IO ()
main = do
    fn <- newEmptyMVar
    putMVar fn $ do
        putStrLn "First!"
        swapMVar fn (putStrLn "Again!")
            >> return ()
    
    join $ readMVar fn
    join $ readMVar fn
    join $ readMVar fn