Close resources in Haskell in a more-or-less safe way.

resources.hs

import           Control.Monad (unless)
import           Data.IORef

data Resource = Resource
    { resourceName   :: String
    , resourceClosed :: IORef Bool
    }

mkResource :: String -> IO Resource
mkResource name = do
    closed <- newIORef False
    let rs = Resource name closed
    _ <- mkWeakIORef closed (closeResource rs)
    return rs

closeResource :: Resource -> IO ()
closeResource rs = do
    putStrLn $ "closeResource: closing " ++ resourceName rs
    needToClose <- atomicModifyIORef (resourceClosed rs) $ \closed ->
        if closed then (True, False) else (True, True)
    if needToClose
        then putStrLn "closeResource: now closing"
        else putStrLn "closeResource: already closed!"

main :: IO ()
main = do
    rs <- mkResource "test"
    loop
    closeResource rs
  where
    loop = do
        ln <- getLine
        unless (ln == "") $ do
            putStrLn $ "Yeah, " ++ ln
            loop