What happens to child threads when their parent dies in GHC Haskell?

Question

The documentation for forkIO says

GHC note: the new thread inherits the masked state of the parent (see mask).

The newly created thread has an exce         


        

Answer 1

This is an answer inspired by Zeta's. It uses a free monad transformer to avoid explicit nesting of computations, and the withAsync function from the async package instead of forkRunDie.

module Main  where

import Control.Monad
import Control.Monad.Trans
import Control.Monad.Trans.Free (FreeT,liftF,iterT)
import Control.Concurrent
import Control.Concurrent.Async (withAsync)
import Control.Exception

type DaemonIO = FreeT ((,) (IO ())) IO

launch :: IO () -> DaemonIO ()
launch a = liftF (a,()) 

runDaemonIO :: DaemonIO a -> IO a
runDaemonIO = iterT $ \(action,rest) -> withAsync action $ \_ -> rest

main :: IO ()
main = do
    let delaySeconds n = threadDelay $ n * (10^6)
    runDaemonIO $ do
        launch $ (forever $ delaySeconds 1 >> print "Pseudo child 1") 
                 `finally` putStrLn "killed 1!"
        launch $ (forever $ delaySeconds 1 >> print "Pseudo child 2") 
                 `finally` putStrLn "killed 2!"
        liftIO $ delaySeconds 10
        liftIO $ putStrLn "done!!!"