From my experiements with a small program (below) I conclude that the default handler is installed on every thread, and real_handler is installed *on the main thread only* *below* the default handler, so

Hello all, I'm not sure I understand how an `ExitCode` exception is used to terminate a running program. Can anyone clarify? When we call `exitWith`, what we're doing is `throw`ing an `ExitCode` exception: https://www.stackage.org/haddock/lts-22.38/base-4.18.2.1/src/System.Exit.htm... That exception must be caught somewhere, and used to cause the running program to exit with the exit code in question. But where? It's not in the default `uncaughtExceptionHandler`: https://hackage.haskell.org/package/base-4.20.0.0/docs/src/GHC-Conc-Sync.htm... That doesn't do anything special with exit code. It seems like real_handler does: https://hackage.haskell.org/package/ghc-internal-9.1001.0/docs/src/GHC.Inter... that if an `ExitCode` exception is thrown the default handler never gets a chance to see it. Is that right? Thanks, Tom {-# LANGUAGE GHC2021 #-} {-# LANGUAGE LambdaCase #-} import System.Environment (getArgs) import System.Exit (ExitCode (ExitFailure)) import Control.Exception (Exception, SomeException (SomeException), throw) import GHC.Conc (forkIO, setUncaughtExceptionHandler, threadDelay) data MyEx = MyEx deriving Show instance Exception MyEx myHandler (SomeException e) = putStrLn $ "In myHandler: " <> show e run f = do setUncaughtExceptionHandler myHandler f main = do getArgs >>= \case -- Exits with exit code 1 ["1"] -> run (throw (ExitFailure 1)) -- Print: In myHandler: MyEx -- Exits with exit code 1 ["2"] -> run (throw MyEx) -- Print: In myHandler: ExitFailure 1 -- Exits with exit code 0 ["3"] -> run (forkIO (throw (ExitFailure 1))) -- Print: In myHandler: MyEx -- Exits with exit code 0 ["4"] -> run (forkIO (throw MyEx)) _ -> error "Need an argument" threadDelay (1000 * 1000)