How is ExitCode handled?

[Tom Ellis]

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.html#exitWith

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.html#setUncaughtExceptionHandler

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.Internal.TopHandler.html#topHandler

>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
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)