How is ExitCode handled?

[Tom Ellis]

On Sun, Oct 20, 2024 at 01:06:01PM +0100, Tom Ellis wrote:
> 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.

Oh, I see that the uncaught exception handler is called in
`reportError`, itself used by `real_handler`, thence `child_handler`,
and `forkIO`.

    https://hackage.haskell.org/package/ghc-internal-9.1001.0/docs/src//GHC.Internal.Conc.Sync.html#reportError

It's also used by the `real_handler` in TopHandler.hs, and thence
`topHandler` and `runIO`.

    https://hackage.haskell.org/package/ghc-internal-9.1001.0/docs/src/GHC.Internal.TopHandler.html

So the uncaught exception handler is not really "installed", it's just
used by the handlers installed by `forkIO` and `runMainIO` (the latter
wraps main, as per the documentation).

So I think I probably understand what's going on now.

Tom