[ghc-steering-committee] Proposal #631: Set program exit code by main return type, recommendation: accept something

Richard Eisenberg reisenberg at janestreet.com
Thu Feb 29 12:38:23 UTC 2024

I haven't followed this proposal closely. But couldn't we have a transition
period toward this eventual goal? That is, introduce a new warning, on by
default, if `main` returns anything other than `()`. That goes for a few
releases. Then we require that the return type of main has an instance of

I'm not worried about changing the behavior of programs that have type IO
ExitCode but expect the program to return 0 unconditionally; that's just
begging for confusion. I am a little worried about breaking programs that
end in an innocent-looking `return 0`, just because some other languages
like to end programs with that phrase. So I'm not sure if we should have an
instance ExitStatus Int (or instance ExitStatus Integer) -- but we probably
should. If a program ends with `return 1`, the programmer probably wants
the OS to return 1 as well.


On Thu, Feb 29, 2024 at 5:29 AM Arnaud Spiwack <arnaud.spiwack at tweag.io>

> Dear all,
> Shea Levy proposes to do something with the values returned by `main`
> https://github.com/ghc-proposals/ghc-proposals/pull/631 .
> The problem is that `main` is allowed to be of type `IO A` for any `A`.
> And GHC will simply drop the value returned by `main`. Shea contends that
> it's surprising. I agree that dropping a value without the compiler being
> explicitly instructed to is surprising. But Shea says that when `A` is
> `ExitCode` this is even more surprising. Namely `main :: IO ExitCode; main
> = return $ Failure 1` actually terminates with exit code 0. And I doubt
> that it's what anybody expects when reading the code.
> The proposal is simple, but I have a lot of comments on it. Sorry about
> that…
> Now, this sort of proposal is tricky. When the current behaviour is
> confusing, we want to change the default. But putting the new default
> behind an extension doesn't really solve the fact that there's a trap. The
> extension is, therefore, unlikely to be well tested before it becomes part
> of the next language edition.
> Shea's main proposition doesn't actually use an extension though. He adds
> a type class `ExitStatus`, and if `ExistStatus A`, then `main :: IO A` uses
> the instance to determine the exit code based on the return value.
> The only change to the current behaviour is that `main :: IO ExitCode`
> instead of always terminating with exit code 0 when returning now
> terminates with the expected error code. The argument for not putting this
> behind an extension is that virtually anybody affected by the change will
> actually have the behaviour they were expecting. But maybe the argument
> isn't strong enough (the changes may be more “interesting” if some library
> exports some `ExistStatus` instance).
> This design of this proposal is inspired by Rust's design. I've asked our
> Rust team, and they certainly seem to have internalised the idea of
> returning an exit code. It really seems a pretty natural feature to have.
> So I'm rather in favour of some flavour of the type class implementation.
> Though have a look at the alternatives, where you'll find other approaches
> such as restricting the type of `main` to unsurprising types.
> One caveat with respect to the main proposal: it is proposed that when no
> `ExistStatus A` is found, then we drop the returned value like today. I
> don't know that it's quite easy to implement this behaviour. But it can be
> recovered by a catch-all overlapping instance, so maybe it's a better way
> to specify the desired behaviour.
> --
> Arnaud Spiwack
> Director, Research at https://moduscreate.com and https://tweag.io.
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