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

[Simon Peyton Jones]

>
> I left out a key part of my last email -- apologies. I'm floating a
> counter-proposal where we *require* an instance of ExitStatus on the return
> type of `main`, with a transition period. In contrast, my understanding of
> the proposal written is that it would use such an instance if it exists,
> but issue a warning if it doesn't, in perpetuity.


Ah  I had not realised that.

But why?

Rather than answer here (private to SC) why don't you put your proposal on
the discussion thread, say why, and invite feedback.

Simon


On Mon, 4 Mar 2024 at 19:24, Richard Eisenberg <reisenberg at janestreet.com>
wrote:

> I left out a key part of my last email -- apologies. I'm floating a
> counter-proposal where we *require* an instance of ExitStatus on the return
> type of `main`, with a transition period. In contrast, my understanding of
> the proposal written is that it would use such an instance if it exists,
> but issue a warning if it doesn't, in perpetuity.
>
> Richard
>
> On Mon, Mar 4, 2024 at 6:14 AM Simon Peyton Jones <
> simon.peytonjones at gmail.com> wrote:
>
>> 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
>>
>>
>> The proposal specifies that such a program returns `ExitSuccess`, but
>> adds a warning. That seems OK to me; it does not break the program.
>>
>> Oh -- maybe you mean that `return 1` means "return with exit code 1"
>> today.  Is that really true?  I don't think so.
>>
>> Overall this proposal seems fine to me.  I'd be happy to see it done.
>>
>> Simon
>>
>> On Thu, 29 Feb 2024 at 12:38, Richard Eisenberg <
>> reisenberg at janestreet.com> wrote:
>>
>>> 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 ExitStatus.
>>>
>>> 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.
>>>
>>> Richard
>>>
>>> On Thu, Feb 29, 2024 at 5:29 AM Arnaud Spiwack <arnaud.spiwack at tweag.io>
>>> wrote:
>>>
>>>> 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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>>>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>>>>
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>>
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