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

Mon Apr 22 14:15:00 UTC 2024

Ok, so, after an extended period of time (which, obviously, is my fault),
we essentially have one option per opinion (well, I guess 3a and 4 have
two, but that's not really what consensus is like). Simon, the other day,
advised me to reduce the number of choices based on Shea's preferences. I
decided to not include Shea's original proposal of doing the backward
incompatible change without extension as I believe that it is contrary to
the spirit of the language editions and all that.

*I'm calling for a vote* on the three following options. As per our
customs, this is preference voting please order the following options. If
you want to vote against an option, rank it after “That's it” (or omit it
altogether). Explanation of the summary below

I'm leaving the vote open until *Wednesday 1st May*. After which, I'll
tally, and synthesise the committee's final position.

2. [Summary: 00WWWN] No change in behaviour, just add a warning when `main`
has a type that isn't `main :: IO ()` or `main :: IO Void`, very much
including `main :: IO ExitCode` (Shae's second favourite alternative)
3a. [Summary: -XNoWombat: 00WWWW / -XWombat: TTETET] A warning is added as
in 2, but, additionally, an extension is introduced. When the extension is
turned on, we always call the proposed `ExitStatus` type class on the
returned value to determine the program's exit code. (Shae's favourite
alternative)
4. [Summary: -XNoWombat: 00000N / -XWombat 00I00N] No warning is
introduced, but an extension is. When the extension is turned on,
everything is as today, except when `main :: IO ExitCode`, in which case we
program's exit code is the exit code returned by `main` (Simon PJ's
favourite)
That's it

------

The summaries are based on the following examples. Each get a letter
representing the behaviour under the proposal, as described in the legend

The examples:
module Ex1 where {  ..; main :: IO Void }
module Ex2 where {  ..; main :: IO () }
module Ex3 where {  ..; main :: IO Int }
module Ex4 where { ...; main :: IO ExitCode }   -- ExitCode exists already
module Ex5 where { ...; main :: IO Bool }         -- No ExitStatus instance
for Bool
module Ex6 where { ...; data T = ..; main :: IO T; instance ExitStatus T
where ... }

The legend:
- 0: main exits with exit code 0 (success)
- E: type error
- W: a warning is emitted
- T: the `ExitStatus` type class is used to select the exit code.
- I: The exit code is the returned value (only apply to `main :: IO
ExitCode`).
- N: not available under this alternative

On Thu, 28 Mar 2024 at 16:18, Arnaud Spiwack <arnaud.spiwack at tweag.io>
wrote:

> @SImonPJ I didn't include these two options because I hadn't understood
> that they had any backing.
>
> Personally: I don't like (1c) and (4)
>   - (1c) doesn't really address Shea's concern that the behaviour is
> currently surprising, as you need to actively turn an extension on to have
> the new behaviour, so you need to already know that the default behaviour
> is counterintuitive.
>   - (4) is weird without type classes. Like what happens if I `type T =
> ExitCode; main :: IO T`? Certainly `main` must not return with exit code 0.
>
> We are having an issue here, the typical bikeshedding issue I imagine,
> that there's about 1 proposal per member of the committee. I'm not sure how
> to solve this efficiently, but I don't think it'll be easy to drive
> consensus.
>
> I did ask Shea for his favoured options. He told me that if he can't have
> 1a, he prefers 3a (I promise I didn't influence him!).
>
> On Thu, 28 Mar 2024 at 09:18, Simon Marlow <marlowsd at gmail.com> wrote:
>
>> I think we can discount 1a because it doesn't satisfy the stability
>> principles, right?
>>
>> Out of the others, I would probably go with 1b or 3a as the most
>> predictable behaviours. I also like Simon's (4) (gated by an extension,
>> that we hope to enable in GHC2027).
>>
>> Cheers
>> Simon
>>
>> On Tue, 26 Mar 2024 at 09:35, Arnaud Spiwack <arnaud.spiwack at tweag.io>
>> wrote:
>>
>>> Alright, so here are the plausible alternatives
>>>
>>> 1a. New type-class-based behaviour without extension
>>> 1b. New type-class-based behaviour gated by an extension
>>> 2. Just a warning (when main isn't at type IO () or IO Void)
>>> 3a. A warning + the new type-class-based behaviour gated by an
>>> extension. With the extension, types that don't implement the type class
>>> raise an error.
>>> 3b. A warning + the new type-class-based behaviour gated by an
>>> extension. With the extension, types that don't implement the type class
>>> raise a warning (which could have a different phrasing than without the
>>> extension).
>>>
>>> Let's vote!
>>>
>>> On Fri, 22 Mar 2024 at 15:30, Malte Ott <malte.ott at maralorn.de> wrote:
>>>
>>>> On 2024-03-22 08:58, Arnaud Spiwack wrote:
>>>> > @Malte, in my opinion, with the extension on, types which are not
>>>> covered
>>>> > by the type class should error out.
>>>>
>>>> Ah, I see. Well, I am fine either way.
>>>>
>>>> I just don’t see much value in deciding for the user which code
>>>> problems are
>>>> unacceptable. Especially since this will make the corresponding language
>>>> extension more breaking and thus harder to make the default.
>>>> Others have voiced similar opinions in the GitHub thread.
>>>>
>>>> Best
>>>> Malte
>>>> _______________________________________________
>>>> ghc-steering-committee mailing list
>>>> ghc-steering-committee at haskell.org
>>>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>>>>
>>>
>>>
>>> --
>>> Arnaud Spiwack
>>> Director, Research at https://moduscreate.com and https://tweag.io.
>>> _______________________________________________
>>> ghc-steering-committee mailing list
>>> ghc-steering-committee at haskell.org
>>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>>>
>>
>
> --
> Arnaud Spiwack
> Director, Research at https://moduscreate.com and https://tweag.io.
>

-- 
Arnaud Spiwack
Director, Research at https://moduscreate.com and https://tweag.io.
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