[ghc-steering-committee] #571: -Wsevere, Shepherd: Adam (rec: accept)

Tue Sep 19 15:13:35 UTC 2023

My opposition is entirely grounded in the change of default behaviour;
otherwise i am in favour.

On Tue, 19 Sep 2023 at 15:31, Simon Marlow <marlowsd at gmail.com> wrote:

> On Tue, 19 Sept 2023 at 15:26, Simon Peyton Jones <
> simon.peytonjones at gmail.com> wrote:
>
>> I think that the motivation for this proposal is to make it harder to
>> shoot yourself in the foot.
>>
>> Maybe implementing this "severe" *category*, but not changing its *default
>> *to error, would get us some of the way there?  Then "best-practice
>> guidance" could be "use -Werror=severe", and job done.  That's a bit easier
>> to say than saying "use -Werrror=missing-methods -Werror= ..." etc.
>>
>
> Absolutely! I'm only objecting to changing the default. Adding the
> "severe" category by itself is useful, I agree.
>
> Cheers
> Simon
>
>
>>
>> Simon
>>
>> On Tue, 19 Sept 2023 at 14:35, Chris Dornan <chris at chrisdornan.com>
>> wrote:
>>
>>> Like Simon M I habitually develop with -Wall -Werror, and like Moritz I
>>> think we really need to be very careful about deliberately breaking
>>> packages.
>>>
>>> For sure, if we were starting anew I would be be sympathetic to treating
>>> them as errors, but, for me, that isn't a good enough reason to make this
>>> breaking change.
>>>
>>> For this reason I vote against this proposal.
>>>
>>> Chris
>>>
>>> On 19 Sep 2023, at 14:20, Simon Marlow <marlowsd at gmail.com> wrote:
>>>
>>> For those not aware, Hackage right now rejects packages with `-Wall
>>> -Werror` in their ghc-options because warnings change between GHC versions
>>> so this tends to lead to unnecessary breakage. I think that's a good
>>> policy, even though I use `-Wall -Werror` everywhere when developing.
>>>
>>> Interestingly, this proposal creates exactly the same kind of risk, by
>>> making some existing warnings errors by default and introducing the
>>> possibility that the set of warnings treated this way might change in the
>>> future. Admittedly it's a smaller risk than `-Wall -Werror`, but it's still
>>> a risk for developers.
>>>
>>> Also note that `ghc -XHaskell2010` will reject some legal Haskell2010
>>> programs, unless you also say `-Wwarn=severe`. We are normally careful to
>>> document the ways in which GHC deviates from the language definition in the
>>> user guide.
>>>
>>> I can see the motivation, but I have to vote against here. I don't think
>>> we should change the set of programs accepted by the compiler unless
>>> absolutely necessary. If it's legal code today, it should be accepted by
>>> future versions of the compiler unless we have a really good reason to
>>> change that.
>>>
>>> Cheers
>>> Simon
>>>
>>>
>>> On Tue, 19 Sept 2023 at 08:53, Moritz Angermann <
>>> moritz.angermann at gmail.com> wrote:
>>>
>>>> Just to clarify: I am not against change, or evolution.  I'm actually
>>>> looking forward to progress. What I am against ist sudden breakage.
>>>> As such, if there _is_ breakage (clc stackage is a subset), we have to
>>>> assume there will be breakage in production codebases, most
>>>> of which are likely not public.
>>>>
>>>> Can't we have `-Wcompat` enable `-Werror=missing-methods`, and
>>>> `-Werror=missing-fields` (I guess that's the same as `-Werror=sever`?)
>>>> Advertise this prominently in the release notes for GHC 9.10? And then
>>>> enable this fully in GHC 9.14? Though I guess the flag we want
>>>> is really `-Wcompat-error`, or we rather change the notion of -Wcompat
>>>> to also promote warnings to errors early? In any case either the
>>>> current documentation for -Wcompat would need to be adjusted, or we'd
>>>> need something that signals new errors.
>>>>
>>>> Ideally I'd like to see something like a warning for `missing-methods`,
>>>> with an additional note that this will become an error in GHC X.Y,
>>>> and that one can opt into this behaviour by enabling -Wcompat.
>>>>
>>>> My test for support is generally: can I take existing code unmodified,
>>>> swap out the compiler, and it will still compile? That way I can report
>>>> back regressions, bugs, ... early on during alphas, betas, and release
>>>> candidates. Right now I can't. I usually have to wait for x.y.4+. That
>>>> also means the feedback for anyone working on GHC is terrible. You
>>>> won't hear about bugs until late in the release cycle where the
>>>> master branch has moved forward by a lot. At the same time it's painful
>>>> for integrators who end up having to backport and patch old
>>>> branches. https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status
>>>> already states anything but 9.4 and 9.6 won't see any further releases.
>>>> Our current production compiler is 8.10, we could not switch to 9.2 due
>>>> to performance regressions. And finally have almost everything
>>>> compiling with 9.6, but are far from having any form of performance
>>>> profile feedback on 9.6 yet.
>>>>
>>>> Again, I'm not against breakage per-se. I'm against sudden breakage.
>>>> Managed evolution or however we want to call it, is something
>>>> I'd absolutely support!
>>>>
>>>> Best,
>>>>  Moritz
>>>>
>>>> On Tue, 19 Sept 2023 at 15:15, Adam Gundry <adam at well-typed.com> wrote:
>>>>
>>>>> On 18/09/2023 20:28, Richard Eisenberg wrote:
>>>>> >
>>>>> > Bottom line for me: I think we should implement and then experiment.
>>>>> > Given the potentially delicate nature of this, I might even advocate
>>>>> for
>>>>> > implementing this in a release branch, so that as much of Hackage as
>>>>> > possible actually has a hope of compiling. Then test to see where
>>>>> the
>>>>> > breakage occurs. If were happy with the result, rebase the
>>>>> > implementation on master. But I don't want us to get into a state
>>>>> where
>>>>> > we accept, implement, observe moderate breakage, and then blast
>>>>> ahead
>>>>> > because the committee approved the idea.
>>>>>
>>>>> The breakage concern is worth thinking about, I agree, but fortunately
>>>>> in this instance we don't need to wait for an implementation to run an
>>>>> experiment. The change can be relatively effectively simulated by
>>>>> compiling with -Werror=missing-methods -Werror=missing-fields, and
>>>>> indeed Oleg has done so already for clc-stackage as he reports here:
>>>>>
>>>>>
>>>>> https://github.com/ghc-proposals/ghc-proposals/issues/544#issue-1410125536
>>>>>
>>>>>
>>>>> https://github.com/ghc-proposals/ghc-proposals/issues/544#issuecomment-1279948737
>>>>>
>>>>> Out of nearly 3000 packages, he found 22 were broken by
>>>>> -Werror=missing-methods and 9 by -Werror=missing-fields.
>>>>>
>>>>> Adam
>>>>>
>>>>>
>>>>> --
>>>>> Adam Gundry, Haskell Consultant
>>>>> Well-Typed LLP, https://www.well-typed.com/
>>>>>
>>>>> Registered in England & Wales, OC335890
>>>>> 27 Old Gloucester Street, London WC1N 3AX, England
>>>>>
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>>>>>
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