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

Tue Sep 19 16:20:44 UTC 2023

Is the general consensus on this thread that defaults should never change,
even if better defaults are discovered?

On Tue, 19 Sept 2023 at 17:13, Chris Dornan <chris at chrisdornan.com> wrote:

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