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

[Simon Peyton Jones]

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.

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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