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

[Adam Gundry]

Dear Committee,

It seems we are somewhat split on the -Wsevere proposal, even assuming 
it is introduced with a deprecation period (changing the warning text 
and adding the group in advance). There is consensus that adding the 
group by itself is fine, and potentially enabling -Werror=severe under 
GHC2024, but enabling it by default for existing code is more controversial.

Ultimately this is a judgement call about the value of the proposal 
versus the breakage it causes. I remain of the view that it is 
worthwhile. This is not merely about more aggressively enforcing best 
practice. Rather, it eliminates the risk of the following:

* Suppose package A defines a type class, package B depends on package 
A, and package C depends on package B.

* Now package A extends the type class definition with a new method and 
default methods. Everything still compiles; package B now issues a 
-Wmissing-methods warning (but is not actively maintained, and the 
author of package C is unlikely to look at warnings in all their 
dependencies).

* Users of package C have to try to debug an infinite loop.

Given that this change avoids a significant issue, affected code has 
been issuing warnings for many years, and impacted users can very easily 
set -Wwarn=severe either at the package level or the project level, I 
think is worth accepting the backwards compatibility cost and the fact 
that some Haskell2010 code will no longer be accepted by default.

Matt Parsons puts it well in the CLC thread, which is pretty clearly in 
favour of this proposal overall 
(https://github.com/haskell/core-libraries-committee/issues/207#issuecomment-1726091889):

 > I think GHC should strive to make fewer breaking changes, and make 
those changes as easy-to-adopt as possible. But this is not a question 
of GHC breaking things, but rather revealing and providing 
early-diagnosis for already broken things.

Further opinions are welcome, of course.

Adam


On 14/09/2023 09:32, Adam Gundry wrote:
> Dear Committee,
> 
> Joachim, along with Oleg Grenrus, proposes to change -Wmissing-methods 
> and -Wmissing-fields warnings into errors by default (retaining the 
> option to downgrade them). I recommend we accept the proposal.
> 
> Proposal: https://github.com/ghc-proposals/ghc-proposals/pull/571
> Rendered: 
> https://github.com/ghc-proposals/ghc-proposals/blob/wsevere/proposals/0000-severe-warnings.rst
> 
> This is primarily motivated by the fact that when classes have default 
> methods, missing methods can lead to runtime loops, which are generally 
> difficult to debug. Since in practice not all users pay attention to 
> warnings that do not inhibit compilation, it makes sense to identify a 
> class of warnings that are sufficiently serious to require explicit 
> action from the user to silence them.
> 
> Since these warnings are currently not errors by default, library 
> authors experimentally assessing the impact of changes may be lead to 
> assume that introducing new methods/fields does not lead to breakage 
> (because downstream code will still compile). The proposal thus makes it 
> more obvious that adding a new method or field is a breaking change.
> 
> The proposal deliberately causes builds to fail by default for some 
> libraries that currently emit warnings. Oleg has kindly performed impact 
> assessments to identify such libraries, and the breakage of a few 
> packages seems worth the cost.
> 
> It is easy to restore the warnings to their previous classification by 
> passing an option at build time, e.g. using -Wno-error=missing-methods. 
> Users can set such an option in cabal.project or stack.yaml to work 
> around breakage that is not promptly fixed by the library author.
> 
> This change does mean that GHC with -XHaskell98/2010 will by default 
> reject some programs that are explicitly permitted by the Haskell98/2010 
> specification. I recommend we document this infelicity, but accept it, 
> as much of the benefit of the proposal is that it applies by default.
> 
> The proposal establishes the precedent that some warnings may be treated 
> as errors by default, and introduces a warning group -Wsevere to 
> classify them. This seems conceptually useful and gives us the option to 
> extend the -Wsevere set in the future (e.g. as a final stage of 
> deprecation before a feature is removed).
> 
> Thoughts?
> 
> Adam
> 
> 
> On 11/09/2023 20:25, Joachim Breitner wrote:
>> Dear Committee,
>>
>> based on suggestions by Oleg Grenrus, I wrote a proposal to introduce a
>> warning group -Wsevere for on-by-defaults, error-by-default warnings,
>> and initially fill it with missing-methods and missing-fields.
>>
>>
>> https://github.com/ghc-proposals/ghc-proposals/pull/571
>>
>> https://github.com/ghc-proposals/ghc-proposals/blob/wsevere/proposals/0000-severe-warnings.rst
>>
>> I’d like to nominate Adam as the shepherd, who already reviewed it a
>> bit on Github.
>>
>> Please guide us to a conclusion as outlined in
>> https://github.com/ghc-proposals/ghc-proposals#committee-process
>>
>>
>> Cheers,
>> Joachim


-- 
Adam Gundry, Haskell Consultant
Well-Typed LLP, https://www.well-typed.com/

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