Feedback on potential change in boot library

[Adam Gundry]

It's not obvious to me that this is a good idea. Users already complain 
that "upgrading GHC broke my code", when in practice this frequently 
means "when upgrading GHC I used new boot library versions with 
incompatible APIs that broke my code". So having a policy of introducing 
more frequent API changes (and making it harder to switch between GHC 
releases in a single release series) needs clear justification IMHO.

(Of course in the long term we should make it easier for users to pin 
boot library APIs while upgrading GHC independently, but we're not there 
yet.)

I think that by default, it makes sense for a new minor GHC release to 
keep the same API versions of boot libraries, but bump to the latest 
super-minor component (i.e. for an A.B.C.D version, keep A.B.C the same 
but bump D). If a boot library fixes a bug that is critical enough to 
require bumping the version distributed with GHC, ideally there would be 
an API-compatible release of the boot library. Of course that might not 
always be feasible, and it is ultimately a judgement call for the GHC 
maintainers as to which boot library versions are best to ship.

Cheers,

Adam



On 26/11/2024 22:27, Hécate via ghc-devs wrote:
>  From the cabal perspective this great news, as this will allow the 
> feedback cycle between our two projects to be shorter. Glad to see the 
> GHC team is considering this!
> 
> Cheers,
> Hécate
> 
> Le 26/11/2024 à 21:14, Ben Gamari a écrit :
>> tl;dr. We propose that GHC more aggressively bump its boot library
>>         dependencies.
>>
>>
>> Hello all,
>>
>> Historically, GHC minor releases have been quite conservative in bumping
>> boot library versions, generally defaulting to retaining the versions
>> used in the GHC-x.y.1 release unless
>>
>>   * a request is made by a boot library maintainer, or
>>   * we discover an issue which warrants a bump
>>
>> The motivation for this policy is both to
>>
>>   * minimize the potential for regressions in correctness and
>>     performance, and
>>
>>   * reduce the potential for end-user breakage due to interface changes
>>     as even minor releases can result in considerable impact (e.g.
>>     #25411, #22633)
>>
>> While this policy has served us well, its conservative nature demands
>> the vigilence of both GHC and upstream maintainers to ensure that bumps
>> that are truly necessary do not fall through the cracks.
>>
>> As the flaws of this process have been more apparent recently (e.g.
>> #24597), we have done a bit of reflection on how we might improve the
>> status quo. Concretely, I would like feedback on the adopting the
>> following policy going forward:
>>
>>> Unless guidance is provided otherwise by a library maintainer, a GHC
>>> x.y.z release will attempt to ship the newest minor version of each
>>> boot libray in the same major series shipped with GHC x.y.1.
>> I believe this policy would leave less room for human error and open
>> opportunities for automated checking. On the other hand, the more
>> aggressive bumping of submodules may mean that users are hit with more
>> (usually minor) compatibility issues when moving between minor GHC
>> releases.
>>
>> How do others feel about this? We are particularly interested to hear
>> from boot library maintainers but feedback from packagers and users is
>> also very much welcome.
>>
>> Cheers,
>>
>> - Ben
>>

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

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