[ghc-steering-committee] #583: HasField redesign, rec: accept
Adam Gundry
adam at well-typed.com
Tue Oct 3 07:21:07 UTC 2023
The backwards compatibility impact here is really extremely small,
because OverloadedRecordUpdate is essentially unusable at present (as
well as being explicitly documented as subject to change), so nobody
uses it. While someone could implement a warning when the extension is
turned on to say that it will change in the future, I'm not sure I see
much point.
I used https://hackage-search.serokell.io/?q=OverloadedRecordUpdate to
look for occurrences of OverloadedRecordUpdate on Hackage, and I found
precisely one pair of packages using it "for real":
large-anon-0.3.0
large-records-0.4
These packages were created by my colleague Edsko in discussion with me,
and will need to be amended when the proposal is implemented, because
they want to take advantage of the new functionality.
The other packages on Hackage containing the string
OverloadedRecordUpdate are tools that reference all extensions:
Cabal-syntax-3.10.1.0
extensions-0.1.0.0
fourmolu-0.14.0.0
ghc-9.6.3
ghc-boot-th-9.6.3
ghc-exactprint-1.7.0.1
ghc-hs-meta-0.1.2.0
ghc-lib-9.6.2.20230523
ghc-lib-parser-9.6.2.20230523
hackport-0.8.4.0
haskell-src-meta-0.8.12
hindent-6.1.0
hlint-3.6.1
ormolu-0.7.2.0
plus there are a few references in comments:
lifx-lan-0.8.2
optics-core-0.4.1.1
pvector-0.1.1
record-dot-preprocessor-0.2.16
tztime-0.1.1.0
While I generally agree with the "don't abruptly break existing code"
position, in this case I don't think there is code out there to break.
Adam
On 03/10/2023 01:14, Moritz Angermann wrote:
> Arnaud,
>
> thank you for your write up. And sorry that my view seems to be not
> clear. Let me try to explain.
> My position is: I'm against anything that _abruptly_ breaks existing
> code. That's basically all there is to it.
> Therefore I'm strongly against breaking changes without an appropriate
> warning period (from the compiler).
> I also strongly believe that most people do not read documentation, and
> the authoritative messages are
> the ones the compiler produces. As I've alluded to in a different email,
> there are lots of people who work
> on software as a 9-5 job, and don't spend their freetime tinkering and
> playing with languages. They just
> want to get the job done they are assigned. I have a lot of respect for
> them. In my experience they don't
> read compiler release announcements, or even go and read the compiler
> documentation. They are given
> the compiler version the company decided on for use in production.
> That's the tool they use. And that tool
> in my opinion needs to be rock solid, and not break easily across
> versions. Thus yes, I would very much
> like to see us not have breaking changes at all, but I can see that we
> may need breaking changes
> occasionally. In those cases I want it to be very visible to the
> consumers that this breaking change is
> coming towards them (compiler warnings). Giving them some time to adjust
> (migration period), until the
> breaking change happens. Ultimately we should be able to compiler
> existing code that compiles today
> with at least the next compiler without it rejecting the code or needing
> modifications to the code (-Werror
> excluded).
>
> Thus what I'm arguing for is:
> - Let's implement this backwards compatibility.
> - Add compiler warnings about the arguments being swapped in a future
> GHC version. For _a least_ one major release.
> - Make the breaking change in a subsequent release.
>
> Alternatively I could also see:
> - Adding compiler warnings now that the arguments will be swapped in a
> future GHC version (for at least one major release).
> - Implement the breaking change in a subsequent release.
>
> Either of those would be ok with me. Implementing a breaking change from
> one version to the next, without an
> appropriate deprecation/migration period (that is, the compiler will
> warn loudly that changes are coming) is something
> I am _very_ vehemently against.
>
> If the migration/deprecation warnings would provide a link to a GitHub
> ticket or something where more information
> can be found and maybe even a discussion could be had would probably
> also be a good idea.
>
> I hope this helps clarify my position? If not, feel free to ask more,
> I'm also happy to jump onto a call to explain my
> position if needed.
>
> Best,
> Moritz
>
>
> On Mon, 2 Oct 2023 at 23:31, Arnaud Spiwack <arnaud.spiwack at tweag.io
> <mailto:arnaud.spiwack at tweag.io>> wrote:
>
> Sorry I took a little bit of time to react to this, it was a lot to
> take in and I didn't have the mental space last week.
>
> The only person that may have spoken against the current state of
> the proposal is Moritz. Yet I realise that I don't actually know,
> Moritz, what your position is.
>
> To recap: to use -XOverloadedRecordUpdate in current GHC, you need
> to use -XRebindableSyntax and provide a setField function. In the
> new proposal you can use -XOverloadedRecordUpdate without
> -XRebindableSyntax, but when -XRebindableSyntax is on, the setField
> function that you have to provide has its argument swapped. The
> current documentation of OverloadedRecordUpdate has the following
> text at the top “*EXPERIMENTAL* /This design of this extension may
> well change in the future. It would be inadvisable to start using
> this extension for long-lived libraries just yet./”.
>
> Now, I don't quite see how we could have a transition period that
> would allow a smooth transition there. There is no piece of code,
> with RebindableSyntax, that would compile before and after the
> change. So unless I'm missing something the position we can take as
> a committee can be either
> - Let's have the breakage without a transition period
> - Let's not make the breaking change ever and use the earlier
> argument order for set
>
> Which one do you argue for, or am I missing another option?
>
> On Sun, 24 Sept 2023 at 15:36, Eric Seidel <eric at seidel.io
> <mailto:eric at seidel.io>> wrote:
>
> I am in favor of this proposal.
>
> On Thu, Sep 21, 2023, at 03:37, Arnaud Spiwack wrote:
> > Dear all.
> >
> > I submitted my recommendation 3 weeks ago, and only Simon has
> commented
> > yet. Please let me know your thoughts.
> >
> > On Wed, 6 Sept 2023 at 16:27, Arnaud Spiwack
> <arnaud.spiwack at tweag.io <mailto:arnaud.spiwack at tweag.io>> wrote:
> >> Dear all,
> >>
> >> Don't forget to opine here. To reiterate, I really don't
> expect the proposal to be controversial. The text of the
> proposal is rather long, but is made easy to read. So it
> shouldn't take too much of your time.
> >>
> >> /Arnaud
> >>
> >> On Thu, 31 Aug 2023 at 01:03, Simon Peyton Jones
> <simon.peytonjones at gmail.com
> <mailto:simon.peytonjones at gmail.com>> wrote:
> >>> I support acceptance.
> >>>
> >>> Simon
> >>>
> >>> On Wed, 30 Aug 2023 at 16:09, Arnaud Spiwack
> <arnaud.spiwack at tweag.io <mailto:arnaud.spiwack at tweag.io>> wrote:
> >>>> Dear all,
> >>>>
> >>>> [ Proposal #583
> https://github.com/ghc-proposals/ghc-proposals/pull/583
> <https://github.com/ghc-proposals/ghc-proposals/pull/583> ]
> >>>>
> >>>> Our own Adam proposes to amend the design of the highly
> experimental OverloadedRecordUpdate extension as had been
> designed in proposal #158 [
> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0158-record-set-field.rst <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0158-record-set-field.rst> ] and #405 [ https://github.com/ghc-proposals/ghc-proposals/pull/405 <https://github.com/ghc-proposals/ghc-proposals/pull/405> ].
> >>>>
> >>>> Specifically, Adam proposes a modification of the type
> classes that would back the extension.
> >>>>
> >>>> In the previous design the HasField class is defined as a
> lens:
> >>>>
> >>>> class HasField (n :: k) r a | r n -> a
> >>>> hasField :: r -> (a -> r, a)
> >>>>
> >>>> The proposal is to replace it by two classes (slightly
> simplified)
> >>>>
> >>>> class HasField (n :: k) r a | r n -> a
> >>>> hasField :: r -> a
> >>>>
> >>>> class SetField (n::k) r a | r n -> a
> >>>> modifyField :: (a -> a) -> r -> a
> >>>> setField :: a -> r -> a
> >>>>
> >>>> This is originally motivated by some performance
> consideration: the prototype implementation of HasField as a
> lens can be very time consuming because instances of HasFields
> are generated eagerly at record definition sites, whereas the
> simple HasField instances can simply reuse the selectors already
> generated by GHC. But a lot of thoughts have been put into the
> new design, and my summary can certainly not do it justice: the
> proposal is very well argumented.
> >>>>
> >>>> A point I'll make here is that the new design is actually
> parametric in the data representation of the field type.
> Something that wasn't possible in the original design.
> >>>>
> >>>> This proposal is not technically backward compatible,
> because the order of argument in which OverloadedRecordUpdate
> expects the argument of setField is changed. This is not
> essential to the proposal, but this is a more consistent order
> argument with the rest of Haskell. And considering that
> OverloadedRecordUpdate is very loudly advertised as
> experimental, I recommend accepting this breakage.
> >>>>
> >>>> Overall the proposal is actually more backward compatible
> with GHC 9.8 than the original design, as the HasField class is
> left unchanged.
> >>>>
> >>>> Overall, the proposal looks quite reasonable to me, and
> well-argued. I recommend acceptance.
--
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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