Re: [ghc-steering-committee] #583: HasField redesign, rec: accept

Sure. Just to be clear, I'm not trying to set a precedent here! Simon On Tue, 3 Oct 2023 at 09:26, Moritz Angermann wrote:

I’m not going to stay in the way of this proposal. I’hope it’s understandable if I recuse myself.

On Tue, 3 Oct 2023 at 4:18 PM, Simon Peyton Jones < simon.peytonjones@gmail.com> wrote:

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We have to balance the cost of mitigation against the cost of breakage.

- Cost of breakage. Adam has identified only two packages that woud, both of which can adapt promptly. It's true that there is more code out there, but the absence on Hackage is strongly indicative that there is unlikely to be much. - Cost of mitigation. Supporting both behaviours, having extra language flags, that must in turn themselves be deprecated away, are real costs. The suck effort away from more productive activity. - I completely agree that if we had a classification, OverloadedRecordUpdate would have been classified as Experimental, and we would not be discussing deprecation cycles. And signalling what is experimental and what is stable is a *primary *goal of the stability conversation we are having. I agree strongly with Moritz on this point. - However the user manual (Section 6.5.11 https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/overloaded_record_up...) says "*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*". Short of an official classification mechanism, which we don't have yet, it seems hard to imagine a clearer statement than that.

My judgement: in this case we should forge ahead without introducing OverloadedRecordNew and supporting both behaviours. (I have not even begun to think about how hard that would be to implement.) Let's spend our precious cycles elsewhere.

Simon

On Tue, 3 Oct 2023 at 08:48, Moritz Angermann wrote:

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Arnaud,

That is a good argument. Without an opt-in to the new syntax, they can't adapt. I guess this brings us to the extension lifecycle. A solution could be 1. Deprecate OverloadedRecordUpdate in favour of OverloadedRecordUpdateNew. The user can adapt their code and use a different Extension. 2. Eventually Have OverloadedRecordUpdateNew be a synonym for changed OverloadedRecordUpdate. And Deprecate OverloadedRecordUpdateNew over a longer period of time.

I think this actually shows the value a `--std=experimental` could have. If `OverloadedRecordUpdate` was behind `--std=experimental` breaking it would be permissible by design. And we would not have this discussion, and Adam would not need to try to find all the potential breaking codes. This is part of the proposal to make `--std=experimental` a thing, allow fast(er) paced iteration on the experimental side of the compiler, while being extremely explicit about it being experimental features that can break at any time and without warning.

The deprecation process outlined above would be fore non-experimental features. They can still be changed, but with a more rigorous change evolution.

I hope this helps to clarify my position?

Best, Moritz

On Tue, 3 Oct 2023 at 15:31, Arnaud Spiwack wrote:

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@Moritz: very clear thank you, though I'd like to understand how one could adapt during the transition period to an argument swap, would you say? Secondary question: if they can't adapt, isn't there a risk of making the breakage more painful by delaying it, and letting more people use -XOverloadedRecordUpdate with the argument order which isn't the final one?

@Adam: we have to recognise that Hackage is not all the Haskell code. I expect Hackage to be a little more conservative in extensions than end-user applications. As such we're not guaranteed an absence of breakage (it's still probably quite small, but it's hard to quantify).

On Tue, 3 Oct 2023 at 09:21, Adam Gundry wrote:

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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:

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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 mailto:arnaud.spiwack@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 mailto:eric@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 mailto:arnaud.spiwack@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 mailto:simon.peytonjones@gmail.com> wrote: >>> I support acceptance. >>> >>> Simon >>> >>> On Wed, 30 Aug 2023 at 16:09, Arnaud Spiwack mailto:arnaud.spiwack@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-re... < 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

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are generated eagerly at record definition sites, whereas the simple HasField instances can simply reuse the selectors

already

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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

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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

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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

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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/

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