[ghc-steering-committee] #370: Syntax for Modifiers, Recommendation: Acceptance

Fri Dec 11 21:39:39 UTC 2020

On 11 Dec 2020 at 21:35:20, Richard Eisenberg <rae at richarde.dev> wrote:

> Sorry -- I've lost track here a bit. What's the issue with the grammar?
>

In this comment
https://github.com/ghc-proposals/ghc-proposals/pull/370#issuecomment-741785189
:

Syntax: the form btype [ {modifier} -> type ] seems to allow one or zero
> modifiers but not two. Or do I misunderstand the braces? Spell it out please
>

Regards,
Alejandro

> I'm quite happy to label this experimental. The key aspect of the proposal
> is just to outline a way forward with adding syntax.
>
> Richard
>
> On Dec 10, 2020, at 4:29 AM, Alejandro Serrano Mena <trupill at gmail.com>
> wrote:
>
> Should we mark them as “accepted” with the following text?
>
> The Committee accepts this proposal as experimental. This means that the
>> Committee expects changes to this feature in the future, maybe as the
>> result of other accepted proposals.
>>
>
> If you agree, then we can accept this proposal once a little remaining
> issue with the grammar has been clarified.
>
> Regards,
> Alejandro
>
>
> On 10 Dec 2020 at 09:59:02, Simon Peyton Jones <simonpj at microsoft.com>
> wrote:
>
>> That was the point of my previous email: accept, and
>> accept-as-experimental are actually one and the same.
>>
>> Objectively, yes.  But I think it helps users to advertise a feature as
>> experimental.  It’s a signal to users that this feature is, well,
>> experimental.  It is more likely to change.
>>
>> It’s only an indication not a clear distinction.  But I find it helpful.
>>
>> For me, it also reflects how I evaluate the proposal.  For a change to a
>> well-established feature, we have a lot of experience in how people use
>> that feature.
>>
>> For experimental features we don’t.  Example: defaulting for matchability
>> for unsaturated type families.  We don’t have unsaturated type families
>> right now, so we don’t have any code that uses them, and hence zero
>> in-the-wild experience about matchability defaulting.  We shouldn’t spend
>> ages arguing the toss – just trust Csongor’s judgement and give it a try,
>> but advertising that details may change.
>>
>> Simon
>>
>>
>> *From:* Spiwack, Arnaud <arnaud.spiwack at tweag.io>
>> *Sent:* 10 December 2020 08:06
>> *To:* Alejandro Serrano Mena <trupill at gmail.com>
>> *Cc:* Richard Eisenberg <rae at richarde.dev>;
>> ghc-steering-committee at haskell.org; Simon Peyton Jones <
>> simonpj at microsoft.com>
>> *Subject:* Re: [ghc-steering-committee] #370: Syntax for Modifiers,
>> Recommendation: Acceptance
>>
>>
>> That was the point of my previous email: accept, and
>> accept-as-experimental are actually one and the same.
>>
>>
>> What Simon is driving at, I think, is: depending on what the proposal is
>> about, we want to apply more or less strict standard of acceptance (if a
>> proposal is about fixing something in an existing feature, then we better
>> be rather sure that it is indeed an improvement; but if it's about adding
>> something new in an untrodden territory, then we can't really be sure, and
>> it's worth experimenting with).
>>
>>
>> On Wed, Dec 9, 2020 at 9:17 PM Alejandro Serrano Mena <trupill at gmail.com>
>> wrote:
>>
>> Dear all,
>>
>> As the shepherd of this proposal, I’m quite confused about what the
>> outcome should be. The ghc-proposals README states that:
>>
>>
>> Acceptance of the proposal implies that the implementation will be
>> accepted into GHC provided it is well-engineered, well-documented, and does
>> not complicate the code-base too much.
>>
>>
>>
>> Most of the Committee seems to lean towards “this seems OK right now, but
>> we don’t want to be locked” or “mark this as experimental”. However,
>> there’s no such notion as “accept as experimental”. Furthermore, as it
>> stands the proposal gives some syntax, and then asks any new extensions to
>> use that syntax; so it cannot be completely thought as a feature by itself.
>>
>>
>> Regards,
>>
>> Alejandro
>>
>> On 9 Dec 2020 at 15:59:43, Spiwack, Arnaud <arnaud.spiwack at tweag.io>
>> wrote:
>>
>> It's always possible to change. I don't think accepting a proposal means
>> (or ought to mean) that we are locked into anything. Accepting a proposal
>> means that we won't oppose a design-related argument to a PR that
>> implements (part or all of) an accepted proposal.
>>
>>
>> I don't know how to quantify the degree of confidence that we have in the
>> stability of a proposal. Here we are all saying: this is better than
>> anything so far, and we rather need something like this to be a thing, but
>> it's really a shot in the dark. And this lack of confidence will be
>> reflected in the manual description. But even if we are confident in the
>> stability of a proposal, it may very well happen that it changes
>> dramatically, even soon.
>>
>>
>> On Wed, Dec 9, 2020 at 2:55 PM Simon Peyton Jones via
>> ghc-steering-committee <ghc-steering-committee at haskell.org> wrote:
>>
>> I’ve replied on GitHub.  Generally in favour.  But mark it as
>> experimental… I don’t want to be locked into “we decided on this in Dec
>> 2020 so now it’s too late”.  WE can learn from experience.
>>
>>
>> Simon
>>
>>
>> *From:* ghc-steering-committee <
>> ghc-steering-committee-bounces at haskell.org> *On Behalf Of *Alejandro
>> Serrano Mena
>> *Sent:* 03 December 2020 20:17
>> *To:* Richard Eisenberg <rae at richarde.dev>
>> *Cc:* ghc-steering-committee at haskell.org
>> *Subject:* Re: [ghc-steering-committee] #370: Syntax for Modifiers,
>> Recommendation: Acceptance
>>
>>
>> Dear Committee,
>>
>>
>> Richard has requested for us to consider the new version of this
>> proposal. As opposed to the previous version, this one is only about
>> reserving syntax for “modifiers”, which at the beginning would be used for
>> things like linearity or matchability of arrows.
>>
>>
>> I think this is a good proposal, and one which would save us from
>> re-considering syntax for every possible extension (and if linearity
>> appears before the arrow and matchability after it, where would a future
>> dimension go?). Thus I keep recommending acceptance on this new incarnation.
>>
>>
>> Regards,
>>
>> Alejandro
>>
>>
>> On 30 Nov 2020 at 20:52:26, Richard Eisenberg <rae at richarde.dev> wrote:
>>
>> To my surprise, I found myself leaning against. So I updated and
>> simplified the proposal to remove Modifier. This makes modifiers a bit more
>> magical, but more likely to actually work in practice. The type inference
>> story previously may have been intractable.
>>
>>
>> I've requested that the committee consider the updates in parallel with
>> community feedback.
>>
>>
>> Thanks,
>>
>> Richard
>>
>>
>> On Nov 30, 2020, at 2:36 PM, Alejandro Serrano Mena <trupill at gmail.com>
>> wrote:
>>
>>
>> After some discussion in the GitHub thread, changes are going to arrive
>> to the proposal. I think the best is to send the proposal back to the
>> “Needs revision” state.
>>
>>
>> Regards,
>>
>> Alejandro
>>
>>
>> On 29 Nov 2020 at 23:12:44, Eric Seidel <eric at seidel.io> wrote:
>>
>> I left a few comments and questions on the PR itself, but I'm leaning
>> towards rejecting the proposal in its current form as well. This doesn't
>> (yet) feel like a generic mechanism, in particular because the only
>> modifier that has been specified would be deeply wired into GHC itself.
>>
>> On Fri, Nov 27, 2020, at 04:46, Joachim Breitner wrote:
>>
>> Hi,
>>
>>
>>
>> Am Donnerstag, den 26.11.2020, 14:58 -0500 schrieb Alejandro Serrano
>>
>> Mena:
>>
>> > Dear all,
>>
>> > This proposal suggests adding syntax for a general notion of
>>
>> > modifiers, like the ones we’ve been talking about lately affecting
>>
>> > linearity or matchability of arrows. For example, if linear types and
>>
>> > unsaturated families are accepted as they stand, we would have `Int
>>
>> > #1 -> @U Bool` (or something like that), whereas with this proposal
>>
>> > we would have the more uniform `Int %1 %Unmatchable -> Bool`.
>>
>> >
>>
>> > Since the amount of modifiers is likely to increase in the future, I
>>
>> > think it’s a great idea to agree and reserve such syntax, instead of
>>
>> > coming up with different ways on each proposal. I thus recommend
>>
>> > acceptance of this proposal.
>>
>> >
>>
>> > The proposal itself:
>>
>> > (1) introduces syntax for modifiers in types and defines how to
>>
>> > type/kind check them,
>>
>> > (2) reserved such syntax for other uses in declarations and terms.
>>
>> >
>>
>> > I think the proposal still has its merits only with (1), even though
>>
>> > I lean towards accepting both parts of it.
>>
>>
>>
>> I like the idea of reserving syntax here, but parts of the proposal
>>
>> smell a bit like premature generalization to me. Are we confident that
>>
>> all annotations we eventually would like to use with this feature can
>>
>> be expressed as types of a kind that is an instance of Modifier? Or
>>
>> should we reserve the ability to have annotations that don't fit that
>>
>> model?
>>
>>
>>
>> Would we ever have annotation that may affect phases earlier than than
>>
>> typechecking? What if we want to use (%type e) and (%data e) to help
>>
>> with the SingleNamepace issues? Look like useful annotations to me, but
>>
>> I am not sure if they fit the framework proposed here.
>>
>>
>>
>> The fact that we special-case %1 supports that.
>>
>>
>>
>> The proposal explicitly has to state “No modifier polymorphism!”. But
>>
>> isn't that indication that using the type system to model the various
>>
>> modifiers might be the wrong tool?
>>
>>
>>
>> I wonder if there is a way where the %(…) on it’s own only reserve
>>
>> syntax, and the various uses of that syntax can be disambiguated
>>
>> _statically_ based on the content of ….
>>
>>
>>
>> Not great syntax, because not concise, enough, but morally I’d feel
>>
>> more at ease with
>>
>>
>>
>>   Int %(multiplicity Many) -> Int
>>
>>   Int %(multiplicity 1) -> Int
>>
>>   Int %(multiplicity m) -> Int
>>
>>
>>
>> where multiplicity is a modifier keyword, to express the existing
>>
>> features (including implicit generalization of m). Then we can extend
>>
>> this to
>>
>>
>>
>>   Int %oneShot -> Int
>>
>>
>>
>> and
>>
>>
>>
>>   Int %(matchability M) -> Int
>>
>>
>>
>> and maybe even
>>
>>
>>
>>   foo (%type [a]) -- where foo :: forall a -> ()
>>
>>
>>
>> which is a modifier that
>>
>>
>>
>>
>>
>> So at the moment, I am inclined to reject this proposal, until I am
>>
>> convinced that we are not painting ourselves into a “all modifiers are
>>
>> types of special kinds and that’s all the syntax and behaviour we ever
>>
>> need” corner.
>>
>>
>>
>> Minor detail: If we can annotate infix use of the (->) “type operator”,
>>
>> should we also be able to annotate other infix operators, i.e.
>>
>>
>>
>>   constr ::= (btype | ! atype) {modifier} conop (btype | ! atype)
>>
>>   infixexp ::= lexp {modifier} qop infixexp
>>
>>
>>
>>
>>
>>
>>
>> Cheers,
>>
>> Joachim
>>
>>
>>
>>
>>
>> --
>>
>> Joachim Breitner
>>
>>   mail at joachim-breitner.de
>>
>>   http://www.joachim-breitner.de/
>> <https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.joachim-breitner.de%2F&data=04%7C01%7Csimonpj%40microsoft.com%7C97291bd458234e28f0aa08d89ce27f4f%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637431843904133795%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=EOafNT2JtRWPODqAKfG3TNH3xqPEe3%2B5ThuYLbjYPiY%3D&reserved=0>
>>
>>
>>
>>
>>
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>>
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