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

Thu Dec 10 08:05:52 UTC 2020

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/
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>>>
>>>
>>>
>>>
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>>>
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