[ghc-steering-committee] Please review #641: Wildcard binders in type declarations

Fri Apr 19 11:28:27 UTC 2024

I don’t seem to have write access to that sheet, but I’ve requested it now.

I was on the fence here, and would have voted to park it until it was
requested. But as int-index points out, a breaking change to TH is on the
way anyway, and it would be good to improve the completeness at the same
time.

So I vote accept (mildly)

/Matti Palli

On Fri, Apr 19, 2024 at 12:45 Vladislav Zavialov <vlad.z.4096 at gmail.com>
wrote:

> I don't think I have a vote, as my term has ended. Even if I had one, I'd
> abstain because it is my amendment.
>
> However, I'd like to point out two things.
>
> 1. If the proposed amendment is rejected, we /still/ have to change
> template-haskell to implement 425 in its current form. The specification
> allows invisible wildcards `@_`, which can't be represented in
> template-haskell at the moment. So I'd like to ask voting members to take
> that into consideration: this is not an "unforced change" because there is
> a change coming either way.
>
> 2. Simon argues against a new recursive data type in the AST. OK, I can
> see the problem, but please don't forget about non-recursive forms `type T
> _ = rhs` and `type T (_ :: k) = rhs`. Even if there is no user demand for
> nested forms like `type T ((_ :: k1) :: k2) = rhs`, flat wildcard binders
> are a less intrusive addition and I've personally wanted them several
> times. So if the problem is recursion, please consider sending the
> amendment for revision instead of rejecting it.
>
> Thus the voting options should probably be:
>
> a) accept the amendment
> b) revise the amendment to avoid recursive/nested forms
> c) reject the amendment
>
> Vlad
>
> On Fri, Apr 19, 2024 at 12:15 PM Simon Peyton Jones <
> simon.peytonjones at gmail.com> wrote:
>
>> Dear GHC Steering Committee
>>
>> On 2 April I asked:
>>
>> I think it's time to vote.   Please so before Monday morning *[8 April]*.
>>> Thank you!
>>
>>
>>
>> *It is now 19 April, of the eight members of the committee only Malte has
>> voted. *
>>
>> Please please vote.  Today!   Use email and record your vote on the
>> spreadsheet
>> <https://docs.google.com/spreadsheets/d/1e6GdwHmAjeDEUhTvP-b18MDkpTfH3SMHhFu5F3nDIWc/edit?usp=sharing>.
>> Vlad you have a vote; since you are the proposer you may choose to abstain
>> but I think it's up to you.
>>
>> The proposal isn't a huge deal either way, but we owe it to the proposer
>> and to ourselves to deal with our business in a timely way.
>>
>> Simon
>>
>> On Tue, 2 Apr 2024 at 17:25, Simon Peyton Jones <
>> simon.peytonjones at gmail.com> wrote:
>>
>>> Dear GHC Steering Committee
>>>
>>> Vlad proposes to amend proposal #425
>>> <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0425-decl-invis-binders.rst>to
>>> permit more wildcard binder forms in type declarations:
>>>   https://github.com/ghc-proposals/ghc-proposals/pull/641
>>>
>>> See my mail below.   I recommend, fairly strongly, to park this until
>>> there is evidence of need.
>>>
>>>    - it's an unforced change,
>>>    - with no user demand
>>>    - but some real user impact (notably: it will break some TH users)
>>>    - and some implementation cost (modest but very non-zero)
>>>    - aiming to anticipate as-yet-unknown future requirements -- but
>>>    YAGNI <https://martinfowler.com/bliki/Yagni.html>
>>>
>>> I think it's time to vote.   Please so before Monday morning.  Thank you!
>>>
>>> Simon
>>>
>>> On Thu, 21 Mar 2024 at 09:56, Simon Peyton Jones <
>>> simon.peytonjones at gmail.com> wrote:
>>>
>>>> Dear Steering Committee
>>>>
>>>> Vlad proposes to amend proposal #425
>>>> <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0425-decl-invis-binders.rst>to
>>>> permit more wildcard binder forms in type declarations:
>>>>   https://github.com/ghc-proposals/ghc-proposals/pull/641
>>>>
>>>> You may find it easiest to look at the rich diff
>>>> <https://github.com/ghc-proposals/ghc-proposals/pull/641/files?short_path=cb2a762#diff-cb2a762676d938436a07317bbd007570b5efdfa00b40763b897ee920694bcbb5>
>>>> .
>>>>
>>>> This is a pretty small generalisation which would allow
>>>>
>>>> data T (( (a :: k1) :: k2)) = ...
>>>>
>>>> in which the binder has multiple kind signatures and redundant parens.
>>>> The change is *not driven by user need*, but rather solely by
>>>> *uniformity*: these same forms are permitted in function definitions:
>>>>
>>>> f :: forall (a :: k). blah
>>>> f @(((a::k1)::k2))) = ...
>>>>
>>>> is permitted.
>>>>
>>>> It imposes a change on Template Haskell syntax too.
>>>>
>>>> The implementation becomes a bit more complicated; more recursive data
>>>> types, etc.  Nothing hard, but more.
>>>>
>>>> It's not a big deal either way.  Very few people expressed a view on
>>>> GitHub.  My personal view is that the modest (albeit non-zero) gain does
>>>> not justify the definite (albeit modest) pain. I would leave this until
>>>> someone actually wants it.
>>>>
>>>> Vlad argues for future-proofing, but my experience is that an eye to
>>>> the future is sensible when you are making changes anyway; but making
>>>> unforced changes solely for the future risks incurring pain now that, when
>>>> the future comes, turns out to have been a poor investment.  We may have
>>>> correctly anticipated, or we may not.
>>>>
>>>> So my recommendation is to park this until we get a real user demand.
>>>>
>>>> It's a perfectly sensible proposal, but adopting it is a judgement
>>>> call. I'll leave a week for committee responses, and then we can just vote.
>>>>
>>>> Simon
>>>>
>>>> On Thu, 21 Mar 2024 at 08:07, Adam Gundry <adam at well-typed.com> wrote:
>>>>
>>>>> Dear Committee,
>>>>>
>>>>> Vlad proposes to amend proposal #425 to permit more wildcard binder
>>>>> forms in type declarations:
>>>>>
>>>>> https://github.com/ghc-proposals/ghc-proposals/pull/641
>>>>>
>>>>> I'd like to nominate Simon PJ as the shepherd.
>>>>>
>>>>> Please guide us to a conclusion as outlined in
>>>>> https://github.com/ghc-proposals/ghc-proposals#committee-process
>>>>>
>>>>> Cheers,
>>>>>
>>>>> Adam
>>>>>
>>>>>
>>>>> --
>>>>> 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
>>>>> <https://www.google.com/maps/search/27+Old+Gloucester+Street,+London+WC1N+3AX,+England?entry=gmail&source=g>
>>>>> _______________________________________________
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>>>>>
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