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

[Eric Seidel]

This feels like a reasonable generalization, assuming the it doesn't raise any
serious implementation concerns. I'll trust Simon PJ above that it doesn't.

I also agree that an unforced change like this should preferably be bundled
alongside some other motivated change.

I think that still means that *we should accept the proposal*, but the GHC
devs can make a reasoned decision about *when* to implement the change.

On Fri, Apr 19, 2024, at 05:15, Simon Peyton Jones 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
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