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

[Simon Marlow]

I voted to park, as per Simon's recommendation (although if it comes up
again I would be against unless it can be made a non-breaking change
perhaps by using Adam's suggestion of pattern synonyms).

Cheers
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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