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

[Simon Marlow]

What is the impact on Template Haskell? Is it a breaking API change?

On Wed, 27 Mar 2024 at 09:15, Simon Peyton Jones <
simon.peytonjones at gmail.com> wrote:

> Dear Simon, Chris, Eric, Moritz, Matthias
>
> You have not responded to my email below.
>
> I'll call a vote tomorrow, but I would love to hear your opinions as a
> member of the GHC SC.
>
> Thanks
>
> 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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