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

[Chris Dornan]

I don’t have a strong opinion as I don’t do a whole lot of TLP, but consistency and uniformity are generally good, as long as we aren’t disruptive. Simon identifies some implementation complexity but I am not sure that implementation effort is so fungible as people tend to do the things they want to do anyway so we might as we’ll approve the proposal while being clear that it is not a priority — at least until some compelling use cases are brought to our attention.

I am marginally in favour but could be talked out of it if I have misjudged the externalities of approval.

Chris

> On 27 Mar 2024, at 09:14, 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 <mailto: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 <mailto: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/
>>> 
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>>> 27 Old Gloucester Street, London WC1N 3AX, England
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