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

[Malte Ott]

>From my understanding the biggest argument against this is the change in
template-haskell?
I am wondering how many users will actually be affected by that.
TypeAbstractions are quite recent so I wouldn’t be surprised if not much
template-haskell code is using the corresponding constructors.
That might also be an argument to do this change now before the ecosystem has
more time to settle on this.

Simon, I am also curious. Why are you not convinced by the goal to make types
and terms as similar as possible?

Best
Malte

On 2024-03-22 14:23, Arnaud Spiwack wrote:
> I'm happy to follow you on this. Especially since in the future that Vlad
> hopes, where there'd be less difference between terms and types, this
> particular feature may fall naturally, so it may be worth revisiting then
> rather than paying the cost now.
> 
> On Fri, 22 Mar 2024 at 09:53, Simon Peyton Jones <
> simon.peytonjones at gmail.com> wrote:
> 
> > Do you worry about the implementation because of future maintenance costs?
> >> Or because of the immediate cost of developing the feature?
> >
> >
> > Mostly the former.  It's just a bit more un-forced complexity.
> >
> > As far as I can see, there aren't other objections to this design besides
> >> the cost, right? There's no real possibility of an alternate, conflicting
> >> design for data type arguments, is there?
> >>
> >
> > It's just Occam's razor.   No one is asking for this.  And I'm unconvinced
> > by "future proofiing" because it's hard to correctly anticipate the future.
> >
> > Simon
> >
> > On Fri, 22 Mar 2024 at 08:13, Arnaud Spiwack <arnaud.spiwack at tweag.io>
> > wrote:
> >
> >> Simon,
> >>
> >> Do you worry about the implementation because of future maintenance
> >> costs? Or because of the immediate cost of developing the feature?
> >>
> >> As far as I can see, there aren't other objections to this design besides
> >> the cost, right? There's no real possibility of an alternate, conflicting
> >> design for data type arguments, is there?
> >>
> >> On Thu, 21 Mar 2024 at 10:57, 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
> >>>> _______________________________________________
> >>>> ghc-steering-committee mailing list
> >>>> ghc-steering-committee at haskell.org
> >>>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
> >>>>
> >>> _______________________________________________
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> >>>
> >>
> >>
> >> --
> >> Arnaud Spiwack
> >> Director, Research at https://moduscreate.com and https://tweag.io.
> >>
> >
> 
> -- 
> Arnaud Spiwack
> Director, Research at https://moduscreate.com and https://tweag.io.

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