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

Fri Mar 22 18:19:29 UTC 2024

>From my understanding the biggest argument against this is the change in
template-haskell?

Not specifically.  My reservation is that

   - it's an unforced change,
   - with no user demand
   - but some real user impact (you mention TH)
   - and some implementation cost (modest but very non-zero)
   - aiming to anticipate as-yet-unknown future requirements

That's not a combination I like.  Pain now for possible (but uncertain)
gain in the future.

I don't object to making types and terms behave similarly -- indeed I have
invested lots of time working with Richard, Vlad, Andrei and others on
proposals and MRs that move in this direction.  I'm just very unconvinced
about *this *proposal.

One minor point.  In patterns we allow this:
f ((,) @Int @[a] x y) = ...
Here the type arguments are not type variables but full-blown types, and of
course nested parens etc come "for free".  But this proposal concerns data
type declarations in which we definitely don't want fulll-blown types. So
it's more than a "terms and types should be the same" discussion.

Simon

On Fri, 22 Mar 2024 at 14:47, Malte Ott <malte.ott at maralorn.de> wrote:

> 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
> > >>>>
> > >>> _______________________________________________
> > >>> ghc-steering-committee mailing list
> > >>> ghc-steering-committee at haskell.org
> > >>>
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
> > >>>
> > >>
> > >>
> > >> --
> > >> Arnaud Spiwack
> > >> Director, Research at https://moduscreate.com and https://tweag.io.
> > >>
> > >
> >
> > --
> > Arnaud Spiwack
> > Director, Research at https://moduscreate.com and https://tweag.io.
>
> > _______________________________________________
> > ghc-steering-committee mailing list
> > ghc-steering-committee at haskell.org
> > https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>
> _______________________________________________
> ghc-steering-committee mailing list
> ghc-steering-committee at haskell.org
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-steering-committee/attachments/20240322/b7bcfdf6/attachment.html>