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

[Simon Peyton Jones]

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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