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

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@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@gmail.com> wrote:

...

Dear Steering Committee

Vlad proposes to amend proposal #425 https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0425-de...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=cb2... .

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 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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