[ghc-steering-committee] #425: invisible binders in type declarations; rec: accept

[Spiwack, Arnaud]

My weak objections are no match for Simon's strong keenness :-) .

I should say that, egoistically, I'd also like 5,6,7 to happen. I was only
expressing concerns about the price.

On Fri, Oct 22, 2021 at 1:00 PM Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> I’m in strong support.  This tidies up the design nicely.
>
>
>
> I am particularly keen on 5,6,7; indeed I wrote a whole proposal about it,
> which Vlad has #included here
>
>
>
> https://github.com/ghc-proposals/ghc-proposals/pull/386
>
>
>
> Simon
>
>
>
> PS: I am leaving Microsoft at the end of November 2021, at which point
> simonpj at microsoft.com will cease to work.  Use simon.peytonjones at gmail.com
> instead.  (For now, it just forwards to simonpj at microsoft.com.)
>
>
>
> *From:* ghc-steering-committee <ghc-steering-committee-bounces at haskell.org>
> *On Behalf Of *Spiwack, Arnaud
> *Sent:* 21 October 2021 08:03
> *To:* Richard Eisenberg <rae at richarde.dev>
> *Cc:* ghc-steering-committee <ghc-steering-committee at haskell.org>
> *Subject:* Re: [ghc-steering-committee] #425: invisible binders in type
> declarations; rec: accept
>
>
>
> I'm generally in favour. But I'm not convinced that the secondary changes
> (points 4–7) are worth it. They are certainly better place than we are
> today, but are they worth breaking existing code for? Point 5–7 can
> probably be replaced by warnings. I don't know about 4.
>
>
>
> On Tue, Oct 19, 2021 at 11:20 PM Richard Eisenberg <rae at richarde.dev>
> wrote:
>
> Hi all,
>
> I am the shepherd for proposal #425,
> https://github.com/ghc-proposals/ghc-proposals/pull/425
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F425&data=04%7C01%7Csimonpj%40microsoft.com%7C32604dcfdde74a09b92608d99460fef7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637703967362805934%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=qZtG9DdB6krhuKGwEnKuL0bPZr%2FYBr1jgRLIS1tb%2B5s%3D&reserved=0>,
> proposing to add invisible binders in type declarations.
>
> The main payload of the proposal is to allow definitions like
>
> > data Proxy @k (a :: k) = Proxy
>
> instead of today's
>
> > data Proxy (a :: k) = Proxy
>
> which has no explicit binding site for k.
>
> This new syntax solves a number of smallish syntax conundra, as very well
> outlined in the proposal.
>
> In addition, the proposal includes two small unrelated tweaks to the
> syntax of type family instances; these are points (6) and (7) in the
> proposal. Both changes will break some obscure (but still realistic,
> knowing Haskell) programs, but both fixes are backward compatible.
>
> ---
>
> I recommend acceptance. The proposal is motivated nicely (do check out the
> examples) and solves a real problem. The new syntax fits in with other
> similar features. The small cleanups to existing syntax will lead to better
> error messages.
>
> The one drawback, as I see it, is that this proposal includes point (5),
> which is a breaking change to the way type synonyms work. Right now, we can
> say
>
> > type P = (Proxy :: k -> Type)
>
> and GHC will infer P :: forall k. k -> Type. Under this proposal, you
> would have to write
>
> > type P @k = (Proxy :: k -> Type)
>
> bringing k into scope explicitly. The fix is not backward compatible, and
> so I think this proposal should come with a migration strategy, where we
> warn about the former version for some releases before banning it.
> (Continuing to support it is possible, but it's very awkward to have a
> variable mentioned only in a synonym's right-hand side.)
>
> Sorry for the delay in producing this recommendation!
> Richard
> _______________________________________________
> ghc-steering-committee mailing list
> ghc-steering-committee at haskell.org
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
> <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmail.haskell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc-steering-committee&data=04%7C01%7Csimonpj%40microsoft.com%7C32604dcfdde74a09b92608d99460fef7%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637703967362815898%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=PrFP%2BiICXUykrsrJFBKkKzD9i3bS9Jd4YNdbl6CDYp8%3D&reserved=0>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-steering-committee/attachments/20211025/dab0a812/attachment.html>