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

[Simon Peyton Jones]

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<mailto:simonpj at microsoft.com> will cease to work.  Use simon.peytonjones at gmail.com<mailto: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<mailto: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
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