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

Thu Mar 24 22:27:24 UTC 2022

I strongly support

Simon

On Thu, 24 Mar 2022 at 18:47, Richard Eisenberg <rae at richarde.dev> wrote:

> Hi committee,
>
> I re-recommend acceptance for this proposal.
>
> 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.
>
> Along with this primary change, there is some cleanup of (loosely) related
> syntax:
>  - a type synonym declaration is now required to mention all kind
> variables on its left-hand side (previously, you could introduce a kind
> variable only on the right in some circumstances)
>  - arity inference for type synonyms and families with trailing invisible
> quantifiers is simplified (you've never written such a thing, I'm sure --
> and yet GHC currently has special handling for this bit of esoterica)
>  - a type family equation must now mention all kind variables on its
> left-hand side (just like the new rule for type synonyms)
>  - a type family equation must now determine instantiation of all kind
> variables via the left-hand side patterns (this vastly increases
> readability and avoids some confusing behavior)
>
> The cleanups can all break existing code. And so I posted on Discourse
> <https://discourse.haskell.org/t/feedback-requested-about-breakage-type-variable-binders-in-datatypes/4241> seeking
> community feedback. The feedback did not suggest this breakage would cause
> wide harm. And thus this re-recommendation of acceptance.
>
> Please let us know what you think. I will go on holiday after April 4, so
> I hope to accept this proposal then.
>
> Thanks!
> Richard
>
> On Oct 25, 2021, at 3:24 AM, Spiwack, Arnaud <arnaud.spiwack at tweag.io>
> wrote:
>
> 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
>> _______________________________________________
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>>
>>
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