[swift-evolution] [Returned for Revision] SE-0095: Replace protocol<P1, P2> syntax with Any<P1, P2>

Xiaodi Wu xiaodi.wu at gmail.com
Thu Jun 2 01:39:07 CDT 2016

On Thu, Jun 2, 2016 at 12:42 AM, Austin Zheng via swift-evolution <
swift-evolution at swift.org> wrote:

> Excellent.
> I put together a PR with a revised proposal containing the core team's
> recommended approach. If anyone is curious they can see it here:
> https://github.com/austinzheng/swift-evolution/blob/ef6adbe0fe09bff6c44c6aa9d73ee407629235ce/proposals/0095-any-as-existential.md
> Since this is the de-facto second round discussion thread, I'll start with
> my personal opinion (which is *not* reflected in the PR): the '&'
> separators in lieu of commas are a good idea, but I would still prefer the
> types to be wrapped in "Any<>", at least when being used as existentials.
> My reasons:
> - Jordan Rose brought up a good point in one of the discussion threads
> today: a resilience goal is to allow a library to add an associated type to
> a protocol that had none and not have it break user code. If this is true
> whatever syntax is used for existentials in Swift 3 should be a valid
> subset of the generalized existential syntax used to describe protocol
> compositions with no associated types.
> - I would rather have "Any<>" be used consistently across all existential
> types eventually than have it only be used for (e.g.) existential types
> with `where` constraints, or allowing two different representations of the
> same existential type (one with Any, and one without).
> - I think any generalized existential syntax without delimiting markers
> (like angle braces) is harder to read than syntax with such markers, so I
> would prefer a design with those markers.

Agree with your reasons, but I'm still uncomfortable that things inside the
angle brackets would behave differently here than elsewhere. Would it help
to make a keyword out of `any` for existentials? Then you could have this:

func foo(value: any X & Y)

> Austin
> On Jun 1, 2016, at 10:17 PM, Chris Lattner <clattner at apple.com> wrote:
> On Jun 1, 2016, at 9:53 PM, Austin Zheng <austinzheng at gmail.com> wrote:
> This was indeed a very thorough review by the core team. I'll prepare a v2
> proposal with this feedback taken into account so we can continue moving
> things along.
> One quick question - is making whatever syntax is chosen for Swift 3
> "forward-compatible" with a future generalized existential feature a
> concern?
> Yes it is a concern, but we assume that the “X & Y” syntax will always be
> accepted going forward, as sugar for the more general feature that is yet
> to be designed.
> -Chris
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