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

Austin Zheng austinzheng at gmail.com
Thu Jun 2 00:42:38 CDT 2016


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 <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.


> 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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