I definitely agree with your concern about angle brackets being used outside a generic context. However, I think I'd prefer the core team syntax (unadorned "P1 & P2") if delimiters are out of the question for now.
One option might be to use parentheses: Any(X & Y & Z), or square brackets: Any[X & Y & Z]. The former might look too much like a function call, although if we're going to call '&' a type infix operator we might as well stretch the analogy as far as it'll go. I actually like the second approach: you can't define static subscripts (and you wouldn't be able to define them on `Any` even if you could), so it wouldn't occupy an already-extant syntactic slot. Either would give existential types their own characteristic syntax, avoiding the angle brackets issue you brought up. Austin > On Jun 1, 2016, at 11:39 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > On Thu, Jun 2, 2016 at 12:42 AM, Austin Zheng via swift-evolution > <swift-evolution@swift.org <mailto:swift-evolution@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 > > <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) > ``` > > Best, > Austin > >> On Jun 1, 2016, at 10:17 PM, Chris Lattner <clatt...@apple.com >> <mailto:clatt...@apple.com>> wrote: >> >> >>> On Jun 1, 2016, at 9:53 PM, Austin Zheng <austinzh...@gmail.com >>> <mailto:austinzh...@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 > > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> > >
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