[swift-evolution] [swift-evolution-announce] [Review] SE-0095: Replace `protocol<P1, P2>` syntax with `P1 & P2`

Joe Groff jgroff at apple.com
Fri Jun 24 18:00:24 CDT 2016

> On Jun 23, 2016, at 8:55 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org> wrote:
> [Proposal: https://github.com/apple/swift-evolution/blob/master/proposals/0095-any-as-existential.md ]
> I’ve gone on record before as against this syntax, although when I set out earlier today to record my usual rebuttal I found that it really was mostly a matter of taste. Yes, this looks weird to me:
> let callback: (Data) -> NSCoding & NSCopying
> but I’m sure the infix ‘->’ for functions looked weird to everyone the first time they saw it as well, and it really is pretty clear in argument position.

We could conceivably bracket the 'where' constraints somewhere. It's nice not to have to punish the common case syntax. In my personal ideal vision of the world, I'd like to see us support opening existentials via path-dependent types (e.g., let a: Collection; let element: a.Element). If we support them in decl-level 'where' clauses, we provide a nice, clean syntax for complex generic relationships that doesn't require angle brackets or per-existential where clauses at all, something like:

	func intersect(a: Collection, b: Collection) -> Collection
		where a.Element == b.Element, b.Element == return.Element {

which doesn't completely define away the need for 'where' as part of existential types, but would shrink it quite a bit.


> However, I did remember one issue, which was brought up on the previous mega-thread: if we do want to generalize protocol values, we’re going to want something that’s essentially “a type with a ‘where’ clauses in it”. I really don’t want to force people to use a typealias to spell such a type, but at the same time I want that where clause to be clearly attached to the type. (As brought up before the return position of a function is currently ambiguous with SE-0081.)
> Despite the lightweightedness and the well-prepared proposal by Adrian and Austin, the lack of bracketing <> () {} [] leads me to maintain my stance against the proposed syntax.
> Jordan
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