[swift-evolution] [Proposal] Separate protocols and interfaces

Douglas Gregor dgregor at apple.com
Mon Jan 4 12:21:41 CST 2016

> On Jan 3, 2016, at 4:19 PM, David Waite <david at alkaline-solutions.com> wrote:
> This would be wonderful - is it something that could happen in the Swift 3 timeframe?

I hesitate to say “yes” here. I think it fits with the goals of Swift 3, but my main concern is that there isn’t enough engineering bandwidth to implement it for Swift 3.

> Is it something that myself or someone else could work on a formal proposal for?

Yes, absolutely. This is a case where I think it’s useful to design what we want, even if we cannot fit the implementation into the Swift 3 schedule. It’s also a case where the compiler has a lot of the pieces already implemented (with some runtime bits landing soon), so the implementation should not be *that* hard and will likely not require architectural changes.

	- Doug

> -DW
>> On Jan 3, 2016, at 4:17 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> On Jan 3, 2016, at 6:48 AM, Антон Жилин via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> Introduction of interfaces will clean up the current blend of static and dynamic protocols, and solve at least three popular issues.
>>> Please see:
>>> https://github.com/Anton3/swift-evolution/blob/master/proposals/0000-introducing-interfaces.md <https://github.com/Anton3/swift-evolution/blob/master/proposals/0000-introducing-interfaces.md>
>> I am *completely* against this proposal.
>> Fundamentally, you're trying to address the limitation that protocols with Self or associated type requirements can't be existential. But it's just a limitation that isn't (conceptually) that hard to fix: the primary operation you need to work with an existing of such a protocol is to "open" a value of existential type, giving a name to the dynamic type it stores. Let's invent one:
>>   func eq(x: Equatable, y: Equatable) -> Bool {
>>     // give the name T to the dynamic type stored in xT
>>     let xT = open x as T
>>     // is y also storing a T?
>>     guard let yT = y as? T else { return false }
>>     // check whether the Ts are equal
>>     return xT == yT
>>   }
>> Ignore the syntax: semantically, we've gone from a "dynamic" existential thing back to something more "static", just by giving a name to the type. Swift generics aren't really even static in any sense: what the do is give names to the types of values so one can establish relationships among different values. "open..as" would do that for existentials. 
>> Note that ether Swift compilers AST and SIL both have "open existential" operations that do this internally. They have no spelling in Swift code, but they are useful to describe operations on existentials. At present, they cannot be formed when the existential involves a protocol with Self or associated type requirements, but that's a limitation that isn't hard to address. 
>> As for your concerns about knowing when one can dynamically override and when one cannot...  There are issues here that need to be addressed. They aren't significant enough to warrant such a drastic change, and may not even require language changes at all. 
>> 	- Doug
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