[swift-evolution] Subclass Existentials

[Douglas Gregor]

> On Feb 2, 2017, at 2:54 PM, David Smith <david_smith at apple.com> wrote:
> 
>> 
>> On Feb 2, 2017, at 11:20 AM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>>> On Feb 1, 2017, at 11:44 PM, Adrian Zubarev <adrian.zubarev at devandartist.com <mailto:adrian.zubarev at devandartist.com>> wrote:
>>> 
>>> typealias AnyObject = … is nice to have, but how about if we fully drop the class constraint-keyword and generalize AnyObject instead?
>>> 
>> That’s a good point. My *technical* goal is for AnyObject to cease to be a protocol, because it’s really describing something more fundamental (“it’s a class!”). Whether we spell that constraint as “class” or “AnyObject” doesn’t affect that technical goal.
>> 
>> I’d gravitated toward the “class” spelling because the idea of a class constraint seems most naturally described by “class”, and it’s precedented in C#.
>> 
>> However, the changes in SE-0095 <https://github.com/apple/swift-evolution/blob/master/proposals/0095-any-as-existential.md> to make “Any” a more fundamental type (and not just a typealias) definitely open the door to doing the same thing with “AnyObject”—just make it a built-in notion in the language, and the spelling for a class constraint. It *certainly* works better with existentials.
>> 
>>> In the future we might want to add AnyValue with value (semantics) constraint, would that mean that we’d need another keyword there like value?
>>> 
>> “value” would be a terrible keyword, as you know. Point taken :)
>> 
>> If we did something like this, we would probably want it to be akin to ValueSemantics—not just “it’s a struct or enum”, but “it provides value semantics”, because not all structs/enums provide value semantics (but immutable classes do).
>> 
>>> Speaking of the future directions:
>>> 
>>> Now that we’re no longer supporting the idea of Any<…> syntax and any type prefixed with Any seems to be special for its particular usage, could we safely bring the empty Any protocol back (is this somehow ABI related?)?
>>> 
>> From an implementation standpoint, the choice to make AnyObject a magic protocol was a *horrible* decision. We have hacks throughout everything—the compiler, optimizers, runtime, and so on—that specifically check for the magic AnyObject protocol. So, rather than make Any a magic protocol, we need to make AnyObject *not* magic.
>> 
>>> One day after this proposal is accepted, implemented and released, we probably will talk about the where clause for existentials. But since a lot of the existentials will have the form typealias Abc = …, this talk will also include the ability to constrain generic typealiases.
>>> 
>> By “one day” I suspect you mean “some day” rather than “the day after” :)
>> 
>> Yes, I feel like this is a natural direction for existentials to go.
> 
> Looking ahead to when this is on the table, I'm a little worried about the syntactic implications of constrained existentials now that the Any<> syntax doesn't seem to be as popular. The obvious way to go would be
> 
> 'X & Y where …'
> 
> But that leads to ambiguity in function declarations
> 
> func doTheThing<T>() -> X & Y where … where T == …
> 
> This could be resolved by requiring constrained existentials to be typealiased to return them, but I don't think there's any other situations where we require a typealias to use something, and it just feels like a workaround.

Types can be parenthesized, so that’s a workaround. But I too have some concerns here that we’re creating an ambiguity that users will trip over.

	- Doug


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