[swift-evolution] [Proposal draft] Make Optional Requirements Objective-C-only

[Erica Sadun]

> On Apr 25, 2016, at 10:49 AM, Douglas Gregor <dgregor at apple.com> wrote:
>> * Swift already has an `Optional` type. Importing ObjC "optional" protocol requirements is therefore semantically problematic from a Swift development POV. I don't like either the "@objcoptional" or "@objc optional" solutions mentioned upthread. They overload "optional" syntactically and confuse semantics. I think the key words that better describe what's happening in, for example, a `UITableViewDelegate`, are "discretionary" or "elective" implementations.  Swift has renamed lots of Objective C things (waves hi to SE-0005 <https://github.com/apple/swift-evolution/blob/master/proposals/0005-objective-c-name-translation.md>). Why not "optional”?
> 
> If we were adding optional requirements to Swift protocols, I would agree that it makes sense to change the nomenclature to avoid the oxymoron and the confusion with optionals. However, since this is now moving into the realm of “Objective-C compatibility feature”, I think it’s reasonable to retain the existing, Objective-C terminology.
> 
> Also, there is a link between the Optional type and optional requirements: when you reference an optional requirement, you get back an Optional.

Fair enough point but one that doesn't really sway me enough to include a native keyword for an ObjC compatibility feature.

>> * I do *support* retaining `@objc` in some form and I believe it can be addressed in a way that does not appear to be a bug. "Optional protocol conformance" is a behavior that is external to the language. I do not believe would be voluntarily added to Swift should the topic arise. 
> 
> It’s a feature that exists to support compatibility with another language; we would not add it if it not for Objective-C. However, it is a real language feature with different semantics from other language features.

Sounds like we're agreed on this point.

>> Therefore I find it insufficient to introduce attributes like `@elective` or `@discretionary` in order to satisfy non-native requirements. I would prefer to see the @objc attribute be extended to support these and any future Objective-C-specific behaviors: @objc(elective), @objc(importedProtocolSupport: elective), or whatever. While these are wordy, I assume like any other Swift attributes they can be placed on a line before the function declaration, and it would be instantly clear why they've been placed there, and they would not overlap with Swift semantics *or* expectations. I leave the color of the bikeshed as an exercise for the reader.
> 
> Do remember that @objc(something) already has a meaning: it gives the Objective-C name “something” to the entity that the @objc(something) describes.

And this is something I *did* overlook. Is there leeway to add labeled items `@objc(x: y)`?  If so, `@objc(something)` could transition to `@objc(somelabel: something)` and a separate label be used for this.

The key point I want to make is that something that is semantically and syntactically external to the language should enter through a well regulated gateway. That gateway should be marked in some fashion that contextualizes its use and understanding to the foreign source so it's immediately understood to be non-native. It doesn't have to be part of `@objc` but things that aren't Swift native should never have a first class presence in the language. The approach to supporting one non-native language should be extensible to supporting other non-native languages.

-- E

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