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

[Daniel Steinberg]

I am very glad to see that Swift protocols will not support optional requirements.

I wonder, however, if @objc is the wrong label. The requirement is less because of Objective-C and more because of Cocoa/Cocoa Touch APIs. I wonder if it’s useful to separate which things are being implemented because of differences between Swift and Objective-C and which things are being implemented for compatibility with Cocoa APIs which happen to be written in Objective-C.

A second example might be IBOutlets which are vars and have types such as UILabel! because of how storyboards and nibs come to life. Perhaps an @cocoa decoration there might allow them to be let and type UILabel to imply that they should be initialized once and before they are used - a runtime crash at development time in the case of an unconnected outlet would be expected.

In any case, I am generally for the proposal but wondering if an @cocoa tag might be more descriptive than @objc.

Best,

Daniel



> On Apr 25, 2016, at 7:13 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Apr 25, 2016, at 10:49 AM, Douglas Gregor <dgregor at apple.com <mailto: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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