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 > <[email protected]> wrote: > > >> On Apr 25, 2016, at 10:49 AM, Douglas Gregor <[email protected] >> <mailto:[email protected]>> 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 > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution
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