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

[Erica Sadun]

> On Apr 24, 2016, at 3:28 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Apr 22, 2016, at 8:02 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>> 
>> Sent from my iPhone
>> 
>>> On Apr 22, 2016, at 5:56 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> 
>>> Not an expert on Obj-C compatibility in Swift by any means, but this
>>> reads like it's largely a change of nomenclature. To me, though,
>>> `objcoptional` reads exceedingly poorly. Why not emphasize the Obj-C
>>> compatibility angle by requiring the `@objc` attribute to precede each
>>> use of `optional`? (In other words, effectively rename `optional` to
>>> `@objc optional`.)
>> 
>> That is a great idea. 
> 
> Doesn’t this have the same problem as the current (Swift 1/2) implementation?  People will continue to believe that it is a bug that you must specify @objc.
> 
> -Chris

I thought I'd throw a few ideas into the mix. I'm arriving late to the discussion. (I didn't expect the conversation to last this long.) I did take a quick look back through the thread but I may have missed some bits along the way. Apologies in advance for any redundancy:

* Optional requirement is an oxymoron. (This is a redux of my previous contribution to this topic but it's a good starting point.)

* Swift's "optional" protocol implementations are no such thing. They are default implementations that can be overridden. (Tangentially, why not introduce a required "override" keyword for conforming types that implement a version of a member introduced in protocol extensions? This would match the class approach and enhance safety and intent.)

* 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"?

* 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. 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.

-- Erica

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