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

[Erica Sadun]

> On Apr 25, 2016, at 11:49 AM, Douglas Gregor <dgregor at apple.com> wrote:
> 
>> 
>> On Apr 25, 2016, at 10:13 AM, Erica Sadun <erica at ericasadun.com <mailto:erica at ericasadun.com>> 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.
> 
> It’s a contextual keyword, so the impact is far less than a full-fledged keyword (but, point taken).

Ah, right. 

>>>> 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.
> 
> @objc(x: y) looks suspiciously like a typo for the selector @objc(x:y:).

Oh lord yes. Yes, it does. *headdesk*.

>> 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.
> 
> It’s not semantically and syntactically external. It is a real feature with specific, unique type-checking behavior. It is externally-motivated, and limited to interoperability with another language, but that doesn’t make it an external feature in the way that (say) some other tool that generates Swift code is external.

I think I made my points at least in terms of objc optional requirements though: 

1. I think it's a very good thing. 
2. I wish there were a better way to express it.

-- E, cc'ing in Chris who I believe *is* the review manager for SE-0070

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