Note much more then just Apple's frameworks leverage the optional protocol
feature of Objective-C. It really is tied to a feature of objective-c and
its dynamic runtime.
···
On Mon, Apr 25, 2016 at 10:30 PM Daniel Steinberg via swift-evolution < swift-evolution@swift.org> wrote:
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@swift.org> wrote:
On Apr 25, 2016, at 10:49 AM, Douglas Gregor <dgregor@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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