[swift-evolution] [Review] SE-0070: Make Optional Requirements Objective-C only
conductator at ntlworld.com
Wed Apr 27 10:01:37 CDT 2016
Interesting idea. I tried this in a playground to see what the calling syntax would be like:
let delegate = Optional(Delegate())
let default = 0
let value: Int = (try? delegate?.returnFive() ?? default) ?? default
Not quite as elegant at the call site, but it works. Sadly the rethrowing overload of ?? complicates things, I'm not sure of a way to work around that directly.
I do like that types could potentially implement the function and do something, then throw an error rather than returning a value, which would allow them opt into using the default value. It would complicate matters if the conforming types throw multiple errors for whatever reason, though.
From James F
> On 27 Apr 2016, at 15:30, Andrew Bennett <cacoyi at gmail.com> wrote:
> Sorry if this has been discussed, but have you considered dropping optional entirely, making it throw, and a default implementation that throws a selector not found exception?
> This is approximately what you would expect using it in objc. I don't think it has the complexity discussed in the proposals alternatives for other call site issues.
> If it throws you can call with "try?" to get similar functionality in most cases.
> This assumes that respondsToSelector doesn't pick up the Swift default implementation.
>> On Wednesday, 27 April 2016, James Froggatt via swift-evolution <swift-evolution at swift.org> wrote:
>> Thanks again.
>> I had a look at the links in the proposal as you suggested, and I see a lot of people pointing to protocol extensions as a solution (and counter-arguments of the inability to optimise code with this method, which are left unresolved).
>> To make use of the protocol extensions solution, one would have to define the protocol, add a protocol extension which implements every function, then add an empty type which allows access to these implementations.
>> For a recommended alternative, this seems a lot of work. In a world where Swift didn't have Objective C compatibility, and this empty-type workaround to access defaults was the best option available, I'd be inclined to support a proposal to add optional method requirements. It has the added optimisation and easy delegate-swapping relative to closure properties, as you mentioned, and feels less hacky than the closure-function switching suggested in the proposal.
>> The protocol extension + default type would provide a direct alternative, but it leads me to wonder what exactly we're trying to avoid by discouraging optional methods. The potential for unexpected optimisation, which seems to be the primary issue, is unsolved, since the type can check to see if the delegate is its own, default type, and proceed to ignore the method regardless. In exchange, we make things much harder for types simply wishing to have a default value when there is no registered delegate.
>> So what aspect of optional protocol requirements are we actually trying to discourage, which isn't present in protocol extensions?
>> PS. If we're concerned about overlap with protocol extensions: it seems a bit like eliminating functions from the language because they overlap with the more general concept of closures. It's a fine idea, but it seems more reasonable to find a solution that handles both cases conveniently before we start eliminating one of them.
>> From James F
>> > On 26 Apr 2016, at 22:56, Douglas Gregor <dgregor at apple.com> wrote:
>> >> On Apr 26, 2016, at 3:33 AM, James Froggatt <conductator at ntlworld.com> wrote:
>> >> Fair enough. Upon reflection, I think my real issue is somewhat different to what I suggested previously.
>> >> I wasn't intending to suggest such a thing would be practical, just that it would be a decent alternative to optional protocol requirements. The alternative given in the proposal seems to be more of a way to remove optional protocol requirements on the surface, while actually helping to make them a native feature, if you see what I mean. It's not a realistic alternative - it's a worse syntax for the exact same thing, which also comes with awful side-effects for Swift as a whole. No-one would ever seriously consider this as an alternative, yet it's listed as under the heading ‘Alternatives Considered’.
>> > If you follow the swift-evolution discussion links in the proposal, you’ll note that a number of people have proposed exactly what is listed in “Alternatives Considered”. The only truly wacky idea in there is my caller-side default implementations idea, which I covered simply because it was my last stab at eliminating optional requirements before giving up and sequestering them permanently behind “@objc”.
>> >> You say the arguments given against optional closure properties are strong, but I don't they would be nearly as relevant to the case I suggested. By making them properties of the table view, the tableView parameter would be eliminated, meaning the property names could be unique.
>> >> EG:
>> >> var numberOfRows: (inSection: Int) -> Int
>> >> var cellForRow:: (at: NSIndexPath) -> UITableViewCell
>> >> var moveRow: (from: NSIndexPath, to: NSIndexPath)
>> >> This removes the need to add the mentioned workarounds, since a function could be assigned to the closure property just as easily as an inline closure. I feel this is much more worthy of being considered as an alternative. The idea of these proposals is to document why we do things, so at least for someone wondering why we require all this @objc syntax rather than support optional protocol requirements natively, this would actually present them with a viable alternative which could be applied in their APIs.
>> > Doing this implies creating a potentially large number of stored closure properties, which is not as storage-efficient as storing a single delegate reference. Moreover, it makes it harder to set up your customization points: instead of implementing one protocol, you’re writing assignments into some number of stored closure properties. Imaging trying to change the delegate to some other delegate temporarily: you would have to manually store each of the closures into some local structure and introduce your own, except that you can’t get them all because some new version of the platform would add new stored closure properties. Finally, Cocoa just doesn’t work like this, so you would require some massive re-architecture to get there. I don’t see how this is a better design.
>> > - Doug
>> swift-evolution mailing list
>> swift-evolution at swift.org
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