[swift-evolution] Will Swift ever support optional methods without @objc?

[Karl]

> On 15 Nov 2016, at 16:46, Shawn Erickson <shawnce at gmail.com> wrote:
> 
> This has been discussed somewhat heavily in the past and nothing yet has really moved forward on it. I do think a good way of doing something like this would be helpful. I have resulted to defining an interface with an extension that provides empty defaults and for each function a match bool var exists to imply if it exists or not. The code accepting a delegate can probe these bool vars to configure itself to efficiently operate based on knowledge about what the delegate expects (some missing from most proposed solutions other then @objc optional).
> On Tue, Nov 15, 2016 at 6:59 AM Karl via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> On 15 Nov 2016, at 12:22, Haravikk via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>>> On 15 Nov 2016, at 07:53, Rick Mann via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> 
>>>> On Nov 14, 2016, at 22:51 , Charlie Monroe via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> One major example is the NS/UITableViewDataSource or Delegate - there are many many methods that you don't need to implement, hence are optional.
>>>> 
>>>> But I think that this was partially solved by default implementation of protocol methods, which pretty much does what you want...
>>> 
>>> I just realized I only responded to someone else, and not the whole list. It does, but it forces me to make the return value of the protocol method optional, so that the default implementation can return nil. 
>>> 
>>> In the end, I guess that's not so bad, since I'm not happy with the entire approach, but it'll do for now.
>> 
>> What's different about having the method return nil vs being optional? You're attempting to call it either way, and presumably need some means of handling the return value, except in Swift it's all nice and explicit and easy to put in a conditional like:
>> 
>> 	if let result = myObject.someOptionalMethod() { /* Do some stuff */ }
>> 	print(myObject.someOptionalStringMethod() ?? "")
>> 
>> And so-on. If you need a method to be both optional, and return a nilable result then you can use a double optional like so:
>> 
>> 	if let result = myObject.someDoubleOptionalMethod() { // Method was implemented
>> 		if let value = result { // Method returned a value
>> 			/* Do some stuff */
>> 		}
>> 	}
>> 
>> 
>> By defining the methods as returning an Optional and throwing in default implementations you can specify fewer, bigger protocols and make clear what the requirements really are, though personally given the choice I'd prefer a dozen smaller protocols that are absolutely explicit in what they do.
>> 
>> But yeah, I think the tools you need are all there already; maybe there's an argument to be made for allowing default return values on protocol methods, to reduce the boiler-plate?
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> 
> I think there is a difference between:
> 
> - A method which returns an optional result, and
> - An optional method which, if present, always returns a result
> 
> Perhaps not so much of a difference at the usage site (it’s just a question of placing a ? for optional chaining), but semantically and when conforming to the protocol, they mean different things.
> 
> - Karl
> 
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If you don’t mind me asking, what is your use-case?

Even though I think "optional methods" and “methods returning optionals” are different things, I don’t really have any examples where optional methods are better than sub-protocols.

e.g.

```
// Core callbacks
protocol MyDelegate { }

// Optional callbacks, added like a mixin
protocol MyDelegateWithExtras : MyDelegate { }

// Some more optional callbacks
protocol MySubDelegate : MyDelegate {}

class DelegateImpl : MySubDelegate, MyDelegateWithExtras {
  // Implement all core + optional callbacks
}

var d : MyDelegate = DelegateImpl()

if let extras = d as? MyDelegateWithExtras {
    // invoke optional functionality
}
```

I don’t know what the overhead of the as? call is, but it’s almost certainly less than an Obj-C `respondsToSelector` call. Depending on whether you need to swap the delegate for objects of different types, you could also use generics to optimise the checks (and possibly more) away.

- Karl
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