[swift-evolution] [Idea] How to eliminate 'optional' protocol requirements

[Joe Groff]

> On Apr 12, 2016, at 11:24 PM, Douglas Gregor <dgregor at apple.com> wrote:
> 
> 
>> On Apr 11, 2016, at 10:15 AM, Joe Groff <jgroff at apple.com> wrote:
>> 
>> 
>>> On Apr 7, 2016, at 5:12 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> One could perhaps work around (a), (b), and (d) by allowing compound (function-like) names like tableView(_:viewFor:row:) for properties, and work around (c) by allowing a method to satisfy the requirement for a read-only property, but at this point you’ve invented more language hacks than the existing @objc-only optional requirements. So, I don’t think there is a solution here.
>> 
>> To me, compound names for closure properties and satisfying property requirements with methods aren't hacks, they're missing features we ought to support anyway. I strongly prefer implementing those over your proposed solution.
> 
> I haven’t seen these come up in any discussion that wasn’t about mapping Objective-C optional requirements to something else in Swift. What other use cases are you envisioning?

The desire for labeled closure variables has come up a few times. For instance, it would provide a way to give local label names to closure arguments. In C, you can say:

	void doStuff(void (*onCompletion)(void *result, void *error)) {
		if (auto x = /*doStuff*/) {
			onCompletion(/*result*/ x, /*error*/ nullptr);
		} else {
			onCompletion(/*result*/ nullptr, /*error*/ getLastError());
		}
		onCompletion(/*result*/ x, /*error*/ y);
	}

where `result` and `error` don't affect the type, but describe the use of the parameters to the completion closure. It'd be nice to do the same in Swift:

	func doStuff(onCompletion completed(result:error:): (AnyObject?, AnyObject?) -> ()) {
		/*doStuff*/
		completed(result: x, error: nil)
	}

There's also the case of using if let to test for presence of a method, where you'd really like to keep the labels on the local binding:

	if let doStuff(to:with:) = object.doStuff(to:with:) {
		...
	}

We currently lean on the vestige of labeled-tuples-as-function-arguments to do this, but I think we ultimately want to get away from that.

-Joe

>> It sounds to me like a lot of people using optional protocol requirements *want* the locality of control flow visible in the caller, for optimization or other purposes,
> 
> Most of the requests I see for this feature are of the form “this works for @objc protocols, so it should work everywhere,” and most of the push-back I’ve seen against removing ‘optional’ is a concern over interaction with Cocoa. I haven’t gotten the sense that optional requirements are considered to be the best design for any particular task in Swift.
> 
>> and your proposed solution makes this incredibly obscure and magical.
> 
> 
> That’s fair. The mechanism I’m talking about *is* a bit hard to explain—we would need to rely on the diagnostic for cases where one tries to call a method that is caller-defaulted from Swift code, e.g.,
> 
> 	error: method ‘foo(bar:wibble:)’ may not be implemented by the adopting class; add a default implementation via an extension to protocol ‘Foo'
> 
> This would only affect optional requirements of protocols imported from Objective-C. My hypothesis is that those just aren’t used in Swift app code.
> 
> 	- Doug
>