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

[Douglas Gregor]

> On Apr 11, 2016, at 10:30 AM, Matthew Johnson <matthew at anandabits.com> wrote:
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> Sent from my iPad
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>> On Apr 11, 2016, at 12:15 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
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>>> On Apr 7, 2016, at 5:12 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
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>>> 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.
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>> 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. 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, and your proposed solution makes this incredibly obscure and magical.
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> Do you have the same thought for optional closure properties?  If so and heightForRow was an optional closure property it would satisfy all use cases elegantly.  It could have a default implementation that returns nil.  When non-uniform heights are required a normal method implementation can be provided.  Delegates that have uniform row heights some of the time, but not all of the time, would also be supported by implementing the property.

There are still some issues here:

1) It doesn’t handle optional read/write properties at all, because the setter signature would be different. Perhaps some future lens design would make this possible. For now, the workaround would have to be importing the setter as a second optional closure property, I guess. (The current system is similarly broken).

2) For an @objc protocol, you won’t actually be able to fully implement the optional closure property with a property of optional type, because “return nil” in the getter is not the same as “-respondsToSelector: returns false”. Indeed, the getter result type/setter parameter type should be non-optional, so we would (at best) need a special rule that optional closure properties of @objc protocols can only be implemented by non-optional properties of closure type or by methods.

> If we decide to favor this approach it would be really nice to be able to import Cocoa delegate protocols this way.  Is that something that might be feasible?


Yes. If we favor this approach, it should be fairly direct to make imported Objective-C protocols work this way.

	- Doug