[swift-evolution] [Proposal draft] Remove objc requirement for optional protocol methods/properties

[Chris Lattner]

> On Jan 9, 2016, at 2:00 PM, David Scrève via swift-evolution <swift-evolution at swift.org> wrote:
> 
> ## Introduction
> 
> When using protocol, we usually need to use optional method or property. We propose to remove the 
> requirement of @objc keyword for such protocol.

Hi David,

This is certainly a glaring hole in the system, and one we need to discuss.  Here are some problems with making optional requirements a first class thing in Swift:

1. They overlap heavily (but are syntactically privileged) with optional properties, consider the difference between "optional func f() -> Int"  vs "var f : (() -> Int)?  {get}”.

2. Protocols with default implementations provide the vastly most common use-cases for these.

3. They overload “optional” terminology in the language to mean something different.

4. They are non-sensical - how can something be a requirement and be optional? :)



Doug has it on his plate to explore what we can do about this, in the Swift 3 timeframe, but not in the immediate future.  IMO, it would be really great if we could make Objective-C optional requirements disappear in the Clang importer, and transform them into requirements with a default implementation.

-Chris