[swift-evolution] Renaming for Protocol Conformance

[Douglas Gregor]

> On Aug 22, 2016, at 9:59 PM, Jonathan Hull via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hi everyone,
> 
> We talked about this before when we were discussing mixins, and there seemed to be generally positive feelings towards it as a feature for the future.  I am fairly certain this affects the ABI though, so I thought I would bring it up now.
> 
> If two protocols have methods/properties with the same name, but different signatures, we need a way to distinguish between them when attempting to conform to both.
> 
> 	protocol A {
> 		var x:Int {get set}
> 	}
> 
> 	protocol B {
> 		var x:Double {get set}
> 	}

I believe that this can happen, and it is unfortunate that Swift has no mechanism for dealing with it today. However, I agree with Xiaodi that your proposal would be much stronger with real-world examples rather than theoretical ones.

> One possibility is to allow a struct/class/enum to conform to the protocol while renaming one (or both) of the clashing methods:
> 
> 	struct C: A,B {
> 		var x:Int
> 		var y:Double implements B.x
> 	}
> 
> The conforming method/property would still have to have the same signature, but could have a different name (and parameter labels).  It would also allow protocol methods which have identical signatures and semantics, but different names to be implemented using the same method (i.e ‘implements D.z & E.w’).
> 
> When something is cast to the protocol (say ‘as B’), then calling the property (e.g. ‘x’) would end up calling the implementation of the renamed property ( ‘y’ in this example) on the conforming type.

Sure. Calling through the protocol type will get whatever method/property satisfied the protocol requirement. Yes, there are limits here due to protocols with associated types and Self requirements, but I fully expect those to go away at some point with generalized existentials.

> I think we would also want a way to retroactively conform using existing properties/methods in an extension declaring conformance.  Not sure what the best syntax for that would be.  Off the top of my head (though I would love to have something with less cruft):
> 
> 	extension D:B {
> 		@conform(to: B.x, with: D.y)
> 	}
> 
> or maybe just:
> 	
> 	extension D:B {
> 		D.y implements B.x
> 	}
> 	
> 
> All of this is merely to start the discussion, so feel free to propose better syntax or a more elegant solution...


C# has a much narrower solution that lets you qualify the method declaration (rather than doing a full rename), e.g.,

	struct C : A {
	  var x: Int
	  var y: Double
	}

	extension C : B {
	  var B.x: Double {
	    get { return y }
	    set { y = newValue }
	  }
	}

They have some examples at:

	https://msdn.microsoft.com/en-us/library/aa288461(v=vs.71).aspx <https://msdn.microsoft.com/en-us/library/aa288461(v=vs.71).aspx>

One would have to figure out what the name-lookup rules are, of course, but this might allow us to solve the problem without introducing a generalized renaming mechanism.

	- Doug

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