[swift-evolution] Renaming for Protocol Conformance

Jonathan Hull jhull at gbis.com
Mon Aug 22 23:59:20 CDT 2016

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}

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.

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...



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