[swift-evolution] [Pitch] Nested types in protocols (and nesting protocols in types)

[Braeden Profile]

But what would that mean?  If I reference `ProtocolName.InnerType`, that doesn’t always have meaning.  In fact, if you have two different extensions where AssociatedType equals something else, there’s a type ambiguity from other code.  I suspect it would only work if that InnerType was mandated to be `private`.

> On Oct 17, 2016, at 12:44 PM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> That option should not be disallowed. Here is a simple example you might want to build at some point:
> 
> protocol ProtocolName {
>      
>     associatedtype AssociatedType
> }
> 
> extension ProtocolName where AssociatedType == Int {
>   
>     struct InnerType {}
> }
> 
> 
> 
> -- 
> Adrian Zubarev
> Sent with Airmail
> 
> Am 17. Oktober 2016 um 20:30:58, Karl via swift-evolution (swift-evolution at swift.org <mailto:swift-evolution at swift.org>) schrieb:
> 
>> Is your vision that each conforming type would have to provide its own nested type as specified by the protocol?
>> 
>> Or could the protocol itself define a nested type and anything could use it?
>> 
>> protocol FloatingPoint: … {
>>     enum RoundingRule {
>>         // Do I put an implementation here?
>>     }
>> }
>> 
>> No, types which are defined inside the protocol are implemented there. Providing your own types to satisfy a conformance is what associated types are for.
>> 
>> If you wanted something like that, you could do it with a nested protocol + associated type:
>> 
>> protocol FloatingPoint {
>> 
>>     protocol _RoundingRule { func round(_ : Super) -> Super }
>>     associatedType RoundingRule : _RoundingRule
>> }
>> 
>> struct Float : FloatingPoint {
>> 
>>     enum RoundingRule : _RoundingRule {
>>         func round(_ val: Float) -> Float {
>>             /* switch self, perform rounding… */ 
>>         }
>>     }
>> }
>> 
>> That brings up an interesting point, though - we would need a way to refer to the outer protocol (I used “Super” here).
>> 
> 
> 
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