[swift-evolution] [Pitch] Allow nested protocol declarations

[Douglas Gregor]

> On Apr 28, 2016, at 10:15 AM, Brad Hilton via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Type nesting allows some convenient and straightforward semantics that we see inside the Swift standard library such as views on String like String.CharacterView, String.UnicodeScalarView, etc. However a protocol cannot be nested in a type and gives a non-obvious error that the “Declaration is only valid at file scope.” Just as other nested types allow proper contextual scoping, a nested protocol could make a lot sense for a number of patterns. For example, there are many “Delegate” protocols throughout the Cocoa frameworks. Here’s a controller/delegate pattern before and after type nesting:
> 
> // Without type nesting
> 
> protocol MyControllerDelegate : class {
>     
> }
> 
> class MyController {
>     
>     weak var delegate: MyControllerDelegate?
>     
> }
> 
> // With type nesting
> 
> class MyController {
>     
>     weak var delegate: Delegate?
>     
>     protocol Delegate : class {
>         
>     }
>     
> }
> 
> Though the change is mostly semantics, it does allow an explicit association between My Controller and the Delegate instead of only a named association. It also cleans up the module name space like other nested types and makes associated protocols more discoverable in my opinion. 
> 
> I’d love to hear everyone’s thoughts.

Note that this cannot work when any enclosing type is generic, e.g.,

class MyController<T> {
  protocol Delegate {
    // I’ve just created a parameterized protocol!
  }
}

Otherwise, I don’t see any issues with the proposal, and I like that it eliminates a large number of top-level names.

	- Doug


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