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

[Brad Hilton]

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.

Brad Hilton
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