> On Apr 28, 2016, at 10:15 AM, Brad Hilton via swift-evolution
> <[email protected]> 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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