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

[Tony Allevato]

+1, would love to see UIKit's (and other frameworks') delegates/datasources
nested like this one day.

It seems like there's nothing that should prevent nesting like this; it's
just going to produce a different mangled name.


On Thu, 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.
>
> Brad Hilton
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>
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