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

[Brad Hilton]

You're right and since you can't nest any kind of type in a generic type
right, that wouldn't work even with this proposal. However, in the generics
manifesto they did say that they intend to remove that restriction. If they
do lift the restriction, they may have to leave an exception in place for
protocols.

On Thu, Apr 28, 2016 at 5:27 PM, Douglas Gregor <dgregor at apple.com> wrote:

>
> 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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