> On Feb 6, 2017, at 11:12 PM, Karl Wagner via swift-evolution
> <[email protected]> wrote:
>
>
>> On 7 Feb 2017, at 06:05, Slava Pestov <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>>>
>>> On Feb 6, 2017, at 9:00 PM, Karl Wagner via swift-evolution
>>> <[email protected] <mailto:[email protected]>> wrote:
>>> - Nested protocols in generic types are not parameterised by the parent's
>>> generic parameters.
>> So if I write GenericType<Int>.SomeProto and GenericType<String>.SomeProto,
>> is it the same protocol? What about GenericType.SomeProto, is that allowed?
>>
>> Slava
>
> GenericType.SomeProto (without parameters) is the only spelling that is
> allowed. There is no GenericType<Int>.SomeProto.
>
> That way we avoid every bound-generic type creating a new protocol.
I have only had a chance to scan the proposal so I will probably have more
comments after I take a closer look, but I want to respond to this topic right
away.
I understand the reason why you want it to work this way, but think it will be
a significant source of confusion. Protocols will behave differently than any
other entity nested in a generic type. This means the protocols nested in
generic types will likely be counterintuitive for most people.
I know this was the case for me while I was scanning the proposal. I had to
think carefully to understand how you want this to work and why you want this
behavior. That is despite it being a relatively direct consequence of the
Swift’s design for protocols, which uses associated types and intentionally
does not allow generic protocols.
It may be that there isn’t a good way to avoid that and we need to try to
address it through documentation and education. But I think this should
receive careful consideration before we commit to this direction.
Overall the proposal looks really great! I’m really looking forward to this
feature.
> I think it works really nicely when you consider what it would like like with
> existential-based capturing. Notice that there is only one
> ‘MyCollectionView.Source’, and compatibility is determined based on
> existential constraints.
>
> - Karl
>
>
> class MyCollectionView<MediaItem> : UICollectionView {
>
> protocol Source {
> // [implicit] associatedtype MediaItem
> func item(at: Int) -> MediaItem
> var numberOfItems: Int { get }
> }
> var source: Any<MyCollectionView.Source where .MediaItem == MediaItem> //
> Not possible today.
> }
>
> class BookSource: MyCollectionView.Source {
> typealias MediaItem = Book
>
> func item(at: Int) -> Book { /* ... */ }
> var numberOfItems: Int { /* ... */ }
> }
>
> class DummySource<MediaItem>: MyCollectionView.Source where MediaItem:
> DummyConstructable {
> // associatedtype 'MediaItem' bound to generic parameter.
>
> func item(at: Int) -> MediaItem { /* ... */ }
> var numberOfItems: Int { /* ... */ }
> }
>
> MyCollectionView<Book>().source = BookSource()
> MyCollectionView<Book>().source = DummySource<Book>()
> MyCollectionView<Song>().source = DummySource() // type is: DummySource<Song>
> MyCollectionView<Movie>().source = DummySource() // type is:
> DummySource<Movie>
>
>
>
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