[swift-evolution] [RFC][Proposal] Ease restrictions on protocol nesting

[Karl Wagner]

Anybody from core-team able to weigh in: do we want to keep this rule of not allowing parametrised protocols (in which case nesting is "trivial" - capturing is not allowed, only one protocol exists, so it's just namespacing), or is this something which should be deferred?    
  
  
  

  
If possible I would like to include protocols inside of generic types. I kind of like the non-parameterised version. It's elegant and allows you to write generic conformers to the protocol.
  
  

  
  
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> On Feb 7, 2017 at 4:10 pm,  <Matthew Johnson (mailto:matthew at anandabits.com)>  wrote:
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> > On Feb 6, 2017, at 11:12 PM, Karl Wagner via swift-evolution  <swift-evolution at swift.org (mailto:swift-evolution at swift.org)>  wrote:
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> > > On 7 Feb 2017, at 06:05, Slava Pestov  <spestov at apple.com (mailto:spestov at apple.com)>  wrote:
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> > > >  On Feb 6, 2017, at 9:00 PM, Karl Wagner via swift-evolution  <swift-evolution at swift.org (mailto:swift-evolution at swift.org)>  wrote:   
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> > > >  - Nested protocols in generic types are not parameterised by the parent's generic parameters.  
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> > > So if I write GenericType<Int>.SomeProto and GenericType<String>.SomeProto, is it the same protocol? What about GenericType.SomeProto, is that allowed?
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> > > Slava
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> > GenericType.SomeProto (without parameters) is the only spelling that is allowed. There is no GenericType<Int>.SomeProto.
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> > That way we avoid every bound-generic type creating a new protocol.   
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> 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.
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> 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.    
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> 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.
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> 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.
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> Overall the proposal looks really great!    I’m really looking forward to this feature.    
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> > 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.
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> > - Karl
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> >   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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