[swift-evolution] [RFC] #Self

Patrick Smith pgwsmith at gmail.com
Tue May 10 20:45:25 CDT 2016

I think Type fits really well. Because every class, struct, and enum gets a static member called ‘Type’. But you can only use it outside the declaration (String.Type), not inside.

So similar to how types declared inside can be referenced:

struct A {
  enum Kind {
    case cat
    case dog

  var kind: Kind // Use nested type here; I don’t have to write A.Kind

We could use Type in a similar way:

struct B {
  // This gets created automatically for every type, and is accessible today from B.Type
  //metatype Type { … }

  var children: [Type] // Use here in the same way as Kind above

> On 11 May 2016, at 8:32 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> Sent from my iPad
> On May 10, 2016, at 5:24 PM, Hooman Mehr <hooman at mac.com <mailto:hooman at mac.com>> wrote:
>>> On May 10, 2016, at 2:49 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> That said, I’m not sure I understand the concrete use-cases.  When is this concept important?  When is “Self” not good enough?
>>> The only case where there is new functionality is when this is used in a protocol requirement.  I gave an example earlier today.  
>> This functionality is the key: Ability of an open (non-final) class to conform to a protocol that lets it return an instance of the conforming type (itself). Self does not work for that and we can’t change its behavior (or can we?) So one solution seems to be Matt’s proposal. This functionality is important for me and an example use case is class clusters. For the client code it is sealed and acts just like a final class, but internally it may return a subclass that is an implementation detail. We should be able to do this.
> Agree and this is why I am willing to write the proposal for this.  There was a discussion a few months ago about this problem and a few solutions were kicked around.  The biggest problem with this approach at the time was lack of a good name, which I believe we now have in Type.
> I'm going to let the discussion continue for a day or two and will write a proposal if no significant counter arguments arise.
> -Matthew
>> Hooman
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