[swift-evolution] Binding generic types using a metatype parameter

[Joe Groff]

> On Mar 24, 2016, at 4:04 PM, Joanna Carter <joanna at carterconsulting.org.uk <mailto:joanna at carterconsulting.org.uk>> wrote:
> 
> Unfortunately, this doesn't compile…
> 
> extension PropertyType
> {
>  private static func create() -> Property<Self>
>  {
>    return Property<Self>.init()
>                                        ^ '>' is not a postfix unary operator
>  }
> }
> 
> Which is why I found it necessary to add a typealias like so…
> 
> extension PropertyType
> {
>  private static func create() -> Property<Self>
>  {
>    typealias SelfType = Self
> 
>    return Property<SelfType>.init()
>  }
> }

Oops, that's definitely a bug.

> This doesn't compile either…
> 
> struct PropertyFactory
> {
>  static func createBoundPropertywithValueType(valueType: PropertyType.Type) -> PropertyProtocol
>  {
>    return valueType.create()
>                ^ Member 'create' cannot be used on value of protocol type 'PropertyType'; use a generic constraint instead
>  }
> }
> 
> For some reason, as soon as you change the PropertyType extension static func to return PropertyProtocol…
> 
> extension PropertyType
> {
>  private static func create() -> PropertyProtocol
>  {
>    typealias SelfType = Self
> 
>    return Property<SelfType>.init()
>  }
> }
> 
> … then, and only then, the factory's method compiles correctly.

Oops, my fault, this one is by design. Since we're calling through a dynamic type, our type system isn't able to express the result type `Property<valueType>`, so we need to abstract it behind the PropertyProtocol.

> Of course, after all this effort, I still have the problem of how to get the Any.Type from the SubjectType property of a Mirror into a PropertyType.Type.

That one's easy, at least—you can take `Any.Type` and use `as?` to cast it to `PropertyType.Type`.

> What would really make life easier here would be the ability to extend Any.
> 
> Any chance?

I think that'd be cool. Some people fear the power (and ensuing responsibility) that unleashes, though…

-Joe

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160324/1292015d/attachment.html>