[swift-evolution] [Discussion] Seal `T.Type` into `Type<T>`

[Chris Lattner]

> On Jul 8, 2016, at 1:06 PM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hello Swift community, before Swift 3 drops I’d like to discuss if it is reasonable to consider to create a standalone type to drop/seal the T.Type magic?
> 
Hi Adrian,

What’s the motivation for this change?

-Chris

> However this change does not play well with the proposal of dropping the .self compiler magic.
> 
> Some bikeshedding:
> 
> public struct Type<T> : Hashable {
>      
>     // Seal the `.Type` magic into this standalone type.
>     // Disallow `.Type` usage elsewhere
>      
>     public let sealed: T.Type
>      
>     public var hashValue: Int { /* implement somehow */ }
>      
>     public init() {
>          
>         // Or do it somehow different for the sake of dropping `.self` magic
>         self.sealed = T.self
>     }
> }
> 
> public func ==<T>(lhs: Type<T>, rhs: Type<T>) -> Bool {
>      
>     return lhs.hashValue == rhs.hashValue
> }
> Downside of this approach is the accessibility of the .Type instance:
> 
> protocol Initializable {
>     init()
> }
> 
> func foo<T : Initializable>(type: Type<T>) -> T {
>      
>     // It would be nice if we could use `type.init()` instead, but than we
>     // would need more compiler magic :/
>     return type.sealed.init()
> }
> I couldn’t come up with a better name than sealed, if anyone has a better idea feel free to share your thoughts.
> 
> Remember this is a discussion and not (yet) a detailed proposal, where everyone can provide feedback and bikeshedding for a better design.
> 
> 
> 
> 
> -- 
> Adrian Zubarev
> Sent with Airmail
> 
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