What's the state of higher kind types?

Evolution Discussion
13

Parameterized associate types are how ML languages model higher kinded types0, but they're free functions (or functions in a module). I think the equivalent would be making a separate mapper type:

protocol Mappable {
  associatedtype Container<A>
  static func map<Element, Result>(
    _ base: Container<Element>, 
    transform: (Element) -> Result
  ) -> Container<Result>
}

struct ArrayMapper: Mappable {
  typealias Container<A> = [A]
  static func map<Element, Result>(
    _ base: [Element],  
    _ transform: (Element) -> Result
  ) -> [Result]
}

We could then extend array so we still have method syntax:

extension Array {
  func map<Result>(_ transform: (Element) -> Result) -> [Result] {
    ArrayMapper.map(self, transform)
  }
}

I'm not sure how you'd define the protocol so that Container is Self or if it's possible.

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