SE-0216: User-defined dynamically callable types

Evolution Proposal Reviews
20

@Chris_Lattner3 and @dan-zheng, in the proposed solution:

We propose introducing a new @dynamicCallable attribute to the Swift language which may be applied to structs, classes, enums, and protocols. This follows the precedent of SE-0195.

But the @dynamicMemberLookup attribute cannot be applied to protocols, AFAIK.

import Foundation
import JavaScriptCore

@dynamicMemberLookup
public protocol JSDynamicMemberLookup {

  subscript(dynamicMember name: String) -> JSValue { get set }
}

// error: @dynamicMemberLookup attribute requires 'JSDynamicMemberLookup'
// to have a 'subscript(dynamicMember:)' member with a string index

(I was trying to add this to existing Objective-C classes, such as JSContext and JSValue).