prototype

prototype.swift

public protocol PropertyDescribing {
  /// The type or metatype the property belongs to.
  associatedtype Container

  /// The expected static type.
  associatedtype Content
  
  /// The name of the property.
  var name: String { get }
  
  /// The type of the property at runtime.
  /// Assertion: `type` can be exactly `Content`, a subtype of `Content` or 
  /// a type that fits `Content` if `Content` is an existential (incl. `Any`).
  var type: Any.Type { get }
}
public protocol MutablePropertyDescribing: PropertyDescribing {}

public struct PropertyDescriptor<Content, Container>: PropertyDescribing { 
  /// ...

  /// Specializes the descriptor at runtime to `Value`.
  /// `Value` can exactly `type`, a supertype of `type` or an existential (incl. `Any`)
  /// that matches `type`.
  ///
  /// `Value` needn't be inferred from the return type if the specializing function
  /// proposal is accepted.
  func specializing<Value>() -> PropertyDescriptor<Value, Container>
}
public struct MutablePropertyDescriptor<Content, Container>: MutablePropertyDescribing {
  /// ...
  
  /// Ditto.
  func specializing<Value>() -> MutablePropertyDescriptor<Value, Container>
}

@magic protocol Reflectable {
  public static func classProperties: [String: PropertyDescriptor<Any, Self>]
  public static func mutableClassProperties: [String: MutablePropertyDescriptor<Any, Self>]

  public static func properties: [String: PropertyDescriptor<Any, Self>]
  public static func mutableProperties: [String: MutablePropertyDescriptor<Any, Self>]

  /// Getter for instance properties and class properties.
  public func getValue<Descriptor: PropertyDescribing where Descriptor.Container == Self>(using descriptor: Descriptor) -> Descriptor.Content
  public mutating func setValue<Descriptor: MutablePropertyDescribing where Descriptor.Container == Self>(value: Descriptor.Content, using descriptor: Descriptor)

  /// Setter forinstance properties and class properties.
  public static func getValue<Descriptor: PropertyDescribing where Descriptor.Container == Self>(using descriptor: Descriptor) -> Descriptor.Content
  public mutating static func setValue<Descriptor: MutablePropertyDescribing where Descriptor.Container == Self>(value: Descriptor.Content, using descriptor: Descriptor)
}

func callSite() {
  /// Assuming `ReflectableClass` conforms to `CustomStringConvertible`.
  let myObject = ReflectableClass()

  /// "Non-typed" version.
  let descriptor1 = ReflectableClass.properties["description"]!
  let description1: Any = myObject.getValue(using: descriptor1)
  
  /// Typed version.
  let descriptor2 = ReflectableClass.properties["description"]!.specializing<String>()
  let description2: String = myObject.getValue(using: descriptor2)

  /// Let's try to request for a unrelated type.
  let descriptor2 = ReflectableClass.properties["description"]!.specializing<Array<String>>()
  /// Should result in a runtime error: The property `description` of type `String` in `ReflectableClass` is inconvertible to `Array<String>`.

  /// Struct
  let myValue = ReflectableStruct()
  
  let descriptor3 = ReflectableStruct.mutableProperties["message"]!.specializing<String>()
  myValue.setValue("Testing", using: descriptor3) /// Compiler Error: cannot mutate `myValue`.
}