Swift: Accomplishing Dynamic Dispatch on PATs (Protocol with Associated Types)

The problem with this is that any time you add a subclass to Animal, you need to manually update AnyAnimal to handle it.

It’s possible to do this more generically, because you know the animal’s type in the AnyAnimal constructor and that’s sufficient to declare its subsequent behavior:

  struct AnyAnimal: Animal {
    let name: String
    private let walker: () -> Void
    private let eater: (Any) -> Void
    
    init<T: Animal>(_ animal: T) {
      name = animal.name
      walker = { animal.walk() }
      eater = { food in
        guard let f = food as? T.FoodType else { return }
        animal.eat(food: f)
      }
    }
    
    func walk(){ walker() }
    
    func eat(food: Any){ eater(food) }
  }