Embrace Swift generics

27:10 - Complete example

protocol AnimalFeed {
  associatedtype CropType: Crop where CropType.Feed == Self
  static func grow() -> CropType
}

protocol Crop {
  associatedtype Feed: AnimalFeed where Feed.CropType == Self
  func harvest() -> Feed
}

protocol Animal {
  associatedtype Feed: AnimalFeed
  func eat(_ food: Feed)
}

struct Farm {
  func feed(_ animal: some Animal) {
    let crop = type(of: animal).Feed.grow()
    let produce = crop.harvest()
    animal.eat(produce)
  }

  func feedAll(_ animals: [any Animal]) {
    for animal in animals {
      feed(animal)
    }
  }
}

struct Cow: Animal {
  func eat(_ food: Hay) {}
}

struct Hay: AnimalFeed {
  static func grow() -> Alfalfa {
    Alfalfa()
  }
}

struct Alfalfa: Crop {
  func harvest() -> Hay {
    Hay()
  }
}