Example of parametric polymorphism from WWDC '22 Embrace Swift Generics

embrace_swift_generics.swift

// https://developer.apple.com/videos/play/wwdc2022/110352/

/**
 some
    - holds a fixed concrete type
    - guarantees type relationships

 any
    - holds an abrititray concreate type
    - erases type relationships
 */

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

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
}

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 Horse: Animal {
    func eat(_ food: Carrot) { }
}

struct Chicken: Animal {
    func eat(_ food: Grain) { }
}

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

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

struct Carrot: AnimalFeed {
    static func grow() -> Root { .init() }
}

struct Root: Crop {
    func harvest() -> Carrot { .init() }
}

struct Grain: AnimalFeed {
    static func grow() -> Wheat { .init() }
}

struct Wheat: Crop {
    func harvest() -> Grain { .init() }
}