Generic Types are Not That Worse than Protocols

GenericTypesNotThatWorseThanProtocols.swift

// Response to http://www.russbishop.net/swift-associated-types:
// 1. Type parameter leaking is a real problem(the `S` parameter of `buyFoodAndFeed` below can not be omitted — "We have no way to express that we don't care about the Supplement type parameter") but not as severe as is described in the article because non-generic subclasses of generic superclasses do not need type parameter re-specification when being used.
// 2. It is not true that "we have no way to link the type of F to Food". We can link Animal.F to Store.G. See `buyFoodAndFeed` below.

class Food {
    required init() {
        
    }
}

class Grass: Food {
    
}

class Supplement {
    
}

class Salt: Supplement {
    
}

class Animal<F: Food, S: Supplement> {
    func feed(f: F) {
        print("Yummy!")
    }
    
    func supplement(s: S) {
        
    }
}

class Cow: Animal<Grass, Salt> {
    override func feed(grass: Grass) {
        print("Moo!")
    }
}

class Holstein: Cow {
    override func feed(grass: Grass) {
        print("Hoo!")
    }
}

class Store<G> {

}

func buyFoodAndFeed<F, S>(animal: Animal<F, S>, store: Store<F>) {
    animal.feed(F())
}

buyFoodAndFeed(Cow(), store: Store<Grass>())