associatedTypes.swift

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

struct Kibble {}

class Cat: Animal {
    func eat(_ food: Kibble) {
        // code
    }
}

struct DogFood {}

class Dog: Animal {
    func eat(_ food: DogFood) {
        //
    }
}

let cat = Cat()
let dog = Dog()