Protocol with associated type as property

Question

Is it possible to use protocol with associated type as a property to match some constraints?

Here is an example which I'm struggling with:

protocol Animal {

    associatedtype Item

    func doSomething(with item: Item)
}

class Owner<Item> {

    var item: Item
    // I have to be sure that Animal.Item == Item
    var animal: Animal
}

Answer 1

Protocols with associated types cannot be used as the type of a property. Instead of constraining animal 's type, you can try constraining the generic type of Owner :

class Owner<AnimalType> where AnimalType : Animal {
    var item: AnimalType.Item?
    var animal: AnimalType?
}

You don't need Item because you can just use AnimalType.Item for this. That's just how associated types work. Say you want a function that accepts an Item , you can just use AnimalType.Item in place of that:

func someFunc(someParameter: AnimalType.Item) { ... }

Or maybe this might fit your needs?

class Owner<AnimalType, ItemType> where AnimalType : Animal, AnimalType.Item == ItemType {
    var item: ItemType?
    var animal: AnimalType?
}

Answer 2

With Swift 5.7 there is now a concept of a "primary" associatedtype which allows you to do this:

protocol Animal<Item> {
   associatedtype Item 
   func doSomething(with item: Item)
}

You then set it as a property like this:

class Owner<Item> {
    var item: Item
    var animal: any Animal<Item>
}

See https://www.donnywals.com/what-are-primary-associated-types-in-swift-5-7/