Protocol variable implementing another protocol

Question

I'm trying to do something like that, but then, ParentC doesn't conform to Parent because its children member isn't Child but ChildC

Which is weird because ChildC implements Child...

Is this a limit of Swift? Or is there is a way of doing that?

(I do not ask for an alternative solution, I want to know if this is possible)

protocol Parent: Codable {
    var children: [Child] { get }
}

protocol Child: Codable {
    var name: String { get }
}

struct ParentC: Parent {
    var children: [ChildC]
}

struct ChildC: Child {
    var name: String
}

@Hamish Impressive answer, I understand it's not possible, moreover, the alternative proposed in your answer would force to implement `encode` and `decode` of `Codable`, such a shame :( — Tancrede Chazallet, Mar 29 '18 at 11:13
You should be able to get an automatic `Codable` conformance, see for example https://gist.github.com/hamishknight/e64305a10448610a433a6077733888f1 — Hamish, Mar 29 '18 at 11:24
@Hamish that's a great tweak to get it to work (option 1)! Thanks — Tancrede Chazallet, Mar 29 '18 at 11:44
Option 2 is more flexible. Also, @Hamish instead of `gist` link in the comments it should be answer below. — user28434'mstep, Mar 29 '18 at 12:01

score 2 · Answer 1 · answered Mar 30 '18 at 06:29

You can workaround this variance "limitation" by making Parent a protocol with associated type:

protocol Parent: Codable {
    associatedtype ChildType: Child
    var children: [ChildType] { get }
}

This will impact the places you can use Parent though, since protocols with associated types have some restrictions on where to use them.

Protocol variable implementing another protocol

1 Answers1

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