Avoiding Type Erasure when implementing the Repository Pattern

Question

I'm trying to implement the repository pattern in Swift in a generic way. The problem that I'm currently facing is, that it seems like I have to write type erasure wrappers for all my repositories. Am I missing something here? Is there a better way to do this or to make the compiler happy at this point?

// 1
class Item {}

// 2
protocol Repository {
    associatedtype T
}

// 3
protocol AnyItemRepository: Repository where T == Item {}

// 4
class ItemRepository: AnyItemRepository {
    static let shared = ItemRepository()

    private init() {}
}

// 5
class ViewController {

    // 6
    var itemRepository: AnyItemRepository? = ItemRepository.shared

}

1. One of many entities
2. The base repository interface that can be extended if necessary or implemented directly
3. A special item repository interface that guarantees additional functionality on top of the base repository
4. Concrete repository implementation for a specific entity type
5. Some class that needs to access the data
6. Dependency to any item repository. The compiler errors on this line: Protocol 'AnyItemRepository' can only be used as a generic constraint because it has Self or associated type requirements

Answer 1

You don't need the AnyItemRepository type. Just write extension methods on Repository like so:

public extension Repository where T == Item {
   func doSomethingSpecial(with items: [Item]) {
      // blah blah
   }
}

In your view controller, you can't use Repository or AnyItemRepository in this way because these are generic type constraints. You must either use a concrete type or generically parameterize ViewController.

class RepositoryViewController<R>: UIViewController where R: Repository, R.T == Item {
    var itemRepository: R { get }
}

class ViewController: RepositoryViewController<ItemRepository> {
   override var itemRepository: ItemRepository {
      return ItemRepository.shared
   }
}

(The above is untested pseudocode designed to give you the gist. It has never been run by anyone ever and may not even compile.)