Using Some Protocol as a Concrete Type Conforming to Another Protocol Is Not Supported

Using 'Protocol' as a concrete type conforming to protocol 'Protocol' is not supported

Fix is replacing Element: Animal with Element == Animal.

Using some protocol as a concrete type conforming to another protocol is not supported

The underlying reason for the limitation is that Swift doesn't have first-class metatypes. The simplest example is that this doesn't work:

func isEmpty(xs: Array) -> Bool {
    return xs.count == 0
}

In theory, this code could work, and if it did there would be a lot of other types I could make (like Functor and Monad, which really can't be expressed in Swift today). But you can't. You need to help Swift nail this down to a concrete type. Often we do that with generics:

func isEmpty<T>(xs: [T]) -> Bool {
    return xs.count == 0
}

Notice that T is totally redundant here. There is no reason I should have to express it; it's never used. But Swift requires it so it can turn the abstract Array into the concrete [T]. The same is true in your case.

This is a concrete type (well, it's an abstract type that will be turned into a concrete type any time it's instantiated and P is filled in):

class ImplementProtocolA<P : ProtocolA>

This is a fully abstract type that Swift doesn't have any rule to turn into a concrete type:

class ImplementProtocolB : ImplementProtocolA<ProtocolB>

You need to make it concrete. This will compile:

class ImplementProtocolB<T: ProtocolB> : ImplementProtocolA<T> {}

And also:

class UserDemoPresenter<T: GetUserView> : Presenter {
    typealias V = T
}

Just because you're likely to run into the issue later: your life will go much easier if you'll make these structs or final classes. Mixing protocols, generics, and class polymorphism is full of very sharp edges. Sometimes you're lucky and it just won't compile. Sometimes it will call things you don't expect.

You may be interested in A Little Respect for AnySequence which details some related issues.

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private var presenter : UserDemoPresenter<GetUserView>

This is still an abstract type. You mean:

final class Something<T: GetUserView> {
    private var presenter: UserDemoPresenter<T>
}

If that creates a problem, you'll need to create a box. See Protocol doesn't conform to itself? for discussion of how you type-erase so that you can hold abstract types. But you need to work in concrete types. You can't ultimately specialize on a protocol. You must eventually specialize on something concrete in the majority of cases.

Using as a concrete type conforming to protocol AnyObject is not supported

I had the same idea to create weak container with generics.

As result I created wrapper for NSHashTable and did some workaround for your compiler error.

class WeakSet<ObjectType>: SequenceType {

    var count: Int {
        return weakStorage.count
    }

    private let weakStorage = NSHashTable.weakObjectsHashTable()

    func addObject(object: ObjectType) {
        guard object is AnyObject else { fatalError("Object (\(object)) should be subclass of AnyObject") }
        weakStorage.addObject(object as? AnyObject)
    }

    func removeObject(object: ObjectType) {
        guard object is AnyObject else { fatalError("Object (\(object)) should be subclass of AnyObject") }
        weakStorage.removeObject(object as? AnyObject)
    }

    func removeAllObjects() {
        weakStorage.removeAllObjects()
    }

    func containsObject(object: ObjectType) -> Bool {
        guard object is AnyObject else { fatalError("Object (\(object)) should be subclass of AnyObject") }
        return weakStorage.containsObject(object as? AnyObject)
    }

    func generate() -> AnyGenerator<ObjectType> {
        let enumerator = weakStorage.objectEnumerator()
        return anyGenerator {
            return enumerator.nextObject() as! ObjectType?
        }
    }
}

Usage:

protocol MyDelegate : AnyObject {
    func doWork()
}

class MyClass: AnyObject, MyDelegate {
    fun doWork() {
        // Do delegated work.
    }
}

var delegates = WeakSet<MyDelegate>()
delegates.addObject(MyClass())

for delegate in delegates {
    delegate.doWork()
}

It's not the best solution, because WeakSet can be initialized with any type, and if this type doesn't conform to AnyObject protocol then app will crash. But I don't see any better solution right now.

Unable to use protocol as associatedtype in another protocol in Swift

The problem, which David has already alluded to, is that once you constrain a protocol's associatedtype to a specific (non @objc) protocol, you have to use a concrete type to satisfy that requirement.

This is because protocols don't conform to themselves – therefore meaning that you cannot use Address to satisfy the protocol's associated type requirement of a type that conforms to Validator, as Address is not a type that conforms to Validator.

As I demonstrate in my answer here, consider the counter-example of:

protocol Validator {
    init()
}
protocol Address : Validator {}

protocol FormRepresentable {
    associatedtype ValueWrapper: Validator
}

extension FormRepresentable {
    static func foo() {
        // if ValueWrapper were allowed to be an Address or Validator,
        // what instance should we be constructing here?
        // we cannot create an instance of a protocol.
        print(ValueWrapper.init())
    }
}

// therefore, we cannot say:
enum AddressFrom : FormRepresentable {
    typealias ValueWrapper = Address
}

The simplest solution would be to ditch the Validator protocol constraint on your ValueWrapper associated type, allowing you to use an abstract type in the method argument.

protocol FormRepresentable {
    associatedtype ValueWrapper
    func valueForDetail(valueWrapper: ValueWrapper) -> String
}

enum AddressFrom : Int, FormRepresentable {

    // ...

    func valueForDetail(valueWrapper: Address) -> String {
        // ...
    }
}

If you need the associated type constraint, and each AddressFrom instance only expects a single concrete implementation of Address as an input – you could use generics in order for your AddressFrom to be initialised with a given concrete type of address to be used in your method.

protocol FormRepresentable {
    associatedtype ValueWrapper : Validator
    func valueForDetail(valueWrapper: ValueWrapper) -> String
}

enum AddressFrom<T : Address> : Int, FormRepresentable {

    // ...

    func valueForDetail(valueWrapper: T) -> String {
        // ...
    }
}

// replace ShippingAddress with whatever concrete type you want AddressFrom to use
let addressFrom = AddressFrom<ShippingAddress>.Address1

However, if you require both the associated type constraint and each AddressFrom instance must be able to handle an input of any type of Address – you'll have use a type erasure in order to wrap an arbitrary Address in a concrete type.

protocol FormRepresentable {
    associatedtype ValueWrapper : Validator
    func valueForDetail(valueWrapper: ValueWrapper) -> String
}

struct AnyAddress : Address {

    private var _base: Address

    var addressLine1: String {
        get {return _base.addressLine1}
        set {_base.addressLine1 = newValue}
    }
    var country: String {
        get {return _base.country}
        set {_base.country = newValue}
    }
    var city: String {
        get {return _base.city}
        set {_base.city = newValue}
    }

    init(_ base: Address) {
        _base = base
    }
}

enum AddressFrom : Int, FormRepresentable {

    // ...

    func valueForDetail(valueWrapper: AnyAddress) -> String {
        // ...
    }
}

let addressFrom = AddressFrom.Address1

let address = ShippingAddress(addressLine1: "", city: "", country: "")

addressFrom.valueForDetail(AnyAddress(address))

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