Restriction on associatedtype to an Array

Question

I have a protocol with an associatedtype and I made some of my classes conform to it:

protocol MyProtocol {
    associatedtype MyType
    func myFunction()-> MyType
}

struct MyStruct: Codable {
    var myVar: Int
}

class MyClass1: MyProtocol {
    func myFunction()-> [MyStruct]? {
        return [MyStruct(myVar: 1)]
    }
}

class MyClass2: MyProtocol {
    func myFunction()-> Int? {
        return 1
    }
}

Now I want to create a protocol that can only be applied to an object that is conform to MyProtocol and its associatedtype is an array of Codable like so:

protocol MyProtocolCodable where Self: MyProtocol, MyType: [Codable] {}

However I get an error:

Type 'Self.MyType' constrained to non-protocol, non-class type '[Codable]' (aka 'Array')

How can I get around this problem and apply my restriction?

Note: I get the same error when trying to restrict with any kind of Array, but works fine with other types:

protocol MyProtocolCodable where Self: MyProtocol, MyType: [Int] {} 
protocol MyProtocolCodable where Self: MyProtocol, MyType: Array {}

user_8697793 · Answer

This is because Array is not a protocol, it's a struct:

@frozen struct Array

You can try to conform to Sequence instead:

protocol MyProtocolCodable where Self: MyProtocol, MyType: Sequence, MyType.Element: Codable {}

user_6638682 · Answer

You should use '==' to make associatedType constraints, replace yours instead of:

protocol MyProtocolCodable where Self: MyProtocol, MyType == [Codable] {}

Restriction on associatedtype to an Array

2 Answers