Matching enum value in switch when enum implements a protocol

Question

I've got a protocol

protocol P { }

and it's implemented by an enum

enum E: P {
    case a
    case b
}

So far, so good.

I want to be able to receive instances of P, and return a specific value if it's one of E (there will be other enums / structs etc implementing P in the future).

I've tried this:

extension P {

    var value: String {
        switch self {
            case E.a: return "This is an E.a"
            default: return "meh"
        }
    }
}

But this doesn't compile

error: Temp.playground:14:16: error: enum case 'a' is not a member of type 'Self'
    case E.a: return "hello"

I've also tried:

 case is E.a: return "This is an E.a"

which just gives this error:

 error: Temp.playground:14:19: error: enum element 'a' is not a member type of 'E'
     case is E.a: return "hello"
             ~ ^

I know I could do it like this:

switch self {
    case let e as E:
        switch e {
            case E.a: return "hello"
            default: return "meh"
        }
    default: return "meh"
}

but I really really don't want to!

Is there a syntax or technique I'm missing?

Answer 1

You need to match against type E before you can test against the value E.a, but this can be done in a single expression:

extension P {    
    var value: String? {
        switch self {
        case let e as E where e == .a:
            return "hello"
        default:
            return nil
        }
    }
}

Answer 2

You can define your P extension with specific type enum in your case

create extension like this

extension P where Self == E {
    var value:String {
        switch self  {
        case .a  :
                return "A"
        case .b:
            return "b"

        }
    }
}

let e = E.a.value
print(e)

Output :

"A"