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Question

Implementing dynamic member lookup on an enum with associated values leads to build errors when there is more than one keypath function.

I have an enum with multiple associates values, as described below:

@dynamicMemberLookup
enum Media {
    case movie(Movie)
    case book(Book)
    
    subscript<T>(dynamicMember keyPath: KeyPath<Entertainment, T>) -> T {
        switch self {
            case .movie(let movie):
                return movie[keyPath: keyPath]
            case .book(let book):
                return book[keyPath: keyPath]
        }
    }
}

The structs Movie and Book inherit protocols Filmed and Written , which themselves inherit from the more generic protocol Entertainment :

protocol Entertainment {
    var isbn: Int { get }
    var title: String { get }
    var sales: Int { get set }
}

protocol Written: Entertainment {
    var author: String { get }
}

protocol Filmed: Entertainment {
    var actors: [String] { get set }
}

struct Movie: Filmed {
    var isbn: Int
    var title: String
    var sales: Int
    var actors: [String]
}

struct Book: Written {
    var isbn: Int
    var title: String
    var sales: Int
    var author: String
}

So far, this works and has allowed me to access variables such as isbn and title , which are defined in Entertainment , from the top-level Media object through the subscript keypath lookup.

let media: Media = .movie(Movie(isbn: 1011, title: "Top Gun", sales: 1000, actors: ["Tom Cruise", "Miles Teller"]))

print(media.title)
print(media.sales)

Now, when I try to add the subscript function below in Media so that I can optionally access variables conforming to either Filmed or Written , I receive an error for the above print statements stating Ambiguous use of 'subscript(dynamicMember:)'

subscript<T>(dynamicMember keyPath: KeyPath<Filmed, T>) -> T? {
    switch self {
        case .movie(let movie):
            return movie[keyPath: keyPath]
        default:
            return nil
    }
}

In this example, the build errors are as follows:

let media: Media = .movie(Movie(isbn: 1011, title: "Top Gun", sales: 1000, actors: ["Tom Cruise", "Miles Teller"]))

print(media.title) // BUILD ERROR: Ambiguous use of 'subscript(dynamicMember:)'
print(media.sales) // BUILD ERROR: Ambiguous use of 'subscript(dynamicMember:)'
print(media.actors) // This doesn't throw an error and successfully logs

When I remove the second subscript function, the build errors are gone.

Please let me know if I'm doing anything wrong in my implementation here.

Answer 1

Figured out the answer:

By changing Filmed and Written to no longer conform to Entertainment , the build errors were removed. The code as following now works:

protocol Entertainment {
    var isbn: Int { get }
    var title: String { get }
    var sales: Int { get set }
}

protocol Written {
    var author: String { get }
}

protocol Filmed {
    var actors: [String] { get set }
}

struct Movie: Entertainment, Filmed {
    var isbn: Int
    var title: String
    var sales: Int
    var actors: [String]
}

struct Book: Entertainment, Written {
    var isbn: Int
    var title: String
    var sales: Int
    var author: String
}

@dynamicMemberLookup
enum Media {
    case movie(Movie)
    case book(Book)
    
    subscript<T>(dynamicMember keyPath: KeyPath<Entertainment, T>) -> T {
        switch self {
            case .movie(let movie):
                return movie[keyPath: keyPath]
            case .book(let book):
                return book[keyPath: keyPath]
        }
    }

    subscript<T>(dynamicMember keyPath: KeyPath<Filmed, T>) -> T? {
        switch self {
            case .movie(let movie):
                return movie[keyPath: keyPath]
            default:
                return nil
        }
    }
}

Here is the output of the print statements:

let media: Media = .movie(Movie(isbn: 1011, title: "Top Gun", sales: 1000, actors: ["Tom Cruise", "Miles Teller"]))

print(media.title) // OUTPUT: "Tom Cruise"
print(media.sales) // OUTPUT: 1000
print(media.actors) // OUTPUT: Optional<[String]>: ["Tom Cruise", "Miles Teller"]