swift - Can you extend an enum?

Question

I use enums to store string values like this:

    enum Animals: String {
        case descCat = "I has attitude"
        case descDog = "how can I help"
        case descGator = "I will eat you"
        var s: String {
            get {
                return self.rawValue as String
            }
        }
    }

Then I access them like this:

print("Dogs be like:" + Animals.descDog.s)

My question is can I extend enums like any other struct or object so I don't have to add the var s: String {} property to each enum?

question from:https://stackoverflow.com/questions/33444932/can-you-extend-an-enum

Answer 1

You want to add a property to all enums whose raw value is a string? This sounds like a case for constrained protocol extensions!

extension RawRepresentable where RawValue == String {
    var description: String {
        return rawValue
    }
}

This works because all enums with a raw value automatically conform to the RawRepresentable protocol, and said protocol has an associated type RawValue that tells you which type the raw value is.

Now your Animals enum will automatically inherit it:

print(Animals.descCat.description) // -> "I has attitude"

Notice that string enums are themselves already CustomStringConvertible, so they already have a description property (that returns the name of the enum case), and yours doesn't override it:

print(Animals.descCat) // -> "descCat"

If you want your description to override the default, just add a declaration of CustomStringConvertible conformance to your enum:

private enum Animals: String, CustomStringConvertible { /*...*/ }
print(Animals.descCat) // -> "I has attitude"

---

You can also extend this idea to cover other raw value types. For example:

extension RawRepresentable where RawValue: CustomStringConvertible {
    var description: String {
        return rawValue.description
    }
}

Now, you can get automatic descriptions for enums whose raw value is Int or even a custom type (so long as that type has a description of its own).