[swift-users] Making Error sub-enums Equatable

Brent Royal-Gordon brent at architechies.com
Wed May 10 03:23:21 CDT 2017

> On May 8, 2017, at 2:01 AM, Rick Mann via swift-users <swift-users at swift.org> wrote:
> Seriously, I've been googling this for a half-hour, and I can't find an answer (everything that comes up is for ErrorType, absolutely nothing for Error).
> I have an enum:
> enum MyErrors : Error
> {
>    case one(String)
>    case two
>    case three(String)
> }
> let a: MyErrors = .one("foo")
> let b = .two
> let c = .towo
> I want to compare them with ==, and I don't care about the associated types. I can't for the life of me figure out how without an exhaustive switch statement in a == definition. Is that the only way?

Yes, the correct way to compare two enums is with a `switch` statement.

The good news is, Swift's `switch` statement is good enough that these aren't terribly difficult to write. My preferred pattern (given your "ignore the associated type" semantic) is:

	extension MyErrors: Equatable {
		static func == (lhs: MyErrors, rhs: MyErrors) -> Bool {
			switch (lhs, rhs) {
			case (.one, .one):
				return true
			case (.two, .two):
				return true
			case (.three, .three):
				return true
			case (.one, _), (.two, _), (.three, _):
				return false

You do it this way instead of using `default:` so that, if you add another case later, it won't just get matched by the `default:` and always return `false`.

(P.S. I would suggest using a name like `MyError`, not `MyErrors`. A given instance of `MyError` only represents one of the errors, not several of them.)

Brent Royal-Gordon

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