How to test equality of Swift enums with associated values

Question

I want to test the equality of two Swift enum values. For example:

enum SimpleToken {
    case Name(String)
    case Number(Int)
}
let t1 = SimpleToken.Number(123)
let t2 = SimpleToken.Number(123)
XCTAssert(t1 == t2)

However, the compiler won't compile the equality expression:

error: could not find an overload for '==' that accepts the supplied arguments
    XCTAssert(t1 == t2)
    ^~~~~~~~~~~~~~~~~~~

Do I have do define my own overload of the equality operator? I was hoping the Swift compiler would handle it automatically, much like Scala and Ocaml do.

Answer 1

Swift 4.1+

As @jedwidz has helpfully pointed out, from Swift 4.1 (due to SE-0185, Swift also supports synthesizing Equatable and Hashable for enums with associated values.

So if you're on Swift 4.1 or newer, the following will automatically synthesize the necessary methods such that XCTAssert(t1 == t2) works. The key is to add the Equatable protocol to your enum.

enum SimpleToken: Equatable {
    case Name(String)
    case Number(Int)
}
let t1 = SimpleToken.Number(123)
let t2 = SimpleToken.Number(123)

Before Swift 4.1

As others have noted, Swift doesn't synthesize the necessary equality operators automatically. Let me propose a cleaner (IMHO) implementation, though:

enum SimpleToken: Equatable {
    case Name(String)
    case Number(Int)
}

public func ==(lhs: SimpleToken, rhs: SimpleToken) -> Bool {
    switch (lhs, rhs) {
    case let (.Name(a),   .Name(b)),
         let (.Number(a), .Number(b)):
      return a == b
    default:
      return false
    }
}

It's far from ideal — there's a lot of repetition — but at least you don't need to do nested switches with if-statements inside.