Error when trying to follow protocol delegation docs

Meta question: I am kinda lost on how exactly one uses Documentation. Currently it shows me the docs for 5.10 beta. I am on 5.9.2 and I would like to check the docs for that.

For the actual question, I am not sure if this is because of the version mismatch, but I am trying to follow Documentation. The code shown there (complete example as follows) does not work. I get the following error

error: protocol 'Delegate' cannot be nested inside another declaration

But the docs claim

Protocols can be nested inside of type declarations like structures and classes, as long as the outer declaration isn’t generic.

This is the example from the link above. I just added the RandomNumberGenerator and LinearCongruentialGenerator implementation for completeness. I do not think I did a naive copy paste / typo here.

protocol RandomNumberGenerator {
    func random() -> Double

class LinearCongruentialGenerator: RandomNumberGenerator {
    var lastRandom = 42.0
    let m = 139968.0
    let a = 3877.0
    let c = 29573.0
    func random() -> Double {
        lastRandom = ((lastRandom * a + c)
        return lastRandom / m

class DiceGame {
    let sides: Int
    let generator = LinearCongruentialGenerator()
    weak var delegate: Delegate?

    init(sides: Int) {
        self.sides = sides

    func roll() -> Int {
        return Int(generator.random() * Double(sides)) + 1

    func play(rounds: Int) {
        for round in 1...rounds {
            let player1 = roll()
            let player2 = roll()
            if player1 == player2 {
                delegate?.game(self, didEndRound: round, winner: nil)
            } else if player1 > player2 {
                delegate?.game(self, didEndRound: round, winner: 1)
            } else {
                delegate?.game(self, didEndRound: round, winner: 2)

    protocol Delegate: AnyObject {
        func gameDidStart(_ game: DiceGame)
        func game(_ game: DiceGame, didEndRound round: Int, winner: Int?)
        func gameDidEnd(_ game: DiceGame)

The "fix" seems to just move Delegate outside the DiceGame class, but still, curious to know if this is an error due to version mismatch, if so, how exactly do I get the docs for a version I want (5.9.2)?

So I think I answered one half, it is due to version mismatch swift-evolution/proposals/ at main · apple/swift-evolution · GitHub.

The only other question is how do I follow the correct doc version. I find it highly unlikely that there are no official versioned docs.

Good question!

There don't seem to be archived copies of the documentation for prior versions. This has been a longstanding issue: in the past, the ebook version was available simultaneously for both the beta and release versions, but there hasn't been an ebook version lately. You can try to use

Nested protocols are a new feature added to the language in SE-0404, and this part of the book was only updated a few weeks ago. Indeed, in Swift 5.9, you'll need to not nest the Delegate.


Thank you for confirming :slightly_smiling_face:. I will try using the link you mentioned, helpful.

Edit: Thanks for also linking the book repo, I was trying to find the docs in a different repo, had no idea this existed, so thanks again.

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