Why is Swift creating optional parameter without '?'

Question

While trying to implement the "experiment" on page 35 of the Swift programming guide I get a very strange error in the Xcode playground. In the pokerDeck method, on the line beginning with let card... in the innermost loop, I get the error Value of optional type 'Rank?' not unwrapped. How/why is Rank optional?

struct Card {
    var rank : Rank
    var suit : Suit
    init(rank : Rank, suit : Suit) {
        self.rank = rank
        self.suit = suit
    }
    func pokerDeck() -> Card[] {
        let suits = [Suit.Spades, Suit.Clubs, Suit.Hearts, Suit.Diamonds]
        var deck = Card[]()
        for suit in suits {
            for r in 1 ... 13 {
                let card = Card(rank: Rank.fromRaw(r), suit: suit) // ERROR
                deck += card
            }
        }
        return deck
    }
}

The suggested fix by Xcode is to add a !

let card = Card(rank: Rank.fromRaw(r)!, suit: suit)

Which makes the problem even murkier.

Answer 1

The fromRaw() method of an Enum doesn't know what you will pass to it – it can't know if it will succeed on every call – so they designed that method with a return type of Rank? (the return type is an optional)

the ! operator (known as forced unwrapping) forces an optional to be converted into an Enum (and will cause an error if it is nil)