Swift: Nil is incompatible with return type String

Does your function declare an optional return type?

func foo() -> String? { ...

See more on: https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html

NOTE

The concept of optionals doesn’t exist in C or Objective-C. The nearest thing in Objective-C is the ability to return nil from a method that would otherwise return an object, with nil meaning “the absence of a valid object.”

You have to tell the compiler that you want to return nil. How do you that? By assigning ? after your object. For instance, take a look at this code:

func newFriend(friendDictionary: [String : String]) -> Friend? {
    guard let name = friendDictionary["name"], let age = friendDictionary["age"] else {
        return nil
    }
    let address = friendDictionary["address"]
    return Friend(name: name, age: age, address: address)
}

Notice how I needed to tell the compiler that my object Friend, which I'm returning, is an optional Friend?. Otherwise it will throw an error.