[Swift] Extra argument ‘completion’ in call

In iOS 8 SDK Development, the use of a closure to act on a response from the user when we ask them to use their twitter account produces the following compilation error:

Extra argument 'completion' in call

The error is misleading because there is no “extra argument” in the call to “accountStore.requestAccessToAccountsWithType“. Below the problem is on line #6. See it?

func reloadTweets() {
  let accountStore = ACAccountStore()
  let twitterAccountType = accountStore.accountTypeWithAccountTypeIdentifier(ACAccountTypeIdentifierTwitter)
  
  accountStore.requestAccessToAccountsWithType(twitterAccountType, options: nil, completion:
    { (granted: Bool, error: NSError) -> Void in
      if (!granted) {
        println("account access not granted")
      } else {
        println("account access granted")
      }
    })
}

It does not stand out and I only figured it out because I recognized the closure for ‘completion:‘ is a type-alias to ‘ACAccountStoreRequestAccessCompletionHandler‘, which is documented as:

typealias ACAccountStoreRequestAccessCompletionHandler = (Bool, NSError!) -> Void

Adding the exclamation mark (!) after NSError to force unwrap solve the error. The confusing part to me is NSError is not an optional type (at least it is not clear in the definition of NSError and I am not too familiar with Objective-C). Why does it need to be forced unwrapped? Would appreciate any further explanation on this from anyone watching. 🙂