I'm guessing this is a bug (submitted bug #19223563, but if it isn't, you'll be surprised to learn that in Xcode Version 6.1.1 you cannot append to a String from a UITextView or UITextField.
The problem is that both the UITextField and the UITextView are using an implicitly unwrapped optional String (String!) as the type. This is messing up the +=, so you either have to break apart the statement (defeating the purpose of +=). Or you have do a force unwrap before the +=.
If you don't you'll get a strange Swift Compiler Error stating that 'String!' is not identical to 'UInt8'
var numberString: String! = "0" numberString = numberString + "2" numberString! += "1" numberString += "1" //'String!' is not identical to 'UInt8' var textView = UITextView() textView.text += "More text" //'String!' is not identical to 'UInt8' var textField = UITextField() textField.text += "Even more text" //'String!' is not identical to 'UInt8'