import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var display: UILabel?
    var userIsTyping = false

    @IBAction func touchDigit(_ sender: UIButton) {
        let digit = sender.currentTitle!
        if userIsTyping {
            let textCurrentInDisplay = display!.text! // will not be executed warning
            display!.text = textCurrentInDisplay + digit
        }
        else {
           display!.text = digit
            userIsTyping = true;
        }
    }



}