Crude counter view controller

CounterViewController.swift

var appState = 0
let reducer: (Int, Int) -> Int = { (sum, increment) in sum + increment };

class CounterViewController: UIViewController {
    @IBOutlet var inputField: UITextField!
    @IBOutlet var addButton: UIButton!
    @IBOutlet var sumLabel: UILabel!
    @IBAction func addButtonPressed(_ sender: UIButton) {
        appState = reducer(appState, Int(self.inputField.text!)!)
        self.sumLabel.text = "\(appState)"
    }
}