Is this a closure? If so, why?

Question

While preparing an answer to another question, I created one for myself. Consider the following short program.

(ns net.dneclark.JFrameAndTimerDemo
  (:import (javax.swing JLabel JButton JPanel JFrame Timer))
  (:gen-class))

(defn timer-action [label counter]
 (proxy [java.awt.event.ActionListener] []
   (actionPerformed
     [e]
      (.setText label (str "Counter: " (swap! counter inc))))))

(defn timer-fn []
  (let [counter (atom 0)
        label (JLabel. "Counter: 0")
        timer (Timer. 1000 (timer-action label counter))
        panel (doto (JPanel.)
                (.add label))]
    (.start timer)
    (doto (JFrame. "Timer App")
      (.setContentPane panel)
      (.setDefaultCloseOperation JFrame/EXIT_ON_CLOSE)
      (.setLocation 300 300)
      (.setSize 200 200)
      (.setVisible true))))

(defn -main []
  (timer-fn))

In the action listener, 'timer-action', the value of the 'counter' argument is altered. The actual variable is declared in the 'timer-fn' function, but is altered in the listener. In my earlier experience with languages like Pascal, I would have considered 'counter' to be passed by reference. Is that the case here or is this an example of a closure? Something else?

Thanks for the help.

Answer 1

Yes, it's a closure. The lexical context of the handler function definition is preserved, and when it is later invoked it can access and update variables that "live" there.

I'm not sure how to answer the question, "why?" other than to point out that it's simply the way the language is defined to work.