Example of closures in Clojure

adam.clj

(defn do-it []
  (let [a ["a" "b" "c"]
        b ["x" "y" "z"]
        counter (atom 0)]
    (run! (fn [foo]
            (reset! counter 0)
            (run! (fn [bar]
                    (swap! counter inc)
                    (println {:counter @counter
                              :foo foo
                              :bar bar}))
                  b))
          a)))

;; output

{:counter 1, :foo a, :bar x}
{:counter 2, :foo a, :bar y}
{:counter 3, :foo a, :bar z}
{:counter 1, :foo b, :bar x}
{:counter 2, :foo b, :bar y}
{:counter 3, :foo b, :bar z}
{:counter 1, :foo c, :bar x}
{:counter 2, :foo c, :bar y}
{:counter 3, :foo c, :bar z}