A function wrapper that ensures that a function can only be called once

stratch.clj

(ns scratch.core)


(defn say-hi [username]
  (let [s (str "hello, " username)]
    (Thread/sleep (rand 100))
    (println s)
    s))

(comment (say-hi "Steve"))

(defn wrap-once
  "Returns a fn that wraps given function `f` so that:
    - First      call returns {:okay (apply f args)}.
    - Subsequent calls return nil, without executing `f`."
  [f]
  (let [run?_ (atom false)]
    (fn once-fn [& args]
      (when (compare-and-set! run?_ false true)
        {:okay (apply f args)}))))

(comment
  (let [wf (wrap-once say-hi)]
    (println "---")
    [(future (wf "Steve 1"))
     (future (wf "Steve 2"))
     (future (wf "Steve 3"))]))