Singleton functions

singleton-fn.cljs

(defn singleton-fn 
  "Returns a function that creates a resource. When called a second time,
  calls dispose-fn before recreating the resource"
  [create-fn dispose-fn]
  (let [!state (atom nil)]
    (fn [& args]
      (when-let [state @!state]
        (dispose-fn state))
      (let [result (apply create-fn args)]
        (reset! !state (or result ::initialized))))))

;; example

(defonce make-timer (singleton-fn #(js/setInterval (fn [] (js/console.log "tick")) %)
                                  #(js/clearInterval %)))

(def my-timer (make-timer 5000))