(defn add [a b]
  (Thread/sleep 300)
  (+ a b))

;; Thread. takes any function of no arguments and runs it aside
(def worker
  (Thread. (fn [] (println "result:" (add 3 5)))))
(.start worker)

(println "main thread")

;; the answer arrives while the main thread has already moved on;
;; join only holds the exit open long enough to see it printed
(.join worker)
;; output:
;; main thread
;; result: 8