An abstraction over reactive behavior

async.clj

(defmulti then-fn
  (fn [deferred _] (class deferred)))

(defmethod then-fn CompletableFuture [^CompletableFuture future func]
  (.thenAccept future
    (reify Consumer
      (accept [_ result] (func result)))))

(defmacro async [arg & block]
  (if (vector? arg)
    (let [[result deferred] arg]
      `(then-fn ~deferred (fn [~result] ~@block)))
    `(then-fn ~arg (fn [~'_] ~@block))))

(defn double-num-async [n]
  (CompletableFuture/completedFuture (* 2 n)))

(defn -main []
  (async [x (double-num-async 2)]
    (println x))
  (async (double-num-async 3)
    (println "done")))