Concurrency 如何防止未交付的承诺被挂起？

以下是clojure为勇敢而真诚的人提供的一个例子：

(time
 (let [butter-promise (promise)]
   (doseq [butter [yak-butter-international butter-than-nothing baby-got-yak]]
     (future (if-let [satisfactory-butter (satisfactory? (mock-api-call butter))]
               (deliver butter-promise satisfactory-butter))))
   (println "And the winner is:" @butter-promise)))

如果没有一种黄油产品符合我们的标准呢

(let [butter-promise (promise)]
  ;; ...
  (if-let [winner (deref butter-promise 10000 nil)]
    (println "And the winner is:" winner)
    (println "Butter timed out."))