没有deref

Question

来自clojure的勇敢和真实：

(defmacro enqueue
  [q concurrent-promise-name & work]
  (let [concurrent (butlast work)
        serialized (last work)]
    `(let [~concurrent-promise-name (promise)]
       (future (deliver ~concurrent-promise-name (do ~@concurrent)))
       (deref ~q)
       ~serialized
       ~concurrent-promise-name)))
(defmacro wait
  "Sleep `timeout` seconds before evaluating body"
  [timeout & body]
  `(do (Thread/sleep ~timeout) ~@body))
(time @(-> (future (wait 200 (println "'Ello, gov'na!")))
           (enqueue saying (wait 400 "Pip pip!") (println @saying))
           (enqueue saying (wait 100 "Cheerio!") (println @saying))))

如果我评论(deref ~q)行，那么只有“Cheerio！”打印出来。为什么我需要deref来获得其他副作用？

Answer 1

如果您注释掉(deref ~q)，则永远不会评估通过q传递的代码，因此嵌套的期货不会出现。

宏扩展：

(macroexpand '(-> (future (wait 200 (println "'Ello, gov'na!")))
                  (enqueue saying (wait 400 "Pip pip!") (println @saying))
                  (enqueue saying (wait 100 "Cheerio!") (println @saying))))
;;-> ....
(clojure.pprint/pp)

(let*
 [saying (clojure.core/promise)]
 (clojure.core/future
   (clojure.core/deliver saying (do (wait 100 "Cheerio!"))))
 ;; no code ended up here...
 (println @saying)
 saying)