没有取消引用就无法实现副作用

Question

来自 clojure for the brave and true:

(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))))

如果我注释掉 (deref ~q) 行，那么只会打印 "Cheerio!"。为什么我需要在这里取消引用以获得其他副作用？

Answer 1

如果您注释掉 (deref ~q)，通过 q 传递的代码将永远不会被计算，因此嵌套 futures 不会存在。

宏展开：

(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)

没有取消引用就无法实现副作用

Side effects not realized without deref

concurrency

clojure