理解这个 Clojure 代码

Question

我是 Clojure 的新手，我一直在努力实现一些并发代码。我在网上找到了这段代码。有一个警告说这不是在 Clojure 中进行并发的正确方法，但我还是想理解这段代码。

(def *readers* (map #(agent %) '("one" "two" "three")))
(def *writers* (map #(agent %) '("four" "five")))
(def *mutex* (agent :unlocked))
(def *value* 0)

; mutex implementation
(defn lock [state who success-fn fail-fn]
  (send who (if (= state :locked) fail-fn success-fn))
  :locked)

(defn unlock [mutex]
  :unlocked)

; Must be invoked with send-off since this handler blocks
(defn rand-sleep [state next-fn]
  (Thread/sleep (rand-int 5))
  (send *agent* next-fn)
  state)

; Reader functions
(declare try-read)

(defn reader-got-lock [name]
  (println (format "Thread %s says that the value is %d." name *value*))
  (send *mutex* unlock)
  (send-off *agent* rand-sleep try-read)
  name)

(defn reader-did-not-get-lock [name]
  (println (format "Thread %s tried to read the value, but could not." name))
  (send-off *agent* rand-sleep try-read)
  name)

(defn try-read [name]
  (send *mutex* lock *agent* reader-got-lock reader-did-not-get-lock)
  name)

; Writer functions
(declare try-write)

(defn writer-got-lock [name]
  (println (format "Thread %s is taking the lock." name))
  (def *value* (rand-int 10))
  (println (format "Thread %s is changing the value to %d." name *value*))
  (send *mutex* unlock)
  (println (format "Thread %s is relasing the lock." name))
  (send-off *agent* rand-sleep try-write)
  name)

(defn writer-did-not-get-lock [name]
  (println (format "Thread %s tried to write the value, but could not." name))
  (send-off *agent* rand-sleep try-write)
  name)

(defn try-write [name]
  (send *mutex* lock *agent* writer-got-lock writer-did-not-get-lock)
  name)

(dorun (map #(send % try-write) *writers*))
(dorun (map #(send % try-read) *readers*))

特别是我被困在这一行：

(defn try-write [name]
  (send *mutex* lock *agent* writer-got-lock writer-did-not-get-lock)
  name)

它应该锁定互斥体并根据互斥体状态调用 writer-got-lock 或 writer-did-not-get-lock。但是*agent* 是在哪里声明的，还是访问当前代理的快捷方式？那么state是如何传递给锁的，在上面调用的情况下是*agent*还是*agent*是who？

Answer 1

*agent* 是当前代理的快捷方式，由 clojure 隐式提供。 *agent* 作为 who 传递。最终调用大致是这样的：

(lock @*mutex* *agent* writer-got-lock writer-did-not-get-lock)