如何在Clojure中取消引用未来？

Question

我的程序如下工作:两个期货在一些随机等待时间之后将一个特定金额从一个余额A转移到余额B. 每次重复做10次.所以我尝试停止程序并解决未来,但它不起作用,它抛出NullPointerException,因为我deref它.我做错了什么？

我想要一个最终结果,在程序结束前的最后一行显示balanceA和balanceB.现在它输出结果,但它出现在"转移"和"尝试"内.我需要做什么？

谢谢.

  (def balanceA (ref 1000))
  (def balanceB (ref 2000))
  (def agentCount (agent 1))

   ;;transfer function
  (defn transfer [balanceA balanceB amount futureNum waitingTime]
    (dosync(alter balanceA - amount))
    (Thread/sleep waitingTime)
    (dosync(alter balanceB + amount))
    (do(
        (println "Trying" futureNum waitingTime)
        (println "Transfered" futureNum @agentCount)
        (send-off agentCount inc))))

  ;;futureA 
  (dotimes [n 10](def futureA (future (transfer balanceA balanceB 20 1 (rand-int 100)))))

  ;;futureB
  (dotimes [n 10](def futureB (future (transfer balanceA balanceB 15 2 (rand-int 40))(prn "result" @balanceB @balanceB))))

  ;;print out the result
  (println "result" @balanceA @balanceB)

  ;;dereference futures
  (@futureA)
  (@futureB

这是我的输出:

  user=> (load-file "assignment10.clj")
  Result: Trying: Trying: 2650  2000
  2 Trying: 2 Trying: 2 3
   Transfered: 2 1

  user=> Trying: 1 28
  Transfered: 1 2
  Trying: 1 28
   Transfered: 1 3
  Trying:Trying:  21  2218

  Transfered: 2 4
  Transfered: 1 4
 97

  Transfered: 2 6
  10
   Trying: 2 26
  Transfered: 2 7
  Transfered: 2 Trying: 8
  2Trying:Transfered:   227
  2 28
  Transfered:7
  Transfered: 2 9
  2 9
  Trying: 2 33
  Transfered: 2 12
  Trying: 1 49
   Trying: 2 39
  Transfered:Transfered:  21  1313

  Trying: 1 57      
  Transfered: 1 15
  Trying: 1 71
  Transfered: 1 16
  Trying: 1 76
  Transfered: 1 17
  Trying: 1 81
  Transfered: 1 18
  Trying: 1 93
  Transfered: 1 19
  Trying: 1 98
  Transfered: 1 20

Answer 1

有几个地方你有一些额外的()s,其中任何一个都可能导致NullPointerExceptions

(@futureA)

表示首先从ref获取当前值,然后获取该值并将其作为函数调用.

(do(
    (println "Trying" futureNum waitingTime)
    (println "Transfered" futureNum @agentCount)
    (send-off agentCount inc))))

因为它将打印"Trying"作为一个函数调用的结果是一个额外的()s 组do,谁的第一个参数是打印"Transfered"的结果,而第二个参数是调用send-off的结果.由于println总是返回,nil这将导致NPE.似乎这些()都是无意的.

def除了顶级表单之外,调用除了顶级表单之外的任何东西都是不寻常的(除非你正在编写宏).在这种情况下,只有futureA和futureB的最后一次出现才有用.可能值得考虑使用for而不是将所有期货的结果保存在序列中,以便您可以看到它们中的任何一个是否失败,而不是仅仅能够判断最后一个是否失败.