备用版本的交换！也返回换出值

问题描述

我今天在IRC的 #clojure 频道上说了一下，但想在这里详细讨论。基本上，为了更好地理解原子， swap！， deref 和Clojure并发作为一个整体，我想尝试写一个函数，它不仅返回使用 swap！交换的值，而且返回被换出的值。

I talked about this a bit on IRC's #clojure channel today but would like to go more in detail here. Basically, in order to better understand atoms, swap!, deref and Clojure concurrency as a whole, I'd like to try to write a function which not only returns the value that was swapped-in using swap!, but also the value that was swapped out.

(def foo (atom 42))

.
.
.

((fn [a]
  (do
    (println "swapped out: " @a)
    (println "swapped in: "(swap! a rand-int)))) foo)

可以打印：

swapped out:  42
swapped in:   14

但是，如果另一个线程 @a 之间有相同的原子 swap！ > deref ，并调用 swap！，那么我可能会换出不是42的值。

However if another thread does swap! the same atom between the @a deref and the call to swap! then I may be swapping out a value that is not 42.

我如何写一个函数来正确地给出两个值（换出和换入）？

How can I write a function which gives back correctly both values (the swapped out and the swapped in)?

我不在乎各种值，原子确实改变为：我想知道的是什么值换出。

I don't care about the various values that the atom does change to: all I want to know is what was the value swapped out.

这可以使用代码保证不会死锁和如果是为什么？

Can this be written using code that is guaranteed not to deadlock and if so why?

推荐答案

Clojure的 swap！ - 设置。您可以定义一个替换版本，返回您喜欢的任何内容：

Clojure's swap! is just a spinning compare-and-set. You can define an alternate version that returns whatever you like:

(defn alternate-swap [atom f & args]
  (loop []
    (let [old @atom
          new (apply f old args)]
      (if (compare-and-set! atom old new)
        [old new]  ; return value
        (recur)))))

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