clojure功能的目的 [英] purpose of clojure reduced function

问题描述

clojure reduce 函数的目的是什么（在clojure 1.5中添加， https://clojure.github.io/clojure/clojure.core-api.html#clojure.core/reduced ）

What is the purpose of the clojure reduced function (added in clojure 1.5, https://clojure.github.io/clojure/clojure.core-api.html#clojure.core/reduced)

我找不到任何例子。文档说：

I can't find any examples for it. The doc says:

以某种方式换行x，使得reduce将以值x终止。

Wraps x in a way such that a reduce will terminate with the value x.

还有一个已减少？ >

There is also a reduced? which is acquainted to it

如果x是调用减少的结果，则返回true

Returns true if x is the result of a call to reduced

$ b b

当我尝试它，例如与（减少+（减少100）），我得到一个错误，而不是100.也为什么我会减少一些东西，当我知道结果提前？因为它被添加可能有一个原因，但是搜索 clojure减少只包含 reduce 结果。

When I try it out, e.g with (reduce + (reduced 100)), I get an error instead of 100. Also why would I reduce something when I know the result in advance? Since it was added there is likely a reason, but googling for clojure reduced only contains reduce results.

推荐答案

reduced 可让您缩短缩图：

(reduce (fn [acc x]
          (if (> acc 10)
            (reduced acc)
            (+ acc x)))
        0 
        (range 100))
;= 15

（NB。边界情况与（减少0）传递作为初始值不工作的Clojure 1.6）

(NB. the edge case with (reduced 0) passed in as the initial value doesn't work as of Clojure 1.6.)

这很有用，因为 reduce 基于循环非常优雅和非常基于 loop 的基于 reduce 的循环的性能不如常常高于 / recur ），因此尽可能使此模式尽可能广泛适用是好的。短路减少的能力大大增加了可能的应用程序的范围。

This is useful, because reduce-based looping is both very elegant and very performant (so much so that reduce-based loops are not infrequently more performant than the "natural" replacements based on loop/recur), so it's good to make this pattern as broadly applicable as possible. The ability to short circuit reduce vastly increases the range of possible applications.

对于减少？，我发现它对于新数据结构实现 reduce 逻辑时很有用;在正常代码中，我让减少在适当时执行自己的减少检查。

As for reduced?, I find it useful primarily when implementing reduce logic for new data structures; in regular code, I let reduce perform its own reduced? checks where appropriate.

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