未调用传感器的初始化

Question

我正在编写一个自定义转换器作为练习，但我很惊讶地发现它的 0 元初始化函数没有被调用。

为什么？

是否与我使用的聚合函数有关？如果是，哪些会调用 init 函数，为什么其他的不会？

(defn inc-xf [xf]
  "inc-xf should be equivalent to (map inc)"
  (fn
    ;; init
    ([]
     (println "init") ;; <- this is not called (???)
     (xf))

    ;; step
    ([result input]
     (println "step" result input)
     (xf result (inc input)))

    ;; completion
    ([result]
     (println "completion" result)
     (xf result))))

(transduce inc-xf
           +
           100
           [5 5 5])

Answer 1

如果您查看 transduce 的实现，您会看到发生了什么。

(defn transduce
  ;; To get the init value, (f) is used instead of ((xform f))
  ([xform f coll] (transduce xform f (f) coll))
  ([xform f init coll]
   ,,,))

然而，为什么更难回答。

实施零元数的传感器是 requirements for a transducer, but it is never actually used in any transducing context in clojure.core. On the mailing list there's been a post asking the same question as you and proposal of an implementation of transduce that actually uses the init arity. The jira ticket 的一部分，但被拒绝，解释如下：

Rich asked me to decline the ticket because the init arity of the xform should not be involved in the reducing function accumulation. - Alex Miller

那么，如果换能器未在任何地方使用，为什么它是合同的初始部分？ ¯\_(ツ)_/¯

未调用传感器的初始化

Transducer's init not called

clojure

transducer