Clojure reducers 折叠未按预期调用组合函数

Question

我正在尝试使用 clojure reducers 库，但对于何时将组合函数作为 reducers/fold 函数的一部分调用，我有点困惑。为了查看什么时候被调用，我创建了以下示例：

(def input (range 1 100))

(defn combine-f
  ([]
    (println "identity-combine")
    0)
  ([left right]
    (println (str "combine " left " "  right))
    (max (reduce max 0 left)
         (reduce max 0 right))))

(defn reduce-f
  ([]
    (println "identity-reduce")
    0)
  ([result input]
    (println (str "reduce " result " "  input))
    (max result input)))

(clojure.core.reducers/fold 10 combine-f reduce-f input)

;prints
identity-combine
reduce 0 1
reduce 1 2
reduce 2 3
reduce 3 4
.
.
.
reduce 98 99

我期望当 fold 执行时，输入将被分成大约 10 大小的组，每个组使用 reduce-f 减少，然后使用 combine-f 组合。但是运行上面的代码，似乎 combine 函数只作为一个身份被调用一次，并且整个输入使用 reduce-f 减少。谁能解释我为什么会看到这种行为？

谢谢，

马特。

Answer 1

很遗憾，range 目前无法并行实现。 It seems there are foldable implementations around as an enhancement ticket, but I can't seem to find right now why they haven't been accepted. 照原样，folds 在 range 上总是会像直接的 reduce 一样继续进行，除了对 combine 运算符的身份调用。作为比较，向量提供随机访问，因此是可折叠的：

(def input (vec (range 1 50)))

(defn combine-f
  ([]
    (println "identity-combine")
    Long/MIN_VALUE)
  ([left right]
    (println (str "combine " left " "  right))
    (max left right)))

(defn reduce-f
  ([]
    (println "identity-reduce")
    Long/MIN_VALUE)
  ([result input]
    (println (str "reduce " result " "  input))
    (max result input)))

(clojure.core.reducers/fold 10 combine-f reduce-f input)

带输出：

identity-combineidentity-combineidentity-combine


reduce -9223372036854775808 1

reduce -9223372036854775808 25reduce -9223372036854775808 19


reduce -9223372036854775808 13reduce 25 26


reduce 26 27
reduce 1 2

reduce 27 28

reduce 28 29

reduce 29 30



reduce 2 3
reduce 19 20

reduce 3 4

identity-combinereduce 4 5

reduce 5 6reduce 13 14


reduce 14 15


reduce 20 21identity-combine

reduce 21 22

reduce 15 16



reduce -9223372036854775808 31

reduce 22 23reduce 16 17reduce -9223372036854775808 7


reduce 7 8


reduce 8 9

reduce 23 24

reduce 31 32
reduce 17 18


reduce 9 10

reduce 10 11


reduce 11 12

identity-combine
reduce 32 33

combine 18 24


combine 6 12identity-combine
reduce -9223372036854775808 37


reduce 33 34

reduce 37 38reduce -9223372036854775808 43
combine 12 24



reduce 43 44reduce 34 35reduce 38 39

reduce 44 45

reduce 35 36


reduce 45 46
reduce 39 40

reduce 46 47
combine 30 36

reduce 47 48



reduce 48 49

reduce 40 41

reduce 41 42

combine 42 49

combine 36 49
combine 24 49

由于*out* 的非序列化访问，您可能会注意到它更加混乱。
（我需要稍微改变combine-f，因为它正在尝试并在一个长的时间内未能reduce。切换到Long/MIN_VALUE 不会对 this 示例产生太大影响，但它是max 很长，所以我想为什么不呢？）。