clojure - 实现 Clojure 条件/分支转换器

Question

我正在尝试在 Clojure 中制作一个条件转换器，如下所示:

(defn if-xf
  "Takes a predicate and two transducers.
   Returns a new transducer that routes the input to one of the transducers
   depending on the result of the predicate."
  [pred a b]
  (fn [rf]
    (let [arf (a rf)
          brf (b rf)]
      (fn
        ([] (rf))
        ([result]
           (rf result))
        ([result input]
           (if (pred input)
             (arf result input)
             (brf result input)))))))

它非常有用，因为它可以让你做这样的事情:

;; multiply odd numbers by 100, square the evens.  
(= [0 100 4 300 16 500 36 700 64 900]
    (sequence
          (if-xf odd? (map #(* % 100)) (map (fn [x] (* x x))))
          (range 10)))

但是，这种条件转换器不适用于在其 1-arity 分支中执行清理的转换器:

;; negs are multiplied by 100, non-negs are partitioned by 2
;; BUT! where did 6 go?
;; expected: [-600 -500 -400 -300 -200 -100 [0 1] [2 3] [4 5] [6]]
;;
(= [-600 -500 -400 -300 -200 -100 [0 1] [2 3] [4 5]]
 (sequence
  (if-xf neg? (map #(* % 100)) (partition-all 2))
  (range -6 7)))

是否可以调整 if-xf 的定义？处理带有清理的换能器的情况？

我正在尝试这个，但行为很奇怪:

(defn if-xf
  "Takes a predicate and two transducers.
   Returns a new transducer that routes the input to one of the transducers
   depending on the result of the predicate."
  [pred a b]
  (fn [rf]
    (let [arf (a rf)
          brf (b rf)]
      (fn
        ([] (rf))
        ([result]
           (arf result) ;; new!
           (brf result) ;; new!
           (rf result))
        ([result input]
           (if (pred input)
             (arf result input)
             (brf result input)))))))

具体来说，冲洗发生在最后:

;; the [0] at the end should appear just before the 100.
(= [[-6 -5] [-4 -3] [-2 -1] 100 200 300 400 500 600 [0]]
      (sequence
       (if-xf pos? (map #(* % 100)) (partition-all 2))
       (range -6 7)))

有没有办法在不将整个输入序列存储在这个转换器内的本地状态的情况下制作这个分支/条件转换器(即在清理时在 1-arity 分支中进行所有处理)？

Answer 1

这个想法是在每次换能器切换时完成。 IMO 这是没有缓冲的唯一方法:

(defn if-xf
  "Takes a predicate and two transducers.
   Returns a new transducer that routes the input to one of the transducers
   depending on the result of the predicate."
  [pred a b]
  (fn [rf]
    (let [arf (volatile! (a rf))
          brf (volatile! (b rf))
          a? (volatile! nil)]
      (fn
        ([] (rf))
        ([result]
         (let [crf (if @a? @arf @brf)]
           (-> result crf rf)))
        ([result input]
         (let [p? (pred input)
               [xrf crf] (if p? [@arf @brf] [@brf @arf])
               switched? (some-> @a? (not= p?))]
           (if switched?
             (-> result crf (xrf input))
             (xrf result input))
           (vreset! a? p?)))))))
(sequence (if-xf pos? (map #(* % 100)) (partition-all 2)) [0 1 0 1 0 0 0 1])
; => ([0] 100 [0] 100 [0 0] [0] 100)