Clojure中最简单的懒惰功能

Question

我很难理解懒惰.

有人可以帮助我理解为什么我的下面的功能不是懒惰的

(defn my-red
    ([f coll] (my-red f (first coll) (rest coll) ))
    ([f init coll] 
      (let [mr (fn [g i c d] 
            (if (empty? c) d 
        (recur  g (g  i (first c)) (rest c)  (conj d (g  i (first c)) ))))] 
    (lazy-seq (mr f init coll [])))))

而在clojure.org/lazy上给出的这个例子是

(defn filter
  "Returns a lazy sequence of the items in coll for which
  (pred item) returns true. pred must be free of side-effects."
  [pred coll]
  (let [step (fn [p c]
                 (when-let [s (seq c)]
                   (if (p (first s))
                     (cons (first s) (filter p (rest s)))
                     (recur p (rest s)))))]
    (lazy-seq (step pred coll))))

Answer 1

在懒惰filter来自呼叫filter在if递归循环的分支.这就是代码击中另一个代码lazy-seq并停止评估seq直到请求另一个元素的地方.

你的实现只能my-red调用lazy-seq一次,这意味着你的seq根本没有被评估或者被完全评估.