Stream in Clojure

最新推荐文章于 2025-05-14 14:24:01 发布

iteye_15891

最新推荐文章于 2025-05-14 14:24:01 发布

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本文尝试将SICP中无限流的概念移植到Clojure中，并实现了一个无限斐波那契数列生成器。文章讨论了Clojure语言特性带来的挑战，如如何在延迟计算和惰性求值的约束下实现流概念。

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(define fibs
  (stream-cons 0
    (stream-cons 1
      (stream-add (stream-cdr fibs) fibs))))

The code above is an excerpt from [i]SICP[/i] that generates an infinite Fibonacci sequence. I've tried to migrate it to Clojure.

The following code is the implementations of the stream concept mentioned in [i]SICP[/i] and the the Fibonacci function. The essential part is this: (defmacro stream-cons [e s] ...). Because Clojure's cons requires that s to be a ISeq or Seqable, that makes the "(stream-cdr fibs)" part always fails. Because this piece of code is executed before the outer stream-cons ever happened. By the time we execute "(stream-cdr fibs)", we haven't had the value of "fibs" yet.


(defn pair [x y] (cons x [y]))

(defn pair-car [p] (first p))

(defn pair-cdr [p] (second p))

(defmacro stream-cons [e s]
  `(pair ~e (delay ~s)))

(defn stream-car [s]
  (pair-car s))

(defn stream-cdr [s]
  (force (pair-cdr s)))

(defn stream-map
  ([f s]
    (stream-cons
      (f (stream-car s))
      (stream-map f (stream-cdr s))))
  ([f s & ss]
    (let [step (fn step [ss]
                  (when (not (empty? ss))
                    (stream-cons (map stream-car ss) (step (map stream-cdr ss)))))]
      (stream-map #(apply f %) (step (cons s ss))))))

(defn stream-add [& ss]
  (apply stream-map + ss))

(defn take-stream [s count]
  (when (> count 0)
    (stream-cons
      (stream-car s)
      (take-stream (stream-cdr s) (dec count)))))

(defn print-stream [s]
  (let [step (fn step [s]
                (when (not (empty? s))
                  (print (stream-car s) "")
                  (recur (stream-cdr s))))]
    (step s)
    (println)))

(def fibs
  (stream-cons 0
    (stream-cons 1
      (stream-add (stream-cdr fibs) fibs))))

(def ones (stream-cons 1 ones))
(def twos (stream-add ones ones))

(print-stream (take-stream twos 10)) ; => 2 2 2 2 2 2 2 2 2 2
(print-stream (take-stream fibs 10)) ; => 0 1 1 2 3 5 8 13 21 34

Interesting enough that I haven't thought of a way to seamlessly integrate the above stream code into Clojure. If we implement it using (deftype LazyStream [] clojure.lang.Seq ...) that would make cons a [i]function[/i] instead of a macro. Therefore I temporarily think that it's impossible to seamlessly integrate the implementation.