关于尾递归函数的StackOverflowError

Question

当我用它调用它时,下面的Clojure代码会产生java.lang.StackOverflowError(avg-bids 4000 10 5).我试图弄清楚为什么,因为sum-bid被写为尾递归函数,所以应该工作.使用Clojure 1.2.

谁知道为什么会这样？

(ns fixedprice.core
  (:use (incanter core stats charts)))

(def *bid-mean* 100)

(defn bid [x std-dev]
  (sample-normal x :mean *bid-mean* :sd std-dev))

(defn sum-bids [n offers std-dev]
  (loop [n n sum (repeat offers 0)]
    (if (zero? n)
      sum
      (recur (dec n) (map + sum (reductions min (bid offers std-dev)))))))

(defn avg-bids [n offers std-dev]
  (map #(/ % n) (sum-bids n offers std-dev))) 

Answer 1

map是懒惰的,你正在建立一个非常深层嵌套的映射映射recur.回溯有点神秘,但仔细观察,你可以看到地图,地图,地图......

Caused by: java.lang.StackOverflowError
        at clojure.lang.LazySeq.seq(LazySeq.java:56)
        at clojure.lang.RT.seq(RT.java:450)
        at clojure.core$seq.invoke(core.clj:122)
        at clojure.core$map$fn__3699.invoke(core.clj:2099)
        at clojure.lang.LazySeq.sval(LazySeq.java:42)
        at clojure.lang.LazySeq.seq(LazySeq.java:56)
        at clojure.lang.RT.seq(RT.java:450)
        at clojure.core$seq.invoke(core.clj:122)
        at clojure.core$map$fn__3699.invoke(core.clj:2099)
        at clojure.lang.LazySeq.sval(LazySeq.java:42)
        at clojure.lang.LazySeq.seq(LazySeq.java:56)
        at clojure.lang.RT.seq(RT.java:450)
        at clojure.core$seq.invoke(core.clj:122)
        at clojure.core$map$fn__3699.invoke(core.clj:2099)
        at clojure.lang.LazySeq.sval(LazySeq.java:42)
        at clojure.lang.LazySeq.seq(LazySeq.java:56)
        at clojure.lang.RT.seq(RT.java:450)
        at clojure.core$seq.invoke(core.clj:122)
        at clojure.core$map$fn__3699.invoke(core.clj:2099)

修复它的一种方法是doall绕过它来打败懒惰.

(defn sum-bids [n offers std-dev]
  (loop [n n sum (repeat offers 0)]
    (if (zero? n)
      sum
      (recur (dec n) (doall (map + sum (reductions min (bid offers std-dev))))))))

user> (avg-bids 4000 10 5)
(100.07129114746716 97.15856005697917 95.81372899072466 94.89235550905231 94.22478826109985 93.72441188690516 93.32420819224373 92.97449591314158 92.67155818823753 92.37275046342015)