tail-recursive函数上的StackOverflowError [英] StackOverflowError on tail-recursive function

问题描述

当我使用（avg-bids 4000 10 5）调用它时，下面的Clojure代码导致java.lang.StackOverflowError。我试图找出为什么，因为总和出价被写为一个尾递归函数，所以应该工作。使用Clojure 1.2。

The following piece of Clojure code results in java.lang.StackOverflowError when I call it with (avg-bids 4000 10 5). I try to figure out why, since sum-bids is written as a tail-recursive function, so that should work. Using Clojure 1.2.

任何人都知道为什么会发生这种情况？

Anyone knows why this happens?

(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))) 

推荐答案

map 是惰性的，你正在通过 recur 。 backtrace是有点神秘，但仔细看，你可以看到地图，地图，地图...

map is lazy, and you're building a very deeply nested mapping of mappings via recur. The backtrace is a bit cryptic, but look closely and you can see map, map, map...

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在它周围放置 doall 以挫败懒惰。

(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)

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