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.
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)))
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)
One way to fix it is to put doall
around it to defeat laziness.
(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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