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PO(Another) Stack overflow on loop-recur in Clojure
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  1. PO(Another) Stack overflow on loop-recur in Clojure
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    copied!<p>Similar questions: <a href="https://stackoverflow.com/q/9202500/2472391">One</a>, <a href="https://stackoverflow.com/q/10237880/2472391">Two</a>, <a href="https://stackoverflow.com/q/4249926/2472391">Three</a>.</p> <p>I am thoroughly flummoxed here. I'm using the <a href="http://clojure.org/special_forms#Special%20Forms--%28recur%20exprs*%29" rel="nofollow noreferrer">loop-recur</a> form, I'm using <a href="http://clojuredocs.org/clojure_core/clojure.core/doall" rel="nofollow noreferrer">doall</a>, and still I get a stack overflow for large loops. My Clojure version is 1.5.1.</p> <p>Context: I'm training a neural net to mimic XOR. The function <code>xor</code> is the feed-forward function, taking weights and input and returning the result; the function <code>b-xor</code> is the back-propagation function that returns updated weights given the results of the last call to <code>xor</code>.</p> <p>The following loop runs just fine, runs very fast, and returns a result, and based off of the results it returns, it is training the weights perfectly:</p> <pre class="lang-clj prettyprint-override"><code>(loop [res 1 ; &lt;- initial value doesn't matter weights xorw ; &lt;- initial pseudo-random weights k 0] ; &lt;- count (if (= k 1000000) res (let [n (rand-int 4) r (doall (xor weights (first (nth xorset n))))] (recur (doall r) (doall (b-xor weights r (second (nth xorset n)))) (inc k))))) </code></pre> <p>But of course, that only gives me the result of the very last run. Obviously I want to know what weights have been trained to get that result! The following loop, with nothing but the return value changed, overflows:</p> <pre class="lang-clj prettyprint-override"><code>(loop [res 1 weights xorw k 0] (if (= k 1000000) weights ; &lt;- new return value (let [n (rand-int 4) r (doall (xor weights (first (nth xorset n))))] (recur (doall r) (doall (b-xor weights r (second (nth xorset n)))) (inc k))))) </code></pre> <p>This doesn't make sense to me. The entirety of <code>weights</code> gets used in each call to <code>xor</code>. So why could I use <code>weights</code> internally but not print it to the REPL?</p> <p>And as you can see, I've stuck <code>doall</code> in all manner of places, more than I think I should need. XOR is a toy example, so <code>weights</code> and <code>xorset</code> are both very small. I believe the overflow occurs not from the execution of <code>xor</code> and <code>b-xor</code>, but when the REPL tries to print <code>weights</code>, for these two reasons:</p> <p>(1) this loop can go up to 1500 without overflowing the stack.</p> <p>(2) the time the loop runs is consistent with the length of the loop; that is, if I loop to 5000, it runs for half a second and then prints a stack overflow; if I loop to 1000000, it runs for ten seconds and then prints a stack overflow -- again, only if I print <code>weights</code> and not <code>res</code> at the end.</p> <p>(3) EDIT: Also, if I just wrap the loop in <code>(def w ... )</code>, then there is no stack overflow. Attempting to peek at the resulting variable does, though.</p> <pre><code>user=&gt; (clojure.stacktrace/e) java.lang.StackOverflowError: null at clojure.core$seq.invoke (core.clj:133) clojure.core$map$fn__4211.invoke (core.clj:2490) clojure.lang.LazySeq.sval (LazySeq.java:42) clojure.lang.LazySeq.seq (LazySeq.java:60) clojure.lang.RT.seq (RT.java:484) clojure.core$seq.invoke (core.clj:133) clojure.core$map$fn__4211.invoke (core.clj:2490) clojure.lang.LazySeq.sval (LazySeq.java:42) nil </code></pre> <p>Where is the lazy sequence?</p> <p>If you have suggestions for better ways to do this (this is just my on-the-fly REPL code), that'd be great, but I'm really looking for an explanation as to what is happening in this case.</p> <hr> <p>EDIT 2: Definitely (?) a problem with the REPL.</p> <p>This is bizarre. <code>weights</code> is a list containing six lists, four of which are empty. So far, so good. But trying to print one of these empty lists to the screen results in a stack overflow, but only the first time. The second time it prints without throwing any errors. Printing the non-empty lists produces no stack overflow. Now I can move on with my project, but...what on earth is going on here? Any ideas? (Please pardon the following ugliness, but I thought it might be helpful)</p> <pre><code>user=&gt; (def ww (loop etc. etc. )) #'user/ww user=&gt; (def x (first ww)) #'user/x user=&gt; x StackOverflowError clojure.lang.RT.seq (RT.java:484) user=&gt; x () user=&gt; (def x (nth ww 3)) #'user/x user=&gt; x (8.47089879874061 -8.742792338501289 -4.661609290853221) user=&gt; (def ww (loop etc. etc. )) #'user/ww user=&gt; ww StackOverflowError clojure.core/seq (core.clj:133) user=&gt; ww StackOverflowError clojure.core/seq (core.clj:133) user=&gt; ww StackOverflowError clojure.core/seq (core.clj:133) user=&gt; ww StackOverflowError clojure.core/seq (core.clj:133) user=&gt; ww (() () () (8.471553034351501 -8.741870954507117 -4.661171802683782) () (-8.861958958234174 8.828933147027938 18.43649480263751 -4.532462509591159)) </code></pre>
 

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