http://dev.clojure.org/jira This file is an XML representation of an issue en-us 4.4 649 25-07-2011 http://dev.clojure.org/jira/browse/CLJ-232 Clojure <p>Steve Gilardi and I noticed some strange behaviour with local clearing in conjunction with delay:</p> <p> (defn do-something <span class="error">&#91;x&#93;</span><br/> (throw (Exception.)))</p> <p> (defn clear-locals <span class="error">&#91;x&#93;</span><br/> (delay (try (do-something x)<br/> (catch Exception e<br/> (println "argument value:" x)))))</p> <p> (force (clear-locals :argument))<br/> ;; =&gt; argument value: nil</p> <p>It seems the locals are getting cleared a little too aggressively here. I did some digging to discover that delay is a thin wrapper around an fn, but using an fn in this context (rather than a delay) does not trigger the problem. Also I found that throwing an exception directly inside the try block (rather than in a function called from the try block) did not trigger it.</p> <p>I was able to reproduce in the new branch, the master branch, and 1.0.0, so it's not related to the more recent locals-clearing changes.</p> CLJ-232

Locals cleared too aggressively on delay

Defect Closed Completed Rich Hickey None Wed, 30 Dec 2009 17:27:00 -0600 Tue, 24 Aug 2010 05:43:00 -0500 Tue, 24 Aug 2010 05:43:00 -0500 Release 1.2 0 0 <p>Converted from <a href="http://www.assembla.com/spaces/clojure/tickets/232">http://www.assembla.com/spaces/clojure/tickets/232</a><br/> Attachments:<br/> locals.clj - <a href="https://www.assembla.com/spaces/clojure/documents/d6rA1a9ASr3RHaeJe5afGb/download/d6rA1a9ASr3RHaeJe5afGb">https://www.assembla.com/spaces/clojure/documents/d6rA1a9ASr3RHaeJe5afGb/download/d6rA1a9ASr3RHaeJe5afGb</a></p> <p>technomancy said: [<a href="file:d6rA1a9ASr3RHaeJe5afGb">file:d6rA1a9ASr3RHaeJe5afGb</a>]: repro case</p> <p>chouser@n01se.net said: <div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent"> <pre class="code-java">((let [x :foo] (#^{:once <span class="code-keyword">true</span>} fn* [] (<span class="code-keyword">try</span> (#(<span class="code-keyword">throw</span> (Exception.))) (<span class="code-keyword">catch</span> Exception e (println <span class="code-quote">"x:"</span> x)))))) &lt;/code&gt;&lt;/pre&gt; The same problem is visible when a <span class="code-keyword">finally</span> clause uses a closed-over: &lt;pre&gt;&lt;code&gt; ((let [x :foo] (#^{:once <span class="code-keyword">true</span>} fn* [] (<span class="code-keyword">try</span> (#(<span class="code-keyword">throw</span> (Exception.))) (<span class="code-keyword">finally</span> (println <span class="code-quote">"x:"</span> x))))))</pre> </div></div><br/> Both of these print x: nil when they should print x: :foo</p> <p>I think the problem is that, because the last expr in a try block produces the value that will be returned, it is compiled in a "return" context (a.k.a. tail position), therefore the locals are cleared before calling the final function.</p> <p>But if that final call throws an exception, you end up in the catch clause with your locals cleared.</p> <p>emitClearLocals protects against this problem by using localsUsedInCatchFinally, but emitClearCloses (used only when :once is true) does not.</p> <p>The solution is so very far beyond me. I guess the simplest might be to skip clearing of closed-overs that are used in catch/finally clauses, but I don't think those are currently tracked. It looks like closeOver could be made to track these as well, but I'm lost as to the relationships between the various instances in that code.</p> <p>danlarkin said: Kevin Downey and I just stumbled on this problem again. Here's our smallest repro case:</p> <div class="code panel" style="border-width: 1px;"><div class="codeContent panelContent"> <pre class="code-java">(defn throwsomething [] (<span class="code-keyword">throw</span> (Exception.))) (defn foo [bar] @(delay (<span class="code-keyword">try</span> (throwsomething) (<span class="code-keyword">catch</span> Exception e (nil? bar))))) &lt;/code&gt;&lt;/pre&gt; We came up with two workarounds, one is to wrap everything inside the delay in a let capturing the <span class="code-quote">"bar"</span> scope, like <span class="code-keyword">this</span>: &lt;pre&gt;&lt;code&gt; (defn foo [bar] @(delay (let [bar bar] (<span class="code-keyword">try</span> (throwsomething) (<span class="code-keyword">catch</span> Exception e (nil? bar)))))) &lt;/code&gt;&lt;/pre&gt; and the other is to not have the exception-throwing call in the tail position, like <span class="code-keyword">this</span>: &lt;pre&gt;&lt;code&gt; (defn foo [bar] @(delay (<span class="code-keyword">try</span> (let [t (throwsomething)] t) (<span class="code-keyword">catch</span> Exception e (nil? bar)))))</pre> </div></div> <p>richhickey said: (In [<span class="error">&#91;r:93fecbd825c26e2570f8449cd64d0df0cc520c1d&#93;</span>]) fold closes clearing into the path system, fixes #232</p> <p>Branch: master</p> Global Rank