<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta content="text/html;charset=UTF-8" http-equiv="Content-Type"> <title></title> </head> <body bgcolor="#ffffff" text="#000000"> Hi Matthew, You are right. I put the wrong code in the example. Please look at this code: <tt> (define PROOS-STORE (make-hash-table 'equal 'weak)) (define (pobject-from-oid oid _class) (let ((class (proos-class _class))) (if (eq? class #f) (error (string-append "proos class definition for " (symbol->string _class) " not found")) (let* ((obj (hash-table-get PROOS-STORE (list oid) (lambda () #f)))) (oodb-dbg "PROOS-STORE: " oid obj) (if (eq? obj #f) (let ((obj (class 'from-oid oid))) (hash-table-put! PROOS-STORE (-> obj list-poid) (make-weak-box obj)) (oodb-dbg "PROOS-STORE: " oid obj) obj) (weak-box-value obj)))))) </tt>And the session in emacs below: <tt>Welcome to MzScheme version 299.403, Copyright (c) 2004-2005 PLT Scheme, Inc. > > > #t > (require "zk-classes.scm") > (define a (car (psearch (all illness-class-category)))) > (define b (car (psearch (all illness-class-category)))) > (proos-store->list) (((59872449310226502864700443097271) #<weak-box>)) > (collect-garbage) > (proos-store->list) (((59872449310226502864700443097271) #<weak-box>)) > (collect-garbage) > (proos-store->list) (((59872449310226502864700443097271) #<weak-box>)) > (set! a #f) > (set! b #f) > (collect-garbage) > (proos-store->list) () </tt>However when I don't use the (list) around the oid, It won't work. Isn't there a way without the 'equal property on the hash table? Matthew Flatt schreef: <blockquote cite="mid20051122015712.4644A15B5A9@salt.cs.brown.edu" type="cite"> <pre wrap="">At Tue, 22 Nov 2005 01:31:44 +0100, Hans Oesterholt-Dijkema wrote: </pre> <blockquote type="cite"> <blockquote type="cite"> <blockquote type="cite"> <pre wrap="">(define PROOS-STORE (make-hash-table 'equal 'weak)) [...] (hash-table-put! PROOS-STORE (list oid) (make-weak-box obj)) [...] </pre> </blockquote> </blockquote> </blockquote> <pre wrap=""> This doesn't look right. Nothing refers to the key created for the hash-table key (since it's a list created right at at the time that the mapping is installed), so the value should be immediately subject to collection. </pre> <blockquote type="cite"> <pre wrap=""> >(define a (car (psearch (all test-class)))) >(define b (car (psearch (all test-class)))) >(eq? a b) #t >(proos-store->list) ((23423523523234342 <weak-box>)) >(set! a #f) >(set! b #f) >(collect-garbage) >(proos-store->list) () </pre> </blockquote> <pre wrap=""> What happens if you skip the two `set!'s? Maybe it will require an extra `collect-garbage', but unless I'm missing something, you should still get an empty list. </pre> <blockquote type="cite"> <pre wrap="">In my case, there's a circular reference. The oid is stored in the object created by (class 'from-oid oid). </pre> </blockquote> <pre wrap=""> This is called the "key in value" problem. In 299.403 and later, you can solve it using ephemerons (where you're currently using weak boxes). Matthew _________________________________________________ For list-related administrative tasks: <a class="moz-txt-link-freetext" href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </pre> </blockquote> </body> </html>