NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/comp/lang/lisp/2067

Internet Message Format | 1992-07-21 | 1.5 KB

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Path: sparky!uunet!usc!cs.utexas.edu!qt.cs.utexas.edu!yale.edu!jvnc.net!princeton!siemens!volvo!mlg
From: [e-mail]. (Michael Greenberg)
Newsgroups: comp.lang.lisp,comp.lang.clos
Subject: Beware of print-object methods when debugging
Message-ID: <mlg.711741357@volvo>
Date: 21 Jul 92 17:55:57 GMT
Sender: [e-mail]
Followup-To: comp.lang.lisp
Lines: 34


A coworker came into my office this morning to ask for help debugging
his program.  The bug seemed to be in a part of the program that did
processing nodes in a graph. The code seemed to work correctly for all
objects (about 1000 of them) except for one.

Somewhere in the code there was a statement like:

(setf (node-a node) (make-instance 'foo ...))

We would do something like

(dolist (node (all-objects-in-tree))
  (print (node-a node)))

and see the expected results except for one object where 'NIL' was
printed.   The code was pretty complex and there were all sorts of
dependencies between the various objects in the system, so it wasn't
surprising that that was an error somewhere.

I beat on this for a while trying to figure out what was
going on.  30 minutes later he said "By the way, I also have a
question about print-object...".

It turns out the problem was that his print-object method for the
class 'foo' sometimes printed 'NIL'.



--
Michael Greenberg                      email: [e-mail]
Siemens Corporate Research             phone: [phone]
755 College Road East
Princeton, NJ 08540