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Emacs debugging help...

From: eric
Subject: Emacs debugging help...
Date: 24 Oct 2002 14:01:55 -0400
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.2

Hello All,

I've recently decided to begin developing and debugging emacs lisp
programs.  However, I'm not sure how to use the debugger, and, for
anything more complicated than a simple function, the documentation
I've discovered is less than adequate.

The particulars of the current situation are:

I'm using VM for reading mail.  When my sister sends me email from
work, she uses Microsoft Outlook, which writes HTML mail.  Now, I know
this is wrong, and I've tried to convince her otherwise, but at this
point the politics are less interesting to me than the technology.  In
any case, VM tries to display the HTML email with w3, and w3 fails
(somewhere).  However, VM is written well, and catches the error, and
does something else.  I want to figure out what went wrong, that I
might fix it.

VM reports the failure to *Messages* as

Inline HTML display failed: (wrong-type-argument stringp nil)

which is generated in a particular function in a 


Now, when I set 'debug-on-error' to t, Emacs doesn't drop into the
debugger at the time this error is raised.  I guess that's because it
doesn't pick this up as an error, (because the error never makes it to
the toplevel?) and I have no debugging information; just the (sort of)
bad behaviour.

Now, if I put a 


just before the function call in the VM source code, recompile and
reinstall, I get into the (not too well understood by me yet) debugger
at around the right time, but then there's still reams of code to step
through (manually, with the 'd' key).

Clearly, there has to be a better way.  What am I missing?

What I'm looking for are useful tips, tricks, and general debugging
advice, that I might be a more productive and useful member of the

I have the latest VM and w3 sources, and would like to understand
these projects better.  I figure that fixing things which bother me is
a good way to start.


Eric Moncrieff                          

"If, in your bold creative way, you hold yourself free to draw a 
giraffe with a short neck, you will really find that you are not 
free to draw a giraffe."  --G. K. Chesterton
GPG Fingerprint: C278 AECC 3292 C26B E9DA  0FD7 EE20 4AA7 E1AD 0D79

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