Re: match-string debugging problem

[Pascal Bourguignon]

Stephen Berman <Stephen.Berman@gmx.net> writes:

> On 10 Mar 2005 18:06:52 +0100 Pascal Bourguignon <spam@mouse-potato.com> 
> wrote:
> 
> > Stephen Berman <Stephen.Berman@gmx.net> writes:
> >
> [...]
> >>     (let ((mystring-list ()))
> >>       (while (re-search-forward
> >>          (concat "^" (regexp-quote mystring1) "\\(.+\\)"
> >>                  (regexp-quote mystring2) "$")
> >>          (point-max) t)
> >>    (setq mystring-list (append (list (match-string 1)) mystring-list)))
> >>       (insert "\n")
> >>       (setq mystring-list (reverse mystring-list))
> >>       (dolist (elt mystring-list)
> >>    (insert elt " ")))))
> >> 
> >> After evalling this code and typing `M-x mystring-list', buffer *test*
> >> consists of these lines:
> >> 
> >> +++++ test1 ~~~~~
> >> +++++ test2 ~~~~~
> >> +++++ test3 ~~~~~
> >> +++++ test4 ~~~~~
> >> +++++ test5 ~~~~~
> >> test1 test2 test3 test4 test5 
> >> 
> >> The last line indicates that match-string correctly matches the
> >> strings that build mystring-list.  But when I step through the code
> >> with edebug, match-string always returns nil and a wrong-type-argument
> >> error is raised at the insert (since nil is not char-or-string-p).
> >> (Edebug isn't the problem: evalling first the regexp search code in
> >> *test* and then (match-string 1) also returns nil.)  Because of this
> >> I'm having a hard time debugging other code that uses match-string.
> >> Can someone explain what's going on?
> >
> > re-search-forward uses global state (buffer, matched range, etc) as
> > match-data, to communicate with match-string.  When you're debugging,
> > this global state is switched or modified.  One could consider it a
> > bug in the debugger.
> >
> > See: match-data
> >      save-match-data   
> >      save-excursion
> >      save-buffer
> 
> How do you know about this state difference?  

Infered from the doc of re-search-forward, match-string and save-match-data.

> I couldn't find any
> mention of it in the documentation of the functions you mention, nor
> elsewhere in the Elisp manual (CVS version).  I glanced through the
> comments in search.c but didn't see anything obviously (to me)
> relevant.  If it's defined in the C source code I'd appreciate a
> pointer, even though I probably wouldn't be able to understand it at
> present.
> 
> In any case, what I did find is that I can in fact track match-string
> by explicitly entering the ordinary Lisp debugger at that point.  So
> putting `(debug)' before `(match-string 1)' in the above code stops
> execution there and stepping into the function by pressing `d' returns
> the matched string.  So if there's a bug, then it's only in Edebug.  I
> couldn't find any relevant discussion in the Edebug section of the
> manual.  So at the very least, the Elisp documentation could be more
> explicit on this issue.


My guess is that either:

- Edebug matches regexp itself, and therefore erases the match-data, 
  (a save-match-data would be in order in Edebug), or

- Edebug changes the current buffer, and therefore when it executes 
  (match-string 1), this functions refers the match-data of the Edebug 
  buffer instead of that of the original buffer
  (a save-excursion would be in order in Edebug).

I'd bet for the former, since save-excursion is more commonly used
than save-match-data... (save-excursion calls save-buffer).


-- 
__Pascal Bourguignon__                     http://www.informatimago.com/
The rule for today:
Touch my tail, I shred your hand.
New rule tomorrow.