Re: Problems with debug-on-entry in the Lisp debugger.

[Lute Kamstra <Lute.Kamstra.lists@xs4all.nl>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>    I can think of two points in a macro to set a break for the
>>    debugger: just before macro expansion and just after it, right
>>    before the evaluation of the resulting sexp.  In both cases,
>>    hiding the debug-on-entry code from the user of the debugger
>>    seems not possible.
>
> To me "entry of a macro" is very clearly "just before expansion".
> Especially if you think of macro-expansion occurring because of
> explicit calls to macroexpand (e.g. in the byte-compiler) rather
> than as a direct part of interpretation.

I see you implemented this.  This makes debug-on-entry for macros a
lot better, of course.  Thanks.  But the problem I mentioned remains:
the debug-entry-code is visible.  For example:

(defmacro inc (var)
  (list 'setq var (list '1+ var)))
(debug-on-entry 'inc)
(progn (setq x 0) (inc x))

gives a backtrace like this:

------ Buffer: *Backtrace* ------ 
Debugger entered--entering a function:
* (lambda (var) (if (or inhibit-debug-on-entry debugger-jumping-flag) nil (debug ...)) (list (quote setq) var (list ... var)))(x)
  (inc x)
  (progn (setq x 0) (inc x))
  eval((progn (setq x 0) (inc x)))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp)
------ Buffer: *Backtrace* ------ 

>> Shall I go ahead and try to implement this, or do people think this
>> is a bad idea?
>
> I think it's a bad idea.  The interpreter is already way too slow,
> I'd rather not make it worse.

I think the effect on performance will be very minimal.  Do you know
of a good way to test the performance of the interpreter so that I can
measure the impact of my proposed change?  Can you indicate how much
performance loss would be acceptable for you?

Lute.