Re: Making Emacs Lisp easier to debug

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 11 Nov 2023 12:10:33 +0000
> Cc: emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > (I also don't understand why you think this will help with font-lock,
> > nor even how it would work in general, should it be possible.
> 
> With font lock, or any other Lisp hook called from redisplay, it should
> be possible, in a recursive-edit loop, to run edebug, displaying on a
> different frame.  That different frame would be running in the inner
> redisplay while the outer redisplay would be suspended.

Why do you need an inner redisplay for that?

And what will that frame show, given that the outer redisplay is
halfway through fontifying the text? what do you expect to see there,
and why?

> > Re-entering redisplay in the middle of a redisplay cycle means that
> > the outer redisplay didn't finish preparing the glyph matrices, and
> > what do you want the inner redisplay to do in such a case?
> 
> Work with the glyph matrices belonging to the inner redisplay whilst the
> outer one is suspended.

But that will immediately get you into the same problem, since the
offending window will get redisplayed by the inner redisplay, and will
again cause Edebug, etc., ad nauseam.

> As I say, it is not clear whether or not this is possible or
> practicable.  If it were, we could enhance edebug such that a function
> in a font lock pattern, or on, say, window-scroll-functions could be
> edebugged by doing nothing more than instrumenting it with C-u C-M-x.
> Thus Lisp called from redisplay would cease to be an awkward special
> case as far as debugging is concerned.

I think we should start by having a clear idea of what should such an
"inner redisplay" show and how, before we are talking about
implementing it.  My impression from what you wrote is that the idea
is way too vague to discuss the details, and what you call "inner
redisplay" is not what you want at all.