Re: Making Emacs Lisp easier to debug

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sat, Nov 11, 2023 at 13:10:44 +0200, Eli Zaretskii wrote:
> > Date: Sat, 11 Nov 2023 11:04:01 +0000
> > Cc: emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

> > :-)  I really meant has anybody looked into making redisplay reentrant.

> Good luck with that!

> (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.

> 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.

> )

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.

-- 
Alan Mackenzie (Nuremberg, Germany).