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Re: Can't M-x compile-defun `edebug' because dynamic variables are false

From: Alan Mackenzie
Subject: Re: Can't M-x compile-defun `edebug' because dynamic variables are falsely taken as lexical.
Date: Wed, 4 Jan 2017 13:39:48 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

Hello, Stefan.

On Tue, Jan 03, 2017 at 04:48:15PM -0500, Stefan Monnier wrote:
> > It looks like you've already done that, possibly on 2015-10-29.  Looking
> > at the source (now I know what to look for), eval-sexp-add-defvars is
> > also in Emacs 25.1's source for compile-defun.

> Oh, indeed.  Not sure why you're seeing what you're seeing, then.
> At least on Emacs's master branch I can't seem to reproduce your problem
> (don't have a fresh emacs-25 build to try it right now).

For what it's worth, I can't reproduce the problem any more.  I must
have got my Emacs 25.1 into a strange state, somehow.

> > , so the question becomes why is setting the declared_special flag done
> > inside the "if (CONSP (tail))" rather than outside?  I.e. why is it only
> > done when a variable has an initialisation?

> Because this code is only relevant when you evaluate (defvar <foo>),
> whereas here we're not evaluating it, we're only processing it for the
> byte-compiler.

Understood.  Thanks.

> There's also the important difference that a (defvar <foo>) only
> has effect for the code in the same file rather than having a global
> effect, so you can do (defvar toto) and then use `toto` as
> a dynamically-scoped variable in your file without wreaking havoc in all
> other files which happen to also use `toto` as a (lexical) variable.

Ouch!  There seems to be a clash between symbols (which are global,
dynamic) and lexical variables (which are local and "invisible"): the
only way to mark a variable as lexical is at the global level.

However, I'm seeing something else strange (still on 25.1).  I do this:

(i) M-x load-library edebug.
(ii) visit edebug.el.
(iii) instrument `edebug' for edebugging (yes, I know ;-).
(iv) M-: (edebug).
(v) step through the function to just after the let bindings.
(vi) e (special-variable-p 'edebug-breakpoints).

This last returns nil.  This suggests edebug-breakpoints has been bound
as a lexical variable, rather than a dynamic one.  (There is a defvar
for it earlier in the file.)  This is surely not right.

What is going on here?  Is it, perhaps, a bug?

However, edebug.elc still seems to work, though I haven't successfully
mananged to run `edebug' recently.

>         Stefan

Alan Mackenzie (Nuremberg, Germany).

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