GNU bug report logs - #13813
24.3.50; eval-and-compile in macro inhibits let-binding of variable

Previous Next

Full log

Message #8 received at 13813 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: David Engster <deng <at> randomsample.de>
Cc: 13813 <at> debbugs.gnu.org
Subject: Re: bug#13813: 24.3.50;
	eval-and-compile in macro inhibits let-binding of variable
Date: Mon, 25 Feb 2013 15:53:21 -0500

> Recipe:
> * Create a file test.el with the following contents:

> (defvar myflag nil)
> (defmacro mytestmacro ()
>   "An eval-and-compile test."
>   `(eval-and-compile (mytestfun)))
> (defun mytestfun ()
>   (when myflag
>     (message "foo")))
> (let ((myflag t))
>   ;; Should display "foo"
>   (mytestmacro))

> * Now call with Emacs from trunk:
>   emacs --batch -l test.el
> * "foo" should be displayed, but isn't. The Emacs 24.2.93 pretest
>   however works as expected.

OK, you got me, I admit it, my recent change to eval-and-compile makes
it a lie.  Basically, there are now 3 different times:
- compile-time
- load-time (aka "eager-macroexpansion")
- eval-time

Most eval-and-compile are used for definitions which are used in macros
(hence needed for both compile-time and load-time).  For this reason,
I changed recently eval-and-compile to be (in effect)
eval-during-load-and-compile.  I could turn it into
a eval-during-eval-load-and-compile, but then your above test would run
`mytestfun' twice (once with myflag=nil and then once with myflag=t)
which I don't think would please everyone either.

> * Note the following:
>   - It works if you just 'setq' the 'myflag' variable

But not if you (progn (setq myflag t) (mytestmacro)).  The issue is
whether the setq takes place in a separate top-level expression, in
which case it's run before eager-macroexpansion of the expression that
does the (mytestmacro).

>   - The same problem occurs with `eval-when-compile', but everything
>     works with `progn'.

Yes, it's basically the same problem.

> And yes, Stefan, this bug turned up in the EIEIO test suite. :-)

Could you give some details, so we can better assess the best solution?
BTW, I think eval-and-compile should only be used to wrap definitions.
Anything else is asking for trouble.


        Stefan

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.