GNU bug report logs - #12758
letf no longer allows unbound variables

Previous Next

To add a comment to this bug, you must first unarchive it, by sending
a message to control AT debbugs.gnu.org, with unarchive 12758 in the body.
You can then email your comments to 12758 AT debbugs.gnu.org in the normal way.

Toggle the display of automated, internal messages from the tracker.

Message #3 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Glenn Morris <rgm <at> gnu.org>
To: submit <at> debbugs.gnu.org
Subject: letf no longer allows unbound variables
Date: Mon, 29 Oct 2012 03:42:28 -0400

Package: emacs
Version: 24.2.50

In Emacs 24.2, this works:

emacs -Q -l cl

(letf ((b))
  (setq b 99))

Evaluating this returns `99', and leaves `b' unbound. This behaviour is
documented in cl.texi:

   In most cases, the @var{place} must have a well-defined value on
   entry to the @code{letf} form.  The only exceptions are plain
   variables and calls to @code{symbol-value} and @code{symbol-function}.
   If the symbol is not bound on entry, it is simply made unbound by
   @code{makunbound} or @code{fmakunbound} on exit.

In current trunk, it throws an error:

    Symbol's value as variable is void: b

cl-letf does no better than letf.

Message #6 received at 12758 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Glenn Morris <rgm <at> gnu.org>
Cc: 12758 <at> debbugs.gnu.org
Subject: Re: bug#12758: letf no longer allows unbound variables
Date: Mon, 29 Oct 2012 09:46:56 -0400

> Evaluating this returns `99', and leaves `b' unbound. This behaviour is
> documented in cl.texi:

>    In most cases, the @var{place} must have a well-defined value on
>    entry to the @code{letf} form.  The only exceptions are plain
>    variables and calls to @code{symbol-value} and @code{symbol-function}.
>    If the symbol is not bound on entry, it is simply made unbound by
>    @code{makunbound} or @code{fmakunbound} on exit.

I think this was a bad idea, so it indeed doesn't work that way any
longer; more specifically:

- W.r.t symbol-function, this is still true for `letf' but not for
  `cl-letf'.
- W.r.t symbol-value, this is not true any more neither of `letf' nor or
  `cl-letf' (I could change that for letf but in the absence of
  a bug-report pointing to pre-existing code that depends on this
  behavior I'd rather not).
- For (letf ((b 4)) ...) this is still true, because it expands to (let
  ((b 4)) ...).  But for (letf ((b)) ...) it isn't because that expands
  to (let ((b b)) ...).
  When `b' is a lexically-scoped variable, we really can't "fix" it
  because lexical variables don't have a notion of "unbound".


        Stefan

Previous Next

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