Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!labrea!agate!pasteur!ames!ll-xn!mit-eddie!bu-cs!bucsb!jbw
From: jbw@bucsb.UUCP (Joe Wells)
Newsgroups: comp.emacs
Subject: Re: scope rules for "let"
Message-ID: <1482@bucsb.UUCP>
Date: 27 Feb 88 19:34:22 GMT
References: <3292@arthur.cs.purdue.edu>
Reply-To: jbw@bucsb.bu.edu (Joe Wells)
Followup-To: comp.emacs
Organization: Boston Univ Comp. Sci.
Lines: 51
Keywords: lambda binding
Summary: emacs lambda binding bug

In article <3292@arthur.cs.purdue.edu> narten@cs.purdue.EDU (Thomas Narten) writes:
>I have buffer that is readonly for keystrokes, but is made writable by
>functions operating on the buffer. I want the default to be readonly,
>with explicit action required to make the buffer write only.

>(let (buffer-read-only)
>     /* muck with the buffer */
>
>     )

Right in here is where the problem occurs.  If you use (set-buffer
...) inside of the "let", and you don't switch back to the original
buffer before exiting the "let", you get the error.

>I want the buffer-read-only variable to be nil only during the
>invocation of "let", reverting to its non-nil value upon exit. This is
>apparently not taking place. I find that the buffer-read-only is being
>left set to nil.

Actually the "t" value of buffer-read-only is being restored ... in a
different buffer!

>I conclude that "let" is not doing what I want. What I would like is
>for let to push the variables onto the symbol table stack, and use
>dynamic scope rules to have all subsequent references to the variables
>with those name find the "recent" bindings. When "let" exits, I would
>expect the temporary binding of those variables to pop off of the
>stack and the previous meanings be restored.
>
>Why does the above not work?

"buffer-read-only" is a buffer-local variable.  GNU Emacs does not
properly undo lambda bindings of buffer-local variables when you
switch buffers while in the context of the lambda binding.  You have
two options until this bug is fixed:

1) Don't use "let" to temporarily override the value of buffer-local
variables.

2)  Make sure you always exit a "let" with the same current buffer as
you entered it.

BTW, the GNU Emacs function "shrink-window-if-larger-than-buffer" in
electric.el makes this mistake.  I found out the hard way, when
buffer-file-name was being zapped, and I couldn't save my file ...

>Thomas Narten
>narten@cs.purdue.edu or {ihnp4, allegra}!purdue!narten

Joe Wells
jbw@bucsf.bu.edu