Path: utzoo!attcan!uunet!samsung!know!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!psuvax1!rutgers!mcnc!rti!talos!kjones
From: kjones@talos.pm.com (Kyle Jones)
Newsgroups: comp.emacs
Subject: Re: %% vs. @@
Message-ID: <1990Jun27.145013.29422@talos.pm.com>
Date: 27 Jun 90 14:50:13 GMT
References: <87.P022y01JM01@JUTS.ccc.amdahl.com>
Lines: 39

Kimball P Collins writes:
 > The %% (toggle-read-only) capability of emacs does not meet my needs
 > because I want the capability of changing the buffer.
 > 
 > I am looking into implementing an additional feature, which has @@ on
 > the mode line in a different spot (different from where (**, %%) are)
 > and allows you to write into the buffer, but won't let the buffer be
 > saved.  This is somewhat like making a buffer into another scratch
 > buffer for as long as it is in @@ mode.
 > 
 > For this, I need to
 > 
 >         * write toggle-buffer-offer-save

You'll need to do a bit more than just toggle buffer-offer-save.
Emacs will ask to save any buffer that has a filename associated
with it (via buffer-file-name).  So your toggle function will
need to set/reset the value of buffer-file-name for file buffers.

 >         * add mode-line-buffer-offer-save to mode-line so that it
 >         prints "@@" or "--" depending on the value of
 >         mode-line-modified, or another variable
 > 
 > The problem is that mode-line is not flexible about functions that can
 > be called for it.

In fact, no user-specified functions can be called during
mode-line-format processing.  (I suspect that this restriction
exists so that the display routines and other routines that call
them needn't worry about garbage collection.)  But you can
control the mode-line based on the value of Lisp variables, and
that's all you need here.

Try this:

(setq-default mode-line-modified
	      (cons '(buffer-file-name "----"
		      (buffer-offer-save "----" "--@@"))
		    mode-line-modified))