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From: jrferro@athena.mit.edu (Jon R Ferro)
Newsgroups: comp.emacs
Subject: Re: Sections in Gnu Emacs Buffers?
Message-ID: <9134@bloom-beacon.MIT.EDU>
Date: 7 Feb 89 00:07:09 GMT
References: <2846@hplabsz.HPL.HP.COM> <4300032@m.cs.uiuc.edu> <49769@yale-celray.yale.UUCP>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: jrferro@athena.mit.edu (Jon R Ferro)
Organization: Massachusetts Institute of Technology
Lines: 65

>>What kinds of things would you use intervals for?

Is this the right idea?:
The first thing that pops into mind would be to have intervals
automagically set off by comment delimiters, so that I wouldn't have to
think about how to fool C-mode into letting me edit my block comments
the way I like:

...
[C-mode: TAB means indent-current-line]
...
}

/*
**  [Pseudo-Text-Fill-mode with interval variable fill-prefix = "** "]
**  [TAB means tab-to-tab-stop]
** ...
*/

[Back in C-mode]
...


This usage implies some sort of automatic interval-creation mechanism
similar to abbrevs (NOT like the explicit ruler insertion of MacWrite).

I don't know enough about the theoretical operation of major modes to
know if changing to a "pseudo" mode in an interval would be best handled
by actually calling the mode function or just changing all the mode and
keymap variables in the interval.

Another good example is the editing of yacc input files for which emacs
(correctly) use c-mode in the sections which are actually C code, but
where c-mode rather gets in your way during the editing of the grammar
rules. (This would be better handled by something much more
text-mode-ish)

As was mentioned before, interval-local variables would also give a
mechanism for marking off blocks to be displayed bold, underlined, or
\narrower like the MacWrite or TeX commands they would emulate, but I
don't think pushing emacs towards WYSIWYGness should be the reason for
this work, or a good thing at all.

P.S. How would interval boundaries be displayed in the file? Would one
(a) have to find the boundaries via mark-current-interval or
narrow-to-interval and then go back to editing, or (b) would there be a
command like display-current-interval which would invert the current
interval's text until either the next keypress or until the next call to
undisplay-current-interval, or (c) permanent visible markers in the
text?  (My opinion: Both (a) and (b) should be available. (c) loses big.)

Would it even be necessary to display the boundaries of the interval if
the mode-line had some indication of what interval or set of interval
variables is active at the point? If the interval mechanism works as
transparently as the buffer/window mechanism, I can picture never
worrying about it.

Sounds like a very intriguing puzzle and a likely addition to emacs.
Hope these comments help.

--Jon

Jon Ferro
MIT Mathematics
jrferro@athena.mit.edu