Re: bidi-display-reordering is now non-nil by default

[Eli Zaretskii <eliz@gnu.org>]

> From: David Kastrup <dak@gnu.org>
> Date: Sun, 31 Jul 2011 11:54:19 +0200
> 
> > We could do this through the modes (buffer-menu.el already does), but
> > my concern was about modes that are not bundled with Emacs, such as
> > mew, which was what triggered this thread in the first place.
> 
> How about setting bidi-paragraph-direction to nil only in text-mode,
> giving it a non-nil default otherwise?

I actually thought about doing that, but the disadvantage is that some
popular modes where dynamic determination of paragraph direction is
important do not inherit from Text Mode.  Rmail Mode is one example; I
expect Gnus to have similar issues.  IOW, any buffer where email is
displayed for reading would probably be affected.

That's just one consideration that prevented me from doing what you
suggest.  I expect other mode families to exist that have the same
issue.

But if we can cover them somehow, that would be a good solution, I
think.

> > bidi-paragraph-direction is a variable.  Setting a variable cannot
> > hurt, even if the version of Emacs one runs doesn't know about it.  Am
> > I missing something?
> 
> The willingness of maintainers to add code for features that are
> irrelevant to the functionality of their package and their Emacs
> version?

Well, I thought compatibility to Emacs in general and newer versions
in particular should be important, especially if the price is a single
line of Lisp.  But I have no experience in dealing with package
maintainers.

> "My mode does not have bidi, so why should I tell Emacs it has a bidi
> direction when it makes no user-visible difference except avoiding a
> performance bug" is harder to convincingly counter than "My mode does
> not have paragraphs, so why should I tell Emacs it does not have
> paragraphs when it makes a user-visible difference with the paragraph
> movement commands".

Maybe.  Bidi is a feature of display, i.e. it's impossible to
correctly display text in some scripts without it.  With today's deep
and wide m17n, whereby file names and network addresses can use R2L
characters, saying "my mode doesn't have bidi" is tantamount to saying
"my mode doesn't support certain cultures".  Whether this is
reasonable or not, I cannot judge.