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

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: emacs-devel@gnu.org
> Date: Wed, 10 Aug 2011 09:20:34 -0400
> 
> >> Also, while I'm here.  I re-suggest to change the default of
> >> bidi-paragraph-direction to left-to-right, since I think that preserves
> >> backward compatibility a lot better.
> > You never explained what backward compatibility is alluded to here.
> 
> Emacs has done only ever left-to-right until now.  So if we use
> right-to-left where left-to-right was needed, it's a regression, whereas
> if we use  left-to-right where right-to-left was needed it's not.

You are judging this from the POV or a non-R2L reader, who doesn't
really care about correct display of R2L text.  From the POV of R2L
readers, what Emacs did before v24 is a "regression", and the current
behavior is the correct one.

It is true that R2L text will be readable in Emacs 24 even with
left-to-right paragraphs, whereas in previous versions it was
illegible, but it still looks wrong to the R2L eye, and in some cases
(like very long lines that mix R2L and L2R) it's downright annoying.

It would be a real regression if Emacs would sometimes display strict
L2R text as right-to-left paragraphs.  But this is not the case.

IOW, Emacs 24 explicitly breaks "backward compatibility" in this
regard, and for a very good reason.  Some applications need to adapt
to that, true, but this adaptation is the remaining 10% of the long
journey from 100% non-support of bidi to 100% support.

> >> We can set it to nil in the major modes where we know it can be
> >> useful, such as text-mode.
> > What other modes will need this?
> 
> No idea.

Well, then how do we make sure that changing the default to
left-to-right will not make things worse than leaving it at nil?  The
list of modes that need left-to-right paragraphs and do not yet get
them due to inheritance from prog-mode is known (and currently very
short), because people complained.  Relying on similar complaints
after changing the default to left-to-right is not the best idea,
since the number of R2L users is much smaller than that of the other
kind.