Re: [emacs-bidi] Bidirectional editing in Emacs -- main design decisions

[Eli Zaretskii]

> Date: Wed, 18 May 2016 05:29:21 -0700 (MST)
> From: mhsekhavat <address@hidden>
> 
> Eli Zaretskii wrote
> >  If Emacs would follow UAX#9 to the letter, each line could have different
> > base direction,
> >    which is, of course, intolerable.  We could avoid this nonsense by
> >    using the "soft newline" or similar features, but I firmly believe
> >    that Emacs should DTRT with bidirectional text even in the simplest
> >    modes, including the Fundamental mode, where every newline is hard.
> > 
> >    Fortunately, UAX#9 acknowledges that applications could have other
> >    ideas about what is a "paragraph".  It calls this ``higher
> >    protocol''.  So I decided to use such a higher protocol -- namely,
> >    the Emacs definition of a paragraph, as determined by the
> >    `paragraph-start' and `paragraph-separate' regexps.  Therefore, the
> >    first strong directional character after `paragraph-start' or
> >    `paragraph-separate' determines the paragraph direction, and that
> >    direction is kept for all the lines of the paragraph, until another
> >    `paragraph-separate' is found.  (Of course, this means that
> >    inserting a single character near the beginning of a paragraph
> >    might affect the display of all the lines in that paragraph, so
> >    some of the current redisplay optimizations which deal with changes
> >    to a single line need to be disabled in this case.)
> 
> I agree that most of the time, having different base direction for each line
> is nonsense. But there are cases where the text does not have "paragraph"s,
> but consists of a number of sentences, each in a line. So an empty line does
> not semantically mean paragraph separator and a right-to-left line appears
> just after a left-to-right line.

Emacs doesn't support such "paragraphs".

> How can I set variables `paragraph-start` and `paragraph-separate` so that
> each line have different direction? I tried setting `paragraph-start` to "."
> and `paragraph-separate` to "$" but didn't work?

You can't, sorry.  The values are hard-coded, and don't follow the
buffer-local values (I tried doing that once, but ended up with
unusable Emacs in modes that change these variables too much).

What is the real-life use case that requires this?  What kind of text
are we talking about?