Re: BIDI, LaTeX (auctex) and the «evil» backslash

[Eli Zaretskii <eliz@gnu.org>]

> From: Uwe Brauer <oub@mat.ucm.es>
> Date: Sat, 21 May 2016 13:55:18 +0000
> Cc: auctex-devel <auctex-devel@gnu.org>
> 
> In my understanding UTF distinguish between

What is "UTF" in this context?

>     -  LTR chars such as a,b,c
> 
>     -  RLT chars such as א,ב,ג
> 
>     -  «neutral» chars such as (),\ etc.

You should read the description of UBA, the Unicode Bidirectional
Algorithm (which Emacs implements).  There you will see that there are
actually 4 classes of characters:

 . string (LTR and RTL)
 . weak (numbers, number separators, diacriticals)
 . neutral (punctuation and whitespace)
 . formatting control characters (RLM etc.)

So:

  (get-char-code-property ?\\ 'bidi-class) => ON

("ON" stands for "other neutral", see the node "Character Properties"
in the ELisp manual.)

>     -  set bidi-paragraph-direction to left (shown in the next
>        screenshot.) The display is correct, however typing Hebrew, when
>        bidi-paragraph-direction is set to left is as unpleasant as
>        writing English with bidi-paragraph-direction set to right.
> 
>     -  use LRM chars before the backslash (see the last screenshot;
>        having set `glyphless-char-display-control' to `acronym'.
>        This looks well to but adding these chars is cumbersome.
> 
>     -  hack auctex (CC to the auctex list): a new variable is
>        introduced, say bidi-support, which is per default nil, but if it
>        is t, then LRM chars are inserted before a backslash. I am
>        pretty sure the auctex team will not like this idea very much.
> 
>     -  back emacs: in a LaTeX buffer, backslash is considered as LTR, I
>        don't know whether this can be done one the lisp level or whether
>        it can be done at all.
> 
> Comments?

The last one is possible, of course (this is Emacs), but that way lies
madness: arbitrarily changing bidirectional properties of characters
will bite you elsewhere, because the corresponding tables are global.

The other 3 alternatives are indeed the available solutions.
Personally, I recommend the 1st one; I see no problem with typing RTL
text in a left-to-right paragraph (and vice versa), and don't
understand what unpleasant things you bump into when doing that.  TeX
files are fundamentally left-to-right, as any program text, so that
would be my suggestion.