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Re: emacs 24's forward-char vs right-char behavior


From: Eli Zaretskii
Subject: Re: emacs 24's forward-char vs right-char behavior
Date: Wed, 25 Apr 2012 11:21:04 +0300

> From: Xah Lee <address@hidden>
> Date: Wed, 25 Apr 2012 00:43:22 -0700 (PDT)
> 
> Now, paste this sentence in emacs “(كتاب ألف ليلة و ليلة)”. Then, hold
> down right arrow key (which is bound to “right-char”), then when
> cursor moves into the Arabic text, it'll suddenly reverse direction,
> and move right to left, until it reaches the left most arabic char
> sequence, it'll jump back to the english text and continue move right.
> 
> Now, do the same but using “forward-char” 【Ctrl+f】. Actually, the same
> behavior is observed visually!
> 
> from Eli's post, it seems to be the expected behavior.

Indeed, expected behavior.

> But then what's the difference of forward-char and right-char? Am
> totally confused now.

Don't feel bad: this bidi business is complicated, especially for
someone who is not a native speaker of one of the bidi languages.

To see the difference between forward-char and right-char, do this:

  emacs -Q
  C-x b foo RET

Now paste the string "(كتاب ألف ليلة و ليلة)" into the buffer "foo"
you just created, and then try both C-f and <right>.  See the
difference now?

Explanation: the difference only shows up in paragraphs whose "base
direction" is right-to-left.  (See the Emacs manual's "Bidirectional
Editing" node for more about this.)  In the *scratch* buffer, all
paragraphs are forced to be left-to-right, because *scratch* is mostly
used for code snippets.  When you create a new buffer "foo", its
default value of bidi-paragraph-direction is nil, which means Emacs
determines the direction from the text of the paragraph.  Pasting
Arabic text causes Emacs to treat the paragraph as right-to-left and
render it starting at the right margin of the window.  As a side
effect, that affects the behavior of <right> vs forward-char.

> In emacs 23, holding right arrow (or Ctrl+f) simply move cursor to the
> right, ALWAYS. I was expecting this from emacs 24's “right-char”.

Type "C-h k <right>", and you will see that the commands bound to this
key in Emacs 23 and Emacs 24 are different.  Then follow the link to
the code of right-char in Emacs 24, and look at its definition.  I
think the code is self-explanatory.




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