Re: Converting compositions to strings

[Eli Zaretskii <eliz@gnu.org>]

> Cc: emacs-devel@gnu.org
> From: Clément Pit-Claudel <cpitclaudel@gmail.com>
> Date: Mon, 30 Jan 2017 11:39:09 -0500
> 
> On 2017-01-30 10:18, Eli Zaretskii wrote:
> > The information about this can be found in the doc strong of 
> > compose-region, under COMPONENTS.
> 
> Thanks.  This covers only "relative" compositions, right? Is there more information on non-relative compositions somewhere?

No, AFAIK this documents both relative and rule-based compositions.

> > The ones that seem to be relevant to what you are looking for (I'm 
> > guessing) are a small class, and I don't think you can discern them 
> > from the others unless you look at the "rules" parts of a 
> > composition.
> 
> Looking at the rules sounds fine; thanks! I think these compositions form a small class in terms of what Emacs supports, but that class seems to most if not all uses of composition that I see in my daily use of Emacs (where are other compositions used, beyond character spacing in what-cursor-position?).
> 
> Based on your pointers, the following seems to work OK for me (I guess it could be extended to cover the (Bl . Br) case, but I haven't seen that pop up yet in practice).
> 
> (defun esh--parse-composition (components)
>   "Translate composition COMPONENTS into a string."
>   (let ((chars (list (aref components 0)))
>         (nrules (/ (length components) 2)))
>     (dotimes (nrule nrules)
>       (let* ((rule (aref components (+ 1 (* 2 nrule))))
>              (char (aref components (+ 2 (* 2 nrule)))))
>         (pcase rule
>           (`(Br . Bl) (push char chars))
>           (_ (error "Unsupported composition COMPONENTS")))))
>     (concat chars)))

I think if we want to be able to produce text with all the text
properties and overlays expressed explicitly as text, we need to write
a back-end for xdisp.c that "displays" to a buffer.  It shouldn't be
hard, the number of interface routines a back-end needs to support is
not large; see 'struct redisplay_interface' in dispextern.h.