Re: MPS: composition

[Eli Zaretskii <eliz@gnu.org>]

> From: Helmut Eller <eller.helmut@gmail.com>
> Cc: gerd.moellmann@gmail.com,  emacs-devel@gnu.org
> Date: Sun, 28 Apr 2024 20:55:20 +0200
> 
> > Replacing that with an integer would be somewhat tricky.  The
> > requirement here is that if any part of the composition changes, the
> > key should be different, or else the results of the composition will
> > not follow the changes.
> 
> You mean, it is explicitly allowed to change part of the key while it is
> in the hash table?

The composable character sequences and the rules to compose them are
defined by Lisp data structures, and those can be modified by Lisp
programs, yes.

As a simple example, consider automatic compositions, produced by
auto-composition-mode.  These compositions are defined by
composition-function-table.  It is a char-table, where each composable
character has a non-nil entry.  We have such rules defined by default
for several languages which require that, but a Lisp program or the
user are allowed to add or modify those rules as they see fit, and
Emacs should not reuse stale cached compositions when the rules
change.  A simple practical example is to define rules for ligatures,
when certain sequences of ASCII characters, like "-->", are displayed
as corresponding font ligatures.  The rules for ligation, encoded in
composition-function-table, can easily change during an Emacs session.

We have clear-composition-cache to make sure stale cached compositions
don't get in the way, but that is a drastic measure, so we use it very
sparingly.