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Re: Creating a coding system

From: K. Handa
Subject: Re: Creating a coding system
Date: Thu, 25 Dec 2014 00:06:22 +0900

In article <address@hidden>, David Kastrup <address@hidden> writes:
> > The other method is to use CCL (i.e :coding-type 'ccl), but,
> > if the combination of the charset decoding and translation
> > table works, it's faster than running CCL code.

> The translation table is not happy about translating things to nothing.
> Apparently that makes the calculation of the reverse translation go
> wrong.

Ah, perhaps make-translation-table-from-alist should have
optional argument to suppress building of reverse map, say
no-reverse-mmap.  Just for decoding, we don't need a reverse

> If you have time to spare on that topic, I'd rather you spend it on
> putting some more info in the Elisp manual or at least the
> define-ccl-program and define-coding-system doc strings.

Ok, I'll work on it.

> And the following afterthought in define-ccl-program is quite opaque as
> well:

>     TRANSLATE :=
>     LOOKUP :=
>     MAP :=

They were introduced to support Unicode handling in Emacs 22
and prior (i.e. pre-Unicode emacsen).  I think we can get
rid of them (or at least make them more simple and usable).

By the way, long ago, I had a plan of converting restricted
Elisp code to CCL, and hide those criptic CCL programs from
users.  Unfortunately, I got busy for the other work, and
couldn't realize that plan.  :-(

> There is also nothing in the DOC string of `define-coding-system' or the
> Elisp manual that would help in guessing what kind of options to choose
> for what kind of task.

> The purpose/definition of coding-type emacs-mule (particularly post
> Emacs-23) is not given, or what the various options with coding-type
> iso-2022 are.  It is not clear when one would use coding-type raw-text
> and when utf-8 (and how does utf-8 relate to emacs-mule?).

I'll try to document them.


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