@c This is part of the Emacs manual.
@c Copyright (C) 1997 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Mule, Major Modes, Windows, Top
@chapter World Scripts Support
@cindex MULE
@cindex international scripts
@cindex multibyte characters
@cindex encoding of characters

@cindex Chinese
@cindex Greek
@cindex IPA
@cindex Japanese
@cindex Korean
@cindex Cyrillic
@cindex Russian
@c #### It's a lie that this file tells you about Unicode....
@cindex Unicode
  If you build XEmacs using the @code{--with-mule} option, it supports a
wide variety of world scripts, including the Latin script, the Arabic
script, Simplified Chinese (for mainland of China), Traditional Chinese
(for Taiwan and Hong-Kong), the Greek script, the Hebrew script, IPA
symbols, Japanese scripts (Hiragana, Katakana and Kanji), Korean scripts
(Hangul and Hanja) and the Cyrillic script (for Byelorussian, Bulgarian,
Russian, Serbian and Ukrainian).  These features have been merged from
the modified version of Emacs known as MULE (for ``MULti-lingual
Enhancement to GNU Emacs'').

@menu
* Mule Intro::              Basic concepts of Mule.
* Language Environments::   Setting things up for the language you use.
* Input Methods::           Entering text characters not on your keyboard.
* Select Input Method::     Specifying your choice of input methods.
* Coding Systems::          Character set conversion when you read and
                              write files, and so on.
* Recognize Coding::        How XEmacs figures out which conversion to use.
* Unification::             Integrating overlapping character sets.
* Specify Coding::          Various ways to choose which conversion to use.
* Charsets and Coding Systems:: Tables and other reference material.
@end menu

@node Mule Intro, Language Environments, Mule, Mule
@section Introduction: The Wide Variety of Scripts and Codings in Use

  There are hundreds of scripts in use world-wide.  The users of these
scripts have established many more-or-less standard coding systems for
storing text written in them in files.  XEmacs translates between its
internal character encoding and various other coding systems when
reading and writing files, when exchanging data with subprocesses, and
(in some cases) in the @kbd{C-q} command (see below).
@footnote{Historically the internal encoding was a specially designed
encoding, called @dfn{Mule encoding}, intended for easy conversion to
and from versions of ISO 2022.  However, this encoding shares many
properties with UTF-8, and conversion to UTF-8 as the internal code is
proposed.}

@kindex C-h h
@findex view-hello-file
  The command @kbd{C-h h} (@code{view-hello-file}) displays the file
@file{etc/HELLO}, which shows how to say ``hello'' in many languages.
This illustrates various scripts.

  Keyboards, even in the countries where these character sets are used,
generally don't have keys for all the characters in them.  So XEmacs
supports various @dfn{input methods}, typically one for each script or
language, to make it convenient to type them.

@kindex C-x RET
  The prefix key @kbd{C-x @key{RET}} is used for commands that pertain
to world scripts, coding systems, and input methods.


@node Language Environments, Input Methods, Mule Intro, Mule
@section Language Environments
@cindex language environments

  All supported character sets are supported in XEmacs buffers if it is
compiled with mule; there is no need to select a particular language in
order to display its characters in an XEmacs buffer.  However, it is
important to select a @dfn{language environment} in order to set various
defaults.  The language environment really represents a choice of
preferred script (more or less) rather that a choice of language.

  The language environment controls which coding systems to recognize
when reading text (@pxref{Recognize Coding}).  This applies to files,
incoming mail, netnews, and any other text you read into XEmacs.  It may
also specify the default coding system to use when you create a file.
Each language environment also specifies a default input method.

@findex set-language-environment
  The command to select a language environment is @kbd{M-x
set-language-environment}.  It makes no difference which buffer is
current when you use this command, because the effects apply globally to
the XEmacs session.  The supported language environments include:

@quotation
ASCII, Chinese-BIG5, Chinese-GB, Croatian, Cyrillic-ALT, Cyrillic-ISO,
Cyrillic-KOI8, Cyrillic-Win, Czech, English, Ethiopic, French, German,
Greek, Hebrew, IPA, Japanese, Korean, Latin-1, Latin-2, Latin-3, Latin-4,
Latin-5, Norwegian, Polish, Romanian, Slovenian, Thai-XTIS, Vietnamese.
@end quotation

  Some operating systems let you specify the language you are using by
setting locale environment variables.  XEmacs handles one common special
case of this: if your locale name for character types contains the
string @samp{8859-@var{n}}, XEmacs automatically selects the
corresponding language environment.

@kindex C-h L
@findex describe-language-environment
  To display information about the effects of a certain language
environment @var{lang-env}, use the command @kbd{C-h L @var{lang-env}
@key{RET}} (@code{describe-language-environment}).  This tells you which
languages this language environment is useful for, and lists the
character sets, coding systems, and input methods that go with it.  It
also shows some sample text to illustrate scripts used in this language
environment.  By default, this command describes the chosen language
environment.

@node Input Methods, Select Input Method, Language Environments, Mule
@section Input Methods

@cindex input methods
  An @dfn{input method} is a kind of character conversion designed
specifically for interactive input.  In XEmacs, typically each language
has its own input method; sometimes several languages which use the same
characters can share one input method.  A few languages support several
input methods.

  The simplest kind of input method works by mapping ASCII letters into
another alphabet.  This is how the Greek and Russian input methods work.

  A more powerful technique is composition: converting sequences of
characters into one letter.  Many European input methods use composition
to produce a single non-ASCII letter from a sequence that consists of a
letter followed by accent characters.  For example, some methods convert
the sequence @kbd{'a} into a single accented letter.

  The input methods for syllabic scripts typically use mapping followed
by composition.  The input methods for Thai and Korean work this way.
First, letters are mapped into symbols for particular sounds or tone
marks; then, sequences of these which make up a whole syllable are
mapped into one syllable sign.

  Chinese and Japanese require more complex methods.  In Chinese input
methods, first you enter the phonetic spelling of a Chinese word (in
input method @code{chinese-py}, among others), or a sequence of portions
of the character (input methods @code{chinese-4corner} and
@code{chinese-sw}, and others).  Since one phonetic spelling typically
corresponds to many different Chinese characters, you must select one of
the alternatives using special XEmacs commands.  Keys such as @kbd{C-f},
@kbd{C-b}, @kbd{C-n}, @kbd{C-p}, and digits have special definitions in
this situation, used for selecting among the alternatives.  @key{TAB}
displays a buffer showing all the possibilities.

   In Japanese input methods, first you input a whole word using
phonetic spelling; then, after the word is in the buffer, XEmacs
converts it into one or more characters using a large dictionary.  One
phonetic spelling corresponds to many differently written Japanese
words, so you must select one of them; use @kbd{C-n} and @kbd{C-p} to
cycle through the alternatives.

  Sometimes it is useful to cut off input method processing so that the
characters you have just entered will not combine with subsequent
characters.  For example, in input method @code{latin-1-postfix}, the
sequence @kbd{e '} combines to form an @samp{e} with an accent.  What if
you want to enter them as separate characters?

  One way is to type the accent twice; that is a special feature for
entering the separate letter and accent.  For example, @kbd{e ' '} gives
you the two characters @samp{e'}.  Another way is to type another letter
after the @kbd{e}---something that won't combine with that---and
immediately delete it.  For example, you could type @kbd{e e @key{DEL}
'} to get separate @samp{e} and @samp{'}.

  Another method, more general but not quite as easy to type, is to use
@kbd{C-\ C-\} between two characters to stop them from combining.  This
is the command @kbd{C-\} (@code{toggle-input-method}) used twice.
@ifinfo
@xref{Select Input Method}.
@end ifinfo

  @kbd{C-\ C-\} is especially useful inside an incremental search,
because stops waiting for more characters to combine, and starts
searching for what you have already entered.

@vindex input-method-verbose-flag
@vindex input-method-highlight-flag
  The variables @code{input-method-highlight-flag} and
@code{input-method-verbose-flag} control how input methods explain what
is happening.  If @code{input-method-highlight-flag} is non-@code{nil},
the partial sequence is highlighted in the buffer.  If
@code{input-method-verbose-flag} is non-@code{nil}, the list of possible
characters to type next is displayed in the echo area (but not when you
are in the minibuffer).

@node Select Input Method, Coding Systems, Input Methods, Mule
@section Selecting an Input Method

@table @kbd
@item C-\
Enable or disable use of the selected input method.

@item C-x @key{RET} C-\ @var{method} @key{RET}
Select a new input method for the current buffer.

@item C-h I @var{method} @key{RET}
@itemx C-h C-\ @var{method} @key{RET}
@findex describe-input-method
@kindex C-h I
@kindex C-h C-\
Describe the input method @var{method} (@code{describe-input-method}).
By default, it describes the current input method (if any).

@item M-x list-input-methods
Display a list of all the supported input methods.
@end table

@findex select-input-method
@vindex current-input-method
@kindex C-x RET C-\
  To choose an input method for the current buffer, use @kbd{C-x
@key{RET} C-\} (@code{select-input-method}).  This command reads the
input method name with the minibuffer; the name normally starts with the
language environment that it is meant to be used with.  The variable
@code{current-input-method} records which input method is selected.

@findex toggle-input-method
@kindex C-\
  Input methods use various sequences of ASCII characters to stand for
non-ASCII characters.  Sometimes it is useful to turn off the input
method temporarily.  To do this, type @kbd{C-\}
(@code{toggle-input-method}).  To reenable the input method, type
@kbd{C-\} again.

  If you type @kbd{C-\} and you have not yet selected an input method,
it prompts for you to specify one.  This has the same effect as using
@kbd{C-x @key{RET} C-\} to specify an input method.

@vindex default-input-method
  Selecting a language environment specifies a default input method for
use in various buffers.  When you have a default input method, you can
select it in the current buffer by typing @kbd{C-\}.  The variable
@code{default-input-method} specifies the default input method
(@code{nil} means there is none).

@findex quail-set-keyboard-layout
  Some input methods for alphabetic scripts work by (in effect)
remapping the keyboard to emulate various keyboard layouts commonly used
for those scripts.  How to do this remapping properly depends on your
actual keyboard layout.  To specify which layout your keyboard has, use
the command @kbd{M-x quail-set-keyboard-layout}.

@findex list-input-methods
  To display a list of all the supported input methods, type @kbd{M-x
list-input-methods}.  The list gives information about each input
method, including the string that stands for it in the mode line.

@node Coding Systems, Recognize Coding, Select Input Method, Mule
@section Coding Systems
@cindex coding systems

  Users of various languages have established many more-or-less standard
coding systems for representing them.  XEmacs does not use these coding
systems internally; instead, it converts from various coding systems to
its own system when reading data, and converts the internal coding
system to other coding systems when writing data.  Conversion is
possible in reading or writing files, in sending or receiving from the
terminal, and in exchanging data with subprocesses.

  XEmacs assigns a name to each coding system.  Most coding systems are
used for one language, and the name of the coding system starts with the
language name.  Some coding systems are used for several languages;
their names usually start with @samp{iso}.  There are also special
coding systems @code{binary} and @code{no-conversion} which do not
convert printing characters at all.

  In addition to converting various representations of non-ASCII
characters, a coding system can perform end-of-line conversion.  XEmacs
handles three different conventions for how to separate lines in a file:
newline, carriage-return linefeed, and just carriage-return.

@table @kbd
@item C-x @key{RET} C @var{coding} @key{RET}
Describe coding system @var{coding}.

@item C-x @key{RET} C @key{RET}
Describe the coding systems currently in use.

@item M-x list-coding-systems
Display a list of all the supported coding systems.

@item C-u M-x list-coding-systems
Display comprehensive list of specific details of all supported coding
systems.

@end table

@kindex C-x RET C
@findex describe-coding-system
  The command @kbd{C-x RET C} (@code{describe-coding-system}) displays
information about particular coding systems.  You can specify a coding
system name as argument; alternatively, with an empty argument, it
describes the coding systems currently selected for various purposes,
both in the current buffer and as the defaults, and the priority list
for recognizing coding systems (@pxref{Recognize Coding}).

@findex list-coding-systems
  To display a list of all the supported coding systems, type @kbd{M-x
list-coding-systems}.  The list gives information about each coding
system, including the letter that stands for it in the mode line
(@pxref{Mode Line}).

  Each of the coding systems that appear in this list---except for
@code{binary}, which means no conversion of any kind---specifies how and
whether to convert printing characters, but leaves the choice of
end-of-line conversion to be decided based on the contents of each file.
For example, if the file appears to use carriage-return linefeed between
lines, that end-of-line conversion will be used.

  Each of the listed coding systems has three variants which specify
exactly what to do for end-of-line conversion:

@table @code
@item @dots{}-unix
Don't do any end-of-line conversion; assume the file uses
newline to separate lines.  (This is the convention normally used
on Unix and GNU systems.)

@item @dots{}-dos
Assume the file uses carriage-return linefeed to separate lines,
and do the appropriate conversion.  (This is the convention normally used
on Microsoft systems.)

@item @dots{}-mac
Assume the file uses carriage-return to separate lines, and do the
appropriate conversion.  (This is the convention normally used on the
Macintosh system.)
@end table

  These variant coding systems are omitted from the
@code{list-coding-systems} display for brevity, since they are entirely
predictable.  For example, the coding system @code{iso-8859-1} has
variants @code{iso-8859-1-unix}, @code{iso-8859-1-dos} and
@code{iso-8859-1-mac}.

  In contrast, the coding system @code{binary} specifies no character
code conversion at all---none for non-Latin-1 byte values and none for
end of line.  This is useful for reading or writing binary files, tar
files, and other files that must be examined verbatim.

  The easiest way to edit a file with no conversion of any kind is with
the @kbd{M-x find-file-literally} command.  This uses @code{binary}, and
also suppresses other XEmacs features that might convert the file
contents before you see them.  @xref{Visiting}.

  The coding system @code{no-conversion} means that the file contains
non-Latin-1 characters stored with the internal XEmacs encoding.  It
handles end-of-line conversion based on the data encountered, and has
the usual three variants to specify the kind of end-of-line conversion.


@node Recognize Coding, Unification, Coding Systems, Mule
@section Recognizing Coding Systems

  Most of the time, XEmacs can recognize which coding system to use for
any given file--once you have specified your preferences.

  Some coding systems can be recognized or distinguished by which byte
sequences appear in the data.  However, there are coding systems that
cannot be distinguished, not even potentially.  For example, there is no
way to distinguish between Latin-1 and Latin-2; they use the same byte
values with different meanings.

  XEmacs handles this situation by means of a priority list of coding
systems.  Whenever XEmacs reads a file, if you do not specify the coding
system to use, XEmacs checks the data against each coding system,
starting with the first in priority and working down the list, until it
finds a coding system that fits the data.  Then it converts the file
contents assuming that they are represented in this coding system.

  The priority list of coding systems depends on the selected language
environment (@pxref{Language Environments}).  For example, if you use
French, you probably want XEmacs to prefer Latin-1 to Latin-2; if you
use Czech, you probably want Latin-2 to be preferred.  This is one of
the reasons to specify a language environment.

@findex prefer-coding-system
  However, you can alter the priority list in detail with the command
@kbd{M-x prefer-coding-system}.  This command reads the name of a coding
system from the minibuffer, and adds it to the front of the priority
list, so that it is preferred to all others.  If you use this command
several times, each use adds one element to the front of the priority
list.

@vindex file-coding-system-alist
  Sometimes a file name indicates which coding system to use for the
file.  The variable @code{file-coding-system-alist} specifies this
correspondence.  There is a special function
@code{modify-coding-system-alist} for adding elements to this list.  For
example, to read and write all @samp{.txt} using the coding system
@code{china-iso-8bit}, you can execute this Lisp expression:

@smallexample
(modify-coding-system-alist 'file "\\.txt\\'" 'china-iso-8bit)
@end smallexample

@noindent
The first argument should be @code{file}, the second argument should be
a regular expression that determines which files this applies to, and
the third argument says which coding system to use for these files.

@vindex coding
  You can specify the coding system for a particular file using the
@samp{-*-@dots{}-*-} construct at the beginning of a file, or a local
variables list at the end (@pxref{File Variables}).  You do this by
defining a value for the ``variable'' named @code{coding}.  XEmacs does
not really have a variable @code{coding}; instead of setting a variable,
it uses the specified coding system for the file.  For example,
@samp{-*-mode: C; coding: iso-8859-1;-*-} specifies use of the
iso-8859-1 coding system, as well as C mode.

@vindex buffer-file-coding-system
  Once XEmacs has chosen a coding system for a buffer, it stores that
coding system in @code{buffer-file-coding-system} and uses that coding
system, by default, for operations that write from this buffer into a
file.  This includes the commands @code{save-buffer} and
@code{write-region}.  If you want to write files from this buffer using
a different coding system, you can specify a different coding system for
the buffer using @code{set-buffer-file-coding-system} (@pxref{Specify
Coding}).


@node Unification, Specify Coding, Recognize Coding, Mule
@section Character Set Unification

Mule suffers from a design defect that causes it to consider the ISO
Latin character sets to be disjoint.  This results in oddities such as
files containing both ISO 8859/1 and ISO 8859/15 codes, and using ISO
2022 control sequences to switch between them, as well as more
plausible but often unnecessary combinations like ISO 8859/1 with ISO
8859/2.  This can be very annoying when sending messages or even in
simple editing on a single host.  XEmacs works around the problem by
converting as many characters as possible to use a single Latin coded
character set before saving the buffer.

Unification is planned for extension to other character set families,
in particular the Han family of character sets based on the Chinese
ideographic characters.  At least for the Han sets, however, the
unification feature will be disabled by default.

This functionality is based on the @file{latin-unity} package by
Stephen Turnbull @email{stephen@@xemacs.org}, but is somewhat
divergent.  This documentation is also based on the package
documentation, and is likely to be inaccurate because of the different
constraints we place on ``core'' and packaged functionality.

@menu
* Unification Overview::        History and general information.
* Unification Usage::           An overview of operation.
* Unification Configuration::   Configuring unification.
* Unification FAQs::            Questions and answers from the mailing list.
* Unification Theory::          How unification works.
* What Unification Cannot Do for You::  Inherent problems of 8-bit charsets.
@end menu

@node Unification Overview, Unification Usage, Unification, Unification
@subsection An Overview of Character Set Unification

Mule suffers from a design defect that causes it to consider the ISO
Latin character sets to be disjoint.  This manifests itself when a user
enters characters using input methods associated with different coded
character sets into a single buffer.

A very important example involves email.  Many sites, especially in the
U.S., default to use of the ISO 8859/1 coded character set (also called
``Latin 1,'' though these are somewhat different concepts).  However,
ISO 8859/1 provides a generic CURRENCY SIGN character.  Now that the
Euro has become the official currency of most countries in Europe, this
is unsatisfactory (and in practice, useless).  So Europeans generally
use ISO 8859/15, which is nearly identical to ISO 8859/1 for most
languages, except that it substitutes EURO SIGN for CURRENCY SIGN.

Suppose a European user yanks text from a post encoded in ISO 8859/1
into a message composition buffer, and enters some text including the
Euro sign.  Then Mule will consider the buffer to contain both ISO
8859/1 and ISO 8859/15 text, and MUAs such as Gnus will (if naively
programmed) send the message as a multipart mixed MIME body!

This is clearly stupid.  What is not as obvious is that, just as any
European can include American English in their text because ASCII is a
subset of ISO 8859/15, most European languages which use Latin
characters (eg, German and Polish) can typically be mixed while using
only one Latin coded character set (in this case, ISO 8859/2).  However,
this often depends on exactly what text is to be encoded.

Unification works around the problem by converting as many characters as
possible to use a single Latin coded character set before saving the
buffer.


@node Unification Usage, Unification Configuration, Unification Overview, Unification
@subsection Operation of Unification

This is a description of the early hack to include unification in
XEmacs 21.5.  This will almost surely change.

Normally, unification works in the background by installing
@code{unity-sanity-check} on @code{write-region-pre-hook}.
Unification is on by default for the ISO-8859 Latin sets.  The user
activates this functionality for other chacter set families by
invoking @code{enable-unification}, either interactively or in her
init file.  @xref{Init File, , , xemacs}.  Unification can be
deactivated by invoking @code{disable-unification}.

Unification also provides a few functions for remapping or recoding the
buffer by hand.  To @dfn{remap} a character means to change the buffer
representation of the character by using another coded character set.
Remapping never changes the identity of the character, but may involve
altering the code point of the character.  To @dfn{recode} a character
means to simply change the coded character set.  Recoding never alters
the code point of the character, but may change the identity of the
character.  @xref{Unification Theory}.

There are a few variables which determine which coding systems are
always acceptable to unification: @code{unity-ucs-list},
@code{unity-preferred-coding-system-list}, and
@code{unity-preapproved-coding-system-list}.  The last defaults to
@code{(buffer preferred)}, and you should probably avoid changing it
because it short-circuits the sanity check.  If you find you need to
use it, consider reporting it as a bug or request for enhancement.

@menu
* Basic Functionality::            User interface and customization.
* Interactive Usage::              Treating text by hand.
                                   Also documents the hook function(s).
@end menu


@node Basic Functionality, Interactive Usage, , Unification Usage
@subsubsection Basic Functionality

These functions and user options initialize and configure unification.
In normal use, they are not needed.

@strong{These interfaces will change.  Also, the @samp{unity-} prefix
is likely to be changed for many of the variables and functions, as
they are of more general usefulness.}

@defun enable-unification
Set up hooks and initialize variables for unification.

There are no arguments.

This function is idempotent.  It will reinitialize any hooks or variables
that are not in initial state.
@end defun

@defun disable-unification
There are no arguments.

Clean up hooks and void variables used by unification.
@end defun

@c #### several changes should go to latin-unity.texi
@defopt unity-ucs-list
List of universal coding systems recommended for character set unification.

The default value is @code{'(utf-8 iso-2022-7 ctext escape-quoted)}.

Order matters; coding systems earlier in the list will be preferred when
recommending a coding system.  These coding systems will not be used
without querying the user (unless they are also present in
@code{unity-preapproved-coding-system-list}), and follow the
@code{unity-preferred-coding-system-list} in the list of suggested
coding systems.

If none of the preferred coding systems are feasible, the first in
this list will be the default.

Notes on certain coding systems:  @code{escape-quoted} is a special
coding system used for autosaves and compiled Lisp in Mule.  You should
never delete this, although it is rare that a user would want to use it
directly.  Unification does not try to be ``smart'' about other general
ISO 2022 coding systems, such as ISO-2022-JP.  (They are not recognized
as equivalent to @code{iso-2022-7}.)  If your preferred coding system is
one of these, you may consider adding it to @code{unity-ucs-list}.
@end defopt

Coding systems which are not Latin and not in
@code{unity-ucs-list} are handled by short circuiting checks of
coding system against the next two variables.

@defopt unity-preapproved-coding-system-list
List of coding systems used without querying the user if feasible.

The default value is @samp{(buffer-default preferred)}.

The first feasible coding system in this list is used.  The special values
@samp{preferred} and @samp{buffer-default} may be present:

@table @code
@item buffer-default
Use the coding system used by @samp{write-region}, if feasible.

@item preferred
Use the coding system specified by @samp{prefer-coding-system} if feasible.
@end table

"Feasible" means that all characters in the buffer can be represented by
the coding system.  Coding systems in @samp{unity-ucs-list} are
always considered feasible.  Other feasible coding systems are computed
by @samp{unity-representations-feasible-region}.

Note that, by definition, the first universal coding system in this
list shadows all other coding systems.  In particular, if your
preferred coding system is a universal coding system, and
@code{preferred} is a member of this list, unification will blithely
convert all your files to that coding system.  This is considered a
feature, but it may surprise most users.  Users who don't like this
behavior may put @code{preferred} in
@code{unity-preferred-coding-system-list}, but not in
@code{unity-preapproved-coding-system-list}.
@end defopt


@defopt unity-preferred-coding-system-list
List of coding systems suggested to the user if feasible.

The default value is @samp{(iso-8859-1 iso-8859-15 iso-8859-2 iso-8859-3
iso-8859-4 iso-8859-9)}.

If none of the coding systems in
@samp{unity-preapproved-coding-system-list} are feasible, this list
will be recommended to the user, followed by the
@samp{unity-ucs-list} (so those coding systems should not be in
this list).  The first coding system in this list is default.  The
special values @samp{preferred} and @samp{buffer-default} may be
present:

@table @code
@item buffer-default
Use the coding system used by @samp{write-region}, if feasible.

@item preferred
Use the coding system specified by @samp{prefer-coding-system} if feasible.
@end table

"Feasible" means that all characters in the buffer can be represented by
the coding system.  Coding systems in @samp{unity-ucs-list} are
always considered feasible.  Other feasible coding systems are computed
by @samp{unity-representations-feasible-region}.
@end defopt


@defvar unity-iso-8859-1-aliases
List of coding systems to be treated as aliases of ISO 8859/1.

The default value is '(iso-8859-1).

This is not a user variable; to customize input of coding systems or
charsets, @samp{unity-coding-system-alias-alist} or
@samp{unity-charset-alias-alist}.
@end defvar


@node Interactive Usage, , Basic Functionality, Unification Usage
@subsubsection Interactive Usage

First, the hook function @code{unity-sanity-check} is documented.
(It is placed here because it is not an interactive function, and there
is not yet a programmer's section of the manual.)

These functions provide access to internal functionality (such as the
remapping function) and to extra functionality (the recoding functions
and the test function).

@defun unity-sanity-check begin end filename append visit lockname &optional coding-system

Check if @var{coding-system} can represent all characters between
@var{begin} and @var{end}.

For compatibility with old broken versions of @code{write-region},
@var{coding-system} defaults to @code{buffer-file-coding-system}.
@var{filename}, @var{append}, @var{visit}, and @var{lockname} are
ignored.

Return nil if buffer-file-coding-system is not (ISO-2022-compatible)
Latin.  If @code{buffer-file-coding-system} is safe for the charsets
actually present in the buffer, return it.  Otherwise, ask the user to
choose a coding system, and return that.

This function does @emph{not} do the safe thing when
@code{buffer-file-coding-system} is nil (aka no-conversion).  It
considers that ``non-Latin,'' and passes it on to the Mule detection
mechanism.

This function is intended for use as a @code{write-region-pre-hook}.  It
does nothing except return @var{coding-system} if @code{write-region}
handlers are inhibited.
@end defun

@defun unity-buffer-representations-feasible
There are no arguments.

Apply unity-region-representations-feasible to the current buffer.
@end defun

@defun unity-region-representations-feasible begin end &optional buf
Return character sets that can represent the text from @var{begin} to
@var{end} in @var{buf}.

@c #### Fix in latin-unity.texi.
@var{buf} defaults to the current buffer.  Called interactively, will be
applied to the region.  The function assumes @var{begin} <= @var{end}.

The return value is a cons.  The car is the list of character sets
that can individually represent all of the non-ASCII portion of the
buffer, and the cdr is the list of character sets that can
individually represent all of the ASCII portion.

The following is taken from a comment in the source.  Please refer to
the source to be sure of an accurate description.

The basic algorithm is to map over the region, compute the set of
charsets that can represent each character (the ``feasible charset''),
and take the intersection of those sets.

The current implementation takes advantage of the fact that ASCII
characters are common and cannot change asciisets.  Then using
skip-chars-forward makes motion over ASCII subregions very fast.

This same strategy could be applied generally by precomputing classes
of characters equivalent according to their effect on latinsets, and
adding a whole class to the skip-chars-forward string once a member is
found.

Probably efficiency is a function of the number of characters matched,
or maybe the length of the match string?  With @code{skip-category-forward}
over a precomputed category table it should be really fast.  In practice
for Latin character sets there are only 29 classes.
@end defun

@defun unity-remap-region begin end character-set &optional coding-system

Remap characters between @var{begin} and @var{end} to equivalents in
@var{character-set}.  Optional argument @var{coding-system} may be a
coding system name (a symbol) or nil.  Characters with no equivalent are
left as-is.

When called interactively, @var{begin} and @var{end} are set to the
beginning and end, respectively, of the active region, and the function
prompts for @var{character-set}.  The function does completion, knows
how to guess a character set name from a coding system name, and also
provides some common aliases.  See @code{unity-guess-charset}.
There is no way to specify @var{coding-system}, as it has no useful
function interactively.

Return @var{coding-system} if @var{coding-system} can encode all
characters in the region, t if @var{coding-system} is nil and the coding
system with G0 = 'ascii and G1 = @var{character-set} can encode all
characters, and otherwise nil.  Note that a non-null return does
@emph{not} mean it is safe to write the file, only the specified region.
(This behavior is useful for multipart MIME encoding and the like.)

Note:  by default this function is quite fascist about universal coding
systems.  It only admits @samp{utf-8}, @samp{iso-2022-7}, and
@samp{ctext}.  Customize @code{unity-approved-ucs-list} to change
this.

This function remaps characters that are artificially distinguished by Mule
internal code.  It may change the code point as well as the character set.
To recode characters that were decoded in the wrong coding system, use
@code{unity-recode-region}.
@end defun

@defun unity-recode-region begin end wrong-cs right-cs

Recode characters between @var{begin} and @var{end} from @var{wrong-cs}
to @var{right-cs}.

@var{wrong-cs} and @var{right-cs} are character sets.  Characters retain
the same code point but the character set is changed.  Only characters
from @var{wrong-cs} are changed to @var{right-cs}.  The identity of the
character may change.  Note that this could be dangerous, if characters
whose identities you do not want changed are included in the region.
This function cannot guess which characters you want changed, and which
should be left alone.

When called interactively, @var{begin} and @var{end} are set to the
beginning and end, respectively, of the active region, and the function
prompts for @var{wrong-cs} and @var{right-cs}.  The function does
completion, knows how to guess a character set name from a coding system
name, and also provides some common aliases.  See
@code{unity-guess-charset}.

Another way to accomplish this, but using coding systems rather than
character sets to specify the desired recoding, is
@samp{unity-recode-coding-region}.  That function may be faster
but is somewhat more dangerous, because it may recode more than one
character set.

To change from one Mule representation to another without changing identity
of any characters, use @samp{unity-remap-region}.
@end defun

@defun unity-recode-coding-region begin end wrong-cs right-cs

Recode text between @var{begin} and @var{end} from @var{wrong-cs} to
@var{right-cs}.

@var{wrong-cs} and @var{right-cs} are coding systems.  Characters retain
the same code point but the character set is changed.  The identity of
characters may change.  This is an inherently dangerous function;
multilingual text may be recoded in unexpected ways.  #### It's also
dangerous because the coding systems are not sanity-checked in the
current implementation.

When called interactively, @var{begin} and @var{end} are set to the
beginning and end, respectively, of the active region, and the function
prompts for @var{wrong-cs} and @var{right-cs}.  The function does
completion, knows how to guess a coding system name from a character set
name, and also provides some common aliases.  See
@code{unity-guess-coding-system}.

Another, safer, way to accomplish this, using character sets rather
than coding systems to specify the desired recoding, is to use
@code{unity-recode-region}.

To change from one Mule representation to another without changing identity
of any characters, use @code{unity-remap-region}.
@end defun

Helper functions for input of coding system and character set names.

@defun unity-guess-charset candidate
Guess a charset based on the symbol @var{candidate}.

@var{candidate} itself is not tried as the value.

Uses the natural mapping in @samp{unity-cset-codesys-alist}, and
the values in @samp{unity-charset-alias-alist}."
@end defun

@defun unity-guess-coding-system candidate
Guess a coding system based on the symbol @var{candidate}.

@var{candidate} itself is not tried as the value.

Uses the natural mapping in @samp{unity-cset-codesys-alist}, and
the values in @samp{unity-coding-system-alias-alist}."
@end defun

@defun unity-example

A cheesy example for unification.

At present it just makes a multilingual buffer.  To test, setq
buffer-file-coding-system to some value, make the buffer dirty (eg
with RET BackSpace), and save.
@end defun


@node Unification Configuration, Unification FAQs, Unification Usage, Unification
@subsection Configuring Unification for Use

If you want unification to be automatically initialized, invoke
@samp{enable-unification} with no arguments in your init file.
@xref{Init File, , , xemacs}.  If you are using GNU Emacs or an XEmacs
earlier than 21.1, you should also load @file{auto-autoloads} using the
full path (@emph{never} @samp{require} @file{auto-autoloads} libraries).

You may wish to define aliases for commonly used character sets and
coding systems for convenience in input.

@defopt unity-charset-alias-alist
Alist mapping aliases to Mule charset names (symbols)."

The default value is
@example
   ((latin-1 . latin-iso8859-1)
    (latin-2 . latin-iso8859-2)
    (latin-3 . latin-iso8859-3)
    (latin-4 . latin-iso8859-4)
    (latin-5 . latin-iso8859-9)
    (latin-9 . latin-iso8859-15)
    (latin-10 . latin-iso8859-16))
@end example

If a charset does not exist on your system, it will not complete and you
will not be able to enter it in response to prompts.  A real charset
with the same name as an alias in this list will shadow the alias.
@end defopt

@defopt unity-coding-system-alias-alist nil
Alist mapping aliases to Mule coding system names (symbols).

The default value is @samp{nil}.
@end defopt


@node Unification FAQs, Unification Theory, Unification Configuration, Unification
@subsection Frequently Asked Questions About Unification

@enumerate
@item
I'm smarter than XEmacs's unification feature!  How can that be?

Don't be surprised.  Trust yourself.

Unification is very young as yet.  Teach it what you know by
Customizing its variables, and report your changes to the maintainer
(@kbd{M-x report-xemacs-bug RET}).

@item
What is a UCS?

According to ISO 10646, a Universal Coded character Set.  In
XEmacs, it's Universal (Mule) Coding System.
@ref{Coding Systems, , , xemacs}

@item
I know @code{utf-16-le-bom} is a UCS, but unification won't use it.
Why not?

There are an awful lot of UCSes in Mule, and you probably do not want to
ever use, and definitely not be asked about, most of them.  So the
default set includes a few that the author thought plausible, but
they're surely not comprehensive or optimal.

Customize @code{unity-ucs-list} to include the ones you use often, and
report your favorites to the maintainer for consideration for
inclusion in the defaults using @kbd{M-x report-xemacs-bug RET}.
(Note that you @emph{must} include @code{escape-quoted} in this list,
because Mule uses it internally as the coding system for auto-save
files.)

Alternatively, if you just want to use it this one time, simply type
it in at the prompt.  Unification will confirm that is a real coding
system, and then assume that you know what you're doing.

@item
This is crazy: I can't quit XEmacs and get queried on autosaves!  Why?

You probably removed @code{escape-quoted} from
@code{unity-ucs-list}.  Put it back.

@item
Unification is really buggy and I can't get any work done.

First, use @kbd{M-x disable-unification RET}, then report your
problems as a bug (@kbd{M-x report-xemacs-bug RET}).
@end enumerate


@node Unification Theory, What Unification Cannot Do for You, Unification FAQs, Unification
@subsection Unification Theory

Standard encodings suffer from the design defect that they do not
provide a reliable way to recognize which coded character sets in use.
@xref{What Unification Cannot Do for You}.  There are scores of
character sets which can be represented by a single octet (8-bit
byte), whose union contains many hundreds of characters.  Obviously
this results in great confusion, since you can't tell the players
without a scorecard, and there is no scorecard.

There are two ways to solve this problem.  The first is to create a
universal coded character set.  This is the concept behind Unicode.
However, there have been satisfactory (nearly) universal character
sets for several decades, but even today many Westerners resist using
Unicode because they consider its space requirements excessive.  On
the other hand, many Asians dislike Unicode because they consider it
to be incomplete.  (This is partly, but not entirely, political.)

In any case, Unicode only solves the internal representation problem.
Many data sets will contain files in ``legacy'' encodings, and Unicode
does not help distinguish among them.

The second approach is to embed information about the encodings used in
a document in its text.  This approach is taken by the ISO 2022
standard.  This would solve the problem completely from the users' of
view, except that ISO 2022 is basically not implemented at all, in the
sense that few applications or systems implement more than a small
subset of ISO 2022 functionality.  This is due to the fact that
mono-literate users object to the presence of escape sequences in their
texts (which they, with some justification, consider data corruption).
Programmers are more than willing to cater to these users, since
implementing ISO 2022 is a painstaking task.

In fact, Emacs/Mule adopts both of these approaches.  Internally it uses
a universal character set, @dfn{Mule code}.  Externally it uses ISO 2022
techniques both to save files in forms robust to encoding issues, and as
hints when attempting to ``guess'' an unknown encoding.  However, Mule
suffers from a design defect, namely it embeds the character set
information that ISO 2022 attaches to runs of characters by introducing
them with a control sequence in each character.  That causes Mule to
consider the ISO Latin character sets to be disjoint.  This manifests
itself when a user enters characters using input methods associated with
different coded character sets into a single buffer.

There are two problems stemming from this design.  First, Mule
represents the same character in different ways.  Abstractly, ',As(B'
(LATIN SMALL LETTER O WITH ACUTE) can get represented as
[latin-iso8859-1 #x73] or as [latin-iso8859-2 #x73].  So what looks like
',Ass(B' in the display might actually be represented [latin-iso8859-1
#x73][latin-iso8859-2 #x73] in the buffer, and saved as [#xF3 ESC - B
#xF3 ESC - A] in the file.  In some cases this treatment would be
appropriate (consider HYPHEN, MINUS SIGN, EN DASH, EM DASH, and U+4E00
(the CJK ideographic character meaning ``one'')), and although arguably
incorrect it is convenient when mixing the CJK scripts.  But in the case
of the Latin scripts this is wrong.

Worse yet, it is very likely to occur when mixing ``different'' encodings
(such as ISO 8859/1 and ISO 8859/15) that differ only in a few code
points that are almost never used.  A very important example involves
email.  Many sites, especially in the U.S., default to use of the ISO
8859/1 coded character set (also called ``Latin 1,'' though these are
somewhat different concepts).  However, ISO 8859/1 provides a generic
CURRENCY SIGN character.  Now that the Euro has become the official
currency of most countries in Europe, this is unsatisfactory (and in
practice, useless).  So Europeans generally use ISO 8859/15, which is
nearly identical to ISO 8859/1 for most languages, except that it
substitutes EURO SIGN for CURRENCY SIGN.

Suppose a European user yanks text from a post encoded in ISO 8859/1
into a message composition buffer, and enters some text including the
Euro sign.  Then Mule will consider the buffer to contain both ISO
8859/1 and ISO 8859/15 text, and MUAs such as Gnus will (if naively
programmed) send the message as a multipart mixed MIME body!

This is clearly stupid.  What is not as obvious is that, just as any
European can include American English in their text because ASCII is a
subset of ISO 8859/15, most European languages which use Latin
characters (eg, German and Polish) can typically be mixed while using
only one Latin coded character set (in the case of German and Polish,
ISO 8859/2).  However, this often depends on exactly what text is to be
encoded (even for the same pair of languages).

Unification works around the problem by converting as many characters as
possible to use a single Latin coded character set before saving the
buffer.

Because the problem is rarely noticable in editing a buffer, but tends
to manifest when that buffer is exported to a file or process,
unification uses the strategy of examining the buffer prior to export.
If use of multiple Latin coded character sets is detected, unification
attempts to unify them by finding a single coded character set which
contains all of the Latin characters in the buffer.

The primary purpose of unification is to fix the problem by giving the
user the choice to change the representation of all characters to one
character set and give sensible recommendations based on context.  In
the ',As(B' example, either ISO 8859/1 or ISO 8859/2 is satisfactory, and
both will be suggested.  In the EURO SIGN example, only ISO 8859/15
makes sense, and that is what will be recommended.  In both cases, the
user will be reminded that there are universal encodings available.

I call this @dfn{remapping} (from the universal character set to a
particular ISO 8859 coded character set).  It is mere accident that this
letter has the same code point in both character sets.  (Not entirely,
but there are many examples of Latin characters that have different code
points in different Latin-X sets.)

Note that, in the ',As(B' example, that treating the buffer in this way will
result in a representation such as [latin-iso8859-2
#x73][latin-iso8859-2 #x73], and the file will be saved as [#xF3 #xF3].
This is guaranteed to occasionally result in the second problem you
observed, to which we now turn.

This problem is that, although the file is intended to be an
ISO-8859/2-encoded file, in an ISO 8859/1 locale Mule (and every POSIX
compliant program---this is required by the standard, obvious if you
think a bit, @pxref{What Unification Cannot Do for You}) will read that
file as [latin-iso8859-1 #x73] [latin-iso8859-1 #x73].  Of course this
is no problem if all of the characters in the file are contained in ISO
8859/1, but suppose there are some which are not, but are contained in
the (intended) ISO 8859/2.

You now want to fix this, but not by finding the same character in
another set.  Instead, you want to simply change the character set
that Mule associates with that buffer position without changing the
code.  (This is conceptually somewhat distinct from the first problem,
and logically ought to be handled in the code that defines coding
systems.  However, unification is not an unreasonable place for it.)
Unification provides two functions (one fast and dangerous, the other
@c #### fix latin-unity.texi
slower and careful) to handle this.  I call this @dfn{recoding}, because
the transformation actually involves @emph{encoding} the buffer to
file representation, then @emph{decoding} it to buffer representation
(in a different character set).  This cannot be done automatically
because Mule can have no idea what the correct encoding is---after
all, it already gave you its best guess.  @xref{What Unification
Cannot Do for You}.  So these functions must be invoked by the user.
@xref{Interactive Usage}.


@node What Unification Cannot Do for You, , Unification Theory, Unification
@subsection What Unification Cannot Do for You

Unification @strong{cannot} save you if you insist on exporting data in
8-bit encodings in a multilingual environment.  @emph{You will
eventually corrupt data if you do this.}  It is not Mule's, or any
application's, fault.  You will have only yourself to blame; consider
yourself warned.  (It is true that Mule has bugs, which make Mule
somewhat more dangerous and inconvenient than some naive applications.
We're working to address those, but no application can remedy the
inherent defect of 8-bit encodings.)

Use standard universal encodings, preferably Unicode (UTF-8) unless
applicable standards indicate otherwise.  The most important such case
is Internet messages, where MIME should be used, whether or not the
subordinate encoding is a universal encoding.  (Note that since one of
the important provisions of MIME is the @samp{Content-Type} header,
which has the charset parameter, MIME is to be considered a universal
encoding for the purposes of this manual.  Of course, technically
speaking it's neither a coded character set nor a coding extension
technique compliant with ISO 2022.)

As mentioned earlier, the problem is that standard encodings suffer from
the design defect that they do not provide a reliable way to recognize
which coded character sets are in use.  There are scores of character
sets which can be represented by a single octet (8-bit byte), whose
union contains many hundreds of characters.  Thus any 8-bit coded
character set must contain characters that share code points used for
different characters in other coded character sets.

This means that a given file's intended encoding cannot be identified
with 100% reliability unless it contains encoding markers such as those
provided by MIME or ISO 2022.

Unification actually makes it more likely that you will have problems of
this kind.  Traditionally Mule has been ``helpful'' by simply using an
ISO 2022 universal coding system when the current buffer coding system
cannot handle all the characters in the buffer.  This has the effect
that, because the file contains control sequences, it is not recognized
as being in the locale's normal 8-bit encoding.  It may be annoying if
@c #### fix in latin-unity.texi
you are not a Mule expert, but your data is guaranteed to be recoverable
with a tool you already have: Mule.

However, with unification, Mule converts to a single 8-bit character set
when possible.  But typically this will @emph{not} be in your usual
locale.  Ie, the times that an ISO 8859/1 user will need unification is
when there are ISO 8859/2 characters in the buffer.  But then most
likely the file will be saved in a pure 8-bit encoding that is not ISO
8859/1, ie, ISO 8859/2.  Mule's autorecognizer (which is probably the
most sophisticated yet available) cannot tell the difference between ISO
8859/1 and ISO 8859/2, and in a Western European locale will choose the
former even though the latter was intended.  Even the extension
@c #### fix in latin-unity.texi
(``statistical recognition'') planned for XEmacs 22 is unlikely to be
acceptably accurate in the case of mixed codes.

So now consider adding some additional ISO 8859/1 text to the buffer.
If it includes any ISO 8859/1 codes that are used by different
characters in ISO 8859/2, you now have a file that cannot be
mechanically disentangled.  You need a human being who can recognize
that @emph{this is German and Swedish} and stays in Latin-1, while
@emph{that is Polish} and needs to be recoded to Latin-2.

Moral: switch to a universal coded character set, preferably Unicode
using the UTF-8 transformation format.  If you really need the space,
compress your files.


@node Specify Coding, Charsets and Coding Systems, Unification, Mule
@section Specifying a Coding System

  In cases where XEmacs does not automatically choose the right coding
system, you can use these commands to specify one:

@table @kbd
@item C-x @key{RET} f @var{coding} @key{RET}
Use coding system @var{coding} for the visited file
in the current buffer.

@item C-x @key{RET} c @var{coding} @key{RET}
Specify coding system @var{coding} for the immediately following
command.

@item C-x @key{RET} k @var{coding} @key{RET}
Use coding system @var{coding} for keyboard input.  (This feature is
non-functional and is temporarily disabled.)

@item C-x @key{RET} t @var{coding} @key{RET}
Use coding system @var{coding} for terminal output.

@item C-x @key{RET} p @var{coding} @key{RET}
Use coding system @var{coding} for subprocess input and output
in the current buffer.
@end table

@kindex C-x RET f
@findex set-buffer-file-coding-system
  The command @kbd{C-x RET f} (@code{set-buffer-file-coding-system})
specifies the file coding system for the current buffer---in other
words, which coding system to use when saving or rereading the visited
file.  You specify which coding system using the minibuffer.  Since this
command applies to a file you have already visited, it affects only the
way the file is saved.

@kindex C-x RET c
@findex universal-coding-system-argument
  Another way to specify the coding system for a file is when you visit
the file.  First use the command @kbd{C-x @key{RET} c}
(@code{universal-coding-system-argument}); this command uses the
minibuffer to read a coding system name.  After you exit the minibuffer,
the specified coding system is used for @emph{the immediately following
command}.

  So if the immediately following command is @kbd{C-x C-f}, for example,
it reads the file using that coding system (and records the coding
system for when the file is saved).  Or if the immediately following
command is @kbd{C-x C-w}, it writes the file using that coding system.
Other file commands affected by a specified coding system include
@kbd{C-x C-i} and @kbd{C-x C-v}, as well as the other-window variants of
@kbd{C-x C-f}.

  In addition, if you run some file input commands with the precedent
@kbd{C-u}, you can specify coding system to read from minibuffer.  So if
the immediately following command is @kbd{C-x C-f}, for example, it
reads the file using that coding system (and records the coding system
for when the file is saved).  Other file commands affected by a
specified coding system include @kbd{C-x C-i} and @kbd{C-x C-v}, as well
as the other-window variants of @kbd{C-x C-f}.

@vindex default-buffer-file-coding-system
  The variable @code{default-buffer-file-coding-system} specifies the
choice of coding system to use when you create a new file.  It applies
when you find a new file, and when you create a buffer and then save it
in a file.  Selecting a language environment typically sets this
variable to a good choice of default coding system for that language
environment.

@kindex C-x RET t
@findex set-terminal-coding-system
  The command @kbd{C-x @key{RET} t} (@code{set-terminal-coding-system})
specifies the coding system for terminal output.  If you specify a
character code for terminal output, all characters output to the
terminal are translated into that coding system.

  This feature is useful for certain character-only terminals built to
support specific languages or character sets---for example, European
terminals that support one of the ISO Latin character sets.

  By default, output to the terminal is not translated at all.

@kindex C-x RET k
@findex set-keyboard-coding-system
  The command @kbd{C-x @key{RET} k} (@code{set-keyboard-coding-system})
specifies the coding system for keyboard input.  Character-code
translation of keyboard input is useful for terminals with keys that
send non-ASCII graphic characters---for example, some terminals designed
for ISO Latin-1 or subsets of it.

(This feature is non-functional and is temporarily disabled.)

  By default, keyboard input is not translated at all.

  There is a similarity between using a coding system translation for
keyboard input, and using an input method: both define sequences of
keyboard input that translate into single characters.  However, input
methods are designed to be convenient for interactive use by humans, and
the sequences that are translated are typically sequences of ASCII
printing characters.  Coding systems typically translate sequences of
non-graphic characters.

@kindex C-x RET p
@findex set-buffer-process-coding-system
  The command @kbd{C-x @key{RET} p} (@code{set-buffer-process-coding-system})
specifies the coding system for input and output to a subprocess.  This
command applies to the current buffer; normally, each subprocess has its
own buffer, and thus you can use this command to specify translation to
and from a particular subprocess by giving the command in the
corresponding buffer.

  By default, process input and output are not translated at all.

@vindex file-name-coding-system
  The variable @code{file-name-coding-system} specifies a coding system
to use for encoding file names.  If you set the variable to a coding
system name (as a Lisp symbol or a string), XEmacs encodes file names
using that coding system for all file operations.  This makes it
possible to use non-Latin-1 characters in file names---or, at least,
those non-Latin-1 characters which the specified coding system can
encode.  By default, this variable is @code{nil}, which implies that you
cannot use non-Latin-1 characters in file names.


@node Charsets and Coding Systems, , Specify Coding, Mule
@section Charsets and Coding Systems

This section provides reference lists of Mule charsets and coding
systems.  Mule charsets are typically named by character set and
standard.

@table @strong
@item ASCII variants

Identification of equivalent characters in these sets is not properly
implemented.  Unification does not distinguish the two charsets.

@samp{ascii} @samp{latin-jisx0201}

@item Extended Latin

Characters from the following ISO 2022 conformant charsets are
identified with equivalents in other charsets in the group by
unification.

@samp{latin-iso8859-1} @samp{latin-iso8859-15} @samp{latin-iso8859-2}
@samp{latin-iso8859-3} @samp{latin-iso8859-4} @samp{latin-iso8859-9}
@samp{latin-iso8859-13} @samp{latin-iso8859-16}

The follow charsets are Latin variants which are not understood by
unification.  In addition, many of the Asian language standards provide
ASCII, at least, and sometimes other Latin characters.  None of these
are identified with their ISO 8859 equivalents.

@samp{vietnamese-viscii-lower}
@samp{vietnamese-viscii-upper}

@item Other character sets

@samp{arabic-1-column}
@samp{arabic-2-column}
@samp{arabic-digit}
@samp{arabic-iso8859-6}
@samp{chinese-big5-1}
@samp{chinese-big5-2}
@samp{chinese-cns11643-1}
@samp{chinese-cns11643-2}
@samp{chinese-cns11643-3}
@samp{chinese-cns11643-4}
@samp{chinese-cns11643-5}
@samp{chinese-cns11643-6}
@samp{chinese-cns11643-7}
@samp{chinese-gb2312}
@samp{chinese-isoir165}
@samp{cyrillic-iso8859-5}
@samp{ethiopic}
@samp{greek-iso8859-7}
@samp{hebrew-iso8859-8}
@samp{ipa}
@samp{japanese-jisx0208}
@samp{japanese-jisx0208-1978}
@samp{japanese-jisx0212}
@samp{katakana-jisx0201}
@samp{korean-ksc5601}
@samp{sisheng}
@samp{thai-tis620}
@samp{thai-xtis}

@item Non-graphic charsets

@samp{control-1}
@end table

@table @strong
@item No conversion

Some of these coding systems may specify EOL conventions.  Note that
@samp{iso-8859-1} is a no-conversion coding system, not an ISO 2022
coding system.  Although unification attempts to compensate for this, it
is possible that the @samp{iso-8859-1} coding system will behave
differently from other ISO 8859 coding systems.

@samp{binary} @samp{no-conversion} @samp{raw-text} @samp{iso-8859-1}

@item Latin coding systems

These coding systems are all single-byte, 8-bit ISO 2022 coding systems,
combining ASCII in the GL register (bytes with high-bit clear) and an
extended Latin character set in the GR register (bytes with high-bit set).

@samp{iso-8859-15} @samp{iso-8859-2} @samp{iso-8859-3} @samp{iso-8859-4}
@samp{iso-8859-9} @samp{iso-8859-13} @samp{iso-8859-14} @samp{iso-8859-16}

These coding systems are single-byte, 8-bit coding systems that do not
conform to international standards.  They should be avoided in all
potentially multilingual contexts, including any text distributed over
the Internet and World Wide Web.

@samp{windows-1251}

@item Multilingual coding systems

The following ISO-2022-based coding systems are useful for multilingual
text.

@samp{ctext} @samp{iso-2022-lock} @samp{iso-2022-7} @samp{iso-2022-7bit}
@samp{iso-2022-7bit-ss2} @samp{iso-2022-8} @samp{iso-2022-8bit-ss2}

XEmacs also supports Unicode with the Mule-UCS package.  These are the
preferred coding systems for multilingual use.  (There is a possible
exception for texts that mix several Asian ideographic character sets.)

@samp{utf-16-be} @samp{utf-16-be-no-signature} @samp{utf-16-le}
@samp{utf-16-le-no-signature} @samp{utf-7} @samp{utf-7-safe}
@samp{utf-8} @samp{utf-8-ws}

Development versions of XEmacs (the 21.5 series) support Unicode
internally, with (at least) the following coding systems implemented:

@samp{utf-16-be} @samp{utf-16-be-bom} @samp{utf-16-le}
@samp{utf-16-le-bom} @samp{utf-8} @samp{utf-8-bom}

@item Asian ideographic languages

The following coding systems are based on ISO 2022, and are more or less
suitable for encoding multilingual texts.  They all can represent ASCII
at least, and sometimes several other foreign character sets, without
resort to arbitrary ISO 2022 designations.  However, these subsets are
not identified with the corresponding national standards in XEmacs Mule.

@samp{chinese-euc} @samp{cn-big5} @samp{cn-gb-2312} @samp{gb2312}
@samp{hz} @samp{hz-gb-2312} @samp{old-jis} @samp{japanese-euc}
@samp{junet} @samp{euc-japan} @samp{euc-jp} @samp{iso-2022-jp}
@samp{iso-2022-jp-1978-irv} @samp{iso-2022-jp-2} @samp{euc-kr}
@samp{korean-euc} @samp{iso-2022-kr} @samp{iso-2022-int-1}

The following coding systems cannot be used for general multilingual
text and do not cooperate well with other coding systems.

@samp{big5} @samp{shift_jis}

@item Other languages

The following coding systems are based on ISO 2022.  Though none of them
provides any Latin characters beyond ASCII, XEmacs Mule allows (and up
to 21.4 defaults to) use of ISO 2022 control sequences to designate
other character sets for inclusion the text.

@samp{iso-8859-5} @samp{iso-8859-7} @samp{iso-8859-8}
@samp{ctext-hebrew}

The following are character sets that do not conform to ISO 2022 and
thus cannot be safely used in a multilingual context.

@samp{alternativnyj} @samp{koi8-r} @samp{tis-620} @samp{viqr}
@samp{viscii} @samp{vscii}

@item Special coding systems

Mule uses the following coding systems for special purposes.

@samp{automatic-conversion} @samp{undecided} @samp{escape-quoted}

@samp{escape-quoted} is especially important, as it is used internally
as the coding system for autosaved data.

The following coding systems are aliases for others, and are used for
communication with the host operating system.

@samp{file-name} @samp{keyboard} @samp{terminal}

@end table

Mule detection of coding systems is actually limited to detection of
classes of coding systems called @dfn{coding categories}.  These coding
categories are identified by the ISO 2022 control sequences they use, if
any, by their conformance to ISO 2022 restrictions on code points that
may be used, and by characteristic patterns of use of 8-bit code points.

@samp{no-conversion}
@samp{utf-8}
@samp{ucs-4}
@samp{iso-7}
@samp{iso-lock-shift}
@samp{iso-8-1}
@samp{iso-8-2}
@samp{iso-8-designate}
@samp{shift-jis}
@samp{big5}


