World Scripts Support
*********************

If you build XEmacs using the --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'').

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 C-q command (see below). (Historically the internal encoding was a specially designed encoding, called 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.)

The command C-h h (view-hello-file) displays the 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 input methods, typically one for each script or language, to make it convenient to type them.

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

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 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 (see 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.

The command to select a language environment is 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:

     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.

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 8859-N, XEmacs automatically selects the corresponding language environment.

To display information about the effects of a certain language environment LANG-ENV, use the command C-h L LANG-ENV RET (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.

Input Methods
=============

An 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 '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 chinese-py, among others), or a sequence of portions of the character (input methods chinese-4corner and 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 C-f, C-b, C-n, C-p, and digits have special definitions in this situation, used for selecting among the alternatives.  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 C-n and 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 latin-1-postfix, the sequence e ' combines to form an 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, e ' ' gives you the two characters e'.  Another way is to type another letter after the e---something that won't combine with that---and immediately delete it.  For example, you could type e e DEL ' to get separate e and '.

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

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.

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

Selecting an Input Method
=========================

C-\
     Enable or disable use of the selected input method.

C-x RET C-\ METHOD RET
     Select a new input method for the current buffer.

C-h I METHOD RET
C-h C-\ METHOD RET
     Describe the input method METHOD (describe-input-method). By default, it describes the current input method (if any).

M-x list-input-methods
     Display a list of all the supported input methods.

To choose an input method for the current buffer, use C-x RET C-\ (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 current-input-method records which input method is selected.

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 C-\ (toggle-input-method).  To reenable the input method, type C-\ again.

If you type C-\ and you have not yet selected an input method, it prompts for you to specify one.  This has the same effect as using C-x RET C-\ to specify an 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 C-\.  The variable default-input-method specifies the default input method (nil means there is none).

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 M-x quail-set-keyboard-layout.

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

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 iso.  There are also special coding systems binary and 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.

C-x RET C CODING RET
     Describe coding system CODING.

C-x RET C RET
     Describe the coding systems currently in use.

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

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

The command C-x RET C (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 (see Recognize Coding).

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

Each of the coding systems that appear in this list---except for 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:

...-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.)

...-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.)

...-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.)

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

In contrast, the coding system 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 M-x find-file-literally command.  This uses binary, and also suppresses other XEmacs features that might convert the file contents before you see them.  See Visiting.

The coding system 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.

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 (see 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.

However, you can alter the priority list in detail with the command 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.

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

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

The first argument should be 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.

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

Once XEmacs has chosen a coding system for a buffer, it stores that coding system in 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 save-buffer and 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 set-buffer-file-coding-system (see Specify Coding).

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 latin-unity package by Stephen Turnbull 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.

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.

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 unity-sanity-check on write-region-pre-hook. Unification is on by default for the ISO-8859 Latin sets.  The user activates this functionality for other character set families by invoking enable-unification, either interactively or in her init file.  See Init File (xemacs).  Unification can be deactivated by invoking disable-unification.

Unification also provides a few functions for remapping or recoding the buffer by hand.  To 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 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.  See Unification Theory.

There are a few variables which determine which coding systems are always acceptable to unification: unity-ucs-list, unity-preferred-coding-system-list, and unity-preapproved-coding-system-list.  The last defaults to (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.

Basic Functionality
...................

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

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

 -- Function: 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.

 -- Function: disable-unification
     There are no arguments.

     Clean up hooks and void variables used by unification.

 -- User Option: unity-ucs-list
     List of universal coding systems recommended for character set unification.

     The default value is '(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 unity-preapproved-coding-system-list), and follow the 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:  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 iso-2022-7.)  If your preferred coding system is one of these, you may consider adding it to unity-ucs-list.

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

 -- User Option: unity-preapproved-coding-system-list
     List of coding systems used without querying the user if feasible.

     The default value is (buffer-default preferred).

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

     buffer-default
          Use the coding system used by write-region, if feasible.

     preferred
          Use the coding system specified by prefer-coding-system if feasible.

     "Feasible" means that all characters in the buffer can be represented by the coding system.  Coding systems in unity-ucs-list are always considered feasible.  Other feasible coding systems are computed by 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 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 preferred in unity-preferred-coding-system-list, but not in unity-preapproved-coding-system-list.

 -- User Option: unity-preferred-coding-system-list
     List of coding systems suggested to the user if feasible.

     The default value is (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 unity-preapproved-coding-system-list are feasible, this list will be recommended to the user, followed by the 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 preferred and buffer-default may be present:

     buffer-default
          Use the coding system used by write-region, if feasible.

     preferred
          Use the coding system specified by prefer-coding-system if feasible.

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

 -- Variable: 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, unity-coding-system-alias-alist or unity-charset-alias-alist.

Interactive Usage
.................

First, the hook function 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).

 -- Function: unity-sanity-check begin end filename append visit lockname &optional coding-system

     Check if CODING-SYSTEM can represent all characters between BEGIN and END.

     For compatibility with old broken versions of write-region, CODING-SYSTEM defaults to buffer-file-coding-system. FILENAME, APPEND, VISIT, and LOCKNAME are ignored.

     Return nil if buffer-file-coding-system is not (ISO-2022-compatible) Latin.  If 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 not do the safe thing when 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 write-region-pre-hook.  It does nothing except return CODING-SYSTEM if write-region handlers are inhibited.

 -- Function: unity-buffer-representations-feasible
     There are no arguments.

     Apply unity-region-representations-feasible to the current buffer.

 -- Function: unity-region-representations-feasible begin end &optional buf
     Return character sets that can represent the text from BEGIN to END in BUF.

     BUF defaults to the current buffer.  Called interactively, will be applied to the region.  The function assumes BEGIN <= 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 skip-category-forward over a precomputed category table it should be really fast.  In practice for Latin character sets there are only 29 classes.

 -- Function: unity-remap-region begin end character-set &optional coding-system

     Remap characters between BEGIN and END to equivalents in CHARACTER-SET.  Optional argument CODING-SYSTEM may be a coding system name (a symbol) or nil.  Characters with no equivalent are left as-is.

     When called interactively, BEGIN and END are set to the beginning and end, respectively, of the active region, and the function prompts for 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 unity-guess-charset. There is no way to specify CODING-SYSTEM, as it has no useful function interactively.

     Return CODING-SYSTEM if CODING-SYSTEM can encode all characters in the region, t if CODING-SYSTEM is nil and the coding system with G0 = 'ascii and G1 = CHARACTER-SET can encode all characters, and otherwise nil.  Note that a non-null return does 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 utf-8, iso-2022-7, and ctext.  Customize 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 unity-recode-region.

 -- Function: unity-recode-region begin end wrong-cs right-cs

     Recode characters between BEGIN and END from WRONG-CS to RIGHT-CS.

     WRONG-CS and RIGHT-CS are character sets.  Characters retain the same code point but the character set is changed.  Only characters from WRONG-CS are changed to 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, BEGIN and END are set to the beginning and end, respectively, of the active region, and the function prompts for WRONG-CS and 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 unity-guess-charset.

     Another way to accomplish this, but using coding systems rather than character sets to specify the desired recoding, is 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 unity-remap-region.

 -- Function: unity-recode-coding-region begin end wrong-cs right-cs

     Recode text between BEGIN and END from WRONG-CS to RIGHT-CS.

     WRONG-CS and 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, BEGIN and END are set to the beginning and end, respectively, of the active region, and the function prompts for WRONG-CS and 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 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 unity-recode-region.

     To change from one Mule representation to another without changing identity of any characters, use unity-remap-region.

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

 -- Function: unity-guess-charset candidate
     Guess a charset based on the symbol CANDIDATE.

     CANDIDATE itself is not tried as the value.

     Uses the natural mapping in unity-cset-codesys-alist, and the values in unity-charset-alias-alist."

 -- Function: unity-guess-coding-system candidate
     Guess a coding system based on the symbol CANDIDATE.

     CANDIDATE itself is not tried as the value.

     Uses the natural mapping in unity-cset-codesys-alist, and the values in unity-coding-system-alias-alist."

 -- Function: 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.

Configuring Unification for Use
-------------------------------

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

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

 -- User Option: unity-charset-alias-alist
     Alist mapping aliases to Mule charset names (symbols)."

     The default value is

             ((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))

     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.

 -- User Option: unity-coding-system-alias-alist nil
     Alist mapping aliases to Mule coding system names (symbols).

     The default value is nil.

Frequently Asked Questions About Unification
--------------------------------------------

 1. 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 (M-x report-xemacs-bug RET).

 2. What is a UCS?

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

 3. I know 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 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 M-x report-xemacs-bug RET. (Note that you must include 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.

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

     You probably removed escape-quoted from unity-ucs-list.  Put it back.

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

     First, use M-x disable-unification RET, then report your problems as a bug (M-x report-xemacs-bug RET).

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. See 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, 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 noticeable 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 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, see 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 slower and careful) to handle this.  I call this recoding, because the transformation actually involves encoding the buffer to file representation, then 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.  See What Unification Cannot Do for You.  So these functions must be invoked by the user. See Interactive Usage.

What Unification Cannot Do for You
----------------------------------

Unification cannot save you if you insist on exporting data in 8-bit encodings in a multilingual environment.  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 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 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 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 (``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 this is German and Swedish and stays in Latin-1, while 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.

Specifying a Coding System
==========================

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

C-x RET f CODING RET
     Use coding system CODING for the visited file in the current buffer.

C-x RET c CODING RET
     Specify coding system CODING for the immediately following command.

C-x RET k CODING RET
     Use coding system CODING for keyboard input.  (This feature is non-functional and is temporarily disabled.)

C-x RET t CODING RET
     Use coding system CODING for terminal output.

C-x RET p CODING RET
     Use coding system CODING for subprocess input and output in the current buffer.

The command C-x RET f (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.

Another way to specify the coding system for a file is when you visit the file.  First use the command C-x RET c (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 the immediately following command.

So if the immediately following command is 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 C-x C-w, it writes the file using that coding system. Other file commands affected by a specified coding system include C-x C-i and C-x C-v, as well as the other-window variants of C-x C-f.

In addition, if you run some file input commands with the precedent C-u, you can specify coding system to read from minibuffer.  So if the immediately following command is 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 C-x C-i and C-x C-v, as well as the other-window variants of C-x C-f.

The variable 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.

The command C-x RET t (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.

The command C-x RET k (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.

The command C-x RET p (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.

The variable 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 nil, which implies that you cannot use non-Latin-1 characters in file names.

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.

ASCII variants

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

     ascii latin-jisx0201

Extended Latin

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

     latin-iso8859-1 latin-iso8859-15 latin-iso8859-2 latin-iso8859-3 latin-iso8859-4 latin-iso8859-9 latin-iso8859-13 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.

     vietnamese-viscii-lower vietnamese-viscii-upper

Other character sets

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

Non-graphic charsets

     control-1

No conversion

     Some of these coding systems may specify EOL conventions.  Note that 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 iso-8859-1 coding system will behave differently from other ISO 8859 coding systems.

     binary no-conversion raw-text iso-8859-1

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).

     iso-8859-15 iso-8859-2 iso-8859-3 iso-8859-4 iso-8859-9 iso-8859-13 iso-8859-14 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.

     windows-1251

Multilingual coding systems

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

     ctext iso-2022-lock iso-2022-7 iso-2022-7bit iso-2022-7bit-ss2 iso-2022-8 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.)

     utf-16-be utf-16-be-no-signature utf-16-le utf-16-le-no-signature utf-7 utf-7-safe utf-8 utf-8-ws

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

     utf-16-be utf-16-be-bom utf-16-le utf-16-le-bom utf-8 utf-8-bom

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.

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

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

     big5 shift_jis

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.

     iso-8859-5 iso-8859-7 iso-8859-8 ctext-hebrew

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

     alternativnyj koi8-r tis-620 viqr viscii vscii

Special coding systems

     Mule uses the following coding systems for special purposes.

     automatic-conversion undecided escape-quoted

     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.

     file-name keyboard terminal

Mule detection of coding systems is actually limited to detection of classes of coding systems called 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.

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