This file is part of XEmacs.  It documents the latin-unity package, which ensures that wherever possible representations of all Latin characters are drawn from the same 8-bit character set.

Copyright © 2002, 2003 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim or modified copies of this manual provided that the copyright notice above is preserved, and at least one of the numbered paragraphs below up to the phrase "End of permissions notice." is included, under the terms of any of the licenses enumerated below.

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2. The license used for XEmacs documentation (see the file man/xemacs/xemacs.texi distributed with XEmacs).

3. The GNU Free Documentation License (available from the Free Software Foundation at the address above).

End of permissions notice.

Because the XEmacs documentation license and the GNU Free Documentation License are not free software licenses, and are mutually incompatible with each other and with the GNU General Public License, it is desirable that the permissions given here be preserved if possible.  This normally requires that you write fresh text for any additions or modifications to this file, rather than copying code or comments from other sources.  The exceptions are if you are sole copyright holder for the other source, or if you receive permission from the holders of the copyrights for the other sources.

That is, if you merge code or comments from a file licensed to you only under the GNU General Public License you may not distribute under the documentation licenses or with a dual license; such a combination may be distributed under the GNU General Public License only.  Similarly, if you copy text from a file licensed to you only under the XEmacs documentation license or the GNU Free Documentation License you may distribute the result only under the appropriate license.

Latin Unity for Emacsen
***********************

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

This is version 1.1 of the latin-unity manual, last updated on Monday, February 7, 2005.

latin-unity Copying Conditions
******************************

Copyright (C) 2002 Free Software Foundation, Inc.

latin-unity is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version.

latin-unity is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with XEmacs; see the file COPYING. If not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

An Overview of latin-unity
**************************

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.

latin-unity 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 latin-unity
************************

Normally, latin-unity works in the background by installing latin-unity-sanity-check on write-region-pre-hook.  The user activates this functionality by invoking latin-unity-install, either interactively or in her init file. See Init File (xemacs).  latin-unity can be deactivated by invoking latin-unity-uninstall.

latin-unity 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 Theory of Operation.

There are a few variables which determine which coding systems are always acceptable to latin-unity:  latin-unity-ucs-list, latin-unity-preferred-coding-system-list, and latin-unity-preapproved-coding-system-list.  The latter two default to (), and should probably be avoided because they short-circuit the sanity check.  If you find you need to use them, consider reporting it as a bug or request for enhancement.  Because they seem unsafe, the recommended interface is likely to change.

Basic Functionality
===================

These functions and user options initialize and configure latin-unity. In normal use, only a call to latin-unity-install is needed.

 -- Function: latin-unity-install
     Set up hooks and initialize variables for latin-unity.

     There are no arguments.

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

     Note: a quirk in XEmacs means that the cl-macs library cannot be required, it must be loaded explicitly.  This means that if you invoke latin-unity-install in your init file, XEmacs will print

          Loading cl-macs...
          Loading cl-macs...done

     on your console, as these messages have not yet been redirected to the *Message-Log* buffer.

 -- Function: latin-unity-uninstall
     There are no arguments.

     Clean up hooks and void variables used by latin-unity.

 -- User Option: latin-unity-ucs-list
     List of coding systems considered to be universal.

     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 latin-unity-preapproved-coding-system-list), and follow the latin-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 not delete this, although it is rare that a user would want to use it directly.  latin-unity 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 latin-unity-ucs-list. However, this will typically have the side effect that (eg) ISO 8859/1 files will be saved in 7-bit form with ISO 2022 escape sequences.

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

 -- User Option: latin-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 latin-unity-ucs-list are always considered feasible.  Other feasible coding systems are computed by latin-unity-representations-feasible-region.

     Note that 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, latin-unity 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 should put preferred in latin-unity-preferred-coding-system-list.

 -- User Option: latin-unity-preferred-coding-system-list
     List of coding systems suggested 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 latin-unity-preferred-coding-system-list are feasible, this list will be recommended to the user, followed by the latin-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 latin-unity-ucs-list are always considered feasible.  Other feasible coding systems are computed by latin-unity-representations-feasible-region.

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

Interactive Usage
=================

First, the hook function latin-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: latin-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: latin-unity-buffer-representations-feasible

     There are no arguments.

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

 -- Function: latin-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.  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: latin-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 latin-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 latin-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 latin-unity-recode-region.

 -- Function: latin-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 latin-unity-guess-charset.

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

 -- Function: latin-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 latin-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 `latin-unity-recode-region.

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

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

 -- Function: latin-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 latin-unity-cset-codesys-alist, and the values in latin-unity-charset-alias-alist."

 -- Function: latin-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 latin-unity-cset-codesys-alist, and the values in latin-unity-coding-system-alias-alist."

 -- Function: latin-unity-example

     A cheesy example for latin-unity.

     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.

 -- Function: latin-unity-test

     A simple automated test suite for latin-unity.

Installing latin-unity with your (X)Emacs
*****************************************

latin-unity may be installed from XEmacs via the package user interface (accessible from the Tools menu or via M-x list-packages).

You can also download the latin-unity-VERSION-pkg.tar.gz tarball from ftp://ftp.xemacs.org/pub/xemacs/packages/, and simply unpack it in the usual place.

latin-unity sources are available from XEmacs's CVS repository.  The module name is latin-unity.  See http://www.xemacs.org/Develop/cvsaccess.html for more information about XEmacs's CVS repository.

Configuring latin-unity for Use
*******************************

If you want latin-unity to be automatically initialized, invoke latin-unity-install 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: latin-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: latin-unity-coding-system-alias-alist nil
     Alist mapping aliases to Mule coding system names (symbols).

     The default value is nil.

Reporting Bugs and Problems
***************************

Please report bugs to the author, Stephen Turnbull, or to the developers' mailing list, XEmacs Beta.

Suggestions for improvement are welcome at the same addresses.

Frequently Asked Questions
**************************

 1. I'm smarter than latin-unity!  How can that be?

     Don't be surprised.  Trust yourself.

     latin-unity is very young as yet.  Teach it what you know by Customizing its variables, and report your changes to the maintainer (see Bug Reports).

 2. What is a UCS?

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

 3. I know utf-16-le-bom is a UCS, but latin-unity 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 latin-unity-ucs-list to include the ones you use, and report your favorites to the maintainer for consideration for inclusion in the defaults, see Bug Reports.  (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.  latin-unity 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 latin-unity-ucs-list.  Put it back.

 5. latin-unity is really buggy and I can't get any work done.

     First, use M-x latin-unity-uninstall RET, then report your problems as a bug (see Bug Reports).

Theory of Operation
*******************

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 latin-unity 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, 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).

latin-unity 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, the latin-unity package uses the strategy of examining the buffer prior to export.  If use of multiple Latin coded character sets is detected, latin-unity 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 latin-unity 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 latin-unity 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, latin-unity is not an unreasonable place for it.)  latin-unity provides two functions (one fast and dangerous, the other slow 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 latin-unity Cannot Do for You.  So these functions must be invoked by the user.  See Interactive Usage.

What latin-unity Cannot Do for You
**********************************

latin-unity 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.

latin-unity 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 automatically recoverable with a tool you already have: Mule.

However, with latin-unity, 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 latin-unity 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 at all 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.

Interfaces
**********

Various recent---dating from the end of the nineties---ISO 8859 standard language environments used to be provided with this package, but they have been refactored out into their own package, latin-euro-standards, on which this one depends. See the documentation for that package if you're interested in using those from Lisp.

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.  latin-unity 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 latin-unity.

     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 latin-unity.  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 latin-unity 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

Internals
*********

No internals documentation yet.

latin-unity-utils.el provides one utility function.

 -- Function: latin-unity-dump-tables

     Dump the temporary table created by loading latin-unity-utils.el to latin-unity-tables.el.  Loading the latter file initializes latin-unity-equivalences.