17. World Scripts Support

If you compile XEmacs with Mule option, it supports a wide variety of world scripts, including Latin script, as well as Arabic script, Simplified Chinese script (for mainland of China), Traditional Chinese script (for Taiwan and Hong-Kong), Greek script, Hebrew script, IPA symbols, Japanese scripts (Hiragana, Katakana and Kanji), Korean scripts (Hangul and Hanja) and 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”).

17.1 What is Mule?

Mule is the MUltiLingual Extension to XEmacs. It provides facilities not only for handling text written in many different languages, but in fact multilingual texts containing several languages in the same buffer. This goes beyond the simple facilities offered by Unicode for representation of multilingual text. Mule also supports input methods, composing display using fonts in various different encodings, changing character syntax and other editing facilities to correspond to local language usage, and more.

The most obvious problem is that of the different character coding systems used by different languages. ASCII supplies all the characters needed for most computer programming languages and US English (it lacks the currency symbol for British English), but other Western European languages (French, Spanish, German) require more than 96 code positions for accented characters. In fact, even with 8 bits to represent 96 more character (including accented characters and symbols such as currency symbols), some languages’ alphabets remain incomplete (Croatian, Polish). (The 64 "missing characters" are reserved for control characters.) Furthermore, many European languages have their own alphabets, which must conflict with the accented characters since the ASCII characters are needed for computer interaction (error and log messages are typically in ASCII).

For economy of space, historical practice has been for each language to establish its own encoding for the characters it needs. This allows most European languages to represented with one octet (byte) per character. However, many Asian languages have thousands of characters and require two or more octets per character. For multilingual purposes, the ISO 2022 standard establishes escape codes that allow switching encodings in midstream. (It’s also ISO 2022 that establishes the standard that code points 0-31 and 128-159 are control codes.)

However, this is error-prone and complex for internal processing. For this reason XEmacs uses an internal coding system which can encode all of the world’s scripts. Unfortunately, for historical reasons, this code is not Unicode, although we are moving in that direction.

XEmacs translates between the 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). The internal encoding is never visible to the user in a production XEmacs, but unfortunately the process cannot be completely transparent to the user. This is because the same ranges of octets may represent 1-octet ISO-8859-1 (which is satisfactory for most Western European use prior to the introduction of the Euro currency), 1-octet ISO-8859-15 (which substitutes the Euro for the rarely used "generic currency" symbol), 1-octet ISO-8859-5 (Cyrillic), or multioctet EUC-JP (Japanese). There’s no way to tell without being able to read!

A number of heuristics are incorporated in Mule for automatic recognition, there are facilities for the user to set defaults, and where necessary (rarely, we hope) to set coding systems directly.

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.

17.2 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 section Recognizing Coding Systems). 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.

17.3 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 section Selecting an 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).

17.4 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.

17.5 Mule and Fonts

(This section is X11-specific.)

Text in XEmacs buffers is displayed using various faces. In addition to specifying properties of a face, such as font and color, there are some additional properties of Mule charsets that are used in text.

There is currently two properties of a charset that could be adjusted by user: font registry and so called ccl-program.

Font registry is a regular expression matching the font registry field for this character set. For example, both the ascii and latin-iso8859-1 charsets use the registry "ISO8859-1". This field is used to choose an appropriate font when the user gives a general font specification such as ‘-*-courier-medium-r-*-140-*’, i.e. a 14-point upright medium-weight Courier font.

You can set font registry for a charset using ‘set-charset-registry’ function in one of your startup files. This function takes two arguments: character set (as a symbol) and font registry (as a string).

E. g., for Cyrillic texts Mule uses cyrillic-iso8859-5 charset with ‘"ISO8859-5"’ as a default registry, and we want to use ‘"koi8-r"’ instead, because fonts in that encoding are installed on our system. Use:

(set-charset-registry 'cyrillic-iso8859-5 "koi8-r")

(Please note that you probably also want to set font registry for ‘ascii’ charset so that mixed English/Cyrillic texts be displayed using the same font.)

"CCL-programs" are a little special-purpose scripts defined within XEmacs or in some package. Those scripts allow XEmacs to use fonts that are in different encoding from the encoding that is used by Mule for text in buffer. Returning to the above example, we need to somehow tell XEmacs that we have different encodings of fonts and text and so it needs to convert characters between those encodings when displaying. That’s what ‘set-charset-ccl-program’ function is used for. There are quite a few various CCL programs defined within XEmacs, and there is no comprehensive list of them, so you currently have to consult sources.

We know that there is a CCL program called ‘ccl-encode-koi8-r-font’ that is used exactly for needed purpose: to convert characters between ‘ISO8859-5’ encoding and ‘koi8-r’. Use:

(set-charset-ccl-program 'cyrillic-iso8859-5 'ccl-encode-koi8-r-font)

There are several more uses for CCL programs, not related to fonts, but those uses are not described here.

17.6 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-h C coding <RET>

Describe coding system coding.

C-h 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 section Recognizing Coding Systems).

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 section The 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 section Visiting Files.

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.

17.7 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 section 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 section Local Variables in Files). 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 section Specifying a Coding System).

17.8 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.

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.

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

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.

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