path: root/lib/stdlib/doc/src/unicode_usage.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>1999</year>
      <year>2014</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
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    <title>Using Unicode in Erlang</title>
    <prepared>Patrik Nyblom</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>2009-02-25</date>
    <rev>PA1</rev>
    <file>unicode_usage.xml</file>
  </header>
<section>
<title>Unicode Implementation</title>
  <p>Implementing support for Unicode character sets is an ongoing
  process. The Erlang Enhancement Proposal (EEP) 10 outlined the
  basics of Unicode support and also specified a default encoding in
  binaries that all Unicode-aware modules should handle in the
  future.</p>

  <p>The functionality described in EEP10 was implemented in Erlang/OTP
  R13A, but that was by no means the end of it. In Erlang/OTP R14B01 support
  for Unicode file names was added, although it was in no way complete
  and was by default disabled on platforms where no guarantee was given
  for the file name encoding. With Erlang/OTP R16A came support for UTF-8 encoded
  source code, among with enhancements to many of the applications to
  support both Unicode encoded file names as well as support for UTF-8
  encoded files in several circumstances. Most notable is the support
  for UTF-8 in files read by <c>file:consult/1</c>, release handler support
  for UTF-8 and more support for Unicode character sets in the
  I/O-system. In Erlang/OTP 17.0, the encoding default for Erlang source files was
  switched to UTF-8.</p>

  <p>This guide outlines the current Unicode support and gives a couple
  of recipes for working with Unicode data.</p>
</section>
<section>
<title>Understanding Unicode</title>
  <p>Experience with the Unicode support in Erlang has made it
  painfully clear that understanding Unicode characters and encodings
  is not as easy as one would expect. The complexity of the field as
  well as the implications of the standard requires thorough
  understanding of concepts rarely before thought of.</p>

  <p>Furthermore the Erlang implementation requires understanding of
  concepts that never were an issue for many (Erlang) programmers. To
  understand and use Unicode characters requires that you study the
  subject thoroughly, even if you're an experienced programmer.</p>

  <p>As an example, one could contemplate the issue of converting
  between upper and lower case letters. Reading the standard will make
  you realize that, to begin with, there's not a simple one to one
  mapping in all scripts. Take German as an example, where there's a
  letter "ß" (Sharp s) in lower case, but the uppercase equivalent is
  "SS". Or Greek, where "Σ" has two different lowercase forms: "ς" in
  word-final position and "σ" elsewhere. Or Turkish where dotted and
  dot-less "i" both exist in lower case and upper case forms, or
  Cyrillic "I" which usually has no lowercase form. Or of course
  languages that have no concept of upper case (or lower case). So, a
  conversion function will need to know not only one character at a
  time, but possibly the whole sentence, maybe the natural language
  the translation should be in and also take into account differences
  in input and output string length and so on. There is at the time of
  writing no Unicode to_upper/to_lower functionality in Erlang/OTP, but
  there are publicly available libraries that address these issues.</p>

  <p>Another example is the accented characters where the same glyph
  has two different representations. Let's look at the Swedish
  "ö". There's a code point for that in the Unicode standard, but you
  can also write it as "o" followed by U+0308 (Combining Diaeresis,
  with the simplified meaning that the last letter should have a "¨"
  above). They have exactly the same glyph. They are for most
  purposes the same, but they have completely different
  representations. For example MacOS X converts all file names to use
  Combining Diaeresis, while most other programs (including Erlang)
  try to hide that by doing the opposite when for example listing
  directories. However it's done, it's usually important to normalize
  such characters to avoid utter confusion.</p>

  <p>The list of examples can be made as long as the Unicode standard, I
  suspect. The point is that one need a kind of knowledge that was
  never needed when programs only took one or two languages into
  account. The complexity of human languages and scripts, certainly
  has made this a challenge when constructing a universal
  standard. Supporting Unicode properly in your program <em>will</em> require
  effort.</p>

</section>
<section>
<title>What Unicode Is</title>
  <p>Unicode is a standard defining code points (numbers) for all
  known, living or dead, scripts. In principle, every known symbol
  used in any language has a Unicode code point.</p>
  <p>Unicode code points are defined and published by the <em>Unicode
  Consortium</em>, which is a non profit organization.</p>
  <p>Support for Unicode is increasing throughout the world of
  computing, as the benefits of one common character set are
  overwhelming when programs are used in a global environment.</p>
  <p>Along with the base of the standard: the code points for all the
  scripts, there are a couple of <em>encoding standards</em> available.</p>
  <p>It is vital to understand the difference between encodings and
  Unicode characters. Unicode characters are code points according to
  the Unicode standard, while the encodings are ways to represent such
  code points. An encoding is just a standard for representation,
  UTF-8 can for example be used to represent a very limited part of
  the Unicode character set (e.g. ISO-Latin-1), or the full Unicode
  range. It's just an encoding format.</p>
  <p>As long as all character sets were limited to 256 characters,
  each character could be stored in one single byte, so there was more
  or less only one practical encoding for the characters. Encoding
  each character in one byte was so common that the encoding wasn't
  even named. When we now, with the Unicode system, have a lot more
  than 256 characters, we need a common way to represent these. The
  common ways of representing the code points are the encodings. This
  means a whole new concept to the programmer, the concept of
  character representation, which was before a non-issue.</p>

  <p>Different operating systems and tools support different
  encodings. For example Linux and MacOS X has chosen the UTF-8
  encoding, which is backwards compatible with 7-bit ASCII and
  therefore affects programs written in plain English the
  least. Windows on the other hand supports a limited version of
  UTF-16, namely all the code planes where the characters can be
  stored in one single 16-bit entity, which includes most living
  languages.</p>

  <p>The most widely spread encodings are:</p>
  <taglist>
    <tag>Bytewise representation</tag>
    <item>This is not a proper Unicode representation, but the
    representation used for characters before the Unicode standard. It
    can still be used to represent character code points in the Unicode
    standard that have numbers below 256, which corresponds exactly to
    the ISO-Latin-1 character set. In Erlang, this is commonly denoted
    <c>latin1</c> encoding, which is slightly misleading as ISO-Latin-1 is
    a character code range, not an encoding.</item>
    <tag>UTF-8</tag>
    <item>Each character is stored in one to four bytes depending on
    code point. The encoding is backwards compatible with bytewise
    representation of 7-bit ASCII as all 7-bit characters are stored
    in one single byte in UTF-8. The characters beyond code point 127
    are stored in more bytes, letting the most significant bit in the
    first character indicate a multi-byte character. For details on
    the encoding, the RFC is publicly available. Note that UTF-8 is
    <em>not</em> compatible with bytewise representation for
    code points between 128 and 255, so a ISO-Latin-1 bytewise
    representation is not generally compatible with UTF-8.</item>
    <tag>UTF-16</tag>
    <item>This encoding has many similarities to UTF-8, but the basic
    unit is a 16-bit number. This means that all characters occupy at
    least two bytes, some high numbers even four bytes. Some programs,
    libraries and operating systems claiming to use UTF-16 only allows
    for characters that can be stored in one 16-bit entity, which is
    usually sufficient to handle living languages. As the basic unit
    is more than one byte, byte-order issues occur, why UTF-16 exists
    in both a big-endian and little-endian variant. In Erlang, the
    full UTF-16 range is supported when applicable, like in the
    <c>unicode</c> module and in the bit syntax.</item>
    <tag>UTF-32</tag>
    <item>The most straight forward representation. Each character is
    stored in one single 32-bit number. There is no need for escapes
    or any variable amount of entities for one character, all Unicode
    code points can be stored in one single 32-bit entity. As with
    UTF-16, there are byte-order issues, UTF-32 can be both big- and
    little-endian.</item>
    <tag>UCS-4</tag>
    <item>Basically the same as UTF-32, but without some Unicode
    semantics, defined by IEEE and has little use as a separate
    encoding standard. For all normal (and possibly abnormal) usages,
    UTF-32 and UCS-4 are interchangeable.</item>
  </taglist>
  <p>Certain ranges of numbers are left unused in the Unicode standard
  and certain ranges are even deemed invalid. The most notable invalid
  range is 16#D800 - 16#DFFF, as the UTF-16 encoding does not allow
  for encoding of these numbers. It can be speculated that the UTF-16
  encoding standard was, from the beginning, expected to be able to
  hold all Unicode characters in one 16-bit entity, but then had to be
  extended, leaving a hole in the Unicode range to cope with backward
  compatibility.</p>
  <p>Additionally, the code point 16#FEFF is used for byte order marks
  (BOM's) and use of that character is not encouraged in other
  contexts than that. It actually is valid though, as the character
  "ZWNBS" (Zero Width Non Breaking Space). BOM's are used to identify
  encodings and byte order for programs where such parameters are not
  known in advance. Byte order marks are more seldom used than one
  could expect, but their use might become more widely spread as they
  provide the means for programs to make educated guesses about the
  Unicode format of a certain file.</p>
</section>
<section>
  <title>Areas of Unicode Support</title>
  <p>To support Unicode in Erlang, problems in several areas have been
  addressed. Each area is described briefly in this section and more
  thoroughly further down in this document:</p>
  <taglist>
    <tag>Representation</tag>
    <item>To handle Unicode characters in Erlang, we have to have a
    common representation both in lists and binaries. The EEP (10) and
    the subsequent initial implementation in Erlang/OTP R13A settled a standard
    representation of Unicode characters in Erlang.</item>
    <tag>Manipulation</tag>
    <item>The Unicode characters need to be processed by the Erlang
    program, why library functions need to be able to handle them. In
    some cases functionality was added to already existing interfaces
    (as the string module now can handle lists with arbitrary code points),
    in some cases new functionality or options need to be added (as in
    the <c>io</c>-module, the file handling, the <c>unicode</c> module
    and the bit syntax). Today most modules in kernel and STDLIB, as
    well as the VM are Unicode aware.</item>
    <tag>File I/O</tag>
    <item>I/O is by far the most problematic area for Unicode. A file
    is an entity where bytes are stored and the lore of programming
    has been to treat characters and bytes as interchangeable. With
    Unicode characters, you need to decide on an encoding as soon as
    you want to store the data in a file. In Erlang you can open a
    text file with an encoding option, so that you can read characters
    from it rather than bytes, but you can also open a file for
    bytewise I/O. The I/O-system of Erlang has been designed (or at
    least used) in a way where you expect any I/O-server to be
    able to cope with any string data, but that is no longer the case
    when you work with Unicode characters. Handling the fact that you
    need to know the capabilities of the device where your data ends
    up is something new to the Erlang programmer. Furthermore, ports
    in Erlang are byte oriented, so an arbitrary string of (Unicode)
    characters can not be sent to a port without first converting it
    to an encoding of choice.</item>
    <tag>Terminal I/O</tag>
    <item>Terminal I/O is slightly easier than file I/O. The output is
    meant for human reading and is usually Erlang syntax (e.g. in the
    shell). There exists syntactic representation of any Unicode
    character without actually displaying the glyph (instead written
    as <c>\x{</c>HHH<c>}</c>), so Unicode data can usually be displayed
    even if the terminal as such do not support the whole Unicode
    range.</item>
    <tag>File names</tag>
    <item>File names can be stored as Unicode strings, in different
    ways depending on the underlying OS and file system. This can be
    handled fairly easy by a program. The problems arise when the file
    system is not consistent in it's encodings, like for example
    Linux. Linux allows files to be named with any sequence of bytes,
    leaving to each program to interpret those bytes. On systems where
    these "transparent" file names are used, Erlang has to be informed
    about the file name encoding by a startup flag. The default is
    bytewise interpretation, which is actually usually wrong, but
    allows for interpretation of <em>all</em> file names. The concept
    of "raw file names" can be used to handle wrongly encoded
    file names if one enables Unicode file name translation
    (<c>+fnu</c>) on platforms where this is not the default.</item>
    <tag>Source code encoding</tag>
    <item>When it comes to the Erlang source code, there is support
    for the UTF-8 encoding and bytewise encoding. The default in
    Erlang/OTP R16B was bytewise (or latin1) encoding; in Erlang/OTP 17.0
    it was changed to UTF-8. You can control the encoding by a comment like:
<code>
%% -*- coding: utf-8 -*-
</code>
    in the beginning of the file. This of course requires your editor to
    support UTF-8 as well. The same comment is also interpreted by
    functions like <c>file:consult/1</c>, the release handler etc, so that
    you can have all text files in your source directories in UTF-8
    encoding.
    </item>
    <tag>The language</tag>
    <item>Having the source code in UTF-8 also allows you to write
    string literals containing Unicode characters with code points &gt;
    255, although atoms, module names and function names are
    restricted to the ISO-Latin-1 range. Binary
    literals where you use the <c>/utf8</c> type, can also be
    expressed using Unicode characters &gt; 255. Having module names
    using characters other than 7-bit ASCII can cause trouble on
    operating systems with inconsistent file naming schemes, and might
    also hurt portability, so it's not really recommended. It is
    suggested in EEP 40 that the language should also allow for
    Unicode characters &gt; 255 in variable names. Whether to
    implement that EEP or not is yet to be decided.</item>
  </taglist>
</section>
<section>
  <title>Standard Unicode Representation</title>
  <p>In Erlang, strings are actually lists of integers. A string was
  up until Erlang/OTP R13 defined to be encoded in the ISO-latin-1 (ISO8859-1)
  character set, which is, code point by code point, a sub-range of
  the Unicode character set.</p>
  <p>The standard list encoding for strings was therefore easily
  extended to cope with the whole Unicode range: A Unicode string in
  Erlang is simply a list containing integers, each integer being a
  valid Unicode code point and representing one character in the
  Unicode character set.</p>
  <p>Erlang strings in ISO-latin-1 are a subset of Unicode
  strings.</p>
  <p>Only if a string contains code points &lt; 256, can it be
  directly converted to a binary by using
  i.e. <c>erlang:iolist_to_binary/1</c> or can be sent directly to a
  port. If the string contains Unicode characters &gt; 255, an
  encoding has to be decided upon and the string should be converted
  to a binary in the preferred encoding using
  <c>unicode:characters_to_binary/{1,2,3}</c>. Strings are not
  generally lists of bytes, as they were before Erlang/OTP R13. They are lists of
  characters. Characters are not generally bytes, they are Unicode
  code points.</p>

  <p>Binaries are more troublesome. For performance reasons, programs
  often store textual data in binaries instead of lists, mainly
  because they are more compact (one byte per character instead of two
  words per character, as is the case with lists). Using
  <c>erlang:list_to_binary/1</c>, an ISO-Latin-1 Erlang string could
  be converted into a binary, effectively using bytewise encoding -
  one byte per character. This was very convenient for those limited
  Erlang strings, but cannot be done for arbitrary Unicode lists.</p>
  <p>As the UTF-8 encoding is widely spread and provides some backward
  compatibility in the 7-bit ASCII range, it is selected as the
  standard encoding for Unicode characters in binaries for Erlang.</p>
  <p>The standard binary encoding is used whenever a library function
  in Erlang should cope with Unicode data in binaries, but is of
  course not enforced when communicating externally. Functions and
  bit-syntax exist to encode and decode both UTF-8, UTF-16 and UTF-32
  in binaries. Library functions dealing with binaries and Unicode in
  general, however, only deal with the default encoding.</p>

  <p>Character data may be combined from several sources, sometimes
  available in a mix of strings and binaries. Erlang has for long had
  the concept of <c>iodata</c> or <c>iolist</c>s, where binaries and
  lists can be combined to represent a sequence of bytes. In the same
  way, the Unicode aware modules often allow for combinations of
  binaries and lists where the binaries have characters encoded in
  UTF-8 and the lists contain such binaries or numbers representing
  Unicode code points:</p>
  <code type="none">
unicode_binary() = binary() with characters encoded in UTF-8 coding standard

chardata() = charlist() | unicode_binary()

charlist() = maybe_improper_list(char() | unicode_binary() | charlist(),
                                 unicode_binary() | nil())</code>
  <p>The module <seealso
  marker="stdlib:unicode"><c>unicode</c></seealso> in STDLIB even
  supports similar mixes with binaries containing other encodings than
  UTF-8, but that is a special case to allow for conversions to and
  from external data:</p>
  <code type="none">
external_unicode_binary() = binary() with characters coded in
  a user specified Unicode encoding other than UTF-8 (UTF-16 or UTF-32)

external_chardata() = external_charlist() | external_unicode_binary()

external_charlist() = maybe_improper_list(char() |
                                            external_unicode_binary() |
                                            external_charlist(),
                                          external_unicode_binary() | nil())</code>
</section>
<section>
  <title>Basic Language Support</title>
  <p><marker id="unicode_in_erlang"/>As of Erlang/OTP R16 Erlang
  source files can be written in either UTF-8 or bytewise encoding
  (a.k.a. <c>latin1</c> encoding). The details on how to state the encoding
  of an Erlang source file can be found in 
  <seealso marker="stdlib:epp#encoding"><c>epp(3)</c></seealso>. Strings and comments
  can be written using Unicode, but functions still have to be named
  using characters from the ISO-latin-1 character set and atoms are
  restricted to the same ISO-latin-1 range. These restrictions in the
  language are of course independent of the encoding of the source
  file.</p>
  <section>
    <title>Bit-syntax</title>
    <p>The bit-syntax contains types for coping with binary data in the
    three main encodings. The types are named <c>utf8</c>, <c>utf16</c>
    and <c>utf32</c> respectively. The <c>utf16</c> and <c>utf32</c> types
    can be in a big- or little-endian variant:</p>
    <code>
&lt;&lt;Ch/utf8,_/binary&gt;&gt; = Bin1,
&lt;&lt;Ch/utf16-little,_/binary&gt;&gt; = Bin2,
Bin3 = &lt;&lt;$H/utf32-little, $e/utf32-little, $l/utf32-little, $l/utf32-little,
$o/utf32-little&gt;&gt;,</code>
    <p>For convenience, literal strings can be encoded with a Unicode
    encoding in binaries using the following (or similar) syntax:</p>
    <code>
Bin4 = &lt;&lt;"Hello"/utf16&gt;&gt;,</code>
  </section>
  <section>
    <title>String and Character Literals</title>
    <p>For source code, there is an extension to the <c>\</c>OOO
    (backslash followed by three octal numbers) and <c>\x</c>HH
    (backslash followed by <c>x</c>, followed by two hexadecimal
    characters) syntax, namely <c>\x{</c>H ...<c>}</c> (a backslash
    followed by an <c>x</c>, followed by left curly bracket, any
    number of hexadecimal digits and a terminating right curly
    bracket). This allows for entering characters of any code point
    literally in a string even when the encoding of the source file is
    bytewise (<c>latin1</c>).</p>
    <p>In the shell, if using a Unicode input device, or in source
    code stored in UTF-8, <c>$</c> can be followed directly by a
    Unicode character producing an integer. In the following example
    the code point of a Cyrillic <c>с</c> is output:</p>
    <pre>
7> <input>$с.</input>
1089</pre>
  </section>
  <section>
    <title>Heuristic String Detection</title>
    <p>In certain output functions and in the output of return values
    in the shell, Erlang tries to heuristically detect string data in
    lists and binaries. Typically you will see heuristic detection in
    a situation like this:</p>
    <pre>
1> <input>[97,98,99].</input>
"abc"
2> <input>&lt;&lt;97,98,99&gt;&gt;.</input>
&lt;&lt;"abc"&gt;&gt;    
3> <input>&lt;&lt;195,165,195,164,195,182&gt;&gt;.</input>
&lt;&lt;"åäö"/utf8&gt;&gt;</pre>
    <p>Here the shell will detect lists containing printable
    characters or binaries containing printable characters either in
    bytewise or UTF-8 encoding. The question here is: what is a
    printable character? One view would be that anything the Unicode
    standard thinks is printable, will also be printable according to
    the heuristic detection. The result would be that almost any list
    of integers will be deemed a string, resulting in all sorts of
    characters being printed, maybe even characters your terminal does
    not have in its font set (resulting in some generic output you
    probably will not appreciate). Another way is to keep it backwards
    compatible so that only the ISO-Latin-1 character set is used to
    detect a string. A third way would be to let the user decide
    exactly what Unicode ranges are to be viewed as characters. Since
    Erlang/OTP R16B you can select either the whole Unicode range or the
    ISO-Latin-1 range by supplying the startup flag <c>+pc
    </c><i>Range</i>, where <i>Range</i> is either <c>latin1</c> or
    <c>unicode</c>. For backwards compatibility, the default is
    <c>latin1</c>. This only controls how heuristic string detection
    is done. In the future, more ranges are expected to be added, so
    that one can tailor the heuristics to the language and region
    relevant to the user.</p>
    <p>Lets look at an example with the two different startup options:</p>
<pre>
$ <input>erl +pc latin1</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)  
1> <input>[1024].</input>
[1024]
2> <input>[1070,1085,1080,1082,1086,1076].</input>
[1070,1085,1080,1082,1086,1076]
3> <input>[229,228,246].</input>
"åäö"
4> <input>&lt;&lt;208,174,208,189,208,184,208,186,208,190,208,180&gt;&gt;.</input>
&lt;&lt;208,174,208,189,208,184,208,186,208,190,208,180&gt;&gt;
5> <input>&lt;&lt;229/utf8,228/utf8,246/utf8&gt;&gt;.</input>
&lt;&lt;"åäö"/utf8&gt;&gt;
</pre>
<pre>
$ <input>erl +pc unicode</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)  
1> <input>[1024].</input>
"Ѐ"
2> <input>[1070,1085,1080,1082,1086,1076].</input>
"Юникод"
3> <input>[229,228,246].</input>
"åäö"
4> <input>&lt;&lt;208,174,208,189,208,184,208,186,208,190,208,180&gt;&gt;.</input>
&lt;&lt;"Юникод"/utf8&gt;&gt;
5> <input>&lt;&lt;229/utf8,228/utf8,246/utf8&gt;&gt;.</input>
&lt;&lt;"åäö"/utf8&gt;&gt;
</pre>
    <p>In the examples, we can see that the default Erlang shell will
    only interpret characters from the ISO-Latin1 range as printable
    and will only detect lists or binaries with those "printable"
    characters as containing string data. The valid UTF-8 binary
    containing "Юникод", will not be printed as a string. When, on the
    other hand, started with all Unicode characters printable (<c>+pc
    unicode</c>), the shell will output anything containing printable
    Unicode data (in binaries either UTF-8 or bytewise encoded) as
    string data.</p>

    <p>These heuristics are also used by
    <c>io</c>(<c>_lib</c>)<c>:format/2</c> and friends when the
    <c>t</c> modifier is used in conjunction with <c>~p</c> or
    <c>~P</c>:</p>
<pre>
$ <input>erl +pc latin1</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)  
1> <input>io:format("~tp~n",[{&lt;&lt;"åäö"&gt;&gt;, &lt;&lt;"åäö"/utf8&gt;&gt;, &lt;&lt;208,174,208,189,208,184,208,186,208,190,208,180&gt;&gt;}]).</input>
{&lt;&lt;"åäö"&gt;&gt;,&lt;&lt;"åäö"/utf8&gt;&gt;,&lt;&lt;208,174,208,189,208,184,208,186,208,190,208,180&gt;&gt;}
ok
</pre>
<pre>
$ <input>erl +pc unicode</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)  
1> <input>io:format("~tp~n",[{&lt;&lt;"åäö"&gt;&gt;, &lt;&lt;"åäö"/utf8&gt;&gt;, &lt;&lt;208,174,208,189,208,184,208,186,208,190,208,180&gt;&gt;}]).</input>
{&lt;&lt;"åäö"&gt;&gt;,&lt;&lt;"åäö"/utf8&gt;&gt;,&lt;&lt;"Юникод"/utf8&gt;&gt;}
ok
</pre>
    <p>Please observe that this only affects <i>heuristic</i> interpretation
    of lists and binaries on output. For example the <c>~ts</c> format
    sequence does always output a valid lists of characters,
    regardless of the <c>+pc</c> setting, as the programmer has
    explicitly requested string output.</p>
  </section>
</section>
<section>
  <title>The Interactive Shell</title>
  <p>The interactive Erlang shell, when started towards a terminal or
  started using the <c>werl</c> command on windows, can support
  Unicode input and output.</p>
  <p>On Windows, proper operation requires that a suitable font
  is installed and selected for the Erlang application to use. If no
  suitable font is available on your system, try installing the DejaVu
  fonts (<c>dejavu-fonts.org</c>), which are freely available and then
  select that font in the Erlang shell application.</p>
  <p>On Unix-like operating systems, the terminal should be able
  to handle UTF-8 on input and output (modern versions of XTerm, KDE
  konsole and the Gnome terminal do for example) and your locale
  settings have to be proper. As an example, my <c>LANG</c>
  environment variable is set as this:</p>
  <pre>
$ <input>echo $LANG</input>
en_US.UTF-8</pre>
  <p>Actually, most systems handle the <c>LC_CTYPE</c> variable before
  <c>LANG</c>, so if that is set, it has to be set to
  <c>UTF-8</c>:</p>
  <pre>
$ echo <input>$LC_CTYPE</input>
en_US.UTF-8</pre>
  <p>The <c>LANG</c> or <c>LC_CTYPE</c> setting should be consistent
  with what the terminal is capable of, there is no portable way for
  Erlang to ask the actual terminal about its UTF-8 capacity, we have
  to rely on the language and character type settings.</p>
  <p>To investigate what Erlang thinks about the terminal, the
  <c>io:getopts()</c> call can be used when the shell is started:</p>
  <pre>
$ <input>LC_CTYPE=en_US.ISO-8859-1 erl</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)
1> <input>lists:keyfind(encoding, 1, io:getopts()).</input>
{encoding,latin1}
2> <input>q().</input>
ok
$ <input>LC_CTYPE=en_US.UTF-8 erl</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)
1> <input>lists:keyfind(encoding, 1, io:getopts()).</input>
{encoding,unicode}
2></pre>

  <p>When (finally?) everything is in order with the locale settings,
  fonts and the terminal emulator, you probably also have discovered a
  way to input characters in the script you desire. For testing, the
  simplest way is to add some keyboard mappings for other languages,
  usually done with some applet in your desktop environment. In my KDE
  environment, I start the KDE Control Center (Personal Settings),
  select "Regional and Accessibility" and then "Keyboard Layout". On
  Windows XP, I start Control Panel->Regional and Language
  Options, select the Language tab and click the Details... button in
  the square named "Text services and input Languages". Your
  environment probably provides similar means of changing the keyboard
  layout. Make sure you have a way to easily switch back and forth
  between keyboards if you are not used to this, entering commands
  using a Cyrillic character set is, as an example, not easily done in
  the Erlang shell.</p>

  <p>Now you are set up for some Unicode input and output. The
  simplest thing to do is of course to enter a string in the
  shell:</p>

  <pre>
$ <input>erl</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)
1> <input>lists:keyfind(encoding, 1, io:getopts()).</input>
{encoding,unicode}
2> <input>"Юникод".</input>
"Юникод"
3> <input>io:format("~ts~n", [v(2)]).</input>
Юникод
ok
4> </pre>
  <p>While strings can be input as Unicode characters, the language
  elements are still limited to the ISO-latin-1 character set. Only
  character constants and strings are allowed to be beyond that
  range:</p>
  <pre>
$ <input>erl</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)
1> <input>$ξ.</input>
958
2> <input>Юникод.</input>
* 1: illegal character
2> </pre>
</section> 
<section>
  <title>Unicode File Names</title>
  <marker id="unicode_file_names"/>
  <p>Most modern operating systems support Unicode file names in some
  way or another. There are several different ways to do this and
  Erlang by default treats the different approaches differently:</p>
  <taglist>
    <tag>Mandatory Unicode file naming</tag>
    <item>
      <p>Windows and, for most common uses, MacOS X enforces Unicode
      support for file names. All files created in the file system have
      names that can consistently be interpreted. In MacOS X, all file
      names are retrieved in UTF-8 encoding, while Windows has
      selected an approach where each system call handling file names
      has a special Unicode aware variant, giving much the same
      effect. There are no file names on these systems that are not
      Unicode file names, why the default behavior of the Erlang VM is
      to work in &quot;Unicode file name translation mode&quot;,
      meaning that a file name can be given as a Unicode list and that
      will be automatically translated to the proper name encoding for
      the underlying operating and file system.</p>
      <p>Doing i.e. a <c>file:list_dir/1</c> on one of these systems
      may return Unicode lists with code points beyond 255, depending
      on the content of the actual file system.</p>
      <p>As the feature is fairly new, you may still stumble upon non
      core applications that cannot handle being provided with file
      names containing characters with code points larger than 255, but
      the core Erlang system should have no problems with Unicode file
      names.</p>
    </item>
    <tag>Transparent file naming</tag>
    <item>
      <p>Most Unix operating systems have adopted a simpler approach,
      namely that Unicode file naming is not enforced, but by
      convention. Those systems usually use UTF-8 encoding for Unicode
      file names, but do not enforce it. On such a system, a file name
      containing characters having code points between 128 and 255 may
      be named either as plain ISO-latin-1 or using UTF-8 encoding. As
      no consistency is enforced, the Erlang VM can do no consistent
      translation of all file names.</p>

      <p>By default on such systems, Erlang starts in <c>utf8</c> file
      name mode if the terminal supports UTF-8, otherwise in
      <c>latin1</c> mode.</p>

      <p>In the <c>latin1</c> mode, file names are bytewise endcoded.
      This allows for list representation of all file names in
      the system, but, for example, a file named "Östersund.txt", will
      appear in <c>file:list_dir/1</c> as either "Östersund.txt" (if
      the file name was encoded in bytewise ISO-Latin-1 by the program
      creating the file, or more probably as
      <c>[195,150,115,116,101,114,115,117,110,100]</c>, which is a
      list containing UTF-8 bytes - not what you would want... If you
      on the other hand use Unicode file name translation on such a
      system, non-UTF-8 file names will simply be ignored by functions
      like <c>file:list_dir/1</c>. They can be retrieved with
      <c>file:list_dir_all/1</c>, but wrongly encoded file names will
      appear as &quot;raw file names&quot;.</p>

    </item>
  </taglist>

  <p>The Unicode file naming support was introduced with Erlang/OTP
  R14B01. A VM operating in Unicode file name translation mode can
  work with files having names in any language or character set (as
  long as it is supported by the underlying OS and file system). The
  Unicode character list is used to denote file or directory names and
  if the file system content is listed, you will also get
  Unicode lists as return value. The support lies in the Kernel and
  STDLIB modules, why most applications (that does not explicitly
  require the file names to be in the ISO-latin-1 range) will benefit
  from the Unicode support without change.</p>

  <p>On operating systems with mandatory Unicode file names, this
  means that you more easily conform to the file names of other (non
  Erlang) applications, and you can also process file names that, at
  least on Windows, were completely inaccessible (due to having names
  that could not be represented in ISO-latin-1). Also you will avoid
  creating incomprehensible file names on MacOS X as the vfs layer of
  the OS will accept all your file names as UTF-8 and will not rewrite
  them.</p>

  <p>For most systems, turning on Unicode file name translation is no
  problem even if it uses transparent file naming. Very few systems
  have mixed file name encodings. A consistent UTF-8 named system will
  work perfectly in Unicode file name mode. It was still however
  considered experimental in Erlang/OTP R14B01 and is still not the default on
  such systems. Unicode file name translation is turned on with the
  <c>+fnu</c> switch to the On Linux, a VM started without explicitly
  stating the file name translation mode will default to <c>latin1</c>
  as the native file name encoding. On Windows and MacOS X, the
  default behavior is that of Unicode file name translation, why the
  <c>file:native_name_encoding/0</c> by default returns <c>utf8</c> on
  those systems (the fact that Windows actually does not use UTF-8 on
  the file system level can safely be ignored by the Erlang
  programmer). The default behavior can, as stated before, be
  changed using the <c>+fnu</c> or <c>+fnl</c> options to the VM, see
  the <seealso marker="erts:erl"><c>erl</c></seealso> program. If the
  VM is started in Unicode file name translation mode,
  <c>file:native_name_encoding/0</c> will return the atom
  <c>utf8</c>. The <c>+fnu</c> switch can be followed by <c>w</c>,
  <c>i</c> or <c>e</c>, to control how wrongly encoded file names are
  to be reported. <c>w</c> means that a warning is sent to the
  <c>error_logger</c> whenever a wrongly encoded file name is
  "skipped" in directory listings, <c>i</c> means that those wrongly
  encoded file names are silently ignored and <c>e</c> means that the
  API function will return an error whenever a wrongly encoded file
  (or directory) name is encountered. <c>w</c> is the default. Note
  that <c>file:read_link/1</c> will always return an error if the link
  points to an invalid file name.</p>

  <p>In Unicode file name mode, file names given to the BIF
  <c>open_port/2</c> with the option <c>{spawn_executable,...}</c> are
  also interpreted as Unicode. So is the parameter list given in the
  <c>args</c> option available when using <c>spawn_executable</c>. The
  UTF-8 translation of arguments can be avoided using binaries, see
  the discussion about raw file names below.</p>

  <p>It is worth noting that the file <c>encoding</c> options given
  when opening a file has nothing to do with the file <em>name</em>
  encoding convention. You can very well open files containing data
  encoded in UTF-8 but having file names in bytewise (<c>latin1</c>) encoding
  or vice versa.</p>

  <note><p>Erlang drivers and NIF shared objects still can not be
  named with names containing code points beyond 127. This is a known
  limitation to be removed in a future release. Erlang modules however
  can, but it is definitely not a good idea and is still considered
  experimental.</p></note>

<section>
  <title>Notes About Raw File Names</title>
  <marker id="notes-about-raw-filenames"/>
  <p>Raw file names were introduced together with Unicode file name
  support in erts-5.8.2 (Erlang/OTP R14B01). The reason &quot;raw file
  names&quot; was introduced in the system was to be able to
  consistently represent file names given in different encodings on
  the same system. Having the VM automatically translate a file name
  that is not in UTF-8 to a list of Unicode characters might seem
  practical, but this would open up for both duplicate file names and
  other inconsistent behavior. Consider a directory containing a file
  named &quot;björn&quot; in ISO-latin-1, while the Erlang VM is
  operating in Unicode file name mode (and therefore expecting UTF-8
  file naming). The ISO-latin-1 name is not valid UTF-8 and one could
  be tempted to think that automatic conversion in for example
  <c>file:list_dir/1</c> is a good idea. But what would happen if we
  later tried to open the file and have the name as a Unicode list
  (magically converted from the ISO-latin-1 file name)? The VM will
  convert the file name given to UTF-8, as this is the encoding
  expected. Effectively this means trying to open the file named
  &lt;&lt;&quot;björn&quot;/utf8&gt;&gt;. This file does not exist,
  and even if it existed it would not be the same file as the one that
  was listed. We could even create two files named &quot;björn&quot;,
  one named in the UTF-8 encoding and one not. If
  <c>file:list_dir/1</c> would automatically convert the ISO-latin-1
  file name to a list, we would get two identical file names as the
  result. To avoid this, we need to differentiate between file names
  being properly encoded according to the Unicode file naming
  convention (i.e. UTF-8) and file names being invalid under the
  encoding. By the common <c>file:list_dir/1</c> function, the wrongly
  encoded file names are simply ignored in Unicode file name
  translation mode, but by the <c>file:list_dir_all/1</c> function,
  the file names with invalid encoding are returned as &quot;raw&quot;
  file names, i.e. as binaries.</p> 

  <p>The Erlang <c>file</c> module accepts raw file names as
  input. <c>open_port({spawn_executable, ...} ...)</c> also accepts
  them. As mentioned earlier, the arguments given in the option list
  to <c>open_port({spawn_executable, ...}  ...)</c> undergo the same
  conversion as the file names, meaning that the executable will be
  provided with arguments in UTF-8 as well. This translation is
  avoided consistently with how the file names are treated, by giving
  the argument as a binary.</p>

  <p>To force Unicode file name translation mode on systems where this
  is not the default was considered experimental in Erlang/OTP R14B01 due to
  the fact that the initial implementation did not ignore wrongly
  encoded file names, so that raw file names could spread unexpectedly
  throughout the system. Beginning with Erlang/OTP R16B, the wrongly encoded file
  names are only retrieved by special functions
  (e.g. <c>file:list_dir_all/1</c>), so the impact on existing code is
  much lower, why it is now supported. Unicode file name translation
  is expected to be default in future releases.</p>

  <p>Even if you are operating without Unicode file naming translation
  automatically done by the VM, you can access and create files with
  names in UTF-8 encoding by using raw file names encoded as
  UTF-8. Enforcing the UTF-8 encoding regardless of the mode the
  Erlang VM is started in might, in some circumstances be a good idea,
  as the convention of using UTF-8 file names is spreading.</p>
</section>
<section>
  <title>Notes About MacOS X</title>
  <p>MacOS X's vfs layer enforces UTF-8 file names in a quite
  aggressive way. Older versions did this by simply refusing to create
  non UTF-8 conforming file names, while newer versions replace
  offending bytes with the sequence &quot;%HH&quot;, where HH is the
  original character in hexadecimal notation. As Unicode translation
  is enabled by default on MacOS X, the only way to come up against
  this is to either start the VM with the <c>+fnl</c> flag or to use a
  raw file name in bytewise (<c>latin1</c>) encoding. If using a raw
  filename, with a bytewise encoding containing characters between 127
  and 255, to create a file, the file can not be opened using the same
  name as the one used to create it. There is no remedy for this
  behaviour, other than keeping the file names in the right
  encoding.</p>

  <p>MacOS X also reorganizes the names of files so that the
  representation of accents etc is using the "combining characters",
  i.e. the character <c>ö</c> is represented as the code points
  [111,776], where 111 is the character <c>o</c> and 776 is the
  special accent character "combining diaeresis". This way of
  normalizing Unicode is otherwise very seldom used and Erlang
  normalizes those file names in the opposite way upon retrieval, so
  that file names using combining accents are not passed up to the
  Erlang application. In Erlang the file name &quot;björn&quot; is
  retrieved as [98,106,246,114,110], not as [98,106,117,776,114,110],
  even though the file system might think differently. The
  normalization into combining accents are redone when actually
  accessing files, so this can usually be ignored by the Erlang
  programmer.</p>
</section>
</section>
<section>
  <title>Unicode in Environment and Parameters</title>
  <marker id="unicode_in_environment_and_parameters"/>
  <p>Environment variables and their interpretation is handled much in
  the same way as file names. If Unicode file names are enabled,
  environment variables as well as parameters to the Erlang VM are
  expected to be in Unicode.</p>
  <p>If Unicode file names are enabled, the calls to 
  <seealso marker="kernel:os#getenv/0"><c>os:getenv/0</c></seealso>, 
  <seealso marker="kernel:os#getenv/1"><c>os:getenv/1</c></seealso> and
  <seealso marker="kernel:os#putenv/2"><c>os:putenv/2</c></seealso>
  will handle Unicode strings. On Unix-like platforms, the built-in
  functions will translate environment variables in UTF-8 to/from
  Unicode strings, possibly with code points > 255. On Windows the
  Unicode versions of the environment system API will be used, also
  allowing for code points > 255.</p>
  <p>On Unix-like operating systems, parameters are expected to be
  UTF-8 without translation if Unicode file names are enabled.</p>
</section>
<section>
  <title>Unicode-aware Modules</title>
  <p>Most of the modules in Erlang/OTP are of course Unicode-unaware
  in the sense that they have no notion of Unicode and really should
  not have. Typically they handle non-textual or byte-oriented data
  (like <c>gen_tcp</c> etc).</p>
  <p>Modules that actually handle textual data (like <c>io_lib</c>,
  <c>string</c> etc) are sometimes subject to conversion or extension
  to be able to handle Unicode characters.</p>
  <p>Fortunately, most textual data has been stored in lists and range
  checking has been sparse, why modules like <c>string</c> works well
  for Unicode lists with little need for conversion or extension.</p>
  <p>Some modules are however changed to be explicitly
  Unicode-aware. These modules include:</p>
  <taglist>
    <tag><c>unicode</c></tag>
    <item>
      <p>The module <seealso marker="stdlib:unicode"><c>unicode</c></seealso>
      is obviously Unicode-aware. It contains functions for conversion
      between different Unicode formats as well as some utilities for
      identifying byte order marks. Few programs handling Unicode data
      will survive without this module.</p>
    </item>
    <tag><c>io</c></tag>
    <item>
      <p>The <seealso marker="stdlib:io"><c>io</c></seealso> module has been
      extended along with the actual I/O-protocol to handle Unicode
      data. This means that several functions require binaries to be
      in UTF-8 and there are modifiers to formatting control sequences
      to allow for outputting of Unicode strings.</p>
    </item>
    <tag><c>file</c>, <c>group</c>, <c>user</c></tag>
    <item>
      <p>I/O-servers throughout the system are able to handle
      Unicode data and has options for converting data upon actual
      output or input to/from the device. As shown earlier, the
      <seealso marker="stdlib:shell"><c>shell</c></seealso> has support for
      Unicode terminals and the <seealso
      marker="kernel:file"><c>file</c></seealso> module allows for
      translation to and from various Unicode formats on disk.</p>
      <p>The actual reading and writing of files with Unicode data is
      however not best done with the <c>file</c> module as its
      interface is byte oriented. A file opened with a Unicode
      encoding (like UTF-8), is then best read or written using the
      <seealso marker="stdlib:io"><c>io</c></seealso> module.</p>
    </item>
    <tag><c>re</c></tag>
    <item>
      <p>The <seealso marker="stdlib:re"><c>re</c></seealso> module allows
      for matching Unicode strings as a special option. As the library
      is actually centered on matching in binaries, the Unicode
      support is UTF-8-centered.</p>
    </item>
    <tag><c>wx</c></tag>
    <item>
      <p>The <seealso marker="wx:wx"><c>wx</c></seealso> graphical library
      has extensive support for Unicode text</p>
    </item>
  </taglist>
  <p>The module <seealso
  marker="stdlib:string"><c>string</c></seealso> works perfectly for
  Unicode strings as well as for ISO-latin-1 strings with the
  exception of the language-dependent <seealso
  marker="stdlib:string#to_upper/1"><c>to_upper</c></seealso> and
  <seealso marker="stdlib:string#to_lower/1"><c>to_lower</c></seealso>
  functions, which are only correct for the ISO-latin-1 character
  set. Actually they can never function correctly for Unicode
  characters in their current form, as there are language and locale
  issues as well as multi-character mappings to consider when
  converting text between cases. Converting case in an international
  environment is a big subject not yet addressed in OTP.</p>
</section>
<section>
  <title>Unicode Data in Files</title>
  <p>The fact that Erlang as such can handle Unicode data in many forms
  does not automatically mean that the content of any file can be
  Unicode text. The external entities such as ports or I/O-servers are
  not generally Unicode capable.</p>
  <p>Ports are always byte oriented, so before sending data that you
  are not sure is bytewise encoded to a port, make sure to encode it
  in a proper Unicode encoding. Sometimes this will mean that only
  part of the data shall be encoded as e.g. UTF-8, some parts may be
  binary data (like a length indicator) or something else that shall
  not undergo character encoding, so no automatic translation is
  present.</p>
  <p>I/O-servers behave a little differently. The I/O-servers connected
  to terminals (or stdout) can usually cope with Unicode data
  regardless of the <c>encoding</c> option. This is convenient when
  one expects a modern environment but do not want to crash when
  writing to a archaic terminal or pipe. Files on the other hand are
  more picky. A file can have an encoding option which makes it
  generally usable by the io-module (e.g. <c>{encoding,utf8}</c>), but
  is by default opened as a byte oriented file. The <seealso
  marker="kernel:file"><c>file</c></seealso> module is byte oriented, why only
  ISO-Latin-1 characters can be written using that module. The
  <seealso marker="stdlib:io"><c>io</c></seealso> module is the one to use if
  Unicode data is to be output to a file with other <c>encoding</c>
  than <c>latin1</c> (a.k.a. bytewise encoding). It is slightly
  confusing that a file opened with
  e.g. <c>file:open(Name,[read,{encoding,utf8}])</c>, cannot be
  properly read using <c>file:read(File,N)</c> but you have to use the
  <c>io</c> module to retrieve the Unicode data from it. The reason is
  that <c>file:read</c> and <c>file:write</c> (and friends) are purely
  byte oriented, and should so be, as that is the way to access
  files other than text files - byte by byte. Just as with ports, you
  can of course write encoded data into a file by "manually" converting
  the data to the encoding of choice (using the <seealso
  marker="stdlib:unicode"><c>unicode</c></seealso> module or the bit syntax)
  and then output it on a bytewise encoded (<c>latin1</c>) file.</p>
  <p>The rule of thumb is that the <seealso
  marker="kernel:file"><c>file</c></seealso> module should be used for files
  opened for bytewise access (<c>{encoding,latin1}</c>) and the
  <seealso marker="stdlib:io"><c>io</c></seealso> module should be used when
  accessing files with any other encoding
  (e.g. <c>{encoding,uf8}</c>).</p>

  <p>Functions reading Erlang syntax from files generally recognize
  the <c>coding:</c> comment and can therefore handle Unicode data on
  input. When writing Erlang Terms to a file, you should insert
  such comments when applicable:</p>
  <pre>
$ <input>erl +fna +pc unicode</input>
Erlang R16B (erts-5.10.1) [source]  [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1  (abort with ^G)
1> <input>file:write_file("test.term",&lt;&lt;"%% coding: utf-8\n[{\"Юникод\",4711}].\n"/utf8&gt;&gt;).</input>
ok
2> <input>file:consult("test.term").</input>   
{ok,[[{"Юникод",4711}]]}
  </pre>
</section>
<section>
  <title>Summary of Options</title>
  <marker id="unicode_options_summary"/>
  <p>The Unicode support is controlled by both command line switches,
  some standard environment variables and the version of OTP you are
  using. Most options affect mainly the way Unicode data is displayed,
  not the actual functionality of the API's in the standard
  libraries. This means that Erlang programs usually do not
  need to concern themselves with these options, they are more for the
  development environment. An Erlang program can be written so that it
  works well regardless of the type of system or the Unicode options
  that are in effect.</p>

  <p>Here follows a summary of the settings affecting Unicode:</p>
  <taglist>
    <tag>The <c>LANG</c> and <c>LC_CTYPE</c> environment variables</tag>
    <item>
      <p>The language setting in the OS mainly affects the shell. The
      terminal (i.e. the group leader) will operate with <c>{encoding,
      unicode}</c> only if the environment tells it that UTF-8 is
      allowed. This setting should correspond to the actual terminal
      you are using.</p>
      <p>The environment can also affect file name interpretation, if
      Erlang is started with the <c>+fna</c> flag (which is default from
      Erlang/OTP 17.0).</p>
      <p>You can check the setting of this by calling
      <c>io:getopts()</c>, which will give you an option list
      containing <c>{encoding,unicode}</c> or
      <c>{encoding,latin1}</c>.</p>
    </item>
    <tag>The <c>+pc </c>{<c>unicode</c>|<c>latin1</c>} flag to 
    <seealso marker="erts:erl"><c>erl(1)</c></seealso></tag>
    <item>
      <p>This flag affects what is interpreted as string data when
      doing heuristic string detection in the shell and in
      <c>io</c>/<c>io_lib:format</c> with the <c>"~tp"</c> and
      <c>~tP</c> formatting instructions, as described above.</p>
      <p>You can check this option by calling io:printable_range/0,
      which will return <c>unicode</c> or <c>latin1</c>. To be
      compatible with future (expected) extensions to the settings,
      one should rather use <c>io_lib:printable_list/1</c> to check if
      a list is printable according to the setting. That function will
      take into account new possible settings returned from
      <c>io:printable_range/0</c>.</p>
    </item>
    <tag>The <c>+fn</c>{<c>l</c>|<c>a</c>|<c>u</c>}
    [{<c>w</c>|<c>i</c>|<c>e</c>}] 
    flag to <seealso marker="erts:erl"><c>erl(1)</c></seealso></tag>
    <item>
      <p>This flag affects how the file names are to be interpreted. On
      operating systems with transparent file naming, this has to be
      specified to allow for file naming in Unicode characters (and
      for correct interpretation of file names containing characters
      &gt; 255.</p>
      <p><c>+fnl</c> means bytewise interpretation of file names, which
      was the usual way to represent ISO-Latin-1 file names before
      UTF-8 file naming got widespread.</p>
      <p><c>+fnu</c> means that file names are encoded in UTF-8, which
      is nowadays the common scheme (although not enforced).</p>
      <p><c>+fna</c> means that you automatically select between
      <c>+fnl</c> and <c>+fnu</c>, based on the <c>LANG</c> and
      <c>LC_CTYPE</c> environment variables. This is optimistic
      heuristics indeed, nothing enforces a user to have a terminal
      with the same encoding as the file system, but usually, this is
      the case.  This is the default on all Unix-like operating
      systems except MacOS X.</p>

      <p>The file name translation mode can be read with the
      <c>file:native_name_encoding/0</c> function, which returns
      <c>latin1</c> (meaning bytewise encoding) or <c>utf8</c>.</p>
    </item>
    <tag><seealso marker="stdlib:epp#default_encoding/0">
    <c>epp:default_encoding/0</c></seealso></tag>
    <item>
      <p>This function returns the default encoding for Erlang source
      files (if no encoding comment is present) in the currently
      running release. In Erlang/OTP R16B <c>latin1</c> was returned (meaning
      bytewise encoding). In Erlang/OTP 17.0 and forward it returns
      <c>utf8</c>.</p>
      <p>The encoding of each file can be specified using comments as
      described in 
      <seealso marker="stdlib:epp#encoding"><c>epp(3)</c></seealso>.</p>
    </item>
    <tag><seealso marker="stdlib:io#setopts/1"><c>io:setopts/</c>{<c>1</c>,<c>2</c>}</seealso> and the <c>-oldshell</c>/<c>-noshell</c> flags.</tag>
    <item>
      <p>When Erlang is started with <c>-oldshell</c> or
      <c>-noshell</c>, the I/O-server for <c>standard_io</c> is default
      set to bytewise encoding, while an interactive shell defaults to
      what the environment variables says.</p>
      <p>With the <c>io:setopts/2</c> function you can set the
      encoding of a file or other I/O-server. This can also be set when
      opening a file. Setting the terminal (or other
      <c>standard_io</c> server) unconditionally to the option
      <c>{encoding,utf8}</c> will for example make UTF-8 encoded characters
      being written to the device regardless of how Erlang was started or
      the users environment.</p>
      <p>Opening files with <c>encoding</c> option is convenient when
      writing or reading text files in a known encoding.</p>
      <p>You can retrieve the <c>encoding</c> setting for an I/O-server
      using <seealso
      marker="stdlib:io#getopts/1"><c>io:getopts()</c></seealso>.</p>
    </item>
  </taglist>
</section>
<section>
  <title>Recipes</title>
  <p>When starting with Unicode, one often stumbles over some common
  issues. I try to outline some methods of dealing with Unicode data
  in this section.</p>
  <section>
    <title>Byte Order Marks</title>
    <p>A common method of identifying encoding in text-files is to put
    a byte order mark (BOM) first in the file. The BOM is the
    code point 16#FEFF encoded in the same way as the rest of the
    file. If such a file is to be read, the first few bytes (depending
    on encoding) is not part of the actual text. This code outlines
    how to open a file which is believed to have a BOM and set the
    files encoding and position for further sequential reading
    (preferably using the <seealso marker="stdlib:io"><c>io</c></seealso>
    module). Note that error handling is omitted from the code:</p>
<code>
open_bom_file_for_reading(File) -&gt;
    {ok,F} = file:open(File,[read,binary]),
    {ok,Bin} = file:read(F,4),
    {Type,Bytes} = unicode:bom_to_encoding(Bin),
    file:position(F,Bytes),
    io:setopts(F,[{encoding,Type}]),
    {ok,F}.
</code>
    <p>The <c>unicode:bom_to_encoding/1</c> function identifies the
    encoding from a binary of at least four bytes. It returns, along
    with an term suitable for setting the encoding of the file, the
    actual length of the BOM, so that the file position can be set
    accordingly. Note that <c>file:position/2</c> always works on
    byte-offsets, so that the actual byte-length of the BOM is
    needed.</p>
    <p>To open a file for writing and putting the BOM first is even
    simpler:</p>
<code>
open_bom_file_for_writing(File,Encoding) -&gt;
    {ok,F} = file:open(File,[write,binary]),
    ok = file:write(File,unicode:encoding_to_bom(Encoding)),
    io:setopts(F,[{encoding,Encoding}]),
    {ok,F}.
</code>
    <p>In both cases the file is then best processed using the
    <c>io</c> module, as the functions in <c>io</c> can handle code
    points beyond the ISO-latin-1 range.</p>
  </section>
  <section>
    <title>Formatted I/O</title>
    <p>When reading and writing to Unicode-aware entities, like the
    User or a file opened for Unicode translation, you will probably
    want to format text strings using the functions in <seealso
    marker="stdlib:io"><c>io</c></seealso> or <seealso
    marker="stdlib:io_lib"><c>io_lib</c></seealso>. For backward
    compatibility reasons, these functions do not accept just any list
    as a string, but require a special <em>translation modifier</em>
    when working with Unicode texts. The modifier is <c>t</c>. When
    applied to the <c>s</c> control character in a formatting string,
    it accepts all Unicode code points and expect binaries to be in
    UTF-8:</p>
    <pre>
1> <input>io:format("~ts~n",[&lt;&lt;"åäö"/utf8&gt;&gt;]).</input>
åäö
ok
2> <input>io:format("~s~n",[&lt;&lt;"åäö"/utf8&gt;&gt;]).</input>
åäö
ok</pre>
    <p>Obviously the second <c>io:format/2</c> gives undesired output
    because the UTF-8 binary is not in latin1. For backward
    compatibility, the non prefixed <c>s</c> control character expects
    bytewise encoded ISO-latin-1 characters in binaries and lists
    containing only code points &lt; 256.</p>
    <p>As long as the data is always lists, the <c>t</c> modifier can
    be used for any string, but when binary data is involved, care
    must be taken to make the right choice of formatting characters. A
    bytewise encoded binary will also be interpreted as a string and
    printed even when using <c>~ts</c>, but it might be mistaken for a
    valid UTF-8 string and one should therefore avoid using the
    <c>~ts</c> control if the binary contains bytewise encoded
    characters and not UTF-8.</p>
    <p>The function <c>format/2</c> in <c>io_lib</c> behaves
    similarly. This function is defined to return a deep list of
    characters and the output could easily be converted to binary data
    for outputting on a device of any kind by a simple
    <c>erlang:list_to_binary/1</c>. When the translation modifier is
    used, the list can however contain characters that cannot be
    stored in one byte. The call to <c>erlang:list_to_binary/1</c>
    will in that case fail. However, if the I/O server you want to
    communicate with is Unicode-aware, the list returned can still be
    used directly:</p>
<pre>
$ <input>erl +pc unicode</input>
Erlang R16B (erts-5.10.1) [source] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.10.1 (abort with ^G)
1> <input>io_lib:format("~ts~n", ["Γιούνικοντ"]).</input>
["Γιούνικοντ","\n"]
2> <input>io:put_chars(io_lib:format("~ts~n", ["Γιούνικοντ"])).</input>
Γιούνικοντ
ok</pre>
    <p>The Unicode string is returned as a Unicode list, which is
    recognized as such since the Erlang shell uses the Unicode
    encoding (and is started with all Unicode characters considered
    printable). The Unicode list is valid input to the <seealso
    marker="stdlib:io#put_chars/2"><c>io:put_chars/2</c></seealso> function,
    so data can be output on any Unicode capable device. If the device
    is a terminal, characters will be output in the <c>\x{</c>H
    ...<c>}</c> format if encoding is <c>latin1</c> otherwise in UTF-8
    (for the non-interactive terminal - "oldshell" or "noshell") or
    whatever is suitable to show the character properly (for an
    interactive terminal - the regular shell). The bottom line is that
    you can always send Unicode data to the <c>standard_io</c>
    device. Files will however only accept Unicode code points beyond
    ISO-latin-1 if <c>encoding</c> is set to something else than
    <c>latin1</c>.</p>
  </section>
  <section>
    <title>Heuristic Identification of UTF-8</title> 
    <p>While it is
    strongly encouraged that the actual encoding of characters in
    binary data is known prior to processing, that is not always
    possible. On a typical Linux system, there is a mix of UTF-8
    and ISO-latin-1 text files and there are seldom any BOM's in the
    files to identify them.</p>
    <p>UTF-8 is designed in such a way that ISO-latin-1 characters
    with numbers beyond the 7-bit ASCII range are seldom considered
    valid when decoded as UTF-8. Therefore one can usually use
    heuristics to determine if a file is in UTF-8 or if it is encoded
    in ISO-latin-1 (one byte per character) encoding. The
    <c>unicode</c> module can be used to determine if data can be
    interpreted as UTF-8:</p>
    <code>
heuristic_encoding_bin(Bin) when is_binary(Bin) -&gt;
    case unicode:characters_to_binary(Bin,utf8,utf8) of
	Bin ->
	    utf8;
	_ ->
	    latin1
    end.
    </code>
    <p>If one does not have a complete binary of the file content, one
    could instead chunk through the file and check part by part. The
    return-tuple <c>{incomplete,Decoded,Rest}</c> from
    <c>unicode:characters_to_binary/{1,2,3}</c> comes in handy. The
    incomplete rest from one chunk of data read from the file is
    prepended to the next chunk and we therefore circumvent the
    problem of character boundaries when reading chunks of bytes in
    UTF-8 encoding:</p>
    <code>
heuristic_encoding_file(FileName) -&gt;
    {ok,F} = file:open(FileName,[read,binary]),
    loop_through_file(F,&lt;&lt;&gt;&gt;,file:read(F,1024)).

loop_through_file(_,&lt;&lt;&gt;&gt;,eof) -&gt;
    utf8;
loop_through_file(_,_,eof) -&gt;
    latin1;
loop_through_file(F,Acc,{ok,Bin}) when is_binary(Bin) -&gt;
    case unicode:characters_to_binary([Acc,Bin]) of
	{error,_,_} ->
	    latin1;
	{incomplete,_,Rest} ->
	    loop_through_file(F,Rest,file:read(F,1024));
	Res when is_binary(Res) ->
	    loop_through_file(F,&lt;&lt;&gt;&gt;,file:read(F,1024))
    end.
    </code>
    <p>Another option is to try to read the whole file in UTF-8
    encoding and see if it fails. Here we need to read the file using
    <c>io:get_chars/3</c>, as we have to succeed in reading characters
    with a code point over 255:</p>
    <code>
heuristic_encoding_file2(FileName) -&gt;
    {ok,F} = file:open(FileName,[read,binary,{encoding,utf8}]),
    loop_through_file2(F,io:get_chars(F,'',1024)).

loop_through_file2(_,eof) -&gt;
    utf8;
loop_through_file2(_,{error,_Err}) -&gt;
    latin1;
loop_through_file2(F,Bin) when is_binary(Bin) -&gt;
    loop_through_file2(F,io:get_chars(F,'',1024)).
    </code>
  </section>
  <section>
    <title>Lists of UTF-8 Bytes</title>
    <p>For various reasons, you may find yourself having a list of
    UTF-8 bytes. This is not a regular string of Unicode characters as
    each element in the list does not contain one character. Instead
    you get the "raw" UTF-8 encoding that you have in binaries. This
    is easily converted to a proper Unicode string by first converting
    byte per byte into a binary and then converting the binary of
    UTF-8 encoded characters back to a Unicode string:</p>
    <code>
  utf8_list_to_string(StrangeList) ->
    unicode:characters_to_list(list_to_binary(StrangeList)).
    </code>
  </section>
  <section>
    <title>Double UTF-8 Encoding</title>
    <p>When working with binaries, you may get the horrible "double
    UTF-8 encoding", where strange characters are encoded in your
    binaries or files that you did not expect. What you may have got,
    is a UTF-8 encoded binary that is for the second time encoded as
    UTF-8. A common situation is where you read a file, byte by byte,
    but the actual content is already UTF-8. If you then convert the
    bytes to UTF-8, using i.e. the <c>unicode</c> module or by
    writing to a file opened with the <c>{encoding,utf8}</c>
    option. You will have each <i>byte</i> in the in the input file
    encoded as UTF-8, not each character of the original text (one
    character may have been encoded in several bytes). There is no
    real remedy for this other than being very sure of which data is
    actually encoded in which format, and never convert UTF-8 data
    (possibly read byte by byte from a file) into UTF-8 again.</p>
    <p>The by far most common situation where this happens, is when
    you get lists of UTF-8 instead of proper Unicode strings, and then
    convert them to UTF-8 in a binary or on a file:</p>
    <code>
  wrong_thing_to_do() ->
    {ok,Bin} = file:read_file("an_utf8_encoded_file.txt"),
    MyList = binary_to_list(Bin), %% Wrong! It is an utf8 binary!
    {ok,C} = file:open("catastrophe.txt",[write,{encoding,utf8}]), 
    io:put_chars(C,MyList), %% Expects a Unicode string, but get UTF-8
                            %% bytes in a list!
    file:close(C). %% The file catastrophe.txt contains more or less unreadable
                   %% garbage!
    </code>
    <p>Make very sure you know what a binary contains before
    converting it to a string. If no other option exists, try
    heuristics:</p>
    <code>
  if_you_can_not_know() ->
    {ok,Bin} = file:read_file("maybe_utf8_encoded_file.txt"),
    MyList = case unicode:characters_to_list(Bin) of
      L when is_list(L) ->
        L;
      _ ->
        binary_to_list(Bin) %% The file was bytewise encoded
    end,
    %% Now we know that the list is a Unicode string, not a list of UTF-8 bytes
    {ok,G} = file:open("greatness.txt",[write,{encoding,utf8}]), 
    io:put_chars(G,MyList), %% Expects a Unicode string, which is what it gets!
    file:close(G). %% The file contains valid UTF-8 encoded Unicode characters!
    </code>
  </section>
</section>
</chapter>