<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>1996</year>
<year>2012</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
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
The Initial Developer of the Original Code is Ericsson AB.
</legalnotice>
<title>unicode</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
</header>
<module>unicode</module>
<modulesummary>Functions for converting Unicode characters</modulesummary>
<description>
<p>This module contains functions for converting between different character representations. Basically it converts between iso-latin-1 characters and Unicode ditto, but it can also convert between different Unicode encodings (like UTF-8, UTF-16 and UTF-32).</p>
<p>The default Unicode encoding in Erlang is in binaries UTF-8, which is also the format in which built in functions and libraries in OTP expect to find binary Unicode data. In lists, Unicode data is encoded as integers, each integer representing one character and encoded simply as the Unicode codepoint for the character.</p>
<p>Other Unicode encodings than integers representing codepoints or UTF-8 in binaries are referred to as "external encodings". The iso-latin-1 encoding is in binaries and lists referred to as latin1-encoding.</p>
<p>It is recommended to only use external encodings for communication with external entities where this is required. When working inside the Erlang/OTP environment, it is recommended to keep binaries in UTF-8 when representing Unicode characters. Latin1 encoding is supported both for backward compatibility and for communication with external entities not supporting Unicode character sets.</p>
</description>
<datatypes>
<datatype>
<name name="encoding"/>
</datatype>
<datatype>
<name name="endian"/>
</datatype>
<datatype>
<name name="unicode_binary"/>
<desc>
<p>A binary() with characters encoded in the UTF-8 coding standard.</p>
</desc>
</datatype>
<datatype>
<name name="unicode_char"/>
<desc>
<p>An integer() representing a valid unicode codepoint.</p>
</desc>
</datatype>
<datatype>
<name name="chardata"/>
</datatype>
<datatype>
<name name="charlist"/>
<desc>
<p>A unicode_binary is allowed as the tail of the list.</p>
</desc>
</datatype>
<datatype>
<name name="external_unicode_binary"/>
<desc>
<p>A <c>binary()</c> with characters coded in a user specified Unicode
encoding other than UTF-8 (UTF-16 or UTF-32).</p>
</desc>
</datatype>
<datatype>
<name name="external_chardata"/>
</datatype>
<datatype>
<name name="external_charlist"/>
<desc>
<p>An <c>external_unicode_binary()</c> is allowed as the tail
of the list.</p>
</desc>
</datatype>
<datatype>
<name name="latin1_binary"/>
<desc><p>A <c>binary()</c> with characters coded in iso-latin-1.</p>
</desc>
</datatype>
<datatype>
<name name="latin1_char"/>
<desc><p>An <c>integer()</c> representing valid latin1
character (0-255).</p>
</desc>
</datatype>
<datatype>
<name name="latin1_chardata"/>
</datatype>
<datatype>
<name name="latin1_charlist"/>
<desc><p>A <c>latin1_binary()</c> is allowed as the tail of
the list.</p>
</desc>
</datatype>
</datatypes>
<funcs>
<func>
<name name="bom_to_encoding" arity="1"/>
<fsummary>Identify UTF byte order marks in a binary.</fsummary>
<type name="endian"/>
<type_desc variable="Bin">A binary() of byte_size 4 or more.</type_desc>
<desc>
<p>Check for a UTF byte order mark (BOM) in the beginning of a
binary. If the supplied binary <c><anno>Bin</anno></c> begins with a valid
byte order mark for either UTF-8, UTF-16 or UTF-32, the function
returns the encoding identified along with the length of the BOM
in bytes.</p>
<p>If no BOM is found, the function returns <c>{latin1,0}</c></p>
</desc>
</func>
<func>
<name name="characters_to_list" arity="1"/>
<fsummary>Convert a collection of characters to list of Unicode characters</fsummary>
<desc>
<p>Same as characters_to_list(<anno>Data</anno>,unicode).</p>
</desc>
</func>
<func>
<name name="characters_to_list" arity="2"/>
<fsummary>Convert a collection of characters to list of Unicode characters</fsummary>
<desc>
<p>This function converts a possibly deep list of integers and
binaries into a list of integers representing unicode
characters. The binaries in the input may have characters
encoded as latin1 (0 - 255, one character per byte), in which
case the <c><anno>InEncoding</anno></c> parameter should be given as
<c>latin1</c>, or have characters encoded as one of the
UTF-encodings, which is given as the <c><anno>InEncoding</anno></c>
parameter. Only when the <c><anno>InEncoding</anno></c> is one of the UTF
encodings, integers in the list are allowed to be grater than
255.</p>
<p>If <c><anno>InEncoding</anno></c> is <c>latin1</c>, the <c><anno>Data</anno></c> parameter
corresponds to the <c>iodata()</c> type, but for <c>unicode</c>,
the <c><anno>Data</anno></c> parameter can contain integers greater than 255
(unicode characters beyond the iso-latin-1 range), which would
make it invalid as <c>iodata()</c>.</p>
<p>The purpose of the function is mainly to be able to convert
combinations of unicode characters into a pure unicode
string in list representation for further processing. For
writing the data to an external entity, the reverse function
<seealso
marker="#characters_to_binary/3">characters_to_binary/3</seealso>
comes in handy.</p>
<p>The option <c>unicode</c> is an alias for <c>utf8</c>, as this is the
preferred encoding for Unicode characters in
binaries. <c>utf16</c> is an alias for <c>{utf16,big}</c> and
<c>utf32</c> is an alias for <c>{utf32,big}</c>. The <c>big</c>
and <c>little</c> atoms denote big or little endian
encoding.</p>
<p>If for some reason, the data cannot be converted, either
because of illegal unicode/latin1 characters in the list, or
because of invalid UTF encoding in any binaries, an error
tuple is returned. The error tuple contains the tag
<c>error</c>, a list representing the characters that could be
converted before the error occurred and a representation of the
characters including and after the offending integer/bytes. The
last part is mostly for debugging as it still constitutes a
possibly deep and/or mixed list, not necessarily of the same
depth as the original data. The error occurs when traversing the
list and whatever's left to decode is simply returned as is.</p>
<p>However, if the input <c><anno>Data</anno></c> is a pure binary, the third
part of the error tuple is guaranteed to be a binary as
well.</p>
<p>Errors occur for the following reasons:</p>
<list type="bulleted">
<item>Integers out of range - If <c><anno>InEncoding</anno></c> is
<c>latin1</c>, an error occurs whenever an integer greater
than 255 is found in the lists. If <c><anno>InEncoding</anno></c> is
of a Unicode type, an error occurs whenever an integer
<list type="bulleted">
<item>greater than <c>16#10FFFF</c>
(the maximum unicode character),</item>
<item>in the range <c>16#D800</c> to <c>16#DFFF</c>
(invalid range reserved for UTF-16 surrogate pairs)</item>
</list>
is found.
</item>
<item>UTF encoding incorrect - If <c><anno>InEncoding</anno></c> is
one of the UTF types, the bytes in any binaries have to be valid
in that encoding. Errors can occur for various
reasons, including "pure" decoding errors
(like the upper
bits of the bytes being wrong), the bytes are decoded to a
too large number, the bytes are decoded to a code-point in the
invalid unicode
range or encoding is "overlong", meaning that a
number should have been encoded in fewer bytes. The
case of a truncated UTF is handled specially, see the
paragraph about incomplete binaries below. If
<c><anno>InEncoding</anno></c> is <c>latin1</c>, binaries are always valid
as long as they contain whole bytes,
as each byte falls into the valid iso-latin-1 range.</item>
</list>
<p>A special type of error is when no actual invalid integers or
bytes are found, but a trailing <c>binary()</c> consists of too
few bytes to decode the last character. This error might occur
if bytes are read from a file in chunks or binaries in other
ways are split on non UTF character boundaries. In this case an
<c>incomplete</c> tuple is returned instead of the <c>error</c>
tuple. It consists of the same parts as the <c>error</c> tuple, but
the tag is <c>incomplete</c> instead of <c>error</c> and the
last element is always guaranteed to be a binary consisting of
the first part of a (so far) valid UTF character.</p>
<p>If one UTF characters is split over two consecutive
binaries in the <c><anno>Data</anno></c>, the conversion succeeds. This means
that a character can be decoded from a range of binaries as long
as the whole range is given as input without errors
occurring. Example:</p>
<code>
decode_data(Data) ->
case unicode:characters_to_list(Data,unicode) of
{incomplete,Encoded, Rest} ->
More = get_some_more_data(),
Encoded ++ decode_data([Rest, More]);
{error,Encoded,Rest} ->
handle_error(Encoded,Rest);
List ->
List
end.
</code>
<p>Bit-strings that are not whole bytes are however not allowed,
so a UTF character has to be split along 8-bit boundaries to
ever be decoded.</p>
<p>If any parameters are of the wrong type, the list structure
is invalid (a number as tail) or the binaries does not contain
whole bytes (bit-strings), a <c>badarg</c> exception is
thrown.</p>
</desc>
</func>
<func>
<name name="characters_to_binary" arity="1"/>
<fsummary>Convert a collection of characters to an UTF-8 binary</fsummary>
<desc>
<p>Same as characters_to_binary(Data, unicode, unicode).</p>
</desc>
</func>
<func>
<name name="characters_to_binary" arity="2"/>
<fsummary>Convert a collection of characters to an UTF-8 binary</fsummary>
<desc>
<p>Same as characters_to_binary(<anno>Data</anno>, <anno>InEncoding</anno>, unicode).</p>
</desc>
</func>
<func>
<name name="characters_to_binary" arity="3"/>
<fsummary>Convert a collection of characters to an UTF-8 binary</fsummary>
<desc>
<p>This function behaves as <seealso
marker="#characters_to_list/2">
characters_to_list/2</seealso>, but produces an binary
instead of a unicode list. The
<c><anno>InEncoding</anno></c> defines how input is to be interpreted if
binaries are present in the <c>Data</c>, while
<c><anno>OutEncoding</anno></c> defines in what format output is to be
generated.</p>
<p>The option <c>unicode</c> is an alias for <c>utf8</c>, as this is the
preferred encoding for Unicode characters in
binaries. <c>utf16</c> is an alias for <c>{utf16,big}</c> and
<c>utf32</c> is an alias for <c>{utf32,big}</c>. The <c>big</c>
and <c>little</c> atoms denote big or little endian
encoding.</p>
<p>Errors and exceptions occur as in <seealso
marker="#characters_to_list/2">
characters_to_list/2</seealso>, but the second element
in the <c>error</c> or
<c>incomplete</c> tuple will be a <c>binary()</c> and not a
<c>list()</c>.</p>
</desc>
</func>
<func>
<name name="encoding_to_bom" arity="1"/>
<fsummary>Create a binary UTF byte order mark from encoding.</fsummary>
<type_desc variable="Bin">A binary() of byte_size 4 or more.</type_desc>
<desc>
<p>Create an UTF byte order mark (BOM) as a binary from the
supplied <c><anno>InEncoding</anno></c>. The BOM is, if supported at all,
expected to be placed first in UTF encoded files or
messages.</p>
<p>The function returns <c><<>></c> for the
<c>latin1</c> encoding, there is no BOM for ISO-latin-1.</p>
<p>It can be noted that the BOM for UTF-8 is seldom used, and it
is really not a <em>byte order</em> mark. There are obviously no
byte order issues with UTF-8, so the BOM is only there to
differentiate UTF-8 encoding from other UTF formats.</p>
</desc>
</func>
</funcs>
</erlref>
