Implementing support for Unicode character sets is an ongoing process. The Erlang Enhancement Proposal (EEP) 10 outlines the basics of Unicode support and also specifies a default encoding in binaries that all Unicode-aware modules should handle in the future.
The functionality described in EEP10 is implemented in Erlang/OTP as of R13A, but that is by no means the end of it. More functionality will be needed in the future and more OTP-libraries might need updating to cope with Unicode data.
This guide outlines the current Unicode support and gives a couple of recipes for working with Unicode data.
Unicode is a standard defining codepoints (numbers) for all known, living or dead, scripts. In principle, every known symbol used in any language has a Unicode codepoint.
Unicode codepoints are defined and published by the Unicode Consortium, which is a non profit organization.
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.
Along with the base of the standard, the codepoints for all the scripts, there are a couple of encoding standards available. 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.
The most widely spread encodings are:
Certain ranges of characters are left unused 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.
Additionally, the codepoint 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 is becoming more widely spread as they provide the means for programs to make educated guesses about the Unicode format of a certain file.
In Erlang, strings are actually lists of integers. A string is defined to be encoded in the ISO-latin-1 (ISO8859-1) character set, which is, codepoint by codepoint, a sub-range of the Unicode character set.
The standard list encoding for strings is therefore easily extendible to cope with the whole Unicode range: A Unicode string in Erlang is simply a list containing integers, each integer being a valid Unicode codepoint and representing one character in the Unicode character set.
Regular Erlang strings in ISO-latin-1 are a subset of their Unicode strings.
Binaries on the other hand 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
As the UTF-8 encoding is widely spread and provides the most compact storage, it is selected as the standard encoding of Unicode characters in binaries for Erlang.
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.
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
unicode_binary() = binary() with characters encoded in UTF-8 coding standard
unicode_char() = integer() >= 0 representing valid Unicode codepoint
chardata() = charlist() | unicode_binary()
charlist() = [unicode_char() | unicode_binary() | charlist()]
a unicode_binary is allowed as the tail of the list
The module
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() = [unicode_char() |
external_unicode_binary() |
external_charlist()]
an external_unicode_binary() is allowed as the tail of the list
The bit-syntax contains types for coping with binary data in the three main encodings. The types are named
<<Ch/utf8,_/binary>> = Bin1,
<<Ch/utf16-little,_/binary>> = Bin2,
Bin3 = <<$H/utf32-little, $e/utf32-little, $l/utf32-little, $l/utf32-little,
$o/utf32-little>>,
For convenience, literal strings can be encoded with a Unicode encoding in binaries using the following (or similar) syntax:
Bin4 = <<"Hello"/utf16>>,
For source code, there is an extension to the
In the shell, if using a Unicode input device,
7> $с. 1089
The interactive Erlang shell, when started towards a terminal or started using the
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 (
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
$ echo $LANG en_US.UTF-8
Actually, most systems handle the
$ echo $LC_CTYPE en_US.UTF-8
The
To investigate what Erlang thinks about the terminal, the
$ LC_CTYPE=en_US.ISO-8859-1 erl
Erlang R16B (erts-5.10) [source] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.10 (abort with ^G)
1> lists:keyfind(encoding, 1, io:getopts()).
{encoding,latin1}
2> q().
ok
$ LC_CTYPE=en_US.UTF-8 erl
Erlang R16B (erts-5.10) [source] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.10 (abort with ^G)
1> lists:keyfind(encoding, 1, io:getopts()).
{encoding,unicode}
2>
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.
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:
$ erl
Erlang R16B (erts-5.10) [source] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.10 (abort with ^G)
1> lists:keyfind(encoding, 1, io:getopts()).
{encoding,unicode}
2> "уницоде"
"уницоде"
3> io:format("~ts~n", [v(2)]).
уницоде
ok
4>
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:
$ erl Erlang R16B (erts-5.10) [source] [async-threads:0] [hipe] [kernel-poll:false] Eshell V5.10 (abort with ^G) 1> $ξ 958 2> уницоде. * 1: illegal character 2>
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:
Windows and, for most common uses, MacOSX enforces Unicode support for file names. All files created in the filesystem have names that can consistently be interpreted. In MacOSX, 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 "Unicode file name translation mode", 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.
Doing i.e. a
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 codepoints larger than 255, but the core Erlang system should have no problems with Unicode file names.
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 codepoints 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. If the VM would automatically select encoding based on heuristics, one could get unexpected behavior on these systems, therefore file names not being encoded in UTF-8 are returned as "raw file names" if Unicode file naming support is turned on.
A raw file name is not a list, but a binary. Many non core applications still do not handle file names given as binaries, why such raw names are avoided by default. This means that systems having implemented Unicode file naming through transparent file systems and an UTF-8 convention, do not by default have Unicode file naming turned on. Explicitly turning Unicode file name handling on for these types of systems is considered experimental.
The Unicode file naming support was introduced with OTP release R14B01. A VM operating in Unicode file 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 be able to 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.
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 MacOSX as the vfs layer of the OS will accept all your file names as UTF-8 and will not rewrite them.
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 is still however considered experimental in R14B01. Unicode file name translation is turned on with the
In Unicode file name mode, file names given to the BIF
It is worth noting that the file
Erlang drivers and NIF shared objects still can not be named with names containing codepoints 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.
Raw file names is introduced together with Unicode file name support in erts-5.8.2 (OTP R14B01). The reason "raw file names" is introduced in the system is 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 "björn" 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
The core system of Erlang (Kernel and STDLIB) accepts raw file names except for loadable drivers and executables invoked using
To force Unicode file name translation mode on systems where this is not the default is considered experimental in OTP R14B01 due to the raw file names possibly being a new experience to the programmer and that the non core applications of OTP are not tested for compliance with raw file names yet. Unicode file name translation is expected to be default in future releases.
If working with raw file names, one can still conform to the encoding convention of the Erlang VM by using the
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.
MacOSXs 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 "%HH", where HH is the original character in hexadecimal notation. As Unicode translation is enabled by default on MacOSX, the only way to come up against this is to either start the VM with the
MacOSX also reorganizes the names of files so that the representation of accents etc is denormalized, i.e. the character
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.
If Unicode file names are enabled, the calls to
On Unix-like operating systems, parameters are expected to be UTF-8 without translation if Unicode file names are enabled.
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
Modules that actually handle textual data (like
Fortunately, most textual data has been stored in lists and range checking has been sparse, why modules like
Some modules are however changed to be explicitly Unicode-aware. These modules include:
The module
The
I/O-servers throughout the system are able both to handle Unicode data and has options for converting data upon actual output or input to/from the device. As shown earlier, the
The actual reading and writing of files with Unicode data is however not best done with the
The
The
The module
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.
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 codepoint 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
open_bom_file_for_reading(File) ->
{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}.
The
To open a file for writing and putting the BOM first is even simpler:
open_bom_file_for_writing(File,Encoding) ->
{ok,F} = file:open(File,[write,binary]),
ok = file:write(File,unicode:encoding_to_bom(Encoding)),
io:setopts(F,[{encoding,Encoding}]),
{ok,F}.
In both cases the file is then best processed using the
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
1> io:format("~ts~n",[<<"åäö"/utf8>>]).
åäö
ok
2> io:format("~s~n",[<<"åäö"/utf8>>]).
åäö
ok
Obviously the second
As long as the data is always lists, the
The function
$ erl
Erlang R16B (erts-5.10) [source] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.10 (abort with ^G)
1> io_lib:format("~ts~n", ["θνιψοδε"]).
["θνιψοδε","\n"]
2> io:put_chars(io_lib:format("~ts~n", ["θνιψοδε"])).
θνιψοδε
ok
The Unicode string is returned as a Unicode list, which is recognized as such since the Erlang shell uses the Unicode encoding. The Unicode list is valid input to the
While it iss 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.
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
heuristic_encoding_bin(Bin) when is_binary(Bin) ->
case unicode:characters_to_binary(Bin,utf8,utf8) of
Bin ->
utf8;
_ ->
latin1
end.
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
heuristic_encoding_file(FileName) ->
{ok,F} = file:open(FileName,[read,binary]),
loop_through_file(F,<<>>,file:read(F,1024)).
loop_through_file(_,<<>>,eof) ->
utf8;
loop_through_file(_,_,eof) ->
latin1;
loop_through_file(F,Acc,{ok,Bin}) when is_binary(Bin) ->
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,<<>>,file:read(F,1024))
end.
Another option is to try to read the whole file in utf8 encoding and see if it fails. Here we need to read the file using
heuristic_encoding_file2(FileName) ->
{ok,F} = file:open(FileName,[read,binary,{encoding,utf8}]),
loop_through_file2(F,io:get_chars(F,'',1024)).
loop_through_file2(_,eof) ->
utf8;
loop_through_file2(_,{error,_Err}) ->
latin1;
loop_through_file2(F,Bin) when is_binary(Bin) ->
loop_through_file2(F,io:get_chars(F,'',1024)).