Provided by: libintl-perl_1.23-1build1_all 
NAME
Locale::TextDomain - Perl Interface to Uniforum Message Translation
SYNOPSIS
use Locale::TextDomain ('my-package', @locale_dirs);
use Locale::TextDomain qw (my-package);
my $translated = __"Hello World!\n";
my $alt = $__{"Hello World!\n"};
my $alt2 = $__->{"Hello World!\n"};
my @list = (N__"Hello",
N__"World");
printf (__n ("one file read",
"%d files read",
$num_files),
$num_files);
print __nx ("one file read", "{num} files read", $num_files,
num => $num_files);
my $translated_context = __p ("Verb, to view", "View");
printf (__np ("Files read from filesystems",
"one file read",
"%d files read",
$num_files),
$num_files);
print __npx ("Files read from filesystems",
"one file read",
"{num} files read",
$num_files,
num => $num_files);
DESCRIPTION
The module Locale::TextDomain(3pm) provides a high-level interface to Perl message
translation.
Textdomains
When you request a translation for a given string, the system used in libintl-perl follows
a standard strategy to find a suitable message catalog containing the translation: Unless
you explicitely define a name for the message catalog, libintl-perl will assume that your
catalog is called 'messages' (unless you have changed the default value to something else
via Locale::Messages(3pm), method textdomain()).
You might think that his default strategy leaves room for optimization and you are right.
It would be a lot smarter if multiple software packages, all with their individual message
catalogs, could be installed on one system, and it should also be possible that third-
party components of your software (like Perl modules) can load their message catalogs,
too, without interfering with yours.
The solution is clear, you have to assign a unique name to your message database, and you
have to specify that name at run-time. That unique name is the so-called textdomain of
your software package. The name is actually arbitrary but you should follow these best-
practice guidelines to ensure maximum interoperability:
File System Safety
In practice, textdomains get mapped into file names, and you should therefore make
sure that the textdomain you choose is a valid filename on every system that will
run your software.
Case-sensitivity
Textdomains are always case-sensitive (i. e. 'Package' and 'PACKAGE' are not the
same). However, since the message catalogs will be stored on file systems, that
may or may not distinguish case when looking up file names, you should avoid
potential conflicts here.
Textdomain Should Match CPAN Name
If your software is listed as a module on CPAN, you should simply choose the name
on CPAN as your textdomain. The textdomain for libintl-perl is hence
'libintl-perl'. But please replace all periods ('.') in your package name with an
underscore because ...
Internet Domain Names as a Fallback
... if your software is not a module listed on CPAN, as a last resort you should
use the Java(tm) package scheme, i. e. choose an internet domain that you are
owner of (or ask the owner of an internet domain) and concatenate your preferred
textdomain with the reversed internet domain. Example: Your company runs the web-
site 'www.foobar.org' and is the owner of the domain 'foobar.org'. The textdomain
for your company's software 'barfoos' should hence be 'org.foobar.barfoos'.
If your software is likely to be installed in different versions on the same system, it is
probably a good idea to append some version information to your textdomain.
Other systems are less strict with the naming scheme for textdomains but the phenomena
known as Perl is actually a plethora of small, specialized modules and it is probably
wisest to postulate some namespace model in order to avoid chaos.
Binding textdomains to directories
Once the system knows the textdomain of the message that you want to get translated into
the user's language, it still has to find the correct message catalog. By default,
libintl-perl will look up the string in the translation database found in the directories
/usr/share/locale and /usr/local/share/locale (in that order).
It is neither guaranteed that these directories exist on the target machine, nor can you
be sure that the installation routine has write access to these locations. You can
therefore instruct libintl-perl to search other directories prior to the default
directories. Specifying a differnt search directory is called binding a textdomain to a
directory.
Beginning with version 1.20, Locale::TextDomain extends the default strategy by a Perl-
specific approach. If File::ShareDir is installed, it will look in the subdirectories
named locale and LocaleData (in that order) in the directory returned by
"File::ShareDir::dist_dir ($textdomain)" (if File::ShareDir is installed), and check for a
database containing the message for your textdomain there. This allows you to install
your database in the Perl-specific shared directory using Module::Install's
"install_share" directive or the Dist::Zilla ShareDir plugin.
If File::ShareDir is not availabe, or if Locale::TextDomain fails to find the translation
files in the File::ShareDir directory, it will next look in every directory found in the
standard include path @INC, and check for a database containing the message for your
textdomain there. Example: If the path /usr/lib/perl/5.8.0/site_perl is in your @INC, you
can install your translation files in /usr/lib/perl/5.8.0/site_perl/LocaleData, and they
will be found at run-time.
USAGE
It is crucial to remember that you use Locale::TextDomain(3) as specified in the section
"SYNOPSIS", that means you have to use it, not require it. The module behaves quite
differently compared to other modules.
The most significant difference is the meaning of the list passed as an argument to the
use() function. It actually works like this:
use Locale::TextDomain (TEXTDOMAIN, DIRECTORY, ...)
The first argument (the first string passed to use()) is the textdomain of your package,
optionally followed by a list of directories to search instead of the Perl-specific
directories (see above: /LocaleData appended to a File::ShareDir directory and every path
in @INC).
If you are the author of a package 'barfoos', you will probably put the line
use Locale::TextDomain 'barfoos';
resp. for non-CPAN modules
use Locale::TextDomain 'org.foobar.barfoos';
in every module of your package that contains translatable strings. If your module has
been installed properly, including the message catalogs, it will then be able to retrieve
these translations at run-time.
If you have not installed the translation database in a directory LocaleData in the
File::ShareDir directory or the standard include path @INC (or in the system directories
/usr/share/locale resp. /usr/local/share/locale), you have to explicitely specify a
search path by giving the names of directories (as strings!) as additional arguments to
use():
use Locale::TextDomain qw (barfoos ./dir1 ./dir2);
Alternatively you can call the function bindtextdomain() with suitable arguments (see the
entry for bindtextdomain() in "FUNCTIONS" in Locale::Messages). If you do so, you should
pass "undef" as an additional argument in order to avoid unnecessary lookups:
use Locale::TextDomain ('barfoos', undef);
You see that the arguments given to use() have nothing to do with what is imported into
your namespace, but they are rather arguments to textdomain(), resp. bindtextdomain().
Does that mean that Locale::TextDomain exports nothing into your namespace? Umh, not
exactly ... in fact it imports all functions listed below into your namespace, and hence
you should not define conflicting functions (and variables) yourself.
So, why has Locale::TextDomain to be different from other modules? If you have ever
written software in C and prepared it for internationalization (i18n), you will probably
have defined some preprocessor macros like:
#define _(String) dgettext ("my-textdomain", String)
#define N_(String) String
You only have to define that once in C, and the textdomain for your package is
automatically inserted into all gettext functions. In Perl there is no such mechanism (at
least it is not portable, option -P) and using the gettext functions could become quite
cumbersome without some extra fiddling:
print dgettext ("my-textdomain", "Hello world!\n");
This is no fun. In C it would merely be a
printf (_("Hello world!\n"));
Perl has to be more concise and shorter than C ... see the next section for how you can
use Locale::TextDomain to end up in Perl with a mere
print __"Hello World!\n";
EXPORTED FUNCTIONS
All functions have quite funny names on purpose. In fact the purpose for that is quite
clear: They should be short, operator-like, and they should not yell for conflicts with
existing functions in your namespace. You will understand it, when you internationalize
your first Perl program or module. Preparing it is more like marking strings as being
translatable than inserting function calls. Here we go:
__ MSGID
NOTE: This is a double underscore!
The basic and most-used function. It is a short-cut for a call to gettext() resp.
dgettext(), and simply returns the translation for MSGID. If your old code reads like
this:
print "permission denied";
You will now write:
print __"permission denied";
That's all, the string will be output in the user's preferred language, provided that
you have installed a translation for it.
Of course you can also use parentheses:
print __("permission denied");
Or even:
print (__("permission denied"));
In my eyes, the first version without parentheses looks best.
__x MSGID, ID1 => VAL1, ID2 => VAL2, ...
One of the nicest features in Perl is its capability to interpolate variables into
strings:
print "This is the $color $thing.\n";
This nice feature might con you into thinking that you could now write
print __"This is the $color $thing.\n";
Alas, that would be nice, but it is not possible. Remember that the function __()
serves both as an operator for translating strings and as a mark for translatable
strings. If the above string would get extracted from your Perl code, the un-
interpolated form would end up in the message catalog because when parsing your code
it is unpredictable what values the variables $thing and $color will have at run-time
(this fact is most probably one of the reasons you have written your program for).
However, at run-time, Perl will have interpolated the values already before __()
(resp. the underlying gettext() function) has seen the original string. Consequently
something like "This is the red car.\n" will be looked up in the message catalog, it
will not be found (because only "This is the $color $thing.\n" is included in the
database), and the original, untranslated string will be returned. Honestly, because
this is almost always an error, the xgettext(1) program will bail out with a fatal
error when it comes across that string in your code.
There are two workarounds for that:
printf __"This is the %s %s.\n", $color, $thing;
But that has several disadvantages: Your translator will only see the isolated string,
and without the surrounding code it is almost impossible to interpret it correctly.
Of course, GNU emacs and other software capable of editing PO translation files will
allow you to examine the context in the source code, but it is more likely that your
translator will look for a less challenging translation project when she frequently
comes across such messages.
And even if she does understand the underlying programming, what if she has to reorder
the color and the thing like in French:
msgid "This is the red car.\n";
msgstr "Cela est la voiture rouge.\n"
Zut alors! No way! You cannot portably reorder the arguments to printf() and friends
in Perl (it is possible in C, but at the time of this writing not supported in Perl,
and it would lead to other problems anyway).
So what? The Perl backend to GNU gettext has defined an alternative format for
interpolatable strings:
"This is the {color} {thing}.\n";
Instead of Perl variables you use place-holders (legal Perl variables are also legal
place-holders) in curly braces, and then you call
print __x ("This is the {color} {thing}.\n",
thing => $thang,
color => $color);
The function __x() will take the additional hash and replace all occurencies of the
hash keys in curly braces with the corresponding values. Simple, readable,
understandable to translators, what else would you want? And if the translator
forgets, misspells or otherwise messes up some "variables", the msgfmt(1) program,
that is used to compile the textual translation file into its binary representation
will even choke on these errors and refuse to compile the translation.
__n MSGID, MSGID_PLURAL, COUNT
Whew! That looks complicated ... It is best explained with an example. We'll have
another look at your vintage code:
if ($files_deleted > 1) {
print "All files have been deleted.\n";
} else {
print "One file has been deleted.\n";
}
Your intent is clear, you wanted to avoid the cumbersome "1 files deleted". This is
okay for English, but other languages have more than one plural form. For example in
Russian it makes a difference whether you want to say 1 file, 3 files or 6 files. You
will use three different forms of the noun 'file' in each case. [Note: Yep, very
smart you are, the Russian word for 'file' is in fact the English word, and it is an
invariable noun, but if you know that, you will also understand the rest despite this
little simplification ...].
That is the reason for the existance of the function ngettext(), that __n() is a
short-cut for:
print __n"One file has been deleted.\n",
"All files have been deleted.\n",
$files_deleted;
Alternatively:
print __n ("One file has been deleted.\n",
"All files have been deleted.\n",
$files_deleted);
The effect is always the same: libintl-perl will find out which plural form to pick
for your user's language, and the output string will always look okay.
__nx MSGID, MSGID_PLURAL, COUNT, VAR1 => VAL1, VAR2 => VAL2, ...
Bringing it all together:
print __nx ("One file has been deleted.\n",
"{count} files have been deleted.\n",
$num_files,
count => $num_files);
The function __nx() picks the correct plural form (also for English!) and it is
capable of interpolating variables into strings.
Have a close look at the order of arguments: The first argument is the string in the
singular, the second one is the plural string. The third one is an integer indicating
the number of items. This third argument is only used to pick the correct
translation. The optionally following arguments make up the hash used for
interpolation. In the beginning it is often a little confusing that the variable
holding the number of items will usually be repeated somewhere in the interpolation
hash.
__xn MSGID, MSGID_PLURAL, COUNT, VAR1 => VAL1, VAR2 => VAL2, ...
Does exactly the same thing as __nx(). In fact it is a common typo promoted to a
feature.
__p MSGCTXT, MSGID
This is much like __. The "p" stands for "particular", and the MSGCTXT is used to
provide context to the translator. This may be neccessary when your string is short,
and could stand for multiple things. For example:
print __p"Verb, to view", "View";
print __p"Noun, a view", "View";
The above may be "View" entries in a menu, where View->Source and File->View are
different forms of "View", and likely need to be translated differently.
A typical usage are GUI programs. Imagine a program with a main menu and the
notorious "Open" entry in the "File" menu. Now imagine, there is another menu entry
Preferences->Advanced->Policy where you have a choice between the alternatives "Open"
and "Closed". In English, "Open" is the adequate text at both places. In other
languages, it is very likely that you need two different translations. Therefore, you
would now write:
__p"File|", "Open";
__p"Preferences|Advanced|Policy", "Open";
In English, or if no translation can be found, the second argument (MSGID) is
returned.
This function was introduced in libintl-perl 1.17.
__px MSGCTXT, MSGID, VAR1 => VAL1, VAR2 => VAL2, ...
Like __p(), but supports variable substitution in the string, like __x().
print __px("Verb, to view", "View {file}", file => $filename);
See __p() and __x() for more details.
This function was introduced in libintl-perl 1.17.
__np MSGCTXT, MSGID, MSGID_PLURAL, COUNT
This adds context to plural calls. It should not be needed very often, if at all, due
to the __nx() function. The type of variable substitution used in other gettext
libraries (using sprintf-like sybols, like %s or %1) sometimes required context. For a
(bad) example of this:
printf (__np("[count] files have been deleted",
"One file has been deleted.\n",
"%s files have been deleted.\n",
$num_files),
$num_files);
NOTE: The above usage is discouraged. Just use the __nx() call, which provides inline
context via the key names.
This function was introduced in libintl-perl 1.17.
__npx MSGCTXT, MSGID, MSGID_PLURAL, COUNT, VAR1 => VAL1, VAR2 => VAL2, ...
This is provided for comleteness. It adds the variable interpolation into the string
to the previous method, __np().
It's usage would be like so:
print __nx ("Files being permenantly removed",
"One file has been deleted.\n",
"{count} files have been deleted.\n",
$num_files,
count => $num_files);
I cannot think of any situations requiring this, but we can easily support it, so here
it is.
This function was introduced in libintl-perl 1.17.
N__ (ARG1, ARG2, ...)
A no-op function that simply echoes its arguments to the caller. Take the following
piece of Perl:
my @options = (
"Open",
"Save",
"Save As",
);
...
my $option = $options[1];
Now say that you want to have this translatable. You could sometimes simply do:
my @options = (
__"Open",
__"Save",
__"Save As",
);
...
my $option = $options[1];
But often times this will not be what you want, for example when you also need the
unmodified original string. Sometimes it may not even work, for example, when the
preferred user language is not yet determined at the time that the list is
initialized.
In these cases you would write:
my @options = (
N__"Open",
N__"Save",
N__"Save As",
);
...
my $option = __($options[1]);
# or: my $option = dgettext ('my-domain', $options[1]);
Now all the strings in @options will be left alone, since N__() returns its arguments
(one ore more) unmodified. Nevertheless, the string extractor will be able to
recognize the strings as being translatable. And you can still get the translation
later by passing the variable instead of the string to one of the above translation
functions.
N__n (MSGID, MSGID_PLURAL, COUNT)
Does exactly the same as N__(). You will use this form if you have to mark the
strings as having plural forms.
N__p (MSGCTXT, MSGID)
Marks MSGID as N__() does, but in the context MSGCTXT.
N__np (MSGCTXT, MSGID, MSGID_PLURAL, COUNT)
Marks MSGID as N__n() does, but in the context MSGCTXT. =back
EXPORTED VARIABLES
The module exports several variables into your namespace:
%__ A tied hash. Its keys are your original messages, the values are their translations:
my $title = "<h1>$__{'My Homepage'}</h1>";
This is much better for your translation team than
my $title = __"<h1>My Homepage</h1>";
In the second case the HTML code will make it into the translation database and your
translators have to be aware of HTML syntax when translating strings.
Warning: Do not use this hash outside of double-quoted strings! The code in the tied
hash object relies on the correct working of the function caller() (see "perldoc -f
caller"), and this function will report incorrect results if the tied hash value is
the argument to a function from another package, for example:
my $result = Other::Package::do_it ($__{'Some string'});
The tied hash code will see "Other::Package" as the calling package, instead of your
own package. Consequently it will look up the message in the wrong text domain.
There is no workaround for this bug. Therefore:
Never use the tied hash interpolated strings!
$__ A reference to "%__", in case you prefer:
my $title = "<h1>$__->{'My Homepage'}</h1>";
PERFORMANCE
Message translation can be a time-consuming task. Take this little example:
1: use Locale::TextDomain ('my-domain');
2: use POSIX (:locale_h);
3:
4: setlocale (LC_ALL, '');
5: print __"Hello world!\n";
This will usually be quite fast, but in pathological cases it may run for several seconds.
A worst-case scenario would be a Chinese user at a terminal that understands the codeset
Big5-HKSCS. Your translator for Chinese has however chosen to encode the translations in
the codeset EUC-TW.
What will happen at run-time? First, the library will search and load a (maybe large)
message catalog for your textdomain 'my-domain'. Then it will look up the translation for
"Hello world!\n", it will find that it is encoded in EUC-TW. Since that differs from the
output codeset Big5-HKSCS, it will first load a conversion table containing several ten-
thousands of codepoints for EUC-TW, then it does the same with the smaller, but still very
large conversion table for Big5-HKSCS, it will convert the translation on the fly from
EUC-TW into Big5-HKSCS, and finally it will return the converted translation.
A worst-case scenario but realistic. And for these five lines of codes, there is not much
you can do to make it any faster. You should understand, however, when the different
steps will take place, so that you can arrange your code for it.
You have learned in the section "DESCRIPTION" that line 1 is responsible for locating your
message database. However, the use() will do nothing more than remembering your settings.
It will not search any directories, it will not load any catalogs or conversion tables.
Somewhere in your code you will always have a call to POSIX::setlocale(), and the
performance of this call may be time-consuming, depending on the architecture of your
system. On some systems, this will consume very little time, on others it will only
consume a considerable amount of time for the first call, and on others it may always be
time-consuming. Since you cannot know, how setlocale() is implemented on the target
system, you should reduce the calls to setlocale() to a minimum.
Line 5 requests the translation for your string. Only now, the library will actually load
the message catalog, and only now will it load eventually needed conversion tables. And
from now on, all this information will be cached in memory. This strategy is used
throughout libintl-perl, and you may describe it as 'load-on-first-access'. Getting the
next translation will consume very little resources.
However, although the translation retrieval is somewhat obfuscated by an operator-like
function call, it is still a function call, and in fact it even involves a chain of
function calls. Consequently, the following example is probably bad practice:
foreach (1 .. 100_000) {
print __"Hello world!\n";
}
This example introduces a lot of overhead into your program. Better do this:
my $string = __"Hello world!\n";
foreach (1 .. 100_000) {
print $string;
}
The translation will never change, there is no need to retrieve it over and over again.
Although libintl-perl will of course cache the translation read from the file system, you
can still avoid the overhead for the function calls.
AUTHOR
Copyright (C) 2002-2013, Guido Flohr <guido@imperia.net>, all rights reserved. See the
source code for details.
This software is contributed to the Perl community by Imperia (<http://www.imperia.net/>).
SEE ALSO
Locale::Messages(3pm), Locale::gettext_pp(3pm), perl(1), gettext(1), gettext(3)
POD ERRORS
Hey! The above document had some coding errors, which are explained below:
Around line 924:
You forgot a '=back' before '=head1'
Around line 1070:
=cut found outside a pod block. Skipping to next block.