Locale...Domain(3pm) User Contributed Perl Documentation Locale...Domain(3pm)
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 explicitly 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 different 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 available, 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 explicitly 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! While it is possible to reorder the arguments to
printf() and friends, it requires a syntax that is is nothing that
you want to learn.
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 existence 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
necessary 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 __npx ("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)
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.
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>";
CLASS METHODS
The following class methods are defined:
options
Returns a space-separated list of all '--keyword' and all '--flag'
options for xgettext(1), when extracting strings from Perl source
files localized with Locale::TextDomain.
The option should rather be called xgettextDefaultOptions. With
regard to the typical use-case, a shorter name has been picked:
xgettext `perl -MLocale::TextDomain -e 'print Locale::TextDomain->options'`
See
<https://www.gnu.org/software/gettext/manual/html_node/xgettext-Invocation.html>
for more information about the xgettext options '--keyword' and
'--flag'.
If you want to disable the use of the xgettext default keywords,
you should pass an option '--keyword=""' to xgettext before the
options returned by this method.
If you doubt the usefulness of this method, check the output on the
command-line:
perl -MLocale::TextDomain -e 'print Locale::TextDomain->options'
Nothing that you want to type yourself.
This method was added in libintl-perl 1.28.
keywords
Returns a space-separated list of all '--keyword' options for
xgettext(1) so that all translatable strings are properly
extracted.
This method was added in libintl-perl 1.28.
flags
Returns a space-separated list of all '--flag' options for
xgettext(1) so that extracted strings are properly flagged.
This method was added in libintl-perl 1.28.
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) 2002-2017 Guido Flohr <http://www.guido-flohr.net/>
(<mailto:guido.flohr@cantanea.com>), all rights reserved. See the
source code for details!code for details!
SEE ALSO
Locale::Messages(3pm), Locale::gettext_pp(3pm), perl(1), gettext(1),
gettext(3)
perl v5.38.2 2024-03-30 Locale...Domain(3pm)
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