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man : Locale::TextDomain
Locale::TextDomUser3Contributed Perl DocumeLocale::TextDomain(3p)
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");
my @plurals = (N__ ("One world", "{num} worlds"),
N__ ("1 file", "%d files"));
my $question = __x ("Error reading file '{file}': {err}",
file => $file, err => $!);
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);
DESCRIPTION
The module Locale::TextDomain(3pm) provides a high-level interface to
Perl message translation.
Textdomains
When your 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:
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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 CPANS 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.
Locale::TextDomain extends the default strategy by a Perl specific
approach. Unless told otherwise, it will look for a directory
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LocaleData in every component 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::TextDoamin(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 every part of @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 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
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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
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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.
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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 angle brackets, and then
you call
print __x ("This is the {color} {thing}.\n",
thing => $thang,
color => $color);
The function __x() will take the additional hash and replace all
occurencies of the hash keys in angle brackets with the
corresponding values. Simple, readable, understandable to
translators, what else would you want? And if the translator
forgets, misspells or otherwise messes up some "variables", the
msgfmt(1) program, that is used to compile the textual translation
file into its binary representation will even choke on these errors
and refuse to compile the translation.
__n MSGID, MSGID_PLURAL, COUNT
Whew! That looks complicated ... It is best explained with an
example. We'll have another look at your vintage code:
if ($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(),
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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.
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];
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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.
N__n (ARG1, ...)
Does exactly the same as N__(). You will use this form if you have
to mark the strings as having plural forms.
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
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database and your translators have to be aware of HTML syntax when
translating 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 look 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
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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) 2002-2004, Guido Flohr <guidoATimperia.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 866:
=cut found outside a pod block. Skipping to next block.
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