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NAME

Locale::KeyedText - Refer to user messages in programs by keys

DEPENDENCIES

Perl Version: 6

Core Modules: none

Non-Core Modules: none

COPYRIGHT AND LICENSE

This file is part of the Locale::KeyedText library.

Locale::KeyedText is Copyright (c) 2003-2005, Darren R. Duncan. All rights reserved. Address comments, suggestions, and bug reports to perl@DarrenDuncan.net, or visit http://www.DarrenDuncan.net/ for more information.

Locale::KeyedText is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License (LGPL) as published by the Free Software Foundation (http://www.fsf.org/); either version 2.1 of the License, or (at your option) any later version. You should have received a copy of the LGPL as part of the Locale::KeyedText distribution, in the file named "LGPL"; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.

Any versions of Locale::KeyedText that you modify and distribute must carry prominent notices stating that you changed the files and the date of any changes, in addition to preserving this original copyright notice and other credits. Locale::KeyedText is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the LGPL for more details.

While it is by no means required, the copyright holders of Locale::KeyedText would appreciate being informed any time you create a modified version of Locale::KeyedText that you are willing to distribute, because that is a practical way of suggesting improvements to the standard version.

SYNOPSIS

        use Locale::KeyedText;

        main();

        sub main() {
                # Create a translator.
                my $translator = Locale::KeyedText.new_translator( 
                        ['MyLib::Lang::', 'MyApp::Lang::'],  # set package prefixes for localized app components
                        ['Eng', 'Fr', 'De', 'Esp']           # set list of available languages in order of preference
                );

                # This will print 'Enter 2 Numbers' in the first of the four languages that has a matching template available.
                print $translator.translate_message( Locale::KeyedText.new_message( 'MYAPP_PROMPT' ) );

                # Read two numbers from the user.
                my Num ($first, $second) = $*IN;

                # Print a statement giving the operands and their sum.
                MyLib.add_two( $first, $second, $translator );
        }

        module MyLib;

        sub add_two( Num $first, Num $second, Locale::KeyedText::Translator $translator ) {
                my Num $sum = $first + $second;

                # This will print '<FIRST> plus <SECOND> equals <RESULT>' in the first possible language.
                # For example, if the user inputs '3' and '4', it the output will be '3 plus 4 equals 7'.
                print $translator->translate_message( Locale::KeyedText->new_message( 'MYLIB_RESULT', 
                        { 'FIRST' => $first, 'SECOND' => $second, 'RESULT' => $sum } ) );
        }

Note that the above example only shows off a few of Locale::KeyedText's features; for a larger and more complete example, see the EXAMPLE PROGRAM documentation sections further below.

DESCRIPTION

Many times during a program's operation, the program (or a module it uses) will need to display a message to the user, or generate a message to be shown to the user. Sometimes this is an error message of some kind, but it could also be a prompt or response message for interactive systems.

If the program or any of its components are intended for widespread use then it needs to account for a variance of needs between its different users, such as their preferred language of communication, or their privileges regarding access to information details, or their technical skills. For example, a native French or Chinese speaker often prefers to communicate in those languages. Or, when viewing an error message, the application's developer should see more details than joe public would.

Alternately, sometimes a program will raise a condition or error that, while resembling a message that would be shown to a user, is in fact meant to be interpreted by the machine itself and not any human user. In some situations, a shared program component may raise such a condition, and one application may handle it internally, while another one displays it to the user instead.

Locale::KeyedText provides a simple but effective mechanism for applications and modules that empowers single binaries to support N locales or user types simultaneously, and that allows any end users to add support for new languages easily and without a recompile (such as by simply copying files), often even while the program is executing.

Locale::KeyedText gives your application the maximum amount of control as to what the user sees; it never outputs anything by itself to the user, but rather returns its results for calling code to output as it sees fit. It also does not make direct use of environment variables, which can aid in portability.

Locale::KeyedText itself is trivially easy to install, since it is written in pure Perl and it has no external dependencies of any kind.

Practically speaking, Locale::KeyedText doesn't actually do a lot internally; it exists mainly to document a certain localization methodology in an easily accessable manner, such that would not be possible if its functionality was subsumed into a larger module that would otherwise use it. Hereafter, if any other module or application says that it uses Locale::KeyedText, that is a terse way of saying that it subscribes to the localization methodology that is described here, and hence provides these benefits to developers and users alike.

For some practical examples of Locale::KeyedText in use, see my dependent CPAN modules whose problem domain is databases and/or SQL.

CLASSES IN THIS MODULE

This module is implemented by several object-oriented Perl 6 packages, each of which is referred to as a class. They are: Locale::KeyedText (the module's name-sake), Locale::KeyedText::Message (aka Message), and Locale::KeyedText::Translator (aka Translator).

While all 3 of the above classes are implemented in one module for convenience, you should consider all 3 names as being "in use"; do not create any modules or packages yourself that have the same names.

The Message and Translator classes do most of the work and are what you mainly use. The name-sake class mainly exists to guide CPAN in indexing the whole module, but it also provides a few wrapper functions over the other classes for your convenience; you never instantiate an object of Locale::KeyedText itself.

HOW IT WORKS

Modern programs or database systems often refer to an error condition by an internal code which is guaranteed to be unique for a situation, and this is mapped to a user-readable message at some point. For example, Oracle databases often have error codes in a format like 'ORA-03542'. These codes are "machine readable"; any application receiving such a code can identify it easily in its conditional logic, using a simple 'equals', and then the application can "do the right thing". No parsing or ambiguity involved. By contrast, if a program simply returned words for the user, such as 'error opening file', programs would have a harder time figuring out the best way to deal with it. But for displaying to users, easy messages are better.

I have found that when it comes to getting the most accurate program text for users, we still get the best results by having a human being write out that text themselves.

What Locale::KeyedText does is associate each member in a set of key-codes, which are hard-coded into your application or module, with one or more text strings to show human users. This association would normally be stored in a Perl file that defines and returns an anonymous hash definition. While it is obvious that people who would be writing the text would have to know how to edit Perl files, this shouldn't be a problem because Locale::KeyedText is only meant to be used with user text that is associated with hard-coded program conditions. In other words, this user text is *part of the program*, and not the program's users' own data; only someone already involved in making the program would be editing them. At the same time, this information is in separate resource files used by the program, so that if you wanted to upgrade or localize what text the user sees, you only have to update said separate resource files, and not change your main program.

Note that an update is planned for this module that will enable user text to be stored in non-Perl external files, such as a 2-column plain-text format that will be much easier for a non-programmer to edit. But the current Perl-based solution will also be kept due to its more dynamic capabilities.

I was inspired to have this organization partly by how Mac OS X manages its resources. It is the standard practice for Mac OS X programs, including the operating system itself, to have the user language data in separate files (usually XML files I think) from the main program binary. Each user language is in a separate file, and adding a localization to a Mac OS X program is as simple as adding a language file to the program package. No recompilation necessary. This is something that end users could do, although program package installers usually do it. An os-level preference / control-panel displays a list of all the languages your programs do or might have, and lets you arrange the list in order of preference. When you open a program, it will search for language files specific to the program in the order you chose so to pick a supported language closest to your preference. Presumably the messages in these files are looked up by the program using keys. Mac OS X (and the previous non-Unix Mac OS) handles lots of other program resources as data files as well, making them easy to upgrade.

Locale::KeyedText aims to bring this sort of functionality to Perl modules or programs. Your module or program can be distributed with one or more resource files containing text for users, and your program would use associated keys internally.

It is strongly suggested (but not required) that each Perl module which uses this would come up with keys which are unique across all Perl modules (perhaps the key name can start with the module name?). An advantage of this is that, for example, your module could come with a set of user messages, but another module or program which uses yours may wish to override some of your messages, showing other messages instead which are more appropriate to the context in which they are using your module. One can override simply by using the same key code with a new user message in one of their own resource files. At some appropriate place, usually in the main program, Locale::KeyedText can be given input that says what resource files it should use and in what order they should be consulted. When Locale::KeyedText is told to fetch the user message for a certain code, it returns the first one it finds. This also works for the multiple language or permissions issue; simply order the files appropriately in the search list. The analogy is similar to inheriting from multiple modules which have the same method names as you or each other, or having multiple search directories in your path that modules could be installed in.

Generally, when a program module would return a code-key to indicate a condition, often it will also provide some variable values to be interpolated into the user strings; Locale::KeyedText would also handle this.

A program generates a Message that contains all possibly useful details, so that each Template can optionally use them; but often a template will choose to show less than all of the available details depending on the intended viewer.

One of the main distinctions of this approach over similar modules is that text is always looked up by a key which is not meant to be meaningful for a user. Whereas, with the other modules like "gettext" it looks like you are supposed to pass in english text and they translate it, which could produce ambiguous results or associations. Or alternately, the other modules require your text data to be stored in a format other than Perl files. Or alternately they have a compiled C component or otherwise have external dependencies; Locale::KeyedText has no external dependencies (it is very simple).

There are other differences. Where other solutions take variables, they seem to be positional (like with 'sprintf'); whereas, Locale::KeyedText has named variables, which can be used in any order, or not used at all, or used multiple times. Locale::KeyedText is generally a simpler solution than alternatives, and doesn't know about language specific details like encodings or plurality.

My understanding of alternate solutions like "gettext" suggests that they use a compile-time macro-based approach to substitute the user's preferred language into the program code itself, so it then becomes a version of that language. By contrast, Locale::KeyedText does no compile time binding and will support multiple languages or locales simultaneously at run time.

MESSAGE OBJECT PROPERTIES

One object type that this module implements is the Message. It is a simple container which stores data to be used or displayed by your program. Using it has no side effects on your program's environment or its globals; it also has no external dependencies.

A Message object has two main properties:

  • Message Key - A short string which identifies what base message (or type of message) this object is an instance of. The key is intended to be read by a machine and mapped to a user-readable message; the key itself is not meant to be meaningful to a user. Alternately, if you decide to use Message objects like Exceptions, then the key would indicate what condition is being reported.

  • Message Variables - An associative array (hash ref) containing variable names and values that are associated with this Message instance, and can be interpolated into the human-readable version. Each variable name is a machine-readable short string; the allowed variable names you can have depend on the Message Key it is being used with (others are ignored). Each variable value should be a scalar of some kind.

Both a Message object's Message Key property and each of the keys in its Message Variables property must not be an empty string and may only contain Perl "word" (\w) characters.

TEMPLATE OBJECT PROPERTIES

Locale::KeyedText doesn't define any "Template" objects, but it expects you to make modules having a specific simple API that will serve their role. See the SYNOPSIS POD for examples of valid Template modules.

A Template module is very simple, consisting mainly of a data-stuffed hash and an accessor method to read values from it by key. Each template hash key corresponds to the Message Key property of a Message object, and each hash value contains the user-readable message text associated with the Message; this user string may also contain variable names that correspond to Message Variables, which will be substituted at run-time before the text is shown to the user.

Each Template module ideally comes as part of a set, at least one member large, with each set member being an an exclusive alternative for the rest of the set. There is a separate template module for each distinct "user language" (or "user type") for each distinct Message; each file can be shared by multiple Messages but the whole module must represent a single language.

The name of each Template module has two parts, the Set Name and the Member Name. The Set Name comes first and makes up most of the module name; it must be the same for every module in the same set as the current one. The Member Name comes next and is what distinguishes each module from others in its set. For maximum flexibility in their use, the full name of a module consists of the two parts concatenated without any delimiter. This means, for example, that the full module names in a set could be either [Foo::L::Eng, Foo::L::Fre, Foo::L::Ger] or [L::FooEng, L::FooFre, L::FooGer]; the latter is mainy useful if you want modules from multiple sets in the same disk directory. In the first example, the Set Name is "Foo::L::" and in the second it is "L::Foo".

A library could be distributed with a Template module set that is specific to it, another library likewise, and a program which uses both libraries could have yet another set for itself. When the program is run, it would determine either from a user config file or a user interface that the current user is fluent in (and prefers) language A but also understands language B. Later on, if for example the first library generates an error message and wants it shown to the user, the main program would check each of the 3 Template module sets in turn, looking at just the set member for each that corresponds to language A, looking for a match to said error message. If it finds one, then that is displayed; if not, it then checks each set's member for language B and displays that; and so on.

For the present, Locale::KeyedText expects its Template objects to come from Perl modules, but in the future they may alternately be something else, such as XML files.

TRANSLATOR OBJECT PROPERTIES

Another object type that this module implements is the Translator. While it stores some properties for configuration, its main purpose is to convert Message objects on demand into user-readable message strings, using data from external Template objects as a template.

A Translator object has 2 main properties:

  • Template Sets - An ordered array where each element is a Template module Set Name. When we have to translate a message, the corresponding Template modules will be searched in the order they appear in this array until a match for that message is found. Since a program or library may wish to override the user text of another library which it uses, the Template module for the program or first library should appear first in the array. This property is analogous to Perl's @ISA package variable.

  • Template Members - An ordered array where each element is a Template module Member Name and usually corresponds to a language like English or French. The order of these items corresponds to an individual user's (or user role's) preferences such that each says what language they prefer to communicate in, and what their backup choices are, in order, if preferred ones aren't supported by a program or its libraries. When translating a message, a match in found in the most preferred language is used.

Each of a Translator object's Template Sets and Template Members property elements must not be an empty string and may only contain characters that are valid in a Perl package name; the code presently disallows any characters that are not in [a-zA-Z0-9_:].

SYNTAX

This class does not export any functions or methods, so you need to call them using object notation. This means using Class.function() for functions and $object.method() for methods. If you are inheriting this class for your own modules, then that often means something like $self.method().

CONSTRUCTOR WRAPPER FUNCTIONS

These functions are stateless and can be invoked only off of the module name; they are thin wrappers over other methods and exist strictly for convenience.

new_message( MSG_KEY[, MSG_VARS] )

        my $message = Locale::KeyedText.new_message( 'INVALID_FOO_ARG', 
                { 'ARG_NAME' => 'BAR', 'GIVEN_VAL' => $bar_value } );

This function wraps Locale::KeyedText::Message.new( MSG_KEY[, MSG_VARS] ).

new_translator( SET_NAMES, MEMBER_NAMES )

        my $translator = Locale::KeyedText.new_translator( 
                ['Foo::L::','Bar::L::'], ['Eng', 'Fre', 'Ger'] );

This function wraps Locale::KeyedText::Translator.new( SET_NAMES, MEMBER_NAMES ).

MESSAGE CONSTRUCTOR FUNCTIONS AND METHODS

This function/method is stateless and can be invoked off of either the Message class name or an existing Message object, with the same result.

new( MSG_KEY[, MSG_VARS] )

        my $message = Locale::KeyedText::Message.new( 'FOO_GOT_NO_ARGS' );
        my $message2 = Locale::KeyedText::Message.new( 'INVALID_FOO_ARG', 
                { 'ARG_NAME' => 'BAR', 'GIVEN_VAL' => $bar_value } );
        my $message3 = $message.new( 'TABLE_NO_EXIST', { 'GIVEN_TABLE_NAME' => $table_name } );
        my $message4 = Locale::KeyedText::Message.new( 'TABLE_COL_NO_EXIST', 
                { 'GIVEN_TABLE_NAME' => $table_name, 'GIVEN_COL_NAME' => $col_name } );

This function creates a new Locale::KeyedText::Message object and returns it, assuming the method arguments are valid; if they are not, it returns undef. The Message Key property of the new object is set from the MSG_KEY string argument; the optional MSG_VARS hash ref argument sets the "Message Variables" property if provided (it defaults to empty if the argument is undefined).

MESSAGE OBJECT METHODS

These methods are stateful and may only be invoked off of Message objects.

get_message_key()

        my $msg_key = $message.get_message_key();

This method returns the Message Key property of this object.

get_message_variable( VAR_NAME )

        my $value = $message.get_message_variable( 'GIVEN_COL_NAME' );

This method returns the Message Variable value associated with the variable name specified in VAR_NAME.

get_message_variables()

        my %msg_vars = $message.get_message_variables();

This method returns all Message Variable names and values in this object as a hash ref.

TRANSLATOR CONSTRUCTOR FUNCTIONS AND METHODS

This function/method is stateless and can be invoked off of either the Translator class name or an existing Translator object, with the same result.

new_translator( SET_NAMES, MEMBER_NAMES )

        my $translator = Locale::KeyedText::Translator.new( 
                ['Foo::L::','Bar::L::'], ['Eng', 'Fre', 'Ger'] );
        my $translator2 = $translator.new( 'Foo::L::', 'Eng' );

This function creates a new Locale::KeyedText::Translator object and returns it, assuming the method arguments are valid; if they are not, it returns undef. The Template Sets property of the new object is set from the SET_NAMES array ref (or string) argument, and Template Members is set from MEMBER_NAMES.

TRANSLATOR OBJECT METHODS

These methods are stateful and may only be invoked off of Message objects.

get_template_set_names()

        my @set_names = $translator.get_template_set_names();

This method returns all Template Sets elements in this object as an array ref.

get_template_member_names()

        my @member_names = $translator.get_template_member_names();

This method returns all Template Members elements in this object as an array ref.

translate_message( MESSAGE )

        my $user_text_string = $translator.translate_message( $message );

This method takes a (machine-readable) Message object as its MESSAGE argument and returns an equivalent human readable text message string; this assumes that a Template corresponding to the Message could be found using the Translator object's Set and Member properties; if none could be matched, this method returns undef. This method could be considered to implement the 'main' functionality of Locale::KeyedText.

METHODS FOR DEBUGGING

These methods are stateful and may only be invoked off of either Message or Translator objects.

as_string()

        my $dump_string = $message.as_string();
        my $dump_string = $translator.as_string();

This method returns a stringified version of this object which is suitable for debugging purposes (such as to test that the object's contents look good at a glance); no property values are escaped and you shouldn't try to extract them.

EXAMPLE PROGRAM WITH ENTIRELY SEPARATED TEMPLATE MODULE FILES

The following demonstrates a simple library and a simple program that uses it; both are N-multi-lingual and ship with English and French support files. While there is no support file specific to the library for a certain third language, the one with the program also adds support to the library.

Content of shared library file 'MyLib.pm':

        module MyLib;

        use Locale::KeyedText;

        sub my_invert( Str $number ) returns Num {
                $number.defined or throw Locale::KeyedText.new_message( 'MYLIB_MYINV_NO_ARG' );
                $number ~~ m/\d/ or throw Locale::KeyedText.new_message( 
                        'MYLIB_MYINV_BAD_ARG', { 'GIVEN_VALUE' => $number } );
                $number == 0 and throw Locale::KeyedText.new_message( 'MYLIB_MYINV_RES_INF' );
                return 1 / $number;
        }

Content of English language Template file 'MyLib/L/Eng.pm':

        module MyLib::L::Eng;
        my Str %text_strings is constant = (
                'MYLIB_MYINV_NO_ARG' => 'my_invert(): argument NUMBER is missing',
                'MYLIB_MYINV_BAD_ARG' => 'my_invert(): argument NUMBER is not a number, it is "{GIVEN_VALUE}"',
                'MYLIB_MYINV_RES_INF' => 'my_invert(): result is infinite because argument NUMBER is zero',
        );
        sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }

Content of French language (rough manual translation) Template file 'MyLib/L/Fre.pm':

        module MyLib::L::Fre;
        my Str %text_strings is constant = (
                'MYLIB_MYINV_NO_ARG' => 'my_invert(): paramètre NUMBER est manquant',
                'MYLIB_MYINV_BAD_ARG' => 'my_invert(): paramètre NUMBER est ne nombre, il est "{GIVEN_VALUE}"',
                'MYLIB_MYINV_RES_INF' => 'my_invert(): aboutir a est infini parce que paramètre NUMBER est zero',
        );
        sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }

Content of main program 'MyApp.pl':

        use MyLib;
        use Locale::KeyedText;

        main( grep { $_ ~~ m/^<[a-zA-Z]>+$/ } @*ARGS ); # user indicates language as command line argument

        sub main( Str ?@user_lang_prefs = 'Eng' ) {
                my Locale::KeyedText::Translator $translator = Locale::KeyedText.new_translator( 
                        ['MyApp::L::', 'MyLib::L::'], @user_lang_prefs );
                show_message( $translator, Locale::KeyedText.new_message( 'MYAPP_HELLO' ) );
                LOOP: {
                        show_message( $translator, Locale::KeyedText.new_message( 'MYAPP_PROMPT' ) );
                        my Str $user_input = $*IN; $user_input.chomp;
                        $user_input or last LOOP; # user chose to exit program
                        try {
                                my Num $result = MyLib.my_invert( $user_input );
                                show_message( $translator, Locale::KeyedText.new_message( 'MYAPP_RESULT', 
                                        { 'ORIGINAL' => $user_input, 'INVERTED' => $result } ) );
                                CATCH {
                                        show_message( $translator, $! ); # input error, detected by library
                                }
                        };
                        redo LOOP;
                }
                show_message( $translator, Locale::KeyedText.new_message( 'MYAPP_GOODBYE' ) );
        }

        sub show_message( Locale::KeyedText::Translator $translator, Locale::KeyedText::Message $message ) returns Str {
                my Str $user_text = $translator.translate_message( $message );
                unless( $user_text ) {
                        print $*ERR "internal error: can't find user text for a message: \n"~
                                "   "~$message.as_string()~"\n"~
                                "   "~$translator.as_string()~"\n";
                        exit;
                }
                print $*OUT $user_text~"\n";
        }

Content of English language Template file 'MyApp/L/Eng.pm':

        module MyApp::L::Eng;
        my Str %text_strings is constant = (
                'MYAPP_HELLO' => 'Welcome to MyApp.',
                'MYAPP_GOODBYE' => 'Goodbye!',
                'MYAPP_PROMPT' => 'Enter a number to be inverted, or press ENTER to quit.',
                'MYAPP_RESULT' => 'The inverse of "{ORIGINAL}" is "{INVERTED}".',
        );
        sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }

Content of French language (rough manual translation) Template file 'MyApp/L/Fre.pm':

        module MyApp::L::Fre;
        my Str %text_strings is constant = (
                'MYAPP_HELLO' => 'Bienvenue allé MyApp.',
                'MYAPP_GOODBYE' => 'Salut!',
                'MYAPP_PROMPT' => 'Fournir nombre être inverser, ou appuyer sur ENTER être arrêter.',
                'MYAPP_RESULT' => 'Renversement "{ORIGINAL}" est "{INVERTED}".',
        );
        sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }

Content of alternate text Template file 'MyApp/L/Homer.pm':

        module MyApp::L::Homer;
        my Str %text_strings is constant = (
                'MYAPP_HELLO' => 'Light goes on!',
                'MYAPP_GOODBYE' => 'Light goes off!',
                'MYAPP_PROMPT' => 'Give me a county thingy, or push that big button instead.',
                'MYAPP_RESULT' => 'Turn "{ORIGINAL}" upside down and get "{INVERTED}", not "{ORIGINAL}".',
                'MYLIB_MYINV_NO_ARG' => 'Why you little ...!',
                'MYLIB_MYINV_BAD_ARG' => '"{GIVEN_VALUE}" isn\'t a county thingy!',
                'MYLIB_MYINV_RES_INF' => 'Don\'t you give me a big donut!',
        );
        sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }

ALTERNATE EXAMPLE PROGRAM WITH MOSTLY INTEGRATED TEMPLATE MODULES

A feature extension to Locale::KeyedText allows you to store your Template class packages inside the same files as your other program code, rather than the Templates being in their own files. This feature is in recognition to developers that want to reduce as much as possible the number of separate files in their program distribution, at the cost of not being able to update user text or add support for new languages separately from updating the program code files themselves (one of Locale::KeyedText's original design principles).

Keep in mind that both methods of storing Template class packages can be used at the same time. Translator.translate_message() will first check for an embedded package by the appropriate name and use that if it exists; if one does not then it will try to use the external file, as is standard practice.

The following is an altered version of the SYNOPSIS documentation that shows Template class packages for MyLib and MyApp embedded in the code files rather than being separate; this example totals 3 files instead of the old 7 files. Actually, it shows both methods together, with 4 embedded, 1 separate.

Content of shared library file 'MyLib.pm':

        module MyLib {
                use Locale::KeyedText;
                sub my_invert( Str $number ) returns Num {
                        $number.defined or throw Locale::KeyedText.new_message( 'MYLIB_MYINV_NO_ARG' );
                        $number ~~ m/\d/ or throw Locale::KeyedText.new_message( 
                                'MYLIB_MYINV_BAD_ARG', { 'GIVEN_VALUE' => $number } );
                        $number == 0 and throw Locale::KeyedText.new_message( 'MYLIB_MYINV_RES_INF' );
                        return 1 / $number;
                }
        }

        module MyLib::L::Eng {
                my Str %text_strings is constant = (
                        'MYLIB_MYINV_NO_ARG' => 'my_invert(): argument NUMBER is missing',
                        'MYLIB_MYINV_BAD_ARG' => 'my_invert(): argument NUMBER is not a number, it is "{GIVEN_VALUE}"',
                        'MYLIB_MYINV_RES_INF' => 'my_invert(): result is infinite because argument NUMBER is zero',
                );
                sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }
        }

        module MyLib::L::Fre {
                my Str %text_strings is constant = (
                        'MYLIB_MYINV_NO_ARG' => 'my_invert(): paramètre NUMBER est manquant',
                        'MYLIB_MYINV_BAD_ARG' => 'my_invert(): paramètre NUMBER est ne nombre, il est "{GIVEN_VALUE}"',
                        'MYLIB_MYINV_RES_INF' => 'my_invert(): aboutir a est infini parce que paramètre NUMBER est zero',
                );
                sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }
        }

Content of main program 'MyApp.pl':

        use MyLib;
        use Locale::KeyedText;

        main( grep { $_ ~~ m/^<[a-zA-Z]>+$/ } @*ARGS ); # user indicates language as command line argument

        sub main( Str ?@user_lang_prefs = 'Eng' ) {
                my Locale::KeyedText::Translator $translator = Locale::KeyedText.new_translator( 
                        ['MyApp::L::', 'MyLib::L::'], @user_lang_prefs );
                show_message( $translator, Locale::KeyedText.new_message( 'MYAPP_HELLO' ) );
                LOOP: {
                        show_message( $translator, Locale::KeyedText.new_message( 'MYAPP_PROMPT' ) );
                        my Str $user_input = $*IN; $user_input.chomp;
                        $user_input or last LOOP; # user chose to exit program
                        try {
                                my Num $result = MyLib.my_invert( $user_input );
                                show_message( $translator, Locale::KeyedText.new_message( 'MYAPP_RESULT', 
                                        { 'ORIGINAL' => $user_input, 'INVERTED' => $result } ) );
                                CATCH {
                                        show_message( $translator, $! ); # input error, detected by library
                                }
                        };
                        redo LOOP;
                }
                show_message( $translator, Locale::KeyedText.new_message( 'MYAPP_GOODBYE' ) );
        }

        sub show_message( Locale::KeyedText::Translator $translator, Locale::KeyedText::Message $message ) returns Str {
                my Str $user_text = $translator.translate_message( $message );
                unless( $user_text ) {
                        print $*ERR "internal error: can't find user text for a message: \n"~
                                "   "~$message.as_string()~"\n"~
                                "   "~$translator.as_string()~"\n";
                        exit;
                }
                print $*OUT $user_text~"\n";
        }

        module MyApp::L::Eng {
                my Str %text_strings is constant = (
                        'MYAPP_HELLO' => 'Welcome to MyApp.',
                        'MYAPP_GOODBYE' => 'Goodbye!',
                        'MYAPP_PROMPT' => 'Enter a number to be inverted, or press ENTER to quit.',
                        'MYAPP_RESULT' => 'The inverse of "{ORIGINAL}" is "{INVERTED}".',
                );
                sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }
        }

        module MyApp::L::Fre {
                my Str %text_strings is constant = (
                        'MYAPP_HELLO' => 'Bienvenue allé MyApp.',
                        'MYAPP_GOODBYE' => 'Salut!',
                        'MYAPP_PROMPT' => 'Fournir nombre être inverser, ou appuyer sur ENTER être arrêter.',
                        'MYAPP_RESULT' => 'Renversement "{ORIGINAL}" est "{INVERTED}".',
                );
                sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }
        }

Content of alternate text Template file 'MyApp/L/Homer.pm':

        module MyApp::L::Homer;
        my Str %text_strings is constant = (
                'MYAPP_HELLO' => 'Light goes on!',
                'MYAPP_GOODBYE' => 'Light goes off!',
                'MYAPP_PROMPT' => 'Give me a county thingy, or push that big button instead.',
                'MYAPP_RESULT' => 'Turn "{ORIGINAL}" upside down and get "{INVERTED}", not "{ORIGINAL}".',
                'MYLIB_MYINV_NO_ARG' => 'Why you little ...!',
                'MYLIB_MYINV_BAD_ARG' => '"{GIVEN_VALUE}" isn\'t a county thingy!',
                'MYLIB_MYINV_RES_INF' => 'Don\'t you give me a big donut!',
        );
        sub get_text_by_key( Str $msg_key ) returns Str { return %text_strings{$msg_key}; }

BUGS

I only have superficial memory recall of non-English languages. I made the French language example in the SYNOPSIS by manually translating the English one with a printed on paper English to French dictionary; it probably contains multiple grammatical errors (not that the English was well-formed either).

CAVEATS

All Locale::KeyedText functions and methods currently will fail silently if they are given bad input; they will not throw any exceptions. (At the same time, however, the objects will always be internally consistent and can continue to be used.) This means, for example, if translate_message() fails because it tries to use a Template module (whose name you provide) that doesn't exist or that doesn't have the required API, it will return the same undef value that it returns if all named Template modules are correct but no match for the Message argument is found; it will do likewise if the given argument isn't a valid Message object. Locale::KeyedText will not provide any specifics as to why it failed. Depending on your usage, that may be exactly what you want, or it may not be. A large part of the reason for this silence is that Locale::KeyedText itself is supposed to be very simple and internally language independent; a thrown plain Perl exception would contain some detail in a specific user language. Likewise, a thrown Message object exception would require external files itself to resolve them, leading to recursive complexity. Suggestions for an alternate "proper" solution are welcome; meanwhile, the current solution seems best to me.

CREDITS

Besides myself as the creator ...

* 2004.07.26 - Thanks to Jason Martin (jhmartin@toger.us) for suggesting a feature, along with providing sample usage and patch code, that supports embedding of Template class packages in the same files as program code, rather than requiring separate files.

* 2005.03.21 - Thanks to Stevan Little (stevan@iinteractive.com) for feedback towards improving this module's documentation, particularly towards using a much shorter SYNOPSIS, so that it is easier for newcomers to understand the module at a glance, and not be intimidated by large amounts of detailed information.

SEE ALSO

Locale::Maketext, Locale::gettext, Locale::PGetText, DBIx::BabelKit.

1 POD Error

The following errors were encountered while parsing the POD:

Around line 680:

Non-ASCII character seen before =encoding in 'paramètre'. Assuming UTF-8