View on
MetaCPAN is shutting down
For details read Perl NOC. After June 25th this page will redirect to
Darren Duncan > Locale-KeyedText-v1.73.0 > Locale::KeyedText



Annotate this POD

View/Report Bugs
Module Version: 1.73.0   Source   Latest Release: Locale-KeyedText-2.0.0


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


This document describes Locale::KeyedText version 1.73.0.

It also describes the same-number versions of Locale::KeyedText::Message ("Message") and Locale::KeyedText::Translator ("Translator").

Note that the "Locale::KeyedText" package serves only as the name-sake representative for this whole file, which can be referenced as a unit by documentation or 'use' statements or Perl archive indexes. Aside from 'use' statements, you should never refer directly to "Locale::KeyedText" in your code; instead refer to other above-named packages in this file.


    use Locale::KeyedText;


    sub main {
        # Create a translator.
        my $translator = Locale::KeyedText::Translator->new({
            'set_names' => ['MyLib::Lang::', 'MyApp::Lang::'],
                # set package prefixes for localized app components
            'member_names' => ['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(
                'msg_key' => 'MYAPP_PROMPT' }) );

        # Read two numbers from the user.
        my ($first, $second) = <STDIN>;

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

    package MyLib; # module

    sub add_two {
        my (undef, $first, $second, $translator) = @_;
        my $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::Message->new({ 'msg_key' => 'MYLIB_RESULT',
                'msg_vars' => { 'FIRST' => $first, 'SECOND' => $second,
                'RESULT' => $sum } }) );



Many times during a program's operation, the program (or a package 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 packages 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.

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 package that would otherwise use it. Hereafter, if any other package 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 the /examples directory of this distribution. Or, see my dependent CPAN packages whose problem domain is databases and/or SQL.

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 package, 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 Locale::KeyedText 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 packages or programs. Your package 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 package which uses this would come up with keys which are unique across all Perl packages (perhaps the key name can start with the package name?). An advantage of this is that, for example, your package could come with a set of user messages, but another package 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 package. 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 packages which have the same method names as you or each other, or having multiple search directories in your path that packages could be installed in.

Generally, when a program package 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.

Compared to Other Solutions

One of the main distinctions of this approach over similar packages is that text is always looked up by a key which is not meant to be meaningful for a user. Whereas, with the other packages 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 packages 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 non-trivial external dependencies; Locale::KeyedText has no non-trivial 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.


The interface of Locale::KeyedText is entirely object-oriented; you use it by creating objects from its member classes, usually invoking new() on the appropriate class name, and then invoking methods on those objects. All of their attributes are private, so you must use accessor methods. Locale::KeyedText does not declare any subroutines or export such.

The usual way that Locale::KeyedText indicates a failure is to throw an exception; most often this is due to invalid input. If an invoked routine simply returns, you can assume that it has succeeded, even if the return value is undefined.

The Locale::KeyedText::Message Class

A Message object is a simple container which stores data to be used or displayed by your program. The Message class is pure and deterministic, such that all of its class and object methods will each return the same result and/or make the same change to an object when the permutation of its arguments and any invocant object's attributes is identical; they do not interact with the outside environment at all.

A Message object has two main attributes:

$!msg_key - Message Key

Str - This uniquely identifies the type of message that the object represents (or gives the name of a condition being reported, if it is used as an exception payload). 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. The Message Key can be any defined and non-empty string.

%!msg_vars - Message Variables

Hash(Str) of Any - This contains zero or more variable names and values that are associated with the message, 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 name can be any defined and non-empty string, and each variable value can be anything at all. Note that while the Hash itself is copied on input and output, any variable values which are references will be passed by reference, so you may store references to other objects in them if you wish.

This is the main Message constructor method:

new( :$msg_key!, :%msg_vars? )

This method creates and returns a new Locale::KeyedText::Message object. The Message Key attribute of the new object is set from the named parameter $msg_key (a string); the optional named parameter %msg_vars (a hash ref) sets the "Message Variables" attribute if the corresponding argument is provided (it defaults to empty if the argument is not provided).

Some example usage:

    my $message = Locale::KeyedText::Message->new({
        'msg_key' => 'FOO_GOT_NO_ARGS' });
    my $message2 = Locale::KeyedText::Message->new({
        'msg_key' => 'TABLE_COL_NO_EXIST',
        'msg_vars' => {
            'GIVEN_TABLE_NAME' => $table_name,
            'GIVEN_COL_NAME' => $col_name,
        } });

Note that a Message object does not permit changes to its attributes; they must all be set when the object is constructed. If you want to conceptually change an existing Message object, you must create a new object that is a clone of the first but for the changes.

A Message object has these methods:


This method returns a deep copy of this Message as a Hash ref of 2 elements, which correspond to the 2 named parameters of new().


This method returns the Message Key attribute of its object.

get_msg_var( $var_name! )

This method returns the Message Variable value (a string) associated with the variable name specified in the positional parameter $var_name (a string).


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


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 attribute values are escaped and you shouldn't try to extract them. This method is also defined as the implicit handler when coercing this object to a string.

The Template Modules

Locale::KeyedText doesn't define any "Template" modules, but it expects you to make modules having a specific simple API that will serve their role.

For example, inside the text Template file "MyApp/L/" you can have:

    use Readonly;
    Readonly my %TEXT_STRINGS => (
        'MYAPP_HELLO' => q[Welcome to MyApp.],
        'MYAPP_GOODBYE' => q[Goodbye!],
            => q[Enter a number to be inverted, or press ENTER to quit.],
        'MYAPP_RESULT' => q[The inverse of "<ORIGINAL>" is "<INVERTED>".],

    { package MyApp::L::Eng; # module
        sub get_text_by_key {
            my (undef, $msg_key) = @_;
            return $TEXT_STRINGS{$msg_key};
    } # module MyApp::L::Eng

And inside the text Template file "MyApp/L/" you can have:

    use Readonly;
    Readonly my %TEXT_STRINGS => (
        'MYAPP_HELLO' => q[Bienvenue allé MyApp.],
        'MYAPP_GOODBYE' => q[Salut!],
            => q[Fournir nombre être inverser, ou appuyer sur]
               . q[ ENTER être arrêter.],
        'MYAPP_RESULT' => q[Renversement "<ORIGINAL>" est "<INVERTED>".],

    { package MyApp::L::Fre; # module
        sub get_text_by_key {
            my (undef, $msg_key) = @_;
            return $TEXT_STRINGS{$msg_key};
    } # module MyApp::L::Fre

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 attribute 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 modules to come from Perl modules, but in the future they may alternately be something else, such as XML or tab-delimited plain text files.

The Locale::KeyedText::Translator Class

While a Translator object stores some attributes for configuration, its main purpose is to convert Message objects on demand into user-readable message strings, using data from external Template modules as a template. The Translator class as a whole is not pure and deterministic because it invokes user-defined external files for reading, mainly in the translate_message() method, but it has no other side effects.

A Translator object has 2 main attributes:

@!set_names - Set Names

Array of Str - This stores an ordered list of one or more elements 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. Each Set Name can be any defined and non-empty string.

@!member_names - Member Names

Array of Str - This stores an ordered list one or more elements 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 Set Name can be any defined and non-empty string.

This is the main Translator constructor method:

new( :@set_names!, :@member_names! )

This method creates and returns a new Locale::KeyedText::Translator object. The Set Names property of the new object is set from the named parameter @set_names (an array ref), and Member Names is set from the named parameter @member_names (an array ref).

Some example usage:

    my $translator = Locale::KeyedText::Translator->new({
        'set_names' => ['Foo::L::','Bar::L::'],
        'member_names' => ['Eng', 'Fre', 'Ger'] });
    my $translator2 = Locale::KeyedText::Translator->new({
        'set_names' => ['Foo::L::'], 'member_names' => ['Eng'] });

Note that a Translator object does not permit changes to its attributes; they must all be set when the object is constructed. If you want to conceptually change an existing Translator object, you must create a new object that is a clone of the first but for the changes.

A Translator object has these methods:


This method returns a deep copy of this Translator as a Hash ref of 2 elements, which correspond to the 2 named parameters of new().


This method returns all Set Names elements in this object as an array ref.


This method returns all Member Names elements in this object as an array ref.


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 attribute values are escaped and you shouldn't try to extract them. This method is also defined as the implicit handler when coercing this object to a string.


This method returns an array ref having all combinations of this object's Set Names and Member Names elements, concatenated in the form "<Set><Member>". All combinations having the same Member Name are adjacent to each other in the output; for example, with Sets of ['MyApp','MyLib'] and Members of ['Eng','Fre'], the resulting list is ['MyAppEng','MyLibEng','MyAppFre','MyLibFre']. This method is used internally by translate_message() to produce the list of Template module names that it will search.

translate_message( $message! )

This method takes a (machine-readable) Message object as its positional parameter $message 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.

Some example usage:

    my $user_text_string = $translator->translate_message( $message );

The Translator class also has these utility methods, which are all used by translate_message() to handle the trickier parts of its work:

template_module_is_loaded( $module_name! )

This method takes the name of a Perl package in its positional parameter $module_name (a string) and checks whether or not it has already been loaded, returning true if so and false if not.

load_template_module( $module_name! )

This method takes the name of a Perl package in its positional parameter $module_name (a string) and tries to load it using 'require'.

get_template_text_from_loaded_module( $module_name!, $msg_key! )

This method takes the name of a Perl package in its positional parameter $module_name (a string), and a Message Key in its positional parameter $msg_key (a string). Assuming that a Perl module by the given module name is already loaded, it tries to invoke $module_name.get_text_by_key( $msg_key ) and return that subroutine's result, which is a Template text string if the module recognizes $msg_key, and the undefined value if not.

interpolate_vars_into_template_text( $text!, %msg_vars! )

This method takes a defined (but possibly empty) Template text string in its positional parameter $text (a string), and a Message Variables hash ref in its positional parameter %msg_vars. It returns a copy of $text modified by interpolating the %msg_vars into it, where each variable value is substituted for any occurance of its corresponding variable name that is bounded by '<' and '>'. For example, given "Hello <place>!" in $text and "{ 'place' => 'World' }" in %msg_vars, it will return "Hello World!". All occurances of any given variable name will be replaced, non-recursively, and any "<foo>" not matched by a variable name will be left intact.


This documentation is pending.


This documentation is pending.


This file requires any version of Perl 5.x.y that is at least 5.8.1.

It also requires the Perl 5 packages version and only, which would conceptually be built-in to Perl, but aren't, so they are on CPAN instead.

It also requires these Perl 5 packages that are on CPAN: Readonly-(1.03...).

It also requires these Perl 5 packages that are on CPAN: Class::Std-(0.0.8...), Class::Std::Utils-(0.0.2...).

It also requires these Perl 5 packages that are bundled with Perl: Scalar::Util.


None reported.


These Perl 5 packages are the initial main dependents of Locale::KeyedText: Rosetta::Model, Rosetta, Rosetta::Validator, Rosetta::Engine::Example, Rosetta::Shell.

These Perl 5 packages work to solve similar problems as Locale::KeyedText: Locale::Maketext, Locale::gettext, Locale::PGetText, DBIx::BabelKit.


This documentation is pending.


Darren R. Duncan (


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

Locale::KeyedText is Copyright (c) 2003-2006, Darren R. Duncan. All rights reserved. Address comments, suggestions, and bug reports to, or visit 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 (; 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., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, 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.


Jason Martin (

On 2004.07.26, suggested a feature, and provided sample usage and patch code, that supports embedding of Template modules into the same files as program code, rather than requiring separate files.

Stevan Little (

On 2005.03.21, provided feedback towards improving this module's documentation, particularly towards using a much shorter and non-intimidating SYNOPSIS.

syntax highlighting: