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Andy Wardley
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NAME

Badger::Modules - a module for loading modules

SYNOPSIS

Example 1 - using a Badger::Modules object

    use Badger::Modules;
    
    my $modules = Badger::Modules->new( 
        path => ['My::Example','Your::Example'],
    );
    
    # load either My::Example::Foo or Your::Example::Foo
    my $foo_module = $modules->module('foo');
    
    # module() returns the module (class) name that was loaded
    my $foo_object = $foo_module->new;

Example 2 - creating a Badger::Modules subclass

    package My::Project::Modules;
    use base 'Badger::Modules';
    our $PATH = 'My::Project';      # or a list reference
    1;

Example 3 - using the Badger::Modules subclass

    use My::Project::Modules;

    # either by creating an object...
    my $modules = My::Project::Modules->new;
    my $module  = $modules->module('foo')
        || die $modules->reason;
    
    # ...or by calling class methods
    my $module  = My::Project::Modules->module('foo')
        || die $modules->reason;
    

Example 4 - pre-loading modules (TODO)

    # This doesn't work yet
    use My::Project::Module
        preload => 'foo bar baz';
    

Example 4 - pre-loading modules and exporting constants (MAYBE)

    # Not sure about this.
    use My::Project::Module
        modules => 'foo bar baz';
    
    my $object = FOO_MODULE->new;

DESCRIPTION

Badger::Modules is a module for dynamically loading other modules that live under a common namespace or namespaces.

It is ideally suited for loading plugin extension modules into an application when you don't know in advance which modules may be required. An example of such a module is Badger::Codecs which loads Badger::Codec modules for encoding and decoding data in various formats.

It can also be useful even when you do know what modules you want to use, or think you do. Delegating the task of loading modules to a central Badger::Modules object allows you to easily change the modules that are loaded at a later date, simply by changing the configuration for the Badger::Modules object.

Badger::Modules can be used as a stand-alone module or as a base class for creating specialised sub-classes of your own. A subclass of Badger::Modules can pre-define default values for configuration options. For example, you might want to create a Your::App::Plugins module that is configured to load modules under the Your::App::Plugin namespace by default.

Subclasses can also override methods to change the way it works or affect what happens after a module is loaded. The Badger::Factory module is an example of such a module. It provides additional methods for dynamically creating objects and relies on the underlying functionality provided by Badger::Modules to ensure that the relevant modules are loaded.

What's the Problem?

Consider the following code fragment showing a subroutine that creates and uses a Your::App::Widget object.

    use Your::App::Widget;
    
    sub some_code {
        my $widget = Your::App::Widget->new;
        $widget->do_something;
    }

One of the benefits of object oriented programming is that objects of equivalent types are interchangeable. That means that we should be able to replace the Your::App::Widget object with a different implementation as long as it has the same interface in terms of the methods it implements. In strictly typed programming languages this equivalence is enforced rigidly, by requiring that both objects share a common base class, expose the same interface, implement a particular role, or some other mechanism. In loosely typed languages like Perl, we have to rely on duck typing: if it looks like a duck, floats like a duck and quacks like a duck then it is a duck (or is close enough to being a duck for practical purposes).

For example, we might want to use a dummy widget object for test purposes.

    use Your::App::MockObject::Widget;
    
    sub some_code {
        my $widget = Your::App::MockObject::Widget->new;
        $widget->do_something;
    }

Or perhaps use a C implementation of a module on platforms that support it.

    use Your::App::XS::Widget;
    
    sub some_code {
        my $widget = Your::App::XS::Widget->new;
        $widget->do_something;
    }

Or maybe an implementation with additional debugging facilities for use during development, but not in production code.

    use Your::App::Developer::Widget;
    
    sub some_code {
        my $widget = Your::App::Developer::Widget->new;
        $widget->do_something;
    }

By now the problem should be apparent. To use a different implementation of the widget object we have to go and manually change the code. Every occurrence of Your::App::Widget in every module of your application must be changed to the new module name. Of course, if you were doing this in real life you would probably end up defining a variable to store the name of the relevant class. Something like this perhaps.

    use Your::App::Widget;
    our $WIDGET_CLASS = 'Your::App::Widget';
    
    sub some_code {
        my $widget = $WIDGET_CLASS->new;
        $widget->do_something;
    }

This works well in simple cases. However, if you've designed your application to be suitably modular (thereby promoting reusability of the individual components and extensibility of the system as a whole) then you may have a whole bunch of different modules to load, all of which need similar variables.

    use Your::App::Widget;
    use Your::App::Doodah;
    use Your::App::Thingy;
    our $WIDGET_CLASS = 'Your::App::Widget';
    our $DOODAH_CLASS = 'Your::App::Doodah';
    our $THINGY_CLASS = 'Your::App::Thingy';

Not only is the repetition of Your::App in the above code a red flag for refactoring in itself, but we also have to consider the issue of sharing these variables among the various modules that might need access to them. But before we fall too deep into that rabbit hole, let's jump through the looking glass and see how Badger::Modules can be used to tackle the problem.

Using Badger::Modules

Badger::Modules can be used as a stand-alone module for loading other modules in a particular namespace. The following example creates a Badger::Modules object for loading modules under the Your::App namespace.

    use Badger::Modules;
    
    my $modules = Badger::Modules->new( 
        path => 'Your::App' 
    );

Here we've created a Badger::Modules object which loads modules under the Your::App namespace. To create a Your::App::Widget object we can now write the following code.

    my $wclass = $modules->module('Widget');
    my $widget = $wclass->new;

The module() method maps the argument passed to a full class name (Your::App::Widget), loads the module if it hasn't already been loaded and then returns the class name. Of course we could combine this into a single expression:

    my $widget = $modules->module('Widget')->new;

The path configuration option can be specified as a reference to a list of namespaces. The Badger::Modules module will try each in turn until it finds a matching module.

    my $modules = Badger::Modules->new( 
        path => ['My::App','Your::App'],
    );

Now when you request the Widget module you'll get My::App::Widget returned if it exists or Your::App::Widget if it doesn't.

If neither is available then an error will be thrown as a Badger::Exception object containing an error message of the format module not found: Widget. You can set the item configuration option to something other than module to change this message. For example, setting the item to plugin will generate a plugin not found: Widget message.

    my $modules = Badger::Modules->new( 
        item => 'plugin',
        path => ['My::App::Plugin','Your::App::Plugin'],
    );

If you would rather have the module() method return undef to indicate that a module can't be found then set the tolerant configuration option to any true value.

    my $modules = Badger::Modules->new( 
        path     => ['My::App','Your::App'],
        tolerant => 1,
    );

It's then up to you to check the return value and handle the case where it is undefined. The error() method (inherited from the Badger::Base base class) can be used to return an error message for the purposes of friendly error reporting.

    my $module = $modules->module('Widget')
        || die $modules->error;         # module not found: Widget

Any other errors encountered while loading a module will be reported using croak, regardless of the tolerant option. These usually indicate syntax errors requiring immediate attention and thereby warrant the full backtrace that croak provides.

Mapping Names

[ROUGH DRAFT]

Name is tried as-is first.

    Your::App + Widget = Your::App::Widget

Then we try camel casing it.

    Your::App + nice_widget = Your::App::NiceWidget

This allows us to specify names in lower case with underscores separating words and have them automatically mapped to the correct CamelCase representation for module names.

Lower case + underscores not only looks nicer (IMHO, YMMV) but can also help to eliminate problems on filesystems like HFS that are case insensitive by default. If you're relying on the difference between say, CGI and cgi in a module name then you're going to have a world of pain the first time you (or someone else) tries to use that code on a shiny new Mac. And yes, that's me speaking from personal experience :-)

You may think this is a brain-dead stupid thing to do. You may be right. But there are brain-dead stupid filesystems out there that we have to accommodate.

Defining a Badger::Modules Subclass

The Badger::Modules module can be used as a base class for your own module-loading modules. Here's a complete example.

    package My::App::Plugins;
    use base 'Badger::Modules';
    our $PATH = ['My::App::Plugin', 'Your::App::Plugin'];
    1;

The $PATH package variable can be defined to provide the default search path. The $ITEM, $ITEMS, $NAMES and $TOLERANT package variables (not shown) can also be used to set the default values for the corresponding configuration options.

You can then use your subclass like this:

    use My::App::Plugins;
    my $plugins = My::App::Plugins->new;
    my $plugin  = $plugins->module('example');

This will load either My::App::Plugin::Example or Your::App::Plugin::Example, or throw an error to report that the example module can't be loaded.

You can provide additional configuration options when you create your subclass object. Any path elements specified will be searched after those defined in the $PATH package variable.

    use My::App::Plugins;
    my $plugins = My::App::Plugins->new(
        path => 'Our::App::Plugins',
    );
    my $plugin  = $plugins->module('example');

This will load My::App::Plugin::Example, Your::App::Plugin::Example, Our::App::Plugin::Example or throw an error.

Using Badger::Modules as a Singleton

You can call the module() method as a class method against Badger::Modules or any subclass of it.

    use My::App::Plugins;
    my $plugin  = My::App::Plugins->module('example');

In this case the module() method fetches a singleton prototype object to use (creating it via a call to new(), if necessary). The same prototype object will be re-used for any subsequent class methods.

CONFIGURATION OPTIONS

path / module_path

This options allows you to specify one or more base namespaces to search for modules. Multiple values can be specified by reference to an array.

    # single path location
    my $modules = Badger::Modules->new(
        path => 'My::Modules',
    );

    # multiple path locations
    my $modules = Badger::Modules->new(
        path => ['My::Modules', 'Your::Modules'],
    );

The module_path option is an alias for path.

    my $modules = Badger::Modules->new(
        module_path => ['My::Modules', 'Your::Modules'],
    );

If the item configuration option is specified then the name of the module_path option will be changed accordingly.

    # setting item to 'wibble' provides 'wibble_path' as alias to 'path'
    my $modules = Badger::Modules->new(
        item        => 'wibble',
        wibble_path => ['My::Modules', 'Your::Modules'],
    );

An $ITEM package variable defined in a subclass module has the same effect.

    package My::App::Plugins;
    use base 'Badger::Modules';
    our $ITEM = 'plugin';
    1;
    
    package main;
    use My::App::Plugins;
    
    my $plugins = My::App::Plugins->new(
        plugin_path => ['My::Plugin', 'Your::Plugin'],
    );

item

This option can be used to change the name of the items that the module loads. The default value is the generic term module. You may wish to set it to something else for the sake of having more meaningful configuration options, error messages, etc.

    my $modules = Badger::Modules->new(
        item        => 'wibble',
        wibble_path => ['My::Modules', 'Your::Modules'],
    );

A default value can be provided by a $ITEM package variable in a subclass of Badger::Modules.

    package My::App::Plugins;
    use base 'Badger::Modules';
    our $ITEM = 'plugin';
    1;

Another effect of setting item is that it allows you to specify the path option using the item name as a prefix.

    my $modules = Badger::Modules->new(
        item        => 'plugin',
        plugin_path => ['My::App::Plugin', 'Your::App::Plugin'],
    );

This can be useful if you've got several different module loaders in an application and want to avoid confusion between the different path configuration options.

items

This can be used to specify the correct plural form of the item name for those cases where the singular form does not pluralise regularly (where "regularly" is defined as something that the plural() function can handle.

    my $modules = Badger::Modules->new(
        # highly contrived example
        item  => 'attorney_general',
        items => 'attorneys_general',
    );

A default value can be provided by a $ITEMS package variable in a subclass of Badger::Modules.

    package My::App::Plugins;
    use base 'Badger::Modules';
    our $ITEM  = 'attorney_general';
    our $ITEMS = 'attorneys_general';
    1;

Note that this isn't used in the base class, but some subclasses rely on it to generate useful error messages.

names

This can be used to provide an explicit mapping for module names that may be requested via the module() method. The default behaviour is to camel case module names that are separated by underscores. For example, requesting a foo_bar module will look for a FooBar module in any of the path locations.

This work well enough for most modules, but some do not capitalise consistently. Modules whose names contain acronyms like URL are typically prone to a dose of fail.

    $module = $modules->module('url');      # looks for XXX::Url not XXX::URL

If you specify the name in the correct capitalisation then you'll have no problem.

    $module = $modules->module('URL');

If like me you prefer to use case-insensitive throughout and leave it up to the module loader to worry about the correct capitalisation then the names option is your friend. You can use to define any number of simple aliases for the module() method to use.

    $modules = Badger::Modules->new(
        path  => ['My::Plugin', 'Your::Plugin'],
        names => {
            url => 'URL',
            cgi => 'CGI',
            foo => 'iFoo::XS'
        }
    );

Note that the values specified in the names hash array are partial module names. They will still be applied to the base paths specified in the path option to generate complete candidate module paths.

tolerant

This option affects what happens when a module requested via the module() method cannot be found. In the usual case, the tolerant option is not set and the module() method will throw a "module not found: XXX" error. If the tolerant option is set then the method will instead return undef

METHODS

new()

Constructor method to create a new Badger::Modules object. Inherited from the Badger::Base base class.

module($name)

Method to load a module identified by $name.

[ROUGH DRAFT]

    * name is aliased via names lookup table

    * name is expanded to various possible capitalisations
    
    * each base namespace in path is tried...
    
    * with each name...
    
    * until one is located and loaded, in which case found() is called
      (or failed() if an error occurred while loading the module)
    
    * or we exhaust all possibilities, in which case not_found() is called.
    

modules()

This method can be used to get or set the internal mapping of names to modules. It's not used at present... there's some more refactoring to be done with Badger::Factory to sort out how this is going to work.

path()

Method to get or set the module search path. It returns a reference to a list of the current search path namespaces when called without any arguments.

    my $path = $modules->path;

It can be called with arguments to set a new search path. One or more modules namespaces can be specified as arguments:

    $modules->path('My::App', 'Your::App');

These can also be specified as a reference to an array.

    # either
    $modules->path(['My::App', 'Your::App']);
    
    # or
    @namespaces = ('My::App', 'Your::App');
    $modules->path(\@namespaces);

names()

INTERNAL METHODS

init_modules($config)

Internal initialisation method used to prepare newly created Badger::Modules objects. The init() method is an alias to init_modules() for the default new() method inherited from the Badger::Base base class to call.

If you subclass the Badger::Modules module and define your own init() method then it should call the init_modules() to perform the base class initialisation either before, after or in between blocks of your own code.

    package Your::Modules;
    use base 'Badger::Modules';
    
    sub init {
        my ($self, $config) = @_;
        # your code here
        $self->init_modules($config);
        # more of your code here
        return $self;
    }

This has the same effect as calling $self->SUPER::init($config) but with less ambiguity in the face of multiple inheritance (usually considered a bad thing to be avoided wherever possible) or in obscure cases where you are monkeying around with the heritage (i.e. base classes) of a module and Perl can't reliably resolve the correct init() method at compile time.

module_names($name)

This method maps the name passed as an argument to the correct case (or variations of case) for Perl modules.

TODO: More on this. Method should possibly be renamed to expand_names()?

found($name,$module)

This method is called by the module() method when a requested module is found. The implementation in the base class simply returns the module name passed to it as the second argument. This becomes the return value for the successful invocation of the module() method.

Subclasses may redefine this method to perform some other functionality.

not_found($name)

This method is called by the module() method when a requested module cannot be found. The default behaviour for this implementation in the base class throws an error (via the error method inherited from the Badger::Base base class). If the tolerant configuration option is set to a true value then it instead returns undef by calling the decline() method, also inherited from Badger::Base.

Subclasses may redefine this method to perform some other functionality.

failed($message)

This method is used internally to report a failure to load a module.

AUTHOR

Andy Wardley http://wardley.org/

COPYRIGHT

Copyright (C) 2006-2010 Andy Wardley. All Rights Reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO

Badger::Factory