NAME

       Module::Optional - Breaking module dependency chains

SYNOPSIS

         use Bar::Dummy qw();
         use Module::Optional Bar;

ABSTRACT

       This module provides a way of using a module which may or may not be installed on the target machine. If
       the module is available it behaves as a straight use. If the module is not available, subs are repointed
       to their equivalents in a dummy namespace.

DESCRIPTION

       Suppose you are the developer of module "Foo", which uses functionality from the highly controversial
       module "Bar". You actually quite like "Bar", and want to reuse its functionality in your "Foo" module.
       But, many people will refuse to install "Foo" as it needs "Bar". Maybe "Bar" is failing tests or is
       misbehaving on some platforms.

       Making "Bar" an optional module will allow users to run "Foo" that don't have "Bar" installed. For
       Module::Build users, this involves changing the status of the "Bar" dependency from "requires" to
       "recommends".

       To use this module, you need to set up a namespace "Bar::Dummy". The recommended way of doing this is to
       ship lib/Bar/Dummy.pm with your module.  This could be shipped as a standalone module. A dummy module for
       "Params::Validate" is shipped with Module::Optional, as this was the original motivation for the module.
       If there are other common candidates for dummying, petition me, and I'll include them in the
       Module::Optional distribution.

   Using an optional module
       Place the lines of code in the following order:

         use Bar::Dummy qw();
         use Module::Optional qw(Bar quux wibble wobble);

       Always set up the dummy module first, but don't import anything - this is to avoid warnings about
       redefined subroutines if the real Bar is installed on the target machine. Module::Optional will do the
       importing: quux wibble and wobble from the real Bar if it exists, or from Bar::Dummy if it doesn't.

   Asking for a module version
       If you need a version of the module or later, this can be done thus:

         use Bar::Dummy qw();
         use Module::Optional qw(Bar 0.07 quux wibble wobble);

       If version 0.07 or later of Bar is not available, the dummy is used.

   Suppressing the module
       You will probably be developing your module on a platform that does have Bar installed (I hope). However,
       you need to be able to tell what happens on systems without Bar. To do this, run the following (example
       is Unix):

         MODULE_OPTIONAL_SKIP=1 make test

       You also want to do this in tests for the dummy module that you are providing. (You are providing tests
       for this module?) This can easily be done with a begin block at the top of the test:

         BEGIN {
             local $ENV{MODULE_OPTIONAL_SKIP} = 1;
             use Module::Optional qw(Params::Validate);
         }

   Writing a ::Dummy Module
       You provide a namespace suffixed with ::Dummy containing subs corresponding to all the subs and method
       calls for the optional module. You should also provide the same exports as the module itself performs.

       Adhere strictly to any prototypes in the optional module.

       An example of a dummy module is Params::Validate::Dummy, provided in this distribution.

INTERNALS

       Module::Optional performs two types of redirection for the missing module.  Firstly via @ISA inheritance
       - Foo::Bar inherits from Foo::Bar::Dummy.

       Secondly, an AUTOLOAD method is added to Foo::Bar, which will catch calls to subs in this namespace.

BUGS

       Please report bugs to rt.cpan.org by posting to bugs-module-optional@rt.cpan.org or visiting
       https://rt.cpan.org/Public/Dist/Display.html?Name=Module-Optional.

AUTHOR

               Ivor Williams
               ivorw-mod-opt at xemaps.com

COPYRIGHT

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

       The full text of the license can be found in the LICENSE file included with this module.

SEE ALSO

       Test::MockModule, Module::Pluggable, Module::Build.