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.