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.