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.