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Alt - Alternate Module Implementations



    PERL_ALT_INSTALL=OVERWRITE cpanm Alt::IO::All::Redux


Alt:: is the namespace for alternate implementations of CPAN modules.

The purpose of the Alt module is to provide documentation explaining the Alt concept, how it works, and guidelines for using it well.


For a given piece of software, CPAN only allows for one implementation of a given module/distribution name.

GitHub on the other hand, is not limited this way. Any author can make a fork, since GitHub repos are namespaced by author id.

On CPAN, even the author(s) of the module in question is limited by this, as they cannot release newer or older forks of their code, without introducing a new name.


A module Foo::Bar is distributed on CPAN as Foo-Bar. It may have submodules like Foo::Bar::Baz.

To make an alternate CPAN version, leave everything exactly the same, except distribute the new version as Alt-Foo-Bar-AltIdentifier.

When a user installs your module like so:

    PERL_ALT_INSTALL=OVERWRITE cpanm Alt::Foo::Bar::better

they will get your version of the Foo::Bar framework (Foo::Bar, Foo::Bar::Baz).

Obviously, this completely overlays the old Foo::Bar install, but that's the whole idea. The user isn't surprised by this because they just asked for an Alternate implementation. If they don't like it, they can simply reinstall the original Foo-Bar, or try some other alternate.


This idea is new, and the details should be sorted out through proper discussions. Pull requests welcome.

Here are the basic guidelines for best using the Alt namespace:

Name Creation

Names for alternate modules should be minted like this:


For instance, if MSTROUT wants to make an alternate IO-All distribution to have it be Moo-based, he might call it:


He might also just call it:


By having 'Alt' at the start, it guarantees that it does not mess with future IO::All development. The "phrase" at the end can be anything unique to CPAN, but should describe the spirit of the alternate. If the alternate is meant to be short-lived, it can just be the author's CPAN id.

Makefile.PL Changes

Due to experience with problems, it is important to make your Alt module not install without explicit direction. You can accomplish this easily in a Makefile.PL, with something like this:

    my $alt = $ENV{PERL_ALT_INSTALL} || '';
    $WriteMakefileArgs{DESTDIR} =
      $alt ? $alt eq 'OVERWRITE' ? '' : $alt : 'no-install-alt';

Similar techniques should be available for other module release frameworks.

Module for CPAN Indexing

You will need to provide a module like Alt::IO::All::MSTROUT so that CPAN will index something that can cause your distribution to get installed by people:


Since you are adding this module, you should add some doc to it explaining your alternate version's improvements.

The Alt:: module can be as simple as this:

    package Alt::IO::All::MSTROUT;
    our $VERSION = '0.01';

It is important to use the no_index directive on the modules you are providing an alternates for. This is especially important if you are the author of the original, as PAUSE will reindex CPAN to your Alt- version which defeats the purpose. Even if you are not the same author, it will make your index reports not show failures.


It is important to not declare a $VERSION in any of the modules that you are providing alternates for. This will help ensure that your alternate module does not satisfy the version requirements for something that wants the real module.

If you want to depend on the alternate versions, then set the dependency on the Alt:: module.

NOTE: If you provide an alternate Foo::Bar (with no VERSION) it will satisfy the version requirements for someone who requires Foo::Bar => 0. In a sense, depending on version 0 means that alternates are OK.

use the Alt

You should add this line to your alternate modules:

    use Alt::IO::All::MSTROUT;

That way the Alt:: module gets loaded any time you use IO::All (with the alternate version installed). This gives debugging clues since the Alt:: module is now in %INC.

Other Concerns

If you have em, I(ngy) would like to know them. Discuss on #toolchain on for now.


The Alt- concept was thought up by Ingy as he tried to figure out how to revamp the somewhat popular IO::All and modules. Alternates can now be released and alpha/beta tested, while the originals remain stable.

When Alt-IO-All-new is "community approved" it can replace IO-All. If people want the old code, they can can install Alt-IO-All-old.


Ingy döt Net <>


Copyright 2012-2014. Ingy döt Net.