History

Since the original development of Alien::Base, Module::Build, on which Alien::Base::ModuleBuild is based, has been removed from the core of Perl. It seemed worthwhile to write a replacement installer that works with ExtUtils::MakeMaker which IS still bundled with the Perl core. Because this is a significant undertaking it is my intention to integrate the many lessons learned by Joel Berger, myself and the "Alien::Base team" as possible. If the interface seems good then it is because I've stolen the ideas from some pretty good places.

Philosophy

Ideally the code for an Alien::Base based Alien should simply inherit from Alien::Base, like so:

 package Alien::libfoo;
 
 use parent qw( Alien::Base );
 
 1;

The detection logic should be done by the installer code (alienfile and Alien::Build) and saved into runtime properties (see "runtime_prop" in Alien::Build). And as much as possible the runtime should be implemented in the base class (Alien::Base). Where reasonable, the base class should be expanded to meet the needs of this arrangement.

when downloading a package grab the latest version

If the maintainer of an Alien disappears for a while, and if the version downloaded during a "share" install is hardcoded in the alienfile, it can be problematic for end-users.

There are exceptions, of course, in particular when a package provides a very unstable interface from version to version it makes sense to hard code the version and for the Alien developer and Alien consumer developer to coordinate closely.

when installing a package the operating system as a whole should not be affected

The convenience of using an Alien is that a user of a CPAN module that consumes an Alien doesn't need to know the exact incantation to install the libraries on which it depends (or indeed it may not be easily installed through the package manager anyway).

As a corollary, a user of a CPAN module that consumes an Alien module shouldn't expect operating system level packages to be installed, or for these packages to be installed in common system level directories, like "/usr/local" or "/opt". Instead a "share" directory associated with the Perl install and Alien module should be used.

Plugins that require user opt-in could be written to prompt a user to automatically install operating system packages, but this should never be done by default or without consent by the user.

avoid dependencies

One of the challenges with Alien development is that you are by the nature of the problem, trying to make everyone happy. Developers working out of CPAN just want stuff to work, and some build environments can be hostile in terms of tool availability, so for reliability you end up pulling a lot of dependencies. On the other hand, operating system vendors who are building Perl modules usually want to use the system version of a library so that they do not have to patch libraries in multiple places. Such vendors have to package any extra dependencies and having to do so for packages that the don't even use makes them understandably unhappy.

As general policy the Alien::Build core should have as few dependencies as possible, and should only pull extra dependencies if they are needed. Where dependencies cannot be avoidable, popular and reliable CPAN modules, which are already available as packages in the major Linux vendors (Debian, Red Hat) should be preferred.

As such Alien::Build is hyper aggressive at using dynamic prerequisites.

interface agnostic

One of the challenges with Alien::Buil::ModuleBuild was that Module::Build was pulled from the core. In addition, there is a degree of hostility toward Module::Build in some corners of the Perl community. I agree with Joel Berger's rationale for choosing Module::Build at the time, as I believe its interface more easily lends itself to building Alien distributions.

That said, an important feature of Alien::Build is that it is installer agnostic. Although it is initially designed to work with ExtUtils::MakeMaker, it has been designed from the ground up to work with any installer (Perl, or otherwise).

As an extension of this, although Alien::Build may have external CPAN dependencies, they should not be exposed to developers USING Alien::Build. As an example, Path::Tiny is used heavily internally because it does what File::Spec does, plus the things that it doesn't, and uses forward slashes on Windows (backslashes are the "correct separator on windows, but actually using them tends to break everything). However, there aren't any interfaces in Alien::Build that will return a Path::Tiny object (or if there are, then this is a bug).

This means that if we ever need to port Alien::Build to a platform that doesn't support Path::Tiny (such as VMS), then it may require some work to Alien::Build itself, modules that USE Alien::Build shouldn't need to be modified.

plugable

The actual logic that probes the system, downloads source and builds it should be as pluggable as possible. One of the challenges with Alien::Build::ModuleBuild was that it was designed to work well with software that works with "autoconf" and "pkg-config". While you can build with other tools, you have to know a bit of how the installer logic works, and which hooks need to be tweaked.

Alien::Build has plugins for "autoconf", "pkgconf" (successor of "pkg-config"), vanilla Makefiles, and CMake. If your build system doesn't have a plugin, then all you have to do is write one! Plugins that prove their worth may be merged into the Alien::Build core. Plugins that after a while feel like maybe not such a good idea may be removed from the core, or even from CPAN itself.

In addition, Alien::Build has a special type of plugin, called a negotiator which picks the best plugin for the particular environment that it is running in. This way, as development of the negotiator and plugins develop over time modules that use Alien::Build will benefit, without having to change the way they interface with Alien::Build