Prerequisites and Features

Prerequisites are grouped into features, and the user could choose yes/no on each one's dependencies; the module writer may also supply a boolean value via "-default" to specify the default choice.

The Core Features marked by the name "-core" will double-check with the user, if the user chooses not to install the mandatory modules. This differs from the pre-0.26 'silent install' behaviour.

Starting from version 0.27, if "-core" is set to the string "all" (case-insensitive), every feature will be considered mandatory.

The dependencies are expressed as pairs of "Module" => "version" inside an array reference. If the order does not matter, and there are no "-default", "-tests" or "-skiptests" directives for that feature, you may also use a hash reference.

The Installation Process

Once Module::AutoInstall has determined which module(s) are needed, it checks whether it's running under the CPAN shell and should therefore let CPAN handle the dependency.

Finally, the "WriteMakefile()" is overridden to perform some additional checks, as well as skips tests associated with disabled features by the "-tests" option.

The actual installation happens at the end of the "make config" target; both "make test" and "make install" will trigger the installation of required modules.

If it's not running under CPAN, the installer will probe for an active connection by trying to resolve the domain "cpan.org", and check for the user's permission to use CPAN. If all went well, a separate
CPAN instance is created to install the required modules.

If you have the CPANPLUS package installed in your system, it is preferred by default over CPAN; it also accepts some extra options (e.g. "-target => 'skiptest', -skiptest => 1" to skip testing).

All modules scheduled to be installed will be deleted from %INC first, so ExtUtils::MakeMaker will check the newly installed modules.

Additionally, you could use the "make installdeps" target to install the modules, and the "make checkdeps" target to check dependencies without actually installing them; the "perl Makefile.PL --checkdeps" command has an equivalent effect.

If the Makefile.PL itself needs to use an independent module (e.g. Acme::KillarApp, v1.21 or greater), then use something like below:

    BEGIN {
        require Module::AutoInstall;
        # the first argument is an arrayref of the -config flags
        Module::AutoInstall->install([], 'Acme::KillerApp' => 1.21);
    }
    use Acme::KillerApp 1.21;
    Module::AutoInstall->import(
        # ... arguments as usual ...
    );

Note the version test in the use clause; if you are so close to the cutting edge that Acme::KillerApp 1.20 is the latest version on CPAN, this will prevent your module from going awry.

User-Defined Hooks

User-defined pre-installation and post-installation hooks are available via "MY::preinstall" and "MY::postinstall" subroutines, as shown below:

    # pre-install handler; takes $module_name and $version
    sub MY::preinstall  { return 1; } # return false to skip install
    # post-install handler; takes $module_name, $version, $success
    sub MY::postinstall { return; }   # the return value doesn't matter

Note that since Module::AutoInstall performs installation at the time of "use" (i.e. before perl parses the remainder of Makefile.PL), you have to declare those two handlers before the "use" statement for them to take effect.

If the user did not choose to install a module or it already exists on the system, neither of the handlers is invoked. Both handlers are invoked exactly once for each module when installation is attempted.

"MY::preinstall" takes two arguments, $module_name and $version; if it returns a false value, installation for that module will be skipped, and "MY::postinstall" won't be called at all.

"MY::postinstall" takes three arguments, $module_name, $version and $success. The last one denotes whether the installation succeeded or not: 1 means installation completed successfully, 0 means failure during install, and "undef" means that the installation was not attempted at all, possibly due to connection problems, or that module does not exist on CPAN at all.

Customized "MY::postamble"

Starting from version 0.43, Module::AutoInstall supports modules that require a "MY::postamble" subroutine in their Makefile.PL. The user-defined "MY::postamble", if present, is responsible for calling "Module::AutoInstall::postamble" and include the output in its return value.

For example, the DBD::* (database driver) modules for the Perl DBI are required to include the postamble generated by the function "dbd_postamble", so their Makefile.PL may contain lines like this:

    sub MY::postamble {
        return &Module::AutoInstall::postamble . &dbd_postamble;
    }

Note that the Module::AutoInstall module does not export the "postamble" function, so the name should always be fully qualified.