Perl Module Dependencies: how to require the latest, and nothing less

Posted in: Technical Track

Recently, hanekomu was contemplating how to make subsequent installs of a Task::BeLike module upgrade its dependencies to their latest version.

The idea is intriguing. It’s not something you want to do for a typical module, but it makes sense in the context of Task::BeLike. If you care enough about a module to put it in your Task::BeLike, you probably care enough to want to upgrade when there’s a new version out there.

Alas, I think hanekomu’s proposed way of doing it is flawed (mind you, the debate is still going on as of the writing of this entry, and I can very well still be proven wrong). But after some pondeferous chin scratching, I might have come with a cunning alternative to it.

Let’s say that in your Build.PL (the logic would be the same for a Makefile.PL) you have your dependencies stashed in %dependencies. Something akin to:

%dependencies = (
    XML::LibXML      => 0,          # any version will do
    XML::XPathScript => '1.42',     # 1.42 or higher
    Moose            => 'latest',   # nothing but the shiniest!

All we want to do, really, is to switch the latest for, well, the latest version available. Surprisingly, that something that is almost as easy to do than to say:

for my $mod ( keys %dependencies ) {
    next unless $dependencies{$mod} eq 'latest';

    require CPANPLUS::Backend;
    state $cb = CPANPLUS::Backend->new;

    $dependencies{$mod} = $cb->module_tree( $mod )->package_version;

Yes, that’s really all there is to it. A little further hacking later, I have incorporated the functionality to my own Task::BeLike::YANICK module. The way I implemented it, installing the module the usual way will yield no surprise (i.e., dependencies already present are not going to be updated). But if the environment variable TASK_UPGRADE is set to true, like so:

TASK_UPGRADE=1 cpan -f Task::BeLike::YANICK

. . . then the magic is going to be unleashed (the -f is to force the re-install, if the Task has already been installed before).

Alternatively, just to know which dependencies are out-of-date, one can also extract the distribution and do a

perl ./Build.PL --upgrade
./Build prereq_report

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