Hooking into @INC

2014-06-05

In UAV::Pilot, there’s a shell called uav that’s meant to be an easy way to mess around. It takes arbitrary Perl commands and runs them through eval(). By loading up libraries into its context namespace, we can provide commands for all your basic UAV needs.

The old way of loading these libraries was to write them as a normal module, except without a package statement at the top. When you loaded a library, UAV::Pilot::Command would go digging around in the distro’s share dir. Once found, it slurps in the file, adds a package for the namespace we want, and hands it to eval().

This worked OK, but it had the downside that for development, you couldn’t make in-place fixes to the library. File::ShareDir gives you back the path to the distro’s system share directory, so you had to install the development distribution first and then find out if your fixes worked.

Then, I saw this tidbit in the perldelta for 5.20:

“Since Perl v5.10, it has been possible for subroutines in @INC to return a reference to a scalar holding initial source code to prepend to the file. This is now documented.”

Specifically, it’s documented in perlfunc under the require entry. But what I really wanted was the method of sticking a subroutine in @INC.

When you do this, the subroutine is passed a reference to itself, and a path to the file. It should return a list of up to four things:

1. Reference to a scalar, which is text that will be prepended to the module
2. Filehandle to read from
3. Subref, which if there is no filehandle above, will be called in a loop to get the module text until it returns 0
4. Subroutine state var

For my purposes, I only needed to return the first two.

This means the Commands modules can be in the regular module paths (though still without a package statement). This is was implemented in UAV::Pilot v1.0_0 trial release, and UAV::Pilot::ARDrone has been ported to the new way, too. WumpusRover should be forthcoming.

I’m letting the CPAN smoke testers go over the trial release before I put the big official v1.0 on the module, but we’re looking good so far.