The 2003 Perl Advent Calendar
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On the 25th day of Advent my True Language brought to me..

So, I've written about a hundred modules in my advent calendar. That's a lot of modules. That's a lot of stuff to install.

Even though I love modules - they're little bundles of prewritten goodness - it's a major pain installing them on all the machines I'm going to use, and remembering what modules are needed for what scripts need what modules. It's even worse when you're handing scripts to other people to run, as they often have no clue either how to install modules and, if they do, what side effects installing the modules will have on their system.

We're Perl programmers, and we believe being lazy is a virtue. I can't be bothered to install a whole bunch of stuff every time I want to try out a new program just because it happens to be written in Perl. What I want is something that I can double click.

PAR is the tool to do that.

If you're a Java programmer, then you know what a JAR file is. It's a Java ARchive. Basically it's a ZIP file that contains a bunch of compiled Java code (class files,) and you can use them with java like so:

  bash$ java -jar myjarfile.jar

And lo, since the jar contains everything you need to run the code it just works, without you having to install anything else.

PAR files, Perl ARchive files, are much the same idea. Except rather than bytecode PAR files contain Perl source code and binary code for any compiled C code that you're shipping with your module. What this means that if you've got PAR installed and someone ships you a PAR file you can (assuming it's been designed this way) execute it like this:

  bash$ perl -MPAR myparfile.par

And that's it. No more module installation. No more worrying about dependencies. Assuming the author of the PAR file has done their bit there should be everything you need inside the one zip file that the clever PAR module should be able to extract on it's own.

Installing PAR

On a Windows box, with the latest ActivePerl installation from ActiveState installing a copy of PAR is simple; All you need do is use the PPM system which installs binary modules directly from the precompiled ActiveState repository

  ppm> install PAR

No compiler, make, or any other tool required.

On other platforms (that have such useful tools already) you can install the module just like any other, by using the cpan or cpanp commands to load the CPAN or CPANPLUS shells and typing install PAR.

Writing Your Own Par Bundles

Let's write a simple Tk program that has two boxes input and output, and a button that when pressed runs the input though HTML::Entities to encode all the characters in it that can be displayed in HTML and spit the resulting text to the output box:

  # turn on perl's safety features 
  use strict;
  use warnings;
  # use our modules
  use Tk;
  use HTML::Entities;
  # create the main window
  my $mw = MainWindow->new;
  $mw->title("HTML Converter");
  # create the text boxes
  my $input  = $mw->Text()->pack;
  my $output = $mw->Text()->pack;
  # connect the output to our private filehandle
  tie *TT, ref $output, $output;
  # button and code to execute when clicked
  $mw->Button(-text => "Convert", -command => sub {
    # get the text from the input box
    my $input_text = $input->get("1.0", 'end');
    # delete everything from the output
    $output->delete("1.0", 'end'); 
    # run it though HTML::Entities;
    print TT encode_entities($input_text);
  # go into the main loop

We save this script as Making a bundle of all the dependencies this script needs can use is easy; We use the pp command line program:

  C:\WINDOWS\DESKTOP> pp -p -o ent.par

This creates ent.par. If you decompress this bundle using WinZip (it's just a zip file) you'll see that it contains a few core modules, Tk, and some extra files:
  META.yml is obviously your script. is the script that is executed when the distribution is run from the command line (in this case it just calls Manifest is a straight list of all the things that are meant to be in the bundle (so you can tell if something's missing.) And META.yml contains meta information about the distribution (what version of PAR created it, dependencies information, etc) in machine/human readable YAML format.

One thing that we immediately notice is that this par file contains all the binary code that's needed to run Tk. This is compiled for our version of perl on our platform. If someone else has compiled a different perl for our machine this par archive code won't work. Likewise my par bundle with binary components won't function on a Linux box. It is possible to have a par bundle that contains multiple versions of the binary code for separate machines and for par to do the right thing, but I'm not going to cover it here. For now you need to remember that if you're using binary modules - things like Tk - it'll only work on the platform you developed it on. Of course, if your code uses no binary modules, then the one archive will work anywhere.

Anyway, you can now execute your bundle on any machine with a compatible Perl installed by doing:

  C:\WINDOWS\DESKTOP> perl -MPAR ent.par

Being Smart

PAR is smart when it comes to dependencies. It works out what you need to bundle with your application by examining your source code and then using some careful heuristics to figure out exactly what you need. This means that if you're using a module that has dependences outside of Perl, then in all the common cases it'll include these in the bundle too. Isn't it clever? To do this it uses the

  • Module::ScanDeps
  • module, also available from CPAN.

    PAR is also intelligent enough to throw away any of the unneeded parts of the module, and by this I mean documentation. If you look at any of the modules in the archive you'll see that they no longer contain any pod code. This means for small distributions (where you're not passing the whole of Tk around) you can get very tiny par files.

    Standalone Code

    This is all very well - it's certainly removed the headache of installing things left right and centre. But of course, there's still two problems. Firstly, the target computer must have a copy of perl installed - and the right version of perl for your code - and secondly they must have a copy of PAR installed. This is no good for the click and drool school; They'll never understand the "first go to ActiveState, download this particular perl, then type ppm at the prompt and then install PAR at the ppm prompt" bootstrapping process. It's also no good if the people at the other end don't have access to the Internet, or don't have the necessary setup to install PPM. If we're shipping people software on CD we can't make any assumptions about their computer beyond the operating system they're running.

    What we need to send them is a double clickable executable. Never fear, PAR is more than capable of this task. As well as creating a PAR file itself it can also use the same techniques to create one executable that contains all the things you'd normally put in a PAR file plus a copy of perl itself and the necessary code to run PAR related code.

    For example, to convert the above script into a completely independent executable, which bundles perl, the Template Toolkit, Tk, and, of course, PAR with it you need to do something like:

      C:\WINDOWS\DESKTOP> pp -o ent.exe

    This doesn't actually recompile perl, but rather takes the existing installation and re-encodes it into a shippable state along with everything else. This means you don't need a compiler on your computer (hooray) and also the whole process is much much quicker (It'd take about an hour to compile and test perl on the Windows box I'm currently using, where it takes about twenty seconds to make the executable with PAR)

    The resulting ent.exe executable can now be simple double clicked to be executed - it's a standard windows executable. Of course, you don't have to make your own, you can download my copy

  • here
  • (warning: 1.25MB)

  • ActiveState's ActivePerl
  • Module::ScanDeps