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masak (6289)

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Been programming Perl since 2001. Found Perl 6 somewhere around 2004, and fell in love. Now developing November (a Perl 6 wiki), Druid (a Perl 6 board game), pls (a Perl 6 project installer), GGE (a regex engine), and Yapsi (a Perl 6 implementation). Heavy user of and irregular committer to Rakudo.

Journal of masak (6289)

Saturday July 03, 2010
08:28 PM

Dreaming in mixins

[ #40434 ]

Working with pls (a next-gen project installer for the Perl 6 ecosystem), I had a few classes with code like this:

class POC::Tester does App::Pls::Tester {
    method test($project --> Result) {
        my $target-dir = "cache/$project<name>";
        if "$target-dir/Makefile" !~~ :e {
            return failure;
        unless run-logged( relative-to($target-dir, "make test"),
                           :step('test'), :$project ) {
            return failure;

        return success;

(success and failure are Result enum values defined elsewhere. They felt like pleasant documentation, and when return type checking works, they'll even help catch errors!)

Now, I wanted to add super-simple progress diagnostics to this method. I wanted an announce-start-of('test', $project); at the start of the module, and either an announce-end-of('test', success); or an announce-end-of('test', failure);, depending on the success or failure of the method.

I have a low threshold for boilerplate. After realizing that I'd have to manually add those calls in the beginning of the method, and before each return — and not only in this method, but in several others — I thought "man, I shouldn't have to tolerate this. This is Perl 6, it should be able to do better!"

So I thought about what I really wanted to do. I wanted some sort of... method wrapper. Didn't really want a subclass, and a regular role wouldn't cut it (because class methods override same-named role methods).

Then it struck me: mixins. Did those already work in Rakudo? Oh well, try it and see. So I created this role:

role POC::TestAnnouncer {
    method test($project --> Result) {
        announce-start-of('test', $project&lt;name&gt;);
        my $result = callsame;
        announce-end-of('test', $result);
        return $result;

And then, later: does POC::TestAnnouncer

And it worked! On the first attempt! jnthn++!

(If you're wondering what in the above method that does the wrapping — it's the callsame call in the middle. It delegates back to the overridden method. Note that with this tactic, I get to write my announce-start-of and announce-end-of calls exactly once. I don't have to go hunting for all the various places in the original code where a return is made.)

I guess this counts as using mixins to do Aspect-Oriented Programming. This way of working certainly makes the code less scattered and tangled.

So, in this file, I currently have a veritable curry of dependency injection, behavior-adding roles, lexical subs inside methods, AOP-esque mixins, and a MAIN sub. They mix together to create something really tasty. And it all runs, today, under Rakudo HEAD.

As jnthn said earlier today, it's pretty cool that a script of 400 LoC, together with a 230-LoC module, make up a whole working installer. With so little code, it almost doesn't feel like coding.

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  • With Perl 5 that would look like either:

    package POC::TestAnnouncer;

    use Moose::Role;

    before test => sub {
      my $project = $_[1];
      announce-start-of('test', $project{name});

    after test => sub {
      my $project = $_[1];
      announce-end-of('test', $project{name});

    And then there's the more powerful around:

    package POC::TestAnnouncer;

    use Moose::Role;

    around test => sub {
      my $fn = shift;
      my $self = shift;
      my $project = $_[0];
      announce-start-of('test', $project{name});
    • Thanks. I was only aware of how Moose did it to the extent that people come into the #perl6 channel sometimes and ask "what's the equivalent to Moose's before/after/around in Perl 6?" and we answer "[call|next][same|with]".

      (The "call-" methods are returning calls, and the "next-" methods are tailcalls. "-same" sends along the original arguments, and "-with" allows you to send new ones.)

      What's really neat, and what I still haven't quite gotten my head wrapped around, is that these four routines are used in t

      • Oh, huh, and I should probably add that mixins may look like they do a variant of wrapping in Perl 6, but they really work by creating an anonymous subclass to the class of the object, and re-blessing the object to that subclass. So, they work by inheritance, not wrapping.

        The fact that both of these use "callsame;" to delegate to the original routine means that I can use mixins and think they are wrappers, and they'll still behave as I expect. That's why I like the above unification.

        • The Moose version Frew posted uses Roles. So applying this "mixin" at Runtime to an instance will also derive an anonymous subclass with the Role applied and re-bless an instance into that subclass.

        • Also playing about some I came up with something nearly identical using MooseX::Declare's syntax.

          role POC::TestAnnouncer {
                  use mro;

                  method test ($project) {
                      announce-start-of('test', $project{name});
                      my $result = $self->maybe::next::method($project); # callsame
                      announce-end-of('test', $result);

          • I mean $.callsame rather than .callsame. Thanks to TimToady and TiMBuS for clarifying things at least enough that I understand that much.