Recently in moose Category

The talks of the last perl Monger meeting are now available online for your viewing pleasure. In addition of the classic decks of slides, thanks to Andrew Ross of the FOSSLC we also have full-fledged recordings of the talks:

• Tim Procter's ''Portable Perl Daemons'': audio-visual extravaganza or just the slides.

• Your truly introduction to Moose: audio-visual extravaganza or just the slides.

Enjoy!

(by the by, I joined the fun and accepted the Perl Iron Man Challenge.)

As assiduous readers of my blog may recall, last year I gave a 35 minutes Perl introduction at OSBootCamp 8. For unphantomable reasons, I've been asked to come back and recidive for FOSSLC Summercamp 2009, which is happening on the 13, 14 and 15 of May.

This time, my talk is going to be an introduction to Moose. Why Moose? Since my goal is to hook unsuspecting students on Perl, I figured the easiest way to tip them to the dark side would be to show those java-tainted young minds the latest and shiniest OO coolness that the Perl community has to offer.

Sorry, what? ... Didn't I mention in a previous blog entry that I prefered Object::InsideOut over Moose?

Ah... Hum... Well, yes, true, I might have said that... (darn those assiduous readers)

Mind you, I still think that Object::InsideOut is a very fine module indeed, but several things made me progressively slide in the Moose camp:

• Moose has an incredibly strong momentum in the community.
  Notwithstanding the fact that when so many bright people get excited about something, it's usually for a good reason, it also means that Moose development is thriving. And not only core development, but extensions are by now abounding.
  And this, for my little lazy heart, is true bliss. The MooseX namespace is quickly growing into a mini OO-CPAN.

• At the time, I was arguing with Yuval that I had no need for those cute 'before' and 'after' method modifiers. Ah! That was before I seriously tried using them...

• Roles. Those things are dangerously addictive. And they do such a damned good job of simplifying one's code.

• MooseX::Method::Signatures, Moose types, coercion, and the myriad of other things that all work together to move the tedious bits of coding out of the way, and to fragment the good parts into small, easy to understand/maintain pieces.

• Meta-programming. Pure mad scientist type of fun. How can I possibly resist?

So yeah, I call on my right to change my volatile mind. Color me a new Moose adept. And, hopefully, I'll similarly be able to color some more converts this Friday. Wish me luck.

This week I finally got a good excuse to go on and dabble with Moose meta-programming. In my WWW::Ohloh::API classes, a lot of attributes take their values from an initial xml snippet. The original, naive way those values were extracted was along the lines of:

has xml_src => (
    is   => 'ro',
    isa => 'XML::LibXML::Node',
);

has created_at => (
    is         => 'ro', 
    lazy      => 1,
    default => sub {
        return $_[0]->xml_src->findvalue( 'created_at' );
    }
)

has sender_account_name => (
    is          => 'ro', 
    lazy      => 1,
    default => sub {
        return $_[0]->xml_src->findvalue( 'sender_account_name' );
    }
);

Not harrowingly horrible, but rather repetitive. I figured out it would be much niftier to provide the name of the attribute holding the source xml and the pertinent xpath, and let Moose magic take care of the rest. Something like:

has created_at => (
    metaclass => 'XMLExtract',
    is => 'ro', 
    xml_src => 'xml_src',
    xpath => 'created_at',
)

Well, turns out that once one knows how, this is fairly easy to do:

package XMLExtract;

use Moose;

extends 'Moose::Meta::Attribute';

has 'xml_src' => ( isa => 'Str', );

has xpath => ( isa => 'Str', );

has '+lazy' => ( default => 1 );

before '_process_options' => sub {
    my ( $class, $name, $options ) = @_;

    die "attribute '$name' in class '$class' must be lazy-evaluated\n"
    if defined $options->{lazy} and not $options->{lazy};

    my $src = $options->{xml_src} ||= 'xml_src';
    my $xpath = $options->{xpath} ||= $name;

    $options->{default} = sub {
        return $_[0]->$src->findvalue($xpath);
    };

};

1;

If you notice, I've set 'xml_src' and 'xpath' to default to 'xml_src' and the name of the current attribute, so that the code can be further simplified to

has xml_src => (
    is   => 'ro',
    isa => 'XML::LibXML::Node',
);

has created_at => (
    metaclass => 'XMLExtract',
    is => 'ro', 
)

has sender_account_name => (
    metaclass => 'XMLExtract',
    is => 'ro', 
);

Short, sweet and to the point. Now we're talking code I like!