Question

chromatic’s recent blog got me curious about the Moose subroutine has. I was looking at the Moose source code and noticed that inside the has subroutine, there is a $meta variable unpacked from @_. Where does $meta come from? I’ve started wading through the various Moose and Class::MOP modules. In many subroutines, it seems that $meta is commonly found as the first argument in @_, even though it is not specifically passed to it as an argument.

Edit: Here is the original source code for the has subroutine:

sub has {
    my $meta = shift;
    my $name = shift;

    Moose->throw_error('Usage: has \'name\' => ( key => value, ... )')
        if @_ % 2 == 1;

    my %options = ( definition_context => Moose::Util::_caller_info(), @_ );
    my $attrs = ( ref($name) eq 'ARRAY' ) ? $name : [ ($name) ];
    $meta->add_attribute( $_, %options ) for @$attrs;
}

Answer 1

molecules comment in ysth answer:

I’m not sure how the has subroutine gets converted to this closure, but this definitely shows the curried nature of the imported has

Here is (hopefully!) a simple example of how this could be achieved (however I suspect Moose does it in a much more complex and better way!)

Meta.pm

package Meta;

sub new {
    my $class = shift;
    bless { @_ }, $class;
}

sub has {
    my $meta = shift;
    print "Given => @_ \n";
    print "meta $_ => ", $meta->{$_}, "\n" for keys %$meta;
}

1;

Import.pm

package Import;
use strict;
use warnings;
use Meta;

# some arbitrary meta info!
our $Meta = Meta->new( a => 'A', b => 'B' );

sub import {
    my $caller = caller;

    # import 'has' into caller namespace
    no strict 'refs';
    *{$caller . '::has'} = sub { $Meta->has(@_) };
}

1;

meta_has.pl

use strict;
use warnings;
use Import;

has name => ( is => 'rw', isa => 'Int' );

Now if you run meta_has.pl you will get:

# Given => name is rw isa Int 
# meta a => A
# meta b => B

Hope that helps.

/I3az/