How do I handle optional parameters in Moose?

Question

I'm currently starting with Perl OOP using the "Moose" package.

The compiler complains that it "Can't modify non-lvalue subroutine call at Parser.pm line 16."

I don't quite understand why I can't just assign a new object. I guess there is a better or more valid way to do optional parameters with Moose?

    #!/usr/bin/perl -w

package Parser;

use Moose;

require URLSpan;

require WWW::Mechanize;

has 'urlspan' => (is => 'rw', isa => 'URLSpan', required => 1);
has 'mech' => (is => 'rw', isa => 'WWW::Mechanize');

sub BUILD {
    my $self = shift;
    if(!$self->mech) {
        warn("no Mech set for " . $self->urlspan->name);
        $self->mech = WWW::Mechanize->new(agent => 'Mozilla/5.0 (Windows; U; Windows NT 6.0; de; rv:1.9.0.6) Gecko/2009011913 Firefox/3.0.4',
                                         stack_depth => 1
                                         ); #line 16
        }

}

Answer 1

$self->mech is a method call; you can't really treat it like a field in a C struct. If you want to set it, you need to pass the new object to it.

        $self->mech( 
            WWW::Mechanize->new(
                agent => 'xyz',
                stack_depth => 1
            )
        );

Answer 2

Probably the preferred Moose way of doing this is to set lazy_build on the attribute:

has 'mech' => (is => 'rw', isa => 'WWW::Mechanize', lazy_build => 1);
sub _build_mech {
     warn("no Mech set for " . $self->urlspan->name);
     WWW::Mechanize->new(
           agent => 'Mozilla/5.0 (Windows; U; Windows NT 6.0; de; rv:1.9.0.6)'.
                    ' Gecko/2009011913 Firefox/3.0.4',
           stack_depth => 1
     );
}

This will allow the attribute 'mech' to get populated the first time it is called, unless otherwise set by the constructor or by the accessor (since it's still 'rw').