changing moose object classes on the go

[tomgracey]

Supposing I have a dog

my $dog = DOG->new; Then later I decide my dog is in fact a labrador. I could do bless $dog, 'LABRADOR';

which (i think) would be fine as LABRADOR inherits from DOG

but Moose has things like

has color => (is => 'rw', isa => 'Str', default => 'golden')

If I simply bless my $dog into LABRADOR then he won't be golden. How can I resolve this situation?

(I guess what I'm really asking is - is there a way to change the class of an object that already exists under Moose? Or perhaps this is not the done thing?)

[moritz]

If I simply bless my $dog into LABRADOR then he won't be golden. How can I resolve this situation?

By creating a new LABRADOR object, and using it instead of the old object. You can reuse the attributes from the original DOG object.

Seriously, changing the type of an object is a an usual requirement, and might point to a flaw in your overall design. If a dog is a labrador, it's already born a labrador. Or phrased differently, time-dependent properties should not encoded in the type.

[blue_cowdawg]

(I guess what I'm really asking is - is there a way to change the class of an object that already exists under Moose? Or perhaps this is not the done thing?)

Pretty much the same sort of question I asked a while ago here. The quick answer is as follows:

package Dog;
use Moose;
has 'breed' => ( is => 'ro',isa=>'String');
has 'color' => ( is => 'rw', isa=>'String');

sub BUILD { 
     my $self =shift;
     my $class = "Dog::" . $self->breed();

     $class->meta->rebless_instance($self);

} 
1;

and somewhere in user space:

| handwaving here..
use Dog;

    # updated this on 2013-04-24, error in original code
    my $dog = Dog->new(breed=>'Poodle');
|
|

[tomgracey]

Hi Peter

Realise its a while ago and I forgot to thank you for this - basically ->rebless_instance($self) was the answer. Thanks enormously!

[Brutha]

I had a similar problem. I receive a minimal abstract object and changed it into a specialized one to make life easier. What I did was creating a new class "Labrador" with the superclass "Dog" and a role "LabradorMethodsAndAtributes". Well, in my case it wasn't dogs.

Shortened, adjusted, unrunable code:

my $class_name = "Felidae::Tiger";
    
# Does the class already exist?
return $class_name
    if is_class_loaded($class_name);

# No, create it
my $superclass  = "Felidae";
my $role_name   = "Felidae::Role::Tiger";

# create the Role
my $role = Moose::Meta::Role->create($role_name);

# A leopard does not have stripes but spots
$role->add_attribute( "stripecount",
                      ( is => 'rw',
                        isa => 'Int',
                        ... # builder, lazy etc
                     ) );

my $my_class = Moose::Meta::Class->create(
                            $class_name,
                            superclasses => [$superclass],
                            roles        => [$role_name],
                    );
my $garfield = $class_name->new($felidae_obj);

Now you have an object which isa "Felidae" and a "Tiger" and does "stripecount".