Deep cloning Moose object with attributes that are ArrayRef[Object] and Set::Object

Question

I've got a Moose object:

class My::Game {
  has 'players' => (isa => 'Set::Object', ...)
  has 'action_sequence' => (isa => 'ArrayRef[My::Game::Action]', ...)
}

Now I want to be able to clone this object with a call like $game2 = $game->clone; How do I deep clone it so that the objects in the ArrayRef are cloned? And more trickily, the Set::Object?

I've looked at MooseX::Clone, but I'm unclear how to apply it to this case. Example code would be appreciated.

Thanks!

Answer 1

I haven't used any of the pieces here (MooseX::Clone, MooseX::Compile, and Set::Object), so here's just a rough outline of where I'd start, from my review of the docs and general knowledge of Moose architecture:

• MooseX::Clone gives your object a clone() method, which will then call clone() on each of your attributes with the 'Clone' trait.
• so, you need to add a clone() method to both of your attributes...
• it would be my ambitious hope that MooseX::Clone can handle native attribute arrayrefs, but since it probably doesn't, you may get to extend that feature yourself (i.e. if 'action_sequence' has the option traits => ['Array'], it is smart enough that the clone method is actually defined via handles => { clone => [ 'map', 'clone' ] } -- i.e. clone the attribute by calling clone() on each of the member elements) -- here is likely where you would be submitting your first patch
• Set::Object's documentation suggests that you can clone objects reliably with Storable::dclone (which you should verify, and flog the author and/or supply patches if this is not correct)
• and now you need to add a clone() method to the Set::Object attribute, which the docs suggest you do with MooseX::Compile (and here is where you should come on irc.perl.org #moose and give us a blow-by-blow of your progress, so we can make a documentary about it later) :)

Answer 2

Turns out that simply adding the MooseX::Clone role to the class provides a clone() method that recursively clones attributes.

• For hashref/arrayref attributes, it copies structures.
• For scalars (including references) it simply does a shallow copy of the reference.
• If you add traits => ['Clone'] to the attribute, it will recursively clone the attribute by calling clone() on the attribute value.

To support cloning Set::Object, I ended up creating a trait called CloneByCoercion by subclassing the Clone trait, parameterized with the type to coerce to/from before cloning.

So to use it, I wrote:

has 'blah' => (isa => 'Set::Object', is => rw,
  traits => ['CloneByCoercion' => {to=>'ArrayRef'}]
);

MooseX::Types::Set::Object provides coercions to and from ArrayRef (although I needed to patch a bug in it: the coercion to ArrayRef should return a reference, not a list)

I also modified MooseX::Clone to keep an objects-seen hash, so that it supports cloning interlinked object structures with circular references.

I'll eventually get around to putting this stuff up on CPAN or submitting patches to the modules.