How do I create an instance of value from the attribute's meta object with Moose?

Question

I'm working on a serialization tool using Moose to read and write a file that conforms to a nonstandard format. Right now, I determine how to load the next item based on the default values for the objects in the class, but that has its own drawbacks. Instead, I'd like to be able to use information in the attribute meta-class to generate a new value of the right type. I suspect that there's a way to determine what the 'isa' restriction is and derive a generator from it, but I saw no particular methods in Moose::Meta::Attribute or Class::MOP::Attribute that could help me.

Here's a bit further of an example. Let's say I have the following class:

package Example;
use Moose;

use My::Trait::Order;
use My::Class;

with 'My::Role::Load', 'My::Role::Save';

has 'foo' => (
    traits => [ 'Order' ],
    isa => 'Num',
    is => 'rw',
    default => 0,
    order => 1,
);

has 'bar' => (
    traits => [ 'Order' ],
    isa => 'ArrayRef[Str]',
    is => 'rw',
    default => sub { [ map { "" } 1..8 ] }
    order => 2,
);

has 'baz' => (
    traits => [ 'Order' ],
    isa => 'Custom::Class',
    is => 'rw',
    default => sub { Custom::Class->new() },
    order => 3,
);

__PACKAGE__->meta->make_immutable;
1;

(Further explanation: My::Role::Load and My::Role::Save implement the serialization roles for this file type. They iterate over the attributes of the class they're attached to, and look at the attribute classes for an order to serialize in.)

In the My::Role::Load role, I can iterate over the meta object for the class, looking at all the attributes available to me, and picking only those that have my Order trait:

package My::Role::Load;
use Moose;    

...

sub load {
    my ($self, $path) = @_;

    foreach my $attribute ( $self->meta->get_all_attributes ) {
        if (does_role($attribute, 'My::Trait::Order') ) {
            $self->load_attribute($attribute) # do the loading
        }
    }
}

Now, I need to know the isa of the attribute that the meta-attribute represents. Right now, I test that by getting an instance of it, and testing it with something that's kind of like this:

use 5.010_001; # need smartmatch fix.
...
sub load_attribute {
    my ($self, $attribute, $fh) = @_;
    my $value = $attribute->get_value($self); # <-- ERROR PRONE PROBLEM HERE!
    if (ref($value) && ! blessed($value)) { # get the arrayref types.
        given (ref($value)) {
            when('ARRAY') { 
                $self->load_array($attribute);
            }
            when('HASH') {
                $self->load_hash($attribute);
            }
            default {
                confess "unable to serialize ref of type '$_'";
            }
        }
    }
    else {
        when (\&blessed) {
            confess "don't know how to load it if it doesn't 'load'."
                if ! $_->can('load');
            $_->load();
        }
        default {
            $attribute->set_value($self, <$fh>);
        }
    }
}

But, as you can see at # <-- ERROR PRONE PROBLEM HERE!, this whole process relies on there being a value in the attribute to begin with! If the value is undef, I have no indication as to what to load. I'd like to replace the $attribute->get_value($self) with a way to get information about the type of value that needs to be loaded instead. My problem is that the docs I linked to above for the Class::MOP::Attribute and the Moose::Meta::Attribute don't seem to have any way of getting at the type of object that the attribute is supposed to get.

The type information for an attribute is basically what I'm trying to get at.

(Note to future readers: the answer here got me started, but is not the final solution in of itself. You will have to dig into the Moose::Meta::TypeConstraint classes to actually do what I'm looking for here.)

Answer 1

I'm not quite sure I understand (perhaps you can include some pseudocode that demonstrates what you are looking for), but it sounds like you could possibly get the behaviour you want by defining a new attribute trait: set up your attribute so that a bunch of methods on the class delegate to the attribute's object (isa => 'MySerializer', handles => [ qw(methods) ]).

You might possibly also need to subclass Moose::Meta::Class (or better, add a role to it) which augments the behaviour of add_attribute().

Edit: If you look at the source for Moose::Meta::Attribute (specifically the _process_options method), you will see that the isa option is processed by Moose::Util::TypeConstraints to return the actual type to be stored in the type_constraint field in the object. This will be a Moose::Meta::TypeConstraint::Class object, which you can make calls like is_a_type_of() against.

This field is available via the type_constraint method in Moose::Meta::Attribute. See Moose::Meta::TypeConstraint for all the interfaces available to you for checking an attributes's type.

Answer 2

Not sure I follow what you are after and perhaps Coercions might do what you want?

However to get the attributes isa:

{
    package Foo;
    use Moose;

    has 'bar' => ( isa => 'Str', is => 'rw' );
}


my $foo = Foo->new;

say $foo->meta->get_attribute('bar')->type_constraint;   # => 'Str'

/I3az/

Answer 3

Out of curiosity why not use/extend MooseX::Storage? It does Serialization, and has for about two and a half years. At the very least MooseX::Storage will help you by showing how a (well tested and production ready) serialization engine for Moose is written.