Re: Moose "unions" and inheritance

[tobyink]

Firstly, when overriding an attribute definition in a child class, use:

has '+data' => ( ... );

This allows you to inherit attribute options from the parent class, so you don't need to redeclare, say, is => 'rw' (because the parent class has already declared that).

There isn't an especially elegant way of adding to the parent attribute's type constraints. This is the best I could do. I'm using MooseX::Types here because it makes things a little prettier. It's possible to do the same mucking around with Moose::Util::TypeConstraints but who's got the patience?!

use v5.14;
use strict;
use warnings;

package KeyAtom {
    use Moose;
    use MooseX::Types::Moose -all;

    has data => (
        is      => 'rw',
        isa     => Str | RegexpRef ,
    );
}

package ValAtom {
    use Moose;
    use MooseX::Types::Moose -all;

    extends 'KeyAtom';
    
    sub _existing_constraint {
        my ($class, $attr) = @_;
        return $class->meta->find_attribute_by_name($attr)->type_const
+raint;
    }

    has '+data' => (
        isa     => __PACKAGE__->_existing_constraint('data') | ArrayRe
+f | HashRef,
    );
}

ValAtom->new(data => 'Hello');      # Str
ValAtom->new(data => qr{Hello});    # RegexpRef
ValAtom->new(data => []);           # ArrayRef
ValAtom->new(data => {});           # HashRef
ValAtom->new(data => \*STDOUT);     # none of the above... crash!