Question

I have a Moose object with a non-required attribute:

has 'optional_attr' => (
    is       => 'ro',
    isa      => 'MyCoolType',
    required => 0,
);

How can I confess if I ever try to read this attribute while it’s not set?

Answer 1

How about:

lazy    => 1,
default => sub { confess "not set" },

You might want to throw in a predicate, too:

predicate => 'has_optional_attr',

so you can find out if it’s set without dying.

There’s also MooseX::LazyRequire, which lets you say just:

use MooseX::LazyRequire;

has 'optional_attr' => (
    is            => 'ro',
    isa           => 'MyCoolType',
    lazy_required => 1,
);

Under the hood, it uses the same trick I suggested, but it looks more elegant in your class.