Accessing the caller from a Puppet parser function

demo init.pp

class demo::volatile {
        demo::volatile_res { 'demo': }
}

define demo::volatile_res {
        $result = volatile_function($name)
        notify { $result: }
}

terminal

$ puppet apply -e 'include demo::volatile'
Error: Demo::Volatile_res[demo]: an error happened at /home/ffrank/.puppet/modules/demo/manifests/init.pp:21 on node geras.localdomain
Wrapped exception:
Demo::Volatile_res[demo]: an error happened

volatile_function.rb

module Puppet::Parser::Functions
  newfunction(:volatile_function, :type => :rvalue) do |args|
    raise "#{@resource.ref}: an error happened"
  end 
end