to_xml

# note that last item is User class
a = [10, [20, 30, ["helo", "world"]], [1.2, 1.3], User]

puts a.to_xml

<?xml version="1.0" encoding="UTF-8"?>
<records type="array">
  <record type="integer">10</record>
  <records type="array">
    <record type="integer">20</record>
    <record type="integer">30</record>
    <strings type="array">
      <record>helo</record>
      <record>world</record>
    </strings>
  </records>
  <floats type="array">
    <record type="float">1.2</record>
    <record type="float">1.3</record>
  </floats>
  <record>User</record>
</records>