Caml: ref in module

Learning
1

Hi
I can’t understand why we use ref in modules,
in my book :
module Element = struct type t = int end;;
module Queue =
struct
type element = Element.t
type queue = element list ref
let create() = ((ref []) : queue)
end;;

Can you explain to me the “ref” in module Queue?

Thank you in advance.

2

First, you should probably use Markdown formatting to offset code in your question.

Second, see the paragraphs on “references” near the end of: http://caml.inria.fr/pub/docs/manual-ocaml/coreexamples.html#sec12