OCaml tip - To test equality of self referencing data, we can't use (=) since it causes Out_of_memory. In this specific case, we should use (==).

selfref.ml

type 'a ring_t = {
  value: 'a;
  mutable next: 'a ring_t;
  mutable prev: 'a ring_t;
}

let init value =
  let rec x = {value; next = x; prev = x} in
  x

let () =
  let x = init 3
  and y = init 8
  and yesno yn = (if yn then "yes" else "no")
  in
  (
    Printf.printf "x == y ? --> %s\n" (yesno (x == y));
      (* --> no *)
    Printf.printf "x == x ? --> %s\n" (yesno (x == x));
      (* --> yes *)

    Printf.printf "x = y ? --> %s\n" (yesno (x = y));
      (* --> no *)
    Printf.printf "x = x ? --> %s\n" (yesno (x = x))
      (* Fatal error: exception Out_of_memory *)
  )