How does one implement cyclic doubly-linked list?

Learning
1

Is it possible to create a cyclic doubly linked list with this type declaration ?

type 'a dlist =
  | Nil
  | Cons of 'a dlist Lazy.t * 'a * 'a dlist Lazy.t

This could fit for the creation of a singleton list
let singl v = let rec cell = Cons (lazy cell, v, lazy cell) in cell

But I can’t figure out how to cons an element to the list in O(1), as in singly-linked lists.

2

Since your doubly linked list is immutable, it is not possible to add an element to the cycle without making a new copy of the whole cycle. An O(1) cons is thus impossible.