let rec zlist_of_list lis = lazy(
match lis with
[] -> Empty
| h::res -> Node(h, zlist_of_list res)
);;

…that is, we return a lazy thunk which is going to match either the empty list or split off the head and return a node with that first element in the head and call itself recursively on the tail to make the zlist be lazily created one link at a time.

list_of_zlist looks like this:

let rec list_of_zlist zl =
let hd = Lazy.force zl in
match hd with
Empty -> []
| Node(h, t) -> h::(list_of_zlist t);;

We force the head of the list, then see if there’s more we need to force and do so if we have to by recursive call.