Thank you for the correction and citation.

>    Keiko Nakata wrote:
>    > I think that functors cannot be recursive in the current OCaml.

Yes, I was incorrect.
We have: 

module rec F: functor (X:sig end) -> sig val f : int -> int end = 
  functor(X:sig end) -> struct  
    let f x = if x = 0 then 0 else M.f (x-1)
  end

and M : sig val f: int -> int end = F(M)

This should imply interesting examples are available. 

Best,
Keiko