> How do I do this:
> 
> type foo = B of bar
> type bar = F of foo
> 
> That is, I want two types to refer to each other.

It's exactly similar to value definitions: put a ``and'' between the
two recursive definitions.

type foo = B of bar
and bar = F of foo

Then you can define recursively a f of type foo and a b of type bar as in:

let rec f = B b
and b = F f;;

val f : foo =
 B
  (F
    (B
      (F
        (B
          (F
            (B
              (F ...
)))))))
val b : bar =
  F
   (B
     (F
       (B
         (F
           (B
             (F
               (B
                 (F ...
))))))))

Hope this helps,
        
Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/