I was trying to use a recursive module but was getting unexpected 
"Undefined_recursive_modules" exceptions.  After a long debugging 
session, I discovered the source of my problem to be exception 
declarations in the module.  I don't intuitively understand why they 
cause a problem, and as I cannot find documentation of the behavior, I 
am wondering if it might be a bug.

Here is the example.  This is essentially the example in the manual, but 
I have added an exception to the module:

module rec A : sig
   type t = Leaf of int | Node of ASet.t
   exception Fail
   val compare : t -> t -> int
end = struct
   type t = Leaf of int | Node of ASet.t
   exception Fail
   let rec compare = (* suitable definition *)
end
and ASet : Set.S with type elt = A.t = Set.Make(A)

Then we can try to use it:

# let x = A.Leaf(3);;
val x : A.t = A.Leaf 3
# let s = ASet.add x ASet.empty;;
val s : ASet.t = <abstr>
# let s' = ASet.add x s;;
Exception: Undefined_recursive_module ("recmodtest.ml", 6, 6).

If we remove the "exception Fail" from the signature, everything works 
just fine.  Is this behavior correct?  (I am using OCaml 3.08.1)

Aaron

--
Aaron Bohannon
http://www.cis.upenn.edu/~bohannon/