For what it's worth (which may not be much), it is possible to write
classes in OCAML that are downcastable.  Here's how.

-- 
Tim Freeman       
tim@fungible.com
GPG public key fingerprint ECDF 46F8 3B80 BB9E 575D  7180 76DF FE00 34B1 5C78 

module type Foo = sig
  class type superclass = object
    method data: exn
  end
            
  class subclass_1: string -> object
    inherit superclass
    method s: string
  end
                    
  exception Subclass_1 of subclass_1
    
  class subclass_2: int -> object
    inherit superclass
    method i: int
  end
                    
  exception Subclass_2 of subclass_2
    
  val x: superclass
end

module Foo: Foo = struct

  class type superclass = object
    method data: exn
      (* You can put more methods here if you like. *)
  end
            
  class subclass_1_impl (s: string) (makeexn: subclass_1_impl -> exn) =
  object (self: 'self)
    constraint 'self = #superclass
    method data: exn = makeexn (self :> subclass_1_impl)
    method s: string = s
  end
    
  exception Subclass_1 of subclass_1_impl
    
  class subclass_1 (s: string) = subclass_1_impl s (fun sc -> Subclass_1 sc)
    
  class subclass_2_impl (i: int) (makeexn: subclass_2_impl -> exn) =
  object (self: 'self)
    constraint 'self = #superclass
    method data: exn = makeexn (self :> subclass_2_impl)
    method i: int = i
  end
    
  exception Subclass_2 of subclass_2_impl
    
  class subclass_2 (i: int) = subclass_2_impl i (fun sc -> Subclass_2 sc)
    
  let _ = Random.self_init ()

  let x: superclass =
    if Random.bits () mod 2 = 0 then
      (new subclass_1 "blort" :> superclass)
    else
      (new subclass_2 17 :> superclass)
      
  let _ =
    match x#data with
        Subclass_1 s1 -> Format.printf "s is %s.\n@?" s1#s
      | Subclass_2 s2 -> Format.printf "i is %d.\n@?" s2#i
      | _ -> failwith "Downcast failed"
end
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