On Thu, 26 Sep 2002, Michael Vanier wrote:
> I've run into a design problem that I think requires downcasting, and I
> wanted to see if anyone on the list has any ideas about an alternative
> approach, since AFAIK downcasting is impossible in the current ocaml object
> system (at least without nasty Obj.magic hacks).  I'll try to simplify the
> problem as much as possible, but this is still pretty long.

I'm not so sure about your specific problem, but when I've wanted my class 
hierarchy to behave more like a sum type I've simply grafted a sum type over the 
set of class types (or virtual classes). Here's a sketch for a composite 

type ('a, 'b) node = Leaf_node of 'a | Hier_node of 'b
and ('a, 'b) instance =
    CellRef of Atom.t * ('a, 'b) node * Geometry.placement
  | CellArrayRef of
      Atom.t * ('a, 'b) node * Geometry.placement * Gdsii.colrow * ipoint * ipoint

class virtual leaf_intf  =
  object
    method virtual full_view : (leaf_intf, hier_intf) node
    (* ... etc... *)
  end 
and virtual hier_intf =
  object
    inherit leaf_intf
    method virtual leaf_view : (leaf_intf, hier_intf) node
    method virtual insts : (leaf_intf, hier_intf) instance list
  end

let as_leaf : (leaf_intf,hier_intf) node -> leaf_intf = function 
    Leaf_node(l) -> (l :> leaf_intf)
  | Hier_node(h) -> (h :> leaf_intf)

let  as_hier : (leaf_intf,hier_intf) node -> hier_intf = function 
  (* ...etc... *)

Obviously, this only works well if you have a fairly small number of 
"abstract base classes", since each one is a type parameter. You don't even need 
to use parametric polymorphism, and you could just declare a type specific to 
this class hierarchy; take a look at 

http://cristal.inria.fr/~remy/cours/appsem/ocaml049.html#toc22

where Didier Remy gives an example using variants and projection functions. 

Also, the weak library mentioned elsewhere looks promising. 

As I get older and lazier, I wonder more if a lot of the OO approaches are 
really easier to grasp. A lot of people are going right for the O before going 
through Caml, and that seems wrong. 

-- Brian


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