Re: Objects as sums

[Jerome Vouillon <Jerome.Vouillon@inria.fr>]

On Sat, Nov 28, 1998 at 01:46:12PM +0300, Anton Moscal wrote:

[...]
> This is not solution of my task. I want to simulate by ocaml classes the
> following C++ program:
> 
> # include <stdlib.h>
> 
> enum {NODE, LEAF};
> class Node;
> class Leaf;
> class Tree { public:
>   virtual int tag () = 0;
>   virtual Node * node () { abort (); return 0; }
>   virtual Leaf * leaf () { abort (); return 0; }
> };
> 
> class Leaf: public Tree { public:
>   int val;
>   int tag () { return LEAF; }
>   Leaf * leaf () { return this; }
> };
> 
> class Node: public Tree { public:
>   Tree * l, * r;
>   int tag () { return NODE; }
>   Node * node () { return this; }
> };
> 
> int sum (Tree * tree)
> {
>   switch (tree -> tag ()) 
>     {
>     case LEAF: return tree -> leaf () -> val;
>     case NODE: return sum (tree -> node () -> l) + sum (tree -> node () -> r);
>     }
> }
>
[...]
> 
> PS: this trick implements type-safe conversion down to objects hierarchy.
>     Question of my interest is the following: whether such
>     conversion is possible or not?

Yes, it is possible to translate the program into Ocaml.

The difficulty you have probably encounter is that in class "leaf",
for instance, the type of self is not "leaf", but a more general type.
The implementation of method "full_object" below must therefore
contain a coercion. As it is not possible to coerce to a type that is
not yet completely defined (this is the case for type "leaf" at this
point), I used an extra class "leaf_intf" to provide a type for the
coercion.

I have used a single method "full_object" rather than two methods
"node" and "leaf". This way, there is no need to raise any
exception. But this change was not necessary.

Regards,

-- Jérôme

type ('a, 'b) alt = First of 'a | Second of 'b;;

class virtual tree = object
  method virtual full_object : (leaf_intf, node_intf) alt
end and virtual leaf_intf = object
  inherit tree
  method virtual v : int
end and virtual node_intf = object
  inherit tree
  method virtual l : tree
  method virtual r : tree
end;;

class leaf v0 = object (self)
  inherit leaf_intf
  method full_object = First (self :> leaf_intf)
  method v = v0
end;;

class node l0 r0 = object (self)
  inherit node_intf
  method full_object = Second (self :> node_intf)
  method l :> tree = l0
  method r :> tree = r0
end;;

let rec sum t =
  match t#full_object with
    First t' ->
      t'#v
  | Second t' ->
      sum (t'#l) + sum (t'#r);;

# sum (new node (new leaf 3) (new node (new leaf 1) (new leaf 2)) :> tree);;
- : int = 6