Date: Sat, 28 Nov 1998 13:46:12 +0300 (MSK)
From: Anton Moscal <msk@post.tepkom.ru>
To: Didier.Remy@inria.fr
Subject: Re: Objects as sums
In-Reply-To: <19981126165842.07547@morgon.inria.fr>
On Thu, 26 Nov 1998, Didier Remy wrote:
> class a = object (self : 'a)
> method b () = ((assert false): 'a)
> method c () = ((assert false): 'a)
> end;;
>
> Here, the methods b and c return an object of the same type as their own
> type. In particular, in a subclass, they will return an object of the type
> of objects of the subclass...
>
> Then, the rest of the example works unchanged.
>
> class c = object (self)
> inherit a
> method c () = self
> end;;
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);
}
}
This program may be easily translated to ocaml as follows:
type tree = Leaf of int | Tree of tree * tree
let rec sum = function
Leaf v -> v
| Tree (l, r) -> sum l + sum r
but I would like to have solution with classes.
Your proposals do not work: type of (tree#node ()) will have `tree' type
and will fail on selection of a method, specific for node.
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?
Regards,
Anton
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