From: Didier Le Botlan <didier.le-botlan@polytechnique.org>
To: Nobuyuki Tomizawa <tomizawa@ccm.cl.nec.co.jp>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Question: 'instanceof'-like like primitive in OCaml
Date: Thu, 05 Apr 2001 21:09:54 +0200 [thread overview]
Message-ID: <3ACCC302.3FE661E5@polytechnique.org> (raw)
In-Reply-To: <FNEDLLDONJLKENCGLOOIKENGCJAA.tomizawa@ccm.cl.nec.co.jp>
Hello,
Because of strong typing, heterogeneous lists are not allowed (which is
actually good).
As far as I understand, what you need is
- some kind of "instanceof"
- some kind of dynamic cast (depending on the result of "instanceof").
You have many ways to get instanceof :
For example you can declare a virtual method "instanceOfSpecific : bool"
in base class, and each subclass must define it.
Dynamic cast is not possible here because a base object cannot be seen
as a "derived2" object in general.
You can declare a method "specific" which raises an exception (default
behaviour) and which is overriden in class "derived2" to do what
expected.
Thus, if you incorrectly call "specific" method on a "derived1" object,
an exception is raised. This corresponds somehow to an incorrect dynamic
cast in Java.
Finally, here is a short example :
class virtual base =
object
method common x = x+1
method virtual isSpecific : bool
(* 'specific' returns a string. failwith is polymorph, we must help
type checker. *)
method specific = ((failwith "specific is not defined !!!") : string)
end
class derived1 =
object
inherit base
method isSpecific = false
end
class derived2 message =
object
inherit base
method isSpecific = true
method specific = "Hello " ^ message
end
let example =
print_endline "Starting...";
let obj1 = new derived1
and obj2 = new derived2 "World"
and obj3 = new derived2 "Folks" in
let list = [ obj1 ; obj2 ; obj3; obj2 ; obj1 ] in
let apply_specific obj =
if obj#isSpecific then print_endline obj#specific
else print_endline "No Specific"
in
List.iter apply_specific list
compile :
ocamlc -o exam exam.ml
and try...
Nobuyuki Tomizawa wrote:
> I'm a novice OCaml programmer and have a question about heterogeneous
> list and "downward cast".
>
> Here is a pseudo Java code (I have):
>
> class base {
> void common() {...};
> }
>
> class derived1 extends base {
> }
>
> class derived2 extends base {
> void specific() {..};
> }
>
> and what I want to do are:
>
> * make the list which typed as "base list".
> * call `derived2#specific' method if the element in the list is
> an instance of 'derived2'.
>
> But, OCaml seems not to have Java's `instanceof'-like primitive and/or
> downward-cast primitive.
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next prev parent reply other threads:[~2001-04-08 20:07 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-04-04 13:03 Nobuyuki Tomizawa
2001-04-05 18:15 ` Brian Rogoff
2001-04-05 19:09 ` Didier Le Botlan [this message]
2001-04-06 9:14 ` Gerd Stolpmann
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