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From: "Nobuyuki Tomizawa" <tomizawa@ccm.cl.nec.co.jp>
To: <caml-list@inria.fr>
Subject: [Caml-list] Question: 'instanceof'-like like primitive in OCaml
Date: Wed, 4 Apr 2001 22:03:58 +0900
Message-ID: <FNEDLLDONJLKENCGLOOIKENGCJAA.tomizawa@ccm.cl.nec.co.jp>
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Dear all,

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.

My solution is to use variant type for the list and identify the class
using pattern matching:

    type tag = Derived2 of d2 | DontCare of b;;

    let l = [ Derived2(new d2); DontCare(new d1 :> b)] in ...;;

But I feel this solution is awkward because we have to define variant
type for each classes I want to treat them as specific.

Could you please tell me more 'smart' answer or another way in OCaml
style?

Thanks in advance.

-- Nobuyuki
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