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Date: Fri, 19 Sep 2003 10:21:56 -0400
To: caml-list@inria.fr
Subject: [Caml-list] Subclasses and pattern matching
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From: katre <katre@henchmonkey.org>
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Hello again!

I'm working on a project where I want to create a list of data, which is
all similar but not identical.  In other languages with which I am
familiar, I would do this with a base class and a few subclasses, as
such:

class virtual abstractClass = 
    object

    method virtual to_string : unit -> string
end

class subclass1 = 
    object
    inherit abstractClass

    method thing () = "subclass1 thing"
    method to_string () = "subclass 1"
end

class subclass2 = 
    object
    inherit abstractClass

    method thing2 () = "subclass2 thing"
    method to_string () = "subclass 2"
end

And I would create the list as

let l = [ (new subclass1 :> abstractClass); (new subclass :> abstractClass); (new subclass1 :> abstractClass); (new subclass2 :> abstractClass) ];

This seems to work fine.

However, I want to use pattern matching to go over this list.  I'm not
sure how this would work in ocaml.  My ideal function would look
something like this:

let rec show_list l =
    match l with
        [] -> ()
        | (h ofclass subclass1)::t -> print_string (h#thing()); show_list t
        | (h ofclass subclass2)::t -> print_string (h#thing2()); show_list t
        | (h ofclass abstractClass)::t -> print_string ("Unknown object " ^ (h#to_string())); show_list t


Is this possible?  What would the syntax be?

katre

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