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[Caml-list] Subclasses and pattern matching
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Date: -- (:)
From: Claude Marche <Claude.Marche@l...>
Subject: Re: [Caml-list] Subclasses and pattern matching

>>>>> "katre" == katre  <katre@henchmonkey.org> writes:

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

    katre> [...]

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

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


    katre> Is this possible?  What would the syntax be?

You have to choose between using a union type and then
program with pattern matching, or choose classes and then use
inheritance. In your case, with classes :


class virtual abstractClass = 
    object
    method virtual to_string : unit -> string
    method virtual thing : unit -> string
end

class subclass1 = 
    object
    inherit abstractClass
    val i = 0
    method thing () = "I'm an integer whose value is " ^ (string_of_int i)
    method to_string () = "subclass 1"
end

class subclass2 = 
    object
    inherit abstractClass
    val v = 1.0
    method thing () =  "I'm an float whose value is " ^ (string_of_float v)
    method to_string () = "subclass 2"
end


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

let show_list =
  List.iter (fun o -> print_string (o#thing()));;

show_list l;;

Hope this helps,


-- 
| Claude Marché           | mailto:Claude.Marche@lri.fr |
| LRI - Bât. 490          | http://www.lri.fr/~marche/  |
| Université de Paris-Sud | phoneto: +33 1 69 15 64 85  |
| F-91405 ORSAY Cedex     | faxto: +33 1 69 15 65 86    |

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