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Date: -- (:)
From: SooHyoung Oh <shoh@d...>
Subject: [Caml-list] Weak type, Open type

Hi, Ocamlers!

While I've studying design pattern with ocaml,
I made the followding 3 classes (PART1):
(1) class virtual entry: base class
(2) class file: subclass of "entry" class
(3) class dir: subclass of "entry" class

I made some code using these classes on top level and it's ok. (PART 2).
While I translate the top level code into new class definition (PART3), it
makes some error.
The error message is on (PART 4).

It seems that (PART2) is weak type and (PART 3) is open type.

[Q 1] Is (PART2) is correct? I don't know why it doesn't make type error.
[Q 2] (depend on Q 1)
Can I translate (PART2) into new class definition like (PART 3) without type
error?
Then how?

============ (PART 1) class definition ===============
class virtual entry () =
object
    method virtual name: unit -> string
end;;

class file name =
object(self)
    inherit entry ()
    method name () = name
    method file_size () = 100
end;;

class dir name =
object(self)
    inherit entry ()
    method name () = name
    method dir_size () = 3000
end;;

=============== (PART 2) NO ERROR with weak type (?) ===================
let file_list = ref ([]: #entry list);;
let add e = file_list := (e:>entry) :: !file_list;;
let _ = add (new dir "root");;
let _ = add (new file "hello.txt");;

================ (PART 3) ERROR with open type (?) ================
class dir2 name =
object(self)
    inherit entry ()

    val file_list = ref ([]: #entry list)

    method name () = name
    method add e = file_list := (e:>entry) :: !file_list
end;;

================ (PART 4) error message  ================
    Some type variables are unbound in this type:
      class dir2 :
        string ->
        object
         val file_list : entry list ref
          method add : #entry -> unit
          method name : unit -> string
       end
    The method add has type #entry -> unit where .. is unbound

---
SooHyoung Oh
tel: 02)583-8709, 042)861-8649
cell. phone: 011-453-4303
web: http://www.duonix.com


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