Thanks for you reply.
Fortunately, I'm using Ocaml 3.05 and polymophic method works well.
But coding as your comments on weak type makes an error. See followings.

> From: "SooHyoung Oh" <shoh@duonix.com>
>
> > 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.
>
> Not weak: file_list has a weak type variable, but it gets immediatly
> instanciated to entry; and add is really polymorphic.
>
> > [Q 1] Is (PART2) is correct? I don't know why it doesn't make type
error.
> Yes. What's wrong?
> Note that your #entry is meaningless, since it is immediatly
> instanciated to entry. You could as well write entry list from the
> beginning, or nothing at all.
>

The types of (PART2) are
    val file_list : _#entry list ref = {contents = []}
    val add : #entry -> unit = <fun>
After removing '#':
    val file_list : entry list ref = {contents = []}
    val add : _#entry -> unit = <fun>

Using "entry" instead of "#entry" produces the following:
    This expression has type
      file = < file_size : unit -> int; name : unit -> string >
    but is here used with type < dir_size : unit -> int; name : unit ->
string >
    Only the second object type has a method dir_size

[Deleted ...]


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