From: Brian Rogoff <bpr@artisan.com>
To: erayo@cs.bilkent.edu.tr
Cc: garrigue@kurims.kyoto-u.ac.jp, jhw@wetware.com, caml-list@inria.fr
Subject: Re: [Caml-list] subtyping and inheritance (bug?)
Date: Tue, 25 Jun 2002 14:33:41 -0700 [thread overview]
Message-ID: <15640.57781.702368.271986@granite.artisan.com> (raw)
In-Reply-To: <200206251522.28391.erayo@cs.bilkent.edu.tr>
Eray Ozkural writes:
> On Tuesday 25 June 2002 05:54, Jacques Garrigue wrote:
> > Sure. The children_set method contains a contravariant occurence of 'self.
> > It cannot be compatible with its subclasses. You don't need to have
> > mutable data to have variance incompatibilities.
> > No bug, business as usual.
>
> Which programming style would express the intended semantics, then? I guess we
> would need a slightly different formulation.
Yes, you need a different formulation. If you want to have a tree class but
want to create a family of color_trees which can be coerced to each other
and put on the same list, something like this
class ['a] tree =
object
val v = (None : string option)
val ch = ([] : 'a list)
method value_get = v
method value_set x = {< v = x >}
method children_get = ch
method children_set x = {< ch = x >}
end;;
class color_tree =
object
inherit [color_tree] tree
method color = "green"
end;;
class pine_tree =
object
val mutable age = 0.0
inherit color_tree
method color = "evergreen"
method age = age
end;;
class fake_tree =
object
inherit color_tree
method color = "olivegreen"
method material = "fantastic plastic"
end;;
now these can all be coreced to color_tree, and they have a colortree list
type on method children_get.
let colortree = new color_tree;;
let pinetree = new pine_tree;;
let faketree = new fake_tree;;
let trees = [colortree;
(pinetree :> color_tree);
(faketree :> color_tree)];;
faketree#children_set trees;;
If you create a new class,
class palm_tree =
object
inherit [palm_tree] tree
method color = "palmgreen"
method fronds = true
end;;
then you won't be able to do the coercion.
let palmtree = new palm_tree;;
(palmtree :> color_tree);; (* ERROR! *)
-- Brian
-------------------
To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
prev parent reply other threads:[~2002-06-25 21:35 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-06-25 2:46 james woodyatt
2002-06-25 2:54 ` Jacques Garrigue
2002-06-25 12:22 ` Eray Ozkural
2002-06-25 21:33 ` Brian Rogoff [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=15640.57781.702368.271986@granite.artisan.com \
--to=bpr@artisan.com \
--cc=caml-list@inria.fr \
--cc=erayo@cs.bilkent.edu.tr \
--cc=garrigue@kurims.kyoto-u.ac.jp \
--cc=jhw@wetware.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).