From: Didier Cassirame <didier.cassirame@gmail.com>
To: Christopher Zimmermann <madroach@gmerlin.de>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Re: typing mutually recursive classes
Date: Wed, 24 Oct 2012 22:35:51 +0200 [thread overview]
Message-ID: <CA+LkvyodGYtzSwN=UZLcx6sKWwdmg8W6rH3RkphMHYFZZMEPmg@mail.gmail.com> (raw)
In-Reply-To: <20121024213213.2e447165b7c4ea9ad9d8e154@gmerlin.de>
[-- Attachment #1: Type: text/plain, Size: 3972 bytes --]
Hi,
Your first two examples compile without problem on my system, using either
v. 3.11.2 or 4.00.
If you use a parameterized registry class on the element type, you will not
be able to store polymorphic values, which is, I reckon, what you
probably want to achieve.
I don't think there's a way to avoid casting the parameters of set to the
#element superclass outside the set method. If you only have one final
element class, just use the parameterized version of the registry, or
replace the register method by a private register_elem method, and add a
public register method in the subclass, doing the cast.
Something like that :
module P =
struct
class element id (registry :registry) =
object
method registry = registry
end
and registry =
object
val mutable set : element list = []
method register_elem : element -> unit =
fun s ->
set <- s :: set
end
end
module I =
struct
class element id registry =
let r = (registry :> P.registry) in
object(self)
inherit P.element id r as super
end
class registry =
object(self)
inherit P.registry as super
method register : element -> unit =
fun x -> super#register_elem (x :> P.element)
end
end
Cheers,
Didier
2012/10/24 Christopher Zimmermann <madroach@gmerlin.de>
> On Wed, 24 Oct 2012 20:40:27 +0200
> Gabriel Scherer <gabriel.scherer@gmail.com> wrote:
>
> > I don't really understand what you are trying to achieve with this
> > #foo types.
>
> It's the simplest statement possible to demonstrate the typing error I
> ran into. Even simpler:
>
> class type a = object end
> and b =
> object
> method foo: 'a. (#a as 'a) -> unit
> end
>
> fails, but
>
> class type a = object end
> class type b =
> object
> method foo: 'a. (#a as 'a) -> unit
> end
>
>
> works fine. Why?
>
> > What would even be the type of "set" in your example? You
> > cannot hold a mutable reference to a polymorphic type (#element list),
> > so I'm not sure what you would have but (element list).
> That's correct. It should read
> val mutable set = []
> method register :'a. (#element as 'a) -> unit =
> fun s ->
> set <- (s : #element :> element) :: set
>
> > If you're
> > going to coerce your elements into the common (element) supertype
> > anyway, why insist on having flexible bounds? You could just use
> > (registry) and (element), coerce when needed (foo :> element), and get
> > rid of those pesky typing issues.
>
> That's my current workaround for this issue. But I would prefer a
> solution where the coercion happens in the registry.
>
> >
> > On Wed, Oct 24, 2012 at 8:03 PM, Christopher Zimmermann
> > <madroach@gmerlin.de> wrote:
> > > Hi,
> > >
> > > I have a problem with typing a system of mutually recursive classes.
> > >
> > > This piece of code fails to compile:
> > >
> > > class a =
> > > object end
> > > and b =
> > > object
> > > method foo: a -> int =
> > > fun s -> 3
> > > end;;
> > >
> > > Error: The universal type variable 'a cannot be generalized:
> > > it escapes its scope.
> > >
> > >
> > > But this compiles fine:
> > >
> > > class a =
> > > object end
> > > class b =
> > > object
> > > method foo: 'a. (#a as 'a) -> int =
> > > fun s -> 3
> > > end;;
> > >
> > >
> > > What I actually want to do is this:
> > >
> > > class element id (registry :#registry) =
> > > object
> > > method registry = registry
> > > end
> > >
> > > and registry =
> > > object
> > > val set = []
> > > method register :'a. (#element as 'a) -> unit =
> > > fun s ->
> > > set <- s :: set
> > > end
> > >
> > >
> > > Any ideas how to do this without parametrizing the classes?
> > >
> > > Christopher
>
>
> --
> Caml-list mailing list. Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs
>
[-- Attachment #2: Type: text/html, Size: 6208 bytes --]
next prev parent reply other threads:[~2012-10-24 20:36 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-24 18:03 [Caml-list] " Christopher Zimmermann
2012-10-24 18:40 ` Gabriel Scherer
2012-10-24 19:32 ` [Caml-list] " Christopher Zimmermann
2012-10-24 20:35 ` Didier Cassirame [this message]
2012-10-24 21:30 ` Didier Cassirame
[not found] ` <CAPFanBHdh8xKsZC6fs2oXEOitjKLhZzVUdVmYT+6d0jL46YM0w@mail.gmail.com>
2012-10-24 21:04 ` [Caml-list] " Gabriel Scherer
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='CA+LkvyodGYtzSwN=UZLcx6sKWwdmg8W6rH3RkphMHYFZZMEPmg@mail.gmail.com' \
--to=didier.cassirame@gmail.com \
--cc=caml-list@inria.fr \
--cc=madroach@gmerlin.de \
/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).