From: "Nicolás Ojeda Bär" <nicolas.ojeda.bar@lexifi.com>
To: "Richard W.M. Jones" <rich@annexia.org>
Cc: Gabriel Scherer <gabriel.scherer@gmail.com>,
caml users <caml-list@inria.fr>
Subject: Re: [Caml-list] List of structurally typed objects
Date: Wed, 22 Feb 2017 18:43:06 +0100 [thread overview]
Message-ID: <CADK7aFOAtvmdA_NjhBuRe4=7ZjUgTXHxgwj5XqLDYwvn5vkjAA@mail.gmail.com> (raw)
In-Reply-To: <20170222173645.GF28111@annexia.org>
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Hi Richard,
Note that you can still write < hello: string; .. > in the signature of
register_obj and do the cast before adding your object to the list:
let register_obj name obj = objs := (name, (obj :> < hello: string >)) ::
!objs
This way you do not need to downcast at each call-site of register_obj.
Cheers,
Nicolas
On Wed, Feb 22, 2017 at 6:36 PM, Richard W.M. Jones <rich@annexia.org>
wrote:
> On Wed, Feb 22, 2017 at 12:09:37PM -0500, Gabriel Scherer wrote:
> > The signature you demand is actually incorrect, because the return
> > type of get_obj is to permissive: it claims that it can return *any*
> > ('a obj), so you could register an object with just the hello method
> > and get back an object with more methods.
> >
> > On the other hand, if you use the monomorphic < hello : string > type
> > in the interface, then it is type-correct and the implementation you
> > provide is accepted. It is explicit in the fact that you can't recover
> > the extra methods of the object once they have been stored in this
> > lowest-common-denominator container.
>
> Ah, I see.
>
> Modifying the code to use the monomorphic type works, but I have to
> downcast the objects to the right type. For reference, the working
> code is below.
>
> Thanks,
>
> Rich.
>
> list_fns.mli --------------------------------------------------------
> type obj = < hello : string >
> val register_obj : string -> obj -> unit
> val get_obj : string -> obj
>
> list_fns.ml ---------------------------------------------------------
> type obj = < hello : string >
> let objs = ref []
> let register_obj name obj = objs := (name, obj) :: !objs
> let get_obj name = List.assoc name !objs
>
> test.ml -------------------------------------------------------------
> class foo = object
> method hello = "hello from foo"
> method goodbye () = print_endline "goodbye"
> end
>
> class bar = object
> method hello = "hello from bar"
> end
>
> let () =
> let o1 = new foo in
> let o2 = new bar in
> List_fns.register_obj "o1" (o1 :> List_fns.obj);
> List_fns.register_obj "o2" (o2 :> List_fns.obj);
> print_endline ("calling o1: " ^ (List_fns.get_obj "o1")#hello);
> print_endline ("calling o2: " ^ (List_fns.get_obj "o2")#hello)
> --------------------------------------------------------------------
>
> --
> 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
>
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next prev parent reply other threads:[~2017-02-22 17:43 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-22 17:01 Richard W.M. Jones
2017-02-22 17:09 ` Gabriel Scherer
2017-02-22 17:36 ` Richard W.M. Jones
2017-02-22 17:43 ` Nicolás Ojeda Bär [this message]
2017-02-22 17:53 ` Richard W.M. Jones
2017-02-22 17:28 ` Damien Guichard
2017-02-22 17:38 ` Richard W.M. Jones
2017-02-22 18:08 ` Ivan Gotovchits
2017-02-22 19:04 ` Mikhail Mandrykin
2017-02-22 19:21 ` [Caml-list] Warning for unused variables Aymeric Fromherz
2017-02-22 19:43 ` Gabriel Scherer
2017-02-22 19:59 ` Aymeric Fromherz
2017-02-22 20:01 ` Gabriel Scherer
2017-02-22 20:17 ` Aymeric Fromherz
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