From: Francois Pottier <francois.pottier@inria.fr>
To: Ohad Rodeh <ORODEH@il.ibm.com>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] mutability analysis too strict?
Date: Mon, 10 Dec 2001 11:50:40 +0100 [thread overview]
Message-ID: <20011210115040.A31678@pauillac.inria.fr> (raw)
In-Reply-To: <OFF96B3E7D.145DD6FC-ONC2256B1E.00379C10@telaviv.ibm.com>; from ORODEH@il.ibm.com on Mon, Dec 10, 2001 at 12:13:19PM +0200
On Mon, Dec 10, 2001 at 12:13:19PM +0200, Ohad Rodeh wrote:
>
> How do you propose building a repository of polymorphic values then?
Unless I'm mistaken, the short answer is, you can't. As pointed out by
Rémi, your proposed interface:
module type Repos = sig
val put : 'a -> 'b -> unit
val get : 'a -> 'b
end
is inherently unsafe. One solution is to use several repositories
(i.e. several hash tables). You can wrap them up into a single module if you
wish, but the client will then need to tell which repository (i.e. which hash
table) he wishes to access:
module type Repositories = sig
type ('a, 'b) repository
val put : ('a, 'b) repository -> 'a -> 'b -> unit
val get : ('a, 'b) repository -> 'a -> 'b
end
Not very useful, though, since this merely paraphrases the interface for hash
tables.
Another solution is to create a single repository where keys and/or objects
belong to a suitably defined `universal' (also known as `dynamic') type. That
is, the repository would be a hash table of type [(univ, univ) Hashtbl.t],
where the abstract type [univ] supports the following operations:
type univ
type 'a tag
val new_tag: unit -> 'a tag
val in: 'a tag -> 'a -> univ
val out: 'a tag -> univ -> 'a
A value of arbitrary type can be turned into a [univ] value by attaching it
with an explicit tag, using [in]. It can later be retrieved by matching it
against the same tag, using [out]. ([out] may fail dynamically if the tags do
not match.) [out] is type-safe because it requires a tag; it would be unsound
for [out] to have type unit -> 'a (same problem as for [get] above).
One possible implementation for [univ], where a tag is implemented as a
reference cell, was posted to the list a while ago, but I can't find it at the
moment. Another possible implementation implements tags using exception names.
(In this variant, the type [tag] disappears, and [new_tag] is a functor which
produces specialized versions of [in] and [out].) These solutions are rather
tricky to implement; furthermore, they are inefficient and unsafe, because
[out] involves a run-time check. You are probably better off using several
hash tables.
--
François Pottier
Francois.Pottier@inria.fr
http://pauillac.inria.fr/~fpottier/
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next prev parent reply other threads:[~2001-12-10 10:50 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-12-10 10:13 Ohad Rodeh
2001-12-10 10:30 ` Basile STARYNKEVITCH
2001-12-10 10:50 ` Francois Pottier [this message]
2001-12-10 11:21 ` Mark Seaborn
-- strict thread matches above, loose matches on Subject: below --
2001-12-10 9:12 Ohad Rodeh
2001-12-10 9:28 ` Remi VANICAT
2001-12-09 15:43 Ohad Rodeh
2001-12-10 8:13 ` Francois Pottier
2001-12-09 12:14 [Caml-list] Mutability " Ohad Rodeh
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