From: Pierre Weis <Pierre.Weis@inria.fr>
To: orodeh@cs.huji.ac.il (Ohad Rodeh)
Cc: caml-list@inria.fr
Subject: Re: Ref polymorphism restriction
Date: Mon, 25 Dec 2000 23:09:40 +0100 (MET) [thread overview]
Message-ID: <200012252209.XAA11293@pauillac.inria.fr> (raw)
In-Reply-To: <Pine.LNX.4.10.10012250930420.7348-100000@zigzag.cs.huji.ac.il> from Ohad Rodeh at "Dec 25, 100 09:34:13 am"
> List,
> The following program does not compile, with a
> rather strange error message.
>
> module type SIG =
> sig
> val l : '_a list ref
> val f : '_a -> unit
> end
Warning, there is a strange feature here that makes your '_a as type
schemes paramters (generalized variables) and not type variables:
Objective Caml version 3.00
# module type SIG = sig
val l : '_a list ref
val f : '_a -> unit
end;;
module type SIG = sig val l : 'a list ref val f : 'a -> unit end
#
In effect 'ident means a type scheme parameters. When we introduced
type variables, we denote them by an _ prefix, that was not allowed in
identifiers (as the first character) at that time. Now, identifiers
are allowed to start with an _. Hence '_foo is a perfectly valid
polymorphic type parameter!
> module M : SIG =
> struct
> let l = ref []
>
> let f a = l:= a::!l
> end
>
>
> Modules do not match:
> sig val l : '_a list ref val f : '_a -> unit end
> is not included in
> SIG
> Values do not match:
> val l : '_a list ref
> is not included in
> val l : 'a list ref
>
> Note that the signature for list l IS '_a list ref.
As mentioned above, no: it is 'a list ref. This explains the
typechecker message.
> I suppose this is due to the restriction on polymorphic references.
> Is the OCaml restriction such that references may only be local to
> a single module? Is there a different restriction?
>
> Ohad.
>
> ----------------------------------------------------------
> orodeh@cs.huji.ac.il
> www.cs.huji.ac.il/~orodeh
Anyway, your right: type variables ('_a types) should not escape the
scope of a module.
Merry Christmas,
Pierre Weis
INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/
prev parent reply other threads:[~2000-12-25 22:09 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2000-12-25 7:34 Ohad Rodeh
2000-12-25 22:09 ` Pierre Weis [this message]
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