From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
To: wlovas@stwing.upenn.edu
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Type constraints
Date: Wed, 08 Dec 2004 09:27:24 +0900 (JST) [thread overview]
Message-ID: <20041208.092724.63079073.garrigue@math.nagoya-u.ac.jp> (raw)
In-Reply-To: <20041207181344.GA14891@force.stwing.upenn.edu>
From: William Lovas <wlovas@stwing.upenn.edu>
> On Tue, Dec 07, 2004 at 06:44:36PM +0100, Damien Doligez wrote:
> >
> > On 7 Dec 2004, at 15:57, Andreas Rossberg wrote:
> >
> > >Is this really a counter-example? I don't see any problem with making
> > >it polymorphic - it evaluates to ref, and ref can happily be
> > >polymorphic.
> >
> > Yes, well I simplified it a bit too much. Try this instead:
> >
> > let module M = struct let v = ref [] end in M.v;;
>
> I'm still not convinced. Yes, the type variable should not be generalized
> in the above, by analogy with:
>
> # ref [];;
> - : '_a list ref = {contents = []}
>
> But the `let module' in question -- or one similar in spirit, at least --
>
> # let module M = struct let v = fun x -> x end in M.v;;
> - : '_a -> '_a = <fun>
>
> is analogous to the expression
>
> # fun x -> x
> - : 'a -> 'a = <fun>
>
> in which the type variable *is* generalized.
Analogies don't help you here, because the typechecker doesn't work by
analogies, but by explicit rules.
If you're curious, there is a function is_nonexpansive in
typing/typecore.ml. Only expressions for which this function returns
true will be generalized. (This is a direct implementation of the
syntactic value-generalization scheme.)
Now, this function doesn't now about Texp_letmodule, so any use of
this construct will never be generalized. I don't know exactly why
this was omitted, but I see the combination of two possible reasons:
this requires some amount of extra code, and one must assess its
validity. Yet I suppose this could be done.
By the way, the code is already there for immediate objects, so the
alternative approach with polymorphic methods does work (but generates
more code).
# let o = object method v : 'a. 'a -> 'a = fun x -> x end in
fun x -> o#v x;;
- : 'a -> 'a = <fun>
Jacques Garrigue
next prev parent reply other threads:[~2004-12-08 0:27 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-12-06 19:55 Jim Farrand
2004-12-07 7:12 ` [Caml-list] " Alain Frisch
2004-12-07 13:43 ` Damien Doligez
2004-12-07 14:57 ` Andreas Rossberg
2004-12-07 17:44 ` Damien Doligez
2004-12-07 18:08 ` Alain Frisch
2004-12-07 21:04 ` Damien Doligez
2004-12-07 21:43 ` Alain Frisch
2004-12-08 3:30 ` nakata keiko
[not found] ` <8002B033-4906-11D9-8195-000D9345235C@inria.fr>
2004-12-09 0:56 ` nakata keiko
2004-12-09 1:27 ` Jacques Garrigue
2004-12-08 10:53 ` Damien Doligez
2004-12-08 12:39 ` Alain Frisch
2004-12-08 14:23 ` Jacques Garrigue
2004-12-09 3:07 ` skaller
2004-12-09 4:53 ` Jacques Garrigue
2004-12-08 16:10 ` Xavier Leroy
2004-12-07 18:13 ` William Lovas
2004-12-08 0:27 ` Jacques Garrigue [this message]
2004-12-07 18:41 ` Boris Yakobowski
2004-12-07 19:38 ` Jim Farrand
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