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What does underscore mean in this type expression?
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Date: -- (:)
From: Richard Jones <rich@a...>
Subject: Re: [Caml-list] What does underscore mean in this type expression?
On Sat, Aug 30, 2008 at 03:57:54PM +0200, Christophe TROESTLER wrote:
> On Sat, 30 Aug 2008 14:03:10 +0100, Richard Jones wrote:
> > 
> > > # let x = ref `X;;
> > > val x : _[> `X ] ref = {contents = `X}
> > 
> > OK ... so how to get rid of the error?  Even supplying a full type for
> > get_domains in the implementation doesn't get rid of it.
> > 
> > Here's the code if anyone wishes to try:
> > 
> >   http://www.annexia.org/tmp/libvirt.mli
> >   http://www.annexia.org/tmp/libvirt.ml
> > 
> >   $ ocamlc -c libvirt.mli
> >   $ ocamlc -c libvirt.ml
> 
> Here is the crux of your problem
> 
> let f = let a = ref 0 in fun (x : [> `R ]) ->  x;;
> 
> If you hoist [let have_list_all_domains = ref None] outside [let
> get_domains], then it works fine.  The type checker could certainly be
> slightly improved [1] there but I do not know how difficult it is.

Yes, that fixed it, thanks.

> [1] And maybe also the compiler: a rough check seem to indicate that
> hiding the variable in the definition of f is slightly slower. 

It use this idiom quite a lot to emulate private (static) variables.

Rich.

-- 
Richard Jones
Red Hat