RE: [Caml-list] Re: module typing issue

[David Allsopp <dra-news@metastack.com>]

Guillaume Yziquel wrote:
> Le Monday 28 Mar 2011 à 09:53:52 (+0200), Alain Frisch a écrit :
> > On 03/25/2011 08:01 PM, Guillaume Yziquel wrote:
> > >type error = private int
> > >
> > >external get_error : unit ->  error = "zzz"
> > >
> > >You get some error level that you can pattern match as an int, and
> > >this also enforces that all values of type error come from a call to
> > >get_error.
> >
> > You need to coerce explicitly from type error to type int (... :>
> > int), so this is not really different from defining error as an
> > abstract type and exposing a function error->int.
> >
> > -- Alain
> 
> Yes, doesn't work for private ints. However, it works for stuff like that,
> though it's not exactly the same thing as a 'private int':
> 
> # type t = private A | B;;
> type t = private A | B
> # let x : t = Obj.magic 0;;
> val x : t = A
> # match x with A -> true | _ -> false;;
> - : bool = true
> # A;;
> Error: Cannot create values of the private type t
> 
> I thought it also worked on for ints, and in a sense, I'm surprised that
> you cannot pattern match private ints as ints without a coercion.

I asked a similar question when private ints were introduced in OCaml 3.11 - full explanation from Jacques as to why in: https://sympa-roc.inria.fr/wws/arc/caml-list/2008-10/msg00375.html


David