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From: "David Allsopp" <dra-news@metastack.com>
To: "'David Rajchenbach-Teller'" <David.Teller@univ-orleans.fr>,
	"=?iso-8859-1?Q?'Gr=E9goire_Seux'?=" <kamaradclimber@gmail.com>
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Subject: RE: [Caml-list] define incompatible type
Date: Fri, 12 Feb 2010 10:59:02 -0000
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X-Spam: no; 0.00; gregoire:01 ocaml:01 integers:01 ocaml:01 integers:01 compiler:01 sig:01 val:01 struct:01 eliminates:98 wrote:01 incompatible:01 caml-list:01 constructor:01 coercion:01 

David Rajchenbach-Teller wrote:
> =A0=A0 =A0 Hi Gr=E9goire,
>=A0It's not directly possible in OCaml, but there are at least three =
methods
for doing what you
> want.
>
> The first one is to wrap your integers behind a constructor, e.g.

<snip>

You can also use (post OCaml 3.11.0) private types if you want to be =
able to
use the ID values as integers but only explicitly. While I'm sure that =
the
ocaml compiler eliminates calls to the identity function, I like the
elegance that the conversion from id to int is a type coercion instead =
of a
function call. In real world uses, chances are that of_int is a useful
function doing actual work and not the identity function!

module User :
  sig
    type id =3D private int
    val of_int : int -> id
  end =3D
  struct
    type id =3D int
    let of_int x =3D x
  end;;

let a =3D User.of_int 57
and b =3D User.of_int 57;;

a =3D b;;           (* true *)
a =3D 57;;          (* type error *)
(a :> int) =3D 57;; (* true *)


David