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Date: Sat, 31 Jan 2009 08:55:22 -0800 (PST)
From: Dario Teixeira <darioteixeira@yahoo.com>
Subject: Union of type variables
To: caml-list@yquem.inria.fr
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X-Spam: no; 0.00; kludge:01 sig:01 val:01 foo:01 val:01 foo:01 struct:01 foobar:01 foobar:01 kludge:01 presupposes:01 variants:01 polymorphic:01 int:01 int:01 

Hi,

Consider the following toy experiment with phantom types.  Note that the sh=
own
implementation of function "union" is far from optimal; though it takes onl=
y
two arguments, these are being passed as a list.  This is basically just a
kludge that forces the phantom type in the return type to be also a union.

module M:
sig
=09type 'a t

=09val a: int -> [> `Foo ] t
=09val b: int -> [> `Foo ] t
=09val c: int -> [> `Bar ] t
=09val d: int -> [> `Bar ] t

=09val union: 'a t list -> 'a t
end =3D
struct
=09type foobar_t =3D
=09=09| A of int
=09=09| B of int
=09=09| C of int
=09=09| D of int
=09=09| Union of foobar_t * foobar_t

=09type 'a t =3D foobar_t

=09let a x =3D A x
=09let b x =3D B x
=09let c x =3D C x
=09let d x =3D D x

=09let union (x :: y :: []) =3D Union (x, y)
end


Obviously I would like to get rid of this kludge.  The signature and
implementation for "union" should be something like the (syntactically
incorrect) code below.  But is it at all possible to declare an union
of type variables?  (which presupposes they are polymorphic variants)

val union: 'x t -> 'y t -> [> 'x | 'y ] t
let union x y =3D Union (x, y)

Thanks in advance!
Best regards,
Dario Teixeira
=0A=0A=0A