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Union of type variables
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Date: -- (:)
From: Dario Teixeira <darioteixeira@y...>
Subject: Union of type variables

Consider the following toy experiment with phantom types.  Note that the shown
implementation of function "union" is far from optimal; though it takes only
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:
	type 'a t

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

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

	type 'a t = foobar_t

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

	let union (x :: y :: []) = Union (x, y)

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 = Union (x, y)

Thanks in advance!
Best regards,
Dario Teixeira