]> Hi, 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: sig 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 = struct 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) 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 = Union (x, y) Thanks in advance! Best regards, Dario Teixeira