polyvariant_indices.ml

type any = [ `A | `B | `C ]

type _ t =
| A : [> `A ] t
| B: [> `B ] t
| C: [> `C ] t

let tag (type a) (x: a t) = match x with
| A -> 0
| B -> 1
| C -> 2

let l : any t list = [ A; B; C ]

let only_a_or_b (x: [`A | `B ] t) = match x with
| A -> 0
| B -> 1
| _ -> .

let test = only_a_or_b A