Hello,

I want to define a functor that defines a type t and functions on that type such that t is a subtype of a polymorphic variant type.

For instance, if we have a type

type t_full = [ `A of int | `B of string | `C of int*int | `D ]

I want to be able to define a functor similar to this

module type T_SUBTYPE =
sig

  type t_full = [ `A of int | `B of string | `C of int*int | `D ]

  type t = [< t_full ] (* Not accepted by OCaml *)
end

module OpsOnTSubtype (T: T_SUBTYPE) =
struct
type t = T.t

let f : [> t ] -> int =
function
| `A i -> i
| `B s -> String.length s
| `C (l,r) -> l + r
| `D -> 0
end

However, I don’t know how to define the module signature T_SUBTYPE.

Is this possible in OCaml ?

Thanks in advance,

Frédéric