Comment by nukifw

- You can use GADTs (https://ocaml.org/manual/5.2/gadts-tutorial.html) and indexes to give a concrete type to every constructors:

  ```ocaml
  type _ treated_as = 
   | Int : int -> int treated_as
   | Float : float -> float treated_as

  let f (Int x) = x + 1 (* val f : int treated_as -> int *)
  ```

- You can use the structurale nature of polymorphic variants (https://ocaml.org/manual/5.1/polyvariant.html)

  ```ocaml
  let f = function 
  | `Foo x -> string_of_int (x + 1) 
  | `Bar x -> x ^ "Hello"
  (* val f : [< `Foo of int | `Bar of string] -> string` *)

  let g = function
  | `Foo _ -> ()
  | _ -> () 
  (* val g : [> `Foo of 'a ] -> unit *)
  ```

(Notice the difference between `>` and `<` in the signature?)

And since OCaml has also an object model, you can also encoding sum and sealing using modules (and private type abreviation).