(* reactive.ml *)
module Declarative = struct
  module Map = Hmap

  type ex_index = Map.Key.t

  type _ type_ =
  | T_String : string type_
  | T_Tuple_2 : ('a type_ * 'b type_) -> ('a * 'b) type_

  type _ constructor =
  | C_String : string -> string constructor
  | C_Tuple_2 : ('a constructor * 'b constructor) -> ('a * 'b) constructor

  type _ reactive =
  | R_String : string Variable.t -> string reactive
  | R_Tuple_2 : (('a , 'b) Tuple_2.t * 'a reactive * 'b reactive) -> ('a * 'b) reactive

  let get_var : type a . a reactive -> a Variable.t = function
  | R_String x -> x
  | R_Tuple_2 (tpl , _ , _) -> (
    let updater_forth (a , b) = (a , b) in
    let updater_back (a , b) = (a , b) in
    Tuple_2.map_from ~updater_forth ~updater_back tpl
  )

  let rec make : type a . a constructor -> a reactive = function
  | C_String str -> (
    let v = Variable.make str in
    R_String v
  )
  | C_Tuple_2 (a , b) -> (
    let a' = make a in
    let b' = make b in
    let tpl = Tuple_2.make ~bidirectional:true (get_var a' , get_var b') in
    R_Tuple_2 (tpl , a' , b')
  )

end

(* main.ml *)
let () =
  let (R_Tuple_2 (fullName , R_String firstName , R_String lastName)) = R.Declarative.make @@ C_Tuple_2 (C_String "Will" , C_String "Smith") in
  ...