Hiding private variants v1

pv.ml

module type S = sig

  type priv
  type exposed = [ `A | `Priv of priv ]

  val mkB : unit -> priv
  
  val f : priv -> priv
  val g : priv -> string

  val extract : priv -> exposed
  val inject : exposed -> priv
  
end

module M : S = struct

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

  let mkB () = `B
  
  let f = function
      `A -> `B
    | `B -> `C
    | `C -> `A

  let g = function
      `A -> "A"
    | `B -> "B"
    | `C -> "C"

  type exposed = [ `A | `Priv of priv ]

  let extract = function
    | `A -> `A
    | p  -> `Priv p

  let inject = function
    | `A -> `A
    | `Priv p -> p
  
end

let _ =
  M.( inject (`Priv (mkB ())) |> f |> g)
  |> print_endline