test.ml

module type Witness = sig type 'a key type 'a value end

module type Witnessed = sig
  type 'a key
  type 'a value
  type t
  type ('a, 'b) conv = {
    key : 'c . 'c key -> 'a;
    value : 'c . 'c value -> 'b;
  }
  val box : 'a key -> 'a value -> t
  val conv : ('a, 'b) conv -> t -> ('a * 'b)
end

module MAKE(W : Witness) : Witnessed
  with type 'a key = 'a W.key
   and type 'a value = 'a W.value =
struct
  include W

  type t = Box : 'a key * 'a value -> t
  let box k v = Box (k, v)
  type ('a, 'b) conv = {
    key : 'c . 'c key -> 'a;
    value : 'c . 'c value -> 'b;
  }
  let conv conv (Box (k, v)) = (conv.key k, conv.value v)
end

type _ token
type _ attrib

module W = struct
  type 'a key = 'a token
  type 'a value = 'a attrib
end

module Boxed = struct
  include MAKE(W)
  type lexeme = t
  type ('a, 'b) lexeme_conv = ('a, 'b) conv
end