(* Attempt 1: Does not work, because type not exported *)

module type IntervalSig =
  sig
    type t
    val create : int -> int -> t
  end

module Interval : IntervalSig =
  struct
    type t = Empty | Range of int * int
    let create lo hi = if (lo <= hi) then Range(lo, hi) else Empty
  end

module type ExtendedIntervalSig =
  sig
    include IntervalSig
    val contains : t - > int -> bool
  end

module ExtendedInterval : ExtendedIntervalSig =
  struct
    include Interval
    let contains intvl x =
      match intvl with
      | Empty -> false
      | Range(lo, hi) -> lo <= x && x <= hi
  end

(* Attempt 2: Works, becuase implementation exported as part of signature *)

module type IntervalSig =
  sig
    type t = Empty | Range of int * int
    val create : int -> int -> t
  end

module Interval : IntervalSig =
  struct
    type t = Empty | Range of int * int
    let create lo hi = if (lo <= hi) then Range(lo, hi) else Empty
  end

module type ExtendedIntervalSig =
  sig
    include IntervalSig
    val contains : t -> int -> bool
  end

module ExtendedInterval : ExtendedIntervalSig =
  struct
    include Interval
    let contains intvl x =
      match intvl with
      | Empty -> false
      | Range(lo, hi) -> lo <= x && x <= hi
  end

(* Attempt 3: Does not work, because open only imports the names, but does not extend the structure *)

module type IntervalSig =
  sig
    type t = Empty | Range of int * int
    val create : int -> int -> t
  end

module Interval : IntervalSig =
  struct
    type t = Empty | Range of int * int
    let create lo hi = if (lo <= hi) then Range(lo, hi) else Empty
  end

module type ExtendedIntervalSig =
  sig
    include IntervalSig
    val contains : t -> int -> bool
  end

module ExtendedInterval : ExtendedIntervalSig =
  struct
    open Interval
    let contains intvl x =
      match intvl with
      | Empty -> false
      | Range(lo, hi) -> lo <= x && x <= hi
  end

(**

- open, include
  - open is a mechanism to extend the scope. Makes names available, but does not re-export them.
  - include is a mechanism to extend the structure / signature. Re-exports all names.
- #use, #load
  - #use refers to syntactic copy-paste.

*)

(** Classes vs modules

class Rectangle {
  int x, y, len, height;
  int area() {
  }
}

Rectangle r = new Rectangle(x=3, y=2, len=5, height=6)

*)