counter.ml

module type COUNTER = sig
  val inc : unit -> int
end
;;

module CounterImpl = struct
  let data = ref 0

  let inc () = data := !data + 1; !data
end
;;

module CounterImpl2 = struct
  include CounterImpl
end;;

module Counter : COUNTER = CounterImpl
;;

let counter = (module CounterImpl : COUNTER)
;;

module Counter2 = (val counter : COUNTER)
;;

(* type counter = { mutable value : int };;
 *
 * module type Counter = sig
 *   type t
 *
 *   val inc : t -> unit
 *   val dec : t -> unit
 *   val get : t -> int
 * end;;
 *
 * module CounterImpl : (Counter with type t = counter) = struct
 *   type t = counter
 *
 *   let inc c = c.value <- c.value + 1
 *   let dec c = c.value <- c.value - 1
 *   let get c = c.value
 * end;; *)