what.mli

module Iterator: sig
  type 'a t

  type 'a result =
    | Done
    | Continue of 'a

  val next: 'a t -> 'a result
end

module type Iterable = sig
  type t
  type value
  val iterator: t -> value Iterator.t
end

module Make (I: Iterable): sig
  val iterate: I.t -> f:(I.value -> unit) -> unit
end