module type SOURCE =
  sig
    type element
    type iterator
    type leftovers

    type result =
      | Cont of element * iterator
      | Done of leftovers

    val get : iterator -> result
  end

module type SINK =
  sig
    type element
    type iterator
    type leftovers

    type result =
      | Cont of iterator
      | Done of leftovers

    val put : iterator * element -> result
  end

module Transform (Source : SOURCE) (Sink : SINK) =
  struct
    type result =
      | Depleted of Source.leftovers * Sink.iterator
      | Saturated of Source.iterator * Sink.leftovers

    let transform f =
      let rec loop (source, sink) =
        match Source.get source with
        | Done leftovers -> Depleted (leftovers, sink)
        | Cont (element, source) ->
           match Sink.put (sink, f element) with
           | Done leftovers -> Saturated (source, leftovers)
           | Cont sink -> loop (source, sink)
  end