Biocaml 0.4-dev : Biocaml_lines.Transform.group2

let group2 () =

  let queue : (item * item) Queue.t= Queue.create () in

  let item1 = ref None in

  Biocaml_transform.make ~name:"group2"

   ~feed:(function item -> match !item1 with

    | Some item1' -> (

      Queue.enqueue queue (item1', item);

      item1 := None

     )

    | None -> item1 := Some item

   )

   ~next:(fun stopped -> match Queue.dequeue queue with

    | Some ij -> `output (Ok ij)

    | None ->

     if not stopped then

      `not_ready

     else

      (match !item1 with

       | None -> `end_of_stream

       | Some _ -> `output (Error `premature_end_of_input)

      )

   )

   ()