explicit.scala

private def processBatch(results: Future[Seq[String]], batch: Seq[String]): Future[Seq[String]] = {
  // when the results for the previous batches have been completed, start a new batch
  results.flatMap { responses =>
    // create the web requests for this batch
    val batchFutures: Seq[Future[String]] = batch.map(ws.url(_).get().map(_.body))

    // sequence the futures for this batch into a singe future
    val batchFuture: Future[Seq[String]] = Future.sequence(batchFutures)

    // when this batch is complete, append the responses to the existing responses
    batchFuture.map { batchResponses =>
      Logger.info("Finished a batch")
      responses ++ batchResponses
    }
  }
}

implicit.scala

private def processBatch(results: Future[Seq[String]], batch: Seq[String]): Future[Seq[String]] = {
  // when the results for the previous batches have been completed, start a new batch
  for {
    responses <- results
    // sequence the futures for this batch into a singe future
    batchFuture = Future.sequence(batch.map(ws.url(_).get().map(_.body)))
    batchResponses <- batchFuture
  } yield {
    // when this batch is complete, append the responses to the existing responses
    Logger.info("Finished a batch")
    responses ++ batchResponses
  }
}