Crawler.scala

def handleMessage(msg: CrawlerMessage, 
                  data: CrawlerData
                 ): IO[Nothing, CrawlerData] = msg match {
  case Start(url) =>
    crawlUrl(data, url)

  case CrawlResult(url, links) =>
    val data2 = data.copy(inProgress = data.inProgress - url)

    links.foldM(data2) {
      case (d, link) =>
        val d2 = d.copy(referenceCount = d.referenceCount.updated(
          link.host, d.referenceCount.getOrElse(link.host, 0) + 1))
        crawlUrl(d2, link)
    }
}