An example of refactoring nested login in the Ensime Search Service with `XorT`

SearchService.scala

      import cats.data.Xor, cats.syntax.xor._, cats.data.XorT

      //Some type aliases for semantic purposes
      
      type IndexedSymbols = Iterable[(FileObject, List[FqnSymbol])]

      type IndexingError = (String, Throwable)
      
      type ProcessResult[A] = XorT[Future, IndexingError, A]

      // run a future lifting its success and failure to a disjoint union left and right
      def runF[A](f : Future[A])(errorMsg : String): ProcessResult[A] =
        XorT(f map (_.right) recover {
          case NonFatal(e) => (errorMsg, e).left
        })

      def extractSymbols: ProcessResult[IndexedSymbols] =
        runF(Future.traverse(batch) { case (_, f) =>
          if (!f.exists()) Future.successful(f -> Nil)
          else searchService.extractSymbolsFromClassOrJar(f).map(f -> _)
        })(s"failed to index batch of ${batch.size} files")

      def removeStaleEntries(indexed: IndexedSymbols): ProcessResult[Int] =
        runF(searchService.delete(indexed.map(_._1)(collection.breakOut))
        )(s"failed to remove stale entries in ${batch.size} files")

      def persistEntries(indexed : IndexedSymbols): ProcessResult[Iterable[Int]] = runF(Future.sequence(indexed.collect {
        case (file, syms @ _ :: _) =>
          searchService.persist(FileCheck(file), syms, commitIndex = true)
      }).map(_.flatten))(s"failed persisting entries")

      // compute over each process with a bias on the right side
      val op: ProcessResult[(IndexedSymbols, Int, Iterable[Int])] = for {
        indexedSymbols <- extractSymbols
        removedCount <- removeStaleEntries(indexedSymbols)
        persisted <- persistEntries(indexedSymbols)
      } yield (indexedSymbols, removedCount, persisted)
      
      // inform of the Op outcome
      op.value map {
        case Xor.Left((msg, ex)) => log.error(msg, ex)
        case Xor.Right((indexedSymbols, removedCount, persisted)) =>
          log.debug(s"Process completed for [$indexedSymbols]. Removed : $removedCount, Persisted : $persisted")
      }