thing.scala

  def index(id: UUIDFor[File]): F[ValidationNel[StoreTreeError, List[DatabaseFile]]] = {
    for {
      maybeDbFile <- mToF(songDb.getDirectory(id))
      dbFile <- maybeDbFile.getOrElseF(F.raiseError[Directory](IndexTargetNotFoundInDatabase(id)))

      maybeFileSystemTree <-nToF(fileTree(apiKeyToPath(dbFile.apiKey.value), None))
      fileSystemTree <- maybeFileSystemTree.getOrElseF(F.raiseError[FileTree](IndexTargetNotFoundInFileSystem(dbFile.apiKey.value)))

      storedTree <- mToF(storeTree(fileSystemTree, None))
    } yield storedTree
  }