def addWishToBoard(wish: Wish, boardId: BoardId, userId: UserId): Option[Board]

private var boards = List.empty[Board]

def addWishToBoard(wish: Wish, boardId: BoardId, userId: UserId): Option[Board] = {
    val board = find(boardId)
      .filter(_.ownerId == userId)
      .map(board => board.copy(wishes = wish :: board.wishes.toList))

    boards.foreach(board => boards = board :: boards)

    board
  }