期货让我有些困惑,尝试过flatMap身份

Question

我在尝试编写返回Future [Map [Int,Long]]的方法时遇到问题.

我正在进行一些迭代,回到未来[未来[..]].所以我尝试了flatMap身份.

请参阅下面的我现在收到的代码和错误消息.我不知道此刻发生了什么.

def aggregate(permissions: Vector[Permission]): Map[Int, Long]

def calculate(roleProfile: RoleProfile): Future[Map[Int, Long]] = {
  val roleIds = roleProfile.roles.map(r => r.id)
  db.run(permissionDao.getByRoles(roleIds.toList)).map {
    permissions =>
      aggregate(permissions)
  }
}

def generate(roleGroupId: Int): Future[Map[Int, Long]] = {
  for {
    roleGroup <- roleGroupService.getById(roleGroupId)
    roles <- roleGroupService.getRolesByGroupId(roleGroupId)
  } yield {
    calculate(RoleProfile(roleGroup.get.id, roles.toSet)) //flatMap identity
  }
}

我收到方法'计算'的错误消息:

type mismatch;
[error]  found   : scala.concurrent.Future[Map[Int,Long]]
[error]  required: Map[Int,Long]
[error]       calculate(RoleProfile(roleGroup.get.id, roles.toSet)) //flatMap identity

现在,如果删除'flatMap identity'的注释,我会收到此错误:

type mismatch;
[error]  found   : Map[Int,Long] => Map[Int,Long]
[error]  required: Map[Int,Long] => scala.concurrent.Future[?]
[error]       calculate(RoleProfile(roleGroup.get.id, roles.toSet)) flatMap identity

我很困惑,我怎么能得到它来返回Future [Map [Int,Long]].

更重要的是,这里发生的事情我似乎并不理解.如果你非常感激,请把事情搞清楚.

Answer 1

因为calculate,就像你的getByIdetc调用一样Future,你应该能够简单地将它添加到for-comprehension的主要部分,例如:

def generate(roleGroupId: Int): Future[Map[Int, Long]] = {
  for {
    roleGroup <- roleGroupService.getById(roleGroupId)
    roles <- roleGroupService.getRolesByGroupId(roleGroupId)
    profile <- calculate(RoleProfile(roleGroup.get.id, roles.toSet))
  } yield { profile }
}