Scala State monad - 结合不同的状态类型

Question

我正在围绕状态单子(monad)包裹我的头。琐碎的例子很容易理解。我现在正在转向域对象是复合的现实世界案例。例如，使用以下域对象(它们没有多大意义，只是纯粹的例子):

case class Master(workers: Map[String, Worker])
case class Worker(elapsed: Long, result: Vector[String])
case class Message(workerId: String, work: String, elapsed: Long)

考虑 Worker如S输入 State[S, +A] monad 很容易写一些像这样的组合子:

type WorkerState[+A] = State[Worker, A]
def update(message: Message): WorkerState[Unit] = State.modify { w =>
    w.copy(elapsed = w.elapsed + message.elapsed,
           result = w.result :+ message.work)
}
def getWork: WorkerState[Vector[String]] = State { w => (w.result, w) }
def getElapsed: WorkerState[Long] = State { w => (w.elapsed, w) }
def updateAndGetElapsed(message: Message): WorkerState[Long] = for {
    _ <- update(message)
    elapsed <- getElapsed
} yield elapsed
// etc.

将这些与 Master 结合起来的惯用方式是什么？状态组合器？例如

type MasterState[+A] = State[Master, A]
def updateAndGetElapsedTime(message: Message): MasterState[Option[Long]]

我可以这样实现:

def updateAndGetElapsedTime(message: Message): MasterState[Option[Long]] =   
    State { m =>
        m.workers.get(message.workerId) match {
            case None => (None, m)
            case Some(w) =>
                val (t, newW) = updateAndGetElapsed(message).run(w)
                (Some(t), m.copy(m.workers.updated(message.workerId, newW))
        }
    }

我不喜欢的是我必须在最后一个变压器中手动运行 State monad。我的现实世界的例子涉及更多。使用这种方法，它很快就会变得困惑。

是否有更惯用的方式来运行这种增量更新？

Answer 1

通过结合镜头和状态单子(monad)可以很好地做到这一点。首先是设置(我已经轻轻地编辑了你的设置以使其与 Scalaz 7.1 一起编译):

case class Master(workers: Map[String, Worker])
case class Worker(elapsed: Long, result: Vector[String])
case class Message(workerId: String, work: String, elapsed: Long)

import scalaz._, Scalaz._

type WorkerState[A] = State[Worker, A]

def update(message: Message): WorkerState[Unit] = State.modify { w =>
  w.copy(
    elapsed = w.elapsed + message.elapsed,
    result = w.result :+ message.work
  )
}

def getWork: WorkerState[Vector[String]] = State.gets(_.result)
def getElapsed: WorkerState[Long] = State.gets(_.elapsed)
def updateAndGetElapsed(message: Message): WorkerState[Long] = for {
  _ <- update(message)
  elapsed <- getElapsed
} yield elapsed

现在有几个通用镜头可以让我们看到Master的内部。 :

val workersLens: Lens[Master, Map[String, Worker]] = Lens.lensu(
  (m, ws) => m.copy(workers = ws),
  _.workers
)

def workerLens(workerId: String): PLens[Master, Worker] =
  workersLens.partial andThen PLens.mapVPLens(workerId)

然后我们基本上完成了:

def updateAndGetElapsedTime(message: Message): State[Master, Option[Long]] =
  workerLens(message.workerId) %%= updateAndGetElapsed(message)

这里 %%=只是告诉我们一旦我们通过镜头放大到适当的 worker 后要执行什么状态操作。