带有 scalaz 流的 Monad 转换器

Question

在此代码段中 y.run 没有进行类型检查。

object Test {

  type StateStringTask[A] = StateStringT[Task, A]
  type StateStringT[M[_], A] = StateT[M, String, A]

  val x: Process[Task, Unit] = ???

  val y: Process[StateStringTask, Unit] = ???

  x.run // This typechecks

  y.run // This fails
}

编译器显示此错误：

could not find implicit value for parameter C: scalaz.Catchable[[x]Test.StateStringTask[x]]

我是否必须为 StateStringTask 创建一个 Catchable 实例？我怎么做？或者当运行 a Process?

时，是否有更简单的方法来处理有状态效果

Answer 1

我想这是次优的，但我通过使 StateStringTask 成为 Catchable 的实例得到它：

implicit val stateStringTaskInstance: Catchable[StateStringTask] =
  new Catchable[StateStringTask] {
    // `a.attempt` Whosebugs, don't ask me why :)
    def attempt[A](a: StateStringTask[A]): StateStringTask[Throwable \/ A] = a >>= (
      x => Catchable[Task].attempt(Applicative[Task].pure(x)).liftM[StateStringT]
    )
    def fail[A](err: Throwable) = Catchable[Task].fail(err).liftM[StateStringT]
  }

要提升 Process 上的StateT，效果为Task。例如：

  def received(queue: Queue[Event]): Process[StateStringTask, Event] = {
    val toStateStringTask = new (Task ~> StateStringTask) {
      def apply[A](t: Task[A]): StateStringTask[A] = t.liftM[StateStringT]
    }
    // queue.dequeue: Process[Task, Event]
    queue.dequeue.translate(toStateStringTask)
  }

带有 scalaz 流的 Monad 转换器

Monad transformers with scalaz-streams

scala

monad-transformers

scalaz-stream