Question

在此代码段cleanup_old_logs_service()中没有进行类型检查。

y.run

编译器显示此错误：

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 }

我是否必须为could not find implicit value for parameter C: scalaz.Catchable[[x]Test.StateStringTask[x]]创建Catchable个实例？我怎么做？或者在运行StateStringTask？

时是否有更简单的方法来处理有状态效果

Answer 1

我想这不是最理想的但我通过ThingWithD.__init__() StateStringTask的实例来实现它：

Catchable

提升 implicit val stateStringTaskInstance: Catchable[StateStringTask] = new Catchable[StateStringTask] { // `a.attempt` stackoverflows, 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] } StateT Process效果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)
  }

Monad变压器与scalaz流

1 个答案: