它是什么样的实现？

Question

On the akka website , you can the following definition:

sealed trait AccountCommand[Reply] extends ExpectingReply[Reply]
final case class Withdraw(amount: BigDecimal)(override val replyTo: ActorRef[OperationResult])
    extends AccountCommand[OperationResult]

How to assign an ActorRef to replyTo ?

I've taken the code as an example and try to implement on own type:

case object Channel (override val replyTo: ActorRef[SendMessage])
  extends ExpectingReply[SendMessage]

and the compiler complains.

The way, that the compiler does not complain:

case object Channel extends ExpectingReply[SendMessage] {
  override def replyTo: ActorRef[SendMessage] = ???
}  

Is the example on akka website wrong?

Answer 1

If your message does not have fields, you can use a case class without parameters:

case class Channel()(override val replyTo: ActorRef[SendMessage])
  extends ExpectingReply[SendMessage]

If you're asking about how to create an instance of such a class, then you should use the standard multi-parameter function call syntax:

val msg = Channel()(sendMessageActorRef)

That being said, it looks to me that specifically in that part of the Akka API that you linked to, you don't have to explicitly provide any ActorRef s, Effect s seem to do it kind of automatically.