Scala Trait type mismatch

I have some Kafka Channel hierarchy that I am using in my project:

My base trait is:

trait SendChannel[A, B] extends CommunicationChannel {
  def send(data:A): B
}

Now I have a common kafka send Channel like

trait CommonKafkaSendChannel[A, B, Return] extends SendChannel[A, Return] {
val channelProps: KafkaSendChannelProperties
val kafkaProducer: Producer[String, B]
 override def close(): Unit = kafkaProducer.close()
}

Now there are 2 variants of CommanKafkaSendChannel, one is with callback and one is with Future:

trait KafkaSendChannelWithFuture[A, B] extends CommonKafkaSendChannel[A, B, Future[RecordMetadata]] {
override def send(data: A): Future[RecordMetadata] = Future {
  kafkaProducer.send(new ProducerRecord[String, B](channelProps.topic)).get
}
}

KafkaSendChannelWithCallback definition:

object KafkaSendChannelWithCallback {

def apply[A, B](oChannelProps: KafkaSendChannelProperties,
              oKafkaProducer: Producer[String, B],
              oCallback: Callback): KafkaSendChannelWithCallback[A, B] =
new KafkaSendChannelWithCallback[A,B] {
  override val channelProps: KafkaSendChannelProperties = oChannelProps
  override val kafkaProducer: Producer[String, B] = oKafkaProducer
  override val callback: Callback = oCallback
}
}

trait KafkaSendChannelWithCallback[A, B] extends CommonKafkaSendChannel[A, B, Unit] {

  val callback: Callback

override def send(data: A): Unit =
kafkaProducer.send(new ProducerRecord[String, B](channelProps.topic), callback)
}

Now based on the configuration value I select the proper type of channel on run time like below. I am creating actor with right type of channel which will send the data to kafka:

  val sendChannel = kafkaChannel.channel(config, actorSystem).fold(
    error => {
      logger.error("Exception while instantiating the KafkaSendChannel")
      throw error
    },
    success => success
  )

actor = actorSystem.actorOf(IngestionActor.props(config, sendChannel), name = ACTOR_NAME)

Actor definition:

object IngestionRouterActor {
  def props[V](config: Config, sendChannel: SendChannel[V, Unit]): Props =
Props(classOf[IngestionActor[V]], config, sendChannel)
}

The problem is when I use KafkaSendChannelWithCallback my code compiles properly however when I use KafkaSendChannelWithFuture it gives me below error on actor = declaration:

[error]IngestionActor.scala:32: pattern type is incompatible with expected type; [error] found : KafkaSendChannelWithFuture[String,V] [error] required: SendChannel[V,Unit]

As both the channel definitions are extended from SendChannel, this code should have compiled without any error. I am not sure why it is not compiling. Thanks

Solution

The Props for IngestionActor takes a SendChannel[V, Unit]. Passing in a KafkaSendChannelWithCallback to this argument works because it's a SendChannel[V, Unit].

On the other hand, KafkaSendChannelWithFuture is a SendChannel[V, Future[RecordMetadata]]. A SendChannel[V, Future[RecordMetadata]] is not a SendChannel[V, Unit].

One option is to redefine the Props to take a SendChannel[V, Any], since Any is a supertype of both Unit and Future:

def props[V](config: Config, sendChannel: SendChannel[V, Any]): Props = ???

At this point, the compiler is still unhappy because SendChannel, being a generic type, is invariant by default. In other words, neither SendChannel[V, Unit] nor SendChannel[V, Future[RecordMetadata]] are of type SendChannel[V, Any].

To change that, make SendChannel covariant on the second type parameter (by adding a + in front of the B):

trait SendChannel[A, +B] extends CommunicationChannel {
  def send(data: A): B
}