Does not conform the expected type

Question

I have following code snippet:

  def determineProducerType(keySerializer: KkSerializer)(valueSerializer: KkSerializer)(props: Properties)
  : Eval[KafkaProducer[java.lang.Object, java.lang.Object]] = (keySerializer, valueSerializer) match {

    case (KkStringSeDe, KkStringSeDe) => Later(new KafkaProducer[String, String](props))
    case (KkStringSeDe, KkByteArraySeDe) => Later(new KafkaProducer[String, Byte](props))
    case (KkStringSeDe, KkIntegerSeDe) => Later(new KafkaProducer[String, Integer](props))
    case (KkStringSeDe, KkLongSeDe) => Later(new KafkaProducer[String, Long](props))

  }

The compiler complains:

[info] Compiling 2 Scala sources to /home/developer/Desktop/scala/PureProducer/target/scala-2.12/classes ...
[error] /home/developer/Desktop/scala/PureProducer/src/main/scala/TheProducer.scala:113:48: type mismatch;
[error]  found   : org.apache.kafka.clients.producer.KafkaProducer[String,String]
[error]  required: org.apache.kafka.clients.producer.KafkaProducer[Object,Object]
[error] Note: String <: Object, but Java-defined class KafkaProducer is invariant in type K.
[error] You may wish to investigate a wildcard type such as `_ <: Object`. (SLS 3.2.10)
[error] Note: String <: Object, but Java-defined class KafkaProducer is invariant in type V.
[error] You may wish to investigate a wildcard type such as `_ <: Object`. (SLS 3.2.10)
[error]     case (KkStringSeDe, KkStringSeDe) => Later(new KafkaProducer[String, String](props))
[error]                                                ^
[error] /home/developer/Desktop/scala/PureProducer/src/main/scala/TheProducer.scala:114:51: type mismatch;
[error]  found   : org.apache.kafka.clients.producer.KafkaProducer[String,Byte]
[error]  required: org.apache.kafka.clients.producer.KafkaProducer[Object,Object]
[error]     case (KkStringSeDe, KkByteArraySeDe) => Later(new KafkaProducer[String, Byte](props))
[error]                                                   ^
[error] /home/developer/Desktop/scala/PureProducer/src/main/scala/TheProducer.scala:115:49: type mismatch;
[error]  found   : org.apache.kafka.clients.producer.KafkaProducer[String,Integer]
[error]  required: org.apache.kafka.clients.producer.KafkaProducer[Object,Object]
[error] Note: String <: Object, but Java-defined class KafkaProducer is invariant in type K.
[error] You may wish to investigate a wildcard type such as `_ <: Object`. (SLS 3.2.10)
[error] Note: Integer <: Object, but Java-defined class KafkaProducer is invariant in type V.
[error] You may wish to investigate a wildcard type such as `_ <: Object`. (SLS 3.2.10)
[error]     case (KkStringSeDe, KkIntegerSeDe) => Later(new KafkaProducer[String, Integer](props))
[error]                                                 ^
[error] /home/developer/Desktop/scala/PureProducer/src/main/scala/TheProducer.scala:116:46: type mismatch;
[error]  found   : org.apache.kafka.clients.producer.KafkaProducer[String,Long]
[error]  required: org.apache.kafka.clients.producer.KafkaProducer[Object,Object]
[error]     case (KkStringSeDe, KkLongSeDe) => Later(new KafkaProducer[String, Long](props))
[error]                                              ^
[error] four errors found
[error] (compile:compileIncremental) Compilation failed
[error] Total time: 2 s, completed Nov 12, 2017 10:39:14 AM

What I am trying to do is, I defined a sum type:

sealed trait KkSerializer
case object KkStringSeDe extends KkSerializer
case object KkByteArraySeDe extends KkSerializer
case object KkIntegerSeDe extends  KkSerializer
case object KkLongSeDe extends KkSerializer

When it matches an appropriate sum type, then it should returns corresponding type.

Create an instance of KafkaProducer is:

val producer = new KafkaProducer[String, String](props) 

How to solve it?

Answer 1

I think in this case you can just use path dependent types to get what you want:

sealed trait KkSerializer { type Out }
case object KkStringSeDe extends KkSerializer {
  type Out = String
}
case object KkByteArraySeDe extends KkSerializer {
  type Out = Byte
}

def determineProducerType(k: KkSerializer)(v: KkSerializer)(props: Properties): Eval[KafkaProducer[k.Out, v.Out]] =
  Later(new KafkaProducer[k.Out, v.Out](props))