Ambiguous Implicit Values for Case Class Option Parameter - scala

Question

I am going through some exercises I have invented on case classes and typeclasses. One of the problems I have faced is the following:
object Example extends App {
sealed trait Serializer[T] {
def serialize(seq: List[T]): String
}
implicit object StringSerializer extends Serializer[String] {
def serialize(seq: List[String]): String = seq.toString()
}
implicit object IntSerializer extends Serializer[Int] {
def serialize(seq: List[Int]): String = seq.toString()
}
case class Marker[T: Serializer](lst: Option[List[T]] = None)
Marker() // ambiguous implicit values: here...
}
Now this gives an error about ambiguous implicit values. I think this is related to a question I have asked before (albeit a different error message):
Type erasure in a nested list with a given context bound
Am I correct in that it is the same process at work here, even though the error message is different?

Answer 1

Compiler can't infer T. Try to specify T explicitly
Marker[String]() // compiles
Marker[Int]() // compiles
When you provide lst it can infer T itself
Marker(Some(List(1, 2)))
Marker(Some(List("a", "b")))
For the same reason
Marker(Option.empty[List[Int]])
Marker(Option.empty[List[String]])
Marker[Int](None)
Marker[String](None)
Marker(None: Option[List[Int]])
Marker(None: Option[List[String]])
compile while Marker(None) doesn't.
Alternatively you can prioritize your implicits
trait LowPrioritySerializer {
implicit object StringSerializer extends Serializer[String] {
def serialize(seq: List[String]): String = seq.toString()
}
}
object Serializer extends LowPrioritySerializer {
implicit object IntSerializer extends Serializer[Int] {
def serialize(seq: List[Int]): String = seq.toString()
}
}
Then IntSerializer will be tried firstly and StringSerializer will be tried secondly if IntSerializer didn't work (if type is different).