Ambiguous Implicit Values for Case Class Option Parameter

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?

What do you want your Marker instance to be for, Int or List? — user, Jul 01 '20 at 14:48
I was hoping that marker could be either Int or List, and only when a user passes in a list would the type be set. In my mind I assumed the compiler would just not bother when the option was equal to None. — finite_diffidence, Jul 01 '20 at 14:52
Well, the `lst` field is not mutable, so there was no way anyone could pass a different list in later — user, Jul 01 '20 at 14:53

Dmytro Mitin · Answer 1 · 2020-07-01T15:02:41.663

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).

aha I see now. Thanks!! – finite_diffidence Jul 01 '20 at 14:54 — finite_diffidence, Jul 01 '20 at 14:54

Ambiguous Implicit Values for Case Class Option Parameter

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