Using parameterized branch of ADT in function

Question

Here is the code:

sealed trait Tr
final case class A(a: String) extends Tr
final case class B(b: String) extends Tr

def markWithType[Type <: Tr](tr: Type): Type = tr match {
  //Compile error
  //Expression of type A doesn't conform to expected type Type
  case A(a) => A("A:" + a) 

  //Compile error
  //Expression of type B doesn't conform to expected type Type
  case B(b) => B("B:" + b)
}

The problem is it does not compile. I want to preserve Type <: Tr and make it compiling successfully. Is there a way to do that?

I'm pretty sure shapeless can be helpful here.

https://stackoverflow.com/questions/61668592/why-cant-i-return-a-concrete-subtype-of-a-if-a-generic-subtype-of-a-is-declared/ — Dmytro Mitin, Jun 18 '20 at 15:06
https://stackoverflow.com/questions/52479695/type-mismatch-on-abstract-type-used-in-pattern-matching — Dmytro Mitin, Jun 18 '20 at 15:07

score 2 · Accepted Answer · answered Jun 18 '20 at 15:06

You can go with simple overloading.

sealed trait Tr {
  def markWithType: Tr
}

final case class A(a: String) extends Tr {
  override def markWithType: A = A(s"A: ${a}")
}

final case class B(b: String) extends Tr {
  override def markWithType: B = B(s"B: ${b}")
}

Another option would be a typeclass but I believe that would be an overkill in this case.

Using parameterized branch of ADT in function

1 Answers1