Scala error when providing subclass instance in place of superclass?

April 2, 2022

I am just trying things out in scala and I wrote this code

object Main:
  def main(args: Array[String]): Unit =
    val dInt: Data[Int] = IntData(1)
    val dString: Data[String] = StringData("hello")
    val Data(deconstructedInt) = dInt // unapply
    val Data(deconstructedString) = dString // unapply
    println(deconstructedInt)
    println(deconstructedString)


sealed trait Data[+T]:
  def get: T

case class IntData(get: Int) extends Data[Int]

case class StringData(get: String) extends Data[String]

object Data:
  def apply[T](input: T): Data[T] = input match {
    case i: Int => IntData(i) //compile error here
    case s: String => StringData(s) //compile error here
  }

  def unapply[T](d: Data[T]): Option[String] = d match {
    case IntData(get) => Some(s"int data => get = $get")
    case StringData(get) => Some(s"string data => get = $get")
  }

I get at the location commented in the code this error

Found:    IntData
Required: Data[T]
    case i: Int => IntData(i)

why I am getting this error, isn’t IntData (or StringData) a subclass of Data ?

>Solution :

IntData is a subtype of Data[Int]. So if T is not Int, IntData is not a subtype of Data[T]. Now, you might say, if input matches the first case, then clearly T is Int. But the compiler is not smart to realise this!

You could try using Scala 3’s new match types:

type DataOf[T] = T match {
  case Int => IntData
  case String => StringData
}

def apply[T](input: T): DataOf[T] = input match {
  case i: Int => IntData(i)
  case s: String => StringData(s)
}