sealed trait Data
case class StringData(string: String) extends Data
case class IntData(int: Int) extends Data

trait Reader[A] {
  def read(data: Data): A
}

implicit val stringReader: Reader[String] = {
  case StringData(string) => string
  case _                  => sys.error("not a string")
}
implicit val intReader: Reader[Int] = {
  case IntData(int) => int
  case _            => sys.error("not an int")
}

implicit def fromData[A: Reader](data: Data): A =
  implicitly[Reader[A]].read(data)

val str: String = StringData("foo")
val int: Int = IntData(420)