gadt.scala

sealed trait Accent[A]
final case class Zed(n: Int) extends Accent[Int]
final case class Zee(s: String) extends Accent[String]

object Example {
  def twice[A](accent: Accent[A]): Accent[A] = accent match {
    case Zed(n) => Zed(n + n)
    case Zee(s) => Zee(s + s)
  }

  def empty[A](accent: Accent[A]): Accent[A] = accent match {
    case Zed(n) => Zed(0)
    case Zee(s) => Zee("")
  }

 /*
   As expected you can't do this....

   [vex:gadt] 2048$ scalac -Ywarn-all -Xlint gadt.scala
   gadt.scala:20: error: type mismatch;
    found   : Zee
    required: Accent[A]
       case Zed(n) => Zee("")
                         ^
   gadt.scala:21: error: type mismatch;
    found   : Zed
    required: Accent[A]
       case Zee(s) => Zed(0)
                         ^
   two errors found
 */

/*
  def notOk[A](accent: Accent[A]): Accent[A] = accent match {
    case Zed(n) => Zee("")
    case Zee(s) => Zed(0)
  }
*/
}