Basic GADT in Scala

GADT.scala

// Scala version of code from https://en.wikipedia.org/wiki/Generalized_algebraic_data_type

sealed abstract class Expr[A]

final case class EBool(a: Boolean) extends Expr[Boolean]

final case class EInt(a: Int) extends Expr[Int]

final case class EEqual(a: Expr[Int], b: Expr[Int]) extends Expr[Boolean]

def eval[A](e: Expr[A]): A = e match {
  case EBool(a)     => a
  case EInt(a)      => a
  case EEqual(a, b) => eval(a) == eval(b)
}

val expr1 = EEqual(EInt(2), EInt(3))

val ret = eval(expr1)

val ret2 = eval(EEqual(EInt(4), EInt(4)))