sealed abstract class Expr[T] case class IntExpr(value: Int) extends Expr[Int] case class BooleanExpr(value: Boolean) extends Expr[Boolean] case class IfExpr[A](cond: Expr[Boolean], then: Expr[A], otherwise: Expr[A]) extends Expr[A] case class AddExpr(left: Expr[Int], right: Expr[Int]) extends Expr[Int] object Main extends App { def eval[A](expr: Expr[A]): A = expr match { case IntExpr(x) => x case BooleanExpr(x) => x case IfExpr(cond, then, otherwise) => eval(if (eval(cond)) then else otherwise) case AddExpr(left, right) => eval(left) + eval(right) } println(eval(IfExpr(BooleanExpr(false), IntExpr(1), AddExpr(IntExpr(2), IntExpr(3))))) }