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abstract class Term[T]
case class Lit(x: Int) extends Term[Int]
case class Succ(t: Term[Int]) extends Term[Int]
case class IsZero(t: Term[Int]) extends Term[Boolean]
case class If[T](c: Term[Boolean],
t1: Term[T],
t2: Term[T]) extends Term[T]
object Test extends Application {
def eval[T](t: Term[T]): T = t match {
case Lit(n) => n
case Succ(u) => eval(u) + 1
case IsZero(u) => eval(u) == 0
case If(c, u1, u2) => eval(if (eval(c)) u1 else u2)
}
println(eval(If(IsZero(Lit(1)), Lit(41), Succ(Lit(41)))))
}
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