gadts.scala

来自「JAVA 语言的函数式编程扩展」· SCALA 代码 · 共 20 行

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)
  }
  Console.println(
    eval(If(IsZero(Lit(1)), Lit(41), Succ(Lit(41)))))
}