如何通过我自己的方法实现List的“head :: tail”模式匹配？

Question

我想在我的代码中添加一个'+：'方法来实现模式匹配，就像提供的List一样

case head::tail => _

所以，我尝试编写这些代码

abstract class Num{
     def +: (b:Int):Num
}
case class Entity(a:Int, t:Num) extends Num{
    def +: (b:Int):Num = new Entity(b,this)
    override def toString:String = "Entity(" + a + "," + t + ")"
}
case object ZERO extends Num{
    def +: (b:Int):Num = new Entity(b,this)
    override def toString:String = "ZERO"
}


def toIntList(n:Num):List[Int] = n match{
    case ZERO => Nil
    case (head:Int) +: tail => head :: toIntList(tail)
}

但它们效果不好......

scala> val two = 2 +: ( 1 +: ZERO)
two: Num = Entity(2,Entity(1,ZERO))

scala> toIntList(two)
scala.MatchError: Entity(2,Entity(1,ZERO)) (of class Entity)
    at .toIntList(<console>:11)
    at .<init>(<console>:14)
    at .<clinit>(<console>)
    at .<init>(<console>:7)
    at .<clinit>(<console>)
    at $print(<console>)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)