的确 leading 以及 trailing 是 Queue 类，但默认情况下它们是私有的。把它们公之于众 val 注解：
之前：

scala> class Queue[T] private (leading: List[T], trailing: List[T]){
     |   private def mirror =
     |     if(leading.isEmpty) new Queue(trailing.reverse, Nil)
     |     else this
     |   def head = mirror.leading.head
     |   def tail = {
     |     val q = mirror
     |     new Queue(q.leading.tail, q.trailing)
     |   }
     |   def append[T](x: T)={
     |     new Queue(leading, x::trailing)
     |   }
     | }
<console>:15: error: value leading is not a member of Queue[T]
         def head = mirror.leading.head
                           ^
<console>:18: error: value leading is not a member of Queue[T]
           new Queue(q.leading.tail, q.trailing)
                       ^
<console>:18: error: value trailing is not a member of Queue[T]
           new Queue(q.leading.tail, q.trailing)
                                       ^

之后：

scala> class Queue[T] private (val leading: List[T], val trailing: List[T]){
     |   private def mirror =
     |     if(leading.isEmpty) new Queue(trailing.reverse, Nil)
     |     else this
     |   def head = mirror.leading.head
     |   def tail = {
     |     val q = mirror
     |     new Queue(q.leading.tail, q.trailing)
     |   }
     |   def append[T](x: T)={
     |     new Queue(leading, x::trailing)
     |   }
     | }
defined class Queue

您可以在这里详细了解构造函数和getter。相关要点是：
参数无 val 或者 var 是私有值，仅在类中可见。