书上的例子(Programming in scala2),提示enqueue方法没有实现.

gistfile1.txt

trait Queue[T] {
	def head: T
	def tail: Queue[T]
	def enqueue(x: T): Queue[T]
}
object Queue {
  
	
	private class QueueImpl[T] (
	    private val leading: List[T],
	    private val trailing: List[T]
	    ) extends Queue[T] {
	  
		def mirror = 
		  if (leading.isEmpty) 
		    new QueueImpl(trailing.reverse, Nil)
		  else 
		    this
		  
		def head: T = mirror.leading.head
		def tail: QueueImpl[T] = {
		  val q = mirror
		  new QueueImpl(q.leading.tail, q.trailing)
		}
		
		def enqueue(x: T) = new QueueImpl(leading, x :: trailing)
	}
}

class StrangeIntQueue extends Queue[Int] {
	override def enqueue(x: Int) = {
		println(math.sqrt(x))
		super.enqueue(x)
	}
}

会提示<console>:11: error: method enqueue in trait Queue is accessed from super. It
y not be abstract unless it is overridden by a member declared `abstract' and
verride'
           super.enqueue(x)