为什么Nil无法创建？

Question

Having the error:

Object creation impossible, since member node: Option[(A, MyModule.List[A])] in MyModule.List is not defined

 sealed trait List[+A] { def node: Option[(A, List[A])] def isEmpty = node.isEmpty } abstract class Cons[+A](head: A, tail: List[A]) extends List[A] object Nil extends List[Nothing] object List { def empty[A] = new List[A] { def node = None } def cons[A](head: A, tail: List[A]) = new List[A] { def node = Some((head, tail)) } def apply[A](as: A*):List[A] = { if (as.isEmpty) Nil else Cons(as.head, apply(as.tail: _*)) 

In this case, how to implement? I wonder what functions should be put in trait and what functions should be put in Company Object.

Answer 1

Here is correct implementation:

sealed trait List[+A] {
  def node: Option[(A, List[A])]

  def isEmpty = node.isEmpty
}

case class Cons[+A](head: A, tail: List[A]) extends List[A] {
  override def node: Option[(A, List[A])] = Some((head, tail))
}

object Nil extends List[Nothing] {
  override def node: Option[(Nothing, List[Nothing])] = None
}

object List {
  def empty[A] = new List[A] {
    def node = None
  }

  def cons[A](head: A, tail: List[A]) = new List[A] {
    def node = Some((head, tail))
  }

  def apply[A](as: A*): List[A] = {
    if (as.isEmpty) Nil
    else Cons(as.head, apply(as.tail: _*))
  }
}

I see few problems here:

1) You have two implementation of cons you can implement cons method simpler:

  def cons[A](head: A, tail: List[A]) = Cons(head, tail)

2) Same with empty

  def empty[A] = Nil