scala - 如何在 Scala 中创建泛型类型的对象/单例？

Question

在下面显示的代码中，我如何转换 EmptyTree反对(单例)？

trait Tree[T] {
    def contains(num: T): Boolean
    def inc( num: T ): Tree[T]
  }


class EmptyTree[T <% Ordered[T] ] extends Tree[T] {
    def contains(num:T):Boolean = false
    def inc(num:T):Tree[T] = {
        new DataTree(num, new EmptyTree, new EmptyTree)
    }
    override def toString = "."
}

class DataTree[T <% Ordered[T] ](val x:T, val left:Tree[T], val right:Tree[T]) extends Tree[T] {

    def contains(num:T):Boolean = {
        if( num < x ) left.contains(x)
        else if ( num > x ) right.contains(x)
        else true
    }
    def inc(num:T):Tree[T] = {
        if(num < x ) new DataTree(x, left.inc(num), right)
        else if ( num > x ) new DataTree(x, left, right.inc(num))
        else this
    }
    override def toString = "{" + left + x + right + "}"
}


val t = new DataTree(20, new EmptyTree[Int], new EmptyTree[Int])
                                                //> t  : greeting.Test.DataTree[Int] = {.20.}
val p = t.inc(10)                               //> p  : greeting.Test.Tree[Int] = {{.10.}20.}
val a = p.inc(30)                               //> a  : greeting.Test.Tree[Int] = {{.10.}20{.30.}}
val s = a.inc(5)                                //> s  : greeting.Test.Tree[Int] = {{{.5.}10.}20{.30.}}
val m = s.inc(11)                               //> m  : greeting.Test.Tree[Int] = {{{.5.}10{.11.}}20{.30.}}

Answer 1

让我详细说明阿列克谢的回答。这是带有一些代码样式改进的完整实现:

首先通过了解其协方差来定义您的特征:

 trait Tree[+T] {
    def contains[U >: T : Ordering](num: U): Boolean

    def inc[U >: T : Ordering](num: U): Tree[U]
  }

接下来定义您的所有树的子类型对象

  case object EmptyTree extends Tree[Nothing] {
    def contains[U >: Nothing : Ordering](num: U): Boolean = false
    def inc[U >: Nothing : Ordering](num: U): Tree[U] =
      DataTree(num, EmptyTree, EmptyTree)
    override def toString = "."
  }

现在更改您的一般情况实现:

  case class DataTree[T: Ordering](x: T, left: Tree[T], right: Tree[T]) extends Tree[T] {
    import Ordering.Implicits._
    def contains[U >: T : Ordering](num: U): Boolean = 
      if (num < x) left.contains(x)
      else if (num > x) right.contains(x)
      else true

    def inc[U >: T : Ordering](num: U): Tree[U] = 
      if (num < x) DataTree(x, left.inc(num), right)
      else if (num > x) DataTree(x, left, right.inc(num))
      else this

    override def toString = "{" + left + x + right + "}"
  }

自从我更换了您的 Ordered 后，您可能会有点沮丧。与 Ordering ，但你应该知道 view bounds are deprecated