Question

这是我的档案

trait Set[T] {
    def contains(x: T): Boolean
    def incl(x: T): Set[T]
    def union(that: Set[T]): Set[T]
}

class Empty[T] extends Set[T] {
    override def toString = "."
    def contains(x: T): Boolean = false
    def incl(x: T): Set[T] = new NonEmpty[T](x, new Empty[T], new Empty[T])
    def union(that: Set[T]): Set[T] = that
}

class NonEmpty[T](elem: T, left: Set[T], right: Set[T]) extends Set[T] {
    override def toString = "{" + left + elem + right + "}"

    def contains(x: T): Boolean =
        if (x < elem) left contains x
        else if (x > elem) right contains x
        else true

    def incl(x: T): Set[T] =
         if (x < elem) new NonEmpty(elem, left incl x, right)
         else if (x > elem) new NonEmpty(elem, left, right incl x)
         else this

    def union(that: Set[T]): Set[T] =
        ((left union right) union that) incl elem
}

我正在使用“：paste”方法，因为：加载不起作用。但是我收到以下错误

<console>:25: error: value < is not a member of type parameter T
               if (x < elem) left contains x
                     ^
<console>:26: error: value > is not a member of type parameter T
               else if (x > elem) right contains x
                          ^
<console>:30: error: value < is not a member of type parameter T
                if (x < elem) new NonEmpty(elem, left incl x, right)
                      ^
<console>:31: error: value > is not a member of type parameter T
                else if (x > elem) new NonEmpty(elem, left, right incl x)

我确定这个文件是正确的，因为它来自类示例，并且在教授使用时它在课堂上工作...

有帮助吗？

Answer 1

您收到该错误，因为并非所有类型T都定义了>，<等。

您可能希望T成为Ordered或隐式转换为Ordered，因此可以定义所有内容。

这应修复错误消息：

class NonEmpty[T <% Ordered[T]](elem: T, left: Set[T], right: Set[T]) extends Set[T] {
    override def toString = "{" + left + elem + right + "}"

    def contains(x: T): Boolean =
        if (x < elem) left contains x
        else if (x > elem) right contains x
        else true

    def incl(x: T): Set[T] =
        if (x < elem) new NonEmpty(elem, left incl x, right)
        else if (x > elem) new NonEmpty(elem, left, right incl x)
        else this

    def union(that: Set[T]): Set[T] =
        ((left union right) union that) incl elem
}

T <% S（视图边界）表示类型T必须可转换为S，因此它必须是S的子类型或已定义隐式转换

this queston的接受者答案更详细地解释了它。

Answer 2

自＆＃34; view bounds＆＃34;已在Scala 2.11中弃用，这是使用＆＃34; context bounds＆＃34;的通用二进制搜索树的替代实现。如下：

object GenericBinarySearchTree {

abstract class Set[T] {
  def incl(x: T): Set[T]
  def contains(x: T): Boolean
  def union(that: Set[T]): Set[T]
}

type L[X] = X => Ordered[X]

class Empty[T : L] extends Set[T] {

  override def toString = "."

  def incl(x: T): Set[T] = new NonEmpty(x, new Empty, new Empty)

  def contains(x: T): Boolean = false

  def union(that: Set[T]): Set[T] = that
}

class NonEmpty[T : L](elem: T, left: Set[T], right: Set[T]) extends Set[T] {  

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

  def incl(x: T): Set[T] = {
    if(x > elem) new NonEmpty(elem, left, right.incl(x))
    else if(x < elem) new NonEmpty(elem, left.incl(x), right)
    else this
  }

  def contains(x: T): Boolean = {
    if(x == elem) true 
    else if(x > elem) right.contains(x)
    else left.contains(x)
  } 

  def union(that: Set[T]): Set[T] = ((left union right) union that) incl elem
}

}

Scala REPL“错误：值＆gt;不是类型参数T的成员”

2 个答案: