Scala REPL"错误:值>不是类型参数T的成员"

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,并且当教授使用时它在课堂上工作...

有帮助吗？

Answer 1

你得到这个错误,因为不是每个类型T都有>,<等定义.

您可能想要的是T成为Ordered或可隐式地转换为某些内容Ordered,因此定义了所有这些内容.

这应该修复错误消息:

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)表示该类型T必须可转换为S,因此它必须是子类型S或已定义隐式转换.

这个问题的接受者答案更详细地解释了它.