检查范围是否在Scala中包含值的通用方法 [英] Generic way to check if range contains value in Scala

问题描述

我想编写一个通用类，该通用类包含一个范围的端点，但是通用版本会引发编译错误: value> =不是类型参数A的成员

I'd like to write a generic class that holds the endpoints of a range, but the generic version kicks back a compilation error: value >= is not a member of type parameter A

final case class MinMax[A <: Comparable[A]](min: A, max: A) {
  def contains[B <: Comparable[A]](v: B): Boolean = {
    (min <= v) && (max >= v)
  }
}

特定版本按预期工作:

final case class MinMax(min: Int, max: Int) {
  def contains(v: Int): Boolean = {
    (min <= v) && (max >= v)
  }
}

MinMax(1, 3).contains(2) // true
MinMax(1, 3).contains(5) // false

推荐答案

您离得太近了.

在 Scala 中，我们 订购 ，它是 typeclass ，表示可以比较相等且小于&大于.

In Scala we have Ordering, which is a typeclass, to represent types that can be compared for equality and less than & greater than.

因此，您的代码可以这样写:

Thus, your code can be written like this:

// Works for any type A, as long as the compiler can prove that the exists an order for that type.
final case class MinMax[A](min: A, max: A)(implicit ord: Ordering[A]) {
  import ord._ // This is want brings into scope operators like <= & >=

  def contains(v: A): Boolean =
    (min <= v) && (max >= v)
}

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