斯卡拉选项？为什么 Some 不继承 AnyVal ，为什么它不是值类型？

Question

为什么Scala Some不继承AnyVal，使用值类型特性，节省装箱成本？

就像是:

sealed trait TestOption[+A] extends Any {
  def isEmpty: Boolean
  def get: A
}

final case class TestSome[+A](val x: A) extends AnyVal with TestOption[A] {
  def isEmpty = false
  def get = x
} 

case object TestNone extends TestOption[Nothing] {
  def isEmpty = true
  def get = throw new NoSuchElementException("None.get")
}

Answer 1

来自 Value Classes 的文档:

Summary of Limitations

A value class

• must have only a primary constructor with exactly one public, val parameter whose type is not a value class. (From Scala 2.11.0, the parameter may be non-public.)
• may not have specialized type parameters.
• may not have nested or local classes, traits, or objects
• may not define a equals or hashCode method.
• must be a top-level class or a member of a statically accessible object
• can only have defs as members. In particular, it cannot have lazy vals, vars, or vals as members.
• cannot be extended by another class.

粗体不包括 Some从作为当前编写的值类。其中的第二个可能可以通过轻微的更改来解决，但前者会杀死它。您将无法在 Some 中包装另一个值类如果 Some 本身成为一个值类。