上类型边界优于子类型的优势 [英] Advantage of Upper Type Bound over Subtyping

问题描述

在这个例子中，使用类型上限比只使用类型有什么优势?

What is the advantage of using upper type bounds over just type in this example?

为什么人们喜欢这个

trait Pages[T <: Page] {
  val pages: Seq[T]
  def find(id: String): Option[T] = pages.find(_.id == id)
  def all: Seq[T] = pages
}

在此之上:

trait Pages {
  val pages: Seq[Page]
  def find(id: String): Option[Page] = pages.find(_.id == id)
  def all: Seq[Page] = pages
}

推荐答案

这和你第二个例子的优势一样

It's the same as the advantage of your second example over

trait Pages {
  val pages: Seq[Any]
  def find(id: String): Option[Any] = pages.find(_.id == id)
  def all: Seq[Any] = pages
}

:更精确的类型，允许您不混淆Page的不同子类型，无需强制转换即可访问MyPage的操作等

: more precise types, allowing you not to mix up different subtypes of Page, to access operations of MyPage without casting, etc.

另外，如果你有第一个而需要第二个，你可以使用 Pages[Page];如果你有第二个并且需要第一个，那你就不走运了.

Also, if you have the first and need the second, you can just use Pages[Page]; if you have the second and need the first, you are out of luck.

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