如何声明一个可能返回3种不同类型的方法？

Question

我想定义一个方法,但它的返回类型有3种.

def findSelectedItem: ??? = { ... }

在???这里可能是Category,Section,Page,但我不知道如何找到合适的类型来代表它.

如果它只是2,我可以使用Either[Type1, Type2],但它现在是3.

我是否需要声明类似Either但有3种类型变量的东西？或者我已经可以使用了什么？

Answer 1

你可以嵌套Either:

特定

case class Category(name: String)
case class Section(name: String)
case class Page(name: String)

和这样的方法:

def f(name: String): Either[Category, Either[Section, Page]] = {
  name match {
    case "c" =>
      Left(Category(name))

    case "s" =>
      Right(Left(Section(name)))

    case "p" =>
      Right(Right(Page(name)))
  }
}

然后你可以在结果上"模式匹配":

Seq("c", "s", "p").map(f).foreach {
  case Left(c) => println("C")
  case Right(Left(s)) => println("S")
  case Right(Right(p)) => println("p")
}

作为替代方案,创建自己的Either3:

case class Either3[+A, +B, +C](left: Option[A], middle: Option[B], 
                               right: Option[C])

object Left3 {
  def apply[A, B, C](a: A): Either3[A, B, C] = {
    Either3(Some(a), None, None)
  }

  def unapply[A, B, C](e: Either3[A, B, C]): Option[A] = {
    e.left
  }
}

object Middle3 {
  def apply[A, B, C](b: B): Either3[A, B, C] = {
    Either3(None, Some(b), None)
  }

  def unapply[A, B, C](e: Either3[A, B, C]): Option[B] = {
    e.middle
  }
}

object Right3 {
  def apply[A, B, C](c: C): Either3[A, B, C] = {
    Either3(None, None, Some(c))
  }

  def unapply[A, B, C](e: Either3[A, B, C]): Option[C] = {
    e.right
  }
}

然后

def f(name: String): Either3[Category, Section, Page] = {
  name match {
    case "c" =>
      Left3(Category(name))

    case "s" =>
      Middle3(Section(name))

    case "p" =>
      Right3(Page(name))
  }
}

和...一起

Seq("c", "s", "p").map(f).foreach {
  case Left3(c) => println("C")
  case Middle3(s) => println("S")
  case Right3(p) => println("p")
}