在性状中使用未定义的

Question

我有一个特点

trait A {
    def doSomething(a: Seq[Int]): Seq[String] = {
        a.map {
            case AA(s) => s // want to use unapply defined in trait (this(AA) not allowed)
            case _ => "idc"
        }
    }
    
    def unapply(a: Int): Option[String] = getString(a)

    
    def getString(a: Int): Option[String] = {
        a match {
            case 1 => Some("one")
            case 2 => Some("two")
            case _ => None
        }
    }
}

object AA extends A
object AA2 extends A {
    override def getString(a: Int): Option[String] = {
        super.getString(a).orElse{
            a match {
                case 3 => Some("three")
                case 4 => Some("four")
                case _ => None
            }
        }
    } 
}

object MyClass {
    def main(args: Array[String]) {
        println(AA.doSomething(Seq(1,2,3,4,5))); // Output: List(one, two, idc, idc, idc)
        println(AA2.doSomething(Seq(1,2,3,4,5))); // Expect Output: List(one, two, three, four, idc) but get List(one, two, idc, idc, idc)
    }
}

这里的问题是，如果不创建一个提取器对象，我就不能使用在该特性中定义的未应用程序。

我想用这个特性覆盖不同对象中的getString方法。

Answer 1

您可以使用自我类型来引用自己。

trait A { self =>
  final def doSomething(a: Seq[Int]): Seq[String] =
    a.map {
      case self(s) => s
      case _       => "idc"
    }

  final def unapply(a: Int): Option[String] =
    getString(a)

  def getString(a: Int): Option[String] =
    a match {
      case 1 => Some("one")
      case 2 => Some("two")
      case _ => None
    }
}

就像预期的那样。

运行https://scastie.scala-lang.org/BalmungSan/MnpHz3HGSKSerX5sSoVwaw的代码。

Answer 2

我用的一个解决办法是

trait A {
    def doSomething(a: Seq[Int]): Seq[String] = {
        a.map {
            case Extractor(s) => s
            case _ => "idc"
        }
    }
    
    object Extractor {
        def unapply(a: Int): Option[String] = getString(a)
    }

    
    def getString(a: Int): Option[String] = {
        a match {
            case 1 => Some("one")
            case 2 => Some("two")
            case _ => None
        }
    }
}

object AA extends A
object AA2 extends A {
    override def getString(a: Int): Option[String] = {
        super.getString(a).orElse{
            a match {
                case 3 => Some("three")
                case 4 => Some("four")
                case _ => None
            }
        }
    } 
}

object MyClass {
    def main(args: Array[String]) {
        println(AA.doSomething(Seq(1,2,3,4,5))); // Output: List(one, two, idc, idc, idc)
        println(AA2.doSomething(Seq(1,2,3,4,5))); // Output: List(one, two, three, four, idc)
    }
}