Scala 2.12：无法在提取器对象中使用取消应用

Question

I am following through Scala Tour provided on docs.scala-lang.org . I am stuck at the extractor objects tutorial: https://docs.scala-lang.org/tour/extractor-objects.html

Here is the code I am trying to compile:

object IdGenerator {
  private val id: AtomicInteger = new AtomicInteger
  def apply(name: String): String = id.incrementAndGet + "--" + name
  def unapply(genID: String): Option[String] = {
    val idParts = genID.split("--")
    if (idParts.head.nonEmpty && idParts.tail.nonEmpty)
      Some(idParts(0))
    else
      None
  }
}

println(IdGenerator("ABC"))
println(IdGenerator("DEF"))
println(IdGenerator("XYZ"))

IdGenerator(idName) = IdGenerator("ABC")
println(idName)
println(IdGenerator.unapply(IdGenerator("ABC")))

Here is the error:

D:\MyApps\ScalaPrac\helloworld\hello\src\main\scala\example\Hello.scala:68:5: value update is not a member of object example.IdGenerator
IdGenerator(idName) = IdGenerator("ABC")

It says that value update is not a member of object . Sure, it isn't. But I am not asking it to look for update method, I want it to look for unapply instead.

Answer 1

IdGenerator(idName) = x looks like an assignment, but it actually is syntax sugar for IdGenerator.update(idName, x) . That explains the error message you get.

You need to use the val keyword to extract idName :

val IdGenerator(idName) = IdGenerator("ABC")