How to pattern-match an `Option` in the underlying?

Question

It says in Scala, pattern-matching will be converted to an invocation of unapply(...):Option[...] .

Here I defined an object with unapply :

object Hello {
  def unapply(s:String): Option[String] = Some(s)
}

Then I can use it in pattern-matching:

"hello" match {
   case Hello(s) => println(s)
}

Actually(maybe) it will be converted to:

Hello.unapply("hello") match {
  case Some(s) => println(s)
  case _ =>
}

But the Some here is also a case class , so the Some.unapply will be called.

That should be infinite. Which is not possible.

So how do Scala pattern matching the Option in the underlying?

Answer 1

The result of unapply is simply not pattern-matched again. Instead, the methods isEmpty and get are called. So your example would give:

val temp = Hello.unapply("hello")
if (!temp.isEmpty) {
  val s = temp.get
  println(s)
} else {
  throw new MatchError()
}

Since 2.11, it is not even necessary to return an Option , specifically. Any class with a method isEmpty: Boolean and a method get: A will do.