I'm trying to understand the implementation of Lists in Scala. In particular I'm trying to get my head around how you can write match expressions using an infix operator, for example:
a match {
case Nil => "An empty list"
case x :: Nil => "A list without a tail"
case x :: xs => "A list with a tail"
}
How is the match expression allowed to be x :: xs rather than List(x, xs)?
Jay Conrad's answer is almost right. The important thing is that somewhere there is an object named :: which implements the unapply method, returning type Option[(A, List[A])]. Thusly:
object :: {
def unapply[A](ls: List[A]): Option[(A, A)] = {
if (ls.empty) None
else Some((ls.head, ls.tail))
}
}
// case objects get unapply for free
case object Nil extends List[Nothing]
In the case of :: and List, this object happens to come out of the fact that :: is a case class which extends the List trait. However, as the above example shows, it doesn't have to be a case class at all.