Question

What is the best Scala idiom for matching a value to the n-th element of a list?

The naive approach obviously doesn’t work:

scala> val list = List(5,6,7)
list: List[Int] = List(5, 6, 7)

scala> val x = 7
x: Int = 7

scala> x match { case list(2) => true; case _ => false }
<console>:10: error: value list is not a case class constructor, nor does it have an       unapply/unapplySeq method
   x match { case list(2) => true; case _ => false }

To clarify- this question is not about how to compare a value to the n-th element of a list – it is specifically about whether it can be done using matching.

Answer 1

Behold, the power of instance extractors! (the Regex class in the stdlib works similarly)

case class Nth[A](which: Int) {
  def unapply(in: List[A]): Option[A] = if (in.size >= which+1) Some(in(which)) else None
}

val second = Nth[Int](1)

List(2,4,6) match {
  case second(4) => println("yep!")
  case x => println("nope!")
}