MatchError when match receives an IndexedSeq but not a LinearSeq

Question

Is there a reason that match written against Seq would work differently on IndexedSeq types than the way it does on LinearSeq

Answer 1

You can't use :: for anything other than List. The Vector is failing to match because :: is a case class that extends List, so its unapply method does not work for Vector.

val a :: b = List(1,2,3)    // fine
val a :: b = Vector(1,2,3)  // error

But you can define your own extractor that works for all sequences:

object +: {
  def unapply[T](s: Seq[T]) =
    s.headOption.map(head => (head, s.tail))
}

So you can do:

val a +: b = List(1,2,3)   // fine
val a +: b = Vector(1,2,3) // fine