Why can't I match on a Stream?

Question

Specifically:

scala> def f(n: Seq[Any]) = n match { 
     case Nil => "Empty" 
     case h :: t => "Non-empty" 
}
f: (n: Seq[Any])String

scala> f(Stream())
res1: String = Empty

scala> f(List(1))
res17: String = Non-empty

scala> f(Stream(1))
scala.MatchError: Stream(1, ?) (of class scala.collection.immutable.Stream$Cons)
  at .f(<console>:13)
  ... 33 elided

There are a lot of other ways to implement this, but at written the code was statically safe and failed at run time. What's going on?

Answer 1

For Stream, the concat symbol should be #::, the pattern match should like:

  def f(n: Seq[Any]) = n match {
    case Nil => "Empty"
    case h :: t => "Non-empty"
    case h #:: t => "Non-empty stream"
  }

for :: is for List / Seq type(List extends from Seq:) ), see:

final case class ::[B](override val head: B, private[scala] var tl: List[B]) extends List[B] {