How can I omit this Nil Case

Question

I am mucking around with scala implementing some common algorithms. While attempting to recreate a bubble sort I ran into this issue

Here is an implementation of an the inner loop that bubbles the value to the top:

def pass(xs:List[Int]):List[Int] = xs match { 
  case Nil => Nil 
  case x::Nil => x::Nil 
  case l::r::xs if(l>r) => r::pass(l::xs)
  case l::r::xs => l::pass(r::xs)
}

My issue is with case Nil => Nil. I understand that I need this is because I could apply Nil to this function. Is there a way to ensure that Nil can't be provided as an argument in a manner that would satisfy the compiler so I can eliminate this case?

Answer 1

List has two subtypes, Nil and ::, so :: represents a list that has at least one element.

def pass(xs: ::[Int]):List[Int] = xs match { 
  case x::Nil => x::Nil 
  case l::r::xs if(l>r) => r::pass(new ::(l,xs))
  case l::r::xs => l::pass(new ::(r, xs))
}