Why is Nil required at the end of a list built using the cons operator

Question

I\'m learning Scala at the moment (Programming Scala, 2nd Edition, Odersky).

When building a list using the cons operator we have to write:

val l = 1         


        

Answer 1

The signature of :: is roughly:

case class ::[E](hd: E, tl: List[E]) extends List[E]

// which generates this automatically:

object :: {
    def apply[E](hd: E, tl: List[E]): ::[E]
}

The signature of Nil is roughly:

object Nil extends List[Nothing]

As you see, :: takes an element and a list. 4 is not a list, while Nil is.