Lists

A fundamental data structure is the immutable linked list. It is composed of two parts:

1. Nil: the empty set
2. Cons: a cell containing an element and the rest of the list

trait IntList ...
class Cons(val head: Int, val tail: IntList) extends IntList ...
class Nil extends IntList

The use of val is syntactic sugar for:

class Cons(_head: Int, _tail: IntList) extends IntList {
      val head = _head
      val tail = _tail
}

This saves the use of _head and _tail.