lower-type-bounds.md

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While upper type bounds limit a type to a subtype of another type, lower type bounds declare a type to be a supertype of another type. The term B >: A expresses that the type parameter B or the abstract type B refer to a supertype of type A. In most cases, A will be the type parameter of the class and B will be the type parameter of a method.

Here is an example where this is useful:

trait Node[+B] {
  def prepend(elem: B): Node[B]
}

case class ListNode[+B](h: B, t: Node[B]) extends Node[B] {
  def prepend(elem: B): ListNode[B] = ListNode(elem, this)
  def head: B = h
  def tail: Node[B] = t
}

case class Nil[+B]() extends Node[B] {
  def prepend(elem: B): ListNode[B] = ListNode(elem, this)
}

This program implements a singly-linked list. Nil represents an empty element (i.e. an empty list). class ListNode is a node which contains an element of type B (head) and a reference to the rest of the list (tail). The class Node and its subtypes are covariant because we have +B.

However, this program does not compile because the parameter elem in prepend is of type B, which we declared covariant. This doesn't work because functions are contravariant in their parameter types and covariant in their result types.

To fix this, we need to flip the variance of the type of the parameter elem in prepend. We do this by introducing a new type parameter U that has B as a lower type bound.

trait Node[+B] {
  def prepend[U >: B](elem: U): Node[U]
}

case class ListNode[+B](h: B, t: Node[B]) extends Node[B] {
  def prepend[U >: B](elem: U): ListNode[U] = ListNode(elem, this)
  def head: B = h
  def tail: Node[B] = t
}

case class Nil[+B]() extends Node[B] {
  def prepend[U >: B](elem: U): ListNode[U] = ListNode(elem, this)
}

Now we can do the following:

trait Bird
case class AfricanSwallow() extends Bird
case class EuropeanSwallow() extends Bird


val africanSwallowList= ListNode[AfricanSwallow](AfricanSwallow(), Nil())
val birdList: Node[Bird] = africanSwallowList
birdList.prepend(new EuropeanSwallow)

The Node[Bird] can be assigned the africanSwallowList but then accept EuropeanSwallows.