Swift Functional Programming - Second Edition

Stacks

A stack is a collection that is based on the Last In First Out (LIFO) policy.

The following figure presents a sample stack:

To implement a simple functional stack, we need to provide push, pop, isEmpty, and size operations. We implemented a functional LinkedList in the previous section, which can be used to implement a simple functional stack with the following operations:

• push: The cons operation in LinkedList
• pop:
• isEmpty: The isEmpty operation in LinkedList
• size: The size method in LinkedList

As seen here, the only operation that is missing is pop. Let's implement that:

func pop() -> (element: Element, list: Stack)? {  switch ...