Exercise 1.4.10

Stacks

We mentioned before that cons, car, cdr and friends are purely functional. But, sometimes, you want to destructively modify a list you've defined. For instance, in order to implement a mutable stack. In order to destructively cons elements onto a list, use push.

* (defvar *stack* nil)
*stack*

* (push 1 *stack*)
(1)

* *stack*
(1)

* (push 2 *stack*)
(2 1)

* (push 3 *stack*)
(3 2 1)

* *stack*
(3 2 1)