Problem Statement

A stack is a container of elements with   last in, first out   access policy.   Sometimes it also called LIFO. The stack is accessed through its top. The basic stack operations are:

Sometimes the last pushed stack element is made accessible for immutable access (for read) or mutable access (for write):

Stacks allow a very simple hardware implementation. They are common in almost all processors. In programming, stacks are also very popular for their way (LIFO) of resource management, usually memory. Nested scopes of language objects are naturally implemented by a stack (sometimes by multiple stacks). This is a classical way to implement local variables of a re-entrant or recursive subprogram. Stacks are also used to describe a formal computational framework. See stack machine. Many algorithms in pattern matching, compiler construction (e.g. recursive descent parsers), and machine learning (e.g. based on tree traversal) have a natural representation in terms of stacks.

Create a stack supporting the basic operations: push, pop, empty.

Let's start with the solution:

; build stack [0 1 ... 9 (top)] from a list
(list->stack (iota 10) 'my-stack)
(stack-top 'my-stack) → 9
(pop 'my-stack)  → 9
(stack-top 'my-stack) → 8
(push 'my-stack '🐸) ; any kind of lisp object in the stack
(stack-empty? 'my-stack) → #f
(stack->list 'my-stack) ; convert stack to list
    → (0 1 2 3 4 5 6 7 8 🐸)
(stack-swap 'my-stack) ; swaps two last items 
    → 8 ; new top
(stack->list 'my-stack)
     → (0 1 2 3 4 5 6 7 🐸 8) ; swapped
(while (not (stack-empty? 'my-stack)) (pop 'my-stack)) ; pop until empty
(stack-empty? 'my-stack)  → #t ; true

(push 'my-stack 7)
my-stack ; a stack is not a variable, nor a symbol - cannot be evaluated
   ⛔ error: #|user| : unbound variable : my-stack
(stack-top 'my-stack)  → 7