(in-package #:cl-forth)

(define-condition stack-error (error)
  ((message :initarg :message
            :accessor error-message)))

(define-condition stack-underflow (stack-error)
  ((message :initarg :message
            :accessor underflow-message)))

(defclass stack ()
  ((contents :initform '()
             :initarg :initial-contents
             :accessor contents)))

(defmethod initialize-instance :after ((obj stack) &key initial-element &allow-other-keys)
  (when initial-element
    (setf (contents obj) (list initial-element))))

(defmethod push-to-stack ((obj stack) value)
  "Push VALUE onto the stack in the STACK."
  (push value (contents obj)))

(defmethod pop-from-stack ((obj stack))
  "Pop a value from the stack in the STACK."
  (pop (contents obj)))

(defmethod peek-stack ((obj stack))
  "Peek at the top value of the stack in the STACK."
  (car (contents obj)))

(defmethod print-object ((obj stack) stream)
  (print-unreadable-object (obj stream :type t)
    (format stream "(~{~a~^ ~})" (contents obj))))

;; Example usage:
(defun stack-test ()
  (let ((my-stack (make-instance 'stack)))
    (format t "Empty Stack: ~a~%" my-stack)
    (push-to-stack my-stack 10)
    (push-to-stack my-stack 20)
    (format t "Top element: ~a~%" (peek-stack my-stack))
    (format t "Popped element: ~a~%" (pop-from-stack my-stack))
    (format t "Stack after pop: ~a~%" my-stack)))