bit ugly but simplest possible example of a meaningful pushdown automata parser

hippie-prompt.cl

(defpackage :hippie-prompt
  (:use :common-lisp)
  (:export :prompt
	   :set-prompt))

(in-package :hippie-prompt)

(defun symbolize (ln)
  (intern (string-upcase (coerce ln 'string))))

(defun symbolize-stack (stack &optional (stack2 (list)))
  (if (symbolp (car stack))
      (cons (symbolize stack2) stack)
      (symbolize-stack (cdr stack) (cons (car stack) stack2))))

(defun symbolize-prompt-input (input &optional (stack (list 'empty-stack)))
  (cond
    ((null input)
     (symbolize-stack stack))
    ((whitespacep (car input))
     (if (letterp (car stack))
	 (symbolize-prompt-input (cdr input) (symbolize-stack stack))
	 (symbolize-prompt-input (cdr input) stack)))
    ((letterp (car input)) (symbolize-prompt-input (cdr input) (cons (car input) stack)))
    (t (symbolize-prompt-input (cdr input) stack))))

(defun letterp (char)
  (member char (coerce "abcdefghijklmnopqrstuvwxyz" 'list)))

(defun whitespacep (char)
  (or (eq char #\Space)
      (eq char #\Tab)))

(defun empty-symbolp (symbol)
  (equal "" (symbol-name symbol)))

(defun remove-empty-symbols (ln)
  (remove-if #'empty-symbolp ln))

(defvar *prompt* "> ")

(defun prompt ()
  (princ *prompt*)
  (cdr (reverse (remove-empty-symbols (symbolize-prompt-input (coerce (read-line) 'list))))))