Common Lisp: Easy macro introduction

file_access_macro.lisp

;; =============================================================
;; A file simulator is used to create an example problem 
;; that helps demonstrate the use of macros in common lisp
;; =============================================================
;; Interfaces to file I/Os expose methods like
;; `open`, `close`, `read`
;; not closing a file can lead to issues where the machine runs
;; out of memory, or unreliable state of the file 
;; (due to unwanted side-effects)
;;
;; ------------------------------------
;; try {
;;   file = File("/path/to/file").open()
;;   data = file.read()
;; } catch (...) {
;;   ...
;; } finally {
;;   file.close()
;; }
;; ------------------------------------
;;
;; The above pseudo-code is therefore a common pattern, 
;; luckily languages provide syntactic sugar for the same
;; for example, Python's `with`
;;
;; ------------------------------------
;; with open("path/to/file", "mode") as f:
;;    data = f.read()
;; ------------------------------------
;; 
;; A clearer syntax which even closes the file for us
;; Let's imagine that such syntax is not available in clisp
;; and create the same using clisp macros

(defclass file-sim ()
  ;; -bytes: An array of bits in the file
  ;; -state: "open" if the file can be read / "closed" if the file is closed
         ((bytes
          :initarg :bytes)
          (state
           :initarg :state)))

(defmethod open-file ((file file-sim))
  ;; method to change the state of the file
         (print "open")
         (setf (slot-value file 'state) "open"))

(defmethod read-file ((file file-sim))
  ;; method to read the bytes in the file
         (cond
           ;; Check the state of the file
           ((equal (slot-value file 'state) "open") (print (slot-value file 'bytes)))
           ((equal (slot-value file 'state) "closed") "Use the open-file method on the file before reading")))

(defmethod close-file ((file file-sim))
  ;; method to change the state of the file to "closed"
          (print "closed")
          (setf (slot-value file 'state) "closed"))

;; =============================================================
;; To do the same using a function we would write something like
;; ------------------------------------
;; (defun (file)
;;    (progn 
;;      (open-file file)
;;      (read-file file)
;;      (close-file file)))
;; ------------------------------------
;; To create a macro that outputs this particular function's 
;; code, we can use the (`)[backticks] to create the
;; template and fill in the variables using (,)[comma]
;; @body allows us to add in any method other than `read-file`
;; =============================================================

(defmacro with (file &body body)
         `(progn
            (open-file ,file)
            ,@body
            (close-file ,file)))

;; =============================================================
;; Now we can use this macro like:
;; ------------------------------------
;; (with *test-file* 
;;    (read-file *test-file*))
;; ------------------------------------
;; =============================================================