elisp-macro-demo.el

elisp-macro-demo.el

;;; Example 1
(defmacro inc (var)
  (list 'setq var (list '1+ var)))

(macroexpand '(inc a))
==> (setq a (1+ a))

(defmacro inc2 (var1 var2)
  (list 'progn (list 'inc var1) (list 'inc var2)))

(let ((a 1) (b 2))
  (inc2 a b))

(macroexpand '(inc2 r s))
(progn (inc r) (inc s))

;;; Example 2
(defmacro multidefvar (&rest vars)
  (let ((forms (mapcar (lambda (var) `(defvar ,var)) vars)))
    `(progn ,@forms)))

(macroexpand '(multidefvar a b c d))
==> (progn (defvar a) (defvar b) (defvar c) (defvar d))

`(progn ,(list '1+ 2)
        ,(list '1- 23))

;;; Example 3
;;; (elisp) Argument Evaluation

(defmacro for2 (var from init to final do &rest body)
  "Execute a simple for loop.
For example, (for i from 1 to 10 do (print i))."
  (list 'let (list (list var init))
        (cons 'while
              (cons (list '<= var final)
                    (append body (list (list 'inc var)))))))

(for2 i from 1 to 3 do
      (setq square (* i i))
      (princ (format "\n%d %d" i square)))

1 1
2 4
3 9nil

(macroexpand '(for2 i from 1 to 3 do
                    (setq square (* i i))
                    (princ (format "\n%d %d" i square))))

==> (let ((i 1)) (while (<= i 3) (setq square (* i i)) (princ (format "
%d %d" i square)) (inc i)))

(defmacro for3 (var from init to final do &rest body)
  "Execute a simple for loop.
For example, (for i from 1 to 10 do (print i))."
  `(let ((,var ,init))
     (while (<= ,var ,final)
       ,@body
       (inc ,var))))

(for3 i from 1 to 3 do
      (setq square (* i i))
      (princ (format "\n%d %d" i square)))

1 1
2 4
3 9nil

(macroexpand '(for3 i from 1 to 3 do
                    (setq square (* i i))
                    (princ (format "\n%d %d" i square))))
===> (let ((i 1)) (while (<= i 3) (setq square (* i i)) (princ (format "
%d %d" i square)) (inc i)))

;;; Correctly expansion
(let ((i 1)
      (max 3))
  (while (<= i max)
    (setq square (* i i))
    (princ (format "\n%d      %d" i square))
    (inc i)))

1      1
2      4
3      9nil


(defmacro for4 (var from init to final do &rest body)
  `(let ((,var ,init)
         (max ,final))
     (while (<= ,var max)
       ,@body
       (inc ,var))))

;;; introduce a local variables

(defmacro for5 (var from init to final do &rest body)
  (let ((tempvar (make-symbol "max")))
    `(let ((,var ,init)
           (,tempvar ,final))
       (while (<= ,var ,tempvar)
         ,@body
         (inc ,var)))))

(let ((max 0))
  (for4 x from 0 to 3 do
        (princ (format "\n[%d] %d %d" max x (* x x))))
  (message "%d" max))

[3] 0 0
[3] 1 1
[3] 2 4
[3] 3 9"0"


(let ((max 0))
  (for5 x from 0 to 3 do
        (princ (format "\n[%d] %d %d" max x (* x x))))
  (message "%d" max))


[0] 0 0
[0] 1 1
[0] 2 4
[0] 3 9"0"