elisp-demo-01.el

elisp-demo-01.el

;; 1. (interactive)
;; "P": prefix argument
;; "B": Buffer name
;; "b": should be a >exiting buffer
;; "r": Point and the mark
;; "*": Signal an error if the current buffer is read-only.  Special.
(defun add-by-two (number)
  "Add NUMBER by two."
  (interactive "P\nbF buffer: ")                     ; Enable the prefix argument
  (message "The result is %d." (+ number 2)))
(add-by-two 3)

;; 2. (save-excursion ...)
;; Save mark, point and current buffer
(save-excursion
  (switch-to-buffer-other-window "*test-buffer*")
  (goto-char (point-min))
  (erase-buffer)
  (insert ";;=>")
  (other-window 1))

(defun my-beginning-of-buffer ()
  "Move point to the beginning of the buffer;
leave mark at previous position."
  (interactive)
  (push-mark)
  (goto-char (point-min)))

(defun my-mark-whole-buffer ()
  "Put point at beginning and mark at end of buffer."
  (interactive)
  (push-mark (point))
  (push-mark (point-max) nil t)
  (goto-char (point-min)))

(defun my-append-to-buffer (buffer start end)
  "Append to specified buffer the text of the region.
It is inserted into that buffer before its point.

When calling from a program, give these arguments:
BUFFER (of buffer name, START and END.
START and END specify the portion of the current buffer to copied."
  (interactive
   (list (read-buffer "Append to buffer: " (other-buffer
                                            (current-buffer) t))
         (region-beginning) (region-end)))
  (let ((oldbuf (current-buffer)))
    (save-excursion
      (let* ((append-to (get-buffer-create buffer))
             (windows (get-buffer-window-list append-to t t))
             point)
        (set-buffer append-to)
        (setq point (point))
        (barf-if-buffer-read-only)      ;error message when finds read-only buffer
        (insert-buffer-substring oldbuf start end)
        (dolist (window windows)
          (when (= (window-point window) point)
            (set-window-point window (point))))))))

(defun buffer-exist? (buffer-name)
  "Use `if' and `get-buffer' to test if BUFFER exist."
  (interactive)
  (if (get-buffer buffer-name)
      (message "Yes, the buffer '%s' exist." buffer-name)
    (message "No, the buffer '%s' doesn't exist." buffer-name)))

(buffer-exist? (current-buffer))

;; 3. with-current-buffer
;; tell computer to work on another buffer not user
;; is used to replace "save-excursion + set-buffer"

(defun copy-to-buffer (buffer start end)
  "Copy to specified buffer the text of the region.
It is inserted into that buffer, replacing existing text there.

When calling from a program, give three arguments:
BUFFER (or buffer name), START and END.
START and END specify the portion of the current buffer to be copied."
  (interactive "BCopy to buffer: \nr")
  (let ((oldbuf (current-buffer)))      ; user seem buffer now called old buffer
    (with-current-buffer (get-buffer-create buffer)
      (barf-if-buffer-read-only)
      (erase-buffer)
      (save-excursion                   ;with current user seen buffer
        (insert-buffer-substring oldbuf start end)))))

(defun my-insert-buffer (buffer)
  "Insert after point the contents of BUFFER.
Puts mark after the insert text.
BUFFER may be a buffer of a buffer name."
  (interactive "*bInsert buffer: ")
  (or (bufferp buffer)
      (setq buffer (get-buffer buffer)))
  (let (start end newmark)              ;bind 3 local vars to nil
    (save-excursion
      (save-excursion                   ;go to source buffer
        (set-buffer buffer)
        (setq start (point-min)
              end (point-max)))
      (insert-buffer-substring buffer start end) ;go to destination buffer
      (setq newmark (point)))
    (push-mark newmark)))

;; 4. or
(or (bufferp (get-buffer "*scratch2*"))
    "Nope")
(if (bufferp (get-buffer "*scratch2*"))
    "Yep"
  "Nope")
(unless (bufferp (get-buffer "*scratch2*"))
  "Nope")
(when (not (bufferp (get-buffer "*scratch2*")))
  "Nope")