with-current-buffer mystery

with-current-buffer mystery

[Ryan Yeske]

Hello,

I'm writing some code that appends text from a process into a buffer.
I copied the filter code from the info node (elisp)Filter Functions:

     (defun ordinary-insertion-filter (proc string)
       (with-current-buffer (process-buffer proc)
         (let ((moving (= (point) (process-mark proc))))
           (save-excursion
             ;; Insert the text, advancing the process marker.
             (goto-char (process-mark proc))
             (insert string)
             (set-marker (process-mark proc) (point)))
           (if moving (goto-char (process-mark proc))))))

The text below this code fragment says:

        The reason to use `with-current-buffer', rather than using
        `save-excursion' to save and restore the current buffer, is so
        as to preserve the change in point made by the second call to
        `goto-char'.

Right.  So `with-current-buffer' should preserve the current buffer,
but allow changes to the point?

When I eval the following expressions, I found that it seems to depend
on whether or not the buffer is visible.

;; create a dummy buffer with text
(progn
  (pop-to-buffer (get-buffer-create "*dummy*"))
  (erase-buffer)
  (insert "1\n2\n3\n")
  (goto-char (point-min)))

Notice point is at the beginning of buffer.  Eval the next
expression with *dummy* still visible in other-window.

;; goto end of buffer and insert a 4, note where point is
(with-current-buffer (get-buffer-create "*dummy*")
  (goto-char (point-max))
  (insert "4")
  (newline))

The 4 is inserted at the correct position, but point did not
permanently change.  Switch to that buffer and check.

(pop-to-buffer "*dummy*")

So, maybe I'm misunderstanding `with-current-buffer'.

If you repeat the second step, but with *dummy* buried:

(delete-other-windows)

(with-current-buffer "*dummy*"
  (goto-char (point-max))
  (insert "5")
  (newline))

(pop-to-buffer "*dummy*")

The 5 is in the right place, and the point is at the end.

What is going on?

Any help or explanations would be greatly appreciated.

Ryan

Re: with-current-buffer mystery

[Johan Bockgård]

Ryan Yeske <rcyeske@gmail.com> writes:

> When I eval the following expressions, I found that it seems to
> depend on whether or not the buffer is visible.
[...]
> Any help or explanations would be greatly appreciated.

(info "(elisp)Window Point")

-- 
Johan Bockgård

Re: with-current-buffer mystery

[Ryan Yeske]

bojohan+news@dd.chalmers.se (Johan Bockgård) writes:

> Ryan Yeske <rcyeske@gmail.com> writes:
> 
> > When I eval the following expressions, I found that it seems to
> > depend on whether or not the buffer is visible.
> [...]
> > Any help or explanations would be greatly appreciated.
> 
> (info "(elisp)Window Point")

Thanks, Johan.  This is what I was missing.

Ryan