After understanding how
append-to-buffer works, it is easy to
copy-to-buffer. This function copies text into a
buffer, but instead of adding to the second buffer, it replaces all the
previous text in the second buffer.
The body of
copy-to-buffer looks like this,
... (interactive "BCopy to buffer: \nr") (let ((oldbuf (current-buffer))) (with-current-buffer (get-buffer-create buffer) (barf-if-buffer-read-only) (erase-buffer) (save-excursion (insert-buffer-substring oldbuf start end)))))
copy-to-buffer function has a simpler
The definition then says
(with-current-buffer (get-buffer-create buffer) ...
First, look at the earliest inner expression; that is evaluated first.
That expression starts with
get-buffer-create buffer. The
function tells the computer to use the buffer with the name specified
as the one to which you are copying, or if such a buffer does not
exist, to create it. Then, the
evaluates its body with that buffer temporarily current.
(This demonstrates another way to shift the computer's attention but
not the user's. The
append-to-buffer function showed how to do
the same with
with-current-buffer is a newer, and arguably easier,
barf-if-buffer-read-only function sends you an error
message saying the buffer is read-only if you cannot modify it.
The next line has the
erase-buffer function as its sole
contents. That function erases the buffer.
Finally, the last two lines contain the
insert-buffer-substring as its body.
insert-buffer-substring expression copies the text from
the buffer you are in (and you have not seen the computer shift its
attention, so you don't know that that buffer is now called
Incidentally, this is what is meant by “replacement”. To replace text, Emacs erases the previous text and then inserts new text.
In outline, the body of
copy-to-buffer looks like this:
current-buffer) (with-the-buffer-you-are-copying-to (but-do-not-erase-or-copy-to-a-read-only-buffer) (erase-buffer) (save-excursion insert-substring-from-