Re: Problem with simple script to clean out an ERC buffer

[Joost Kremers]

On Wed, Jun 06 2018, J. David Boyd wrote:
> Yes there is, but it doesn't do anything:  erc-cmd-CLEAR
>
> Supposed to "Clear the window content", but the code reads:
>
> (defun erc-cmd-CLEAR ()
>   "Clear the window content."
>   (recenter 0)
>   t)
> (put 'erc-cmd-CLEAR 'process-not-needed t)
>
> So not only does it not clear the window, but it seems to be 
> disabled.

No, the erc source mentions that the `process-not-needed' property 
just indicates that the command can be run when the erc process is 
not alive. So it's not disabled.

Since erc.el is part of Emacs, you might ask on the emacs-devel 
list why this command doesn't seem to do what it advertises and 
whether it should be changed to actually "clear the window 
content".

> Anyway, I like to clear out the buffer so I can reduce my memory 
> footprint.
> If there is anything important in it, I usually grab it and put 
> it into an org
> file.

As for your original question, your function has a bug:

```

(defun clearERCbuffer()
 (interactive)
 (previous-line)
 (end-of-line)
 (set-mark(point))
 (beginning-of-buffer)
 (exchange-point-and-mark)
 ;; (set-text-properties (point-min) (point-max) nil nil )
 (delete-region (point-min) (point-max)))

```

The final function call is `(delete-region (point-min) 
(point-max))`, which means that all the point, mark and region 
magic you did before that is moot, because you're trying to delete 
the entire buffer.

Which is probably why your getting an error: in the part that you 
are deleting there's most likely (I don't use erc, so I can't 
check) something (a prompt perhaps) that is indeed read-only. What 
you probably want is simply `(delete-region)`, except that START 
and END aren't optional.

So, I would do something like (untested):

```
(defun clearERCbuffer()
 (interactive)
 (save-mark-and-excursion
   (forward-line -1)
   (end-of-line)
   (delete-region (point-min) (point))))
```

Some remarks:

- `save-mark-and-excursion` saves point and mark and restores them 
 after executing its body. This means that point should be back 
 to where it was before you called the function. Note that if 
 you're on Emacs 25, you should replace `save-mark-and-excursion` 
 with `save-excursion`.
- The doc string of `previous-line` suggests it shouldn't be used 
 in Lisp programs, so I used `(forward-line -1)` instead.
- With `delete-region`, there's no need to create an active region 
 first. By (temporarily, due to `save-mark-and-excursion`) 
 positioning the cursor at the end of the previous line, it's 
 possible to simply use (point) as the END argument to 
 `delete-region`.

HTH

--
Joost Kremers
Life has its moments