Re: custom function getting initialized randomly

[Michael Maurer <maurer.michael@gmail.com>]

On Mon, 27 Mar 2023 at 17:21, <tomas@tuxteam.de> wrote:
>
> On Mon, Mar 27, 2023 at 02:56:01PM +0100, Richard Copley wrote:
> > On Mon, 27 Mar 2023 at 10:21, Michael Maurer <maurer.michael@gmail.com> wrote:
> > >
> > > Hello,
> > >
> > > I wrote the following function to copy the kill-ring to the scratch
> > > buffer, but it only gets initialized sometimes at the start of emacs,
> > > sometimes not. Why?
> > >
> > > (defun copy-to-scratch-on-kill
> > > ()
> > > "Copy every kill (delete or yank) to *scratch* buffer."
> > > (let
> > >     ((cur-kill-string
> > >       (current-kill 0 t)))
> > >   (when
> > >       (and
> > >        (not
> > > (equal cur-kill-string ""))
> > >        (not
> > > (equal cur-kill-string prev-kill-string))
> > >        (get-buffer "*scratch*"))
> > >     (with-current-buffer "*scratch*"
> > >       (goto-char
> > >        (point-max))
> > >       (insert cur-kill-string "\n")
> > >       (goto-char
> > >        (point-max))))
> > >   (setq prev-kill-string cur-kill-string)))
> > >
> > > (add-hook 'post-command-hook #'copy-to-scratch-on-kill)
> > >
> >
> > If an error propagates from the hook function (most likely "Kill ring
> > is empty", signalled by `current-kill`) then Emacs removes the
> > function from the hook. See the docstring for variable
> > `post-command-hook`.
>
> There's more: where does the variable `prev-kill-string' come from?
>
>  - if nobody has set it previously, this will be the error
>    killing your hook (this is my hunch)
>  - if somebody has, you are lucky if that somebody was you
>    (otherwise you are into an entertaining debugging session
>
> If you are using a global variable like that, it makes sense
> to declare, document and perhaps initialize it with `defvar'
>
> Another possibility would be to hide it away in a closure [1]
> (you are using lexical binding, aren't you?).  Just ask around
> here if the concept is unfamiliar to you.
>

You're right, that's something I forgot to paste.
The function is preceded by (defvar prev-kill-string "")
I'll take a look at closures and see how far I get on my own, ty.