Re: the hand that feeds you

[Kevin Rodgers]

Harald Hanche-Olsen wrote:
> + Andreas Seik <andreas_neu@gmxpro.de>:
>
> | Hello Newsgroup,
> |
> | i need code in a hook that is executed only once,
> | so i tested the following:
> |
> |
> | (defvar my-hook '())
> | (defun kill-myself ()
> |  (insert "only once")(remove-hook 'my-hook 'kill-myself))
> | (add-hook 'my-hook 'kill-myself)
> | (run-hooks 'my-hook);; i think it might crash here, but it does not
> | (run-hooks 'my-hook)
> |
> | it, seems to work, but i do not trust it.
> | is it not "biting the hand that feeds you"
>
> It works, even in the presence of more hooks on the hook variable.
> But I agree it looks scary, and it seems to rely on the implementation
> of run-hooks and remove-hook:  In all likelihood, run-hooks will just
> cdr down the list, running the car of the rest each time.  And
> remove-hook will not modify the cons cell containing kill-myself as
> its car and the remaining list as its cdr, so all is well.
>
> But it's bad coding style, because you must reason out all that to see
> why it works.  And it is conceivable, if unlikely, that the
> implementation one of these functions will change so that it no longer
> works.  Better then, to let kill-myself really to commit suicide,
> rather than just to remove itself from a hook:
>
> (defun kill-myself ()
>   (insert "only once")
>   (defun kill-myself ()))

That is just as dependent on the implementation of defun as the original
is on the implementation of run-hooks and remove-hooks, isn't it?
Although I guess you could argue that it's only fragile with respect to
1 special form instead of 2 functions.

BTW, this is truly suicidal:

(defun kill-myself ()
  (insert "only once")
  (unintern 'kill-myself))

But maybe (fmakunbound 'kill-myself) or
(fset 'kill-myself (symbol-function 'ignore)) is good enough.  :-)

All I wanted was a Pepsi,
--
Kevin