the hand that feeds you

the hand that feeds you

[Andreas Seik]

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"

thank you
Andreas

Re: the hand that feeds you

[Harald Hanche-Olsen]

+ 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 ()))

-- 
* Harald Hanche-Olsen     <URL:http://www.math.ntnu.no/~hanche/>
- It is undesirable to believe a proposition
  when there is no ground whatsoever for supposing it is true.
  -- Bertrand Russell

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

Re: the hand that feeds you

[wenbinye]

how about define the function like this:

(defvar kill-myself nil)
(defun kill-myself ()
  (unless kill-myself
    (insert "only once")
    (setq kill-myself t)))

Andreas Seik wrote:
> 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"
> 
> thank you
> Andreas

Re: the hand that feeds you

[Harald Hanche-Olsen]

+ Kevin Rodgers <ihs_4664@yahoo.com>:

| Harald Hanche-Olsen wrote:
|> (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?

Er, no, it is only dependent on the specification of defun.  Notably,
that the inner defun does not define a local function, as it might in
some lisp dialects (but not elisp).

| BTW, this is truly suicidal:
|
| (defun kill-myself ()
|    (insert "only once")
|    (unintern 'kill-myself))

Not suicidal at all.  The uninterned symbol remains in the hook, and
its function definition will get called.

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

That will cause run-hooks to fail on the second try.

-- 
* Harald Hanche-Olsen     <URL:http://www.math.ntnu.no/~hanche/>
- It is undesirable to believe a proposition
  when there is no ground whatsoever for supposing it is true.
  -- Bertrand Russell