Re: Prefix-Arg (non-interactive!) in Info

["Andreas Röhler" <andreas.roehler@easy-emacs.de>]

Am 13.08.2010 19:04, schrieb Memnon Anon:
> Eli Zaretskii<eliz@gnu.org>  writes:
>
>> Let's try a different approach: which function would you like to call
>> non-interactively, passing it some value of the prefix arg?  (Please
>> don't say "it's not important which function": my point is _precisely_
>> that it _is_ important, because the answer to your question is
>> specific to the function you want to invoke.)
>
> The function asked for was org-clock-in.
>
> ,----[ org-clock-in ]
> | org-clock-in is an interactive compiled Lisp function.
> |
> | (org-clock-in&optional SELECT START-TIME)
> |
> | Start the clock on the current item.
> | If necessary, clock-out of the currently active clock.
> | With a prefix argument SELECT (C-u), offer a list of recently clocked tasks to
> | clock into.  When SELECT is C-u C-u, clock into the current task and mark
> | is as the default task, a special task that will always be offered in
> | the clocking selection, associated with the letter `d'.
> `----
>
> I knew `C-u' = 4 (and not "t" as someone suggested), so I tried
> (global-set-key (kbd "<F12>") (lambda () (interactive) (org-clock-in 4)))
>                                                          ^^^^^^^^^^^^^^^
> This did not work, but I was hardly surprised.
> It never does on the first approach :)
>
> ===
> (Well, I had this example in mind: (info "(eintr)Interactive multiply-by-seven")
>
> (defun multiply-by-seven (number)       ; Interactive version.
>         "Multiply NUMBER by seven."
>         (interactive "p")
>         (message "The result is %d" (* 7 number)))
>
> (multiply-by-seven 2) -->  14
> M-8 M-x multiply-by-seven -->  56
> C-u M-x multiply-by-seven -->  28
> Works for this function...
> ===
>
> So I tried to figure out in what form org-clock-in wanted its argument
> of 4 passed in.
>
> And I found nothing, until I turned to google.
>
> ...
>
> Now I am really confused:
> I expected this to work, but it does not:
> (multiply-by-seven '(4)).
>
> Oh well, I still don't get it.
>
> But
> (global-set-key (kbd "<F12>") (lambda () (interactive) (org-clock-in
> '(4))))
> works, the questioner has an answer; problem solved. And I will
> dedicate some time to learn elisp some, soon (at least the basics);
> and I will start with a thorough reading of the elisp intro.
>
> Up to now, I could fix my and others problems by looking at working code
> and fiddling until mine worked finally, too ... somehow; but I am
> getting the impression this takes way more time on the long run than
> learning the beast properly.
>
> Memnon
>
>

Hi,

remember having that kind of diffcults too when starting with Emacs.
Somehow that issue may fit into another thread at emacs-devel "Emacs 
learning curve."

Difficulty was the notion of "Universal argument".
 From now:

it's a kind of interface from the Emacs Lisp side providing an input 
facility bound to a predefined key.

Basically it's a tool to toggle, a switch taking args with it.

You must only think of it, when writing something taking user input.
There are more provisions for user input.

Universal argument is just one.


Andreas