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Re: Accessing arg with (interactive "P")

From: Emanuel Berg
Subject: Re: Accessing arg with (interactive "P")
Date: Wed, 06 Jul 2022 03:16:17 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

carlmarcos--- via Users list for the GNU Emacs text editor wrote:

> I have the following elisp function, using
> `current-prefix-arg` to decide subsequent processing.

You can do that but better is to use the argument directly
just as you normally would,

(defun poke (&optional n)
  (interactive "P")
  (if n
      (message "n is set")
    (message "n is nil") ))

Try M-x poke RET and C-u M-x poke RET.

Note that this also works from Lisp which ignores the
interactive spec. Try eval these

  (poke 1)

With (poke) you see that optional arguments defaults to nil.
If you intends to use the arg, and nil doesn't make sense, you
must explicitely set it to some default that does, say 0 which
is common:

  (or n (setq n 0))

But in our example nil is fine so no need to do that.

> Having made the command examine it directly, the usual
> method for accessing it is with (interactive "P").
> The problem is that I do not know how to examine the prefix
> arg through the interactive clause.

Try evaluating the interactive form, for example

  (interactive "P") ; evals to (nil)

so the list element(s) is/are assigned to the formal
parameters, they become the arguments to the function.

Eval this:

  (interactive "sString: \nnNumber: ")

It will assign the data you input to the formal parameters of
a function that looks like this for example

  (defun xy-problem (str num) ... )

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