Re: Rationale behind conversion of a nil prefix arg to numeric 1

[Charles Millar <millarc@verizon.net>]

On 09/08/2016 09:10 AM, Kaushal Modi wrote:
> On Wed, Sep 7, 2016 at 10:41 PM Charles Millar <millarc@verizon.net 
> <mailto:millarc@verizon.net>> wrote:
>
>     I have read this thread as well as the other suggested readings. 
>
>     If the
>     purpose of either p or P is to pass arguments, I do not understand the
>     rationale of allowing C-u when using the interactive upper case P. 
>
>
> That's useful if the user wants the function to behave different based 
> on if the arg is a list or not.
>
>     As
>     noted above, It returns an integer as the only element in a list,
>     which
>     if evaluated returns an error.
>
>
> If the arg is a list (C-u), you have to treat it as a list.. e.g. use 
> (car arg).
>
>     Is there a use for this? 
>
>
> It's up to the user on how they want to treat different arg values.
>
>     Such as you may
>     want an error?
>
>
> Here's a dummy example:
>
> (defun foo (arg)
>   (interactive "P")
>   (message (concat "Arg is "
>                    (cond
>                     ((null arg)
>                      "nil")
>                     ((listp arg)
>                      (format "a list with element %d" (car arg)))
>                     (t
>                      (format "a number %d" arg))))))
> (global-set-key (kbd "C-c '") #'foo)''''
>
>
> After evaluating the above, do
>
> - C-c '
> - C-0 C-c ' or M-0 C-c '
> - C-1 C-c ' or M-1 C-c '
> - ..
> - C-u C-c '
> - C-4 C-c ' or M-4 C-c '
> - C-u C-u C-c '
> - C-1 C-6 C-c ' or M-1 M-6 C-c '
>
> If the user wishes, they can make the foo function behave differently 
> for each of the above bullets.
>
> So it eventually boils down to what the user wants.. do they want the 
> default arg to be nil or 1, do they want to support C-u and C-4 args 
> in different manner, etc. Based on that, they can choose to use the 
> "p" or "P" version of interactive.
> -- 

Thank you, Kaushal!

I suspect that the following has been said many times on this and other 
lists - "An example, as is a picture, is worth a 1000 words."

Charlie Millar