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

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

On 04.09.2016 20:33, Eli Zaretskii wrote:
>> From: Andreas Röhler <andreas.roehler@easy-emacs.de>
>> Date: Sun, 4 Sep 2016 19:11:31 +0200
>>
>>>> I am still wondering about the
>>>> rationale of representing nil as the number one. Does this make sense,
>>>> or is it useful, in some way?
>>> Yes.  Most commands use the argument as a repeat count, so having it
>>> default to one makes perfect sense.
>>>
>> There a difference between "p" sending 1 and nil - which is the result
>> from non-interactive use.
> "p" is documented as the _numeric_ value of the argument.  So it
> obviously cannot yield nil.

The ideosyncrasy: it yields the same value for arg = 1 as if no arg were 
given:

(defun foo (&optional arg)
   (interactive "p")
   (message "My ARG: %s" arg)
   (message "Value of arg: %s" (prefix-numeric-value arg)))


;; called without ARG
(foo)
      ==> "My ARG: nil"
      ==> "Value of arg: 1"

;; called without ARG 1
(foo 1)

      ==> "My ARG: 1"
      ==> "Value of arg: 1"


Regardless of its purpose - such coding style should not be encouraged.

It would be easy and consistent to write

(or arg (setq arg 1))