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

[Andreas Röhler]

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))