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

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

On 04.09.2016 16:23, Eli Zaretskii wrote:
>> Date: Sun, 04 Sep 2016 14:23:35 +0200
>> From: florian@fsavigny.de (Florian v. Savigny)
>>
>> Naively, I used the numeric conversion of the prefix arg, i.e.
>>
>>     (interactive "p")
>>
>> which passes the prefix arg converted to a number. However, if I call
>> the function with no prefix argument, which is, expectedly, nil in raw
>> form, it converts this to the number 1. (This is what
>> `prefix-numeric-value' does, as explained in the docstring.)
>>
>> While I understand that this is how it is done, I am quite puzzled
>> about the logic behind this. It would seem intuitive to me to convert
>> nil to either the number 0 or, again, nil, but never to the number
>> 1. And practically, converting nil to 1 has the consequence that
>> calling the command with no prefix arg:
>>
>>     M-x command
>>
>> is exactly the same as calling it with a prefix arg of 1:
>>
>>     C-u 1 M-x command
>>
>> because the prefix arg converted to a number is 1 in both cases, which
>> reduces the number of possibilities of calling the command by one.
>>
>> I can circumvent this (and get the behaviour that I would expect) by
>> writing the function with
>>
>>     (if current-prefix-arg (prefix-numeric-value current-prefix-arg))
>>
>> in an explicit interactive list, but 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.
If a function needs a repeat count, it should provide some.