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RE: Rationale behind conversion of a nil prefix arg to numeric 1
From: |
Drew Adams |
Subject: |
RE: Rationale behind conversion of a nil prefix arg to numeric 1 |
Date: |
Mon, 5 Sep 2016 09:32:17 -0700 (PDT) |
> > Commands that need to distinguish between those should not use "p".
>
> Exactly.
> Just treat the "p" (lowercase p) interactive form as the special case
> where you only need to deal with numeric arguments and the default numeric
> argument is 1.
>
> If you need to deal with numeric, nil and other non-numeric arguments like
> (4), (16), etc, use the "P" (uppercase p) interactive form. The "P"
> interactive form passes the args to the function in their raw, untouched
> form.
What Eli and Kaushal said.
Again, this is a _feature_. You just need to learn what it is about
(and what it is not about). The doc is quite clear - give it a try.
In particular, if your code wants to know WHETHER a prefix argument
was EXPLICITLY provided by the USER, then you need to test the RAW
prefix argument. If you then want to know what the NUMERIC value
is, use `prefix-numeric-value':
(defun foo (arg)
(interactive "P") ; <=== uppercase P: RAW prefix arg
(if (not arg)
(no-prefix-arg-provided---usual/default-case)
(let ((nval (prefix-numeric-value arg)))
(cond ((= 42 nval) (answer-to-everything-it-seems))
((> nval 0) (at-least-its-positive))
((< nval 0) (so-negative!))
(t (nothing-at-all))))))
If you want to check for PARTICULAR non-nil raw prefix values:
(if (not arg)
(no-prefix-arg-provided---usual/default-case)
(cond ((= arg '-) (handle-M--))
((= arg '-42) (handle-M---4-2-or-C-u---4-2)) ; etc.
((= arg '4) (handle-M-4--or-C-u-4))
((and (consp arg) (= 4 (car arg))) (handle-plain-C-u))
((and (consp arg) (= 16 (car arg))) (handle-plain-C-u-C-u))
...))
Note that `C-u' gives `(4)' as the raw prefix arg, and `C-u 4' gives
`4' as the raw prefix arg. And `prefix-numeric-value' gives `4' in
both cases.
Experiment:
(defun foo (arg)
"Show the raw prefix arg and its numeric value."
(interactive "P")
(message "ARG: %S, Numeric value: %S"
arg (prefix-numeric-value arg)))
The case that this thread is about is just this one: `M-x foo'.
(Much ado about nothing.)
- Rationale behind conversion of a nil prefix arg to numeric 1, Florian v. Savigny, 2016/09/04
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Eli Zaretskii, 2016/09/04
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Andreas Röhler, 2016/09/04
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Eli Zaretskii, 2016/09/04
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Andreas Röhler, 2016/09/05
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Eli Zaretskii, 2016/09/05
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Kaushal Modi, 2016/09/05
- RE: Rationale behind conversion of a nil prefix arg to numeric 1,
Drew Adams <=
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Andreas Röhler, 2016/09/05
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Eli Zaretskii, 2016/09/05
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, tomas, 2016/09/06
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Florian v. Savigny, 2016/09/06
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, tomas, 2016/09/06
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Andreas Röhler, 2016/09/06
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Florian v. Savigny, 2016/09/06
- RE: Rationale behind conversion of a nil prefix arg to numeric 1, Drew Adams, 2016/09/06
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Kaushal Modi, 2016/09/06
- Re: Rationale behind conversion of a nil prefix arg to numeric 1, Andreas Röhler, 2016/09/06