RE: [External] : Re: Making a function than can only be used interactively

[Drew Adams <drew.adams@oracle.com>]

>> Dunno whether it's a coincidence (I'm guessing no),
>> but in the past couple of weeks there've been a
>> boatload of similar questions on emacs.SE. Here
>> are some of them:
>
> I saw them and got the same kind of problem of how
> to write interactive function.

I can tell - by your just now writing this, even after
reading my message:

> I thought that because the prefix argument i[s] used
> first (using C-u N myfunc) then it also has to be
> the first argument in the function declaration.

So you _saw_ my comments on emacs.SE, but it's not
clear that you _read_ them.

Do yourself a favor.  Read this  s l o w l y  . . .

    A "prefix argument" is NOT an argument
    to the function (command).  It's NOT.

That is, it _need not be_.  It's _not automatically_
used as any of the function's args.  It _could_ be
passed as one or more of the function's args.  There's
NO necessary or logical connection between the "prefix
argument" and the function's arguments.

If this isn't yet clear, please read that again.  And
again ... till it's clear.

A prefix argument results from a _user action_.  It
makes a value available when the function's code is
evaluated.  If that value isn't passed as one of the
function's arguments then the function body (or code
it invokes...) can obtain it using the global variable
`current-prefix-arg' - see `C-h v current-prefix-arg'.

And you say:

> because the prefix argument i[s] used first (using
> C-u N myfunc)

No, it's not "used first".  It's _not used at all_
... unless the function's code actually, explicitly
uses it somehow.  A user can hit `C-u' all day long
with zero effect, if the current command doesn't make
any use of the prefix arg.

How can a command (function) explicitly use the prefix
arg?  (1) in an `interactive' spec, (2) in the body,
or (3) by passing its value as an argument.

To use the prefix arg in an `interactive' spec you can
(1) use `P' or `p' in a string argument (any number of
times), or (2) use `current-prefix-arg' in a list arg
(any number of times).

To use it in the body explicitly, use variable
`current-prefix-arg'.

How else can a function use the prefix arg?  (3) Pass
its value as one or more of the function's args.

> Quite complicated thing.

No, not really.  I think you've just been misled by
the name "prefix ARGUMENT".

Nothing to be ashamed of.  But now you know - it is
NOT AN ARGUMENT to the command - not unless (1) the
command's `interactive' spec provides its value as
one of the command's arguments OR (2) code that
invokes the command (function) passes it as one
of the arguments.

I don't think any of us have been able to help much
by replying to vague requests for guidance about
when to use this or that (optional arg, prefix arg,
`interactive', ...).

And the reason, I think, is because the request is
misguided (an X-Y question).  I'm guessing it's the
name "prefix argument" that's got you (and perhaps
others) twisted in a knot by mis-suggesting that it
represents something that has something to do with
an argument to the command (function).  IT DOESN'T.

Now go outside and have fun. ;-)
___

Rereading the Emacs manual about "prefix argument"
now, I can see where someone might get the wrong
idea.  It tries to present a command to users as
something more abstract than the Lisp (or C) function
that implements it.  And so it talks about a "prefix
argument" being provided to the command first, and
"minibuffer args" possibly being provided afterward.

IOW, from a user, non-Lisp point of view, you can
think that you first provide an "argument" to a
command using `C-u' etc.  And then the command might
prompt you for other info ("arguments") in the
minibuffer.

I think that's what that language is about.  But it
can (apparently) create a disconnect when someone
then tries to move to a Lisp understanding, with the
notion of a command as a function (an interactive
function), which gets passed arguments.
___


[
NOTE: If some code in a command (in the `interactive'
spec or in the body), or in any code invoked by the
command, invokes another command interactively, or lets
a user invoke another command interactively, then the
use of that other command invocation redefines the
`current-prefix-arg' value.

And yes, _that_ can get a bit complicated.  If you're
interested in this, and you feel up to it, then dig
into the Elisp manual, node `Command Loop':

https://www.gnu.org/software/emacs/manual/html_node/elisp/Command-Loop.html

]