Re: Making a function than can only be used interactively

[Christopher Dimech]

> Sent: Friday, July 08, 2022 at 6:55 PM
> From: "Yuri Khan" <yuri.v.khan@gmail.com>
> To: "help-gnu-emacs" <help-gnu-emacs@gnu.org>
> Subject: Re: Making a function than can only be used interactively
>
> On Fri, 8 Jul 2022 at 13:31, Emanuel Berg <incal@dataswamp.org> wrote:
> 
> > > It does not matter for interactive use. [...] ‘&optional’
> > > comes into play if you use this function non-interactively,
> > > from Lisp
> >
> > No, it matters. One example how it matters is that optional
> > arguments defaults to nil.
> 
> You’re right, with an interactive specification that evaluates to a
> list it matters, because the list you return may or may not have as
> many arguments as the function takes.
> 
> My point was that in the specific case of a three-argument function
> and a three-item string-valued interactive specification, &optional
> does not matter for interactive use.
> 
> 
> > Check out this file and in particular example 4 which doesn't
> > make sense to me?
> >
> > ;; DWIM example 1, from Lisp ignore region if set
> > ;; example 2, use the region if available from Lisp as well
> >
> > (defun test-dwim (&optional beg end)
> >   (interactive (when (use-region-p)
> >                  (list (region-beginning) (region-end)) ))
> 
> Here you have two cases. If the region is active, you produce a
> two-element list, otherwise, a 0-element list. The function signature
> allows 0..2 arguments, so it works in either case.
> 
> > ;; example 3, one call to `use-region-p' is enough
> >
> > (defun test-dwim-3 (re &optional beg end)
> >   (interactive `(,(read-regexp "re: ")
> >                  ,@(when (use-region-p)
> >                      (list (region-beginning) (region-end)) )))
> 
> Mostly same, except you build a list of 3 or 1 elements, and the
> function accepts 1..3 arguments.
> 
> > ;; example 4, let's do that with the `interactive' spec
> > ;; string. but without `use-region-p' it doesn't reset after
> > ;; I clear the region, or that's what I thought happened
> > ;; anyway :) so this doesn't work as intended, which
> > ;; `test-dwim-3' does, supposedly the worse one.
> >
> > (defun test-dwim-4 (re &optional beg end)
> >   (interactive "sre: \nr")
> 
> Here you use a string interactive spec which always produces 3
> elements. In non-interactive use, it will work if called as
> (test-dwim-4 "^foo$"), (test-dwim-4 "^foo$" 42), or (test-dwim-4
> "^foo$" 42 69).
> 
> As to your “clearing” the region, Emacs always maintains the point and
> mark positions, and the ‘r’ interactive spec code ignores the region
> activation flag and always passes the point and mark. (This could be
> considered a bug, but I see no good alternative behavior, except maybe
> passing two nils if the region is not active.)
> 

When using the prefix argument, what makes sense?  A mandatory or an optional 
declaration for the prefix argument?