Re: Making a function than can only be used interactively

[carlmarcos--- via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

Jul 8, 2022, 06:55 by yuri.v.khan@gmail.com:

> 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.
>

Using a list in the most general way to use interactive.  I need specific rules 
for using a list, how to decide whether the arguments should be mandatory
or optional.


> 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.)
>