Handling nil values in optional argument

Handling nil values in optional argument

[Heime]

I have the following tool, but I want that if I pass nil in the third argument,
the function does not assume that fyr was not supplied, but that fyr is actually
to be used as a nil value. 

(defun mundu (monbf funcs labels &optional fyr)

  (unless fyr (setq fyr t))

RE: [External] : Handling nil values in optional argument

[Drew Adams]

> I have the following tool, but I want that if I pass nil in the third
> argument,
> the function does not assume that fyr was not supplied, but that fyr is
> actually
> to be used as a nil value.
> 
> (defun mundu (monbf funcs labels &optional fyr)
>   (unless fyr (setq fyr t))

You can't.  nil is the default value for an optional argument.

Instead, your code can use something other than nil for the value that you pass as an explicit argument.  It can use the singleton list (nil), for example.

Re: [External] : Handling nil values in optional argument

[Michael Heerdegen via Users list for the GNU Emacs text editor]

Drew Adams <drew.adams@oracle.com> writes:

> Instead, your code can use something other than nil for the value that
> you pass as an explicit argument.  It can use the singleton list
> (nil), for example.

Alternatively one can give the defun an &rest argument list and look at
the length of the specified list in the body.  The argument variables
would have to be assigned in the body.  Use an advertised calling
convention to get argument name support back for eldoc etc.

I guess that's more or less what's cl-defun also does.  So one can also
use cl-defun.  Or just do as Drew said.


Michael.

Re: [External] : Handling nil values in optional argument

[Heime]

On Friday, July 12th, 2024 at 4:37 PM, Michael Heerdegen via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org> wrote:

> Drew Adams drew.adams@oracle.com writes:
> 
> > Instead, your code can use something other than nil for the value that
> > you pass as an explicit argument. It can use the singleton list
> > (nil), for example.
> 
> 
> Alternatively one can give the defun an &rest argument list and look at
> the length of the specified list in the body. The argument variables
> would have to be assigned in the body. Use an advertised calling
> convention to get argument name support back for eldoc etc.
> 
> I guess that's more or less what's cl-defun also does. So one can also
> use cl-defun. Or just do as Drew said.
> 
> 
> Michael.

Can you show an example of using &rest for the specified purpose. 

Re: [External] : Handling nil values in optional argument

[Michael Heerdegen via Users list for the GNU Emacs text editor]

Heime <heimeborgia@protonmail.com> writes:

> Can you show an example of using &rest for the specified purpose. 

Here's a template of a function accepting two and optionally a third
argument:

#+begin_src emacs-lisp
(defun my-function (a b &rest r)
  (declare (advertised-calling-convention
            (a b &optional c) nil))
  (when (cdr r)
    (signal 'wrong-number-of-arguments
            (list '(2 . 3) (+ 2 (length r)))))
  (let ((c (car r)))
    ;; put the function body here...
    (message "a: %S; b: %S; c: %s"
             a b (if r (prin1-to-string c) "not specified"))))
#+end_src


Michael.