Re: (interactive) and &optional

[Philip Kaludercic <philipk@posteo.net>]

Dr Rainer Woitok <rainer.woitok@gmail.com> writes:

> Greetings,
>
> In an attempt  to write a function  with and optional argument  which is
> both,  callable from Lisp and via "M-x",  I ran into some unexpected (by
> me) problems.  Consider the following function:
>
>    (defun fun (&optional arg)
>    (interactive "Sarg: ")
>    (message "%s:%s.\n" 'val (symbol-name arg)))

This would have done the same thing:

     (message "val: %s.\n" arg)

>
> Calling "M-: (fun 'a)" returns
>
>    "val:a.
>    "
>
> including the double quotes,  while calling "M-x fun" and then typing "a
> RET" at the prompt returns
>
>    val:a.\n

That is because `message' returns the message as a string when
evaluated, and using M-: you first have `message' do it's thing, then
return the string it printed (using the printed representation for a
string, which includes queotes).  You can verify this by looking up the
message log using C-h e.

> without double quotes.  Apart from perhaps the double quotes, this is
> what I had expected.  Likewise, calling "M-: (fun)" returns
>
>   "val:nil.
>   "
>
> as expected,  while calling "M-x fun" and then just typing  "RET" at the
> prompt returns
>
>   val:.\n
>
> that is, an empty symbol or string.

Right.

> Am I really expected in a function that is both,  callable from Lisp and
> via "M-x", to code something along the lines of
>
>    (cond ((or (null arg) (string-empty-p arg)) 'default-val)
>          (arg))
>
> to check whether or not an optional argument has been passed?  Are there
> any more elegant ways to achieve this?

The interactive spec can either take a string as you did here, or an
expression that will be evaluated to generate a list of arguments.  So
the following will read a string (not a symbol), check if the string is
empty (I don't think the nil check is necessary) in which case the
symbol `default-var' is returned, otherwise we use `intern' to request a
symbol with the name of whatever we just queried the user:

--8<---------------cut here---------------start------------->8---
(defun fun (&optional arg)
  (interactive (list (let ((name (read-string "arg: ")))
		       (if (string-empty-p name)
			   'default-val
			 (intern name)))))
  (message "val: %s.\n" arg))
--8<---------------cut here---------------end--------------->8---

> Any pointers welcome :-)
>
> Sincerely,
>   Rainer

-- 
Philip Kaludercic