Re: About a programming tip

["Andreas Röhler" <andreas.roehler@easy-emacs.de>]

Am 21.02.2013 07:42, schrieb Thierry Volpiatto:
> Xue Fuqiao <xfq.free@gmail.com> writes:
>
>> In (info "(elisp) Programming Tips"):
>>
>>     * In `interactive', if you use a Lisp expression to produce a list
>>       of arguments, don't try to provide the "correct" default values for
>>       region or position arguments.
>> [...]
>>       You do not need to take such precautions when you use interactive
>>       specs `d', `m' and `r', because they make special arrangements to
>>       recompute the argument values on repetition of the command.
>>
>> I'm confused with these two sentences.  The first sentence says that
>> "don't try to provide the 'correct' default values for region or
>> position arguments", but the second sentence says that "you don't need
>> to take such precautions when you use interactive specs `d', `m' and
>> `r'".  IIRC the specs `d', `m' and `r' are "region or position
>> arguments".  Where do I understand wrong, or is it a bug?  Thanks.
>
> (defun foo-1 (beg end)
>    (interactive (list (read-number "Beg: " (region-beginning))
>                       (read-number "End: " (region-end))))
>    (message "%S" (buffer-substring beg end)))
>
> (defun foo (beg end)
>    (interactive "r")
>    (message "%S" (buffer-substring beg end)))
>
> In other words you don't have to use foo-1, it is easier to use foo.
>

IMO these remarks are misleading indeed, as the form "r" will raise an error if no region exists.
Quite often I need a command, which works on region if existing, but uses some default if not - for example
at the range of the function at point than.

For example if it should work at the whole buffer by default, find in convenient to write

(defun foo (&optional beg end)
   " "
   (interactive "*")
   (let ((beg (cond (beg)
                    ((region-active-p)
                     (region-beginning))
                    (t (point-min))))
         (end (cond (end (copy-marker end))
                    ((region-active-p)
                     (copy-marker (region-end)))
                    (t (copy-marker (point-max))))))