Re: default value with interactive

[Michael Heerdegen <michael_heerdegen@web.de>]

Emanuel Berg <moasen@zoho.com> writes:

> I'm preparing one of my packs for MELPA so I go
> thru the list of formal style
> suggestions [1] ...
>
> One I don't know how to do is when there is
> a default value. The info node that is
> referenced in the suggestion list, namely
>
>     (info "(elisp) Programming Tips")
>
> says the prompt string should look like this
>
>     Enter the answer (default 42):
>
> However then it says it should be implemented
> like this
>
>     (defun foo (pos)
>       (interactive
>         (list (if SPECIFIED SPECIFIED-POS)))
>       (unless pos (setq pos DEFAULT-POS))
>       ...)
>
> Note that DEFAULT-POS in this example does not
> appear in the `interactive' form! So how does
> it appear in the prompt string at all?
>
> And even if it did, how do you then get around
> it without having to compute/fetch DEFAULT-POS
> two times? One time for the prompt string, and
> then again to set pos (in the abstract example
> above)?
>
> Is there a real-code example somewhere when
> this is done the right way?

It depends on you case.  How does it look like?  I think there is no
general answer.

In most situations, the prompt will rather look like

 "Position (default: point)"

instead of

 "Position (default 15327)".

In other situations, it is not really a problem to let `interactive'
return the default value, because when you repeat the command, you want
to specify the argument anyway, and a reasonable "default" is not
well-defined when repeating.

In some cases, maybe your case is such a case, it is better to read the
arguments in the body of the function and test with
`called-interactively-p' whether that is necessary.


Michael.