Defaulting optional numeric and string arguments

Defaulting optional numeric and string arguments

[Heime]

I have an optional numeric argument n passed to a function. 

Am having difficulty setting a default for it to cater for both the 
interactive and non-interactive cases.

Is the below how to do it ?   

  (setq n (if (or (not n) (not numberp n)) 0 n))

I also have an optional string bfrn, to which I am using

  (setq bfrn  (if (or (not bfrn) (string= bfrn "")) "Context" bfrn))

Re: Defaulting optional numeric and string arguments

[Stefan Monnier via Users list for the GNU Emacs text editor]

> I have an optional numeric argument n passed to a function. 
>
> Am having difficulty setting a default for it to cater for both the 
> interactive and non-interactive cases.
>
> Is the below how to do it ?   
>
>   (setq n (if (or (not n) (not numberp n)) 0 n))

Traditionally the default numeric value for an absent prefix arg is
1 rather than 0.  So if you can arrange to use 1 as default, it'll
integrate better into the overall Emacs experience.

Also you can simply the above to

    (unless (numberp n) (setq n 0))

> I also have an optional string bfrn, to which I am using
>   (setq bfrn  (if (or (not bfrn) (string= bfrn "")) "Context" bfrn))

I'd recommend to ignore the "" case (i.e. change your callers and/or the
interactive spec so they don't accidentally send the empty string
(e.g. use `read-string` rather than `read-from-minibuffer`)) and
just use:

    (unless bfrn (setq bfrn "Context"))


- Stefan

Re: Defaulting optional numeric and string arguments

[Heime]

On Friday, September 13th, 2024 at 7:17 AM, Stefan Monnier via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org> wrote:

> > I have an optional numeric argument n passed to a function.
> > 
> > Am having difficulty setting a default for it to cater for both the
> > interactive and non-interactive cases.
> > 
> > Is the below how to do it ?
> > 
> > (setq n (if (or (not n) (not numberp n)) 0 n))
> 
> 
> Traditionally the default numeric value for an absent prefix arg is
> 1 rather than 0. So if you can arrange to use 1 as default, it'll
> integrate better into the overall Emacs experience.
> 
> Also you can simply the above to
> 
> (unless (numberp n) (setq n 0))
> 
> > I also have an optional string bfrn, to which I am using
> > (setq bfrn (if (or (not bfrn) (string= bfrn "")) "Context" bfrn))
> 
> 
> I'd recommend to ignore the "" case (i.e. change your callers and/or the
> interactive spec so they don't accidentally send the empty string
> (e.g. use `read-string` rather than `read-from-minibuffer`)) and
> just use:
> 
> (unless bfrn (setq bfrn "Context"))
 
If user does not pass anything to read-string, would the value not be nothing.
I would like a value to be used as it should store a buffer name.
 
> - Stefan

Re: Defaulting optional numeric and string arguments

[Stefan Monnier]

> If user does not pass anything to `read-string`, would the value not be nothing.

Not if you provide a sane `default-value` argument.


        Stefan

Re: Defaulting optional numeric and string arguments

[Heime]

On Friday, September 13th, 2024 at 8:18 AM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:

> > If user does not pass anything to `read-string`, would the value not be nothing.
> 
> Not if you provide a sane `default-value` argument.
> 
> Stefan

There is the additional problem when using 

(interactive "nNumber of Context Lines: \ns Buffer Name: " )

Which does not apply any defaults