R devel - Oct 2010 - What do you call the value that represents a missing argument?

Hi all,

What's the official name for the value that represents a missing argument?

e.g.
formals(plot)$x
str(formals(plot)$x)
deparse(formals(plot)$x)
is.symbol(formals(plot)$x)

What's the correct way to create an object like this?  (for example if
you are manipulating the formals of a function to add an argument with
no default value, as in http://stackoverflow.com/questions/3892580/).
as.symbol("") returns an error.  Both substitute() and bquote() return
that object, but it's not obvious if this is on purpose.

Hadley


-- 
Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/

On Fri, 8 Oct 2010, Hadley Wickham wrote:
> Hi all,
>
> What's the official name for the value that represents a missing
argument?
>
> e.g.
> formals(plot)$x
See ?list

It is a 'dotted pair list'

Are you looking for 'alist'?

"alist handles its arguments as if they described function arguments. So 
the values are not evaluated, and tagged arguments with no value are 
allowed whereas list simply ignores them. alist is most often used in 
conjunction with formals."
> alist(x=)$x==formals(plot)$x
[1] TRUE
>
HTH,

Chuck
> str(formals(plot)$x)
> deparse(formals(plot)$x)
> is.symbol(formals(plot)$x)
>
> What's the correct way to create an object like this?  (for example if
> you are manipulating the formals of a function to add an argument with
> no default value, as in http://stackoverflow.com/questions/3892580/).
> as.symbol("") returns an error.  Both substitute() and bquote()
return
> that object, but it's not obvious if this is on purpose.
>
> Hadley
>
>
> -- 
> Assistant Professor / Dobelman Family Junior Chair
> Department of Statistics / Rice University
> http://had.co.nz/
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901

Hadley Wickham <hadley at rice.edu> writes:
> What's the correct way to create an object like this?  (for example if
> you are manipulating the formals of a function to add an argument with
> no default value, as in
http://stackoverflow.com/questions/3892580/).as.symbol("") returns an
error.  Both substitute() and bquote() return
> that object, but it's not obvious if this is on purpose.
The question on stackoverflow was concerned with the creation of such an object:

Obvious way:
x <- alist(x=)$x

Because the object like that seems to be a zero length name 

as.symbol("") #should be able to produce it

quote() #is another meaningful candidate which does not work

instead substitute() and bquote() works fine. Funny thing is that the behavior
of bquote() and quote() are not expected to be different in this case.

Vitally.

>
> Hadley