[Rd] What do you call the value that represents a missing argument?

Vitally S. spinuvit.list at gmail.com
Sat Oct 9 11:33:38 CEST 2010

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.


> Hadley

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