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

[Hadley Wickham]

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/