NULL represents the null object in R: it is a reserved
word. NULL is often returned by expressions and functions
whose value is undefined.
an object to be tested or coerced.
as.null ignores its argument and returns NULL. is.null returns TRUE if its argument's value
is NULL and FALSE otherwise.
NULL can be indexed (see Extract) in just about any
syntactically legal way: whether it makes sense or not, the result is
always NULL. Objects with value NULL can be changed by
replacement operators and will be coerced to the type of the
right-hand side. NULL is also used as the empty pairlist: see the
examples. Because pairlists are often promoted to lists, you may
encounter NULL being promoted to an empty list. Objects with value NULL cannot have attributes as there is only
one null object: attempts to assign them are either an error
(attr) or promote the object to an empty list with
attribute(s) (attributes and structure).
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
The New S Language.
Wadsworth & Brooks/Cole.