Question

What is the difference between NULL and character(0) | integer(0) etc?

> identical(NULL, character(0))
[1] FALSE

> is.null(integer(0))
[1] FALSE

> str(character(0))
 chr(0) 

> str(NULL)
 NULL

In general it seems you can pass NULL as parameters into functions, and that an empty vector is generally returned as character(0), integer(0), etc.

Why is this the case? Come to think of it, is there a test for zero-ness, a la is.integer0?

Answer 1

The R Language Definition has this on NULL:

There is a special object called NULL. It is used whenever there is a need to indicate or
specify that an object is absent. It should not be confused with a vector or list of zero
length. The NULL object has no type and no modifiable properties. There is only one NULL
object in R, to which all instances refer. To test for NULL use is.null. You cannot set
attributes on NULL.

So by definition NULL is very different to zero length vectors. A zero length vector very much isn’t absent. NULL is really a catch-all for something that is absent or not set, but not missing-ness, which is the job of NA. There is an exception, the zero-length pairlist, as mentioned by @Owen. The Language Definition states:

A zero-length pairlist is NULL, as would be expected in Lisp but in contrast to a zero-length list.

which highlights the exception in this case.

To test for a zero-length vector use something like if(length(foo) == 0L) for example. And combine that with a class check (is.character(foo)) if you want a specific type of zero length vector.