is.recursive: Is an Object Atomic or Recursive?
In robertzk/monadicbase: The R Base Package

Description Usage Arguments Details References See Also Examples

is.atomic returns TRUE if x is of an atomic type (or NULL) and FALSE otherwise.

is.recursive returns TRUE if x has a recursive (list-like) structure and FALSE otherwise.

1 2	is.atomic(x) is.recursive(x)

`x`	object to be tested.

is.atomic is true for the atomic types ("logical", "integer", "numeric", "complex", "character" and "raw") and NULL.

Most types of objects are regarded as recursive. Exceptions are the atomic types, NULL, symbols (as given by as.name), S4 objects with slots, external pointers, and—rarely visible from R—weak references and byte code, see typeof.

It is common to call the atomic types ‘atomic vectors’, but note that is.vector imposes further restrictions: an object can be atomic but not a vector (in that sense).

These are primitive functions.

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

is.list, is.language, etc, and the demo("is.things").

require(stats)

is.a.r <- function(x) c(is.atomic(x), is.recursive(x))

is.a.r(c(a = 1, b = 3)) # TRUE FALSE
is.a.r(list())          # FALSE TRUE - a list is a list
is.a.r(list(2))         # FALSE TRUE
is.a.r(lm)              # FALSE TRUE
is.a.r(y ~ x)           # FALSE TRUE
is.a.r(expression(x+1)) # FALSE TRUE
is.a.r(quote(exp))      # FALSE FALSE