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The following predicates test whether a Lisp object is an atom, is a
cons cell or is a list, or whether it is the distinguished object
nil
. (Many of these predicates can be defined in terms of the
others, but they are used so often that it is worth having all of them.)
This function returns t
if object is a cons cell, nil
otherwise. nil
is not a cons cell, although it is a list.
This function returns t
if object is an atom, nil
otherwise. All objects except cons cells are atoms. The symbol
nil
is an atom and is also a list; it is the only Lisp object
that is both.
(atom object) ≡ (not (consp object))
This function returns t
if object is a cons cell or
nil
. Otherwise, it returns nil
.
(listp '(1)) ⇒ t
(listp '()) ⇒ t
This function is the opposite of listp
: it returns t
if
object is not a list. Otherwise, it returns nil
.
(listp object) ≡ (not (nlistp object))
This function returns t
if object is nil
, and
returns nil
otherwise. This function is identical to not
,
but as a matter of clarity we use null
when object is
considered a list and not
when it is considered a truth value
(see not
in Combining Conditions).
(null '(1)) ⇒ nil
(null '()) ⇒ t
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