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null

null System Class

Class Precedence List:

null, symbol, list, sequence, t

Description:

The only object of type null is nil, which represents the empty list and can also be notated ().

See Also:

Section 2.3.4 (Symbols as Tokens), Section 2.4.1 (Left-Parenthesis), Section 22.1.3.3 (Printing Symbols)

Expanded Reference: null (System Class)​

The null type​

The type null has exactly one object: NIL. NIL is simultaneously the empty list, the boolean false, and a symbol.

(typep nil 'null)
=> T

(typep '() 'null)
=> T

(typep t 'null)
=> NIL

Class precedence list​

The class precedence list for null is: null, symbol, list, sequence, t. NIL is both a symbol and a list.

(typep nil 'symbol)
=> T

(typep nil 'list)
=> T

(typep nil 'sequence)
=> T

NIL as empty list and false value​

NIL plays dual roles in Common Lisp: it is the empty list and the canonical false value.

;; As the empty list
(length nil)
=> 0

(append nil '(a b))
=> (A B)

;; As false
(if nil "true" "false")
=> "false"

(not nil)
=> T

Null is the only type with a single member​

;; Only NIL is of type null
(remove-if-not (lambda (x) (typep x 'null))
               '(0 nil "" () t))
=> (NIL NIL)

Relationship to cons and list​

The types cons and null form an exhaustive partition of the type list. Every list is either a cons or null.

(subtypep 'null 'list)
=> T
=> T

(subtypep 'null 'cons)
=> NIL
=> T