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atom

atom Type

Supertypes:

atom, t

Description:

It is equivalent to (not cons).

Expanded Reference: atom (Type)​

Type definition​

The type atom is equivalent to (not cons). It includes everything that is not a cons cell: numbers, strings, symbols, arrays, characters, NIL, and so on.

(typep 42 'atom)
=> T

(typep "hello" 'atom)
=> T

(typep 'foo 'atom)
=> T

(typep '(1 2) 'atom)
=> NIL

NIL is an atom​

NIL (the empty list) is an atom, not a cons. This is an important distinction since NIL is also a list.

(typep nil 'atom)
=> T

(typep '() 'atom)
=> T

Using atom type in declarations​

The atom type can be used in type checks and declarations.

(let ((items '(1 "two" (3 4) x nil #\a)))
  (remove-if-not (lambda (x) (typep x 'atom)) items))
=> (1 "two" X NIL #\a)

Relationship to cons type​

The types atom and cons form an exhaustive partition of the type t. Every object is either an atom or a cons, but never both.

(let ((obj '(a . b)))
  (values (typep obj 'atom) (typep obj 'cons)))
=> NIL
=> T

(let ((obj 42))
  (values (typep obj 'atom) (typep obj 'cons)))
=> T
=> NIL