identity

identity Function

Syntax:

identity object → object

Arguments and Values:

object—an object.

Description:

Returns its argument object.

Examples:

(identity 101) → 101 
(mapcan #’identity (list (list 1 2 3) ’(4 5 6))) → (1 2 3 4 5 6) 

Notes:

identity is intended for use with functions that require a function as an argument.

(eql x (identity x)) returns true for all possible values of x, but (eq x (identity x)) might return false when x is a number or character .

identity could be defined by

(defun identity (x) x)

Expanded Reference: identity

identity simply returns its argument unchanged. It is the identity function.

(identity 42) → 42
(identity "hello") → "hello"
(identity nil) → NIL
(identity '(a b c)) → (A B C)

A classic use of identity is with mapcan to concatenate a list of lists (flatten one level).

(mapcan #'identity (list (list 1 2 3) (list 4 5 6)))
=> (1 2 3 4 5 6)

identity is useful as a :key argument when you want to operate on elements directly. Many functions default to identity for their :key.

(sort (list 3 1 4 1 5 9) #'< :key #'identity)
=> (1 1 3 4 5 9)

identity can serve as a predicate to filter out NIL values, since any non-NIL value is truthy.

(remove-if-not #'identity '(1 nil 2 nil 3))
=> (1 2 3)

(remove nil '(a nil b nil c) :key #'identity)
=> (A B C)

When a higher-order function requires a function argument but you do not want any transformation, identity is the right choice.

(mapcar #'identity '(a b c d))
=> (A B C D)

(reduce #'append '((1 2) (3 4) (5 6))
        :key #'identity)
=> (1 2 3 4 5 6)