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constantly

constantly Function

Syntax:

constantly value → function

Arguments and Values:

value—an object.

function—a function.

Description:

constantly returns a function that accepts any number of arguments, that has no side-effects, and that always returns value.

Examples:

(mapcar (constantly 3) ’(a b c d)) → (3 3 3 3) 
(defmacro with-vars (vars &body forms) 
  ‘((lambda ,vars ,@forms) ,@(mapcar (constantly nil) vars))) 
→ WITH-VARS 
(macroexpand ’(with-vars (a b) (setq a 3 b (\* a a)) (list a b))) 
→ ((LAMBDA (A B) (SETQ A 3 B (\* A A)) (LIST A B)) NIL NIL), *true*

See Also:

identity

Notes:

constantly could be defined by:

(defun constantly (object)

#’(lambda (&rest arguments) object))

every, some, notevery, notany

Expanded Reference: constantly​

Basic usage​

constantly returns a function that always returns the given value, regardless of what arguments it receives.

(funcall (constantly 42)) → 42
(funcall (constantly 42) 'a 'b 'c) → 42
(funcall (constantly nil) 1 2 3) → NIL

Using constantly with mapcar​

constantly is useful when you need to generate a sequence of the same value.

(mapcar (constantly 0) '(a b c d e))
=> (0 0 0 0 0)

(mapcar (constantly :default) '(1 2 3))
=> (:DEFAULT :DEFAULT :DEFAULT)

As a callback that ignores its arguments​

When an API requires a function argument but you want to return a fixed value, constantly is the right tool.

;; Replace every element in a tree with the same value
(subst-if :replaced (constantly t) '(a (b c) (d (e f))))
=> :REPLACED

;; Use with maphash to collect all keys
(let ((ht (make-hash-table)))
  (setf (gethash 'a ht) 1)
  (setf (gethash 'b ht) 2)
  (maphash (constantly nil) ht)) → NIL

Building default value functions​

constantly can provide default values in functional interfaces.

(let ((get-default (constantly "N/A")))
  (mapcar (lambda (x)
            (if (numberp x) x (funcall get-default)))
          '(1 :missing 3 :missing 5)))
;; => (1 "N/A" 3 "N/A" 5)