constantly value ⇒ function
constantly returns a function that accepts any number of arguments, that has no side-effects, and that always returns value.
(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
constantly could be defined by:
(defun constantly (object)
#'(lambda (&rest arguments) object))