How to require keyword arguments in Common Lisp? -

given

(defun show-arg (a)   (format t "a ~a~%" a))  (defun show-key (&key a)   (format t "a ~a~%" a)) 

evaluating

(show-arg) 

will lead error saying "invalid number of arguments: 0", where

(show-key) 

will display a nil

how can show-key signal error show-arg does? there way other using (unless (error "a required")) in function body? fond of keyword arguments , use them constantly, , want them required.

keyword arguments optional, need manually check if they're given , signal error if needed. better not require keyword arguments though. compiler won't recognize them required , won't given error message missing arguments @ compile time.

if want require them, can specify arguments 3 element list; first element being argument, second default value , third variable true if argument given. checking third element better checking keyword itself, because can tell difference between nil default, , nil user gave argument.

(defun foo (&key (keyarg nil keyargp))   (unless keyargp (error "keyarg required."))   (* keyarg 2)) 

edit

now think bit more, there way compile time errors missing keyword arguments. define compiler macro function:

(defun foo (&key b c d)   (* b c d))  (define-compiler-macro foo (&whole whole &key (a nil ap) (b nil bp)                                    (c nil cp) (d nil dp))   (declare (ignore b c d))   (unless (and ap bp cp dp)     (error "missing arguments..."))   whole)