Lacking Natural Simplicity

Random musings on books, code, and tabletop games.

Fun with Lisp: notf and cdrf

T. Kurt Bond

2008-08-15 18:59

I actually use these in Emacs Lisp, for which you'd need a (require 'cl)), but the define-modify-macro is originally from Common Lisp.

These are trivial, but useful.

(define-modify-macro notf (&rest args) not)
(setq x nil)
(notf x)
x ;;=> t
(notf x)
x ;;=> nil
(setq x ‘(a (b (c t))))
(notf (car (cdr (cadadr x))))
x ;;=> (a (b (c nil)))
(notf (car (cdr (cadadr x))))
x ;;=> (a (b (c t)))
(setq x [1 2 3 t 5])
(notf (aref x 3))
x ;;=> [1 2 3 nil 5]
(notf (aref x 3))
x ;;=> [1 2 3 t 5]

(define-modify-macro cdrf (&rest args) cdr)
(setq x ‘(:a :b :c :d 1 2 3))
(defun frob (keyword) ‘nothing-now)
(while (member (car x) ‘(:a :b :c :d))
  (frob (car x))
  (cdrf x))
x ;;=> (1 2 3)

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