Using Lisp Macros to Fact-Check Code at Compile Time

flags.cl

;;
;; This code was extracted from a larger project
;; and used in a Twitter thread on using macros
;; to fact-check programmers at compile-time.
;;
;; https://twitter.com/iamjameshunt/status/1470845641027129345
;;
  
(defun -flags-for (num)
  (format nil "~{~A~}"
    (remove-if #'null
      (list
        (and (evenp num) #\e)
        (and (oddp  num) #\o)
        (and (> num 100) #\>)
        (and (> num 900) #\e) ; [e]ven bigger
                              ; (also: wrong)
        (and (< num  20) #\<)))))

(defmacro flag! (&body body)
  (let ((seen (make-hash-table)))
  `(format nil "~{~A~}"
     (remove-if #'null
        (list
          ,@(let ((forms nil))
              (loop for (flag test) in body
                    do
                    (if (nth-value 1 (gethash flag seen))
                      (error (format nil "already seen flag ~S" flag)))
                    (setf (gethash flag seen) t)
                    (setf forms (cons `(and ,test ,flag) forms)))
              forms))))))

(defun flags-for (num)
  (flag!
    (#\e (evenp num))
    (#\o (oddp  num))
    (#\> (> num 100))
    (#\< (< num  20))))