if-and-aif.lisp

(defmacro execution-time (&body body)
  "Return the number of milliseconds it takes to execute `body`. Also
returns the result as the second value."
  (let ((tm (gensym))
        (res (gensym)))
    `(let* ((,tm (get-internal-real-time))
            (,res (progn ,@body))
            (,tm (floor (* 1000 (- (get-internal-real-time) ,tm))
                        internal-time-units-per-second)))
       (values (float ,tm) ,res))))

(defmacro aif (test then &optional else)
  `(let ((it ,test))
     (if it
         ,then
         ,else)))

(defun test-a ()
  (aif (random 100)
       'true
       'false))

(defun test ()
  (if (random 100)
      'true
      'false))

(let ((anaphoric (/ (loop repeat 5 sum (execution-time (loop repeat 10000000 do (test-a)))) 5))
      (not-anaphoric (/ (loop repeat 5 sum (execution-time (loop repeat 10000000 do (test)))) 5)))
  (format t "~&Anaphoric:     ~A msec / Not-Anaphoric: ~A msec~%It's ~A % slower."
          anaphoric
          not-anaphoric
          (* (/ (- anaphoric not-anaphoric) not-anaphoric) 100)))

I'm voting against modifying if. It is so basic construct, so I'd not expect it has any side effects. It is a good thing to have aif yet. Maybe it is worth to rename it to if-it to make the side-effect clearer?