Writing a destructive macro or function like incf?

Question

I need an incf function which does some bounds checking during the increment:

val := val + delta
if val >= 1.0
   then return 1.0
   else ret         


        

Answer 1

You may want to be careful about val, if you want it to be a place which can have side effects:

(defmacro incf-bounded (form delta &environment env)
  (multiple-value-bind (temps vals vars writer reader)
      (get-setf-expansion form env)
    `(let* (,@(mapcar #'list temps vals)
            (,(first vars) (min (+ ,delta ,reader) 1.0))) ;Edited, see comments.
       ,writer)))

Try it with e.g.

(let ((list (list 0 0.5 1)))
  (loop with i = -1 repeat 3 do (incf-bounded (nth (incf i) list) 0.5))
  list)

(This looks needlessly complicated, because I wanted a side effect in the first argument to incf-bounded.)