Plaster

common-lisp
(defmacro plusplus (place &optional delta-form) "Destructively increment place value; However, return the original value." (let ((place-before (gensym))) `(let ((,place-before ,place)) (incf ,place ,delta-form) ,place-before))) ;;; This macro has a multiple evaluation bug (of the place's subforms)! I is ;;; incremented twice and ends up (1) reading the second element but (2) writing ;;; to the third element. (let ((list (list 1 2 3)) (i 0)) (let ((old (plusplus (nth (incf i) list) 3))) (values old i list))) ;; => 2, 2, (1 2 6) ;;; A version that uses DEFINE-MODIFY-MACRO and avoids the multiple evaluation ;;; bug, but returns the new value instead of the old value (DEFINE-MODIFY-MACRO ;;; therefore isn't suitable here). (defun plusplus1-fn (value delta) (+ value delta)) (define-modify-macro plusplus1 (delta) plusplus1-fn) (let ((list (list 1 2 3)) (i 0)) (let ((old (plusplus1 (nth (incf i) list) 3))) (values old i list))) ;; => 5, 1, (1 5 3) ;;; A correct version that manually uses GET-SETF-EXPANSION to access the place. (defmacro plusplus2 (place delta &environment env) (let ((old (gensym))) (multiple-value-bind (temps exprs stores set get) (get-setf-expansion place env) `(let* (,@(mapcar #'list temps exprs) (,old ,get) (,(first stores) (+ ,get ,delta))) ,set ,old)))) (let ((list (list 1 2 3)) (i 0)) (let ((old (plusplus2 (nth (incf i) list) 3))) (values old i list))) ;; => 2, 1, (1, 5, 3)