destructive version of `do' macro

my-do.scm

;; destructive version of `do' macro

;; (my-do ([x 1 y] [y 1 (+ x y)])
;;        ((> x 30) #f)
;;   (print x))
;;
;; is expanded as below:
;; 
;; (let ([x 1] [y 1])
;;   (let loop ()
;;     (if (> x 30)
;;       (begin #f)
;;       (begin
;;         (print x)
;;         (let ([temp0 y] [temp1 (+ x y)])
;;           (set! x temp0)
;;           (set! y temp1)
;;           (loop))))))
(define-syntax my-do
  (syntax-rules ()
    [(_ ([var init step] ...) (pred finished ...) body ...)
     (let ([var init] ...)
       (let loop ()
         (if pred
           (begin finished ...)
           (begin
             body ...
             (my-do-aux loop ((var step) ...) ())))))]))

;; (my-do-aux loop ([x y] [y (+ x y)]) ())
;; => (let ([temp0 y] [temp1 (+ x y)])
;;      (set! x temp0)
;;      (set! y temp1)
;;      (loop))
;; , where temp0 and temp1 are auto-generated symbols
(define-syntax my-do-aux
  (syntax-rules ()
    [(_ loop () ((var temp step) ...))
     (let [(temp step) ...]
       (set! var temp) ...
       (loop))]
    [(_ loop ((var step) rest ...) (acc ...))
     (my-do-aux loop (rest ...) (acc ... (var temp step)))]))