gistfile1.rkt

(define-struct just-a-function ([l #:mutable] r) #:property prop:procedure 2)
(define (define#-help proc)
             (let* ((me (box #f))
                    (f (just-a-function proc (lambda args (apply (just-a-function-l (unbox me)) args)))))
               (set-box! me f)
               (values f (lambda (new-proc) (set-just-a-function-l! f new-proc)))))

(define-syntax define# (syntax-rules () ((_ ((name magic) args ...) body)
                                                  (define-values (name magic)
                                                    (define#-help (lambda (args ...) body))))))

(define# ((foo magic) x) (sqr x))
(foo 9)
(magic sqrt)
(foo 9)