#lang racket
 
(require (only-in srfi/1 alist-cons alist-delete))
(provide *p-list* keyword-get putprop! remprop! symbol-plist)
 
(define *p-list* '())
 
(define (symbol-plist sym)
  (cond ((assq sym *p-list*) => (lambda (x)
                                  (cdr x)))
        (else null)))
 
(define keyword-get
  (case-lambda
    ((args kw not-found)
     (cond ((memq kw (symbol-plist args))
            => (lambda (x)
                 (cadr x)))
           (else not-found)))
    ((args kw) (keyword-get args kw #f))))
 
(define (remprop! sym indicator)
  (and (keyword-get sym indicator)
       (let-values (((head tail)
                     (let ((lst (symbol-plist sym)))
                       (split-at lst (index-of lst indicator eq?)))))
         (let ((datum (append head (cddr tail))))
           (set! *p-list* (alist-cons sym datum
                                      (alist-delete sym *p-list*)))
           #t))))
 
(define (putprop! atom val property p-list)
  (when (keyword-get atom property)
    (remprop! atom property))
  (let ((datum (append `(,property ,val) (symbol-plist atom))))
    (set! *p-list* (alist-cons atom datum
                               (alist-delete atom *p-list*)))
    val))

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(putprop! 'Mary 28 '#:age)
(putprop! 'Mary 'female '#:sex)
(putprop! 'Mary '(Bill Susan Alice) '#:children)
(putprop! 'Mary 'lawyer '#:occupation)
(printf "~a~%" (keyword-get 'Mary '#:age))
(printf "~a~%" (keyword-get 'Mary '#:children))
(printf "~a~%" (keyword-get 'Mary '#:hobby))
(putprop! 'Mary (add1 (keyword-get 'Mary '#:age)) '#:age)
(printf "~a~%" (keyword-get 'Mary '#:age))