[racket] question about macro failing due to hygiene(?)

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Why doesn't this work (expand: unbound identifier in module in: a):


#lang racket

(struct variable ((value #:mutable) rule))

(define-syntax make-store
  (syntax-rules ()
    ((_ ((_id_ _rule_) ...))
     (letrec-syntaxes+values
         (((make-variable)
           (syntax-rules ()
             ((_ #f) (variable #f #f))
             ((_ __rule__)
              (variable
               #f
               (lambda ()
                 (let-syntax
                     ((_id_
                       (syntax-id-rules ()
                         (_ (hash-ref store '_id_))))
                      ...)
                   __rule__)))))))
       (((store)
         (make-hash
          `((_id_ . ,(make-variable _rule_)) ...))))
       store))))

(define simple-arith
  (make-store
   ((a #f)
    (b (* 3 a))
    (c (+ 5 a b))
    (d (let ((a 9)) (- c a))) )))


while this does work (but doesn't do what I want):


#lang racket

(struct variable ((value #:mutable) rule))

(define-syntax make-store
  (syntax-rules ()
    ((_ ((_id_ _rule_) ...))
     (letrec
         ((store
           (make-hash
            `((_id_
               .
               ,(variable
                 #f
                 (lambda ()
                   (let-syntax
                       ((_id_
                         (syntax-id-rules ()
                           (_ (hash-ref store '_id_))))
                        ...)
                     _rule_)))) ...))))
       store)) ))

(define simple-arith
  (make-store
   ((a #f)
    (b (* 3 a))
    (c (+ 5 a b))
    (d (let ((a 9)) (- c a))) )))

(variable-rule (hash-ref simple-arith 'a)) ; I want this to be #f


Marijn
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