references.scm

#lang racket
(require (except-in eopl #%module-begin))

(local-require "errors.scm")

(provide reference? newref deref setref!)

;;;;;;;;;;;;;;;; references ;;;;;;;;;;;;;;;;

(define empty-store
  (lambda () '()))

(define the-store 'uninitialized)

(define initialize-store!
  (lambda ()
    (set! the-store (empty-store))))

(initialize-store!)

(define reference?
  (lambda (v) (integer? v)))

(define newref
  (lambda (val)
    (let ((next-ref (length the-store)))
        (set! the-store (append the-store (list val)))
          next-ref)))

(define deref
  (lambda (ref)
    (list-ref the-store ref)))

(define setref!
  (lambda (ref val)
    (set! the-store
      (letrec ((setref-inner
        (lambda (store1 ref1)
          (cond
            ((null? store1)
              (error-invalid-reference))
            ((zero? ref1)
              (cons val (cdr store1)))
            (else
              (cons
                (car store1)
                (setref-inner
                (cdr store1) (- ref1 1))))))))
        (setref-inner the-store ref)))))