iterating reverse string

in-string-rev.rkt

#lang racket

(define (in-string/rev s)
  (define stop 'stop)
  (define n (string-length s))
  (define i (sub1 n))
  (define (next!)
    (cond
      [(< i 0)
       stop]
      [else
       (let ([result (string-ref s i)])
         (set! i (sub1 i))
         result)]))
  (in-producer next! stop))

(for ([ch (in-string/rev "hello world")])
  (displayln ch))