> (define s "a")
> (string-ref s 0)
#\a
> (string-set! s 0 #\b)
> s
"b"

#lang racket

(define s (make-string 3 #\*))
(string-set! s 0 #\a)
(string-set! s 1 #\b)
(display s)

#lang r5rs

(define s (make-string 3 #\*))
(string-set! s 0 #\a)
(string-set! s 1 #\b)
(display s)

#lang r6rs

(import (rnrs)
        (rnrs mutable-strings (6)))

(define s (make-string 3 #\*))
(string-set! s 0 #\a)
(string-set! s 1 #\b)
(display s)

~ $ gsi
Gambit v4.6.5

> (define s "***")
> (string-set! s 0 #\a)
> (string-set! s 1 #\b)
> (display s)
ab*
> (string-set! "***" 1 #\b) 
>   ; никаких ошибок

#lang r5rs

(define (print-by proc)
  (lambda (s)
    (proc s)
    (newline)
    (newline)))

(define new-line (string #\Newline))

(for-each (print-by display)
          (list "abc\ndef"
                (string-append "abc" "\n" "def")
                (string-append "abc" new-line "def")))

(for-each (print-by write)
          (list "abc\ndef"
                (string-append "abc" "\n" "def")
                (string-append "abc" new-line "def")))

; =>

Добро пожаловать в DrRacket, версия 5.2.1 [3m].
Язык: r5rs; memory limit: 1024 MB.
abc
def

abc
def

abc
def

"abc\ndef"

"abc\ndef"

"abc\ndef"

>