the basics principles of set! in scheme

gistfile1.scm

(require r5rs)

(define xs '(1 2 3 4 5)) ;; xs is a list of numbers
xs
(set! xs "a list of numbers") ;; xs is now "a list of numbers"
xs

(define (update-1 arg)
  (set! arg "this has only changed the value of 'arg' within 'update-1'")
  arg)

(update-1 xs)
xs

(define (update-2)
  ;; to use set! on a gloabl scale we need to know the 'name' of the variable
  ;; we want to change... 
  (set! xs "this has actaully changed the value of xs!")
  xs)
  
(update-2)
xs

(define (fix-life)
  ;; set! requires the name of the variable
  ;; meaning has not been defined,
  ;; if called will result in an error
  ;; if it has been defined then it will 
  ;; change it to 42
  (set! meaning 42))

(define meaning "of life")
meaning
(fix-life)
meaning