;; same as (list-set! l elt val)
;; however it works even if
;; - l isn't a list - defaults to '()
;; - l is too short for elt - will extend l with filler #f elements
(define (list-set-safe! l elt val)
  (let ((lst (if (list? l) l (list))))
    (if (> (length lst) elt)
        (list-set! lst elt val)
        (append! lst
                 (make-list (- elt (length lst)) #f)
                 (list val)))
    lst))

(define (test-list-set-safe)
  (define l #f)
  (list-set-safe! l 1 9)
  (test-equal "list-set-safe 1 to 9"
    (list #f 9)
    l)
  (set! l (list 1 2))
  (list-set-safe! l 0 9)
  (test-equal "list-set-safe 0 to 9"
    (list 9 2)
    l)
  (list-set-safe! l 1 9)
  (test-equal "list-set-safe 1 to 9"
    (list 9 9)
    l)
  (list-set-safe! l 2 9)
  (test-equal "list-set-safe 2 to 9"
    (list 9 9 9)
    l)
  (list-set-safe! l 4 9)
  (test-equal "list-set-safe 4 to 9"
    (list 9 9 9 #f 9)
    l)
  (test-error
   "list-set-safe -1 to 9"
   'out-of-range
   (list-set-safe! l -1 9)))