(define (length list)
  (foldr (lambda (x counter) (+ 1 counter))
         0
         list))

(define (my-map f lst)
  (foldr (lambda (x accum) (append (list (f x)) accum))
         '()
         lst))

(define (filter predicate lst)
  (foldr (lambda (x accum) (if (predicate x) (append (list x) accum) accum))
         '()
         lst))