#lang racket


(define L '(1 2 3 4 5 6 7 8 9))

(define (mean1 L)
  (/ (foldl + 0 L) (length L)))

(define (mean2 L)
  (let ([res
         (foldr (λ (e acc) ; acc este (suma . len)
                  (let* ([sum-so-far (car acc)]
                         [len-so-far (cdr acc)]
                         [new-sum (+ sum-so-far e)]
                         [new-len (add1 len-so-far)]
                         )
                    (cons new-sum new-len)
                    ))
                (cons 0 0)
                L)])
    (/ (car res) (cdr res))))


(define (composite-fold f1 f2 v01 v02)
  (λ (L)
    (foldr
     (λ (e acc)
       (cons (f1 e (car acc)) (f2 e (cdr acc))))
     (cons v01 v02)
     L)))

(define (selY x y) y)
(define (mean3 L)
  (let ((res
         ((composite-fold
          +
          (compose add1 selY)
          0 0) L)
         ))
    (/ (car res) (cdr res))))


(mean1 L)
(mean2 L)
(mean3 L)