#lang racket

(provide lcons lnull lnull? lcar lcdr)


(define (lcons x f) (mcons x f))

(define lnull null)

(define lnull? null?)

(define (lcar xs) (mcar xs))

(define (lcdr xs) 
  (let ([x (mcdr xs)])
    (cond [(not (mpair? x)) (set-mcdr! xs (x))]))
  (mcdr xs))

(define (from n)
  (lcons n (lambda () (from (+ n 1)))))

(define nats
  (from 0))

(define (lnth n xs)
  (cond [(= n 0) (lcar xs)]
        [else (lnth (- n 1) (lcdr xs))]))

(define (lfilter p xs)
  (cond [(lnull? xs) lnull]
        [(p (lcar xs))
         (lcons (lcar xs) (lambda () (lfilter p (lcdr xs))))]
        [else (lfilter p (lcdr xs))]))

(define (prime? n)
  (define (factors i)
    (cond [(>= i n) (list n)]
          [(= (modulo n i) 0) (cons i (factors (+ i 1)))]
          [else (factors (+ i 1))]))
  (= (length (factors 1)) 2))

(define primes (lfilter prime? (from 2)))