;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-lambda-reader.ss" "lang")((modname tail_recursion_full) (read-case-sensitive #t) (teachpacks ((lib "image.ss" "teachpack" "2htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "image.ss" "teachpack" "2htdp")))))

;; parity : list-of-boolean -> boolean
;; returns true if there are an even number of booleans, and false otherwise
(define (parity l)
  (cond
    [(empty? l) true]
    [else (xor (first l)
                   (parity (rest l)))]))

;; xor : boolean boolean -> boolean
(define (xor b1 b2)
  (or (and b2 (not b1))
      (and b1 (not b2))))

(check-expect (parity (list false true false false))
              false)

(check-expect (parity2 (list false true false false))
              false)

(define (parity2 l) (parity/acc l true))

(define (parity/acc l acc)
  (cond
    [(empty? l) acc]
    [else (parity/acc (rest l) (xor (first l) acc))]))