htdp 2e. exercise 125 #racket

Exercise-125_sum.rkt

#lang racket

(require lang/htdp-beginner)

; Htdp 2e. Exercise 125
;
; A List-of-amounts is one of:
; – empty
; – (cons PositiveNumber List-of-amounts)
; interp. a List-of-amounts represents some amounts of money

; List -> Boolean
(check-expect (List-of-amounts? empty) true)
(check-expect (List-of-amounts? (cons 1 empty)) true)
(check-expect (List-of-amounts? 1) false)
(check-expect (List-of-amounts? "1") false)
(check-expect (List-of-amounts? (cons -1 (cons 1 empty))) false)
(check-expect (List-of-amounts? (cons 1 (cons -1 empty))) false)
(define (List-of-amounts? l) (cond
                               [(empty? l) true]
                               [(not (cons? l)) false]
                               [(and (> (first l) 0) (List-of-amounts? (rest l))) true]
                               [else false]))


; List -> Number-Or-False
; caculater sum of List-of-amounts
(check-expect (sum (cons 6 empty)) 6)
(check-expect (sum (cons 2 (cons 3 empty))) 5)
(check-expect (sum (cons -1 empty)) false)
(check-expect (sum empty) 0)
(define (sum l) (cond
                  [(empty? l) 0]
                  [(not (List-of-amounts? l)) false]
                  [else (+ (first l) (sum (rest l)))]))


; License: 0BSD