code from class, 2019-01-09

2019-01-09.rkt

#lang typed/racket

(require typed/rackunit)

;; given a number, return the fizzbuzz game string
(define (fizzbuzz-num [n : Natural]) : String
  (cond [(= 0 (modulo n 15)) "fizzbuzz"]
        [(= 0 (modulo n 3)) "fizz"]
        [(= 0 (modulo n 5)) "buzz"]
        [else (number->string n)]))

(check-equal? (fizzbuzz-num 9) "fizz")
(check-equal? (fizzbuzz-num 10) "buzz")
(check-equal? (fizzbuzz-num 11) "11")
(check-equal? (fizzbuzz-num 15) "fizzbuzz")



(define-type Text (U Poem Essay))
(struct Poem ([title : String] [lines : Natural] [rhymes? : Boolean])
  #:transparent)
(struct Essay ([title : String] [thesis : String])
  #:transparent)

(Poem "My Left Arm" 6 #f)
(Essay "My Right Arm" "My right hand is better than my left.")

;; does the string start with q?
(define (starts-with-q? [s : String])
  (equal? (string-ref s 0) #\Q))

;; return the first character of the title
(define (text-first-char [t : Text]) : Char
  (match t
    ;; silly extra match clause to illustrate how the `?` pattern works:
    [(Poem (? starts-with-q? _) _ _) #\Q]
    [(Poem t _ _)  (string-ref t 0)]
    [(Essay t _)   (string-ref t 0)]))

(check-equal? (text-first-char (Poem "The Lion Roars"
                                     12 #t))
              #\T)
(check-equal? (text-first-char (Essay "On Liberty"
                                      "Politics is complicated."))
              #\O)