code from class, 2013-10-11. Requires today's version of rsound at least, to include piano sounds.

random-piano-notes.rkt

(require rsound)
(require rsound/piano-tones)
(require 2htdp/universe)
(require 2htdp/image)

;(play (piano-tone 62))


(define (both a b) b)

;; a world is zero

;; a note-num is a number

;; a triad-choice is one of 
;; - 0, indicating the "root",
;; - 1, indicating the "third",
;; - 2, rep. the "fifth", or
;; - 3, rep. the "octave"

;; nth-triad-note : returns the nth triad note
;; note-num triad-choice -> note-num
(define (nth-triad-note center choice)
  (cond [(= choice 0) center]
        [(= choice 1) (+ center 4)]
        [(= choice 2) (+ center 7)]
        [(= choice 3) (+ center 12)]))

(check-expect (nth-triad-note 23 0) 23)
(check-expect (nth-triad-note 23 1) 27)
(check-expect (nth-triad-note 23 2) 30)
(check-expect (nth-triad-note 23 3) 35)
(check-expect (nth-triad-note 25 3) 37)

;; note-num key -> note-num
;; toggle the world between 60 and 67
(define (key-handler w k)
  (cond [(= w 60) 67]
        [(= w 67) 60]))

(check-expect (key-handler 60 "p") 67)
(check-expect (key-handler 67 "p") 60)


;; world -> world
;; plays a note
(define (play-note w)
  (both (play (piano-tone
               (nth-triad-note w (random 4))))
        w))

#;(check-expect (play-note 28) 28)


(define (bogus-draw dc)
  (empty-scene 100 100))

(big-bang 60
          [to-draw bogus-draw]
          [on-tick play-note 1/4]
          [on-key key-handler])