球拍彩虹形状递归

Question

I am trying to make a recursive function in Racket where from a list of colours, the function will print out multiple shapes in that order of colours. However, the function is not printing anything but "Empty List".

#lang slideshow
(require 2htdp/image)

(define a (list "red" "Orange" "Yellow" "Green" "Blue" "Purple"))

(define (Rainbow lst)
  (cond
   [(empty? lst) "Empty List"]
   [else (circle 20 "solid" (first lst))
         (Rainbow (rest lst))]))

(Rainbow a)

Answer 1

The displaying in REPL is done when a function returns an image. Your call (Rainbow a) can be rewritten as sequence of (begin (circle 20 "solid" ...) ...) ( cond has implicit begin, so in each recursive step, one begin is added), finished with "Empty List" and begin returns last expression, so "Empty List" is finally returned:

> (begin (circle 20 "solid" "Red")
         (begin (circle 20 "solid" "Orange")
                (begin (circle 20 "solid" "Yellow")
                       "Empty List")))

"Empty List"

But you can use print and print that image into REPL:

#lang slideshow
(require 2htdp/image)

(define a (list "Red" "Orange" "Yellow" "Green" "Blue" "Purple"))

(define (Rainbow lst)
  (cond
   [(empty? lst) (void)]
   [else (print (circle 20 "solid" (first lst)))  
         (Rainbow (rest lst))]))

(Rainbow a)

Note that I used void , so my REPL contains only colored circles and no other text.