Racket Rainbow Shape Recursion-CodePudding

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)

CodePudding user response：

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.