I am working through Realm of Racket chapter 5, last exercise was: Find an
image of a locomotive and create an animation which wraps around to the left
side of the screen after passing the right margin.
I have a solution which is below, but have some questions:
1. Is the solution below a reasonable one? How could I have done it better?
2. I don't really understand what is happening with the big-bang on-tick. The
big-bang function is passed 50 as the initial state. Does the big-bang
function somehow internally keep the state variable cached and then it gets
updated by the change-state function. That bit seems a little confusing to me.
Or should I just read more of the book until it is clearer?
My background is imperative languages, eg C, C++, Java, python, etc. Not sure
if that helps.
My solution to problem:
#lang racket
(require 2htdp/universe 2htdp/image)
(define LOCO-IMAGE <loco-image-here>)
(define WIDTH 800)
(define HEIGHT 200)
(define (change-state current-state)
(cond
[(>= current-state 750) 0]
[else (+ current-state 3)]))
(define (draw-a-ufo-onto-an-empty-scene current-state)
(place-image LOCO-IMAGE current-state (/ HEIGHT 2)
(empty-scene WIDTH HEIGHT)))
(big-bang 50
(on-tick change-state)
(to-draw draw-a-ufo-onto-an-empty-scene))
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