Why is racket passing #<void> to my function instead of the state? - racket

Question

I'm learning Racket, and using the book Realm Of Racket. I reached the point where they show you how to animate an image moving down the window, and then suggest you try modifying it to react to a left or right-arrow key press to move the image sideways.
Seemed simple enough...
#lang racket
(require 2htdp/universe 2htdp/image)
(define WIDTH 500)
(define DX 0)
(define HEIGHT 500)
(define IMAGE .) ; The image doesn't show up here
(define (add3-to-state current-state)
(+ current-state 3))
(define (draw-image current-state)
(place-image IMAGE (+ DX (/ WIDTH 2)) current-state (empty-scene WIDTH HEIGHT)))
(define (state-is-500 current-state)
(>= current-state 459))
(define (set-delta current-state key)
(set! DX (+ DX 10)))
(big-bang 0
(on-tick add3-to-state)
(on-key set-delta)
(to-draw draw-image)
(stop-when state-is-500)
)
When I run this, the image moves downwards as before, but as soon as I hit a key, I get an error...
>=: contract violation
expected: real?
given: #<void>
...in the state-is-500 function.
Anyone able to explain why this is happening? As far as I can see, my function for on-key has the same form as the ones they show in the book.
I tried modifying the state-is-500 function to look like this...
(define (state-is-500 current-state)
(printf "~a\n" current-state)
(>= current-state 459))
...and the output I got was...
102
105
#<void>
...before the error happened.
So it looks like #<void> is being passed to the function, but why?
Thanks

Answer 1

The issue is that big-bang expects the function call on-key to return the new state of the game. You are not retuning anything thus the frameworks reports <void>
Thus if the state does not change the you should just return the input state e.g.
(define (set-delta current-state key)
current-state)
Also this framework and Racket itself makes you want to not mutate variables, that is you don't assign to them, or at least you assign to them only once. Thus set! is not something you should use often.
In this case the state of the app that the book provides is just the vertical distance so is just a single number as shown in add3-to-state. The original state is passed in as the first parameter to big-bang.
The change you have to make is that the current state is not just the up-down movement but also the side to side. This movement shows up as the second parameter to place-image. So you have to have a state that can provide the second and third parameters to that.