Project 2: Build a Snake Game | Racket Projects

Code on GitHub: All examples from this tutorial are available at racket-programming-series/projects/snake-game — clone the repo and follow along!

Games are great for learning because they force you to manage mutable state without becoming messy. In Racket, we do this functionally!

The "World" Concept

We define a State.

Snake position (list of coordinates)
Food position
Direction

#lang racket
(require 2htdp/universe 2htdp/image)

(struct posn (x y) #:transparent)
(struct game (snake food dir) #:transparent)

(define SEG-SIZE 20)
(define WIDTH 400)
(define HEIGHT 400)

The Tick Handler (Game Logic)

Every "tick" (frame), we calculate the new world.

(define (next-state g)
  (define head (first (game-snake g)))
  (define new-head (move head (game-dir g)))
  
  (if (equal? new-head (game-food g))
      ; Eat! Grow snake.
      (game (cons new-head (game-snake g))
            (posn (random 20) (random 20))
            (game-dir g))
      ; Move!
      (game (cons new-head (drop-last (game-snake g)))
            (game-food g)
            (game-dir g))))

The Draw Handler

Convert state to an image.

(define (draw-game g)
  (place-image (circle 10 "solid" "green")
               (* (posn-x (game-food g)) SEG-SIZE)
               (* (posn-y (game-food g)) SEG-SIZE)
               (empty-scene WIDTH HEIGHT)))

Start!

(big-bang (game (list (posn 10 10)) (posn 5 5) 'right)
          (on-tick next-state 0.1)
          (to-draw draw-game)
          (on-key handle-key))

Summary

In 50 lines, we have a reactive game loop. Racket's universe handles the messy event loop, letting you focus purely on the logic: State -> State.

Quick Quiz

Project 2: Build a Snake Game | Racket Projects

The "World" Concept

The Tick Handler (Game Logic)

The Draw Handler

Start!

Summary

Which library provides the 'big-bang' function for creating interactive worlds?

Md Nasim Sheikh

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