Project 2: Build a Snake Game | Racket Projects

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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