Lab 11 The Lambda Lab

Purpose: To practice the use of lambda in a fun and aesthetically pleasing way.

We Are Many. We Are Lambda.

Goals: Practice using functions as data. Practice designing functions that produce functions. Practice using lambda to create anonymous functions. Practice with world programs.

We will design a game in which you can create many balls which move across the screen in various ways. The player can add new balls by clicking on the screen.

Starter Code: Below is the data definition for a Ball.

; A Ball is a (make-ball Nat Mode Color [Nat -> Posn])

(define-struct ball [r mode color placement])

; INTERPRETATION:

; - r is the ball's radius

; - mode is the ball's mode

; - color is the ball's color

; - placement is a function that, given the current time,

;   outputs a new coordinate for the ball to be drawn at

; A Mode is one of:

; - "solid"

; - "outline"

(define BALL-1 (make-ball 3 "solid" "red" (λ (x) (make-posn 5 x))))

(define BALL-2 (make-ball 3 "solid" "blue" (λ (x) (make-posn (+ x 2) 3))))

Starter Code: Below is the data definition for a BallWorld, as well as the main function and some graphical constants.

; A BallWorld is a (make-world Nat [List-of Ball])

(define-struct ball-world [t balls])

; - where t is the amount of time that has passed

; - and balls is the balls of the world

(define WORLD-1 (make-ball-world 0 '()))

(define WORLD-2 (make-ball-world 10 (list BALL-1 BALL-2)))

; main : [List-of Ball] -> World

; Run the game with this list of initial balls

(define (main init-list)

(big-bang (make-ball-world 0 init-list)

[on-tick (λ (bw) (make-ball-world (add1 (ball-world-t bw))

(ball-world-balls bw)))]

[to-draw draw]

[on-mouse place-ball]))

(define WIDTH 500)

(define HEIGHT 500)

(define BG (empty-scene WIDTH HEIGHT))

Exercise 1 Design the function draw-ball that given a Ball, a Posn, and an image, draws the ball at that point on that image.

Exercise 2 Design the function make-drawer that given a time will create a function that takes a Ball and an image and will draw it. Hint: Use draw-ball.

Exercise 3 Design the function draw that draws the ball world. Look at the helpers you’ve just written: especially the signature for make-drawer. What does it remind you of? How can you use that to help you?

Exercise 4 Design the BallGenerator move-vertically.

Exercise 5 Create a constant GENERATORS, which is a [List-of BallGenerator] that will keep track of all the BallGenerators you’ve made thusfar.

Exercise 6 Design the function place-ball, that given a BallWorld, x- and y- coordinate, and MouseEvent will output a BallWorld with a Ball added to it if the person clicked (this is represented by the MouseEvent "button-down"). Feel free to use any radius, color, and mode you like.

As far as the ball’s placement: look at the data definition of a Ball, the signature of this function, and the data definition of a BallGenerator. Feel free to use any BallGenerator you like, as long as it’s used correctly!

Exercise 7 Now comes the fun stuff. Design a function select-random, that given any non-empty list will select a random element from the list.

Exercise 8 Update place-ball to select a random BallGenerator from the GENERATORS you previously defined.

Exercise 9 Update place-ball to make a radius of random size. Make sure you don’t make radiuses of size 0, though, those aren’t very fun!

Exercise 10 Update place-ball to use a random make-color.

Exercise 11 Update place-ball by changing "button-down" to "drag".