Elm example

Circles.elm

import Keyboard
import List
import Window
import Color
import Random

input = dropRepeats Keyboard.space

ticks = fps 30

stepBall delta { x, y, color } = {x = x + delta / 5, y = y, color = color}
stepBalls (space, randBall, delta) balls =
  if space then randBall :: List.map (stepBall delta) balls
  else List.map (stepBall delta) balls

randColor = lift3 Color.rgb (Random.range 0 255 ticks) (Random.range 0 255 ticks) (Random.range 0 255 ticks)
randBall  = lift2 (\f c -> {x=0, y=f * 400, color=c}) (Random.float ticks) randColor

renderBall {x, y, color} = circle 8 |> filled color |> move (x, y)
render (w, h) balls = collage w h (map renderBall balls)

signalZip3 = lift3 (\x y z -> (x, y, z))

main =
  lift2 render Window.dimensions
    (foldp stepBalls [] (signalZip3 input randBall ticks))