Bouncing circle in Elm, try it here: http://elm-lang.org/try

bouncingCircle.elm

import Color exposing (red)
import Graphics.Collage exposing  (circle, collage, Form, filled, move)
import Graphics.Element exposing (Element)
import List exposing (repeat)
import Signal exposing (Signal, (<~), foldp)
import Time exposing (fps)

w : Float
w = 500
h : Float
h = 500

type alias Point  a = {a | x: Float, y: Float}
type alias Moving a = {a | vx: Float, vy: Float}

update : Float -> Moving (Point a) -> Moving (Point a)
update dt p = {p |
      x <-      p.x + p.vx * dt
    , y <-      p.y + p.vy * dt
    ,vx <- if | mustBounce -(w/2) (w/2) p.x p.vx -> negate p.vx
              | otherwise                        -> p.vx
    ,vy <- if | mustBounce -(h/2) (h/2) p.y p.vy -> negate p.vy
              | otherwise                        -> p.vy}

mustBounce : Float -> Float -> Float -> Float -> Bool
mustBounce min max position speed =
    (position <= min && speed < 0) || (position >= max && speed > 0)

renderPoint : Point a -> Form
renderPoint {x, y} =
    circle 10 |> filled red
              |> move (x, y)

render : List Form -> Element
render = collage (round w) (round h)

main : Signal Element
main = render << repeat 1 << renderPoint <~ foldp update {x=0, y=100, vx=0.2, vy=-0.2} (fps 60)