Bouncing ball

bouncing.elm

main = lift scene state

state = foldp step initial (fps 60)

initial = State 0 -70 100 8 0

data State = State Float Float Float Float Float

scene (State t x y vx vy) = collage 400 400 [box, ball x y]

box = filled (hsl (pi/2) 0.5 0.5)
        <| path [ (-90,100), (-100,100), (-100,-50),(100,-50),
                  (100,100), (90,100), (90,-40), (-90,-40), (-90,50)]

ball x y = move (x,y)
      <| filled (hsl 0 0.5 0.5)
      <| circle 10
            
step _ (State t x y vx vy) =
  let vy2 = if y < -27 then -vy*0.95 else vy-0.2
      vx2 = (if x < -80 then -vx else
              (if x >=80 then -vx else vx)) * (if y < -27 then 0.8 else 1)
  in State t (x+vx2) (y+vy2) vx2 vy2