gistfile1.txt

import Graphics.Gloss.Interface.Pure.Game
import Graphics.Gloss.Data.Bitmap

import Data.Monoid

history = 10
cr = green
mult = -1/3
main = loadBMP "explosion4.bmp" >>= \explosion -> play
    (InWindow "Velocity" (500,500) (0,0))
    black
    120
    ((0,0), replicate history ((0,0),1), 10)
    (\w@(p@(px,py),l,et) -> let ((xf, yf),_) = head l in let ((xi,yi),_) = last l in let t = sum $ tail $ map snd l in color cr $ Line [p, (px + mult*(xf-xi)/t, py + mult*(yf-yi)/t)] <> (if et < 1 then translate px py explosion else mempty))
    (\e w@(p,l,et) -> case e of
        EventMotion p' -> (p',l,et)
        EventKey (MouseButton _) Up _ p' -> (p', l, 0)
        _ -> w
        )
    (\t (p,l,et) -> (p,(p,t):(init l),et+t))