ReactiveSnake.elm

import List
import Mouse
import Window

--MODEL
defaultBall = {x = 0, y = 0, v = toFloat 0, angle = toFloat 0}

--UPDATE
location t b = { b | x <- b.x - t * b.v * cos b.angle,
                     y <- b.y - t * b.v * sin b.angle }

velocity (x,y) b = { b | v <- sqrt((b.y-y)^2 + (b.x-x)^2)/10, 
                         angle <- atan2 (b.y-y) (b.x-x)}
updateBall (t, xy) = location t . velocity xy 

--INPUT
input = let delta = lift (\t -> t/20) (fps 40)
        in sampleOn delta (lift2 (,) delta ((keepWhen Mouse.isDown (0,0) (translate <~ Mouse.position ~ Window.dimensions))))

--DISPLAY
display balls (w, h) (x,y)=
                    flow down
                    [
                      [markdown|_Press and drag using mouse left button_|],
                      collage w h
                        <| List.map (\b -> move (b.x, b.y) <| filled red (circle 10.0) ) balls
                    ]
ball = foldp updateBall defaultBall input
balls = combine <| List.map (\d -> delay (d*100) ball) [0..4]
main = lift3 display balls Window.dimensions (floatify <~ Mouse.position)

--HELPERS
floatify (x,y) = (toFloat x, toFloat y)
translate (x,y) (w,h)= (toFloat x - toFloat w/2, toFloat h/2 - toFloat y + 50)