Cats and mice

ball.elm

import Mouse
import Window

ball = {x = 0.0, y = 0.0 }

type Pair = (Float, Float)

unit (x, y) = let d = sqrt(x^2 + y^2) in (x/d, y/d)
(-->) {x, y} (tx, ty) = unit (tx - x, ty - y)

update: Pair -> {x: Float, y: Float} -> {x: Float, y: Float}
update (dx,dy) b = {b | x <- b.x + dx, y <- b.y + dy }

fixit (a,b) = (toFloat a, toFloat b)
fpos = fixit <~ Mouse.position

err: Signal Pair
err = lift2 (-->) (sampleOn (fps 25) balls) fpos
balls = foldp update ball err

main = lift2 scene balls Window.dimensions

scene {x, y} (w, h) =
        let dx = x - toFloat w / 2
            dy = toFloat h / 2 - y
       in collage w h 
          [ circle 30  |> filled orange
                       |> move (dx, dy)
           ]