[dllthomas]

Jtsummers gave some links, but in the interest of having something here:

    data Surface = Surface -- presumably provided by drawing backend
    data Shape = Shape
        { shapeGetCenter :: (Int, Int)
        , shapeSetCenter :: Int -> Int -> Shape
        , shapeDraw :: Surface -> IO ()
        , shapeScale :: Double -> Shape
        , shapeGetAABB :: (Int, Int, Int, Int)
        , shapeShow :: String
        }

    instance Show Shape where
        show = shapeShow

    circle r x y = Shape
        { shapeGetCenter = (x, y)
        , shapeSetCenter = circle r
        , shapeDraw = undefined -- draw shape to surface
        , shapeScale = \ f -> circle (f * r) x y
        , shapeGetAABB =
            ( floor $ fromIntegral x - r/2
            , floor $ fromIntegral y - r/2
            , ceiling $ fromIntegral x + r/2
            , ceiling $ fromIntegral y + r/2
            )
        , shapeShow = unwords [ "circle", show r, show x, show y ]
        }

    box w h theta x y = Shape
        { shapeGetCenter = (x, y)
        , shapeSetCenter = box w h theta
        , shapeDraw = undefined -- draw shape to surface
        , shapeScale = \ f -> box (w * f) (h * f) theta x y
        , shapeGetAABB =
            let w' = h * abs (sin theta)
                    + w * abs (cos theta)
                h' = w * abs (sin theta)
                    + h * abs (cos theta)
             in ( floor $ fromIntegral x - w'/2
                , floor $ fromIntegral y - h'/2
                , ceiling $ fromIntegral x + w'/2
                , ceiling $ fromIntegral y + h'/2
                )
        , shapeShow = unwords [ "box", show w, show theta, show x, show y ]
        }