[loup-vaillant]

I believe phantom type should help with your example. At it's most naive, you need a type for the points in window space, and a second type for the points in drawing space. If you give the sprite a window_point, it will complain about only accepting drawing_points. Now such errors are confined to a conversion function.

With phantom types, a the Point "type" would be a function of types to types. It helps when some operation work on all kinds of points: they can be polymorphic with respect to the additional type.

  data Point a = Point Int Int

  draw_sprite  :: Point Drawing -> sprite -> Io ()
  draw_button  :: Point Window  -> sprite -> Io ()

  translate_point :: Point a -> Point b -> Point a
  translate_point (Point x y) (Point xt yt) =
      Point (x + xt) (y + yt)