On Mar 17, 2010, at 10:27 PM, Darrin Chandler wrote: Let's go back to your original code: data Point = Cartesian (Cartesian_coord, Cartesian_coord) | Spherical (Latitude, Longitude) type Center = Point type Radius = Float data Shape = Circle Center Radius | Polygon [Point] normalize_shape :: Shape -> Shape normalize_shape Circle c r = Circle c r normalize_shape Polygon ps = Polygon $ fmap normalize_point ps where normalize_point = something appropriate for the function. In fact, you could lift this into a higher order function, that takes a normalize_point function as an argument: normalize_shape :: (Point -> Point) -> Shape -> Shape normalize_shape f (Circle c r)= Circle (f c) r normalize_shape f (Polygon ps) = Polygon $ fmap f ps Now, I'm not suggesting that you should always normalize shapes, as I had with normalize_point before. But this combinator captures some nice, generic logic. For example, you can do stuff like: cartesian_shape :: Shape -> Shape cartesian_shape = normalize_shape cartesian_point where ... normalize_shape is the sort of function you would use while defining a function, and possibly provide function specific behavior in the function's where clause. double_shape :: Shape -> Shape double_shape (Circle c r) = Circle c (2 * r) double_shape (Polygon ps) = Polygon $ normalize_shape (double_point . cartesian_point) ps where double_point Cartesian (x, y) = Cartesian (sqrt(2) * x, sqrt(2) * y)