Why is it illegal to store values of differing types, but which instance the same class, into a list? e.g. a = [ 1, 2.0 ] :: Num a => [a] After all, sometimes all you need to know about a list is that all the elements support a common set of operations. If I'm implementing a 3d renderer for example, I'd like to have class Renderable a where render :: a -> RasterImage scene :: Renderable a => [a] Instead of hardcoding a bunch of types as being Renderable, as in data Renderable = Point Something | Line Something | Polygon Something scene :: [Renderable] Or maybe data Point = Point Something data Line = Line Something data Polygon = Polygon Something scene :: { points :: [Point], lines :: [Line], polygons :: [Polygons] } Is there a way of achieving what I want to do? Existentials maybe? I'm still learning the basic stuff and don't grok existentials at all, but I even if I use those, I'll still have to wrap things up in a constructor, won't I? Thanks a bunch, TJ