On Fri, 19 Oct 2007, TJ wrote:
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]
This signature is valid, but it means that all list elements must be of the same Renderable type.
Instead of hardcoding a bunch of types as being Renderable, as in
data Renderable = Point Something | Line Something | Polygon Something
scene :: [Renderable]
You could let the user plug together the alternatives for Renderable. That is, declare the class Renderable and let the user define and instantiate data Figure = Point Something | Line Something | Polygon Something or data Shape = Point Something | Line Something | Polygon Something | Spline Something or whatever he needs. That's a Haskell 98 solution.
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?
I assume, that you could use http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html...