Henning Thielemann:
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.
Yes, that's exactly the restriction I'm unhappy about.
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
But if I already have the types Point, Line, and Polygon, and I want to create a "union type" Figure as above, then my code will look like this: data Point = Point Something data Line = Line Something data Polygon = Polygon Something data Figure = FPoint Point | FLine Line | FPolygon Polygon aFigure = FPoint Point Something aListOfFigures = [FPoint Point Something, FPolygon Polygon Something, FLine Line Something]
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...
That's a nice page :) From a quick reading, the best I came up with was this: data R = forall a. Renderable a => V a instance Show R where render (R a) = render a Which is precisely what I meant when I said that I'd still have to wrap things up in a constructor. Is this hidden type variable thing what "existential types" mean? OT: forall just introduces a new type variable, right? Thanks, TJ