On 10/22/07, Tim Docker
TJ:
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]
Everyone has launched into explanations of how to use existentials to do this, but you may be happy in just haskell 98. In the above, you could just have:
scene :: [RasterImage]
Laziness will ensure that the computation/storage of the images will not occur until they are used.
Tim
Ah... indeed it can, in this case. It won't work if class Renderable also has a method for saving to file, etc, I suppose, unless scene :: [(RasterImage,IO (),...whatever other operations...)] Thanks for the heads up :) TJ