On Tue, Jul 20, 2010 at 07:47:34PM +0100, Malcolm Wallace wrote:
As Igloo notes, GADTs are a more viable way to achieve the storing of the dictionary within the constructor, e.g.
data OrdTree a where Leaf :: a -> OrdTree a Branch :: Ord a => OrdTree a -> OrdTree a -> OrdTree a
I was thinking about this case the other day from an implementation point of view, wouldn't it annotate every subnode of a tree with an extra ord instance, which are guarenteed to be the same anyway? Something like import Data.Set data OrdSet a where OrdSet :: Ord a => Set a -> OrdSet a then using the normal set operations seems like it would be more efficient. It seems like it would be easy to accidentally end up with extra instances being passed around with the current GADT system. Another alternate proposal for what we might do with data contexts that was floating around was to have them allow smaller type signatures. data Ord a => Set a = .. would mean that whenever you see a 'Set a' the 'Ord a' is implied and need not be explicitly stated. so insert :: a -> Set a -> Set a is shorthand for insert :: Ord a => a -> Set a -> Set a John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/