On 2/28/06, Johannes Waldmann
Malcolm Wallace wrote:
But if contexts-on-datatypes worked correctly,
data Set a = Ord a => ....
then even the "real" map from Data.Set:
map :: (Ord a, Ord b) => (a -> b) -> Set a -> Set b
could be an instance method of Functor.
I'd love that. But I don't quite understand: do you think this is/should be possible with: current Haskell? Haskell-Prime? Current ghc (what extensions)?
as Oleg: {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} module Map where import qualified Data.Set class MyMap f a b where myMap :: (a -> b) -> f a -> f b instance (Functor f) => MyMap f a b where myMap = fmap instance (Ord a, Ord b) => MyMap Data.Set.Set a b where myMap = Data.Set.map Jim