One way to do roughly what you want is to pass the dictionary yourself:
>data EqDict a = EqDict { > leq :: a -> a -> Bool } > >data EqList a = EqList (EqDict a) [a] > >test :: EqList a -> EqList a -> Bool >test (EqList dict (a0:as)) (EqList _ (b0:bs)) = (leq dict) a0 b0
This is like the `object-oriented approach' in John Hughes' paper. Let's switch to the set example in his paper:
> data EqDict a = EqDict { isEq:: a -> a -> Bool }
> data Set a = Set (EqDict a) [a]So, to make it a functor, as I originally wanted:
> instance Functor Set where > fmap f (Set dict ls) = Set dict' ls' where > ls' = nubBy (isEq dict') ls > dict' = ???
There really isn't a way to define dict' for the resultant Set.
-Arjun _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
