Re: [Haskell-cafe] Instances of constrained datatypes

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