Can you not define functor like Hughes defines a restricted monad (section 3 in the paper)... Keean Arjun Guha wrote:
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