Question

I am attempting to implement the Functor fmap over a Data.Map.Map, but I am getting an error. I’m sure I don’t need to convert the Map to and from a List in order to get this working, but this is the best I’ve come up with so far.

class Functor' f where
    fmap' :: (a -> b) -> f a -> f b

instance Functor' (Map.Map k) where
    fmap' f m
        | Map.null m = Map.empty
        | otherwise = let x:xs = Map.toList m
                          mtail = Map.fromList xs
                          a = fst x
                          b = snd x
                      in  Map.insert a (f b) (fmap f mtail)

The error:

No instance for (Ord k)
  arising from a use of `Map.fromList'
In the expression: Map.fromList xs
In an equation for `mtail': mtail = Map.fromList xs
In the expression:
  let
    x : xs = Map.toList m
    mtail = Map.fromList xs
    a = fst x
    ....
  in Map.insert a (f b) (fmap f mtail)

Any ideas?

Answer 1

The error is due to not assigning the Ord predicate to the type-variable k. Just do this:

instance Ord k => Functor' (Map.Map k) where