data Tree a = Empty | Leaf a | Node (Tree a) a (Tree a)

instance Functor Tree where

  fmap g Empty          = Empty

  fmap g (Leaf x)       = Leaf (g x)

  fmap g (Node t1 x t2) = Node (fmap g t1) (g x) (fmap g t2)

instance Foldable Tree where

   foldMap f Empty = mempty

   foldMap f (Leaf x) = f x

   foldMap f (Node l k r) = foldMap f l `mappend` f k `mappend` foldMap f r

instance (Monoid (f (Tree a))) => Traversable (Tree (f a)) where

  sequenceA Empty          = pure Empty

  sequenceA (Leaf f)       = Leaf <$> f

  sequenceA (Node t1 f t2) = (sequenceA t1) `mappend` (Leaf <$> f) `mappend` (sequenceA t2)

The first argument of ‘Traversable’ should have kind ‘* -> *’, but ‘Tree (f a)’ has kind ‘*’

In the instance declaration for ‘Traversable (Tree (f a))’