[Haskell-beginners] Applicative on Tree

[Mike Houghton]

Hi,

I’m looking at 

data Tree a = Node a [Tree a] deriving (Show)

and trying to write the Applicative instance.
I have I think a correct Functor :


instance Functor (Tree) where
	fmap f (Node x tr) = Node (f x) ( fmap (fmap f) tr)

but I’m well and truly stuck on Applicative :)

I have


instance Applicative (Tree) where
	pure x = Node x []
	(Node f []) <*> tr = fmap f tr
	(Node f fs) <*> tr@(Node x xs) = ?????

I cant get rid of the question marks! :) 
any pointers would be really appreciated!

Thanks