Haskell Lens Tutorial with traverse

I'm trying to follow this tutorial: http://blog.jakubarnold.cz/2014/08/06/lens-tutorial-stab-traversal-part-2.html

I'm using the following code that I load into ghci:

{-# LANGUAGE RankNTypes, ScopedTypeVariables  #-}import Control.Applicativeimport Data.Functor.Identityimport Data.Traversable-- Define Lens type.type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t type Lens' s a = Lens s s a a -- Lens view function. Omitting other functions for brevity.view :: Lens s t a b -> s -> aview ln x = getConst $ ln Const x-- Tutorial sample data typesdata User = User String [Post] deriving Showdata Post = Post String deriving Show-- Tutorial sample datajohn = User "John" $ map (Post) ["abc","def","xyz"]albert = User "Albert" $ map (Post) ["ghi","jkl","mno"]users = [john, albert]-- A lensposts :: Lens' User [Post]posts fn (User n ps) = fmap (\newPosts -> User n newPosts) $ fn ps

From there, simple stuff like this works:

view posts john

However, when I try to do the next step it does not work:

view (traverse.posts) users

I get:

Could not deduce (Applicative f) arising from a use of ‘traverse’from the context (Functor f)  bound by a type expected by the context:             Functor f => ([Post] -> f [Post]) -> [User] -> f [User]  at <interactive>:58:1-27Possible fix:  add (Applicative f) to the context of    a type expected by the context:      Functor f => ([Post] -> f [Post]) -> [User] -> f [User]In the first argument of ‘(.)’, namely ‘traverse’In the first argument of ‘view’, namely ‘(traverse . posts)’In the expression: view (traverse . posts) users

I see that Lens has a type constraint of Functor and traverse has a more constrained type constraint on f as an Applicative. Why exactly isn't this working and why did the blog tutorial suggest that it works?