Haskell Lens Tutorial with Traverse

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

I am using the following code which I load into ghci:

{-# LANGUAGE RankNTypes, ScopedTypeVariables  #-}

import Control.Applicative
import Data.Functor.Identity
import 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 -> a
view ln x = getConst $ ln Const x

-- Tutorial sample data types
data User = User String [Post] deriving Show
data Post = Post String deriving Show

-- Tutorial sample data
john = User "John" $ map (Post) ["abc","def","xyz"]
albert = User "Albert" $ map (Post) ["ghi","jkl","mno"]
users = [john, albert]

-- A lens
posts :: Lens' User [Post]
posts fn (User n ps) = fmap (\newPosts -> User n newPosts) $ fn ps

      

        
        
        
      

    

From there, simple things like this:

view posts john

      

        
        
        
      

    

However, when I try the next step, it doesn't 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>:58:1-27
Possible 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 the lens has a Functor constraint and the traverse has a more constrained f-type constraint as an applicative. Why exactly does this not work, and why does the blog tutorial suggest it works?