title: How to type hinte higher-order functions in python 3.6?
body: <p>In the following code:</p>
  
  <pre><code>def b(i: int) -&gt; int:
      return i
  
  def a(i: int, b: ?) -&gt; int:
      return i + b(i)
  
  print(a(1, b))
  </code></pre>
  
  <p>How do we type hinting the function <code>b: ?</code> that is a parameter of <code>a</code>?  Thank you.</p>