title: Haskell maps returning a monad
body: <p>The lookup function in Data.Map and Data.IntMap currently return values wrapped in Maybe with
  the type signature</p>
  
  <pre><code>lookup  :: Ord  k =&gt; k -&gt; Map  k a -&gt; Maybe  a
  </code></pre>
  
  <p>It used to have the more general type of</p>
  
  <pre><code>lookup  :: (Monad  m, Ord  k) =&gt; k -&gt; Map  k a -&gt; m a
  </code></pre>
  
  <p>I realize the former likely reduces the need of extra type specification, but the latter would make it much more general and allow lookup to be used in list comprehensions. Is there any way to mimic this behavior with the newer version, or would I have to use an older version of the library?</p>