title: Haskell Hidden Packages: Data.HashSet
body: <p>I'd like to use Data.HashSet in Haskell. So I put <code>import Data.HashSet</code> at the beginning of my program. GHCi complains: <code>Could not find module Data.HashSet</code>.</p>
  
  <p>My questions are:</p>
  
  <ul>
  <li>How can I get Data.HashSet to work?</li>
  <li>I've read somewhere that this is part of a hidden module or package. Why is the module hidden? Does "hidden" mean that I should not use it?</li>
  <li>If I should not use it, is there a better alternative for a haskell data structure with a near-constant lookup time?</li>
  </ul>