title: Create a type that is a subset of an other type
body: <p>How can you create a type that is a subset of an other type? I want a string type that only contains alphanumeric characters. </p>
  
  <p>So I want something like this</p>
  
  <pre><code>type AlphNumString = [AlphaNumChar]
  data AlphaNumChar = ???? filter (isAlphaNum) Char ????
  </code></pre>