title: An UU parser recognizing just the empty string input?
body: <p>I need a value of type <code>Parser ()</code> which would succeed (and return <code>()</code>) on empty (lenght 0) input, and fail in all other cases.</p>
  
  <p><code>pSatisfy (const False)</code> doesn't quite do what's required. <code>pEnd</code> doesn't even seem appropriate for this purpose.</p>
  
  <hr>
  
  <p><code>pExact 0 pAscii</code> might be the exact "by-definition" solution. Still doesn't seem to work:</p>
  
  <pre><code>ghci&gt; runParser "&lt;input&gt;" (pSymbol "aaa" &lt;|&gt; pSymbol "bbb" &lt;|&gt; pExact 0 pAscii) ""
  *** Exception: ambiguous parser?
  </code></pre>