title: Haskell type signature with multiple class constraints
body: <p>How can I have multiple class constraints, so if <code>A</code> is an <code>Eq</code> and <code>B</code> is a <code>Num</code>, I could say either </p>
  
  <pre><code>f :: Eq a =&gt; a -&gt; b`
  </code></pre>
  
  <p>or </p>
  
  <pre><code>f :: Num b =&gt; a -&gt; b
  </code></pre>
  
  <p>So, how can I have <code>Eq a =&gt;</code> and <code>Num b =&gt;</code> at the same time?</p>
  
  <ul>
  <li><code>f :: Eq a =&gt; Num b =&gt; a -&gt; b</code>,</li>
  <li><code>f :: Eq a -&gt; Num b =&gt; a -&gt; b</code>, and</li>
  <li><code>f :: Eq a, Num b =&gt; a -&gt; b</code></li>
  </ul>
  
  <p>didn't do what I wanted.</p>