de class="type">'a list -&gt; 'a</code></pre><div class="info ">
<div class="info-desc">
<p>Return the first element of the given list. Raise
   <code class="code"><span class="constructor">Failure</span>&nbsp;<span class="string">"hd"</span></code> if the list is empty.</p>
</div>
</div>

<pre><span id="VALcompare_lengths"><span class="keyword">val</span> compare_lengths</span> : <code class="type">'a list -&gt; 'b list -&gt; int</code></pre><div class="info ">
<div class="info-desc">
<p>Compare the length