t.email (string_is_email &quot;Not an email&quot;)</code></pre></dd></dl><dl><dt class="spec value" id="val-optional"><a href="#val-optional" class="anchor"></a><code><span class="keyword">val</span> optional : <span><span>(<span class="type-var">'i</span>, <span class="type-var">'o</span>, <span class="type-var">'err</span>)</span> <a href="index.html#type-validator">validator</a></span> <span>&#45;&gt;</span> <span><span class="type-var">'i</span> option</span> <span>&#45;&gt;</span> <span><span>(<span>