3&quot;)
  |&gt; validate input.email (string_is_email &quot;Must be email&quot;)
  |&gt; keep input.message</code></pre></dd></dl><dl><dt class="spec value" id="val-validate"><a href="#val-validate" class="anchor"></a><code><span class="keyword">val</span> validate : <span class="type-var">'input</span> <span>&#45;&gt;</span> <span><span>(<span class="type-var">'input</span>, <span class="type-var">'output</span>, <span class="type-var">'err</span>)</span> <a href="index.html#type-validator">validator</a