one. Returns the value if Some.</p><pre><code class="ml">let validator input =
  let open Validator in
  build build_valid
  |&gt; validate input.name (option_is_some &quot;Must be present&quot;)</code></pre></dd></dl><dl><dt class="spec value" id="val-string_is_not_empty"><a href="#val-string_is_not_empty" class="anchor"></a><code><span class="keyword">val</span> string_is_not_empty : <span><span>(string, string, <span class="type-var">'err</span>)</span> <a href="index.html#type-validator_builder">valid