th : int <span>&#45;&gt;</span> <span><span>(<span><span class="type-var">'a</span> list</span>, <span><span class="type-var">'a</span> list</span>, <span class="type-var">'err</span>)</span> <a href="index.html#type-validator_builder">validator_builder</a></span></code></dt><dd><p>Validate max length of a list</p><pre><code class="ml">let validator input =
  let open Validator in
  build build_valid
  |&gt; validate input.hobbies (list_has_max_length 12 &quot;Max 12 hobbies&quot;)</code></pre></dd></dl><