essage (optional message_validator)</code></pre></dd></dl><dl><dt class="spec value" id="val-keep"><a href="#val-keep" class="anchor"></a><code><span class="keyword">val</span> keep : <span class="type-var">'a</span> <span>&#45;&gt;</span> <span><span>(<span class="type-var">'a</span> <span>&#45;&gt;</span> <span class="type-var">'next_acc</span>, <span><span class="type-var">'e</span> <a href="index.html#type-errors">errors</a></span>)</span> Stdlib.result</span> <span>&#45;&gt;</span> <span><span>(<span 