" id="val-describe"><a href="#val-describe" class="anchor"></a><code><span class="keyword">val</span> describe : <a href="index.html#module-F">F</a>.formatter <span>&#45;&gt;</span> <a href="index.html#type-t">t</a> <span>&#45;&gt;</span> unit</code></dt></dl><dl><dt class="spec type" id="type-must_be_valid_reason"><a href="#type-must_be_valid_reason" class="anchor"></a><code><span class="keyword">type</span> must_be_valid_reason</code><code> = </code><table class="variant"><tr id="type-must_be_valid_reason