ved">F</span>.formatter <span class="arrow">&#45;&gt;</span></span>
  <span><span class="type-var">'t</span> <span class="arrow">&#45;&gt;</span></span>
  unit</span></code></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-filter"><a href="#val-filter" class="anchor"></a><code><span><span class="keyword">val</span> filter : 
  <span><span class="label">fold</span>:<span><span>(<span class="type-var">'t</span>, <span class="type-var">'a</span>, <span class="type-var">'accum</span>)<