pe-t">t</a></span> <span class="arrow">&#45;&gt;</span></span> <span><span class="label">f</span>:<span>(<span><span class="label">key</span>:<a href="../index.html#type-t">t</a> <span class="arrow">&#45;&gt;</span></span> <span><span class="label">data</span>:<span class="type-var">'a</span> <span class="arrow">&#45;&gt;</span></span> unit)</span> <span class="arrow">&#45;&gt;</span></span> unit</span></code></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-fold"><a href="#val-fol