><a href="#val-fold" class="anchor"></a><code><span class="keyword">val</span> fold : <span>(<a href="index.html#type-key">key</a> <span>&#45;&gt;</span> <span class="type-var">'a</span> <span>&#45;&gt;</span> <span class="type-var">'b</span> <span>&#45;&gt;</span> <span class="type-var">'b</span>)</span> <span>&#45;&gt;</span> <span><span class="type-var">'a</span> <a href="index.html#type-t">t</a></span> <span>&#45;&gt;</span> <span class="type-var">'b</span> <span>&#45;&gt;</span> <span class="type-var">