de> is often thought of as applying <code>f</code> &quot;in&quot; <code>T</code>.</p><p>The function <code>map</code> is the mapping of arrows, taking every arrow <code>'a -&gt; 'b</code> to an arrow <code>'a t -&gt; 'b t</code>.</p></dd></dl></details></div></div></div><dl><dt class="spec value" id="val-(&lt;@&gt;)"><a href="#val-(&lt;@&gt;)" class="anchor"></a><code><span class="keyword">val</span> (&lt;@&gt;) : <span>(<span class="type-var">'a</span> <span>&#45;&gt;</span> <span class="type-var">'b</span