an></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><span class="type-var">'b</span> <a href="index.html#type-t">t</a></span></code></dt><dd><p><code>map ~f</code> maps the function <code>f : 'a -&gt; 'b</code> to a function <code>'f T : 'a T -&gt; 'b T</code>.</p><p>As an example, if <code>T (x : u) : u t</code> then <code>map ~(f:u -&gt; v) (T x)</code> is <code>T (f
      x) : v t</code>. As a result, <code>map</co