> That's it: a tiny adaptor that makes a bare function satisfy an interface. How is that possible?

No, why is it necessary?

In Java 8:

  public interface Handler {
      void serveHTTP(ResponseWriter w, Request r);
  }
  
  public class MyApplication {
      public static void myHandler(ResponseWriter w, Request r) {
          w.write("An even easier webserver?");
      }
  
      public static void main(String... args) {
          Http.handle("/", MyApplication::myHandler);
      }
  }

No adaptor needed!

Or even:

  public class MyApplication {
      public static void main(String... args) {
          Http.handle("/", (w, r) -> w.write("An even easier webserver?"));
      }
  }

And please bear in mind that this is Java, the slowest-moving and least sophisticated of contemporary languages! Why does Go require so much more boilerplate than Java?