functional deferred construction

[jknack]

Deferred API work as callback where we must call resolve or reject method.

{
   get("/deferred", promise(deferred -> {
      deferred.resolve("OK");
   }));
}

This programming model makes perfect sense to integrate reactive/async libraries like rx, reactor, etc... but is kind of verbose for using within an Executor:

{
   executor(new ForkJoinPool());

   get("/deferred", promise(deferred -> {
      deferred.resolve("OK");
   }));
}

When we could just write:

{
   executor(new ForkJoinPool());

   get("/deferred", deferred(() -> {
     return "OK";
   }));
}

If an error occurs while resolving the value, we automatically catch the exception and call reject.

This is just more syntax sugar, no major change is required.