What is the difference between "Future.successful(None)" and "Future(None)"

Question

Future.apply starts an asynchronous computation whereas Future.successful creates an already completed Future with the specified result.

Now is Future(None) (Future.apply(None)) less efficient than Future.successful(None)?

Solution 1

Future.apply(None) creates asynchronous computation and executes it. It means that extra lambda object is created and extra task is scheduled (however trivial task).

Future.successful(None) just produces already completed future. It is more efficient.

Solution 2

I don't think that Future(None) gives a big overhead, but still in it's default implementation each call to apply spawns a new task for a ForkJoin thread pool, whereas Future.successful(None) completes immediately. And each call to map or flatMap on the future creates a new task for the poll, which also gives some overhead, so you might want to take a look at scalaz Future/Task implementation which handles such details more carefully.

Comments

[Fatih Donmez]

Can you give same real world example for Future.successful? I can't understand why we need that, except then existing interface waiting Future[T] and we want to pass some Future with already known result?

[Casey]

@FatihDonmez That's essentially the use case, yes. Why write two separate interfaces when you can just wrap the known values?