Add Future.block()

[Diagoras]

There currently exists Future.get(), which is close to what I want, but will throw a NoSuchElementException instead of the underlying exception in the event of a failed future. I'd like an easy way to block on a Future, and force it to throw any exception that might have terminated its run.

[danieldietrich]

Hi, thank you for your suggestion. Currently (2.0.5) there is an ugly workaround:

future.await();    // blocks until completed and returns the future instance
future.getValue() // returns Option<Try<T>>
      .get()      // returns a Try<T> because we know that the Option is a Some
      .get();     // returns a Value of type T or throws a NonFatalException that wraps the original exception

With Javaslang 2.1.0 await() will return the Future instance.

Value sits at the top of the Javaslang type hierarchy. Currently it defines an abstract get() that throws NoSuchElementException in the empty case. However, Try extends Value and Try.get() throws a NonFatalException (which can be considered as a bug, it should throw a NoSuchElementException).

However, in 3.0.0 there will be some changes. I think we should allow get() to throw an arbitrary (non-fatal?) exception in the empty case. Additionally we will get rid of the NonFatalException wrapper by sneaky throwing arbitrary exceptions at runtime.

Then your wish could be expressed like this:

// a future Javaslang 3.0.0
future.get();

Until that you could use this helper:

import javaslang.concurrent.Future;
import javaslang.control.Try;

public class Test {

    public static void main(String[] args) {

        Future<String> future1 = Future.successful("ok");
        System.out.println(block(future1));

        Future<String> future2 = Future.failed(new Exception("error"));
        System.out.println(block(future2));
        
    }

    static <T> T block(Future<T> future) {
        future.await();
        final Try<T> result = future.getValue().get();
        if (result.isEmpty()) {
            sneakyThrow(result.getCause());
        }
        return result.get();
    }

    @SuppressWarnings("unchecked")
    static <T extends Throwable> void sneakyThrow(Throwable t) throws T {
        throw (T) t;
    }

}

Adding an additional block() method is currently no option for us because we want to keep the API as thin and simple as possible, await().get() will do the job. But it will take some time until 3.0.0 will be released... Too much to do, too few time.

[danieldietrich]

Fixed with #1976