extern crate tokio_threadpool;
extern crate futures;

use futures::Future;
use futures::future::ok;
use futures::future::Either;
use futures::FutureExt;

fn a() -> impl Future<Item = Result<i32, u64>, Error = String> {
    ok(Ok(5))
}

fn do_it(a: impl Future<Item = Result<i32, u64>, Error = String>) -> impl Future<Item = i32, Error = Either<u64, String>> {
    a.then(|res| match res {
        Ok(Ok(a)) => Ok(a),
        Ok(Err(b)) => Err(Either::Left(b)),
        Err(c) => Err(Either::Right(c))
    })
}

fn main() {

}