Crate future_union

Future Union

When you use impl traits, specifically with futures, sometimes you will want to have a branching expression (e.g. an if or match) in which the different branches return different types that both impl Future. This does not work since in current stable rust impl trait can only refer to a single type.

One solution to this problem is to use futures::future::Either to combine together all your different possible futures together into one type to return. Doing this by hand is really annoying and requires sweeping changes when you change the number of possible branches.

This macro future_union does this automatically. Currently you still have to keep the total count per function, and then also the index (starting from 0). If those values are inaccurate then you’ll get horrible type errors.

Example

use futures::future::{self, Future};
use future_union::future_union;

fn impl_demo(n: usize) -> impl Future<Item=(), Error=()> {
   match n {
       0 => future_union!(3, 0, future::ok(())),
       1 => future_union!(3, 1, future::ok(()).map(|_| ())),
       _ => future_union!(3, 2, future::ok(()).map(|_| ()).map(|_| ())),
   }
}

Future (heh) plans:

• support for futures-0.3
• implement a function attribute macro that detects future_union calls in a given function and automatically adds the correct count and index e.g.:

  use futures::future::Future;
  #[future_union_fn]
  fn impl_demo(n: usize) -> impl Future<Item=(), Error=()> {
      match n {
          0 => future_union_auto!(future::ok(())),
          1 => future_union_auto!(future::ok(()).map(|_| ())),
          _ => future_union_auto!(future::ok(()).map(|_| ()).map(|_| ())),
      }
  }

Contributions welcome!

Macros

future_union