Merge pull request rust-lang#271 from pijul/master

Combinator to merge two futures yielding the same types

src/future/either.rs

@@ -0,0 +1,23 @@
+use {Future, Poll};
+/// Combines two different futures yielding the same item and error
+/// types into a single type.
+pub enum Either<A, B> {
+    /// First branch of the type
+    A(A),
+    /// Second branch of the type
+    B(B),
+}
+
+impl<A, B, Item, Error> Future for Either<A, B>
+    where A: Future<Item = Item, Error = Error>,
+          B: Future<Item = Item, Error = Error>
+{
+    type Item = Item;
+    type Error = Error;
+    fn poll(&mut self) -> Poll<Item, Error> {
+        match *self {
+            Either::A(ref mut a) => a.poll(),
+            Either::B(ref mut b) => b.poll(),
+        }
+    }
+}

src/future/mod.rs

@@ -47,6 +47,7 @@ mod map_err;
 mod or_else;
 mod select;
 mod then;
+mod either;
 
 // impl details
 mod chain;
@@ -62,6 +63,7 @@ pub use self::map_err::MapErr;
 pub use self::or_else::OrElse;
 pub use self::select::{Select, SelectNext};
 pub use self::then::Then;
+pub use self::either::Either;
 
 if_std! {
     mod catch_unwind;