Add a select combinator for streams

Similar to merge but requires the items/errors to be the same and then only
yields one at a time.

Closes rust-lang#239

src/stream/mod.rs

@@ -31,6 +31,7 @@ mod merge;
 mod once;
 mod or_else;
 mod peek;
+mod select;
 mod skip;
 mod skip_while;
 mod take;
@@ -50,12 +51,13 @@ pub use self::map_err::MapErr;
 pub use self::merge::{Merge, MergedItem};
 pub use self::once::{Once, once};
 pub use self::or_else::OrElse;
+pub use self::peek::Peekable;
+pub use self::select::Select;
 pub use self::skip::Skip;
 pub use self::skip_while::SkipWhile;
 pub use self::take::Take;
 pub use self::then::Then;
 pub use self::zip::Zip;
-pub use self::peek::Peekable;
 
 if_std! {
     use std;
@@ -801,6 +803,24 @@ pub trait Stream {
     {
         chunks::new(self, capacity)
     }
+
+    /// Creates a stream that selects the next element from either this stream
+    /// or the provided one, whichever is ready first.
+    ///
+    /// This combinator will attempt to pull items from both streams. Each
+    /// stream will be polled in a round-robin fashion, and whenever a stream is
+    /// ready to yield an item that item is yielded.
+    ///
+    /// The `select` function is similar to `merge` except that it requires both
+    /// streams to have the same item and error types.
+    ///
+    /// Error are passed through from either stream.
+    fn select<S>(self, other: S) -> Select<Self, S>
+        where S: Stream<Item = Self::Item, Error = Self::Error>,
+              Self: Sized,
+    {
+        select::new(self, other)
+    }
 }
 
 impl<'a, S: ?Sized + Stream> Stream for &'a mut S {

src/stream/select.rs

@@ -0,0 +1,64 @@
+use {Poll, Async};
+use stream::{Stream, Fuse};
+
+/// An adapter for merging the output of two streams.
+///
+/// The merged stream produces items from either of the underlying streams as
+/// they become available, and the streams are polled in a round-robin fashion.
+/// Errors, however, are not merged: you get at most one error at a time.
+#[must_use = "streams do nothing unless polled"]
+pub struct Select<S1, S2> {
+    stream1: Fuse<S1>,
+    stream2: Fuse<S2>,
+    flag: bool,
+}
+
+pub fn new<S1, S2>(stream1: S1, stream2: S2) -> Select<S1, S2>
+    where S1: Stream,
+          S2: Stream<Item = S1::Item, Error = S1::Error>
+{
+    Select {
+        stream1: stream1.fuse(),
+        stream2: stream2.fuse(),
+        flag: false,
+    }
+}
+
+impl<S1, S2> Stream for Select<S1, S2>
+    where S1: Stream,
+          S2: Stream<Item = S1::Item, Error = S1::Error>
+{
+    type Item = S1::Item;
+    type Error = S1::Error;
+
+    fn poll(&mut self) -> Poll<Option<S1::Item>, S1::Error> {
+        let (a, b) = if self.flag {
+            (&mut self.stream2 as &mut Stream<Item=_, Error=_>,
+             &mut self.stream1 as &mut Stream<Item=_, Error=_>)
+        } else {
+            (&mut self.stream1 as &mut Stream<Item=_, Error=_>,
+             &mut self.stream2 as &mut Stream<Item=_, Error=_>)
+        };
+        self.flag = !self.flag;
+
+        let a_done = match try!(a.poll()) {
+            Async::Ready(Some(item)) => return Ok(Some(item).into()),
+            Async::Ready(None) => true,
+            Async::NotReady => false,
+        };
+
+        match try!(b.poll()) {
+            Async::Ready(Some(item)) => {
+                // If the other stream isn't finished yet, give them a chance to
+                // go first next time as we pulled something off `b`.
+                if !a_done {
+                    self.flag = !self.flag;
+                }
+                return Ok(Some(item).into())
+            }
+            Async::Ready(None) if a_done => Ok(None.into()),
+            Async::Ready(None) => Ok(Async::NotReady),
+            Async::NotReady => Ok(Async::NotReady),
+        }
+    }
+}

tests/stream.rs

@@ -287,3 +287,18 @@ fn chunks() {
 fn chunks_panic_on_cap_zero() {
     let _ = list().chunks(0);
 }
+
+#[test]
+fn select() {
+    let a = iter(vec![Ok::<_, u32>(1), Ok(2), Ok(3)]);
+    let b = iter(vec![Ok(4), Ok(5), Ok(6)]);
+    assert_done(|| a.select(b).collect(), Ok(vec![1, 4, 2, 5, 3, 6]));
+
+    let a = iter(vec![Ok::<_, u32>(1), Ok(2), Ok(3)]);
+    let b = iter(vec![Ok(1), Ok(2)]);
+    assert_done(|| a.select(b).collect(), Ok(vec![1, 1, 2, 2, 3]));
+
+    let a = iter(vec![Ok(1), Ok(2)]);
+    let b = iter(vec![Ok::<_, u32>(1), Ok(2), Ok(3)]);
+    assert_done(|| a.select(b).collect(), Ok(vec![1, 1, 2, 2, 3]));
+}