Add Never type versions of Finished and Failed

Author: cramertj

src/lib.rs

@@ -177,18 +177,22 @@ pub use poll::Poll;
 // Primitive futures
 mod done;
 mod empty;
+#[cfg(not(feature = "never"))]
 mod failed;
+#[cfg(not(feature = "never"))]
 mod finished;
 mod lazy;
 #[cfg(feature = "never")]
 mod never;
 pub use done::{done, Done};
 pub use empty::{empty, Empty};
+#[cfg(not(feature = "never"))]
 pub use failed::{failed, Failed};
+#[cfg(not(feature = "never"))]
 pub use finished::{finished, Finished};
 pub use lazy::{lazy, Lazy};
 #[cfg(feature = "never")]
-pub use never::{never, Never};
+pub use never::{never, Never, failed, Failed, finished, Finished};
 
 // combinators
 mod and_then;

src/never.rs

@@ -3,6 +3,7 @@ use {Future, Poll};
 /// A future which is never resolved.
 ///
 /// This future can be created with the `empty` function.
+#[derive(Copy, Clone)]
 pub struct Never {}
 
 /// Creates a future which never resolves, representing a computation that never
@@ -23,3 +24,68 @@ impl Future for Never {
         Poll::NotReady
     }
 }
+
+/// A future representing a finished but erroneous computation.
+///
+/// Created by the `failed` function.
+pub struct Failed<E> {
+    e: Option<E>,
+}
+
+/// Creates a "leaf future" from an immediate value of a failed computation.
+///
+/// The returned future is similar to `done` where it will immediately run a
+/// scheduled callback with the provided value.
+///
+/// # Examples
+///
+/// ```
+/// use futures::*;
+///
+/// let future_of_err_1 = failed::<u32, u32>(1);
+/// ```
+pub fn failed<T, E>(e: E) -> impl Future<Item = T, Error = E> {
+    Failed { e: Some(e) }.map(|x| x)
+}
+
+impl<E> Future for Failed<E> {
+    type Item = !;
+    type Error = E;
+
+    fn poll(&mut self) -> Poll<!, E> {
+        Poll::Err(self.e.take().expect("cannot poll Failed twice"))
+    }
+}
+
+/// A future representing a finished successful computation.
+///
+/// Created by the `finished` function.
+pub struct Finished<T> {
+    t: Option<T>,
+}
+
+/// Creates a "leaf future" from an immediate value of a finished and
+/// successful computation.
+///
+/// The returned future is similar to `done` where it will immediately run a
+/// scheduled callback with the provided value.
+///
+/// # Examples
+///
+/// ```
+/// use futures::*;
+///
+/// let future_of_1 = finished::<u32, u32>(1);
+/// ```
+pub fn finished<T, E>(t: T) -> impl Future<Item = T, Error = E> {
+    Finished { t: Some(t) }.map_err(|x| x)
+}
+
+impl<T> Future for Finished<T> {
+    type Item = T;
+    type Error = !;
+
+    fn poll(&mut self) -> Poll<T, !> {
+        Poll::Ok(self.t.take().expect("cannot poll Finished twice"))
+    }
+}

tests/all.rs

@@ -107,6 +107,7 @@ fn test_finished() {
     assert_done(|| failed(1), err(1));
 }
 
+#[cfg(not(feature = "never"))]
 #[test]
 fn flatten() {
     fn finished<T: Send + 'static>(a: T) -> Finished<T, u32> {
@@ -125,6 +126,25 @@ fn flatten() {
     assert_empty(|| empty::<i32, u32>().map(finished).flatten());
 }
 
+#[cfg(feature = "never")]
+#[test]
+fn flatten() {
+    fn finished<T: Send + 'static>(a: T) -> impl Future<Item = T, Error = u32> {
+        futures::finished(a)
+    }
+    fn failed<E: Send + 'static>(b: E) -> impl Future<Item = i32, Error = E> {
+        futures::failed(b)
+    }
+
+    assert_done(|| finished(finished(1)).flatten(), ok(1));
+    assert_done(|| finished(failed(1)).flatten(), err(1));
+    assert_done(|| failed(1u32).map(finished).flatten(), err(1));
+    assert_done(|| futures::finished::<_, u8>(futures::finished::<_, u32>(1))
+                           .flatten(), ok(1));
+    assert_empty(|| finished(empty::<i32, u32>()).flatten());
+    assert_empty(|| empty::<i32, u32>().map(finished).flatten());
+}
+
 #[test]
 fn smoke_oneshot() {
     assert_done(|| {