Adds stream::repeat_with

src/stream/mod.rs

@@ -27,6 +27,7 @@ pub use empty::{empty, Empty};
 pub use from_fn::{from_fn, FromFn};
 pub use once::{once, Once};
 pub use repeat::{repeat, Repeat};
+pub use repeat_with::{repeat_with, RepeatWith};
 pub use stream::{
     Chain, Filter, Fuse, Inspect, Scan, Skip, SkipWhile, StepBy, Stream, Take, TakeWhile, Zip,
 };
@@ -37,6 +38,7 @@ mod empty;
 mod from_fn;
 mod once;
 mod repeat;
+mod repeat_with;
 
 cfg_if! {
     if #[cfg(any(feature = "unstable", feature = "docs"))] {

src/stream/repeat_with.rs

@@ -0,0 +1,103 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+
+use crate::future::Future;
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+/// A stream that repeats elements of type `T` endlessly by applying a provided closure.
+///
+/// This stream is constructed by the [`repeat_with`] function.
+///
+/// [`repeat_with`]: fn.repeat_with.html
+#[derive(Debug)]
+pub struct RepeatWith<F, Fut, A> {
+    f: F,
+    future: Option<Fut>,
+    __a: PhantomData<A>,
+}
+
+/// Creates a new stream that repeats elements of type `A` endlessly by applying the provided closure.
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// # fn main() { async_std::task::block_on(async {
+/// #
+/// use async_std::prelude::*;
+/// use async_std::stream;
+///
+/// let s = stream::repeat_with(|| async { 1 });
+///
+/// pin_utils::pin_mut!(s);
+///
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(1));
+/// # }) }
+/// ```
+///
+/// Going finite:
+///
+/// ```
+/// # fn main() { async_std::task::block_on(async {
+/// #
+/// use async_std::prelude::*;
+/// use async_std::stream;
+///
+/// let s = stream::repeat_with(|| async { 1u8 }).take(2);
+///
+/// pin_utils::pin_mut!(s);
+///
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, None);
+/// # }) }
+/// ```
+pub fn repeat_with<F, Fut, A>(repeater: F) -> RepeatWith<F, Fut, A>
+where
+    F: FnMut() -> Fut,
+    Fut: Future<Output = A>,
+{
+    RepeatWith {
+        f: repeater,
+        future: None,
+        __a: PhantomData,
+    }
+}
+
+impl<F, Fut, A> RepeatWith<F, Fut, A> {
+    pin_utils::unsafe_unpinned!(f: F);
+    pin_utils::unsafe_pinned!(future: Option<Fut>);
+}
+
+impl<F, Fut, A> Stream for RepeatWith<F, Fut, A>
+where
+    F: FnMut() -> Fut,
+    Fut: Future<Output = A>,
+{
+    type Item = A;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        loop {
+            match &self.future {
+                Some(_) => {
+                    let res =
+                        futures_core::ready!(self.as_mut().future().as_pin_mut().unwrap().poll(cx));
+
+                    self.as_mut().future().set(None);
+
+                    return Poll::Ready(Some(res));
+                }
+                None => {
+                    let fut = (self.as_mut().f())();
+
+                    self.as_mut().future().set(Some(fut));
+                }
+            }
+        }
+    }
+}