Re-add and_then and or_else to TryStreamExt

taiki-e · cramertj · commit 428405564130 · 2019-04-19T09:32:07.000-07:00
diff --git a/futures-util/src/try_stream/and_then.rs b/futures-util/src/try_stream/and_then.rs
@@ -0,0 +1,97 @@
+use core::pin::Pin;
+use futures_core::future::TryFuture;
+use futures_core::stream::{Stream, TryStream};
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+use pin_utils::{unsafe_pinned, unsafe_unpinned};
+
+/// Stream for the [`and_then`](super::TryStreamExt::and_then) method.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct AndThen<St, Fut, F> {
+    stream: St,
+    future: Option<Fut>,
+    f: F,
+}
+
+impl<St, Fut, F> AndThen<St, Fut, F>
+    where St: TryStream,
+          F: FnMut(St::Ok) -> Fut,
+          Fut: TryFuture<Error = St::Error>,
+{
+    unsafe_pinned!(stream: St);
+    unsafe_pinned!(future: Option<Fut>);
+    unsafe_unpinned!(f: F);
+
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Self { stream, future: None, f }
+    }
+
+    /// Acquires a reference to the underlying stream that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> &St {
+        &self.stream
+    }
+
+    /// Acquires a mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_mut(&mut self) -> &mut St {
+        &mut self.stream
+    }
+
+    /// Consumes this combinator, returning the underlying stream.
+    ///
+    /// Note that this may discard intermediate state of this combinator, so
+    /// care should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> St {
+        self.stream
+    }
+}
+
+impl<St, Fut, F> Stream for AndThen<St, Fut, F>
+    where St: TryStream,
+          F: FnMut(St::Ok) -> Fut,
+          Fut: TryFuture<Error = St::Error>,
+{
+    type Item = Result<Fut::Ok, St::Error>;
+
+    fn poll_next(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Self::Item>> {
+        if self.future.is_none() {
+            let item = match ready!(self.as_mut().stream().try_poll_next(cx)?) {
+                None => return Poll::Ready(None),
+                Some(e) => e,
+            };
+            let fut = (self.as_mut().f())(item);
+            self.as_mut().future().set(Some(fut));
+        }
+
+        assert!(self.future.is_some());
+        match ready!(self.as_mut().future().as_pin_mut().unwrap().try_poll(cx)) {
+            Ok(e) => {
+                self.as_mut().future().set(None);
+                Poll::Ready(Some(Ok(e)))
+            }
+            Err(e) => {
+                self.as_mut().future().set(None);
+                Poll::Ready(Some(Err(e)))
+            }
+        }
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+impl<S, Fut, F, Item> Sink<Item> for AndThen<S, Fut, F>
+    where S: TryStream + Sink<Item>,
+          F: FnMut(S::Ok) -> Fut,
+          Fut: TryFuture<Error = S::Error>,
+{
+    type SinkError = S::SinkError;
+
+    delegate_sink!(stream, Item);
+}
diff --git a/futures-util/src/try_stream/mod.rs b/futures-util/src/try_stream/mod.rs
@@ -11,6 +11,9 @@ use futures_core::task::{Context, Poll};
 #[cfg(feature = "compat")]
 use crate::compat::Compat;
 
+mod and_then;
+pub use self::and_then::AndThen;
+
 mod err_into;
 pub use self::err_into::ErrInto;
 
@@ -23,6 +26,9 @@ pub use self::map_ok::MapOk;
 mod map_err;
 pub use self::map_err::MapErr;
 
+mod or_else;
+pub use self::or_else::OrElse;
+
 mod try_next;
 pub use self::try_next::TryNext;
 
@@ -145,6 +151,77 @@ pub trait TryStreamExt: TryStream {
         MapErr::new(self, f)
     }
 
+    /// Chain on a computation for when a value is ready, passing the successful
+    /// results to the provided closure `f`.
+    ///
+    /// This function can be used to run a unit of work when the next successful
+    /// value on a stream is ready. The closure provided will be yielded a value
+    /// when ready, and the returned future will then be run to completion to
+    /// produce the next value on this stream.
+    ///
+    /// Any errors produced by this stream will not be passed to the closure,
+    /// and will be passed through.
+    ///
+    /// The returned value of the closure must implement the `TryFuture` trait
+    /// and can represent some more work to be done before the composed stream
+    /// is finished.
+    ///
+    /// Note that this function consumes the receiving stream and returns a
+    /// wrapped version of it.
+    ///
+    /// To process the entire stream and return a single future representing
+    /// success or error, use `try_for_each` instead.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use futures::channel::mpsc;
+    /// use futures::future;
+    /// use futures::stream::TryStreamExt;
+    ///
+    /// let (_tx, rx) = mpsc::channel::<Result<i32, ()>>(1);
+    ///
+    /// let rx = rx.and_then(|result| {
+    ///     future::ok(if result % 2 == 0 {
+    ///         Some(result)
+    ///     } else {
+    ///         None
+    ///     })
+    /// });
+    /// ```
+    fn and_then<Fut, F>(self, f: F) -> AndThen<Self, Fut, F>
+        where F: FnMut(Self::Ok) -> Fut,
+              Fut: TryFuture<Error = Self::Error>,
+              Self: Sized,
+    {
+        AndThen::new(self, f)
+    }
+
+    /// Chain on a computation for when an error happens, passing the
+    /// erroneous result to the provided closure `f`.
+    ///
+    /// This function can be used to run a unit of work and attempt to recover from
+    /// an error if one happens. The closure provided will be yielded an error
+    /// when one appears, and the returned future will then be run to completion
+    /// to produce the next value on this stream.
+    ///
+    /// Any successful values produced by this stream will not be passed to the
+    /// closure, and will be passed through.
+    ///
+    /// The returned value of the closure must implement the [`TryFuture`] trait
+    /// and can represent some more work to be done before the composed stream
+    /// is finished.
+    ///
+    /// Note that this function consumes the receiving stream and returns a
+    /// wrapped version of it.
+    fn or_else<Fut, F>(self, f: F) -> OrElse<Self, Fut, F>
+        where F: FnMut(Self::Error) -> Fut,
+              Fut: TryFuture<Ok = Self::Ok>,
+              Self: Sized,
+    {
+        OrElse::new(self, f)
+    }
+
     /// Wraps a [`TryStream`] into a type that implements
     /// [`Stream`](futures_core::Stream)
     ///
diff --git a/futures-util/src/try_stream/or_else.rs b/futures-util/src/try_stream/or_else.rs
@@ -0,0 +1,98 @@
+use core::pin::Pin;
+use futures_core::future::TryFuture;
+use futures_core::stream::{Stream, TryStream};
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+use pin_utils::{unsafe_pinned, unsafe_unpinned};
+
+/// Stream for the [`or_else`](super::TryStreamExt::or_else) method.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct OrElse<St, Fut, F> {
+    stream: St,
+    future: Option<Fut>,
+    f: F,
+}
+
+impl<St, Fut, F> OrElse<St, Fut, F>
+    where St: TryStream,
+          F: FnMut(St::Error) -> Fut,
+          Fut: TryFuture<Ok = St::Ok>,
+{
+    unsafe_pinned!(stream: St);
+    unsafe_pinned!(future: Option<Fut>);
+    unsafe_unpinned!(f: F);
+
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Self { stream, future: None, f }
+    }
+
+    /// Acquires a reference to the underlying stream that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> &St {
+        &self.stream
+    }
+
+    /// Acquires a mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_mut(&mut self) -> &mut St {
+        &mut self.stream
+    }
+
+    /// Consumes this combinator, returning the underlying stream.
+    ///
+    /// Note that this may discard intermediate state of this combinator, so
+    /// care should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> St {
+        self.stream
+    }
+}
+
+impl<St, Fut, F> Stream for OrElse<St, Fut, F>
+    where St: TryStream,
+          F: FnMut(St::Error) -> Fut,
+          Fut: TryFuture<Ok = St::Ok>,
+{
+    type Item = Result<St::Ok, Fut::Error>;
+
+    fn poll_next(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Self::Item>> {
+        if self.future.is_none() {
+            let item = match ready!(self.as_mut().stream().try_poll_next(cx)) {
+                None => return Poll::Ready(None),
+                Some(Ok(e)) => return Poll::Ready(Some(Ok(e))),
+                Some(Err(e)) => e,
+            };
+            let fut = (self.as_mut().f())(item);
+            self.as_mut().future().set(Some(fut));
+        }
+
+        assert!(self.future.is_some());
+        match ready!(self.as_mut().future().as_pin_mut().unwrap().try_poll(cx)) {
+            Ok(e) => {
+                self.as_mut().future().set(None);
+                Poll::Ready(Some(Ok(e)))
+            }
+            Err(e) => {
+                self.as_mut().future().set(None);
+                Poll::Ready(Some(Err(e)))
+            }
+        }
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+impl<S, Fut, F, Item> Sink<Item> for OrElse<S, Fut, F>
+    where S: TryStream + Sink<Item>,
+          F: FnMut(S::Error) -> Fut,
+          Fut: TryFuture<Ok = S::Ok>,
+{
+    type SinkError = S::SinkError;
+
+    delegate_sink!(stream, Item);
+}
diff --git a/futures/src/lib.rs b/futures/src/lib.rs
@@ -383,11 +383,11 @@ pub mod stream {
 
     pub use futures_util::try_stream::{
         TryStreamExt,
-        ErrInto, MapOk, MapErr,
+        AndThen, ErrInto, MapOk, MapErr, OrElse,
         TryNext, TryForEach, TryFilterMap,
         TryCollect, TryFold, TrySkipWhile,
         IntoStream,
-        // ToDo: AndThen, InspectErr, OrElse
+        // ToDo: InspectErr
     };
 
     #[cfg_attr(
@@ -398,7 +398,6 @@ pub mod stream {
     pub use futures_util::try_stream::{
         // For TryStreamExt:
         TryBufferUnordered, TryForEachConcurrent,
-        // ToDo: AndThen, InspectErr, OrElse
     };
 
     #[cfg(feature = "std")]
