Implemented StreamExt::try_fold (#344)

src/stream/stream/mod.rs

@@ -49,6 +49,7 @@ mod skip_while;
 mod step_by;
 mod take;
 mod take_while;
+mod try_fold;
 mod try_for_each;
 mod zip;
 
@@ -69,6 +70,7 @@ use min_by::MinByFuture;
 use next::NextFuture;
 use nth::NthFuture;
 use partial_cmp::PartialCmpFuture;
+use try_fold::TryFoldFuture;
 use try_for_each::TryForEeachFuture;
 
 pub use chain::Chain;
@@ -1042,6 +1044,46 @@ extension_trait! {
             Skip::new(self, n)
         }
 
+        #[doc = r#"
+            A combinator that applies a function as long as it returns successfully, producing a single, final value.
+            Immediately returns the error when the function returns unsuccessfully.
+
+            # Examples
+
+            Basic usage:
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::prelude::*;
+            use std::collections::VecDeque;
+
+            let s: VecDeque<usize> = vec![1, 2, 3].into_iter().collect();
+            let sum = s.try_fold(0, |acc, v| {
+                if (acc+v) % 2 == 1 {
+                    Ok(v+3)
+                } else {
+                    Err("fail")
+                }
+            }).await;
+
+            assert_eq!(sum, Err("fail"));
+            #
+            # }) }
+            ```
+        "#]
+        fn try_fold<B, F, T, E>(
+            self,
+            init: T,
+            f: F,
+        ) -> impl Future<Output = Result<T, E>> [TryFoldFuture<Self, F, T>]
+        where
+            Self: Sized,
+            F: FnMut(B, Self::Item) -> Result<T, E>,
+        {
+            TryFoldFuture::new(self, init, f)
+        }
+
         #[doc = r#"
             Applies a falliable function to each element in a stream, stopping at first error and returning it.
 

src/stream/stream/try_fold.rs

@@ -0,0 +1,59 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+
+use crate::future::Future;
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct TryFoldFuture<S, F, T> {
+    stream: S,
+    f: F,
+    acc: Option<T>,
+    __t: PhantomData<T>,
+}
+
+impl<S, F, T> TryFoldFuture<S, F, T> {
+    pin_utils::unsafe_pinned!(stream: S);
+    pin_utils::unsafe_unpinned!(f: F);
+    pin_utils::unsafe_unpinned!(acc: Option<T>);
+
+    pub(super) fn new(stream: S, init: T, f: F) -> Self {
+        TryFoldFuture {
+            stream,
+            f,
+            acc: Some(init),
+            __t: PhantomData,
+        }
+    }
+}
+
+impl<S, F, T, E> Future for TryFoldFuture<S, F, T>
+where
+    S: Stream + Sized,
+    F: FnMut(T, S::Item) -> Result<T, E>,
+{
+    type Output = Result<T, E>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        loop {
+            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+
+            match next {
+                Some(v) => {
+                    let old = self.as_mut().acc().take().unwrap();
+                    let new = (self.as_mut().f())(old, v);
+
+                    match new {
+                        Ok(o) => {
+                            *self.as_mut().acc() = Some(o);
+                        }
+                        Err(e) => return Poll::Ready(Err(e)),
+                    }
+                }
+                None => return Poll::Ready(Ok(self.as_mut().acc().take().unwrap())),
+            }
+        }
+    }
+}