Adds try_for_each combinator

src/lib.rs

@@ -47,6 +47,7 @@
 #![doc(test(attr(allow(unused_extern_crates, unused_variables))))]
 #![doc(html_logo_url = "https://async.rs/images/logo--hero.svg")]
 #![recursion_limit = "1024"]
+#![feature(try_trait)]
 
 use cfg_if::cfg_if;
 

src/stream/stream/mod.rs

@@ -40,6 +40,7 @@ mod skip;
 mod skip_while;
 mod step_by;
 mod take;
+mod try_for_each;
 mod zip;
 
 use all::AllFuture;
@@ -52,6 +53,7 @@ use fold::FoldFuture;
 use min_by::MinByFuture;
 use next::NextFuture;
 use nth::NthFuture;
+use try_for_each::TryForEeachFuture;
 
 pub use chain::Chain;
 pub use filter::Filter;
@@ -66,6 +68,7 @@ pub use zip::Zip;
 
 use std::cmp::Ordering;
 use std::marker::PhantomData;
+use std::ops::Try;
 
 use cfg_if::cfg_if;
 
@@ -921,6 +924,52 @@ extension_trait! {
             Skip::new(self, n)
         }
 
+        #[doc = r#"
+            Applies a falliable function to each element in a stream, stopping at first error and returning it.
+
+            # Examples
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use std::collections::VecDeque;
+            use std::sync::mpsc::channel;
+            use async_std::prelude::*;
+
+            let (tx, rx) = channel();
+
+            let s: VecDeque<usize> = vec![1, 2, 3].into_iter().collect();
+            let s = s.try_for_each(|v| {
+                if v % 2 == 1 {
+                    tx.clone().send(v).unwrap();
+                    Ok(())
+                } else {
+                    Err("even")
+                }
+            });
+
+            let res = s.await;
+            drop(tx);
+            let values: Vec<_> = rx.iter().collect();
+
+            assert_eq!(values, vec![1]);
+            assert_eq!(res, Err("even"));
+            #
+            # }) }
+            ```
+        "#]
+        fn try_for_each<F, R>(
+            self,
+            f: F,
+        ) -> impl Future<Output = R> [TryForEeachFuture<Self, F, Self::Item, R>]
+        where
+            Self: Sized,
+            F: FnMut(Self::Item) -> R,
+            R: Try<Ok = ()>,
+        {
+            TryForEeachFuture::new(self, f)
+        }
+
         #[doc = r#"
             'Zips up' two streams into a single stream of pairs.
 

src/stream/stream/try_for_each.rs

@@ -0,0 +1,56 @@
+use std::marker::PhantomData;
+use std::ops::Try;
+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 TryForEeachFuture<S, F, T, R> {
+    stream: S,
+    f: F,
+    __from: PhantomData<T>,
+    __to: PhantomData<R>,
+}
+
+impl<S, F, T, R> TryForEeachFuture<S, F, T, R> {
+    pin_utils::unsafe_pinned!(stream: S);
+    pin_utils::unsafe_unpinned!(f: F);
+
+    pub(crate) fn new(stream: S, f: F) -> Self {
+        TryForEeachFuture {
+            stream,
+            f,
+            __from: PhantomData,
+            __to: PhantomData,
+        }
+    }
+}
+
+impl<S, F, R> Future for TryForEeachFuture<S, F, S::Item, R>
+where
+    S: Stream,
+    S::Item: std::fmt::Debug,
+    F: FnMut(S::Item) -> R,
+    R: Try<Ok = ()>,
+{
+    type Output = R;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        loop {
+            let item = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+
+            match item {
+                None => return Poll::Ready(R::from_ok(())),
+                Some(v) => {
+                    let res = (self.as_mut().f())(v);
+                    if let Err(e) = res.into_result() {
+                        return Poll::Ready(R::from_error(e));
+                    }
+                }
+            }
+        }
+    }
+}