Merge #163

163: adds stream::filter_map combinator r=yoshuawuyts a=montekki

Implements a `flat_map` combinator. Currently the same about `ret!` as in #162 .

Also the naming should probably be `FilterMapStream`, but in that case `Take` stream should also change it's name i guess.

Stdlib: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.flat_map
Ref: #129 

Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>

src/stream/stream/filter_map.rs

@@ -0,0 +1,48 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+/// A stream that both filters and maps.
+#[derive(Clone, Debug)]
+pub struct FilterMap<S, F, T, B> {
+    stream: S,
+    f: F,
+    __from: PhantomData<T>,
+    __to: PhantomData<B>,
+}
+
+impl<S, F, T, B> FilterMap<S, F, T, B> {
+    pin_utils::unsafe_pinned!(stream: S);
+    pin_utils::unsafe_unpinned!(f: F);
+
+    pub(crate) fn new(stream: S, f: F) -> Self {
+        FilterMap {
+            stream,
+            f,
+            __from: PhantomData,
+            __to: PhantomData,
+        }
+    }
+}
+
+impl<S, F, B> futures_core::stream::Stream for FilterMap<S, F, S::Item, B>
+where
+    S: futures_core::stream::Stream,
+    F: FnMut(S::Item) -> Option<B>,
+{
+    type Item = B;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+        match next {
+            Some(v) => match (self.as_mut().f())(v) {
+                Some(b) => Poll::Ready(Some(b)),
+                None => {
+                    cx.waker().wake_by_ref();
+                    Poll::Pending
+                }
+            },
+            None => Poll::Ready(None),
+        }
+    }
+}

src/stream/stream/mod.rs

@@ -23,6 +23,7 @@
 
 mod all;
 mod any;
+mod filter_map;
 mod min_by;
 mod next;
 mod nth;
@@ -32,6 +33,7 @@ pub use take::Take;
 
 use all::AllFuture;
 use any::AnyFuture;
+use filter_map::FilterMap;
 use min_by::MinByFuture;
 use next::NextFuture;
 use nth::NthFuture;
@@ -130,6 +132,43 @@ pub trait Stream {
         }
     }
 
+    /// Both filters and maps a stream.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use std::collections::VecDeque;
+    /// use async_std::stream::Stream;
+    ///
+    /// let s: VecDeque<&str> = vec!["1", "lol", "3", "NaN", "5"].into_iter().collect();
+    ///
+    /// let mut parsed = s.filter_map(|a| a.parse::<u32>().ok());
+    ///
+    /// let one = parsed.next().await;
+    /// assert_eq!(one, Some(1));
+    ///
+    /// let three = parsed.next().await;
+    /// assert_eq!(three, Some(3));
+    ///
+    /// let five = parsed.next().await;
+    /// assert_eq!(five, Some(5));
+    ///
+    /// let end = parsed.next().await;
+    /// assert_eq!(end, None);
+    /// #
+    /// # }) }
+    fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F, Self::Item, B>
+    where
+        Self: Sized,
+        F: FnMut(Self::Item) -> Option<B>,
+    {
+        FilterMap::new(self, f)
+    }
+
     /// Returns the element that gives the minimum value with respect to the
     /// specified comparison function. If several elements are equally minimum,
     /// the first element is returned. If the stream is empty, `None` is returned.