Adds stream map combinator

Author: montekki

src/stream/stream/map.rs

@@ -0,0 +1,41 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+#[doc(hidden)]
+#[allow(missing_debug_implementations)]
+pub struct Map<S, F, T, B> {
+    stream: S,
+    f: F,
+    __from: PhantomData<T>,
+    __to: PhantomData<B>,
+}
+
+impl<S, F, T, B> Map<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 {
+        Map {
+            stream,
+            f,
+            __from: PhantomData,
+            __to: PhantomData,
+        }
+    }
+}
+
+impl<S, F, B> Stream for Map<S, F, S::Item, B>
+where
+    S: Stream,
+    F: FnMut(S::Item) -> 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));
+        Poll::Ready(next.map(self.as_mut().f()))
+    }
+}

src/stream/stream/mod.rs

@@ -32,6 +32,7 @@ mod find_map;
 mod fold;
 mod fuse;
 mod inspect;
+mod map;
 mod min_by;
 mod next;
 mod nth;
@@ -57,6 +58,7 @@ pub use chain::Chain;
 pub use filter::Filter;
 pub use fuse::Fuse;
 pub use inspect::Inspect;
+pub use map::Map;
 pub use scan::Scan;
 pub use skip::Skip;
 pub use skip_while::SkipWhile;
@@ -334,6 +336,37 @@ extension_trait! {
             Enumerate::new(self)
         }
 
+        #[doc = r#"
+            Takes a closure and creates a stream that calls that closure on every element of this stream.
+
+            # Examples
+
+            ```
+            # fn main() { async_std::task::block_on(async {
+            #
+            use async_std::prelude::*;
+            use std::collections::VecDeque;
+
+            let s: VecDeque<_> = vec![1, 2, 3].into_iter().collect();
+            let mut s = s.map(|x| 2 * x);
+
+            assert_eq!(s.next().await, Some(2));
+            assert_eq!(s.next().await, Some(4));
+            assert_eq!(s.next().await, Some(6));
+            assert_eq!(s.next().await, None);
+
+            #
+            # }) }
+            ```
+        "#]
+        fn map<B, F>(self, f: F) -> Map<Self, F, Self::Item, B>
+        where
+            Self: Sized,
+            F: FnMut(Self::Item) -> B,
+        {
+            Map::new(self, f)
+        }
+
         #[doc = r#"
             A combinator that does something with each element in the stream, passing the value
             on.