First attempt at successor

src/stream/stream/mod.rs

@@ -57,6 +57,7 @@ mod partial_cmp;
 mod position;
 mod scan;
 mod skip;
+mod successor;
 mod skip_while;
 mod step_by;
 mod take;

src/stream/stream/successor.rs

@@ -0,0 +1,59 @@
+use std::pin::Pin;
+use std::marker::PhantomData;
+
+use crate::future::Future;
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+#[derive(Debug)]
+pub struct Successor<F, Fut, T> 
+where Fut: Future<Output=T>
+{
+    successor: F,
+    next: T,
+    _marker: PhantomData<Fut>
+}
+
+pub fn successor<F, Fut, T>(func: F, start: T) -> Successor<F, Fut, T>
+where
+    F: FnMut(T) -> Fut,
+    Fut: Future<Output = T>,
+    T: Copy,
+    {
+        Successor {
+            successor: func,
+            next: start,
+            _marker: PhantomData,
+        }
+    }
+
+impl <F, Fut, T> Successor<F, Fut, T>
+where
+    F: FnMut(T) -> Fut,
+    Fut: Future<Output = T>,
+    T: Copy,
+
+{
+    pin_utils::unsafe_unpinned!(successor: F);
+    pin_utils::unsafe_unpinned!(next: T);
+}
+
+impl <F, Fut, T> Stream for Successor<F, Fut, T>
+where
+    Fut: Future<Output = T>,
+    F: FnMut(T) -> Fut,
+    T: Copy,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+
+        match self.as_mut().successor()(self.next).poll(cx) {
+            Poll::Pending => Poll::Pending,
+            Poll::Ready(val) => {
+                self.next = val;
+                Poll::Ready(Some(val))
+            }
+        }
+    }
+}