The futures crate provides another useful abstraction for a lazily evaluated series of events, called Stream. If Future corresponds to Result, a Stream corresponds to Iterator. Semantically, they are very similar to futures, and they look like this:
trait Stream { type Item; type Error; fn poll(& mut self) -> Poll<Option<Self::Item>, Self::Error>; ...}
The only difference here is that the return type is wrapped in an Option, exactly like the Iterator trait. Thus, a None here would indicate that the stream has terminated. Also, all streams are futures and can be converted using into_future. Let us look at an example of using this construct. We will partially reuse our collatz example from a previous chapter. ...
