How do I convert an iterator into a stream on success or an empty stream on failure?

#[macro_use] extern crate log; extern crate futures; // 0.1.21 extern crate tokio; use futures::prelude::*; use futures::{future, stream}; use std::fmt::Debug; use std::net::{SocketAddr, ToSocketAddrs}; fn resolve(addrs: impl ToSocketAddrs + Debug) -> impl Stream<Item = SocketAddr, Error = ()> { match addrs.to_socket_addrs() { Ok(iter) => stream::unfold(iter, |iter| match iter.next() { Some(a) => Some(future::ok((a, iter))), None => None, }), Err(e) => { error!("could not resolve socket addresses {:?}: {:?}", addrs, e); stream::empty() } } } fn main() { let task = resolve("1.2.3.4:12345") .map_err(|e| error!("{:?}", e)) .for_each(|addr| info!("{:?}", addr)) .fold(); tokio::run(task); }

error[E0308]: match arms have incompatible types --> src/main.rs:12:5 | 12 | / match addrs.to_socket_addrs() { 13 | | Ok(iter) => stream::unfold(iter, |iter| match iter.next() { 14 | | Some(a) => Some(future::ok((a, iter))), 15 | | None => None, ... | 20 | | } 21 | | } | |_____^ expected struct `futures::stream::Unfold`, found struct `futures::stream::Empty` | = note: expected type `futures::stream::Unfold<<impl ToSocketAddrs + Debug as std::net::ToSocketAddrs>::Iter, [closure@src/main.rs:13:42: 16:10], futures::FutureResult<(std::net::SocketAddr, <impl ToSocketAddrs + Debug as std::net::ToSocketAddrs>::Iter), _>>` found type `futures::stream::Empty<_, _>` note: match arm with an incompatible type --> src/main.rs:17:19 | 17 | Err(e) => { | ___________________^ 18 | | error!("could not resolve socket addresses {:?}: {:?}", addrs, e); 19 | | stream::empty() 20 | | } | |_________^ error[E0277]: the trait bound `(): futures::Future` is not satisfied --> src/main.rs:27:10 | 27 | .for_each(|addr| info!("{:?}", addr)) | ^^^^^^^^ the trait `futures::Future` is not implemented for `()` | = note: required because of the requirements on the impl of `futures::IntoFuture` for `()` error[E0599]: no method named `fold` found for type `futures::stream::ForEach<futures::stream::MapErr<impl futures::Stream, [closure@src/main.rs:26:18: 26:39]>, [closure@src/main.rs:27:19: 27:45], ()>` in the current scope --> src/main.rs:28:10 | 28 | .fold(); | ^^^^ | = note: the method `fold` exists but the following trait bounds were not satisfied: `&mut futures::stream::ForEach<futures::stream::MapErr<impl futures::Stream, [closure@src/main.rs:26:18: 26:39]>, [closure@src/main.rs:27:19: 27:45], ()> : futures::Stream` `&mut futures::stream::ForEach<futures::stream::MapErr<impl futures::Stream, [closure@src/main.rs:26:18: 26:39]>, [closure@src/main.rs:27:19: 27:45], ()> : std::iter::Iterator`

Rust is a statically typed language which means that the return type of a function has to be a single type, known at compile time. You are attempting to return multiple types, decided at runtime.

The closest solution to your original is to always return the Unfold stream:

fn resolve(addrs: impl ToSocketAddrs) -> impl Stream<Item = SocketAddr, Error = ()> {
    stream::unfold(addrs.to_socket_addrs(), |r| {
        match r {
            Ok(mut iter) => iter.next().map(|addr| future::ok((addr, Ok(iter)))),
            Err(_) => None,
        }
    })
}

But why reinvent the wheel?

futures::stream::iter_ok

Converts an Iterator into a Stream which is always ready to yield the next value.

Subsequent versions of the futures crate implement Stream for Either, which makes this very elegant:

fn resolve(addrs: impl ToSocketAddrs) -> impl Stream<Item = SocketAddr, Error = ()> {
    match addrs.to_socket_addrs() {
        Ok(iter) => stream::iter_ok(iter).left_stream(),
        Err(_) => stream::empty().right_stream(),
    }
}

It's straightforward to backport this functionality to futures 0.1 (maybe someone should submit it as a PR for those who are stuck on 0.1...):

enum MyEither<L, R> {
    Left(L),
    Right(R),
}

impl<L, R> Stream for MyEither<L, R>
where
    L: Stream,
    R: Stream<Item = L::Item, Error = L::Error>,
{
    type Item = L::Item;
    type Error = L::Error;

    fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
        match self {
            MyEither::Left(l) => l.poll(),
            MyEither::Right(r) => r.poll(),
        }
    }
}

trait EitherStreamExt {
    fn left_stream<R>(self) -> MyEither<Self, R>
    where
        Self: Sized;
    fn right_stream<L>(self) -> MyEither<L, Self>
    where
        Self: Sized;
}

impl<S: Stream> EitherStreamExt for S {
    fn left_stream<R>(self) -> MyEither<Self, R> {
        MyEither::Left(self)
    }
    fn right_stream<L>(self) -> MyEither<L, Self> {
        MyEither::Right(self)
    }
}

Even better, use the fact that Result is an iterator and Stream::flatten exists:

fn resolve(addrs: impl ToSocketAddrs) -> impl Stream<Item = SocketAddr, Error = ()> {
    stream::iter_ok(addrs.to_socket_addrs())
        .map(stream::iter_ok)
        .flatten()
}

Or if you really want to print errors:

fn resolve(addrs: impl ToSocketAddrs) -> impl Stream<Item = SocketAddr, Error = ()> {
    stream::once(addrs.to_socket_addrs())
        .map(stream::iter_ok)
        .map_err(|e| eprintln!("err: {}", e))
        .flatten()
}

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