1//! Module defining an Either type.
2use std::{
3 future::Future,
4 io::SeekFrom,
5 pin::Pin,
6 task::{Context, Poll},
7};
8use tokio::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, ReadBuf, Result};
9
10/// Combines two different futures, streams, or sinks having the same associated types into a single type.
11///
12/// This type implements common asynchronous traits such as [`Future`] and those in Tokio.
13///
14/// [`Future`]: std::future::Future
15///
16/// # Example
17///
18/// The following code will not work:
19///
20/// ```compile_fail
21/// # fn some_condition() -> bool { true }
22/// # async fn some_async_function() -> u32 { 10 }
23/// # async fn other_async_function() -> u32 { 20 }
24/// #[tokio::main]
25/// async fn main() {
26/// let result = if some_condition() {
27/// some_async_function()
28/// } else {
29/// other_async_function() // <- Will print: "`if` and `else` have incompatible types"
30/// };
31///
32/// println!("Result is {}", result.await);
33/// }
34/// ```
35///
36// This is because although the output types for both futures is the same, the exact future
37// types are different, but the compiler must be able to choose a single type for the
38// `result` variable.
39///
40/// When the output type is the same, we can wrap each future in `Either` to avoid the
41/// issue:
42///
43/// ```
44/// use tokio_util::either::Either;
45/// # fn some_condition() -> bool { true }
46/// # async fn some_async_function() -> u32 { 10 }
47/// # async fn other_async_function() -> u32 { 20 }
48///
49/// #[tokio::main]
50/// async fn main() {
51/// let result = if some_condition() {
52/// Either::Left(some_async_function())
53/// } else {
54/// Either::Right(other_async_function())
55/// };
56///
57/// let value = result.await;
58/// println!("Result is {}", value);
59/// # assert_eq!(value, 10);
60/// }
61/// ```
62#[allow(missing_docs)] // Doc-comments for variants in this particular case don't make much sense.
63#[derive(Debug, Clone)]
64pub enum Either<L, R> {
65 Left(L),
66 Right(R),
67}
68
69/// A small helper macro which reduces amount of boilerplate in the actual trait method implementation.
70/// It takes an invocation of method as an argument (e.g. `self.poll(cx)`), and redirects it to either
71/// enum variant held in `self`.
72macro_rules! delegate_call {
73 ($self:ident.$method:ident($($args:ident),+)) => {
74 unsafe {
75 match $self.get_unchecked_mut() {
76 Self::Left(l) => Pin::new_unchecked(l).$method($($args),+),
77 Self::Right(r) => Pin::new_unchecked(r).$method($($args),+),
78 }
79 }
80 }
81}
82
83impl<L, R, O> Future for Either<L, R>
84where
85 L: Future<Output = O>,
86 R: Future<Output = O>,
87{
88 type Output = O;
89
90 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
91 delegate_call!(self.poll(cx))
92 }
93}
94
95impl<L, R> AsyncRead for Either<L, R>
96where
97 L: AsyncRead,
98 R: AsyncRead,
99{
100 fn poll_read(
101 self: Pin<&mut Self>,
102 cx: &mut Context<'_>,
103 buf: &mut ReadBuf<'_>,
104 ) -> Poll<Result<()>> {
105 delegate_call!(self.poll_read(cx, buf))
106 }
107}
108
109impl<L, R> AsyncBufRead for Either<L, R>
110where
111 L: AsyncBufRead,
112 R: AsyncBufRead,
113{
114 fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<&[u8]>> {
115 delegate_call!(self.poll_fill_buf(cx))
116 }
117
118 fn consume(self: Pin<&mut Self>, amt: usize) {
119 delegate_call!(self.consume(amt))
120 }
121}
122
123impl<L, R> AsyncSeek for Either<L, R>
124where
125 L: AsyncSeek,
126 R: AsyncSeek,
127{
128 fn start_seek(self: Pin<&mut Self>, position: SeekFrom) -> Result<()> {
129 delegate_call!(self.start_seek(position))
130 }
131
132 fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<u64>> {
133 delegate_call!(self.poll_complete(cx))
134 }
135}
136
137impl<L, R> AsyncWrite for Either<L, R>
138where
139 L: AsyncWrite,
140 R: AsyncWrite,
141{
142 fn poll_write(self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &[u8]) -> Poll<Result<usize>> {
143 delegate_call!(self.poll_write(cx, buf))
144 }
145
146 fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<tokio::io::Result<()>> {
147 delegate_call!(self.poll_flush(cx))
148 }
149
150 fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<tokio::io::Result<()>> {
151 delegate_call!(self.poll_shutdown(cx))
152 }
153}
154
155impl<L, R> futures_core::stream::Stream for Either<L, R>
156where
157 L: futures_core::stream::Stream,
158 R: futures_core::stream::Stream<Item = L::Item>,
159{
160 type Item = L::Item;
161
162 fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
163 delegate_call!(self.poll_next(cx))
164 }
165}
166
167#[cfg(test)]
168mod tests {
169 use super::*;
170 use tokio::io::{repeat, AsyncReadExt, Repeat};
171 use tokio_stream::{once, Once, StreamExt};
172
173 #[tokio::test]
174 async fn either_is_stream() {
175 let mut either: Either<Once<u32>, Once<u32>> = Either::Left(once(1));
176
177 assert_eq!(Some(1u32), either.next().await);
178 }
179
180 #[tokio::test]
181 async fn either_is_async_read() {
182 let mut buffer = [0; 3];
183 let mut either: Either<Repeat, Repeat> = Either::Right(repeat(0b101));
184
185 either.read_exact(&mut buffer).await.unwrap();
186 assert_eq!(buffer, [0b101, 0b101, 0b101]);
187 }
188}
189