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